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 PL.T. 
 
 DONATIS COMET 
 
 Tromtlie ol3servatioiis of G. P. Eond.at Cambridge. United States 
 Octol)er 4- 185 8. 
 
 Ji. Gwllemm etH. Cleriet del. 
 
 iBecquet a.Taris .
 
 THE HEAVENS 
 
 %\x lllustrattb laiibkok 
 
 POPULAR ASTRONOMY 
 
 AMEDEE GUILLEMIN. 
 
 EDITED BY 
 
 J. NORMAN LOCKYER, F.R.A.S. 
 
 SECOND EDITION. 
 
 LONDON : 
 RICHARD BENTLEY, 
 
 PUBLISHER IN ORDINARY TO HER MAJESTY. 
 
 M.DCCC LXVTI. 
 
 The Bif/ht of Translation is rei^erved.
 
 LONDON: 
 iTRAKGEWAYS & Walden, Trinters, Castlc Street, Leicester Square.
 
 Library 
 
 PREFACE TO THE SECOND EDITION. 
 
 The Author, in wi-iting this popular essay, was ambitious of ob- 
 taining the support of those for whom it was specially written ; 
 that is to say, of the general reader and the young. He has been 
 so generously seconded by the entire press, that in this respect 
 he has succeeded beyond his expectations. He would, therefore, 
 be wanting in gratitude, if he failed to offer in this place his best 
 thanks to those whose praises have been for him so many encourage- 
 ments in the modest path he has chosen. 
 
 But what he did not dare to hope was, to be honoured by the 
 approbation of those who cultivate the highest branches of Astro- 
 nomy ; to see his book presented to the Academy of Sciences by 
 the illustrious Director of the Imperial Observatory, M. Leverrier, 
 and to the Royal Astronomical Society of London by its President, 
 Mr. Warren De La Rue ; to receive in flattering letters the con- 
 gratrdations of illustrious Astronomers, among whom he may be 
 permitted to cite the names of Sir John Herschel, the Rev. W. R. 
 Dawes, Professor Piazzi Smyth, Mr. Warren de La Rue, Mr. Lassell, 
 and Professor G. P. Bond ; and, lastly, to see an English translation 
 
 J undertaken under the editorship of Mr. Lockyer. 
 
 4 This Second Edition of " The Heavens " differs much from the 
 
 UJ 
 
 °^ first one. The Author has availed himself of the reprmt to revise 
 _^ the work with care, and to give, both to the descriptions and 
 
 explanations of the various phenomena, all the clearness of which 
 
 they appear to him susceptible. 
 
 Newly-published memoirs, and recent observations, have enabled 
 
 <..< 'J; O I.; (E-« ''-iL
 
 iv PREFACE TO THE SECOND EDITION. 
 
 him to add, to the different Chapters treating on the Physical 
 Constitution of Stars, all details of a nature likely to interest his 
 readers. The observations of Solar Spots, made by Mr. R. C. 
 Carrington with such admirable perseverance during more than 
 seven years; the work of a like nature by M. Chacornac and 
 Father Secchi, and M. Faye's resume of it ; Mr. Lockyer's Memoir 
 on the planet Mars ; Lord Rosse's new Catalogue of Nebulae, &c., 
 have furnished materials for new drawings faithfully copied from 
 the originals. 
 
 A large number of the wood- engravings have been replaced 
 or retouched, and new plates, printed in colours, added to those 
 which appeared in the first edition. 
 
 L'ENVOI. 
 
 En pr^sentant au public d'Angleterre un livre dont le succes en 
 France est du, sans doute, a la rarete des ouvrages vraiment popu- 
 laires d' Astronomic, j'eprouve le besoin de la mettre sous le pa- 
 tronage du savant distingue qui a bien voulu consentir a en diriger 
 la traduction et la publication. 
 
 C'est done, j)our moi, un devoir, bien doux a remplir du reste, 
 que de temoigner, ici meme, ma profonde gratitude a Mr. JSTorman 
 Lockyer pour les soins qu'il a donnes a ce travail de revision, et 
 surtout pour les importantes et savantes additions dont il a en- 
 richi le texte Fran9ais. Grace a son zele, aussi bienveillant 
 qu'eclaire, je me sens plus a I'aise pour solliciter les suffrages des 
 amis des sciences, en favour d'un livre qui a ete compose et inspire 
 pour le desir d'accroitre le nombre des intelligences aptes a com- 
 prendre et a admirer les sublimes connaissances de rAstronomie, 
 et assez courageuses pour se vouer a leurs progres. 
 
 AMEDEE GUILLEMIN.
 
 PREFACE. 
 
 I AM among those who believe, that the natural and physical 
 sciences possess in themselves attraction sufficient to render any 
 ornament superfluous. This conviction has been my only guide in 
 the conception and writing of this book, which is not, indeed, a 
 scientific one, but a faithful picture of the phenomena offered by the 
 Heavens to man's intelligent admiration. 
 
 My plan was, therefore, ready to my hand, and I had but to 
 follow Natui'e, as now revealed to us by Astronomy, in all her ma- 
 jestic simplicity. All my efibrts have had for their object to repre- 
 sent her in all her details and as a whole. 
 
 I write for those who, though interested in science, have neither 
 the time nor the*wish to become professional astronomers : in a word, 
 for youth and unscientific "children of larger growth." It has 
 been my wish, that The Heaa^ens shoidd be read with something 
 of the charm of a romance, or, at least, with that so powerfvd interest 
 which belongs to travellers' tales of unknown lands. For, after all, 
 is not the mind a traveller, when it follows Science through the 
 far-ofi" regions of the ethereal sky, journepng on from stage to stage, 
 that is, from Sun to Sim, to the very confines of the \-isible Uni- 
 verse ? In our narrative of this journey through the infinite, the 
 reader, it is true, will find no sudden turns of fortune, no unexpected 
 accident to make the heart beat quicker at the thought of the 
 sufierings of one of our feUow-men ; but, on the other hand, it will
 
 yi PREFACE. 
 
 be given him to contemplate the most sublime of all pictures— the 
 majesty of tremendous phenomena, the unalterable and eternal 
 harmony of the laws of Nature. 
 
 What a vast field, moreover, what a magnificent horizon is 
 presented by the Heavens to the most active of human faculties, 
 to the imagination ! When our sight, aided by the most powerful 
 instruments, dives into the depths of space, and finds, instead of 
 feebly strung points of light, worlds like our own, some smaller, 
 some larger than it, a thousand questions rise to our lips. We 
 find oui'selves involuntarily making in thought a hundred travels, 
 more interesting, more strange, more marvellous than those the 
 scene of which lies on our own planet. 
 
 Basing our work on the facts already acquired, we set ourselves 
 to build up our neighbouring worlds ; the configuration of their 
 continents and seas, the rivers which water them, their moun- 
 tains, which are the very skeletons of worlds, the living inhabitants, 
 animal and vegetable, which people them, all present themselves 
 before us in the most varied forms. Forced by an irresistible 
 instinct to people these worlds with free and intelligent beings, we 
 help them at their work, in their wars ; we ask if they, like us, 
 have a history and traditions ; then, the thought that our humanity 
 is but one individual state of being among those, which on all 
 the worlds throughout boundless space work out their destiny, 
 comes to console us ; we are no longer alone to seek after truth, 
 and the realization of justice and goodness. 
 
 These, doubtless, are questions concerning which Astronomy 
 brings no message to us, and which will long, possibly always, 
 continue in the domain of conjecture. Therefore, we have not dwelt 
 upon them in this book, leaving the reader to solve them as his 
 imagination may lead him. But the coldest mind, the mind least 
 accessible to the suggestions of fancy, cannot entirely banish them. 
 In spite of itself, there comes a moment, an hour of reverie, when 
 it too propounds the same problems ; and truly we cannot wish 
 it otherwise. Does it not afibrd one proof the more, of the truth 
 of what day by day becomes more evident, that science borders 
 on poetry?
 
 PREFACE. Vii 
 
 In order to make Astronomy accessible to all, it was necessary 
 to banish from the work the mathematical portion of the science, 
 which forms the essential element in the special treatises on the 
 subject. But, on the other hand, the most interesting details 
 relative to the constitution of the worlds which people space, the 
 most recent observations made by the magnificent instruments 
 now erected in the Observatories of Europe and America, occupy 
 a large place in this phj^sical description of the Universe. 
 
 One word on the sources whence I have selected materials for 
 the book. 
 
 I wished to place this attempt to popularize science on a level 
 with the most recent and most authentic discoveries. I therefore 
 addressed myself directly to the most illustrious astronomers, both 
 in the Old and New "Worlds. All of them have liberally lent me 
 the aid of their knowledge ; original memoirs, photographs, have 
 been forwarded to me from the various scientific centres, with a 
 generosity for which I must here pubKcly express my extreme 
 gratitude. Nor have I lacked encoiiragement and advice. The 
 venerable patriarch of contemporary astronomers. Sir John Herschel, 
 Admiral Smyth, Mr. Warren De La Rue, and Mr. Lassell in 
 England ; the illustrious Director of the Observatory of PoulkoA\'a, 
 M. Otto Struve, in Russia ; M. Littrow, in Germany ; and Pro- 
 fessor G. P. Bond in America, are among foreign astronomers 
 those to whom my best thanks are due for their generous aid. 
 
 In France, M. Leverrier at once placed the library of the 
 Imperial Observatory at my disposal, and gave me permission to 
 make from nature the drawings of the most important instrimients 
 in this magnificent establishment. MM. Laussedat, Chacornac, 
 and Goldschmidt have aided me by their advice, and have 
 communicated to me their observations. 
 
 Nor is this all. I have largely placed under contribution all 
 the ancient and modern publications on Astronomy, the interesting 
 works of Schroter, Laplace, Beer and Madler, the two Struvcs ; 
 Harding's Celestial Atlas, and that constructed by the illustrious 
 astronomer of Bonn, Argelauder ; the special periodical so full of 
 fiicts, the Adronomische Nachrichten of Altona, the Memoirs and
 
 Vlll 
 
 PREFACE. 
 
 Monthlij Notices of the Royal Astronomical Society of London; 
 the works of Airy, Hind, Lord Rosse, and Sir Thomas Maclear ; 
 the publications of the Academy of Sciences of St. Petersburg, 
 Humboldt's Cosmos, Arago's admirable Astronomij ; and, finally, 
 all the precious communications to be found in the ComjJtes Rendus 
 of the French Academy of Sciences, in which the names of such 
 Frenchmen as Arago, Biot, Babinet, Faye, and Delaunay, are found 
 associated with those of all the samns, members of this great re- 
 public of science, who belong to other countries. 
 
 Such have been my feUow-labourers in the preparation of this 
 book. It will be easily understood, that it was not a question 
 of o-atherinff at hazard from the immense collection of ancient 
 and modern works, which form the archives of astronomy : it 
 was necessary that I should choose the most incontested facts, the 
 most recent and most authentic observations ; that I should discuss 
 and compare the numbers, which in Astronomy have such inter- 
 esting meanings ; that I should often go over calculations which 
 lead to them myself: in one word, that I should show to the 
 public, for whom the book has been written, and who have not 
 always the means of verifying an Author's statements, with what 
 respect for the truth, and with what conscientious care, I have 
 acquitted myself of a work so attractive to me. 
 
 It rests with the reader now, to say, whether I have known 
 how to profit by these materials, and whether, like a painter before 
 a beautiful landscape, I have been able to portray beauties of the 
 grandest of all scenes, that of the infinite variety of the stars, 
 moving in concert through boimdless space.
 
 LIST OF ILLUSTRATIONS. 
 
 PLATES. 
 
 Frontispiece. 
 
 I. 
 
 II. 
 
 III. 
 
 IV. 
 
 V. 
 
 VI. 
 
 VII. 
 
 VIIT. 
 
 IX. 
 
 X. 
 
 XI. 
 
 XII. 
 
 XIII. 
 
 XIV. 
 
 XV. 
 
 XVI. 
 
 xvn. 
 xvni. 
 
 XIX. 
 
 XX. 
 
 XXI. 
 
 XXII. 
 
 XXIII. 
 
 Donati's Comet ....... to face 
 
 Plan and inclination of the Planetary orbit.s 
 Sun-spots (Sir J. Herschel) . 
 
 The Zodiacal Light in Japan (PI. II.) 
 
 The Zodiacal Light. Its aspect in Europe 
 
 The Earth viewed from Space 
 
 Orbit of the Earth (The Seasons) 
 
 The Phases of the Moon 
 
 The Full Moon 
 
 Lunar Topography 
 
 Lunar Landscape . 
 
 EcHpse of the Sun, July 18, 1860 . . (PL III.) to face 
 
 Annular and total Eclipses of the Sun 
 
 Total and partial Eclipse of the Moon . (PI. IV.) to face 
 
 Meteor and its Train .... (PI. V.) „ 
 
 Mars (PL VI.) „ 
 
 Jupiter (Warren De La Rue) 
 
 Saturn, after Bond, Struve, and Warren De La Rue 
 
 Forms of Comets 
 
 Halley's Comet 
 
 Donati's Comet, 24th and 26th September, 1858 
 Donati's Comet, October 3, 6, 1858 
 Heads and Tails of the Comets of 1858 and 1861 
 Comparative Dimensions of the Sun, the Planets, and their 
 Satellites .... ... 
 
 XXIV. 
 XXV. 
 
 XXVI. 
 XXVII. 
 XXVIII. 
 
 XXIX. 
 
 The Heavens (looking north). Midnight, Dec. 20 (PL VII.) 
 The Heavens (looking south) „ „ (PI- VII.) 
 
 The Heavens ( „ ) at the Vernal Equinox (PL VIII.) 
 
 The Heavens ( „ ) at the Summer Solstice (PL IX.) 
 
 The Heavens (, „ ) at the Autumnal Equinox (PL X.) 
 
 The Southern Heavens .... (PI. XI.) 
 
 PAGE 
 
 TITLE 
 1 
 
 33 
 
 85 
 87 
 101 
 115 
 127 
 139 
 151 
 157 
 171 
 177 
 183 
 196 
 206 
 229 
 241 
 273 
 279 
 287 
 291 
 295 
 
 301 
 
 TO FACE PACE 
 
 307 
 323 
 325 
 327 
 3.3(J 
 .332
 
 X • LIST OF ILLUSTRATIONS. 
 
 PLATE 
 
 PAGE 
 
 XXX. Coloured Stars (R- XIT.) 356 
 
 XXXI. Star-Clusters 375 
 
 XXXir. Northern Milky Way 383 
 
 XXXIII. Southern Milky Way 385 
 
 XXXIV. Nebulse in Doradus and surrounding « ArgAs .... 409 
 XXXV. Nebula of Orion 413 
 
 XXXVI. Lord Eosse's great Telescope 440 
 
 XXXVII. Foucault's Silver-on-Glass Reflector 491 
 
 XXXVIII. Transit Instruments in the Paris Observatory . . .497 
 XXXIX. Great Equatorial in the Paris Observatory . . . .501 
 
 WOOD -ENGRAVINGS. 
 
 via. 
 
 1. Apparent dimensions of the Solar disk at the Earth's extreme and 
 
 mean distances 18 
 
 2. The apparent size of the Sun as seen from the principal planets . . 19 
 
 3. Dimensions of the Sun deduced from its distance and its apparent 
 
 diameter 22 
 
 4. Comparative dimensions of the Sun and of the orbit of the Moon . 23 
 
 5. Sun-spots observed on the 2nd September, 1839. (Captain Davis.) . 27 
 
 6. Apparent change in the form of spots approaching or receding from 
 
 the centre 28 
 
 7. Difference of time in the apparent rotation of the Sun, and in its real 
 
 rotation 30 
 
 8. Different paths of Sun-spots at different periods of the year . . 31 
 
 9. Sun-spot showing umbra, penumbra, and luminous bridge. (Nasmyth.) 32 
 
 10. Enormous Sun-spots. (Davis.) 36 
 
 11. Transformation of groups of Solar spots in the interval of a rotation. 
 
 Observed by M. Pastorff on 24th May and 21st June, 1828 . . 37 
 
 12. Changes of Solar spots in the interval of one rotation. Details of 
 
 groups A and B. (Fig. 11.) (Pastorff.) 38 
 
 13. Sun-spots surrounded by a platform of faculse. (Capocci.) . . 39 
 
 14. Spot with faculse, May 24, 1863. (Captain Noble.) .... 40 
 
 15. Sun-spot, April 2, 1865. (Lockyer.) 42 
 
 16. Explanation of Sun-spots on Wilson's hypothesis .... 46 
 
 17. Explanation of Sun-spots on Wilson's hypothesis. Appearances pre- • 
 
 seuted by the same spot as seen at the centre and near the limb . 47 
 
 18. Explanation of Sun-spots on Kirchhoff 's hypothesis .... 50 
 
 19. Solar cyclone, May 5, 1857. (Secchi.) 52 
 
 20. Change in shape of a Sun-spot, Oct. 27, 29, 31, and Nov. 2. (Dawes.) 53 
 
 21. Cloud-masses detaching themselves from the penumbra. (Lockyer.) . 54 
 
 22. Phases of Mercury when seen after sunset 64 
 
 23. Phases of Mercury when seen before sunrise 64 
 
 24. Explanation of the Phases of Mercury 65 
 
 25. Apparent dimensions of the disk of Mercury at its extreme and mean 
 
 distances from the Earth 67
 
 LIST OF ILLUSTRATIONS. 
 
 XI 
 
 FIG. 
 
 26. 
 
 27. 
 
 28. 
 
 The size of IMerciiry compared with the Earth 
 Transit of Mercury over the Suu: 1. 12th Novemlier, 1861 
 November, 1868 
 
 2. 5th 
 
 Aspect of the disk of Mercury during its transit over the Sun, 7th 
 May, 1799. Vaporous aureola and bright point on the disk 
 (Schroter.) 
 
 29. Orbit of Mercury 
 
 30. Equatorial band of Mercury. (Schroter.) 
 
 31. Crescent of Mercury, showing irregularities on the terminator and the 
 
 truncation of the Southern Horn. (Schroter.) 
 
 32. Superior and inferior conjunction of Venus. Greatest and least dis 
 
 tance from the Earth 
 
 33. Apparent dimensions of Venus at its extreme and mean distances from 
 
 the Earth 
 
 34. Comparative dimensions of Venus and the Earth 
 
 35. Effect of irradiation ......... 
 
 36. Irregularities in the terminator of Venus. (Schroter.) Spots on its 
 
 two hemispheres. (Bianchini.) 
 
 37. Venus at one of its solstices. Inclination of the axis of rotation 
 
 38. Direction of the axis of the Zodiacal Light 
 
 39. The Earth suspended in space 
 
 40. Curvature of the Surface of the Land. Horizons of the same place at 
 
 different altitudes . 
 
 41. Curvature of the surface of the sea 
 
 42. Curvature of the sea (Explanation) ...... 
 
 43. Elliptical form of the terrestrial meridians. Diminubion in the length 
 
 of a degree from the Pole to the Equator .... 
 
 44. Comparative heights of Mountains ; depth of the 'Seas, and thicknes 
 
 of the solid crust of the Earth ...... 
 
 45. Atmospheric refraction, showing its effect on the apparent places of 
 
 bodies in the celestial vault 
 
 46. Deformation of the Sun's limb at sunset 
 
 47. Compai'ative length of the Sidereal and Solar day 
 
 48. Real Orbit of the Earth, and apparent Orbit of the Sun 
 
 49. The Eai'th at an Equinox. Equality of day and night all over the 
 
 world 
 
 50. The Earth at a Solstice. Unequal day and night 
 
 51. Orbit of the Earth. Varying length of the different seasons 
 
 52. Last phase of the Moon ........ 
 
 53. Orbit of the Moon. Explanation of the Phases .... 
 
 54. Apparent dimensions of the Moon at its extreme and mean distances 
 
 from the Eai'th ......... 
 
 55. Difference of the distances of the Moon, at the horizon and at the 
 
 zenith 
 
 56. Comparative dimensions of the Earth and Moon 
 
 57. The lunar Orbit. 1. Amplified. 2. In its relative dimensions 
 
 58. The curve described in a year by the Moon round the Earth 
 
 59. Rotation of a sphere, supposed to be at rest .... 
 
 60. Actual movement of rotation of the Moon in the interval of a lunation 
 
 61. Mountains of the Moon. View of the region to the south-east of 
 
 Tycho. (Nasmyth.) 
 
 62. The Mountains of the Moon. View of Copernicus. (Nasmyth.) 
 
 PAGE 
 
 67 
 
 68 
 
 70 
 72 
 73 
 
 77 
 
 78 
 78 
 79 
 
 80 
 82 
 89 
 93 
 
 94 
 95 
 95 
 
 97 
 
 99 
 
 104 
 105 
 109 
 113 
 
 118 
 119 
 120 
 126 
 129 
 
 130 
 
 131 
 133 
 134 
 135 
 136 
 136 
 
 144 
 145
 
 xu 
 
 KIG. 
 
 63. 
 
 64. 
 65. 
 66. 
 
 67. 
 68. 
 
 69. 
 
 70. 
 71. 
 72. 
 
 73. 
 
 74. 
 75. 
 
 76. 
 
 77. 
 78. 
 
 79. 
 
 80. 
 81. 
 
 82. 
 83. 
 
 84. 
 
 85. 
 86. 
 
 87. 
 
 Path of the shadow 
 
 LIST OF ILLUSTRATIONS. 
 
 The Peak of Teuerifi'e and its Envh'ons. (Piazzi Smyth.) 
 Topographical rehef of the Isle of Bourbon. (Maillard.) 
 
 General theory of Eclipses 
 
 Total Echpse of the Sun : Theory .... 
 
 Annular Eclipse of the Sun : Theory . 
 
 Total Echpse of the Sun on the 18th July, 1860 
 
 and penumbra on the surface of the Earth 
 
 Total Echpse of the Sun of the 18th July, 1860. Rounded and trun- 
 cated form of the horns of the Solar Crescent. (Laussedat.) . 
 Path of the Moon in the Earth's cone of shade. Total Eclipse . 
 Path of the Moon in the Earth's cone of shade. Partial Eclipse . 
 Radiant Point of Shooting Stars. R.A. 94°, N. Decl. 37°. Radiant 
 
 Point of Meteors observed at Hawkhurst, Nov. 28. (Alexander 
 
 Herschel.) 
 
 Ring of meteorical bodies circulating round the Sun 
 Appearance of a Meteor in a Telescope. (Schmidt.) 
 Mass of meteoric iron, weighing 15 cwt., found in 1828 by Brard at 
 
 Caille (Var) 
 
 Aerolite found at Juvenas (Ardeche), June 5th, 1821 
 
 Orbit and Phases of Mars 
 
 Apparent dimensions of Mars at its mean and extreme distances from 
 
 the Earth 
 
 Mars and the Earth ; comparative dimensions 
 
 Views of Mars at two hours' interval. (Warren De La Rue.) 
 
 Rotation of Mars. Movements of the spots observed during the 
 
 opposition of 1830. (Beer and Miidler.) 
 Inclination of the axis of rotation. Mars at one of its solstices 
 Orbits of Ereia and Polyhymnia, compared together, and with that of 
 
 the Earth 
 
 Comparative dimensions of the Earth, and Juno, Ceres, Pallas, and 
 
 Vesta 
 
 89. 
 
 90. 
 91. 
 92. 
 93, 
 94. 
 
 95, 
 
 96. 
 97. 
 98, 
 
 Echptic chart, from M. Chacornac's " Star Atlas" 
 Discovery of a small planet by means of Ecliptic Charts 
 
 Apparent dimensions of Jupiter at its mean and extreme distances 
 from the Earth 
 
 Comparative dimensions of Jupiter and the Earth 
 
 Rotation of Jupiter. Apparent displacement of two spots in 
 37'" 15' 
 
 Inclination of the axis of Jupiter to the plane of its orbit . 
 
 Jupiter and its Four Satellites 
 
 The Jovian system. Orbits of the Satellites .... 
 
 Eclipses and Transits of the Satellites of Jupiter, seen from the Earth 
 
 Dimensions of the Satellites of Jupiter compared with those of the 
 
 Earth and the Moon 
 
 Apparent dimensions of Saturn at its extreme and mean distances 
 from the Earth, showing also the different appearances presented 
 by its ring-system 
 
 Saturn and the Earth ; Comparative dimensions . . . . 
 
 Bird's-eye view of Saturn and its ring-system 
 
 Saturn, Dec. 26, 1861, at 18" 30'". (Wray.) Luminous appendages 
 of the rmg. The Satellites are also seen apparently on tile ring . 
 
 PAGE 
 
 146 
 147 
 168 
 172 
 172 
 
 173 
 
 176 
 184 
 
 185 
 
 191 
 193 
 195 
 
 198 
 198 
 201 
 
 202 
 204 
 205 
 
 206 
 209 
 
 216 
 
 219 
 221 
 222 
 
 224 
 225 
 
 226 
 227 
 231 
 232 
 234 
 
 235 
 
 239 
 
 240 
 245 
 
 247
 
 LIST OF ILLUSTRATIONS. 
 
 xin 
 
 KIO. 
 
 99. 
 
 100. 
 
 101. 
 
 102, 
 
 103, 
 104. 
 105. 
 
 106. 
 
 107. 
 108. 
 109. 
 
 110. 
 111. 
 
 112. 
 113. 
 
 114. 
 
 115. 
 116. 
 117. 
 118. 
 119. 
 120. 
 121. 
 122. 
 123. 
 
 124. 
 
 125. 
 
 126. 
 127. 
 
 128. 
 129. 
 
 130. 
 131. 
 132. 
 133. 
 134. 
 
 Saturn, Jan 
 (Wray.) 
 
 Explanation of the Phases of Saturn's rings 
 ances of the rings 
 
 1862, at \4^ 0"'. Luminous appendage of the ring. 
 
 Periodical disappear- 
 
 Saturn, Nov. 22, 1848. (Bond.) Luminous points visible near the 
 period of the disappearance of the ring 
 
 Explanation of the brilliant points observed near the epoch of the 
 
 disappearance of the ring •••••... 
 
 Saturn and its satellites. (Sir J. Herschel.) 
 
 Bird's-eye view of the orbits of Saturn's satellites .... 
 The rings seen from Saturn at a latitude of about 28°. Ideal view, 
 
 at midnight, towards the period of the Saturnian solstices . ! 
 The rings seen from Saturn at a latitude of about 28°. Ideal view 
 
 taken at midnight, between the Saturnian equinoxes and the solstices 
 
 Ideal view of a phase of Saturn 
 
 The globe of Saturn, seen from the ring ...... 
 
 Apparent dimensions of Uranus at its mean and extreme distances 
 
 from the Earth •••■•..... 
 
 Comparative size of Uranus and the Earth • . . . . 
 
 Difference between the apparent forms of a flattened globe, seen in 
 
 two different positions ......... 
 
 System of the satellites of Uranus ; relative dimensions of the orbits 
 Inclination of the planes of the orbits of the satellites to the orbit of 
 
 Uranus ........... 
 
 Apparent dimensions of the disk of Neptune at its mean and extrem 
 
 distances from the Earth ....... 
 
 Neptune and the Earth ; comparative dimensions 
 
 Satellite of Neptune ......... 
 
 1. Tailless Comet. 2. Head without tail or nucleus . 
 
 Comet of 1744 (Cheseaux's Comet), with multiple tail 
 
 Forms of cometary orbits ........ 
 
 Comparison of the excentricity of the planetary and cometary orbit 
 Orbits of the periodic Comets ....... 
 
 Subdivision of Gam bart's Comet. (Struve.) .... 
 
 Great Comet of 1811, from a drawing by Admiral Smyth, in the 
 " Speculum Hartwellianum" 
 
 Comet of 1862. Forms and positions of the luminous jets, 23rd 
 August, at one o'clock in the morning, and at nine in the evening 
 
 Comet of 1862. Aspect of the head of the Comet, 23rd August 
 at nine in the evening, and 24th August at the same hour 
 
 Relative brightness of the stars of the first six magnitudes 
 The same part of the constellation of the Twins, as seen through a 
 telescope .......... 
 
 A part of the constellation of the Twins, as seen with the naked eye 
 
 The sky of the horizon of London. Northern circumpolar constella 
 tions ........... 
 
 The heavens of the horizon of London. Equatorial zone. Orion 
 
 The heavens of the horizon of London. Equatorial zone. The Lion 
 
 The heavens of the horizon of London. Equatorial zone . 
 
 The heavens of the horizon of London. The Lyre, Swan, and Eagle 
 
 The heavens of the horizon of London. Equatorial zone . 
 
 248 
 
 248 
 
 249 
 
 250 
 251 
 252 
 
 254 
 
 255 
 256 
 257 
 
 260 
 260 
 
 261 
 261 
 
 262 
 
 266 
 266 
 267 
 271 
 271 
 272 
 276 
 281 
 283 
 
 289 
 
 290 
 
 293 
 312 
 
 314 
 315 
 
 319 
 324 
 325 
 327 
 328 
 329
 
 XIV 
 
 LIST OF ILLUSTRATIONS. 
 
 FIG. 
 
 135. stars invisible at London. Southern circumpolar zone. The Southern 
 
 Cross ....•••••••• 
 
 136. Stars invisible at London. Southern circumpolar zone. Argo . 
 
 137. Stars invisible at London. Southern circumpolar zone 
 
 138. Point of the heavens towards which the Solar System is travelling in 
 
 its movement of revolution ........ 
 
 139. The trapezium of Orion, (Sir J. Herschel.) 
 
 140. Variable star Algol in Perseus 
 
 140a. Variable-star Chart, u Cancri 
 
 141. New and temporary star of 1572, in the constellation of Cassiopea . 
 
 142. Variable star. Eta Argfts 
 
 143. The Pleiades. (Harding.) 
 
 144. The Pleiades as seen with the naked eye 
 
 145. The Hyades. (Harding.) 
 
 146. Prsesepe in Cancer 
 
 147. Star-clusters near «w Centauri. (Sir J. Herschel.) . . . . 
 
 148. Cluster in Aquarius. (Lord Eosse.) 
 
 149. Cluster in Toucan. (Sir J. Herschel.) 
 
 150. Clusters of singular forms. (Sir J. Herschel.) 1. Cluster in Scorpio. 
 
 2. Cluster in the Altar ......... 
 
 151. Nebulae of circular and oval forms. (Sir J. Herschel.) 
 
 152. Annular nebvdse. 1. In Lyra (Sir J. Herschel). 2. The same (Lord 
 
 Eosse). 3. Annular nebula in Cygnus. 4. Li Ophiuchus. 5. In 
 Scorpio. 6; Nebula near y Andromeda) .... 
 
 153. Conical or cometary nebulae. 1. In Eridanus (Sir J. Herschel). 2. In 
 
 the Unicorn (Lord Eosse). 3. In the Great Bear (Sir J. Herschel) 
 
 154. Nebula in Argo. (Sir J. Herschel.) 
 
 155. Nebula in Canes Venatici. (Sir J. Herschel.) . 
 
 156. Spiral form of the nebula in Canes Venatici. (Lord Eosse.) 
 
 157. Spiral nebula in Virgo. (Lord Eosse.) .... 
 
 158. Spiral nebulse. (Lord Eosse.) 1. Of the Lion. 2. Of Pegasus 
 
 159. Nebula in Andromeda ....... 
 
 160. Nebula in Andromeda. (G. P. Bond.) .... 
 
 161. Elliptical annular nebula of the Lion. (Sir J. Herschel.) . 
 
 162. Dumb-bell nebula. (Sir J. Herschel.) .... 
 
 163. Dumb-bell nebula. (Loi'd Eosse.) 
 
 164. Nebula in Sobieski's shield. (Sir. J. Herschel.) 
 
 165. Nebula in Taurus (Crab nebula.) (Lord Eosse.) 
 
 166. Planetary nebulse. (Sir J. Herschel.) 1. In Ursa Major. 2. In 
 
 Pisces. 3. In Andromeda 
 
 167. Planetary nebulsD. (Lord Eosse.) 1. In Ursa Major. 2. In Andromeda 
 
 168. Nebulous stars. (Sir J. Herschel.) 1. In Cygnus. 2. In Perseus 
 
 3. In the Centaur. 4. In Sagittarius. 5. In Auriga. 6. In Andromeda 
 
 169. Nebulous stars. (Lord Eosse.) 1. In Gemini. 2. In Argo 
 
 170. Double and multiple nebulse. (Sir. J. Herschel.) 1 and 4. In Virgo 
 
 2 and 5. In Coma Berenices. 3. In Aquarius. 6. In the Nubecula 
 Major (Clouds of Magellan) 
 
 171. Double nebula. (Lord Eossq.) 
 
 172. Magellanic Clouds. The Small Cloud .... 
 
 173. Magellanic Clouds. The Great Cloud .... 
 
 174. Magellanic Clouds. A portion of the great Cloud. (Sir J. Herschel.) 
 
 333 
 334 
 335 
 
 346 
 349 
 360 
 363 
 365 
 367 
 370 
 370 
 371 
 372 
 377 
 378 
 379 
 
 380 
 397 
 
 398 
 
 399 
 399 
 400 
 401 
 402 
 403 
 404 
 405 
 406 
 406 
 407 
 408 
 411 
 
 416 
 417 
 
 418 
 418 
 
 420 
 421 
 424 
 424 
 425
 
 LIST OF ILI.rSTRATIOXS. 
 
 XY 
 
 FIG. 
 
 175. 
 176. 
 177. 
 178. 
 179. 
 
 180. 
 
 181. 
 182. 
 183. 
 184. 
 185. 
 186. 
 187. 
 188. 
 189. 
 190. 
 191. 
 
 Section of the Milky Way ; position of the Sun in it 
 Method of Constructing an Ellipse .... 
 
 Orbits of equal areas 
 
 Attraction of the Moon on the waters of the sea. Simple lunar tide 
 
 Combined action of the Moon and the Sun on the waters of the sea 
 
 Luni-solar tide of the syzygies. .... 
 
 Neutralizing action of the Moon and Sun on the waters 
 
 Luni-solar tides of the quadratures ... 
 
 Measure of the distance of an inaccessible object 
 Measure of the distance of the Moon 
 Measure of the Sun's distance by the transit of Venus 
 
 Transit of Venus, 1882 
 
 Apparent variation in the height of a tower at different d 
 Theoretical Section of the Astronomical Telescope 
 Section of the Astronomical Telescope 
 
 Newtonian Eeflector 
 
 Gregorian Reflector ....... 
 
 "Front View" Reflector 
 
 Transit Eye-piece 
 
 of the sea 
 
 stances 
 
 PAGE 
 
 432 
 
 444 
 446 
 464 
 
 465 
 
 466 
 474 
 476 
 478 
 480 
 482 
 486 
 488 
 489 
 489 
 490 
 499
 
 CONTENTS. 
 
 PART THE FIRST 
 
 THE SOLAR SYSTEM. 
 
 The bodies wMcli form the Solar System — Direction of movements of rota- 
 tion and revolution — Inclinations of the planes of the orbits of the different 
 planets Page 11 
 
 BOOK THE FIRST. 
 
 THE SUN. 
 
 I. Form and apparent size of the Sun — Its distance from the Earth, and 
 real size — Its surface, volume, mass, and weight 1" 
 
 II. Solar Observation — Sun-spots — The Sun's movement of Rotation — Tlie 
 Kew Photographs of the Sun — The Telescopic appearance of the general sur- 
 face — Mr. Nasmyth's "Willow-leaves" — Opinions of other Astronomers — The 
 appearance of the penumbra — Umbra and penumbra of spots — Enormous spots 
 — Their rapid change of form — Mr. Carrington's researches on their proper 
 motion 26 
 
 III. Theories of the physical constitution of the Sun —Wilson's theory — 
 Kirchhoff's theory— Their antagonism — Opinions of M. Faye and Mr. Herbert 
 Spencer— Solar cyclones — Probable down-rush of clouds into a spot, and conse- 
 quent disappearance — Intensity of the Sun's light and heat ... 44 
 
 h
 
 CONTENTS. 
 
 BOOK THE SECOND. 
 
 THE PLANETS. 
 
 I. Mercury : Apparent movement and phases — Distances from the Sun and 
 and Earth — Form and dimensions ; its transits across the Sun's disk — Length 
 of day and night, seasons and climates — Equatorial belts, atmosphere and moun- 
 tains of Mercury — Mass, density, and force of gravity on its surface . G3 
 
 II. Venus : Distance from the Sun — Apparent and real movements : form 
 of the orbit — Distance of Venus from the Earth — Real dimensions, form, and 
 surface-markings : rotation — Day and night on Venus ; atmosphere, seasons and 
 climates — Physical constitution 75 
 
 III. The Zodiacal Light : Aspect of the Zodiacal Light in the various regions 
 of the Earth — Probable existence of a large luminous ring situated between the 
 Earth and the Sun 85 
 
 IV. The Earth: The Earth suspended in space — Proof of its spheroidal 
 form — Its dimensions, mass, and mean density — Atmospheric refraction; its 
 eflfects on the appearance of the disks of the Sun and Moon ... 92 
 
 V. The Earth : Movement of Rotation — Apparent diurnal movement of the 
 Stars and Sun — Real rotation of the Earth — The difFercucc between sidereal and 
 solar days — The rapidity of the Earth's rotation varies with the latitude . 107 
 
 VI. The Earth : Revolution round the Sun — The year — Dimensions of 
 the Earth's orbit — The seasons — Differences in the length of days and nights 
 according to the seasons and latitudes — Zones and chmates . . . 112 
 
 VII. The Moon : Phases of the Moon — Its movement round the Earth — 
 Dimensions and distances — Recent measures — Effect of irradiation in the case 
 of the Moon — Rotation — Rotation and revolution performed in the same time 
 — The Moon's orbit always concave to the Sun — The Moon's rotation on its 
 axis 124 
 
 VIII. The Moon : Physical Constitution — The aspect of the Moon to the 
 naked eye — The seas or Maria; mountains — Principal mountain chains — 
 Volcanic character of the lunar mountains — The craters Tycho and Copernicus 
 — Walled plains — Annular mountain-ramparts — Craters, peaks, and cones — 
 Terrestrial analogies — Heights and dimensions of the mountains — Bright rays ; 
 Centres from which they emanate ; Mr. Nasmyth's explanation of them — Rilles 
 or furrows — Suggested explanations ; Recent labours of Schmidt . .138 
 
 IX. The Moon: Physical Constitution {Continued) — Ah^Qnce of air and 
 water on the Moon's surface — Has the Moon an atmosphere ? — Aspect of a 
 lunar landscape — The Moon's past history; Professor Fraukland's hypothesis 
 based on traces of glacier-action — The Moon's climate — Days, nights, and sea-
 
 CONTENTS. 
 
 XIX 
 
 SOUS — Extent of the visible and invisible portions of the lunar globe 
 
 Astronomy from a Lunarian's point of view — Lunar photography ; the British 
 Association " Moon Committee" 154 
 
 X. Eclipses of the Sun and Moon : General theory of Eclipses — Eclipse of 
 the Sun can only take place at the time of New Moon — The Eclipses of the 
 Moon happen at opposition — Why each lunation is not accompanied by two 
 Eclipses 1(57 
 
 XL Eclipses op the Sun : Conditions of the possibility and visibility of 
 eclipses of the Sun — Total, annular, and partial Eclipses — Path of the Moon's 
 shadow along the Earth — Longest possible duration of solar Eclipses — The 
 
 corona, red prominences ; they belong to the Sun ; their shape and height 
 
 Influence of the phenomena of Eclipses on living beings .... 171 
 
 XIL Eclipses of the Moon: Conditions of possibility and visibility of 
 Eclipses of the Moon — Partial and total Eclipses — Colour of the lunar disk 
 
 during the phases of a total Eclipse — Periodicity and calculation of Eclipses 
 
 Occultations of the fixed Stars and Planets ...... 183 
 
 XIIL The Meteoric Rings: Shooting or Falling Stars — "Star-showers" — 
 Their numbers — Radiant points — Position, form, and inclination of the Meteoric 
 Rings — Heights, velocities, and weights of shooting stars — Luminous Meteors 
 (Bolides), their telescopic appearance — Meteorites : Professor Maskelyne's clas- 
 sification ; Mr. Scorby's microscopic examination of them, and its results — Re- 
 markable ^leteorites 188 
 
 XIV. Mars: Movement of Mars round the Sun — Mars, in opposition ; Con- 
 ditions necessary for a favourable opposition — Bright and dark spots — Effects 
 of the transit of clouds — Colour of the Planet's disk ; Why Mars is sometimes 
 red — Polar snows — I\Ielting of the Polar snows — Rotation — Seasons and 
 climate 200 
 
 XV. The Minor Planets : Considerable number of celestial bodies circu- 
 lating round the Sun between Mars and Jupiter — Bode's law — Gibers' hypo- 
 thesis — Interlacing of their orbits — Some details on the princijDal Planets of 
 the group, Juno, Pallas, Ceres, and Vesta — How to discover a new Planet 212 
 
 XVI. Jupiter : Distances of Jupiter from the Earth and Sun — Real and 
 apparent dimensions — Movements of rotation ; days and nights — Years and 
 seasons — Dark and light belts on its disk ; atmosphere — Satellites, their move- 
 ments and distances — Eclipses of the Satellites — Their real dimensions 223 
 
 XVII. Saturn : Its exceptional phydical constitution — Distances of Saturn 
 from the Sun and from the Earth — Apparent and real dimensions — Movement 
 of rotation and Polar compression — Days, nights, and seasons — Rings ; Move- 
 ment of rotation — Satellites — Celestial phenomena to an inhabitant of the 
 Planet 237 
 
 XVIII. Uranus : Discovery of Uranus in the last century — Form and di- 
 mensions of its orbit — Its apparent and real dimensions — Its Satellites; 
 Inclinations of their orbits, and directions of their movements . . • 259
 
 XX CONTENTS. 
 
 XIX. Neptune: Discovery of Neptune — The method of discovery — Dis 
 tance -Apparent and real dimensions — Volume and density — SatelUte^ of 
 Neptune . . . • 
 
 264 
 
 BOOK THE THIRD. 
 
 THE COMETS. 
 
 I. Aspect of Comets — Head: nucleus; tails, simple and multiple — How 
 Comets are distinguished from the other bodies of the Solar System — Forms 
 and inclinations of their orbits ; direction of their movements . . .270 
 
 n. Comets of Short Period — Halley's Comet ; its return in 1759 and in 1835 
 — Encke's Comet of short period ; Acceleration of its movement — Division of 
 Gambart'sComet— Elements of the principal periodical Comets . . 277 
 
 ni. Comets of Long Period — Large Comets visible to the naked eye — 
 Physical constitution of Comets ; mass, density — Nature of their light— Danger 
 which might result from the contact of a Comet 285 
 
 PART THE SECOND. 
 
 THE SIDEREAL SYSTEM. 
 
 BOOK THE FIRST. 
 
 THE STARS. 
 
 L The Stars : Scintillation of the Stars — Apparent fixity of their relative 
 distances — Number of Stars visible to the naked eye — Approximate number of 
 Stars visible in telescopes 309 
 
 n. The Constellations : General survey of the Starry Heavens — Constella- 
 tions visible on the horizon of London — Northern Circumpolar Zone . 316 
 
 in. The Constellations {Continued) : Our survey (continued), Equatorial 
 and Southern Constellations visible on the horizon of London . . . 323 
 
 IV. The Constellations {Contimied) : Our survey of the Starry Heavens 
 (continued) — Southern Circumpolar Zone — Stars invisible at London . 332 
 
 V. Distances of the Stars : Distances of some Stars from the Earth — Time 
 required by light to reach the nearest Stars — A rough idea of the dimensions of 
 the visible Universe 337
 
 CONTENTS. Xxi 
 
 VI. MovEiiENTS OF Staks : Stars not immovable in space — Measure of their 
 proper motions ; Velocities of some of them — Translation of the Solar System 
 through space 343 
 
 VII. Double and Multiple Stars : Distinction between optical and physical 
 doubles — Characteristics of the latter — Movements of revolution of double 
 Stars — ilultiple systems 343 
 
 VIII. Coloured Stars : Variety of colours presented by the Stars — Single 
 coloured Stars — Colours of double and multiple Stars — Variations observed 
 in colour — Presumed causes of their changes 355 
 
 IX. Variable Star.s : Periodical changes of brilliancy of Mira Ceti, and 
 Algol, in Perseus — Other variable Stars — Explanation of these changes — 
 Hypotheses — How to observe a " Variable " 359 
 
 X. Temporary Stars : New Stars — Temporary Stars of 1572 — Lost Stars 
 — Explanation of these sudden appearances and disappearances . . 364 
 
 XI. Star Groups : Natural Groupings of Stars — Groups visible to the naked 
 eye — The Pleiades — The Hyades — Praesepe 369 
 
 XII. Star-clusters: Clusters of Stars of globular or spherical form — 
 Enormous number of Stars in certain clusters — Clusters in Perseus, in Ceu- 
 tauras, Toucan, Aquarius, &c. — Curious forms of some clusters . . 374 
 
 XIII. The Milky Way: General aspect of the Milky "Way — Its course 
 through the Northern and Southern Constellations — Resolvability into Stars and 
 Star-clusters — Impenetrability of certain regions of the ]Milky Way . 381 
 
 XIV. Physical and Chemical Constitution of the Stars . . . 388 
 
 BOOK THE SECOND. 
 
 THE XEBUL^. 
 
 I. Nebulae of Regular Form : Circular, elliptical, annular, and spiral 
 Nebula) — Annular Nebula in Lyra — Spiral NebulaB in Canes Venatici, Virgo, 
 and Leo 396 
 
 II. Nebula of Irregular Forms: Large Nebulous masses affecting no 
 symmetrical form — Variation of aspect with instruments employed — Nebulae 
 in the Constellations of Andromeda, the Lion, Fox, Sobieski's Shield and the 
 Bull — Great irregular Nebulae of Orion and Argo 404 
 
 III. Planetapy Nebula and Nebulous Star:! : Planetary Nebulaj — Varia- 
 tion of aspect — Nebulous Stars . . . . . . • .416 
 
 IV. Double and Multiple Nebula : Groups of Nebula — Probabihty of a 
 physical connexion between the components — Multiple Nebute in the larger 
 Magellanic Cloud "^20
 
 xxii CONTENTS. 
 
 V. Magellanic Clouds : Position of tlie two Magellanic Clouds in the 
 Southern sky — Structure of the Little and Great Cloud — Star-clusters, isolated 
 Stars and Nebulcc which they contain 423 
 
 VI. Physical Constitution of Nebula 427 
 
 BOOK THE THIRD. 
 
 STRUCTURE OF THE HEAVENS. 
 
 I. Our own Universe — The Milky "Way : Eeal form of the Stellar stratum 
 which composes the Milky Way — Position of the Sun in the interior of this 
 stratum — General idea of its dimensions 431 
 
 II. Other Universes 435 
 
 PAKT THE THIRD. 
 
 THE LAWS OF ASTRONOMY, 
 
 BOOK THE FIRST. 
 
 Kepler's laws — universal GRA^TTATlo^■. 
 
 I. Kepler's Laws : The Planets describe Ellipses round the Sun — Law of 
 Areas — Counexion between the time of revolution and the mean distances of the 
 Planets from the Sun 443 
 
 II. Universal Gravitation : Gravity on the surface of the Earth ; Law of 
 the diminution of the force of gravity with increased distance — The fall of the 
 Moon towards the Earth — Gravitation is universal — How the Sun and Planets 
 arc weighed ............ 449 
 
 III. Precession of the Equinoxes — Nutation — Planetary perturbation . 455 
 
 IV. The Tides : Phenomena of the ebb and flow, high and low-water — 
 Epochs of spring-tides — Coincidence of the phenomena with the positions of the 
 Moon and Sun — Theory of the tides deduced from the law of gravitation — 
 Combined actions of the Sun and Moon ....... 459 
 
 V. Origin and Formation of the Solar System : Exposition of Laplace's 
 hypothesis of the origin and formation of the Solar World — Primitive Nebula 
 — Luminous Nucleus— Formation of Planets and Satelhtes — Direction of the 
 MovementsofEotation and Revolution 467
 
 CONTENTS. 
 
 XXUl 
 
 BOOK THE SECOND. 
 
 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 I. Celestial Measurkments : General idea of the problem by which the 
 distance of inaccessible objects is determined — Solution of this problem on the 
 Earth's Surface — Distance of the Earth from the Moon — Polar Parallax, Dis- 
 tance of the Sun from the Earth — Stellar Parallax, Distances of the Stars 473 
 
 II. Astronomical Instruments : Visit to an Observatory ; Instruments for 
 obtaining magnified images of celestial objects —The Astronomical Telescope- 
 Newtonian, Herschelian, Gregorian, and Silver-on-glass Reflectors — Instruments 
 used in Observatoi'ies ; Transit Circle, Equatorial ..... 415 
 
 APPENDIX. 
 
 TABLES. 
 
 Abbreviations and Symbols 
 II. Elements of the Planets . 
 
 III. Elements of the Satellites 
 
 IV. The Sun 
 
 V. Additional Elements of the Moon 
 
 VI. Saturn's Rings 
 
 VII. Time 
 
 VIII. Variable Stars .... 
 IX. Celestial Objects 
 X. Test Objects . . , . 
 
 507 
 508 
 509 
 510 
 510 
 511 
 511 
 512 
 515 
 523
 
 I 
 
 The Editor's Additions and Notes are enclosed in brackets [ ]. 
 
 The Notes followed by initials have been contributed re- 
 spectively by 
 
 B. S. . . . Mr. Balfour Stewart. 
 
 W. K. D. . . . Rev. W. R. Dawes. 
 
 T. W. W. . . Rev. T. W. Webb. 
 
 W. R. B. . . . Mr. W. R. BiRT.
 
 .^^' 
 
 ■^x 
 
 PUtnr 0/ ll,e Fd: 
 
 Plot,,- of tlie Ectiptic 
 
 THE SOLAR SYSTEM.
 
 THE HEAVENS. 
 
 "What are the Heavens ? Where the shores of that limitless ocean ; 
 where the bottom of that unfathomable abyss ? 
 
 What are those brilliant points — those innmnerable stars, which, 
 never dim, shine out imceasing-ly from the dark profoiuid ? Are they 
 sbwn broadcast — orderless, \\dih. no other bond save that which 
 perspective lends to them ? Or, if not immovable, as we have so long 
 imagined, if not golden nails fixed to a crystal vault, whither are 
 they bound ? And, finally, what are the parts assigned to the 
 Sun, our Earth, and all the Earths attendant on the glorious orb of 
 day, in this tremendous concert of celestial spheres, — this sublime 
 harmony of the Universe ? 
 
 These are magnificent problems, of which the most fertile imagi- 
 nation would have in vain attempted the solution, if, for the greater 
 glory of the human mind, Astronomy — first-born of the sciences, — 
 had not at length come to our aid. 
 
 How wonderful is the power of man ! Chained down to the sur- 
 face of the Earth, an intelligent atom on a grain of sand lost in the 
 immensity of space, he invents instruments which multiply a thou- 
 sandfold his vision, he soimds the depths of the ether, gauges the 
 visible universe, and counts the myriads of stars which people it; 
 next, studying their most complicated movements, he measures 
 exactly their dimensions and the distances of the nearest of them 
 from the Earth, and next deduces their masses, then, discovering in 
 the seeming disorder of the stellar groupings real bonds of union, 
 he at last evolves order from apparent confusion. 
 
 Nor is this all. Eising by a supreme flight of thought to the 
 most abstract speculations, he discovers the laws which regulate all
 
 THE HEAVENS. 
 
 celestial movements, and defines the nature of the universal force 
 which sustains the worlds. 
 
 Such are the fruits of the unceasing labours of twenty generations 
 of Astronomers. Such the result of the genius and of the patient 
 perseverance of men who have devoted themselves for two thousand 
 years to the study of the phenomena of the Heavens. The Chaldean 
 shepherds were, they say, the first astronomers. We can well believe 
 it. Dwelling in the midst of vast plains, where the mildness of the 
 seasons permitted them to pass the night in the open air, where the 
 clear sky unfolded before them perpetually the most glorious scenes, 
 they ought to have been, and they were, contemplative astronomers. 
 And all of us would be what they were, did not the rigour of our 
 climate and our variable atmosphere so often prevent us observing 
 the Heavens ; and did not, moreover, the turmoil and cares of civilised 
 life deprive us of the necessary leisure. 
 
 Nothing is more fitted to elevate the mind towards the infinite 
 than the pensive contemplation of the starry vaidt in the silent calm 
 of night. A thousand fires sparkle in all parts of the sombre azure 
 of the sky. Varied in colour and brilliancy, some shine with a vivid 
 light, perpetually changing and twinkling; others, again, with a 
 more constant one — more tranquil and soft ; while very many only 
 send us their rays intermittently, as if they could scarce pierce the 
 profundity of space. 
 
 To enjoy this spectacle in all its magnificence, a night must be 
 chosen when the atmosphere is perfectly pure and transparent — one 
 neither illuminated by the Moon, nor by the glimmer of twilight or 
 of dawn. The heavens then resemble an immense sea, the broad 
 expanse of which glitters with gold dust or diamonds. 
 
 In presence of such splendour, the senses, mind, and imagination, 
 are alike enthralled. The impression gathered is an emotion at 
 once profound and religious, an undefinable mixture of admiration, 
 and of calm and tender melancholy. It seems as if these distant 
 worlds, in shining earthwards, put themselves in close communication 
 with our thoughts. 
 
 But Sentiment has but one part in this emotion, and soon 
 Intelligence asserts her sway. It asks how these mja-iads of stars, 
 scattered here and there, can reveal to those who have studied them 
 the structure of the imiverse ; by what method they have succeeded 
 in distinguishing them, in calculating their distances, and deter- 
 mining their movements. Further on, we shall attempt to give an 
 idea of the manner in which these interesting problems have been 
 solved : at present, and before entering into a more detailed description,
 
 THE HEAVENS. 3 
 
 we shall endeavour to sketch with a free hand the panorama of the 
 Universe. 
 
 At a first glance at the starry firmament the stars seem pretty 
 regularly distributed ; nevertheless, look at that whitish, undecided, 
 vapoury glimmer which girdles the heavens as with a belt. It is the 
 Milky Way.* As we approach the borders of this star-cloud in our 
 inspection, the stars appear more and more crowded together, and most 
 of them so small that the eye can scarcely distinguish them. The 
 accumidation of stars in the direction of the Milky Way is more 
 especially visible when we examine the heavens with the aid of a 
 powerful telescope. 
 
 The Milky Way itself is nothing more than an immensely ex- 
 tended zone of stars, that is, of sims ; since, as we know, and as we 
 shall explain in the sequel, each star, from the most brilliant to the 
 faintest, is a sun. 
 
 Here, then, is an immense group, a gigantic assemblage of worlds, 
 which seems to embrace all the Universe, if it be true that the 
 greater nimiber of the scattered stars situated out of the Milky Way, 
 nevertheless form part of it. In reality, this midtitude of millions of 
 suns is divided into numerous and distinct groups, and those into 
 others still more restricted in number, each composed of two or 
 three suns. 
 
 What breadth of space does each of these groups occupy ? What is 
 the measure of the space which holds them all ? The most powerful 
 imagination in vain attempts to answer these questions intelligibly ; 
 here numbers fail us. 
 
 Let us add — without comment in this place, as we shaU return 
 to it, — a fact well proved, and one which will seem strange to many; — 
 
 Our Sun himself is a star of the Milky Way. 
 
 The foregoing, however, is but a first sketch of the structure of 
 the visible Universe. 
 
 In examining attentively every part of the starry vault, a keen 
 eye perceives here and there whitish spots resembling little clouds. 
 One would say they were so many patches detached from the Milky 
 Way, from which, however, they are often very distinct and very 
 distant. The telescope discovers by thousands these cloud-patches, 
 these — to give them their astronomical name — Nehuke. 
 
 [It was formerly imagined, that each of these star-clouds was 
 
 * Via Lactea. It is also called the Galaxy, from yaX«|/«, the Greek word for 
 the same thing.
 
 4 THE HEAVENS. 
 
 nothing more tlian an accumulation of stars, very close together, and 
 very numerous— so many Milky Ways lying outside our own, and for 
 the most part so distant that the most powerful instruments were able 
 only to distinguish a confused glimmering. One of the most im- 
 portant observations of modern times, however, has shown, that many 
 of these nebuleo, including the most glorious one in our northern 
 hemisphere,— that in the sword-handle of Orion,— are but masses 
 of glowing gases. 
 
 Others, again, of these cloud-like masses — cloud-like by reason 
 of their distance— show us, faintly shining on a back-ground of 
 apparent nebiJsB, brilliant stars, larger no doubt, or more brilliant 
 than their fellows, and some of these objects, called " Star- Clusters," 
 which are nearest us, are among the most glorious objects revealed 
 to us by our telescopes.] 
 
 Let us attempt now to conceive what fearful distances separate 
 these archipelagos of worlds from our own ! 
 
 Unfathomable abysses whose unspeakable depths the most power- 
 ful telescopes increase indefinitely! Profounds, endless, bottomless, 
 but lit up by millions of suns ! 
 
 Such appears to us the Universe from the natural observatory 
 where we are placed. But to obtain a more complete idea of its con- 
 stitution, of the infinite variety of its members, we must descend frOm 
 these regions, where the sight and mind are lost, to a group, nearer 
 to us, and therefore more accessible to the investigations of man, — 
 to that group, or system, of which the Earth forms part. 
 
 Of this the Sun is the centre. 
 
 Round this focus of light and heat, but at various distances, revolve 
 more than a hundred secondary bodies, — Planets, some of which are 
 accompanied by smaller ones — Satellites. Not self-luminous, they 
 would be invisible to us, if the light, which they receive from the 
 Sun, were not reflected towards the Earth, making them also appear 
 as luminous points spread over the celestial vault like so many stars. 
 Such would be the appearance of the Earth seen in space, at a dis- 
 tance sufficiently great. 
 
 A common character distinguishes all the celestial bodies that 
 form part of this group— the Solar System— from the multitude of 
 other stars. For while the Suns, composing what is called the 
 Sidereal Universe, are situated at distances seemingly infinite, 
 the bodies composing the group of which we speak are relatively 
 much nearer the Earth, — are, in fact, our neighbours. 
 
 What results from this double fact? Two very simple conse- 
 quences, easily understood.
 
 THE HEAVENS. 5 
 
 The first is, that the stars do not undergo any sensible change 
 of position in the starry vault. Their distance is such, that they 
 appear actually at rest in the depths of space ; hence the term 
 Fixed Stars, — now abandoned, because a minute and elaborate study 
 of their relatiA-e positions has established the fact that the stars really 
 do move in the remote regions of the heavens. The apparent im- 
 mobility of which we have spoken, and which is one of their charac- 
 teristics, is evidenced b}^ the uniformity of appearance preserved 
 for centuries by the artificial groups of stars, to which the name 
 of Constellations has been given. 
 
 I^ow, it is otherwise with the bodies that revolve round our Sun ; 
 they are near enough to the Earth to allow of their displacements 
 in space being perceived in short intervals of time. Travelling, by 
 \drtue of their proper motions along the starry vault, distances 
 which appear greater as their own distance from us is less, these 
 bodies received at the outset the name they have since retained — 
 Planets, or Wandering Stars. 
 
 It is thus, that, when we stand in the middle of an extensive 
 plain, we judge distant objects — those that border the horizon — to be 
 immovable ; whilst we instantl}' perceive the slightest change of place 
 in the near ones. It is true that when we ourselves move, the real 
 movements become complicated with the apparent movements, but 
 the former must be distinguished, if we wish to have an exact idea of 
 the actual course travelled. This complication of the apparent move- 
 ments of the planets, — a necessary consequence of the movement 
 of the Earth, — is one of the most striking testimonies to the 
 reality of the latter ; but it must be also added, that this was pre- 
 cisely the stone of stumbling of ancient astronomy until the time — 
 and that not long ago — when the real movements were made known. 
 It will soon be seen, in the detailed description of each of the 
 planets of the solar system, what wonderful variety reigns within its 
 limits. Movements of rotation, movements of revolution, around the 
 common centre, the duration of these movements, distances, forms and 
 dimensions, distribution of light and heat, all chang-e in passing from 
 one planet to another. And yet, marvellous thing, the same laws 
 govern, all in such a way, that the unity of plan is not less marked 
 than the astonishing variety of the phenomena. 
 
 One circimistance coimnon to all the bodies of the solar system 
 forcibly strikes the imagiaation. It is, that these enormous masses, 
 — these globes, many of which are much heaider than the Earth, and 
 lastly, the Earth itself, not only are suspended in space, but move 
 through the ether with velocities ti-uly stupendous.
 
 6 THE HEAVENS. 
 
 Imagine yourself a spectator, standing immovable in space. A 
 Imninous body appears in the distance, little by little you see it 
 approach and increase in size; its immense circumference, which 
 exceeds a hundred thousand leagues, is in rapid rotation, which 
 makes each point on its periphery travel through nine miles a 
 second. The globe itself at last passes before you, carried through 
 space with a velocity twenty-four times greater than that of a 
 cannon-ball. In such a way Jupiter would appear to you travelling 
 in its orbit. This headlong course woidd banish it for ever, to 
 the most remote regions of the visible universe, if it were not 
 subdued and held by the powerful attraction of a globe a thousand 
 times larger than its own — by the Sun himself. Not only does 
 Astronomy show, by undeniable proofs, the reality of these marvellous 
 movements, — not only has she arrived at the knowledge of their 
 invariable constancy, at least during thousands of centuries ; but she 
 has found in their very rapidity the cause of the stability of all the 
 celestial bodies. 
 
 If there is difficulty in imagining such masses freely circulating 
 in the ether, how much more are we impressed, when we consider 
 that these rapid movements are not confined to the planets ; and 
 when we look upon the Sun with all his retinue, as moving in an orbit 
 yet unknown, himself attracted no doubt by a more powerfid Sun, or 
 by a group of Suns ! All the stars which by reason of their infinite 
 distances appear immovable, move in difierent directions ; and we 
 shall see later, that if these movements are performed with extreme 
 slowness, the slowness is apparent only. In reality, these are the 
 most rapid celestial movements that we know of. 
 
 Thousands of centuries will be necessary before these immense 
 sidereal voyages are accomplished. Their vast periods are to the 
 length of our year, what the dimensions of the Earth are to the 
 distances of the stars ; and, according to the happy expression of 
 Humboldt, they make of the Universe an eternal timekeeper. Thus, 
 in the contemplation of celestial phenomena, the idea of infinite 
 duration impresses itself on the mind with the same irresistible power 
 as the idea of the infinity of space. 
 
 Such is briefly the magnificent field explored by Astronomy. 
 
 Other natural and physical sciences teach us to study nature in 
 its more intimate mysteries ; they unveil to us the molecular con- 
 stitution of bodies ; the play of their combinations and metamor- 
 phoses ; their thousand useful and curious properties ; the development 
 of organized living beings, both vegetable and animal, and even of
 
 THE HEAVENS. 
 
 mail himself, one of whose noblest attributes seems to be the gift of 
 knowing, and who appears by the light of science as the most perfect 
 creation of the organising forces. 
 
 But it is Astronomy that reveals to us the Universe in its majestic 
 whole ; it is she who has made us comprehend its structure, and after 
 having gathered its thousand various elements into a gorgeous picture, 
 has initiated us into the eternal laws that govern the Heavens.
 
 PART THE FIRST. 
 
 THE SOLAR SYSTEM.
 
 PART THE FIRST. 
 
 THE SOLAR SYSTEM. 
 
 The Bodies which form the Solar System — Direction of Movements of Eotation 
 and RevoUitiou — Inclinations of the Planes of the Orbits of the different 
 Planets. 
 
 The group or system of celestial bodies, of whicli the Earth forms 
 part,— a system known in Astronomy under the name of the Solar or 
 Planetary System, is composed, according to our present knowledge, 
 of a hundred and twenty bodies, which may be classed in the follow- 
 ing manner ; taking into account at the same time, both the part 
 which they play in the system, and the order of their distances from 
 the Sun: 
 
 1. A central bodij, relatively immovable in the group ; much 
 larger than all the others, and self-luminous, the Sun ; 
 
 2. Ninety-two secondary bodies, or Planets, situated at increasing dis- 
 tances from the Sun ; revolving round him in orbits nearly circular ; 
 and receiving from him the light which renders them visible to us. 
 The planets may again be divided into three principal groups : 
 
 The smaller planets, those nearest to the central body, are, in 
 the order of their increasing distances from the Sun : Mer- 
 cury, Venus, the Earth, Mars ; 
 
 The larger planets, those most remote from the central body: 
 Jupiter, Saturn, Uranus, Neptune ; 
 
 Lastly, the minor planets, or Asteroids, forming between Mars and 
 Jupiter a ring, which separates the two first grouj)s ; 
 
 Eighty-four small planets are now known, but they are, no doubt, 
 much more numerous ; 
 
 3. Eighteen tertiary bodies, or Satellites, revolving round some of
 
 J2 THE SOLAR SYSTEM. 
 
 the principal planets; such as the Moon, which accompanies the 
 Earth. Jupiter has four such satellites ; Saturn, eight ; Uranus, four ; 
 Neptune, one ; 
 
 4. Nine Comets, the periodical returns of which have been 
 proved by observation, revolving round the Sun in very elongated 
 
 orbits. 
 
 Independently of the hundred and twenty bodies which we 
 have just enumerated, two hundred other comets are known, 
 some of which travel round the Sun in orbits so elongated, and in 
 times so vast, that their return has not yet been proved by obser- 
 vation, although it has been approximately calculated. Others 
 describe curves which may be called infinite, and after having once 
 approached our group, have abandoned it perhaps for ever. A year 
 never passes without new comets being discovered. 
 
 "We must here also mention a nebulous ring of a lenticidar form, 
 the Zodiacal Light, which surrounds the Sun at a certain distance, and 
 the position of which in the system is not yet clearly determined ; 
 and, besides this, one or tv70 or even more rings composed of a mul- 
 titude of small bodies, revealed to us by the appearance and fall of 
 Meteorites, Meteors, and Falling or Shooting Stars. 
 
 The Sun, the planets and their satellites, all assume a sphe- 
 rical form, sometimes flattened at the extremities of one diameter. 
 In the more important of these bodies, movements of rotation have 
 been detected ; these movements all take place in the same direction 
 as that of the Earth. Astronomers, by an analogy based on the laws 
 of mechanics, extend this movement of rotation to all the bodies, which 
 have hitherto bafiled our scrutiny in this particular. A second 
 movement, which we 15all one of revolution or of translation, impels 
 all the planets round the Sim, and all the satellites round their 
 respective planets, in times which vary with the dimensions of the 
 orbits described, by virtue of a remarkable law, the discovery of 
 which we owe to the genius of Kepler.* 
 
 The direction of the movement of revolution is the same for all 
 
 the bodies of the solar system, f and this direction is precisely that of 
 
 • all the movements of rotation. In order that the reader may grasp 
 
 this important point, let him turn to Plate I., which represents the 
 
 * The law, which defines the movement of all celestial bodies, is referred to 
 in Part III., Book i. 
 
 t \yc must except; however, the satellites of Uranus, one of the seven peri- 
 odical comets, and a great number of other comets.
 
 THE SOLAR SYSTEM. 13 
 
 orbits of all the known planets.* The arrow in each case indicates 
 the direction of the planet's revolution round the Sun. Now let us 
 suppose an observer, placed in the centre of the diagram in such a 
 way, that his feet resting on the plane of the paper, his head will be in 
 the northern hemisphere of the heavens. In this position, it is easy 
 to see that the movement indicated by the arrow will take place from 
 the right to the left of the observer. Such is the direction of the 
 movements of revolution of the planetary bodies. 
 
 Let us now compare this movement to the movement of rotation 
 of the Earth on its axis. The centre of our planet is situated on the 
 plane ; the north pole is above it, and the south below it, in such a 
 manner, that the terrestrial rotation which takes place — as proved by 
 the daily movement of the heavens — from west to east, is also to the 
 observer a movement from the right towards the left. If the name 
 of north pole is given to that pole of each of the other planets, 
 which is situated above the plane of which we speak, observation 
 shows that it is alwaj's from right to left, or from west to east, that 
 these planets describe both their movements of rotation, and their 
 movements of translation, round the Sun. 
 
 It is very evident, that if we had supposed the observer standing 
 on the other side of the jDaper, with his head towards the south pole 
 of the heavens, all the movements would have seemed inverted ; 
 that is to say, would take place from left to right, although they 
 would still remain the same, from the point of view which we occupy.* 
 So let us remember, once for all, this fundamental fact of solar astro- 
 nomy, that the movements, both of rotation and of revolution, of the 
 planets and their satellites, are eflPected all in the same direction, 
 that is, from right to left or from west to east. 
 
 The ideal ciu'ves described by the various planets around the 
 Sun, considered at rest, are plain curves, or at least nearly so. This 
 is nearly the same as saying, that the centre of each planetary globe 
 in its movement around the central body, remains alwa3\s in the 
 
 * In this Plate the oi'bits of the planets have been represented by the 
 circumferences of circles, although in reality they are of an elliptical or oval form. 
 Nor is the Sun, as represented in the iigure, exactly in the centre of each orbit ; 
 but it would have been difficult, not to say impossible, to render these differ- 
 ences appreciable on so small a scale. 
 
 That which it is absolutely necessary to show correctly in this representation 
 of the Solar System are the relative distances of the different planets from 
 the common focus. The illustration represents the dimensions of the orbits in 
 their true proportion, with the exception of the orbits of the satellites, the dimen- 
 sions of which have been necessarily enlarged. The positions of the planets are 
 those which these bodies actually occupied in space on the 1st of January, 1865.
 
 14 THE SOLAR SYSTEM. 
 
 same plane. This plane, if prolonged, passes through the centre of 
 the Sun. But the planes of these orbits do not coincide with one 
 another, they are differently inclined to that of the Earth taken 
 as a standard of comparison ; from this it results, that each planet 
 describes half its orbit above the plane of the terrestrial orbit, or, as 
 it is called, thejjlane of the ecliptic, and the other half below it. 
 
 The inclinations, represented in their true proportions in fig. 2 of 
 the Plate we have mentioned, are, moreover, very small, and it follows 
 that as seen from the Earth, the principal planets revolve in a narrow 
 zone of the celestial vault; this has received the name of the 
 Zodiac. 
 
 The solar system seen in section, or in profile so to speak, would 
 appear therefore, to an observer situated at a great distance beyond 
 its limits, as a grouj) of elongated form, having in its centre a lumi- 
 nous point, the Sun, and on both sides of it, a multitude of small 
 stars of unequal brightness — the planets and their satellites, oscilla- 
 ting backwards and forwards in paths nearly rectilinear. 
 
 After having sketched that entire group of celestial bodies, which 
 interests us the most, seeing that our globe is one of its constituent 
 molecules, we will now describe the members of the group one by 
 one, study their movements, and, by the aid of facts furnished by 
 the persevering observations of modern astronomers, examine them, 
 when possible, in their most minute detail. 
 
 We will begin with the Sun.
 
 BOOK THE FIRST. 
 
 THE SUN. 
 
 Or all the stars whicli people the immensity of space, the Sun is the 
 most interesting to us the inhabitants of the Earth. 
 
 It is at once the largest, at least in appearance, the most bril- 
 liant, and that which exercises over oiu- globe the most dominant 
 influence. 
 
 The centre of the movements of all the celestial bodies of the 
 system, of all those which are in fact our neighbours, he is to them 
 and to us the inexhaustible source of light, heat, and life. It is from 
 him that all the energies, developed on the surface of the Earth or on 
 the other planets, — energies manifested under so many various forms 
 — incessantly flow without ever draining their source. 
 
 [And yet all the action on this our Earth is carried on by the 
 two hundred and thirty millionth part of the force radiated by the 
 Sun ; for that is aU the Earth can grasp, as it were, of his rays given 
 out in all directions, and it is by this fraction of his mighty power 
 that aU the Earth's work is done.] 
 
 Lastly, the Sun would seem to be the conmion father of the 
 whole family of bodies that gravitate round him, and which he holds 
 in hand by his powerful attraction. It is from him that at epochs 
 immensely distant from ours, have been thrown out successively, at 
 first under the form of nebulous rings, those agglomerations of matter 
 which have become in the end, by a natural concentration, nearly 
 spherical globes; Jupiter, Saturn, Mars, the Earth, Yenus, are so 
 many children of the Sun. The part played by the Sun in the group, 
 of which he is the centre, we have already stated. Farther on, we
 
 16 THE SOLAR SYSTEM. 
 
 shall see how he figures in the Sidereal Universe ; and we shall find 
 him midst the millions of stars which form the Milky Way. 
 
 Our present object is to study his individuality, to measure his 
 apparent and real dimensions, to study the physics of his surface, 
 and his movement of rotation, and to deduce from aU the facts 
 gathered by the most able and distinguished observers, the structure 
 of this tremendous star, and the most probable conjectures as to its 
 physical constitution. 
 
 i
 
 THE SIX. 
 
 J7 
 
 I. 
 
 Form and apparent Size of the Sun — Its Distance from the Earth, and real 
 Size — Its Surface, Volume, Mass, and Weight. 
 
 As every one knows, the naked eye cannot bear the brightness of the 
 Sun. Nor can this be wondered at, if we remember that the intensity 
 of its light as seen from the Earth, is eight hmidred thousand times 
 greater than that of the full Moon, or twenty-two thousand million 
 times that of the most brilliant star.* To obtain, therefore, a clear idea 
 of its form, we must take advantage of opportunities when clouds, 
 or, better still, dense fogs, interpose themselves between the eye and 
 its radiant body. The use of telescopes would be still more dangerous 
 than that of the imaided sight, if observers did not take the precau- 
 tion to use dark or coloured glasses to shield the eye, inasmuch as 
 lenses and reflectors concentrate to their /be/ a considerable quantity of 
 light- and heat-rays. The eye would be dazzled, or even utterly 
 destroyed, without this indispensable precaution.f 
 
 A first rough glance shows us that the disk of the Sun is circular. 
 But the use of acciu-ate instruments leaves not the least doubt in 
 this respect, and numerous micrometric ^ measurements have proved 
 that all the diameters of the disk are exactly eqvial. The Siui has 
 then the appearance of a perfect Imninous circle, and as it is not less 
 
 * [There is, however, to judge from the different results obtained by different 
 physicists, some uncertainty attaching to these numbers. The comparative 
 brightness of the Sun to Vega in Lyra, as given by different observeis, is as 
 follows : — 
 
 Wollaston 180,000,000,000 -j 
 
 Bond . . 24,000,000,000 Uo 1. 
 
 Clark . . 10,^00,000,000 J 
 
 The intrinsic light of the Sun is another matter ; he would really appear 
 less bright than « Centauri, if we could see both at the same distance.] 
 
 t In our Chapter on Astronomical Instruments in Part III, we shall describe 
 the various methods of solar observation by which these objections are avoided. 
 
 t That is to say, made with micrometers — instruments which serve to 
 measure very small objects and small angles, of which more auun in Fart III- 
 
 c
 
 18 THE SOLAR SYSTEM. 
 
 certain tliat it turns on an axis, and therefore successively presents 
 different faces to us, we can only conclude that its form is m reality 
 that of a perfect sphere without any trace of irregularity or flattening. 
 In the morning when the Sun rises, or in the evening a little 
 time before its setting, if the atmosphere be rather misty, the solar 
 disk may often be observed with the naked eye : it then appears to be 
 larger than usual, and its contour differs sensibly from that of a circle. 
 But these are iUusions, the causes of which we wiU try afterwards to 
 
 explain. 
 
 The apparent dimensions of the Sun do not remain the same 
 throughout the course of a year. The average size is such, that 
 three hundred and sixty disks equal to its own, placed side by side, 
 would cover a half-circle of the celestial vault : its average diameter 
 being about half a degree.* But in winter it appears larger than 
 in summer, at least to the inhabitants of the northern hemisphere of 
 the Earth. In the southern hemisphere it seems larger in summer 
 than in winter, because, of course, our winter is their summer. This 
 change of size must not be attributed to a real change in the 
 dimensions of the Sun. It is easily explained when we know that 
 the annual revolution of the Earth round the central body is effected 
 in a curved path, of which the Sun does not occupy exactly the 
 centre. The distance of the two bodies varies therefore from one 
 day to another, and it is towards the first days of winter in our 
 northern hemisphere that the Earth is at its least distance from the 
 Sun. The different sizes of the solar disk, as seen from the Earth 
 when at its least, mean and greatest distances from it, are shown 
 in the following diagram : — 
 
 1st Janiiaiy. 1st October. 1st July. 
 
 Fig. 1. — Apparent dimensions of the solar disk at tlic Earth's extreme and mean distances.f 
 
 * It is usual in geometry to divide the circumference of the circle into .300 
 equal parts, each of which is called a degree, and is represented thus : 1 °. Each 
 degree is subdivided into 60 minutes, and each minute into 60 seconds : a minute 
 is written 1' ; and a second, 1". 
 
 t If we represent the luminous surface of the Sun by 1000 at its mean 
 distance from the Earth, the numbers 940 and 1072 will represent the same 
 surface as it appears to us at the Sun's greatest distance in .July, and at its least
 
 THE SUN. 
 
 19 
 
 We see, then, that the apparent size of an object varies with the 
 distance : similarl}^ the size of the solar disk ought to vary, seen from 
 each of the planets of the system. The more distant the planet from 
 the Sun, the smaller will the Sun appear. To avoid giving numbers, 
 
 Fig. 2.— The apparent size of the Suu as seen from tlic principal planets. 
 
 distance about the 1st of January. The sanie numbers give us also the rela- 
 tive quantities of heat and of light received by the Earth at these different epochs, 
 so that in summer the Sun warms and lights our globe loss than during winter. 
 This apparent anomaly will be explained when we describe the terrestrial seasons.
 
 20 THE SOLAK SYSTEM. 
 
 whicli would convey no definite idea to the reader, we have included 
 in tlie same diagram (fig. 2) the comparative sizes of the Sun 
 as seen from each of the principal planets at their mean distance. 
 But it must not be forgotten, that if the apparent size varies, the 
 intrinsic intensity of the light remains the same, of course leaving 
 out of consideration absorption by the planetary atmospheres, of the 
 power of which nothing precise is yet known. The quantity of light 
 or of heat received by a planet depends upon the extent of the 
 apparent surface of the solar disk. • 
 
 From Mercury, the planet nearest the Sun, this body is seen with 
 its greatest apparent dimensions ; from Neptune, on the contrary, with 
 its smallest. The luminous surface appears 6670 times larger from 
 the former of these planets than from the latter, situated, as we know, 
 on the confines of our system. When we come to study the physical 
 constitution of these bodies, we shall return to the quantities of light 
 and heat with which the solar effulgence bathes the surface of the 
 jDlanets. We need only say here, that, if to the inhabitants of the 
 Earth, the disk of the Sun presents an apparent surface seven times 
 less than that seen from Mercury, if, in Neptune this surface is further- 
 reduced a thousand times, it still preserves, as seen from this last 
 globe, a brilliancy superior to that of all the bodies, whether planets 
 or stars, that Ave on the Earth see in the heavens, although at this 
 distance the immense kmiinary would appear but as a point, lost 
 amid the innumerable fires of the starry vault. 
 
 The apparent size of an object — in other words, the angle fomied 
 by the visual rays coming from its two extremities to our eye — 
 teaches us, however, nothing of its real size, so long as we are igno- 
 rant of its distance from us. 
 
 What is, then, the distance of the Sun from the Earth and from 
 the other bodies of our planetary system ? 
 
 Let us take, first, the distance of the Earth from the Sun, without 
 considering for the present the particular methods employed to deter- 
 mine it. 
 
 This distance is 95,298,260 miles,* equal to 24,000 (more pre- 
 cisely to 23,984) semi-diameters of our planet. It was about the 
 middle of the last century that this determination was arrived at. 
 
 * It is certain that the distance of the Sun from the Earth as stated above 
 requires to be considerably diminished. The labours of Le Verrier and Hansen, 
 the observations of Mars made by Stone and Winnecke, the new determination of 
 the velocity of light liy Leon Foucault, all point to the necessity for this correc-
 
 THE SUN. 
 
 21 
 
 Yery different from the distance just given was that adopted 
 hypothetically by Pythagoras. This philosopher, who held ideas of 
 the system of the world so similar to those which a lono- series of 
 labour has definitely established, assigned 44,000 miles as our dis- 
 tance from the body, which warms and lights us, a distance which 
 would give as its diameter 75 miles. Bearing these figures in mind, 
 we can understand the ancient assertion which, perhaps, would still 
 astonish many of us, that the Sun is larger than the Peloponnesus.* 
 
 The very large numbers, which are so often met with in Astronomy, 
 leave for the most part only a very vague impression on the mind. 
 It is difficult for the imagination to figure the objects that they 
 
 tion, which will entail a series of modifications in the numbers at present adopted 
 for the various elements of the solar system. 
 
 [The old value and the new value of the Sun's parallax (a word we shall sub- 
 sequently explain), recently obtained, are as follows : — 
 
 Seconds. 
 
 The old value obtained by Bessel from the transit of Venus . . . 8'578 
 
 The new value obtained by Hansen from the Moon's parallactic equation 8-916 
 
 „ „ „ Winnecke from the observations of Mars . 8-964 
 
 „ „ „ Stone 8-930 
 
 „ „ „ Foucault, from the velocity of light . . 8-860 
 
 „ „ „ Leverrier, from the motions of ]\lars, Venus, 
 
 and the j\Ioon ..... 8-950 
 
 This small correction, amounting to only two-fifths of a second of arc, brought 
 to light in the first instance by small disturbances in the motion of the Moon and 
 planets, should, it has been well remarked, inspire astronomers with additional 
 confidence (if that were needed) in the exactness of their science and in the fixed- 
 ness of the laws which bind the Kosmos together. And if, on the other hand, a 
 contrary misgiving is created in other minds from the fact, that this abrupt altera- 
 tion of so important an element as the solar parallax implies an alteration of some 
 four millions of miles in the Sun's reputed distance from our Earth, this mis- 
 giving may, perhaps, be removed by the consideration that after all this 
 improvement of our knowledge amounts to no more than a correction to an 
 observed angle represented by the apparent breadth of a human hair viewed at 
 the distance of about 125 feet.] 
 
 * Before 1769, astronomers had endeavoured to determine the distance of the 
 Sun in vaiious ways. Aristarchus of Samos, and afterwards Ptolemy, Coperni- 
 cus, and Tycho, supposed it equal to 1200 radii of the Earth, nearly 4,800,000 miles, 
 that is to say, twenty times less than the actual distance. Kepler tripled this 
 number. Cassiniand Lacaille approached the nearest to the truth. According to 
 D'Alembert, the latter of these savants valued the distance in question at 21,000 
 terrestrial radii ; Cassini at 28,000. The same author again quotes a distance of 
 12,000 diameters of the Earth, which is precisely that now adopted ; but he does 
 not give the name of the astronomer who arrived at this estimate. Arago, 
 in his " Popular Astronomy," alludes to the measures of Riccioli and HeveHus, 
 giving 7000 and 5200 terrestrial radii respectively ; lastly, those of Richer and 
 Maraldi, deduced from oppositions of Mars, fixed the mean distance of the Sun 
 at 21.712, and at 20,626 terrestrial radii.
 
 22 THE SOLAR SYSTEM. 
 
 represent ; and where it is a question even of moderate distances, it 
 is only by the aid of comparisons that we can arrive at any precise 
 idea. If these distances are greater than those which we can actually 
 see on a terrestrial horizon, say than 25 or 50 miles, the image 
 properly so called vanishes, and we are compelled to have recourse 
 to other means of representation ; for example, we ask how much 
 time a locomotive, going at a known rate, will require to traverse 
 the given distance. The idea of duration comes then in aid of that 
 of space to complete and perfect it. 
 
 Let us see with what exactness we can by this means form a 
 conception of the distance which separates the Earth from the Sun. 
 
 Light — the propagation of which is the most rapid movement 
 known, and which travels at the rate of 192,000 miles in a second 
 of time — requires 8 minutes 17 seconds to flash from the Sun to 
 the Earth. If we suppose the intervening space to contain atmos- 
 pheric air, a sound, with an intensity sufficiently great to put in 
 motion a sphere of such enormous dimensions, woidd take fourteen 
 years and two months to reach our ears, sound travelling, as we know, 
 about 1115 feet a second. 
 
 Finally, an express train going at the rate of about 30 miles 
 an hovir, leaving the Earth the 1st of January, 1866, would not 
 arrive at the Sun until the year 2213, nearly 347 years after the day 
 of its departure. ■ 
 
 We can thus form an idea of the immensity of the chasm which 
 lies between the Sun and our globe, — an immensity, the measure of 
 which is expressed by the round number, so simple in appearance, of 
 95,000,000 miles. It is this number — this 95,000,000 miles— 
 which will henceforth form the unit, the " standard measure," by 
 means of which the other celestial distances will be expressed. 
 
 The distance of the Sun once known, we have only to solve an 
 easy problem in geometry, to deduce its real dimensions from the 
 apparent size of its disk.* We thus know that its diameter is about 
 
 * This problem is so simple, in fact, that we cannot resist the temptation of 
 proving our assertion. Take a disk of white pasteboard, say of about four inches 
 (the French equivalents are given in the figure) in diameter, place it vertically 
 
 1^ 152 000 000 kilom. ^ 
 
 Fig. o.— Dimeiisiuiis of the Sun reduced from its distance and its apparent diameter.
 
 THE SUN. 
 
 23 
 
 112 times (112-06) the diameter of the Earth, or, as it may be ex- 
 pressed in miles, 887,076. The circumference of our great light- 
 giver, therefore, exceeds 2,785,400 miles. 
 
 The Moon, as we shall see in the sequel, revolves round the Earth 
 at a mean distance of 30 diameters of our Earth. If, then, we 
 imagine the centre of the solar orb to coincide with the centre of the 
 Earth, not only would the orbit of the Moon lie entirely within 
 the Sun, but to reach its surface, a distance equal to 26 Earth-dia- 
 meters would stiU remain to be traversed. Fig. 4, drawn to scale, 
 will show this clearly. 
 
 Fig. 4. — Comparative dJmeusious of tlie Sun and of the orbit of the Moon. 
 
 So much, then, for the linear dimensions of our Sun. 
 If we ask what is the extent of the Sun's surface, and what its 
 volume, we find that the first comprises the enormous munber 
 
 and move away from it little by little until its apparent dimensions are precisely 
 the same as those of the Sun, that is, until the pasteboard disk will exactly cover 
 the solar disk. Observation shows that the distance between the eye and the 
 disk is then very nearly 12 yards. 
 
 Now it is easy to see from the preceding figure, that there is between the real 
 dimensions of the pasteboard disk and those of the Sun precisely the same rela- 
 tion as between the distances which separate the observer from each of the two 
 objects in question. The diameter of the Sun is therefore to the diameter of the 
 disk as 95,000,000 miles are to the distance of the disk from the eye. The method 
 employed by astronomers is less elementary ; but in the main it is based ou 
 the same principle.
 
 24 THE SOLAR SYSTEM. 
 
 of 2,471,665,000,000 square miles, that is, 12,611 times the en- 
 tire surface of the terrestrial sphere. If we pass to its volume, 
 it is impossible not to be startled at the colossal number of 
 364>345,641,000,000,000 cubic miles, a number which represents 
 more than 1,400,000 times the cubic contents of the Earth. 
 
 Arago, in his "Popular Astronomy," quotes the following com- 
 parison, well adapted to give an idea of the immensity of this volume : 
 "A certain Professor at Angers, wishing to give his pupils a tangible 
 notion of the size of the Earth compared with that of the Sun, counted 
 the number of grains of wheat of ordinary size contained in a 
 measure called a litre ; he found this to be 10,000 ; consequently a 
 decalitre woidd hold 100,000; a hectolitre 1,000,000, and 14 deca- 
 litres, 1,400,000. After having gathered into one heap the 14 
 decalitres, he held up one grain, and said to his listeners, ' Here is 
 the volume of the Earth, and here is the Sun.' This statement of 
 the case struck his pupils infinitely more than if he had announced 
 it in the abstract numbers, 1 to 1,400,000." 
 
 When we shall have seen what are the actual dimensions of this 
 grain of corn which represents the Earth, we shall be more surprised 
 still, and our imagination will be crushed under the prodigious size 
 of our world's light-giver, which, nevertheless, is but itself a grain of 
 Imninous dust lost in infinite sjjace. 
 
 Our Earth being only one of the members of the planetary family, 
 it would be only natural to extend the comparisons that we have 
 made between its volume and that of the Sun to the princij)al 
 celestial bodies which revolve with it round the central focus. But, 
 in the detailed description that we shall subsequently give of each of 
 these bodies, we shall take occasion to enlarge more on their proper 
 dimensions. To deal with them here as a whole, we may remark that 
 the volimie of the Sun is itself equal to 600 times the united volumes 
 of all the planets and their satellites put together. 
 
 That we have been able, by means of the data suj)j)lied by obser- 
 vation, and the laws of geometry and optics, to measure the true 
 distances of the celestial bodies — at least of those nearest to us ; that 
 from their distances we have determined their dimensions in diameter, 
 in surface, and in volume, is not difiicult to understand, and our 
 readers will doubtless readily admit what we have already said, 
 although we have not finished with the subject, as the question of dis- 
 tance will be again discussed in the third part of this book. 
 
 But that astronomers should pretend even to know the weight of 
 the difierent celestial bodies, and to say how many Earths may be 
 placed in one scale of a balance to hold the Sun in equilibi'ium in
 
 THE SUN. O^ 
 
 the other, will seeui paradoxical, at all events, to many. We shall, 
 farther on, show the possibility of conclusions apparently so audacious, 
 the inquiry into which may seem to border on presumption. We 
 must, however, in the interim, invoke a sentiment which is but 
 rarely required in science — faith in our assertions, not a faith which 
 shelters itself under the impenetrability of the mysterious, but 
 one which will become by future study clear and demonstrated 
 truth. 
 
 Compared with the mass of the Earth, the mass of the Sun is only 
 about 35o,000 times as great, although its volume, as we have seen, 
 is 1,400,000 times larger. This indicates a less density ; and it is 
 found that the matter of which the Sun is composed weighs but little 
 more, volmne for volume, than a quarter of that of Avhich our own 
 globe is formed. The weight of the Sun may thus be expressed in 
 tons : 
 
 2,154,106,580,000,000,000,000,000,000 
 
 It ranks, as we see, among those numbers which present nothing to 
 the mind, and leave the imagination itself powerless. 
 
 We shall find that among the bodies of the solar system, there are 
 man}" planets whose dimensions and masses are considerable when 
 compared to our Earth. The mass of the Sun alone, however, is 
 equal to 750 times the united masses of all the bodies which it main- 
 tains in its sphere of attraction, and to which it dispenses light and 
 heat.
 
 26 'I'HE SOLAR SYSTEM. 
 
 11. 
 
 Solar Observation— Sun-spots— The Sun's movement of Rotation— The Kcvv 
 Photographs of the Sun — The Telescopic appearance of the General Surface — 
 Mr. Nasmyth's ' Willow-leaves' — Opinions of other Astronomers — The appear- 
 ance of the Penumbra — Umbra and Penumbra of Spots — Enormous Spots — 
 Their rapid Change of Form — Mr. Carrington's Researches on their Proper 
 Motion. 
 
 When clouds or mists are thick enough to mitigate the dazzling 
 sjjlendour of the Sun's rays, but yet sufficiently transparent to enable 
 us to see its disk, with its distinctly circular form, the surface of the 
 luminary appears to us uniformly luminous, with no spot dimming 
 its brightness. The same appearance, as every one knows, is pre- 
 sented when we observe it through a plate of black or smoked glass. 
 
 But if, instead of confining ourselves to observations with the 
 naked eye, we examine the body with a telescope of moderate magni- 
 fying power, our eye being properly protected, the enlarged image of 
 the disk will usually appear to us, as if sprinkled with irregularly 
 grouped dark points. These are the " Sun-spots," real movable 
 belongings of the surface of the Sun, the observation of which, as 
 we shall soon see, is surrounded with the greatest interest, as it helps 
 us in the study of the physical constitution of our luminary. 
 
 The following representation of the Sim (fig. 5) will give an idea 
 of the manner in which the spots are distributed, and of their 
 grouping at a given time. 
 
 Let us remark at once, that the number of the spots, their rela- 
 tive positions, and their forms even, vary constantly according to 
 the period of observation. Sometimes, but rarely, the solar disk is 
 perfectly clear, no spot varying the uniformity of its splendour. 
 During a period of ten years, from 1840 to 1850, out of a total 
 number of 1982 days, when the Sun was observed, there were 
 only 372 days on which spots were not observed on its disk. 
 
 As many as 80 spots have been visible at one time. On the other 
 hand, whole years, it is stated, have passed without any being observed.
 
 THE SUN. 
 
 2< 
 
 But we must be allowed to consider the latter fact a negative 
 one, resulting from the Avant of assiduity of observers; for since 
 such astronomers as Schwabe of Dessau, Wolf of Zurich, Carrington 
 of Redhill, Daw^es of Haddenham, [Do La Rue of Cranford, and 
 Stewart of Kew,] have devoted themselves to the continuous obser- 
 vation of these phenomena, the number of days in the year when the 
 disk of the Sun has not presented any spot has always been less 
 than those on which groups have been recognised. 
 
 Fig. 5. — Sun-si)ots observed on the 2ikI September, 1839. (Captain Davis.) 
 
 We shall see, subsequently, that the number of spots follows a 
 certain periodicity, which seems to establish a most interesting cor- 
 relation between Sun-spots and the phenomena of terrestrial mag- 
 netism. 
 
 When observed with care during several consecutive days, the 
 spots are seen to vary in form and position. But midst all these 
 variations, a common movement, a progression of the whole, by virtue 
 of which thev all move in the same direction, can be distinguished.
 
 28 
 
 THE SOLAR SYSTEM. 
 
 From this movement has been deduced the rotation of the sohir 
 globe round an axis that passes through its centre. 
 
 Let us look again through our telescope. The reversed image of 
 the Sun presents itself in such a manner, that the eastern edge — 
 or Umh as it is called by astronomers— occupies the right, the 
 western one the left, whilst the south and north regions of the Sun 
 are, the former at the top, the latter at the bottom of the image, as 
 seen in the telescoiDC. 
 
 Observe a spot on the eastern edge. From one day to another 
 we shall see it progress, and that with gradually increasing rapidity, 
 until it occupies a central position on the disk. Then it will continue 
 to advance towards the left, but, in this second half of its journey, its 
 rapidity will decrease, and the spot will finally disappear on the 
 western border. The same phenomena will take place with all the 
 spots which, at the coimnencement of observation, are scattered over 
 
 Fig. 0. — Apparent change in the form of spots approaching or receding from the centre. 
 
 the solar disk ; all will describe in the same direction with a nearly 
 equal velocity either straight lines or slightly curved ones (according 
 to the period of observation), the convexity of the latter lying in the 
 same direction for all the spots observed at the same time. 
 
 Let us suppose that the particular spot that we have noticed is 
 of an oval form, its greatest length being at right angles to its 
 motion across the Sun at the moment when it appeared at the eastern 
 border. In proportion as it approaches the centre this spot widens, 
 so that it becomes nearly circular, then, having passed the centre, 
 its form becomes more and more oval again, until its disappearance, 
 its apparent size meanwhile in one direction not having sensibly varied. 
 
 Fig. 6 shows the changes of form of which we speak during the
 
 THE SI N. 29 
 
 first or last half of the period of the visibility of the spot. The 
 effects are produced precisely as the laws of perspective demand, if 
 v,e admit that the Siin is of a spherical form, and that the dark spot 
 observed passes over its surface with a uniform movement. 
 
 About fourteen days is the time during which a spot remains 
 visible, and this time is the same for all, although they do not 
 all traverse arcs of precisely the same length on the Sun's surface. 
 
 It is also fourteen days after the disappearance of a spot on the 
 western border before it appears again on the eastern, often changed 
 in form, it is true, but, nevertheless, generally recognisable. 
 
 Precise measurements have proved both the general uniformity 
 and the parallelism of all these movements, although, independently 
 of the rotation of the whole, the spots undergo slight displacements 
 among themselves. 
 
 It was asked at first if the black points which form the spots 
 really belonged to the body of the Sun, whether they were not small 
 bodies revolving like planets aromid the great light-giver, and pre- 
 senting to us their unilluminated faces. But the variation in 
 their apparent rapidity which we have noticed, combined with the 
 change of form which some of them undergo in passing from one 
 border to the other, does not allow us to adopt this h^-pothesis. 
 
 It was also thought possible to explain the movement of the spots 
 from the eastern to the western border, by an actual translation of 
 them across the surface of the Sun, itself immovable. But if so, 
 how shall we account for this absolute uniformity in their move- 
 ments ? 
 
 Thus there are two facts of great importance placed beyond all 
 question by the attentive and continued observation of these black 
 points which are scattered over the surface of the Sun ; on the one 
 hand, the spherical form of the body, on the other the existence of a 
 movement of general rotation. Moreover, this movement takes 
 place from right to left, or from west to east ; that is, as we 
 have already seen, precisely in the same direction as the move- 
 ments, both of rotation or revolution, of the other bodies of the solar 
 system. 
 
 When, three centuries ago, the discoveries of Copernicus at last 
 brought to light the true system of the world, the Sun was 
 promoted from the secondary rank of satellite to the Earth to that 
 of sovereign of the planetary kingdom, and it was imagined that he 
 was enthroned, immovable, in the centre of his court. It was not 
 suspected either that he was whirling through space accomi^anied by 
 his dependants, or that he turned on an axis. AVhy, it was said,
 
 30 
 
 THE SOI,AR SYSTEM. 
 
 this latter movement, in a body which itself is light and heat, and 
 knows only an eternal day ? 
 
 These two movements are, nevertheless, real movements/ The 
 recent progress of Sidereal Astronomy has demonstrated the move- 
 ment of translation of the solar system in sjDace, as the observation 
 of the Sun-spots has proved the rotation of the Sim. 
 
 It was in 1611 that this last and important discovery was made. 
 Before that time, Jordano Bruno and Kej^ler had suspected the 
 movement of rotation ; anticipating, as remarked by Arago, actual 
 observation by their genius, when the astronomer, Jean Fabricius, 
 discovered both the sjDots and their general displacement on the 
 surface of the disk. 
 
 We have said before, that about 28 days elapse between the 
 appearance and disappearance of a spot on the same edge of the Sun ; 
 the time of the actual rotation is actually less by two days.* 
 
 We are here content to state the time of the Sun's rotation 
 thus broadly, as we shall have to return to it by-and-by. 
 
 The Sun's axis of rotation is but slightly inclined (7° 15' according 
 
 * Let us endeavour to understand this important distinction. 
 
 If we take a spot, a, on the accompanying diagram at the moment when, as 
 
 seen from the Eai'th, it coincides 
 with the centre of the Sun, and dis- 
 regard the irregular displacements to 
 which it will be subjected ou the sur- 
 face of that body, an entire rotation 
 will seem to us to be effected when 
 the same spot returns to occupy the 
 same central portion after 27 days 12 
 hours. But, during this time, the 
 Earth, oiu' movable observatory, will 
 be displaced in its orbit, and will have 
 described an arc, from T, its primitive 
 position, to T' its new position. At 
 this moment, the spot has revolved not 
 only back to a again, but also through 
 an additional part of the arc a a', so 
 til at it has actually effected more than 
 an entire rotation. In other words, 
 the point of the surface of the Sun, 
 which corresponded first to the centre 
 of the disk, is now to the east of the 
 new central point a', by the fact of the 
 movement of the Earth. The appar- 
 ent period of the rotation exceeds thus 
 the real period by the time necessary 
 to traverse the path a a'. A simple 
 calculation shows that this period is about two days. 
 
 Fig- ". — DiflFerence of time in the apparent ro 
 tatiou of tlie Suu, and in its real rotation.
 
 THE SUX. 
 
 31 
 
 to Carrington's latest researclies) to the ideal plane in which our 
 Earth moves round the Sun. If this inclination were nil, we should 
 always see the spots moving in right lines over the disk, parallel to 
 the solar equator. But this inclination causes us to be sometimes 
 above and sometimes below the plane of the Sun's equator. Hence 
 the curved paths of the spots at some seasons, the convexity 
 sometimes being towards the north, at others towards the south. 
 But at the two intennediate seasons, that is, on the 6th of June and 
 the 8th of December, the Earth is exactly in the plane of the equator,* 
 and at these times we see the spots moving in straight lines. Fig. 
 8 will render our statements clear. 
 
 Februai-y, 
 
 July. September. Sth December. 
 
 Fig. S. — Different paths of Sun-spots at cliflferent pcriod.= of tlie year. 
 
 Sun-spots are confined in the main to two zones, situated on each 
 side of the equator, and they are seldom observed on other parts of 
 the disk. "Whence it seems to follow, that the phenomena which 
 give rise to them, have a certain relation to the movement of 
 rotation of the solar globe. If the luminous surface of the Sun be 
 an incandescent fluid [or be composed of masses of gas or cloud], 
 it is conceivable that the rapidity of rotation gives rise to a cen- 
 trifugal force which, though absent at the poles, constantly increases 
 towards the equator, where it attains its maximum. Hence arise 
 
 * [The node is situated in heliocentric longitude 73^ 40' for 1850-0 (Carring- 
 ton.]
 
 32 THE SOT.AR SYSTEM. 
 
 currents, whirlwinds, and, no doubt, breaks or rents in the luminous 
 surface. 
 
 The rapidity of rotation increases of course from the poles towards 
 the equator, and is much greater than woiJd be imaginable at first 
 sight to judge from the slowness of its angular movement. A point 
 situated on the solar equator, however, travels with a velocity of 
 4560 miles an hour, or about 1^ miles a second; that is, nearly 
 four times and a half faster than a point situated on the terrestrial 
 equator. 
 
 Now that the spots of the Sun have revealed to us its movement 
 of rotation, and the direction, manner, and duration of that movement, 
 let us study in detail these interesting phenomena, and see w^hat 
 
 
 jM 
 
 mm 
 
 i 
 
 '■':■' 
 
 
 
 
 » 
 
 •i 
 
 
 ^^^^^ ^^■l^a^^^^I 
 
 Hi 
 
 ^^^^ 
 
 
 
 jfm^^^^^^Bmj^^^^^ 
 
 H^^^H 
 
 ^SI^IRt^K 
 
 
 
 ^SSHfvl^l 
 
 WWm 
 
 H^^^^ 
 
 
 
 
 IH 
 
 ^P^^i 
 
 
 
 
 
 ' 
 
 Wl^ 
 
 
 
 # 
 
 
 
 
 Fig. y. — Suu-spot, sb owing uiubra penumbra and lumiuous bridges. (Nasmyth.) 
 
 knowledge we can gather of the physical constitution of this giant of 
 our planetary system. 
 
 Turn to Plate II, which represents a series of Sun-spots. It will 
 be seen that the spots consist almost invariably of one or several dark 
 l^ortions called itmhtw, which seem black when compared with the 
 luminous parts of the disk. 
 
 Around these, a grey tint furrowed with dark striae, forms what 
 is named, improperly, the pouonhra. The majority of spots are 
 composed of one or several nmhne, enclosed in one penumbra. But 
 sometimes spots appear without the greyish envelope, as also occa- 
 sionally penumbra unprovided with umhrce.
 
 PLATE II. 
 
 *|^ 
 
 V ' 
 
 
 
 
 — :-. -is 
 
 ^f^=^^a> 
 
 
 P^ 
 
 A r 
 
 :£ 
 
 
 % 
 
 W 
 
 
 \. 
 
 -^; 
 
 V 
 
 ^ 
 
 m 
 
 
 SUN-SPOTS. (Sir J. Herschel.)
 
 THE SUN. 35 
 
 The forais of the spots, as shown by the drawings placed before 
 the reader, are most varied. The penumbra most frequently repro- 
 duces the principal contours of the imibra, and often presents a 
 great variety of shades, when examined with considerable magni- 
 fying powers. On the exterior edges of the penimibra, the grey 
 tint seems generally the deepest, either by the effect of contrast with 
 the brilliant portions that surround it, or because in reality it 
 possesses at these points a more decided tint. 
 
 Fig. 9 (p. 32) affords a striking example of this aspect of the 
 penumbra. 
 
 This spot presents the peculiarity, not at all unfrequent, that 
 the dark umbra is divided into several fragments by limiinous 
 bridges, spanning it, as it were, from one side of the penumbra to the 
 other. 
 
 The umbra itself is far from offering an uniform black tint. In 
 reality it always presents the aijpearance of varied shades, as if 
 the penumbra and umbra were mingled, and mixed up their tints 
 in varied proportions. 
 
 [^"e owe to the Rev. W. R. Dawes the discovery that the vmhra 
 is but a darker kind of penumbra ; for under the best conditions of 
 air and instrument, he has found it to be in its turn pierced, and 
 affording a \\ew of a much darker portion — which he calls the 
 nucleus — lying underneath. This he finds to be of the most intense 
 blackness ; but in saying this we must warn our readers that such 
 a word as applied to the -Sun is comparative only. Sir J. Herschel 
 has shown, that a ball of ignited quicklime, in a Drummond's 
 oxyhydi"ogen lamp, which itself gives out an apparently near ap- 
 proach to sunlight, when projected on the Sun appears- as a black 
 spot. So the Sun-spots, properly so called, may not be so black 
 after aU !] 
 
 The transits of Mercur}', moreover, over the Sun's disk have 
 taught us that the umbra is less dark than the unilluminated face 
 of a planet. 
 
 We will now speak of the real dimensions of the spots, the 
 successive changes which thej undergo, and what astronomers call 
 their " proper motion," that is, their actual movement on the Sun's 
 surface in any direction. 
 
 The dimensions of the spots are extremely variable, and they 
 sometimes cover enormous areas. It is not uncommon to see one 
 with a surface larger than that of the Earth. Schroter measured 
 one, the extent of which was equivalent to sixteen times the 
 surface embraced by a great circle of our Earth, or four times
 
 36 
 
 THE SOI.AR SYSTEM. 
 
 the entire superficies of our globe; its diameter, therefore, was 
 nearly four times the diameter of the Earth, that is to say, more than 
 29,000 miles. Sir W. Herschel, in 1799, measured a spot consisting of 
 two parts, the diameter of which was not less than 50,000 miles. 
 Some spots observed by Captain Davis on the 30th of August, 1839, 
 show what enormous proportions they sometimes attain. The most 
 extensive was not less than 186,000 miles -in its greatest length, its 
 surface embracing about 25,000,000,000 square miles. 
 
 Fig. 10. — Enormous Suii-spots. (Davis.) 
 
 If the spots are deep rents in the envelope, of what enormous 
 capacity must be those gulfs, those gigantic abysses, at the bottom of 
 which the Earth would only be as a boulder in the crater of a vol- 
 cano ! 
 
 Not only are Sun-spots not permanent — it is rare that one lasts 
 for many successive rotations — but their forms and dimensions difier 
 from one rotation to the other ; sometimes even in the interval of a 
 day.
 
 THE SUN. 37 
 
 The modification undergone by groups of spots, in about the 
 interval of one rotation, can be seen in figs. 11 and 12. 
 
 These difierent groups, though easily recognised again, form, 
 nevertheless, a new ensemble, and the details of the spots are still 
 more modified. 
 
 These changes indicate two j)henomena going on simultaneously, 
 which observers have separately studied. On the one hand, we have 
 here indicated a proper motion of the spots, more or less rapid and 
 distinct from the apparent movement produced by rotation. Accord- 
 ing to Laugier, the proper motion of a spot observed by him was not 
 less than 363 feet a second ; that is to say, three times greater than 
 that of clouds carried along by the most violent hurricane. 
 
 Fig. 11. — Traiisforiiiutiuii of groups o(" Solar spots iu the interval of a rotatidU. 
 Observed by M. Pastorff ou 24th May and 21st June, 1S2S. 
 
 [The proper motion of the spots has recently been inquired into in 
 the most complete manner by Mr. Carrington, who has been content to 
 observe the Sun every fine day for eight and a half years, in order to 
 supply his mite of information towards solving that great question, 
 "What is a Sun?" What he has discovered shows us that there 
 need be no wonder that difierent observers have varied so greatly in 
 the time they have assigned to the Sun's rotation. As our readers 
 already know, that rotation has been deduced from the time taken by 
 the spots to cross the disk. Mr. Carrington now shows that all Sun- 
 spots have a movement of their own, and that the rapidity of this 
 movement varies regularly with their distance from the solar equa- 
 tor. In fact, the spots near the equator travel faster than those 
 away from it, so that if we take an equatorial spot we shall say that 
 the Sun rotates in about 25 days ; and if w^e take one situated half wa\- 
 
 C'iC;
 
 38 
 
 THE SOLAR SYSTEM. 
 
 between the equator and tlie poles (in either hemisphere), we shall 
 say that it rotates in about 28 days. This, truly, is an important 
 stand-point gained, but while it aids our knowledge of the photo- 
 sjjhere — that silver sea over which the spots, like gondolas, so slowly 
 glide — it tells us that of the rotation of the Sun itself lying under- 
 neath this fiery envelope we are yet entirely ignorant, for if it be 
 a solid mass it can only have one period of rotation. Which is it ?] 
 We now come to the other phenomenon indicated. The change of 
 form is not less rapid than the proper movement. Sometimes a spot 
 divides into several separate nuclei ; sometimes many distinct nuclei 
 reimite into one. Arago quotes from Wollaston a curious instance 
 of a spot which seemed to break upon the surface of the solar globe, 
 in the same manner as a fragment of ice thrown on the frozen surface 
 
 Fig 12. — Changes of Solar spots in tlie interval of one rotation. Details of groups A and B. 
 (See last figure.) (Pastorfif.) 
 
 of a sheet of water divides into several pieces, and slides in all 
 directions. 
 
 [Diligent observation, moreover, of the umbra and penumbra with 
 a powerful instrument, reveals to us the fact that change is going on 
 incessantly in the region of the spots. Sometimes, after the lapse of 
 an hour, many changes in detail are noticed : here a portion of the 
 penumbra setting sail across the umbra ; here a portion on the umbra 
 melting from sight ; here, again, an evident change of position and 
 direction in masses which retain their form.] 
 
 Are, then, these spots the only exceptions to the uniform brightness 
 of the Sun's surface ? They are not. 
 
 [Xear the edge of the solar disk, and especially about spots 
 approaching the edge, it is quite easy, even with a small telescope, 
 to discern certain very bright streaks of diversified form, quite dis- 
 tinct in outline, and either entirely separate or coalescing in various
 
 THE SUN. 
 
 39 
 
 ways into ridges and network. These appearances, which have been 
 termed " faculae," are the most brilliant parts of the Sun. Where, 
 near the limb, the spots become invisible, undulated shining ridges 
 still indicate their place — being more remarkable thereabout than 
 elsewhere on the limb, though everywhere traceable in good 
 observing weather. Faculae appear of all magnitudes ; and Professor 
 Phillips, whose description we are quoting, has observed them from 
 barely discernible, softly- gleaming, narrow tracts, 1000 miles long, to 
 continuous, complicated, and heapy ridges, 40,000 miles and more 
 in length, 1000 to 4000 miles broad. By the frequent meeting of 
 the bright ridges, spaces of the Sun's surface are included of various 
 magnitudes and forms, somewhat corresponding to the areas and 
 forms of the irregular spots with penumbroe. They are never 
 regidarly arched, and never formed in straight bands, but always 
 
 i-Mfii--;- ■ ft-ir-- 
 
 W!Sff!.mrMW'^M^7^' 
 
 W 
 
 Fig. 13. — Suu-spots surrouuded by a platform of aculse. (Cai^iu ) 
 
 devious and minutely undulated, like clouds in the evening sky, or 
 irregular ranges of snowy mountains. 
 
 Ridges of this kind often surround a spot, as shown in fig. 13, 
 and hence appear the more conspicuous ; but sometimes there appears 
 a very broad white platform roimd the spot, and from this tlie 
 white criunpled ridges pass in various directions. Towards the 
 Kmb the ridges appear parallel to it ; away from it, this character 
 is exchanged for indeterminate direction and lessened distinctness ; 
 over the remainder of the surface they are much less conspicuous, but 
 can certainly be traced.] 
 
 There would appear to be a close connexion between spots and 
 faculse, for M. Chacornac, an eminent French observer, holds that 
 spots are distributed for the most part in groups, with their greatest
 
 40 
 
 THE SOLAR SYSTEM. 
 
 length parallel to the Sun's Equator, and that the first spot of the 
 group is the blackest, the most regular, and lasts the longest. As 
 the spots on the wake of the first disappear, they give place to faculae, 
 which invade and cover over the regions where the spots showed 
 themselves : then the original spot appears followed by a train of 
 faculae.* 
 
 [The Sun himself has bestowed a great boon upon observational 
 Astronomy. Thanks principally to the labours of Mr. De La Rue, 
 who has now brought the art and science of Astronomical Pho- 
 tography almost to perfection, the 
 Sun, whether brightly shining or 
 hid in dim eclipse, now tells his 
 own story, and j)rints his image on 
 a retina which never forgets and 
 withal so docilely, that each day he 
 shines on the Kew Observatory a 
 young ladyt takes observations 
 which surpass immeasurably in value 
 those made by the hardest-headed 
 astronomers of by- gone times. We 
 have mentioned in a note one fact 
 which these new pictures have taught 
 us : there are others of equal, pos- 
 sibly greater value, which we shall 
 discuss by-and-by. 
 
 So much for the more salient phe- 
 nomena of the Sun's surface, which 
 we can study with our telescopes. 
 There is much more, however, to 
 be inquired into ; and astronomers — 
 so far from being dismayed at the enormous distance of our central 
 luminary, and at the fact, that with our most powerful instruments 
 we can only watch the changes eternally going on on its surface 
 as we could do with the naked eye at a distance of 180,000 miles, — 
 are at the present moment engaged in a discussion on the more 
 minute appearances revealed to us imder the best conditions of air 
 and instrument. 
 
 Pig. 14.— Spot with faculae, May 24, 1863. 
 (Capt. Noble.) 
 
 * A fact agreeing with the deductions made by JkCr. Balfour Stewart from an 
 elaborate study of the Sun- pictures taken at Kew. 
 
 t [Shall we be divulging too much if we recognise here the "quaUfied 
 assistant" of the Kew Reports ?]
 
 THE SUN. 41 
 
 We may begin by saying, that the whole surface of the Sun, 
 except those portions occupied by the spots, is coarsely mottled; 
 and, indeed, the mottled appearance requires no very large amount 
 of optical power to render it visible. It has been often observed 
 with a good refractor of only 2^ inches aperture. Examined, how- 
 ever, with a large instrument, it is seen that the surface is principally 
 made up of luminous masses — described by Sir W. Herschel as 
 "corrugations" and small points of unequal light — imperfectly 
 separated from each other by rows of minute dark dots, called jwres, 
 the intervals between them being extremely small, and occupied 
 by a substance decidedly less luminous than the general surface. 
 Mr. Nasmyth has recently announced his discovery that these pores 
 are the " polygonal interstices between certain Imninous objects of 
 an exceedingly definite shape and general uniformity of size (at least 
 as seen in projection, in the central jjortions of the disk), which is 
 that of the oblong leaves of a willow-tree."* According to other 
 observers, however, these luminous masses present almost every 
 variety of irregular form: they are " rice- grains," granules or gra- 
 nidations, " untidy circular masses," " things twice as long as b^oad," 
 " three times as long as broad," and so on. Mr. Dawes asserts, 
 indeed, that he has seen some nearly in contact differ so greatly in 
 size, that one was four or five times as large as the other ; and 
 while, in a remarkably bright mass, one somewhat resembled a blunt 
 and ill-shaped arrow-head, another, very much smaller, and within 
 b" of it, was an irregidar trapezium, with rounded corners. 
 
 With regard to the general surface of the Sun, therefore, it is 
 not so easy to reconcile the conflicting opinions to which we have 
 alluded. The appearances which, according to Mr. Nasmyth, arise 
 from the interlacing and irregidar arrangement of his " willow-leaves," 
 Mr. Dawes, who is one of the most assiduous observers of the present 
 day and who has closely studied the solar surface, explains very 
 difierentl}'. He looks upon them not as individual and separate 
 bodies of a peculiar nature, but as merely rendering visible to us 
 different conditions as to brightness or elevation of the larger 
 masses forming the mottled surface, "just as the brightest portions 
 of that surface, and the facvdoc also, are different conditions of the 
 general photosi^here." " Their forms and sizes," he says, " are so 
 various as to defy every attempt to describe them by any one 
 appellation or comparison. But the rarest of all forms is the long 
 and narrow." 
 
 * Herschcl's " Outlines of Astronomy," p. (iOo,
 
 42 
 
 THE SOLAR SYSTEM. 
 
 The word " willow-leaf," however, very well paints the appear- 
 ance of the minute details sometimes observed in the penumbrae of 
 spots, which occasionally, as seen in fig. 9, appear to be made up 
 of elongated masses of unequal brightnesses, so arranged that for 
 the most part they point like so many arrows to the centre of the 
 nucleus, giving to the penumbra a radiated appearance. At other 
 times, and sometimes in the same spot, the jagged edge of the 
 penumbra, projecting over the nucleus, has caused Mr. Dawes to 
 liken the interior edge of the penumbra to coarse thatching with 
 straw, the edge of which has been left untrinuned. But other ap- 
 pearances are assumed, depending upon the amount and kind of action 
 
 m 
 
 Sly 
 
 Fig. 15.— Sun-spot, April 2, 1865. (Lockyer.) 
 
 The iuteriov outline of the penumbra and other appearances projected on the umbra are alone in 
 tended to be shown. 
 
 A. Tongue of faoula (?) stretching out into the umbra. B. Clouds (?). 
 
 C. A Promontory in which the "things" are changing the direction of their l.irger axes witli 
 respect to the centre of the spot. 
 
 D. The "things" on the general surface of the Sun ; these are shown by the engraver too regu- 
 larly and too near together, as opposed to the " things" on the penumbra. 
 
 P. Here the penumbra seems composed of layers, and the " things" are arranged like feathers on 
 a duck's wing. 
 
 going on in the spot at the time. This has recently been abundantly 
 demonstrated by Father Secchi. The occasional "willow-leaf" 
 appearance of the penumbra is represented in fig. 15. 
 
 Mr. Dawes has come to the conclusion, that the "granules" or 
 "granulations," are generally larger and brighter, on the brightest 
 parts than on the darkerst ones ; the difierence in brightness of the
 
 THE SUM. 43 
 
 individual " granules " in each part being- much the same as in 
 the different masses themselves ; on each of the larger masses, the 
 individual granules are all very nearly of equal brilliancy throughout 
 the mass to which they belong. They are not in general, if ever, 
 mixed together — some much brighter, and others far less bright, on 
 the same mass. There are also darker or shaded portions between 
 the granules, often pretty thickly covered with dark dots, like 
 stippling with a soft lead-pencil ; these are what have been called 
 "pores" by Sir John Herschel, and " punctulations " by his father. 
 Some of these are almost black, and are like excessively small 
 eruptive spots.]
 
 44 THE SOLAR SYSTEM. 
 
 III. 
 
 Theories of the Physical Constitution of the Sun- -Wilson's Theory — Kirch- 
 hoff's Theory — Their Antagonism — Opinions of ^I. Faye and Mr. Herbert 
 Spencer — Solar Cyclones — Probable downrush of Clouds into a Spot, and 
 Consequent Disappearance — Intensity of the Sun's Light and Heat. 
 
 GrREAT interest, doubtless, attaches to a knowledge of the relative 
 movements of th« celestial bodies, and to the possession of the secret 
 of the successive changes of position of the luminous points in the 
 starry vault, which we are enabled to contemplate. These pheno- 
 mena, studied with admirable perseverance during tAventy centuries, 
 have at length unveiled to us the structure of the Universe, by 
 enabling us to comprehend in all its details that of the system to 
 which the Earth belongs. 
 
 But the domain of Astronomy is not restricted to the study of 
 these general laws, so great in their simplicity. It embraces also all 
 the phenomena appertaining to each celestial body considered singly ; 
 phenomena which, when taken as a whole, allow us to form the most 
 reasonable conjectures as to its particular constitution. The Earth, 
 naturally enough, was the first body of which the physical constitution 
 was studied and known, and this, of course, apart from all astro- 
 nomical considerations, and by direct methods very diflferent from 
 those employed by astronomers. 
 
 The bodies nearest to us and most easy to observe, thanks to their 
 apparent dimensions — the Moon and the Sun — came next in turn. 
 Then followed the several planets of our system ; and at length the 
 investigators of science, overcoming the abysses which separate us 
 from the other systems of the sidereal imiverse, have attacked with 
 success the problems which deal with the physical constitution of 
 the stars and nebula). 
 
 The nature of the light by which a celestial body reveals its 
 existence to us ; its intensity ; the heat which it receives or gives 
 out : the nature of the matter of wliich it is composed ; the various
 
 THE SUN. 45 
 
 phenomena whicli it reveals to us ; the changes of form and of colour 
 which these phenomena undergo ; the succession of day and night and 
 of seasons deduced from its various movements ; the mass ; density ; and 
 force of gravity at its surface — these are some of the principal points, 
 the study of which belongs to that part of our subject called Physical 
 Astronomy ; a part of the science which has from the earliest times 
 been privileged to excite human curiosity to its highest degree. 
 
 "We propose, in the course of this work, to collect together all the 
 facts of this kind with which observers have, up to the present time, 
 enriched our science, in sach a manner as to amply satisfy this legiti- 
 mate curiosity ; and we will begin with the Sim. 
 
 We have been made acquainted with its dimensions, mass, and 
 movement of rotation ; and we have dwelt upon the cui-ious phe- 
 nomena of which the surface of its immense globe is eternally the 
 theatre. "\Ve have now to try to explain these phenomena, and to 
 see in what manner they can be connected with the Sun's constitution. 
 
 The first attempt to do this was made as long ago as 1774, by 
 Alexander Wilson ; and his theory, developed and modified by Bode, 
 Mitchell, and Schroter, and completed by Sir W. Herschel and Dawes, 
 has been confirmed, and partly verified, by the important experiments 
 of Arago. 
 
 According to Wilson's theorj^, the Sun is composed of a dark 
 spherical globe, or at least, a globe not self-luminous, surrounded, 
 at difierent distances, by three atmospheres, or gaseous envelopes, 
 entirely distinct. 
 
 The first atmosphere, the one nearest the central nucleus, is 
 formed of an opaque, cloudy stratum, reflecting light, but giving out 
 none, except that light which it receives itself. 
 
 To this envelope succeeds another, either close to the first, or 
 separated from it by a certain interval. This second atmosphere is 
 self-luminous, being formed of a gas in a permanently incandescent 
 state. The outer surface of this stratum, called the photosphere, gives 
 rise to the visible limits — the well-defined edge, or limb of the sim's disk. 
 
 We have, lastly, a third atmosphere, which is illuminated by the 
 photosphere, is transparent, and siuTomids all the others, and is 
 composed of strata, the density of which decreases as they increase in 
 distance from the central body. 
 
 Let us see now how this hypothesis accounts for the appearances 
 presented by Sun-spots, and the shaded or luminous portions of the 
 remainder of the disk. 
 
 If we imagine that on the surface of the dark nucleus there are 
 formed from time to time gaseous masses, incandescent by reason of
 
 46 THE SOLAR SYSTEM. 
 
 their high temperature ; or again, if there exist on the same surface 
 centres of volcanic disturbance, the eruptions proceeding from these 
 craters, piercing and tearing away successively the two interior 
 atmospheres of the Sun, would produce holes of greater or less extent, 
 openings through which the central nucleus or the overlying umbra 
 could be seen. These openings, therefore, should present generally 
 the form of an irregular cone, widened at the upper part, exposing at 
 
 Fig. 16. — Explanation of Sun-spots on Wilson's hypothesis: a a, photosphere; bb, cloudy stratum ; 
 A, spot with nucleus (umbra) and penumbra ; B, nucleus (umbra) without penumbra ; C, pe- 
 numbra without nucleus (umbra). 
 
 its centre the solid and obscure part of the Sun, and all around this 
 the cloudy atmosphere of a greyish tint. Hence, black spots, sur- 
 rounded with penumbrae. 
 
 But it may happen that the opening thus made in the photosphere 
 will be smaller than that in the cloudy stratum. In this case the 
 black nucleus will be alone visible, and it is thus that a spot without 
 penumbra is explained. If, on the contrary, the rupture in the first 
 envelope closes up before the photosphere, then the obscure body will 
 be invisible, a circumstance which easily explains penumbrse without
 
 THE SOLAR SYSTEM. 47 
 
 II nucleus. These different cases are all represented in tig. 17, where 
 the conditions necessary to present these appearances to an observer 
 on the Earth are indicated. 
 
 When a fissiu'e is \dolently and suddenly produced in a gaseous 
 mass like the photosphere, we must expect to see round the opening 
 a heaping up of the matter of which it is formed, and consequently 
 much greater luminous intensity. 
 
 Such would seem to be the origin of faculao, which generally, as 
 we have seen, surround the spots. 
 
 This theory of the phj'sical constitution of the Sun accounts in a 
 very satisfactory manner for the details of the phenomena observed. 
 The various forms of the spots, their disappearance, their motions even. 
 
 Fig. 17. — Explanation of Sun-spots on Wilson's hypotliesis. Appearances presented by the 
 same spot as seen at the centre and near the limb. 
 
 are easily and naturally explained. The fact often observed, that the 
 nucleus diminishes little by little, and is reduced to a mere point, 
 leaving the penumbra visible sometimes after its disappearance, is 
 explained wonderfully ; it is precisely in this manner that the edges 
 of the two atmospheres should gradually come together when the cause 
 which gave rise to their disturbance diminishes in energy and dis- 
 appears. 
 
 It may also be conceived, that after the disappearance of a spot, 
 the faculae ought still to remain and even to appear more brilliant, 
 since a certain time must be necessary to re-establish the perfect 
 homogeneity of the gaseous strata ; and that the gaseous matter, in 
 filling up the cavity formerly' occupied (apparently) by the imibra or 
 nucleus and the penumbrae, should naturally condense, and thus 
 become more liiminous. 
 
 Besides the ascending currents, the rapidity of which is powerful
 
 48 THE SOLAR SYSTEM. 
 
 enough to pierce the atmospheric envelopes of the Sun, it is thought 
 that there exists a continual agitation in the gaseous strata and on 
 the surface of the photosphere. This surface is not smooth, but 
 furrowed with elevations and depressions in every direction, analogous 
 to the waves of the ocean. Hence the luminous ridges, and darker 
 intervals, and multitude of pores, giving the Sun the mottled aspect 
 before mentioned. 
 
 The apparent changes of form, which result from the rotation of 
 the Smi, now remain to be explained. In fig. 6 are represented 
 the appearances presented by a spot as it travels from the edge to 
 the centre of the disk, or vice versa. The elongated form of a spot 
 at the edges, compared to its rounded form at the centre, is an effect 
 of perspective, and results from the spherical form of the Sim. But 
 this is not all. If the spot and its penumbra are formed by a conical 
 opening, the sloping sides of which reveal to us the thickness of the 
 envelopes,* the portion of the penumbra turned towards the centre 
 will disappear first, while the penumbra on the side nearest the 
 limb will apparently increase. The same appearance wiU be pro- 
 duced at the moment of the appearance of a spot on the eastern limb. 
 This is due to a simple efiect of perspective, which fig. 17 will show 
 at once. 
 
 The preceding theory is entirely founded upon the hypothesis that 
 the light of the Sun does not belong to the nucleus, but that it 
 is radiated by a gas in a state of incandescence, f 
 
 The physical constitution of the Sun is much more simple, if 
 
 * M. Petit, of Toulouse, has succeeded recently in measuring the height of 
 the cloudy stratum which gives rise to the appearance of the penumbra. He has 
 found it to be upwards of 4000 miles. [Professor Phillips has found a much 
 smaller height — 300 miles — to be a probable limit.] 
 
 + That the Sun is not a solid body, at least that its visible surface is not solid, 
 we must admit, in consequence of the extreme mobility displayed by the pheno- 
 mena of that surface. But it is not so evident that it is not an incandescent 
 liquid or body in a state of fusion. This fact has been held to be established 
 by an experiment of Arago's. The optical properties of the luminous rays 
 radiated by au ignited gas are very different from those rays the source of 
 which is a liquid or solid mass, at least if these rays leave the surface of the 
 incandescent body at a very small angle from the limb of a sphere. Whilst the 
 latter rays, examined by means of a very ingenious instrument, called a " polari- 
 scope," by its able inventor, are decomposed into two coloured pencils, the others, 
 in passing through the same artificial medium, remain in their natural state. Now 
 it is precisely the latter phenomenon which is presented by the light emanated 
 from the borders of the Sun. Hence Arago concluded that the luminous surface 
 of the solar globe consists of gas in a state of ignition. But this does not 
 preclude that the interior nucleus may be liquid, that is to say, composed of 
 mineral substances in a state of fusion.
 
 THE SUN. 4<) 
 
 we accept the reasoning of those who adopt a second theory which 
 is less at variance with the ideas held by those unfamiliar with astro- 
 nomy, on the subject of our great luminary. But, perhaps, at least 
 such is our way of viewing it, it scarcely renders a more satisfac- 
 tory exj)lanation than the other of all the observed phenomena, and 
 it leaves without explanation many circiunstances of these pheno- 
 mena. According to it, the Sun is formed of an incandescent nucleus, 
 the direct source of the light and heat which it emits ; whether it be 
 a solid or liquid nucleus matters not. The nucleus is surrounded 
 with a very dense atmosphere, formed of the constituent elements 
 of the body, — elements which the intensity of the temperature main- 
 tains in a gaseous state. 
 
 If partial coolings take place at different points of the atmo- 
 sphere by the action of unknown causes, what happens ? There 
 will be formed at these points precipitations analogous to the clouds 
 of aqueous vapour in the terrestrial atmosphere. Very dense ag- 
 glomerations of vapours in the vesicular state, dark clouds inter- 
 cepting the luminous rays of the body of the Sun, will appear to us 
 as spots on its disk. 
 
 A cloud, once formed, becomes a screen to the upper regions, 
 hence a cooling down of these regions, and the formation of a lighter 
 cloud-screen, less opaque, and which as seen from the Earth will pre- 
 sent the appearance of the penumbrae which surround the spots. 
 
 According to this hypothesis, the aj^parent changes which the spots 
 undergo in moving from the border to the centre, or vice versa, are 
 explained also by an effect of perspective, which fig. 17 will convey 
 to our readers. 
 
 Seen in ground-plan on the centre of the Sun, the spot will seem 
 to occupy the middle of the penumbra ; but in travelling towards the 
 border the part of the upj)er cloud situated towards the centre will 
 be projected on the dark nucleus, and will be confounded with it, 
 whilst the portion of the same cloud towards the limb will apparently 
 increase by exposing to view the thickness of the light cloudy mass 
 which overlies the darker one.* 
 
 * It has been said that tlie second theory leaves some important facts 
 without explanation. It accounts neither for the existence of faculae, nor for the 
 granulations. One does not see also why spots should not be found near the 
 poles, or why, when a spot disappears, the penumbra still subsists after the dis- 
 appearance of the nucleus. It does not explain the difference which exists 
 between the spots without penumbra, and the penumbra deprived of umbra. 
 Besides, a general fact of observation, which seems inexplicable if the spots are 
 clouds in suspension in the solar atmosphere, is, that the spots always disappear 
 
 E
 
 50 
 
 THE SOLAR SYSTEM. 
 
 This theory has been put forward in support of some recent 
 discoveries of great importance to which we must now call attention. 
 
 Every one is acquainted with the glorious coloured band called 
 the spectrum, which is produced by the decomposition of white light 
 by means of a prism. Every one knows, too, that, besides the seven 
 primary colours, there is in the spectrum of sunlight a multitude of 
 dark lines, which completely divide the coloured bands in the direc- 
 tion of its breadth. By comparing these dark lines with the brilliant 
 lines of the different spectra of the light given out by the flames 
 of difierent metallic substances, philosophers have arrived at the 
 knowledge, that t^ie light emitted by the luminous nucleus of the 
 Sun must traverse, before it reaches us, an atmosphere charged with 
 certain metallic vapours. The natui'e of these vapours even has been 
 
 % 
 
 rniw^w 
 
 mm'\\; 
 
 Fig. IS. — Explanation of Sun-spots ou KirchhofF's hyiiothesis. 
 
 determined with precision ; and " spectrum analysis," — the name de- 
 voted to this new and already fertile branch of science, — teaches us 
 that the solar atmosphere contains, in the metallic state, vapours of 
 sodium, iron, nickel, copper, zinc, and barium. The presence of 
 cobalt is doubtful. The presence of gold, silver, mercury, lead, 
 tin, or silicium, so abundant in the terrestrial crust, or of arsenic, 
 antimony, strontimn, cadmium, or lithium, has not yet been proved. 
 As the six metals of the fi'rst series exist in the Sun's atmosphere, 
 they must also exist in the very body of the Sun. 
 
 Here we have, then, a wonderful instance of a celestial body, 
 separated from us by an enormous distance, the constituents of which 
 are studied in their most minute detail, — analysed, if one may so say, 
 
 a little Ijefore ttiey reach the limb, when, according to this hypothesis, they should 
 invariably notch it.
 
 THE SUN. 51 
 
 with the same certainty as if they were put into one of the crucibles of 
 our chemical hiboratories. We shall have more to say about spectrmn 
 analysis when we try to answer the question, " What is a star ? " * 
 
 [But, in the meantime, we may point out what our readers 
 lia^e doubtless discovered for themselves, that these theories are 
 antagonistic in the main. One declares for a cool nucleus, the 
 other for an incandescent one ; one for a gaseous photosphere, the 
 other for a liquid one ; and the experiments made by the polariscope 
 are apparently negatived by those made by the spectroscope. 
 Again, although Wilson's theory accounts for the telescopic appear- 
 ance of spots, it does so on an altogether improbable assumption ; 
 and although Kirchhoff 's theory is more in hannony with our present 
 knowledge, he supports it by a statement as to the spots which is 
 justly ridiculed by all who have ever observed the Sun through a 
 telescope. But are they entirely antagonistic ? Here M. Faye and 
 Mr. Herbert Spencer come to our rescue ; and, in spite of some differ- 
 ences of detail, propound an explanation of the observed phenomena, 
 which certainl}' is in advance of either of those we have hitherto 
 considered, while, very opportunely, the Sun-pictures taken at Kew 
 have put in a mass of new evidence to help us in our inquiries. 
 
 The first important point in this new evidence is, that the Sun 
 himself tells us that his spots are cavities ; this supports the notion 
 alwaj^s held by astronomers and strengthened by the beautiful 
 stereoscopic combinations suggested by De La Rue ; it also equally 
 upsets an important statement made by Kirchhoff. Astronomers, 
 doubtless, woidd have sooner asserted the small mean density of 
 the Sun and its enoimous heat in support of the evidence of their 
 telescopes, if they had not so long held to the theory of the cool and 
 habitable globe underneath. So that Arago's deduction from his 
 expermients on the polarization of the Sun's light — a deduction which 
 supported the theory of the gaseous nature of the photosphere from a 
 new point of view — was doubly welcome. 
 
 M. Faye has removed the grounds for Sir John Herschel's objec- 
 tion to this experiment, and has shown, moreover, that it can be 
 reconciled with Kirchhofi''s spectroscopic one. He considers the forma- 
 tion of a photosphere to be a simple consequence of cooling, and looks 
 
 * According to the recent experiments of Mitscherlicli it is the pure metals, 
 not their chemical combinations, which exist in the solar atmosphere. The 
 bodies which support combustion, such as oxygen and chlorine, do not exist m 
 it, or if they do, they are mixed with combustible bodies in a state of dissociation, 
 of which we have examples when these bodies are raised to a very high 
 temperature.
 
 52 
 
 THE SOLAR SYSTEM. 
 
 upon it, in fact, as the limit which 'separates the intense heat of the 
 interior portions of the Sun from the vacuum and cold of space. 
 
 From this point of view, the beautiful experiments of Arago and 
 Kirchhoff are seen to be no longer contradictory. The term incan- 
 descent gas was not used by Arago in the sense attributed to it now. 
 The flame he used, was that of an ordinary gas-jet, and not the 
 obscure one of a Bunsen's burner, or of a simple gas. Incandescent 
 molecules difiused in a gaseous medium, itself heated to a high 
 temperature, give a continuous spectrum, with the exception of the 
 dark lines due to the absorption of the medium. 
 
 The formation of the photosphere enables us to account for the 
 spots and their movements. The successive layers are constantly 
 traversed by vertical currents, both ascending and descending. In 
 
 Fig. 19.— Solar cyclone, May 5th, 1857. (Secchi.) 
 
 this perpetual agitation we can readily imagine that where the 
 ascending current becomes more intense, the luminous matter of the 
 photosphere is momentarily dissipated. Through this kind of unveil- 
 ing it is not the solid cold and black nucleus of the Sun that we 
 perceive, but the internal ambient, gaseous mass, of which the radiating 
 power at the temperature of the most vivid incandescence is so feeble, 
 in comparison to that of the luminous clouds of the non- gaseous 
 particles, that the difierence of these powers sujffices to explain the 
 striking contrast between the two tones observed in our telescopes. 
 
 This is a great point of M. Faye's theory, although one open to 
 objection. Let us now pass to that of Mr. Herbert Spencer. 
 
 M. Faye, as we have seen, considers the Sun to be at present a 
 gaseous spheroid, having an envelope of metallic matters precipitated 
 in the shape of luminous cloud, the local dispersions of which, caused
 
 THE SUN. 
 
 53 
 
 by currents from within, appear to us as spots. Mr. Spencer, on the 
 contrary, holds that a liquid film exists beneath the visible photosphere. 
 Mr. Spencer's remarks as to possible causes of solar spots are very 
 valuable ; for, whatever theory of their formation be the true one, 
 it is certain that the rapid formation of the spots, their movements, 
 and their disappearances, indicate meteorological phenomena on the 
 most gigantic scale, of which the imagination can scarcely form an 
 
 4^'-/ 
 
 /. 
 
 ^^'>^' \f ■-' 
 
 
 Fig. 20.— Change in shape of a Sun-spot, October 27, 29, 31, and November 2. (Dawes./ 
 
 idea. Immense cyclones pass over the surface of the Sun with fearful 
 rapidity, as is rendered evident by the form and changes of certain 
 spots, as observed by Secchi and others. In one instance, recorded by 
 Mr. Dawes, the rotation of a spot amounted to 110° in six days. 
 
 Here also (fig. 20), in a series of sketches represeaiting a spot 
 as observed by Mr. Dawes, is abundant proof of the rapidity of these 
 movements. The form here indicates clearly the cyclones of which 
 we are about to speak ; although in a manner less precise tlian in
 
 54 THE SOLAR Sl'STEM. 
 
 fig. 19. It should, however, be added, that these cyclonic spots are 
 somewhat rare. 
 
 The spot figured at page 42, also afForded remarkable evidences of 
 rapid change, which seems to put the cloudy nature of the Sim's 
 photosphere beyond all doubt.* It was a spot of the normal 
 character, by no means cyclonic, but with a tongue of what appeared 
 to be a portion of facula, stretching half way into the spot. When 
 the observation commenced, about half-past eleven on the date given, 
 the tongue of facula was extremely brilliant ; by one o'clock, it had 
 become apparently less brilliant than any portion of the penumbra. 
 At the same time it seemed to be ' giving out,' at its end, and a 
 portion of the umbra between it and the penumbra appeared to be 
 veiled with a stratus cloud evolved out of it. 
 After a time, condensation seemed going on 
 on the following portion of the cloudy mass. 
 So that a very brilliant mass of what ap- 
 peared to be facula gradually melted away 
 into umbra, and then the umbra condensed 
 / again ; three or four cloud-masses on the 
 
 Fig. 2i.-cioud-masses detaching iunor cdgc of the pcuumbra were observed 
 themselves from the penumbra, ^q dctach themsclves from it at difiercut 
 
 A very faint one at F. . 
 
 1865, April 2, i2'> 30">. (Lockyer.) pomts, and to travcrso the umbra towards 
 
 the centre of the spot. 
 It has long been taken for granted that there are upward and 
 downward cm-rents on the Sim. And this down-rush into a spot 
 seems proved, for the first time, so far as we know, by the observations 
 to which we are alluding. The fact, also, that this down-rush was 
 accompanied by first a dimming and then a melting of the cloud- 
 masses carried down, was also thought to be established. f 
 
 The cloud-masses, in one region of the penimibra, were also seen 
 to change the direction of their longer axes in about three quarters 
 of an hour with regard to the centre of the spot, in fact they turned 
 round bodily through a considerable angle. Others, projected on the 
 umbra, gradually melted away out of sight. One cloud-mass was 
 distinctly observed to set sail, as it were, over the umbra, and it 
 
 * " Monthly Notices, Royal Astronomical Society," 1865, p. 236. 
 
 t [While this book is passing through the press, Messrs. De La Eue, Stewart, 
 and Loewy, are publishing a paper in which a new theory of Sun-spots is dis- 
 cussed, which is confirmed by the above observation made by Mr. Lockyer. It 
 seems at the same time in accordance with other facts. In this paper all diffei-- 
 ences of luminosity on the surface of the Sun are referred to the same cause, 
 namely, the presence to a greater or less extent of a comparatively cold absorbing 
 atmosphere. — B S.]
 
 I 
 
 THE SUN. 55 
 
 had travelled a considerable distance when the observations were 
 terminated. 
 
 It would seem from these observations, that there is a running down 
 of the shape, as if the cloud-mass seen on the general siu-face of the 
 Sun were gradually drawn out in its journeying towards the umbra. 
 
 Mr. Spencer, basing his reasoning on terrestrial analogies, thus ac- 
 counts for the spots. The central region of a cyclone must be a region 
 of rarefaction, and consequently a region of refrigeration. In an 
 atmosphere of metallic gases rising from a molten surface, and reaching 
 a limit at which condensation takes place, the molecular state, especially 
 towards its upper part, must be such that a moderate diminution of 
 density and fall of temperature will cause precipitation ; that is to say, 
 the rarefied interior of a solar cyclone will be filled with cloud ; con- 
 densation, instead of taking place only at the level of the photosphere, 
 will here extend to a great depth below it. It will be seen that Mr. 
 Spencer, as opposed to Kirchhofi", not only accounts for the formation 
 of a cloud, but places it where the objections made to Kirchhofi''s 
 clouds do not hold good. He next shows that a cloud thus occupying 
 the interior of a cyclone will have a rotatory motion ; and this accords 
 with observation. Being funnel-shaped, as analogy warrants us in 
 assuming, its central parts will be much deeper than its peripheral 
 parts, and therefore more opaque. This, too, corresponds with obser- 
 vation. Nor are we, on this h}^othesis, without some interpretation 
 of the penumbra. If we may suppose the so-called " willow-leaves" — 
 the " things " on the Sun, to be the tops of the currents ascending from 
 the Sun's body, what changes of appearance are they likely to un- 
 dergo in the neighbourhood of a cyclone? For some distance 
 round a cyclone there wiU. be a drawing-in of the superficial gases 
 towards the vortex. All the Imninous spaces of more transparent 
 clouds, forming the adjacent photosphere, will be changed in shape 
 by these centripetal currents ; they will be greatly elongated ; and 
 those pecidiar aspects which the penmnbra presents will so be 
 produced. 
 
 We must now, however, pass from this part of our subject — in- 
 teresting as it is, — and we can do so full of hope, for never before 
 was it engaging the attention of so many minds.] 
 
 In examining with care the contour of the solar disk when the 
 Moon interposes between it and the Earth, as in the case of a total 
 eclipse, there have been observed in the luminous aureola which 
 envelopes the lunar disk, several very curious prominences — some in 
 form of mountains, others of boomerangs, others resembling colunms.
 
 56 THE SOLAK SYSTEM. 
 
 the iqiper part of which appeared out of the perpendicular ; others, 
 again, entirely detached from the disk, seem to float like immense 
 clouds in the atmosphere of the Sun. 
 
 The total eclipse of the 18th of July, 1860, has furnished the 
 most valuable information relating to these strange phenomena, and 
 it may be seen in the facsimile that we shall give, subsequently, 
 of one of the magnificent photographs taken by Mr. Warren De La 
 Eue, what was the aspect of the reddish prominences we have 
 just described. Astronomers hesitated long between opposite ex- 
 planations, some only seeing in these appearances effects pro- 
 duced by the interposition of the Moon, others believing in the 
 objective reality of these phenomena, and looking upon them as 
 agglomerations of matter resting- on the Sun, or susjDended in 
 the external atmosphere which surrounds it at a certain distance. 
 Now, all doubts as to their belonging to the Sun are removed, 
 but we are not much nearer to an explanation of them. Some 
 look upon them as clouds, others as solar aurorse. The fact that they 
 have been seen all round the limb of the Moon, that they exist in 
 all regions near the poles in the same manner as at the equator, 
 would seem to negative all idea of their being in any way connected 
 with the spots or with the causes which give rise to them, although 
 at first the opinion was entertained by some that they were in some 
 manner correlated. 
 
 Let us retiu'n, in order to finish what we have to say of the con- 
 stitution of the Sun,. to the purely physical facts which Astronomy 
 has set forth. 
 
 One word now on the intensity of Sun-light. This intensity 
 is not the same in all parts of the disk. The edges are less luminous 
 than the centre, and Arago valued at one-fortieth the difierence 
 of their intensity, which is much more considerable according to 
 other astronomers, Faye among the number. 
 
 [This fact, fully established for the luminous rays given out by the 
 Sun, applies also to the chemical ones ; but with this difierence, that 
 whereas the light, broadly speaking, diminishes regularly and very 
 gradually, from centre to border, the chemical brightness is much 
 more " patchy," so to speak. Professor E,oscoe, by receiving the 
 image of the Sun on a properly prepared photographic plate, has 
 observed remarkable difierences of this kind ; and, with Mr. De La 
 E-ue, is inclined to attribute to them some connexion with the 
 phenomena of the red prominences to which we have before drawTi 
 attention. The latter distinguished physicist is not without hope of 
 rendering them visible without the intervention of a total eclipse.
 
 THE SUN. 57 
 
 lie imagines that an extension of his beautiful experiment — in which 
 he combines Sun-pictures stereoscopically, and shows the faculae 
 to be above, and the spots below, the general ♦surface, — will enable 
 him to show the so-called red flames as very delicate dark markings 
 on the more brilliant mottled background of the photosphere. 
 These delineations, except with the aid of the stereoscope, would 
 be confounded with the other markings of the Sun's surface, but 
 they would assimie their true aspect and stand out from the rest 
 as soon as two suitable photographic pictures were viewed by the aid 
 of that instrmnent.] 
 
 The gradual diminution of both the Sun's Imninous and chemical 
 brightness towards the limb indicates without doubt the existence 
 of an atmosphere enveloping the body to a great distance. And 
 it is in this envelope, as we have said, that the red clouds observed 
 in total eclipses float. 
 
 According to Sir W. Herschel, the general brightness of the disk 
 being represented by 1000, that of the penumbra is not more than 
 469, and that of the darkest portion of the nucleus as low as 7. 
 
 Considered in each of these points of view, the solar light, as 
 it arrives on the surface of the Earth, is, according to Arago, at 
 least 15,000 times more intense than the flame of a wax-candle. 
 " According to the energy of the battery employed," he adds 
 ("Astronomie Populaire," ii. p. 172), "it is found that the electric 
 light varies in intensity from a fiftieth part to a quarter of that of 
 the Sun." So much for the comparative intensity of Sun-light. 
 
 Compared with the brightness of the full Moon, the light of the 
 Sun, according to Wollaston, is 800,000 times brighter than that of 
 the limar disk ; in other words, 800,000 full Moons would be required 
 in the heavens to produce a day as brilliant as that illimiinated 
 by a cloudless Sun. 
 
 As to the origin of this light, some, as we have seen, attribute it 
 to the incandescence of a gaseous mass, others to that of a solid or 
 liquid nucleus. Other savants again, among whom we must class 
 Sir J. Herschel, regard the solar light as having an electro-magnetic 
 origin, rather than arising from the combustion of solid, liquid, or 
 gaseous matter ; it is, according to them, a perpetual aurora. 
 
 From the intensity of the light, let us pass to the intensity of 
 solar heat. Without any doubt, this heat must be enormous on the 
 surface of the Sun ; and, if we base our estimation on the law of 
 decrease of radiant heat, the conclusion is arrived at that its intensity 
 is about 300,000 times greater than that of the heat received on a given 
 point on the surface of the Earth. The quantify of lieat, incessantly
 
 58 THE SOT.AR SYSTEM. 
 
 radiated into space by the immense focus of our system, has also been 
 calculated. The following comparison made by Sir J. Herschel will 
 give an idea of its calorific activity. Let us imagine a cylindrical 
 pillar of ice, 45 miles in diameter, to be continually darted into 
 the Sun, and that the water produced by its fusion is continually 
 carried off. In order that the heat given off constantly by radi- 
 ation should be wholly expended on its liquefaction, it wovild be 
 necessary to plunge the cylinder of ice into the Svm with the velocity 
 of light, or, in other words, the heat of the Sim can, without diminish- 
 ing its intensity, melt in a second of time a pillar of ice of 1590 
 square miles at its base, and 194,626 miles in height. 
 
 As with the luminous and chemical rays, so with the heat-rays 
 there is a diflFerence in the calorific intensity of the centre and limb, 
 the radiation being greatest from the centre. The polar regions, also, 
 are colder than the equatorial ones ; and Secchi has shown that less 
 heat is radiated from the spots than by the other portions. Sir John 
 Herschel thinks that one of the hemispheres of the Sun is hotter 
 than the opposite one. 
 
 It has been held that there exists a close correlation between the 
 periods of maximum and minimum of the solar spots and the Earth's 
 temperature. There is no doubt an intimate relation existing between 
 them and the Earth's magnetism. This has been proved by delicate 
 researches extending over a long period of years, carried on by such 
 physicists as Major-General Sabine the President of the Royal Society 
 of London, Schwabe of Dessau, and Wolf of Zurich. 
 
 [Thus we come upon another bond of union between the differ- 
 ent members of our system besides gravitation, and there is good 
 reason for believing that our luminary was once caught in the act 
 of creating a magnetic disturbance on our Earth. On the 1st of 
 September, 1859, two astronomers, Messrs. Carrington and Hodgson, 
 were independently observing a large spot, when they noticed a very 
 bright star of light suddenly break out over it, moving with great 
 velocity over the Sun's surface. At the same moment the mag- 
 netograph at Kew, where all the changes in the Earth's magnetism 
 unceasingly register themselves, was violently affected.] 
 
 A question of great interest, of which a solution has lately been 
 attempted, is that of the pemianence or the decrease of the solar 
 heat in the course of ages. 
 
 A philosopher of great eminence — Professor William Thomson — 
 has enunciated the idea that the solar temperature is constantly 
 sustained by a fall of meteorites, the motion of which is transferred 
 into heat at the moment of impact. Whether this theory be true
 
 THE SIIX. 59 
 
 [aud we believe Professor Thomson has abandoned it], or whether 
 the solar globe loses its heat year by year, it is perfectly certain that 
 there will be still sufficient heat left to support life on the Earth and 
 the other planets for millions of years to come — a perspective view 
 which in truth is somewhat consoling. 
 
 The question of the habitability of the Sun has also been agitated : 
 on the hypothesis which makes of this body an incandescent globe, the 
 answer can only be in the negative. We have no idea of an organized 
 being capable of living in a temperature so enormous. But the case 
 is altered if we supj)ose that the solar globe itself is neither very 
 luminous nor incandescent ; if we admit that it can be protected 
 against the radiation of the photosphere by an envelope of great den- 
 sity, which absorbs the light, and is at the same time a non-conductor 
 of heat. Arago remarks, " If any one were to ask me simply the 
 question. Is the Sun inhabited? I should answer that I did not 
 know. But, if he asked me if the Sun could be inhabited by beings 
 organised in a manner analogous to those who people our globe, I 
 should not hesitate to make an affirmative reply." 
 
 Questions of this kind will never be resolved categorically ; their 
 solution, whatever it may be, will remain eternally to humanity in 
 the domain of the probable. But what we must acknowledge, what 
 ought to strike our minds, now so much evidence has been placed 
 before us, is the varied and continual influence of the Sun on the 
 conditions of existence on the surface of our globe. 
 
 He acts on the Earth by his mass, whether he maintains it in its 
 orbit at distances the variation of which is regidated by inflexible 
 laws, or combines his action with that of the Moon, to produce the 
 semi-diurnal oscillatory movement of the waters of the ocean, — the 
 tides. The heat of the solar rays is the principal cause of the per- 
 turbations of equilibrium of the atmospheric strata. It is that which 
 gives rise to the wind, to the aerial and marine currents, to the 
 evaporation of the water of the rivers, of the lakes, of the sea, and 
 which produces a continual circulation of fluids on the surface of the 
 planet. This action is thus found to be the cause of the secidar mo- 
 difications of the geological strata, by the slow but increasing denu- 
 dation of the rocks, and by the transport of material due to currents. 
 It is'the heat and the light of the Sun which everywhere distribute 
 life to the beings of the vegetable and animal world. " At one time," 
 says Humboldt in his Cosmos, " its action manifests itself tranquilly 
 and in silence, by chemical affinities, and determines the divers 
 phenomena of life, in vegetables by the endosmosis of the cellulai- 
 wall, in animals in the tissue of the muscular and nervous fibres ;
 
 60 THE SOLAR SYSTEM. 
 
 at another it fills tlie atmosphere with thunder, waterspouts, and 
 hurricanes. The light-waves do not act only on the world of matter ; 
 they do not confine themselves to decomposing and recomposing 
 substances ; they do not merely draw from the bosom of the Earth the 
 delicate germs of plants, and develope the green matter or chlorophyl 
 in the leaves ; they do not simply tinge the odorous flowers, or repeat 
 thousands and thousands of times the image of the Sun, in the midst 
 of the graceful break of the waves, and on the light stems of the 
 prairie, bent with the breath of the winds. The light of heaven, 
 according to its varying degrees of duration and brilliancy, is also 
 in mysterious relation with the inner man, with the develop- 
 ment more or less decided of his faculties, with the gay or melan- 
 cholic disposition of his mind. This is what Pliny the elder referred 
 to in these words : ' Cceli triditiam discutit sol, cf liumani nuhila 
 animi serenaf."* 
 
 * " The Sim chases sadness from the sky, and dissipates the clouds wliicii 
 dcirken the human heart.".
 
 BOOK THE SECOND. 
 
 THE PLANEIS. 
 
 We have seen that round the Sun — that immense focus of light and 
 heat — revolve at different distances, and in widely varying periods, 
 a midtitude of secondary bodies, and among them our Earth. Some- 
 times solitary, sometimes arranged in groups which reproduce in 
 miniature the solar system itself, these bodies form so many dis- 
 tinct worlds, of which the dimensions, distances, movements, form, 
 structure, and physical constitution, deserve a separate examination 
 and study. 
 
 This study will now occupy us. The numerous phenomena of 
 which these worlds are the theatre — phenomena observed by our 
 astronomers as each planet has glided past us — not only make us 
 acquainted with the mechanism of the system as a whole, but permit 
 us also to examine somewhat closely into the details of the physical 
 organisation of each of these bodies. 
 
 If we look through the most powerful telescopes, we shall see 
 the form of the planets and their characteristic features ; and the 
 markings visible on their disks will tell us if they rotate, and what 
 is the duration of their day and night. The forms and dimensions of 
 the orbits, and the periods of revolution, will give us precise 
 information respecting the succession of seasons and climates, and 
 on the lengths of their years. Even the climatic variations will be 
 partly revealed to us by the degree of inclination of the axis of 
 rotation to the plane in which the body moves round the Sun. 
 
 The presence of satellites will not offer less interest, whether 
 we consider the partial illumination of the planet's night, caused 
 by the reflection from the illimiinated faces of these — in their 
 turn — secondary planets, or the eclipses, necessary consequences,
 
 62 THE SOLAR SYSTEM. 
 
 occurring more or less frequently, of the interposition of an opaque 
 body between the illuminated disk of the planet and the source of 
 light. 
 
 We shall encounter the Earth in our wanderings through the 
 planetary spaces. The study of the astronomical phenomena which 
 relate to it will aiford assistance by no means to be despised in 
 enabling us to comprehend the analogies and differences which these 
 phenomena present in the various planetary worlds. 
 
 Starting, then, on our journey from the Sun, we shall visit in 
 succession all the bodies which revolve round him, following the 
 most natural order, that of their distances.
 
 MERCURY. 63 
 
 I. 
 
 MERCURY. 
 
 Apparent Movement and Phases — Distances from the Sun and Earth — Form and 
 Dimensions ; its Transits across the Sun's Disk — Length of Day and Night, 
 Seasons and CUmates — Equatorial Belts, Atmosphere and Mountains of Mei'- 
 cury — JMass, Density, and Force of Gravity on its surface. 
 
 When the sky is clear, and the atmosphere at the horizon is not 
 too much charged with vapour, there may be perceived sometimes 
 in the evening, after the setting of the sun, a star, whose brilliant 
 twinkKng light renders it conspicuous in the ruddy and faint glimmer 
 of twilight. Its apparent elevation above the horizon, at first small, 
 increases little by little each evening, but it never recedes from the 
 Sun more than 30°. 
 
 This star is the planet Mercury. 
 
 If we continue to observe it on favourable evenings, it will be 
 seen finally to approach the Sun, and, lost in the dazzling bright- 
 ness of his rays, set with him. 
 
 Some days after, in the morning before sunrise, the same star, 
 again emerging from the Sun's rays, will rise earlier and earlier, 
 mounting day by day to a higher elevation above the horizon ; 
 the maximum of this to the east will be precisely equal to that 
 it formerly attained to the west. At last it begins to retrograde, 
 approaching the Sun, until the moment when it again disappears in 
 his rays. Mercury accomplishes then, in this manner, a complete 
 revolution round the Sun ; to us it appears like an oscillation, and 
 one which it repeats eternally ; its duration varies between 10(5 and 
 130 days. 
 
 The ancients, who did not know the true system of the world, 
 deceived by the double appearance of Mercury, sometimes after the 
 setting and sometimes before the rising of the Sun, believed at first
 
 64 
 
 THE SOLAR SYSTEM. 
 
 that two distinct bodies were in question ; they named one Apollo, 
 god of day and light, and the other Mercury, god of thieves. 
 The Indians and the Egyptians also gave it two different names. 
 But observers remarked, at last, that one only of the two bodies was 
 visible at the same time, and that the ajDi^earance of the one coincided 
 very nearly with the disapiDcarance of the other. To conclude their 
 identity from this fact was not a difficult matter. 
 
 Fig. 22. — Phases of Mercury when seen after sunset. 
 
 If, instead of confining ourselves to naked-eye observations, — 
 which, by the way, are by no means easy — we employ a telescope of 
 pretty high magnifying power, it will be found that the form of the 
 planet varies according to the time of observation. This remark also 
 holds good with its apparent size. 
 
 Fig 23. — Phases of Mcrcurj', when seen before sunrise. 
 
 Let us speak first of its form. Mercury, in the course of one of 
 its oscillations, presents phases entirely analogous to those of our 
 Moon. It is at first a limiinous disk, nearly circular, which, by 
 degrees, is reduced on the side towards the east, until not more than 
 a half-circle is visible at the period of its greatest apparent distance 
 from the Sun ; the crescent form lienceforward characterises it more 
 and more, until it is only visible as a fine luminous thread. We give
 
 MEKOURY. 
 
 65 
 
 some of these phases. The progressive increase of its apparent dimen- 
 sions is also shown in exact proportion. 
 
 The same appearances are observed, but in an inverse order, when 
 Mercury is observed during the period in which he is a morning star. 
 
 It is easy to account for these facts which observations have 
 placed before us. The phases prove that Mercurj' has the form of a 
 
 >f" 
 
 Fig. 24. — Explanation of the Phases of Mercury. 
 
 spherical globe, which is not self-luminous. Its movement round 
 the Sun places it, relatively to the Earth, in a series of very different 
 positions, and shows vis portions, sometimes smaller, sometimes larger, 
 of its illmninated half. The same movement varies its distance from 
 the Earth, — this explains the variations in the apparent dimensions 
 of its disk.
 
 66 THE SOLAR SYSTEM. 
 
 On the preceding page is a diagram of the positions of Mercury in 
 different parts of its orbit, during the period of an entire oscillation 
 compared to the successive positions occupied by the Earth. 
 
 When Mercury is in the same direction as the Sun, we say that 
 the planet is in conjunction. It is in superior conjunction when 
 beyond the Sun ; and in inferior conjunction when on our side of it. 
 In the first case, it turns towards us its bright hemisphere ; in the 
 second, its dark one. 
 
 If the Earth itself were immovable, the interval of 106 to 
 130 days, which we have seen to be the period of an entire 
 oscillation, would be also the period of a revolution of Mercury round 
 the Sun. But it is easy to see from the preceding diagram, that, 
 by the time the planet has returned to the same conjunction again, 
 the Earth has travelled onwards in its orbit, and Mercury has, 
 therefore, accomplished more than a complete revolution. 
 
 In reality, the time of a revolution of Mercury is less than that 
 of a complete oscillation ; it is about 88 of our days.* 
 
 If the orbit of Mercury were a perfect circle, its distance from the 
 Sun would not vary. But it is known that the orbits described by 
 the planets are ellipses — oval curves more or less elongated, of which 
 the Sun does not occupy the centre, but one of the foci. 
 
 Amongst the eight principal planets. Mercury's orbit differs most 
 from the circular form. Hence, its distances from the Sun are verj'' 
 vai'iable. While at its greatest distance from the central body its 
 distance is 44,475,000 miles, it approaches at its least distance to 
 within 29,305,000 miles, the difference being over 15 millions of 
 miles. In each of its revolutions Mercury traverses little less than 
 210 millions of miles. 
 
 This gives a velocity of 2,400,000 miles a-day, 100,000 miles an 
 hour, and finally close upon 28 miles a second. 
 
 As we are speaking of distances, let us say a word with regard to 
 those which separate Mercury from the Earth. These vary still more 
 than those we have before mentioned ; and this can easily be conceived, 
 because, firstly, the distance of the planet from the Sun varies ; and 
 secondly, because Mercury, as we have seen, is sometimes between 
 
 * More exactly, 87 days 23 hours 15 minutes and 46 seconds. Such is the 
 precision with which astronomers have succeeded in measuring celestial pheno- 
 mena. This revolution is called a sidereal revolution in contradistinction to the 
 " synodic revolution," because, relatively to the Sun, the planet again occupies 
 the same portion of the heavens. By the synodic revolution of a planet is ex- 
 pressed the interval of time taken to return to the same position relatively to 
 the Sun as seen from the Earth.
 
 JMERCUKY. 67 
 
 our Earth and the common focus ; and sometimes beyond that focus, 
 — the Sun. 
 
 In the first of these positions, Mercury approaches within 
 49,228,000 miles, while in the second it is distant 132,000,000 miles ; 
 these distances vary in the ratio of one to three ; and its apparent 
 diameter changes in inverse proportion to these numbers. 
 
 Fig. 25. — Apparent dimensions of the disk of Mercury at its extreme and mean distances 
 from the Earth. 
 
 As to its real dimensions, they have been easily determined from 
 the two elements which precede ; we allude, on the one hand, to the 
 measure of its apparent diameter ; on the other, to the distance of 
 the planet from the Earth. We hence derive the first physical datum 
 relating to Mercury, that it has the form of a globe, of 3089 miles 
 in diameter: this is about two-fifths of the mean diameter of the 
 Earth. Fig. 26 gives an exact idea of the relative sizes of the two 
 planets. 
 
 Earth. Mercury. 
 
 Fis- 26. — The size of Mercury compared with the Earth. 
 
 Hence it follows that the surface of Mercury is nearly six times 
 and a half less than that of our dwelling-place ; seventeen globes, 
 of the same volume as Mercury, would be required to equal the 
 volume of the Earth. 
 
 Is Mercury of a perfectly spherical form ? It is difficult to be 
 assured of this in observing the planet in its phases. The bright-
 
 68 
 
 THE SOLAR SYSTEM. 
 
 "M"," 'V'TT'lTlnn 
 
 18 61. 
 
 l2.N»v... 
 
 ness of its light is such that precise measures are extremely- 
 difficult.* 
 
 Astronomers have therefore preferred to take advantage of a 
 phenomenon which occurs pretty frequently, and sufficiently so to 
 control the observations. We allude to the passages, or transits, of 
 Mercury across the Sun's disk. 
 
 It must be borne in mind that once in each of its revolutions round 
 the Sun, Mercury passes between the Earth and that radiant body. 
 
 If the plane in which the planet moves were identical with that 
 of the terrestrial orbit, that is to say, coincided with it ; at each 
 
 inferior conjunction 
 Mercury would be pro- 
 jected on the Sun. But 
 this is not the case ; 
 owing to the inclination 
 of the plane of the orbit 
 of Mercury to that of 
 our Earth, sometimes 
 the planet is thrown 
 above the solar disk, 
 sometimes it passes be- 
 low. It happens, how- 
 ever, occasionally, that 
 it is precisely of the 
 same apparent height 
 as the Sun. Mercury is 
 then seen as a black 
 round spot, traversing 
 in the course of several hours f the Sun's disk, which is then partially 
 eclipsed.:}: 
 
 The sharpness of the planet's circidar form, the imiformity of the 
 
 * In 1832, Saturn and Mercury occupied the same region of the heavens, 
 in appearance, that is to say. According to Beer and Madler, who observed them 
 at that time, "Saturn compared to Mercury appeared pale and without hriUiancy, 
 Mercury presented a variable brightness, and remained perfectly visible after 
 the rising of the Sun, whilst Saturn disappeared from the sight. jMercury was 
 illuminated a little more than half." 
 
 t The duration of the transit is very variable. It may last about eight 
 hours. The last passage took place on the 12th of November, 1861. We may 
 add that up to the end of the present century there will be six others, the 
 first of which will take place on the 5th of November, 18C8. The transits of 
 Mercury always occur in May or November. 
 
 X " The first of these observations was made at Paris, by Gassendi, on the 
 7th of November, 1031, and, as mentioned liy him, in accordance with the wish and 
 
 
 5_Nov,.— 
 
 i!iiiJ 1 1; I 
 
 Fig. 27.— Transits of Mercury over the Suu, 1° the 12th 
 November, 1S61, 2° the 5th November, IStiS.
 
 AlERCURY. (39 
 
 movcmoiit of the black spot over the Sun, and, histly, the time of the 
 transit, are circunistanees Avhieh sufficiently prevent the phenomena 
 being confounded with those of solar spots. 
 
 Astronomers have chosen the favourable occasions oiFered by these 
 transits, to measure, by the aid of micrometrical instruments, the 
 apparent diameter of Mercury, from Avhich, by an easy calculation, 
 they have been able to determine its real dimensions. They have, at 
 the same time, observed that the black spot was always perfectly 
 round, that is to say, that the globe shows no trace of flattening.* 
 
 We now know the movement of Mercury round the Sun, the 
 time of its revolution, its distances from the Sun and from the Earth, 
 and, lastly, its dimensions in diameter, in surface, and in volume. It 
 onlj'^ remains now to speak of what is known of its physical con- 
 stitution. The facts that science has succeeded in gathering on this 
 curious and important point of the monography of the planets ought 
 to present a lively interest to us all, by reason of the likeness or 
 contrast which each of these worlds possesses to our own. The manner 
 in which light and heat are distributed on the surfaces of the planetary 
 bodies, the succession of their days, nights, and seasons, the existence 
 or the want of an atmosphere like ours ; lastly, the surface-markings 
 that the telescope has permitted us to observe on their surfaces, are so 
 many valuable particulars which enable us to make the most probable 
 conjectures on the organisation of the li^^ng beings which doubtless 
 people them. Supported by such positive data, imagination can then 
 launch boldly into the field of conjecture. 
 
 The intensity of the light which Mercury receives from the Sun, 
 at its mean distance, is nearly seven times f as great as that with 
 which our globe is illuminated under the same conditions of 
 distance. 
 
 It is not then surprising that the ancients gave Mercury the epithet 
 of " Twinkler" (ff-/7./3wi). This is not all. The laws governing the 
 propagation of radiant heat are the same as those of light. Mercury 
 then receives seven times more heat than the Earth, or, more properly, 
 a heat the intensity of which is in the mean seven times as great. 
 
 To judge by the impression which the light-rays make on our 
 
 suggestion of Kepler, for Kepler had predicted this transit, and had printed or 
 written on it the preceding year, which was that of his death." — UAlembert's 
 Encyclopcedia. 
 
 * A single observation of this kind would not always be conclusive, since 
 Mercury might be in such a position as to present to us one of its poles of 
 rotation. Besides, the flattening might be so slight as not to be measurable at 
 this distance. 
 
 t Exactly, (i-67.
 
 70 THE SOLAR syste:\i. 
 
 eyes, seeing- thut we cannot bear their dazzling briglitness without 
 pain, and, again, by that which they make on our body when it 
 is subjected to their influence, the inhabitants of Mercury sliould 
 be extremely micomfortable. But are they formed like us? and 
 have their senses the same degree of impressionability ? "We know 
 not. Variations of temperature are also disagreeable to us. In 
 this respect, again, we must own that the inhabitants of Mercury 
 have more to suffer than we. Owing to the planet's elongated orbit, 
 we have seen that sometimes it recedes from, and sometimes approaches, 
 the Sun, and that the difference between the extreme distances amounts 
 to fifteen millions of miles. So that whilst at aphelion,* the intensity 
 of the luminous and heat-rays is no more than four times and a half 
 that of the rays received by the Earth ; at perihelion, on the contrary, 
 it rises to more than ten times the same quantity. 
 
 Lastly, and this adds still more to the contrasts of temperature, 
 the variations occur in a period of time less than a quarter of one 
 of our terrestrial years. Presently, we shall see that the seasons 
 present still greater anomalies. 
 
 We must not forget, however, that one circmustance may modify 
 all this to an extent sufficient to render the conditions of vegetable 
 
 and animal life in Mercury either 
 similar to our own or more different 
 still. This circumstance is the ex- 
 istence or absence of a gaseous or 
 vaporous envelope, — in a word, of 
 an atmosphere. Has Mercury, then, 
 an atmosphere ? According to many 
 astronomers. Mercury presented the 
 following aspect (fig. 28) in its 
 transit (1799) across the solar disk. 
 Instead of a black round spot, per- 
 fectly clear and well defined, there 
 
 Fig. '28 —As|iect of the disk of Mercury during '' i xi t i p i 
 
 its transit over the Sun, Ttli May, 1709. WaS SCeU all rOUUd the dlsk ol the 
 
 ^ll^^'^"''"''^^"^'^'^ planet a circular band less luminous 
 
 than the rest of the surface of the 
 Sun, forming a sort of nebulous ring. It was thence inferred that 
 there existed a very high and dense atmosphere. Recent observers 
 have not seen anything like it. But, on the other hand, they have 
 
 * Aphelion — the greatest distance of a body from the Sun ; from awi from, and 
 -i'X'flf the sun. Perihelion — least distance ; from ^rsf' near to, and '^'A.(Of . If the orbit 
 of a body were rigidly circular, we should have neither aphelion nor perihelion 
 points.
 
 MERCDRY. 71 
 
 remarked in the phases of the planet, that the line of separation of 
 the lig-ht and shade, which astronomers call the termhiator, is never 
 very decided, so that the breadth of the hmiinous part seemed 
 diminished. " Hence," say Beer and Madler, " we may conclude 
 that Mercury has a pretty sensible atmosphere." 
 
 If this be so, we can form an idea of the modifications which a 
 somewhat dense atmosphere would induce in the intensity of the light 
 and heat, by comparing the days when on our Earth the sky is clear 
 and without clouds — when the Sun darts its rays on the surface with- 
 out obstacle, with the dark and dull days when the clouds completely 
 hide him from us. The density of the atmospheric envelope Ave see 
 then can strikingly change the effects of solar radiation. Let us 
 compare, for instance, the temperature of one of our valleys with that 
 of the mountainous sunrmits which surround it. It is like passing 
 from summer to the cold of winter, from the burning heat of Jnly to 
 the frost of November, and yet the Sun shines alike on the mountains 
 and on the valley. 
 
 Finally, the chemical composition of the atmosphere of Mercury, 
 the nature of the gases of which it is formed, and which are perhaps 
 very different from the nitrogen and oxygen of our air, are also 
 features which may influence the climate of the planet ; concerning 
 these matters we have no data. Let us confine ourselves, then, 
 to describe the astronomical phenomena of which the influence is 
 incontestable. 
 
 In the first place, let us consider the length of the day. Mercury 
 turns on its axis in 24 hours and b\ minutes, and his year comprises 
 87f of these sidereal rotations. The number of his solar days in this 
 period is therefore 86|, whence results as the length of one of them 
 24 hours and 54 seconds. This is nearly the length of one of our 
 own solar days, so that the organised beings of the two planets have 
 the same periods of light and darkness, of activity and repose. But 
 the relative length of the days and nights in the course of the entire 
 year is much more variable than on the Earth, owing to the great 
 inclination of the axis of Mercury to the plane of its orbit. 
 
 Fig. 29 shows, according to the old observers, at what angle 
 Mercury presents itself to the Sun at the commencement of each of 
 its seasons. 
 
 Very extensive zones around the two poles enjoy at one season, 
 during their summer, continuous day ; at another, durmg their winter, 
 they are plunged in profound darkness. It is only during a short 
 period and near the planet's equinoxes, that these zones see light and 
 darkness succeed in the interval of the same dav.
 
 i Z THE SOLAR SYSTEM. 
 
 The glacial and torrid zones are not distinct on Mercury, and 
 temperate climates do not exist, or rather their zones change their 
 character twice during each revolution. The equatorial regions alone 
 have the advantage of possessing all the year, day and night, light 
 and darkness, and of experiencing heat during the day, cold and calm 
 during the night. It is true, however, that if the Sun towards the 
 equinoxes rises as far as the zenith, it descends nearly to the horizon 
 in the extreme seasons. 
 
 We have said above, that the orbit of Mercury is very elongated, 
 or, in astronomical language, that its excentricity * is considerable. 
 It results that the seasons in Mercury are of very unequal duration, 
 
 
 
 
 Vernal 
 
 
 
 
 
 
 E c^uinoN. 
 
 
 
 
 
 ^'- 
 
 --"^tr--- 
 
 
 
 
 
 
 '"'^^ '" 
 
 ^^^v;^^ ■' 
 
 
 
 f/i, - 
 
 Summer 
 Solstice 
 
 ■ ?t ■ 
 
 i ■ 
 
 1 
 
 ■ V ■ ,', 
 
 ■ ■'^.- 
 
 
 «i 
 
 J 
 
 Winter 
 Solstice 
 
 
 
 %:, 
 
 Autonmal 
 Eq\iinio:!i 
 
 
 1 
 
 Fig. 29— Orbit of Mercury. 
 
 and, seeing that, according as Ave consider the northern or the southera 
 hemisphere, the spring and the summer of the one are the autumn 
 and the winter of the other, a like inequality should exist between 
 the extreme temperatures of the two hemispheres. 
 
 The great proximity of Mercury to the solar rays renders the 
 
 * We have already remarked that it is not in the centre of the oval curve 
 traversed by each planet that the Sun is situated, but in a point by so much 
 more distant from this centre as the curve is more elongated. The name of 
 excentricity is given to the distance between these two points, compared to the 
 half of the greatest diameter of the orbit. Let us add that the place where the 
 Sun is, is called the /'ocws of the curve, and thai the focus is always one of those 
 points in the greater diameter to which we have alluded.
 
 MKKCUKY. 
 
 73 
 
 oLsorviition of tlie planet somewhat difficult ; very little, therefore, 
 i.s known of its surface. One dili,2,-ent observer, Schroter, at the 
 end of the last century, Avas able, how- 
 ever, to observe some dark bands on its 
 disk (fig. 30), which he considered as 
 an equatorial zone ; it was from the di- 
 rection of these bands that he deduced 
 the inclination of the axis of rotation. 
 Besides this, during- the crescent phases 
 many observers, (Schroter, Beer and 
 Miidler) have seen indentations which 
 make the line of separation of light and 
 shade apj)ear jagged ; they also observed 
 that the southern horn of the crescent 
 was truncated (fig. 31). These markings were not always visible, 
 but disappeared, to show themselves anew at intervals, the peri- 
 odicity of which has enabled us to determine the period of rota- 
 
 30. — Equatorial baud of ilorcury. 
 (Schrotor.) 
 
 Fig. 31. — Crescent of Mercury, showing irregularities mi the terminator and the 
 truncation of the Southern Horn. (Schroter.) 
 
 tion of Mercury. They evidently indicate the existence of high 
 mountains, which intercept the light of the Sun, and of valleys 
 plunged in shade, which lie near the parts of the surface of the planet 
 then illuminated. 
 
 Mercurj^ therefore, has mountains. The measurement of the 
 amount of truncation of the crescent, has also shown the height of 
 one of them, and if this measure is not exaggerated it is not less 
 than the o^-^rd part of the diameter of the planet ; that is more than 
 eleven miles! Now the highest mountain known on the Earth, 
 Gaurisankar of the Himalava, is not more than 29,000 feet in vertical
 
 74 THE SOLAR SYSTEM. 
 
 height ; this giant of terrestrial mountains, therefore, does not rise 
 above the sea-level more than the tiVo^^ P^^'*' ^^ ^^^ Earth's 
 diameter. 
 
 Schroter, when examining Mercury during its transit over the 
 Sun on the 7th of May, 1799, saw, or believed that he saw, on the 
 black disk of the planet a luminous point. It has been concluded 
 from this observation, which has not however been confirmed, that 
 there exist on the surface of Mercury active volcanoes. This would 
 be another analogy between the physical constitution of this planet 
 and that of the Earth. 
 
 We have already said, in speaking of the Sun, that astronomical 
 science has succeeded in ascertaining the masses, or the relative weights, 
 of the celestial bodies of the solar system. The mass of Mercury 
 is such that 4,866,000 globes of the same weight as its own would 
 be required to balance the mass of the Sun. As the latter is itself 
 354,936 times as heavy as the mass of the Earth, it follows that the 
 weight of Mercury is the fo'o^^*^' o^' "lore simply fovir-fifths, of the 
 weight of our globe. In comparing these numbers with those which 
 measure the volumes of the two planets, it is found that the matter of 
 which Mercury is formed is much denser than that of which the 
 Earth is composed. Its density is half as much again. It lies be- 
 tween the specific gravity of iron and copper. . . . 
 
 Lastly, there is another physical fact Avhich we must take into 
 account if we would form an idea of the beings which people the 
 planets of the solar system. We refer to the force of gravity on 
 the surface. The influence of this force is all- important ; according 
 as its intensity is greater or less, the muscular movements, for ex- 
 ample, are more or less difficult, requiring an expenditure of force 
 more or less considerable. According to the most recent determi- 
 nations, the force of gravity on Mercury is but three-fifths of what it 
 is on the Earth. To sum up. By the aid of all the astronomical, 
 physical, and meteorological, data which we have reviewed in this 
 study of Mercury, and compared with the corresponding elements of 
 the terrestrial globe, it has been possible for us to point out both the 
 resemblances and difierences of these two worlds, revolving in regions 
 of the heavens, which are, after all, near each other, when we consider 
 the extent of the whole planetary system.
 
 VKNILS. 75 
 
 IL 
 
 VENUS. 
 
 Distance from the Sun — Apparent and real IMovenients ; Form of the Orbit — 
 Distance of Venus from the Earth — Real Dimensions, Form, and Surface- 
 markings ; Rotation — Day and Night on Venus ; Atmosphere, Seasons, and 
 Climates — Physical Constitution. 
 
 The two planets nearest to the Eartli — Mars and Venus, are precisely 
 those which present the most striking- analogies to it ; Mars especially, 
 which we have more particularly studied. This fact is a very natural 
 one, and it will appear to us still more so when we try to form an 
 idea of the origin of the system, according to the views of Laplace. 
 For the present let us study in detail the various phenomena which 
 each planet in turn presents to us. 
 
 Mercury is the first planet which we encountered on leaving the 
 8un. Venus comes after Mercury in the order of distance, whilst of 
 all the principal planets Mercury is that which describes an orbit 
 of the most elongated form, and that by very much. Veims, on 
 the contrary, moves in an orbit the form of which approaches nearest 
 to a perfect circle. There is not between its greatest and least dis- 
 tances from the Sun — between its aphelion and its perihelion, to use 
 the language of astronomers — a difference equal to the j-j^^th part 
 of the maximum distance. 
 
 The mean distance of Venus from the Sun is 68,932,000 miles ; 
 its maximum distance is 69,405,000 miles ; and when nearest to the 
 Sun, it is still removed from him 68,459,000 miles. 
 
 "What is the result of this quasi equality in the movement of 
 Venus? It is, that the quantity of light and heat which it receives 
 from the Sun varies little in the different points of its orbit ; or, what 
 comes to the same thing, in the different sea^ns of its year. Yet,
 
 76 THE SOLAR SYSTEM. 
 
 this quantity is still nearly double in intensity that which our globe 
 receives, — a fact we must take into account when we treat on the 
 physical constitution of the planets. 
 
 Venus, like Mercury, is sometimes an evening, sometimes a 
 morning star. It appears to us to oscillate in the same manner 
 round the Sun. But the amplitude and the duration of its periodical 
 oscillations are much greater. Thus, in that part of its apparent 
 orbit in which it recedes each evening from the setting Sun, it 
 advances at its maximum eastern elongation to a distance of 48°, 
 while that of Mercury is 29". When in the morning before svm- 
 rising, it graduallv leaves that body, its maximum western elongation 
 attains the same value. 
 
 Who does not know the Shejiherd's Star ? Who has not contem- 
 plated its soft and brilliant light, rarely twinkling, and intense enough 
 at times to cast sliadows ? Where a liglit cloud veils that portion of 
 the sky occupied by the planet, a pretty strong glimmer will still 
 indicate its position in the centre of the luminous ring formed by the 
 illuminated molecules of the interposing cloud. The brilliancy of this 
 planet is, indeed, sometimes so intense that in a very clear sky it is 
 visible by day. 
 
 The evening star received from the ancients the name of Vesper, 
 whilst they gave to the morning star the name of Lucifer. The 
 same error whicli led them to double Mercury — if we may be allowed 
 the expression, — made them see in Venus also two distinct bodies. 
 But they at length recognised the identity of the two stars, and Venus 
 eventually replaced Vesper and Lucifer. 
 
 Venus takes 584 days to accomplish an entire oscillation. At 
 the end of this time, it is again found in an identical position with 
 regard to the Sun and the Earth, and recommences this apparent 
 movement round the central body. 
 
 This similarity in the apparent movements of the two planets 
 nearest to the Sun would lead us to infer that their real movements 
 are similar. This is the case. Venus describes round the Sun a 
 curve entirely enclosed by the orbit of the Earth. 
 
 Accordingly, when, instead of observing it with the naked eye we 
 use a telescope of considerable magnifying power, we perceive that it 
 presents phases * like Mercury, and that, like this latter planet, its 
 apparent dimensions vary as in its movement it recedes from or 
 approaches our Earth. We need not here repeat the explanations 
 that we gave in the case of Mercury, inasmuch as they woidd be 
 quite identical foi' Venus. 
 
 * Galileo rccosuibcd tlicui tirst.
 
 VENUS. 
 
 We must not confound the period of a complete oscillation of 
 Venus with the period of its real revolution. As the Earth moves 
 at the same time and in the same direction, the planet requires in 
 reality much more time to return to the same position relatively 
 to the Sun and the Earth than to accouq^lish an entire revolution, or, 
 as it may be expressed, to return to the same part of its orbit. So, 
 while the synodic revolution of Yeniis is accomplislied in about 
 584 days, its sidereal revolution requires only 225 daj's (224*^ 16'' 
 49'" 7^), or less than f ths of one of our years. In this interval, the 
 distance which it travels is upwards of 480 millions of miles, so that 
 its mean velocity is 80,000 miles an hour, or nearly 22 miles a second. 
 We have seen already that Mercury travels at the rate of 28 miles 
 a second; the ^generalization of these facts will show us that the 
 velocity of the planets decreases as we advance from the common 
 centre of their movements. 
 
 Viewed from the mean distance of Venus, the disk of the Sun 
 seems nearly double (in sur- 
 face) its apparent size as seen 
 from the Earth. (See fig. 2, 
 p. 24.) 
 
 A word now on the dif- 
 ferent distances which sepa- 
 rate the two planets when in 
 various positions in their or- 
 bits. 
 
 When Venus is between 
 the Sun and the Earth, it is 
 obviously nearest to us ; and 
 when it is beyond the Sun, 
 it is farthest from us. 
 
 In the first case, to know 
 the distance between the two 
 planets, we must find the 
 difference of their distances 
 from the Sun ; in the second, 
 we must add them togrether. 
 But let us say once for all, 
 
 in respect of Venus as in respect of the other bodies of the sj'stejn, 
 that as the orbits are not circles, but ellipses or ovals, there is for 
 each of the two cases a maximum and a minimimi. We will dwell 
 upon these details when their importance renders it necessarj^. 
 
 The greatest distance of Venus from the Earth varies between 
 
 \?-4 
 
 Fig. 
 
 32. — Superior and inferior conjunction of Venns. 
 Greatest and least distance from tlie Earth.
 
 iO THE SOLAR SYSTE:M. 
 
 166,304,000 and 162,157,000 miles, — its least distance between 
 24,839,000 and 24,293,000 miles. The divergence of these numbers 
 would leave us to believe at first, that the observations of Venus 
 ought to be much more favourable in the case of the short distances. 
 This, however, is not the case. In this part of its orbit, in fact, 
 Venus presents to us more and more of its dark hemisphere, and. 
 
 Fig. 33. — Apparent dimeusious of Venus at its extreme and mean distances rora the Earth. 
 
 besides, its light is extinguished by the brightness of the solar rays. 
 This last circumstance is repeated at the period of its maximum 
 
 Fig. 3t. — Comparative dimensions of Venus and tlie Earth. 
 
 distance, so that it is in the intermediate phases that it is most 
 distinctly \-isible and its light most brilliant. 
 
 From figure 33 we can understand both the change in the ap- 
 parent size, and in the degree of illumination of its disk at its 
 extreme and mean distances from the Earth. The diameter varies 
 nearly in the proportion of the numbers 10, 18, and 65.
 
 VENUS, 
 
 79 
 
 Wlien both the distance of an object and its apparent size are 
 known, nothing is easier, as already remarked, than to determine its 
 real dimensions. It has been calcnlated* that the diameter of the 
 globe of Venus measm-es 8108 miles, which is within the x^goth part 
 of that of the Earth. The dimensions in volume and surface also differ 
 very little from those of the terrestrial spheroid. But up to the 
 present time no perceptible polar compression has been observed. 
 
 This last residt is not at all astonishing, for if such a flattening 
 really existed, if it did not exceed that of the terrestrial j)oles, even 
 the most delicate measures would not be able to detect it. Althouah 
 Venus is one of the nearest planets to the Earth, astronomers have 
 experienced great difficulty in measuring its apparent diameter in a 
 precise manner. This is owing to the astonishing brillianc}^ of the 
 
 Fi? 35. — Effect of irradiation. 
 
 light of Venus, and to the irradiation f which is produced in its 
 image in our instrmnents — a cause of error which it is very difficult 
 to estimate. How can we then be astonished if we are not sure of its 
 diameter within the two^^^ part? 
 
 If the flattening of the globe of the planet is unknown, this is not 
 the case with the period of its rotation, although its determination 
 has also necessitated very delicate observations. 
 
 Venus turns on itself in 23 hours 21 minutes 23 seconds. That is 
 a period less than that of the rotation of the Earth by 35 minutes only. 
 
 * By Sir W. Ilerscliel, Arago, Beer and iNliuUer, &c. 
 
 t The effect of irradiation may be observed in fig. 35. If two circles, one of 
 •which is black, the other white on a black ground, are examined, it will be seen 
 that the last seems perceptibly larger ; and nevertheless their diameters are 
 rigorously the same. This effect is by so much the more perceptible as the 
 light of the object is more intense.
 
 80 
 
 THE SOLAR SYSTEM. 
 
 As in tlie case of Mercuiy, it is by the careful watcliing of the irregu- 
 larities which the illuminated part of the j)lanet presents at the limit 
 of light and shade, and their successive and periodical reappeo ranees, 
 that various astronomers* have been able to measure this period. 
 
 Bianchini, an Italian astronomer of the last century, endeavoured 
 to deduce the rotation of Yenus from observations of the spots which 
 
 Fig. .".I'l. — Irregulavities in tlio tLmiiiiator ol Venus. (Sclirutcr.; 
 Sjiots oil its two liLmisphcrcs. (Bianchini.) 
 
 ho observed on its disk. Although the nvmiber he found is com- 
 pletely at variance with the recognised and adopted period, his 
 observations, nevertheless, have their value, as they give an idea of 
 tlie features which distinguish the surface of the planet.f 
 
 * D. and J. Cassini, Schroter, Vico, Beer and IMadlcr. 
 
 t They were, however, permanent spots, if it be true, as stated by Arago 
 ("Astronomie Populaire," ii. 523), that Bianchini's spots were again seen by 
 Yico, from 1840 to 1842, with all their old forms. We give these spots from 
 Schrtiter's drawings in AphroditographiicJie Fragmente.
 
 VENUS. 81 
 
 Venus, in its various phases, is far from showing- perfectly regular 
 forms. The horns of the crescent, especially the southern one, have 
 nearly always been observed blunted — truncated, so to speak. 
 Schroter also saw a bright point completely separated from the 
 luminous crescent. Fig. 33 shows some of these forms observed by 
 this able astronomer. 
 
 These inequalities, besides serving by their periodical return to 
 enable us to measure the rotation of the planet, are evidences of the 
 irregularities of its surface. Thus the solid ground of Venus is uneven, 
 like that of Mercury and of the Earth ; it is covered with high moun- 
 tains. But is it certain that these asperities attain such a considerable 
 height as is stated ? Do mountains exist on Venus to the vertical 
 elevation of 27 miles ; that is to say, five times higher than the most 
 elevated peak in Thibet, ten times the colossal Mont Blanc ? This is 
 a delicate question which subsequent measurement may perhaps settle. 
 But if the first results were confirmed, we could scarcely help thinking 
 of the strange aspect the mountainous regions of Venus would ofier ; 
 the sublime peaks of our Alpine regions would be but mere 
 mole-hills in comparison. If we refer to the drawings of Schroter 
 (fig. 36), which rejDresent Venus in three of its phases, we shall 
 notice that the luminous part of the disk is far from terminating 
 abruptly along the line of shade. Its light, on the contrary, diminishes 
 gradually ; and this diminution may be entirely explained by the 
 twilight on the planet. The existence of an atmosphere of a con- 
 siderable height has hence been inferred, which, by refracting the rays 
 of the Sun, enables, them to penetrate into regions where that body 
 is already set. Thus the evenings in Venus would be like ours, 
 lighted by twilight, and the mornings by the dawn. 
 
 Venvis, during each of its periodical oscillations, shoidd, one woidd 
 think, when it passes between the Earth and the Sun, be projected on 
 the disk of the latter body. But the occurrence is, on the contrary, 
 rare, because the plane which Venus describes is not coincident with 
 the plane of the Earth's orbit. Sometimes the globe of the planet, 
 always, be it remembered, with its imilluminated half towards us, 
 passes higher than the solar disk, sometimes it passes below. Two 
 transits occur in an interval of eight years, after which they do not 
 again occur until the end of another interval of more than a century. 
 When two transits have taken place both in December, the two fol- 
 lowing invariably occur in June. The last observed were those on the 
 5th of June, 1761, and the 3d of June, 1769. The two next transits will 
 take place on the 8th of December, 1874, and the 6th December, 1882. 
 We shall see further on, that it was by observations of these transits, 
 
 G
 
 82 THE SOLAR SYSTEM. 
 
 made very carefully at different stations on tlie surface of the globe, 
 that the Sun's distance was for the first time calculated. Venus, in 
 traversing the solar disk, appears as a perfectly round black spot. 
 
 Now, in what manner do the days and nights vary, according to 
 the latitudes and the seasons ? This depends both on the way in 
 which Venus in the course of its year presents its polar and equatorial 
 regions to the Sun, and on the relative durations of its two move- 
 ments of rotation aiid revolution. Let us return to some of our 
 former statements. 
 
 Venus turns on itself in 23 hours and 21 minutes and 73 seconds ; 
 this is the duration of its sidereal day.* Its year contains 225 terres- 
 trial days (224"7). It comprises, therefore, 231 entire rotations, or si- 
 dereal days of Venus, which are equivalent to 230 solar days of Venus. 
 Each ordinary day then on Venus consists of 23 hours 26 minutes. 
 
 Fig. 87 — Veiuis at one of its solstices. Inclination of the axis of rotation. 
 
 On the other hand, the nearly circular form of its orbit gives a 
 nearly equal length to the four seasons, and the light and the heat of 
 the Sun are distributed with a like constancy. But that which 
 establishes a marked difference between the terrestrial seasons and 
 climates and those of the planet which we are exploring, is the great 
 inclination of its axis of rotation to the plane of its orbit. In this 
 respect Venus resembles Mercury. Fig. 37 shows the position of the 
 planet at one of its solstices, at the commencement of the summer of 
 the hemisphere which presents its pole to the Sun. At the winter 
 solstice, Venus occupies a diametrically opposed position. It follows 
 
 * On each planet, as on the Earth, we can distiaguitsh the sidereal day, the 
 length of which is identical with tliat of a rotation, and the solar day, a little 
 longer than the sidereal day. We shall explain, in the chapter relating to the 
 Rotation of the Earth, the reason for this essential distinction.
 
 VENUS. 83 
 
 that the polar regions undergo alternately the torrid temperature 
 of summer and the prolonged cold of winter. At the equator, the 
 Sun then hardly rises above the horizon. 
 
 Towards the equinoxes, on the contrary, the regions nearest the 
 equator are exposed to the heat of the Sun, the intensity of which 
 is nearly double the intensity of the solar heat on our globe. Per- 
 haj)s a very dense, cloudy atmosphere, constantly charged with 
 vapours arising from the heat, envelopes the globe of Venus, and 
 thus moderates the rigour of its opposite seasons. A fact which gives 
 to this hypothesis a certain degree of truth is the observation of 
 the transit of Venus over the Sun in 1761. A nebulous ring seemed 
 to surroimd the black disk of the body ; and, moreover, at the 
 moment when it was but partly projected on the Sun, the contour of 
 the exterior limb of the planet was seen edged with a luminous ring. 
 
 These two phenomena are easily explained if we suppose the 
 globe of Venus to be enveloped with a very dense atmosphere. 
 
 There still remain some other interesting physical data bearing 
 on the constitution of Venus. Thus, for instance, calculation has 
 shoAvn that its mass is such, that more than 400,000 globes of the 
 same weight would be required to balance the mass of the Sun ; it is 
 nearly ^Yo^^s of the mass of the Earth. Taking into consideration 
 the difference of the volumes of the two planets, we find that the 
 density of the matter of which Venus is composed, is more than 
 nine- tenths (0*987) of the density of our globe. 
 
 Finally, the force of gravity on its surface is also a little more 
 than nine-tenths of the mean intensity of this force on the surface 
 of the Earth. 
 
 To sum up. The world we have explored resembles in many points, 
 in its dimensions and astronomical and physical constitution, that 
 which we inhabit. And, if we were to accej)t the observations of 
 several astronomers of the 17th and 18th centuries,* it would present 
 an additional resemblance ; as the Moon accompanies the Earth, 
 so also would Venus, according to them, be provided with a satel- 
 lite. But this singular body has not been since seen, and high 
 scientific f authorities are now convinced that the observers in ques- 
 tion were the victims of an optical illusion. It must be confessed 
 that the doubt which still exists on this point is, at least, very 
 curious, and testifies the progress which still remains to be accom- 
 plished in the domain of planetary astronomy. 
 
 [Before we quit Mercury and Venus, we must fiiirly state that 
 
 * D. Cassini, Short, Montaigne, Roedkier, Horrebon, Montbarou, Lambert, 
 t De Lalaude (^Encyclopedie Methodique)
 
 84 THE SOLAR SYSTEM. 
 
 the decision and positiveness with which the physical data are given 
 by the old astronomers, are by no means borne out by modern ob- 
 servation, although we might imagine, to say the least, that if the 
 observations of Schroter and others, faithfully recorded by M. 
 Guillemin, were correct, the vastly superior telescopes of the present 
 day would have verified them. This, however, they have failed to do. 
 The different features, although stated to have been seen by De Vico 
 during the present century, have not once been observed either by 
 Admiral Smyth or the Rev. W. R. Dawes ; indeed, the only physical 
 fact which modern observation has placed before us, and this we owe 
 to Professor Phillips and the Rev. T. W. "Webb, is the possible 
 existence of snow-zones on Venus as on Mars. This, however, is 
 not certain. We must, therefore, caution our readers against re- 
 ceiving absolutely the inferences dra^vn from the old observations. If 
 we cannot see the features on which they were based, we cannot of 
 course verify them.]
 
 PL , II, 
 
 THE ZODIACAL LIGHT AT JAPAN 
 
 Trom ilie ol) serrations of M^ Jones . 
 
 -EGuillemin et K ClerM del. 
 
 Imp .Becq^aet a. Taris
 
 THE ZODIACAL LIGHT. 
 
 III. 
 
 THE ZODIACAL LIGHT. 
 
 Aspect of the Zodiacal Light in the various regions of the Earth — Probable 
 existence of a large Luminous Ring situated between the Earth and the 
 Sun. 
 
 In the evenings, about the time of the vernal equinox — in March 
 and April, when in our climate the twilight is of short duration, if 
 we examine the horizon towards the west, a little after sunset, we 
 may perceive a faint light that rises in the form of a cone among the 
 starry constellations. 
 
 This is what astronomers call the Zodiacal Light. Those un- 
 familiar with it, or little accustomed to the ordinary aspect of the 
 sky, might confuse this glimmering either with the Milky Way, or 
 with the ordinary twilight, or even with an aurora. But, with a 
 little attention, it is impossible to mistake it. 
 
 The triangvilar form of this luminous cone, its elevation, and its 
 inclined position to the horizon, make it a thing apart and one emi- 
 nently deserving particular mention. 
 
 As the days lengthen, and with them the duration of twilight, the 
 zodiacal light disappears, it becomes invisible, at least in our climate. 
 But it may again be seen in the morning, in the east, about the time 
 of the autumnal equinox, in September and October, when the dawn 
 has an equally short duration — again, however, to disappear during 
 the period of long nights and long twilights.* 
 
 * In large towns, the thousands of gas-lamps or other lights render the 
 observation of the Zodiacal Light very difficult, not to say impossible, at all 
 times. On the other hand, in stations conveniently situated, it can be seen at the 
 vai'ious epochs of the year, even in the temperate zones. 
 
 Mr. Heis (of Munstcr) cites some observations made by him in the month of
 
 86 THE SOLAR SYSTEM. 
 
 It is needless to add that the sky must be clear, and the night 
 moonless, for observations of the Zodiacal Light to be possible. 
 
 The brightness of this light is comparable with that of the Milky 
 Way, or with the tails of comets, its transparence is such, that all but 
 the smallest stars are visible through it. Nevertheless, according to 
 Mairan, who occupied himself with this phenomenon in the days 
 most favourable for observation, its light is more intense than that of 
 the Milky Way, and more uniform, generally less white, and in- 
 clining somewhat towards yellow or red in the parts nearest the 
 horizon. It was only towards the apex that he could discern the 
 small stars in the region on which the light was projected. 
 
 This yellowish-red colour was observed also, in 1843, by Arago 
 and other astronomers of the Observatory of Paris, who compared it 
 to the tail of the comet of that same year. Moreover, the same red 
 tint was, in 1707, noticed by Dcrham. 
 
 Now, if from the temperate regions of the two hemispheres we 
 travel towards the tropical zones, the Zodiacal Light increases in 
 intensity and height, and it can be observed throughout the year. 
 The illustrious Humboldt thus relates in his "Cosmos," the impressions 
 made on him in his travels by the sight of this curious phenomenon : 
 " The much greater luminous intensity which the Zodiacal Light 
 presented in Spain, on the coast of Valentia and in the plains oi' 
 New Castile, luid already determined me, before I quitted Europe, to 
 observe it assiduously. The brightness of this light — I should say of 
 this ilhimination — still increased in a surprising manner, as I gra- 
 dually api)roached the equator on the American continent, or on the 
 South Seas. Through the dry and transparent atmosphere of Cumana, 
 on the grassy plains or Llanos of Caracas, on the table-lands of Quito, 
 and on the Mexican lakes, particularly at a height of eight or twelve 
 thousand feet, where I could stay a longer time, I saw the Zodiacal 
 Light sometimes surpass in brilliancy the most striking parts of the 
 Milky Way, comprised between the prow of the Ship and Sagittarius, 
 or, to cite the regions of the sky visible in our hemisphere, between 
 the Eao-le and the Swan."* 
 
 December, in Germany, and Mr. Jones had observed it at the same time in 
 Japan. 
 
 M. Chacornac observed the Zodiacal Light in January and February in Paris 
 and in December in Lyons, in 18G4. A fact little known, established by him, 
 IS, that the light is intense enough to efface stars of the twelfth and thirteenth 
 magnitude. " It is beyond doubt," he writes, " that this phenomenon covers witli 
 ii yellowish red veil the region of the sky on which it is projected." 
 
 * " Co.smos," vol. ii. p. 694.
 
 
 
 < Y 
 
 < i 
 
 Q o 
 
 O S 
 
 N a: 
 
 U o 
 
 I- t
 
 THE ZODIACAL LIGHT. 
 
 89 
 
 Let us see now if it is possible to account for the nature of the 
 Zodiacal Light, which evidently is not a purely meteorological pheno- 
 menon, since its participation in the diurnal movement, its visibility 
 in regions of the Earth very distant one from the othei', nnd, lastly, 
 its nearly invariable inclination along the ecliptic, indicate sufficiently 
 that the cause which produces such appearances lies outside our 
 atmosphere, in the celestial sjDaces. 
 
 Among the explanations that have been given, the most probable 
 one is that which likens the Zodiacal Light to a flattened nebulous ring- 
 surrounding the Sun at 
 some distance. It is to be 
 remarked, that the direc- 
 tion of the axis of the cone, 
 or of the pyramid, pro- 
 longed beloAV the horizon, 
 always passes through the 
 Sun." (Fig. 38.) 
 
 It was believed at first 
 that this direction pre- 
 cisely coincided with the 
 solar equator ; but it 
 seems more certain that 
 it coincides with the plane 
 of the Earth's orbit, or 
 the ecliptic* 
 
 The length of the 
 longer axis of the ring is 
 variable, or, as it may be 
 expressed, the distjance 
 from the summit of the 
 cone to the middle of its 
 base, — to the horizon, — is more or - less considerable, according 
 to the time of observation. Very simple geometric considerations point 
 to the conclusion that the luminous ring sometimes extends as far as 
 the orbit of the Earth, and even surpasses it, sometimes it is enclosed 
 within this same orbit. This may be explained in two ways : either 
 by admitting that the form of the ring is elliptical or oval, or, if it be 
 circular, that the Sun does not lie exactly in the centre. 
 
 Fig. 38.— Uirectiun of tho axis of tho Zodiacal Light. 
 
 * The recent observations of Mr. Heis, at Munster, and of Mr. Jones at 
 Japan, made simultaneously, show, however, the axis of the luminous cone as 
 forming an angle with the latter plane.
 
 90 THE SOLAR SYSTEM. 
 
 Now, what is the nature of this luminous mass ? Must it be con- 
 sidered as a zone of vapours thrown off by the Sun when in the 
 process of consolidation, when our central star passed from a nebulous 
 state to that of a condensed fluid sphere ? This was the opinion of 
 Laplace. 
 
 Another hypothesis, also connected with the first, is, that the 
 Zodiacal Light is fonned of myriads of solid particles, analogous to 
 the aerolites, possessing a general movement, but travelling separately 
 around the focus of our solar world. The light of the ring w^ould 
 be thus produced by the accumidation of this multitude of brilliant 
 points, reflecting towards us the light borrowed by each of them 
 from the Sun. 
 
 This explanation accounts for the variation of the intensity of 
 the Zodiacal Light at different epochs ; it would suffice to admit that 
 the condensation of the particles or the density of the ring is not the 
 same throughout its extent, and that its movement of circidation 
 round the Sun presents successively different parts to the Earth. In 
 this case, it becomes a question whether this lenticular ring of matter 
 is distinct from the zone of aerolites, of which we shall soon speak, and 
 the existence of which seems at length established. 
 
 Lastly, some astronomers regard the Zodiacal Light as a vaporous 
 ring which belongs to the Earth, surrounding it at some distance. 
 I)ut this is an opinion which appears somewhat wild (it can be upset 
 by the most simple geometric consideration), and is utterly at variance 
 with observation. 
 
 We omit to mention various other theories now completclj'^ 
 abandoned. But it must be owned, in concluding what we have to 
 say on this interesting phenomenon, that while the observations re- 
 main so vague and so few in number, we are not yet permitted to 
 pronounce, in a definite way, on its nature. 
 
 Cassini and Mairan have observed in the luminous cone moment- 
 ary sparklings, which they explain by the rapid movements of its 
 particles, alternatively presenting faces of unequal size ; nearly in 
 the same way as one sees the grains of dust sparkling in tlic 
 rays of the Sun when they penetrate into the interior of a dark 
 room. 
 
 This is an explanation which must be presented with the more 
 
 * Whatever may be the true nature of the Zodiacal Light, observation jjrovcs 
 that the substance of which it is composed hes in a region which sometimes 
 extends beyond the Earth's orbit, sometimes Hes within it. Qui' readers will 
 therefore, understand why the description of it is fount! in tins part of the Solar 
 System.
 
 THE ZODIACAL LIGHT. ' 91 
 
 reserve, as the observations of Mairan and Cassini have not been, as 
 far as we know, coiifirined. 
 
 The intermittent brightness described by Humboldt — the sudden 
 undulations which he observed to traverse the luminous pyramid, also 
 await explanation. Arago did not think this fact coidd be explained 
 merely by variations in the strata of our atmosphere.
 
 92 
 
 THE SOLAR SYSTEM. 
 
 V. 
 
 THE EARTH. 
 
 The Earth suspended in Space — Proof of its Spheroidal Form — Its Dimensions, 
 Mass, and Mean Density — Atmospheric Kefraction ; its eflfects on the ap- 
 pearance of the Disks of the Sun and Moon. 
 
 The Earth considered as u celestial body — as a planet — will now be 
 the object of our study. It is the globe we meet with next in our 
 journey from the focus of the solar system. 
 
 The Ear til does not voyage alone, as do Yenus and Mercury ; 
 but, drawing the Moon after it, in its annual course, it is continually 
 escorted by this faithful satellite. It is the first planet that rejoices 
 in such a privilege. 
 
 If the Earth be a body travelling through space, as do the mul- 
 titudes of those which people the heavens, it may be asked, under 
 what aspect it appears to the nearest celestial bodies. This will 
 evidently depend on the distance of the observer. 
 
 The form of the Earth is that of a nearly spherical globe, of 
 which one-half receives the light of the Sun, whilst the other half 
 is plunged in gloom ; to a spectator who moves from it gradually, 
 it would appear under the form of a disk, gradually becoming more 
 and more diminutive, but more and more luminous at the same time ; 
 presenting phases like Mercury and Yenus, according to the relative 
 position of the Earth with regard to the spectator and the Sun. 
 
 At the distance of the Moon, the Earth wiU be seen under 
 the form of a luminous disk, sprinkled with spots, the bright ones 
 marking the continents and islands, and the snow and ice of the 
 poles ; the darker ones indicating the place of the seas ; but besides 
 these pemianent spots, variable and movable ones woxdd be dis- 
 tinguished, produced by the cloudy strata of the atmosphere.
 
 THE EARTH. 
 
 93 
 
 Its apparent diameter would be nearly fonr times that of the 
 Moon, so that, seen at the full, the Earth would shine like thirteen 
 united full moons. At about four times the distance of our satellite, 
 the terrestrial globe would still seem as large as the latter. But as 
 the spectator moved away by degrees, the diameter of the disk would 
 diminish, and would end by becoming insensible. The Earth would 
 then shine in the heavens like a star. 
 
 Fig. 39. — The Earth suspended in Space. 
 
 These statements of Science regarding the form of our globe, 
 and its real dimensions, — statements now familiar to everyone, — are 
 not based on simple analogies. Exact facts, which it is easy to verify, 
 place the rotundity of the Earth beyond doubt, and trigonometrical 
 surveys of extreme precision have determined its true dimensions. 
 Let lis dwell an instant on these different points.
 
 94 
 
 THE SOLAR SYSTEM. 
 
 It Is well known that the horizon of a plain presents the form 
 of a circle, surrounding the observer. If the latter moves, the circle 
 moves also, but its form remains the same, and is modified only 
 when moimtains, or other obstacles of some elevation, limit the 
 view. Out at sea, the ciiTular form of the horizon is still more 
 decided, and changes only near the coasts, the outline of which 
 breaks the regularity. 
 
 Here, then, we obtain a first notion of the rotundity of the Earth, 
 since a sphere is the only body which is presented always to us under 
 the form of a circle, from whatever exterior point of view it is ex- 
 amined. Moreover, it cannot be said that the horizon is formed by 
 the limit of distinct vision, and that it is this which causes the appear- 
 ance of a circular boundary, because the horizon is enlarged when * 
 we mount vertically above the surface of the plain. 
 
 Fig. 40.— Curvature of the Continents. Horizons of the same place at different altitudes. 
 
 The preceding drawing, in which a mountain is figured in the 
 middle of a plain, whose uniform curvature is that of a sphere, will 
 prove our assertion. From the foot of the mountain the spectator 
 will have but a very limited horizon. Let him ascend half-way, his 
 visual radius extends, is inclined below the first horizon, and reveals 
 a more extended circular area. At the summit of the mountain, 
 the horizon still increases, and if the atmosphere be pure, the spec- 
 tator will see numerous objects appear, where from the lower stations 
 the sky alone was visible. 
 
 This extension of the horizon would be inexplicable if the Earth 
 had the form of an extended plane. 
 
 The curvature of the surface of the sea manifests itself in a still
 
 THE EARTH. 
 
 95 
 
 more striking manner. ^Suppose yourself on the coast, at the summit 
 of a high tower, hill, or a steep, rocky shore ; a vessel appears on the 
 horizon, you see only the tops of the masts, the highest sails ; the 
 lower sails and the hull are invisible. As the vessel approaches, its 
 
 Fig. 41. — Curvature of the surface of the sea. 
 
 lower part comes into view above the horizon, and soon it appears 
 entire (fig. 41). 
 
 This fact, of the successive appearances on the surface of the sea, 
 of the different parts of an object, beginning by the highest parts of 
 
 Fig. 42.— Curvature of the Sea. 
 
 it, is manifested in the same manner to the sailors who from the 
 ship observe the land. The explanation is rendered clear in the 
 second sketch, where the course of a vessel, seen in profile, is figured 
 on the convex surface of the sea. 
 
 As the curvature of the ocean is the same in every direction,
 
 96 THE SOLAK SYSTEM. 
 
 it folloAvs that tlie Earth has really the form of a sphere, or at least 
 diifers from it but little. 
 
 We may also mention two proofs of another kind, which, like the 
 preceding ones, are more interesting as facts than as elements of 
 conviction. Who could doubt at the present time of the rotund- 
 ity of the Earth, and of its suspension in space, after so many 
 voyages of circumnavigation, after the daily testimony of the move- 
 ment of the stars, setting on one side of the horizon, to reappear 
 after twenty-four hours on the opposite side ? These are the 
 proofs. 
 
 One of the stars of the northern heavens, — the Pole-star — 
 we shall sj^eak of it again subsequently — remains nearly immovable, 
 and at the same height in the heavens above the horizon of any 
 given place. Now, when we move towards the south, this star by 
 degrees approaches the horizon ; whilst, on the other hand, it rises 
 if we advance to the north. 
 
 This is a fact which can be explained very naturally by the con- 
 vexity of the surface of the Earth, for if this change of height were 
 held to be the result of a real approach of the traveller to, and removal 
 from, the observed star, the known distance of the stars from the 
 Earth shows that the displacement of the observer is, so to speak, 
 indefinitely small, compared to the distance of the star, and cannot 
 in any way account for its apparent movement. Besides, if instead 
 of walking from north to south, the observer travels from east to 
 west, the Pole-star will always appear at the same point of the 
 heavens as referred to the movable horizon, and at the same height 
 above this horizon. But, in this case, it Avill be the hour of the rising 
 and setting of the stars that will vary ; as should happen if the curva- 
 ture of the terrestrial surface is in every direction ; and if, as indeed 
 is known, our globe every day performs an entire rotation round 
 one of its diameters. 
 
 We may announce, then, as a fact, demonstrated by experience and 
 observation, that the Earth, in spite of the irregularities of its sur- 
 face, which seem to us so considerable, is a spheroid, which, seen 
 in space, appears as well defined, regular and smooth, as the disks 
 of the other planets. 
 
 Some numbers relative to the real dimensions of the Earth will 
 support these resvdts, so astonishing to those, who, learning them for 
 the first time, seek to figure them as so many real facts. But before 
 we give them, we will state a little more exactly the form of the 
 Earth, as determined by the most exact measures. Its form is not 
 rigorously spherical ; the diameter or axis, round which the movement
 
 THE EARTH. 97 
 
 of diurnal rotation is performed, is the smaller diameter. Our globe, 
 then, is flattened at the poles, that is to say, at the extremities of the 
 axis. The existence of this flattening has been determined in the 
 following manner. 
 
 Let us consider a meridian, one of the ideal curved lines, indefi- 
 nite in nmnber, which encircle the Earth, each one passing through 
 the two poles. If the Earth were exactly spherical, each meridian 
 would be a circle, that is, if we leave out of consideration the irregu- 
 larities of the Earth's surface. On this hypothesis, the successive 
 verticals which, from the Equator to the Pole, would form between 
 themselves equal angles — an angle of one degree, for instance, — 
 would be equally distant the one from the other. The distances 
 
 Fig. 43. — Elliptical lorni uf the terrestrial niuridiaiis. Diiniiiutiou iii the length of a dugrce 
 from the Pole to the Equator. 
 
 between the feet of the verticals on the surface of the Earth, would 
 be expressed by equal numbers. 
 
 Observation contradicts this supposition, and it has been found 
 that the length of the successive degrees of the meridian increases, 
 in a continuous manner, from the Equator to the Pole.* 
 
 * It is easy to see from fig. 43, that the nieridiau ought, in fact, to jiresent 
 really the form of an ellipse or oval, the greater diameter of which terminates 
 in the Equator, and the smaller in the two poles. In such a curve, the cur- 
 vature, is the more strongly marked, as we consider the arcs nearest the major 
 axis. We shall then recjuire to traverse a shorter distance nearer the Equator to 
 find the same angle between successive verticals than we shall near the Y>o\es. 
 But it is well to rennrk that the fattening is much exaggerated in the drawing. 
 
 H
 
 98 
 
 THE SOI-AR SYSTEM. 
 
 The following table sliows the differences of length of the arcs 
 of a degree, measured in the northern hemisphere of the Earth, 
 at increasing latitudes, that is to say, at gradually increasing distances 
 from the Equator : 
 
 
 Mean Latitude 
 of Arc. 
 
 Length of one 
 
 Degree in English 
 
 Feet. 
 
 India . 
 
 12° 32' 20" 
 
 362,956 
 
 jj 
 
 16 8 21 . . 
 
 303,044 
 
 America 
 
 39 12 . . 
 
 363,786 
 
 Italy . 
 
 42 59 . . 
 
 364,262 
 
 France 
 
 44 51 2 . . 
 
 364,572 
 
 England 
 
 52 2 19 . . 
 
 364,951 
 
 Denmark 
 
 54 8 14 . . 
 
 365,087 
 
 Russia 
 
 5G 3 55 
 
 365,291 
 
 Sweden 
 
 66 20 10 . . 
 
 365,744 
 
 The differences are unmistakable, and their constancy puts the 
 fact of the polar flattening quite beyond doubt. But their relative 
 smallness, — there is onlj^ a difference of 2788 feet between the ex- 
 treme latitudes — proves that the compression is in truth very small, 
 as may be proved by comparing the length of the equatorial diameter 
 and the polar one, deduced from the preceding data : 
 
 Feet. 
 Equatorial diameter = 41,848,380 
 Polar „ = 41,708,710 
 
 Thus, if we represent the Earth by a sphere, a yard in diameter, 
 the polar diameter will be about the -yV^^ P^^'^ of an inch too long. 
 
 [But this is not all. The most recent results arrived at by 
 geodesists have taught us that the Earth is not quite truly repre- 
 sented by an orange, at all events, imless the orange be slightly 
 squeezed, for the equatorial eireunifcrence is not a perfect circle, hut an 
 ellipse, the larger and shorter equatorial diameters being respectively 
 41,852,864 and 41,843,896 feet. That is to say, the equatorial 
 diameter which pierces the Earth from longitude 14° 23' east, to 
 194° 23 east of Greenwich, is two miles longer than that at right 
 angles to it.*] 
 
 What then, on this scale, are the irregularities produced by the 
 mountains and valleys ? what the depths of the seas ? The calcula- 
 tion is easy : Kunchinjinga and Gaurisankar, the giants of the 
 Himalayan range, the highest known mountains of our globe, would 
 
 * Mem. Roy. Ast. Soc, vol. xxix. 1860.
 
 THE EARTH. 
 
 99 
 
 only be elevated above the surface of a sphere of this size the 
 I'oVs^t P^i't of its diameter ; Mont Blanc, about half as much. 
 In chains of mountains of ordinary elevations, the hills and valleys 
 would be unnoticeable. The greatest depth of the ocean would but 
 indent the siu'face about ^-Jy'^.^^th of an inch. Fig. 44 shows on a 
 larger scale the relative dimensions of the height of the mountains, 
 of the depth of the ocean, and the presumed thickness of the terres- 
 trial crust. To obtain these dimensions, we have given the terrestrial 
 globe a diameter of fourteen yards. 
 
 The inequalities of the surface of the Earth have often been com- 
 pared to the roughness of the skin of an orange. It will be seen 
 by the preceding statements how exaggerated this comparison is. 
 
 Fijj. 44. — Comparative heights of Mountains ; depth of the Seas, and thickness of the solid 
 crust of the Earth. 
 
 Our globe, reduced to the dimensions of an orange, would not pre- 
 sent to the naked eye any trace of projection or of depression, nor 
 would it offer any perceptible indication of flattening. 
 
 The study of the structure of the Earth — the configurations of its 
 surface, its watercourses and seas, the geological constitution of its 
 crust, and of the interior nucleus which composes it — presents the 
 highest possible degree of interest, but its consideration would be out 
 of place in the present volume. 
 
 We will only refer to the opinion, now generally entertained of its 
 primitive fluiditj^ ; because this hypothesis has its astronomical contir- 
 mation in the ellipticity which has been detected. It can be demon- 
 strated by the laws of mechanics, that a fluid mass, animated by a
 
 100 THE SOLAR SYSTEM. 
 
 movement of rotation, tends to take the form of a spheroid, flattened 
 at the extremities of the axis around which the movement is 
 effected. 
 
 Among- the planets which we have still to explore, we shall find 
 many which assume, like the Earth, a spheroidal form, but with a 
 much more considerable flattening at their poles. Now, their move- 
 ment of rotation is much more rapid. 
 
 One word more on the form and dimensions of the Earth. 
 
 We shall be able to form an idea of the curvature of the surface 
 of the globe over a limited extent of country, from the following facts : 
 A traveller who starts on a journej^ from a given place, descends 
 more and more below the horizon of that place. When he shall have 
 traversed a degree (69 1 miles), he will actually be more than a 
 thousand yards below the point of departure, disregarding differences 
 of level proceeding from irregularities of surface. The horizon of 
 Paris, prolonged as far as Marseilles, would pass above that town at 
 a height of over 18 miles. 
 
 By reason of the flattening of the poles, the circumference of a 
 meridian is shorter than that of the Equator by nearly 42 miles. 
 
 It follows from the preceding numbers, that the surface of the 
 Earth contains about 196,626,000 square miles. Of this immense 
 surface the oceans embrace more than three-quarters ; the remain- 
 der comprises the continents and islands. It is not a little curious 
 that one hemisphere of the terrestrial globe comprises the land, 
 whilst the other hemisphere is nearly entirely occupied by water. 
 If we adjust a globe in such a way that London would occupy the 
 centre of the visible hemisphere, and place it at some distance from 
 the eye, we shall see on the hemisphere turned towards us the whole 
 of Europe, Asia, Africa, North America, and part of South America. 
 If we place ourselves on the opposite side, with our antipodes in the 
 centre, we shall see a hemisphere, with the exception of New Holland 
 and the lower point of Southern America, nearly entirely covered by 
 the ocean, only here and there scattered with islands. 
 
 The figures in Plate Y give an idea of the distribution of the 
 solid and liquid portions of the terrestrial surface. 
 
 If, from the measures of the surface of the globe, we pass to those 
 of its volmne and w^eight, we arrive at numbers of which it is ex- 
 tremely diflicult to form a definite idea, so much are they beyond our 
 ordinary notions. 
 
 . :;.;But we can conceive a cubic volume of which the length, breadth, 
 and height, are each one mile — this we call a cubic mile, — well, 
 our globe contains 259,800,000,000 of such cubes ! Experiments
 
 PLATE V. 
 
 THE EARTH VIEWED FROM SPACE.
 
 THE EARTH. 103 
 
 and calculations, into the details of wliich we cannot enter in this 
 place, have taught us the mean density of the materials of which our 
 Earth is composed ; we say mean density, because the density varies 
 in different strata. [And, moreover, the solid crust may not extend 
 to the centre — the Earth and other planets may be gigantic bubbles 
 with solid shells.] This mean density then is such that the Earth 
 weighs 5^ times more than an equal volume of water would do. 
 
 Hence we have been able, as it were, to weigh the Earth, and we 
 have found that it weighs 6,069,000,000,000,000,000,000 tons ! This 
 is exclusive of the weight of the air. Does then the air weigh 
 anything ? Yes ; if we suppose it to extend only 37 miles [and 
 we have proof positive that it extends much higher], it will weigh 
 5,178,000,000,000,000 tons; this, however, is, after all, less than 
 the millionth part of the weight of the Earth. 
 
 Such are the dimensions, such is the mass of the planet which 
 serves us as a dwelling-place. What in comparison, and considered 
 merely from a material point of view, are the works of man both 
 individually and collectivelj" ? 
 
 Nevertheless, this sphere which appears to us so colossal, must, 
 after all, be classed only among the smaller planets of our sys- 
 tem, and is but a grain of sand compared with our central Sun, 
 and a mere point lost in the immensity of the sj)ace comprised within 
 the limits of our system. What idea then shall we have of the 
 infinity of space, when, leaving our OAvn system behind, as we 
 shall shortly do, we shall see that even that entire system is but 
 an atom of the Yisible Universe ? 
 
 We have just spoken of the total weight of the atmosphere : this 
 is a point of mere curiosity. But the pressure which this fluid mass 
 exercises on every inch of surface, on organised beings which are 
 enveloped and move in it, on liquids and vapours, is of extreme im- 
 portance to organised life and to the physical conditions under which 
 life is possible. The density of the atmosphere, the law of decrease of 
 this density, both in the lower and upper strata, are so many facts 
 which have an intimate relationship with the temperature of the sur- 
 face at different altitudes, with climate, and consequently with the 
 distribution of animal and vegetable life on the surface of our 
 globe. 
 
 Moreover, there is a relationship no less decided between the 
 constitution of the gaseous envelope in which we live, and the way m 
 which the solar rays traverse it. 
 
 Every one knows that a limiinous ray proceeds in a straight line 
 when it traverses a homogeneous mediimi, that is to say, one of
 
 104 
 
 THE SOLAR SYSTEM. 
 
 unvarying density. The object wliicli this himinous ray brings to our 
 view under these circumstances is exactly where it appears to be. 
 
 If, on the other hand, before it reaches us, the himinous ray has 
 to traverse media of different densities, obliquely, each change of 
 density makes it deviate from its direct route; and when it reaches 
 the eye, the total deviation causes the object not to appear in the true 
 direction which it occupies. Its apparent position does not then 
 indicate its real position. This phenomenon of deviation takes 
 place in the atmosphere, and is called atmoapJieric refraction. The 
 importance of a proper knowledge of refraction in astronomical ob- 
 servations will be understood when we state that all celestial bodies 
 are thus more or less displaced ; the error resulting from this dis- 
 placement is not the same in every part of the celestial vault : it is 
 
 Fig. 40. — AtiHosplicric refraction, showiiif^ its effect on the .ipjwrent places of bodies in 
 the celestial vault 
 
 much more considerable when the strata traversed are thicker, or are 
 presented obliquely to the Imninous rays ; in other words, when the 
 object observed is near the horizon. 
 
 The light proceeding from all celestial bodies in all their positions, 
 therefore, is refracted unequally, and the effect of refraction is to 
 make them appear higher than they really are ; to place them nearer 
 the zenith. 
 
 Hence follows a rather curious fact, namely, that the entire disks of 
 the Sun or Moon remain still visible when they are mathematically 
 set, that is to say, when they are really below the plane of the horizon. 
 The duration of the day is then increased by refraction. The same 
 phenomenon, of course, occurs both morning and evening. 
 
 Refraction still prolongs the day, even when the Sun has actually
 
 THK KAKTII. 
 
 105 
 
 clisappcarod ; the upper strata of the afiuosphere ai'e still ilhiiuiiiatcd, 
 when the surface of the Earth is already in shade. They reflect earth- 
 wards a portion of this liglit, and by this means it is that the night 
 passes into da}^ and day into night, by imperceptible gradations. 
 Such is the cause of the morning- and evening twilight. In tiii(\ the 
 duration of dawn and twiliglit varies according to the soasous and 
 latitudes. 
 
 Not only is the apparent position of bodies altered by the re- 
 fraction of the atmosphere,* but, for the same reason, the form of 
 
 Fi>; 40. — Dclormatiou of the Sun's limb at Sunset. 
 
 those with large disks, as the Sun and Moon, is modified. Hefractiou, 
 the intensity of which, as we have seen, increases as it approaclies 
 nearer the horizon, aflects the lower limbs of these luminaries more 
 strongly than it does the upper ones, so that the body, already flattened 
 in its upper half, is still more so in the loAver one. The sea-view repre- 
 sented in fig. 46 represents this curious phenomenon, which can be 
 
 * Tables of corrections have been calculated for different heights. These 
 tables enable us to find the true position of a celestial body, when the apparent 
 position is known by observation. Nevertheless astronomers avoid observing too 
 near the horizon, and wait until the body, by virtue of its diurnal motion, has 
 attained its maximum height, at the moment of culmination, or of the upper 
 transit of the meridian as it is called.
 
 106 THE SOLAR SYSTEM. 
 
 seen inland as well as at sea, at both the rising and setting of the 
 Sun and Moon. Sometimes this deformation of the solar and lunar 
 disks is far from presenting the regularity and symmetry which 
 are seen in our drawing. The irregularities in the density of the 
 lower strata of the air make the body appear under the most curious 
 aspects. 
 
 [Recent researches into the action of the light-rays of the Sun- 
 beam have made us acquainted with another class of facts of the 
 utmost value to physical astronomers, as bearing upon the atmo- 
 spheres of our sister Planets. Indeed, the more we study the 
 marvellous mechanism of our owai atmosphere, its manipulation, so 
 to speak, of sunshine, the reinforcing, tempering and economizing 
 power it possesses by reason of the aqueous vapours which it con- 
 tains, the more we see that, in spite of many ideas which it will 
 be our duty to lay before our readers, it is not impossible that the 
 actual heat experienced on the surfaces of all our Planets may be 
 vastly different from that to which they would be apparently entitled 
 taking only their distance from the Sun into accomit. It has been 
 stated, that the heat-rays from the Sun pass through space without loss, 
 and become effective in proportion to the density of the atmospheres 
 through which they pass, or the amount of water present in them ; and 
 if so, the projjortion of heat received at ISIercur}^, Venus, Jupiter, and 
 Saturn, may be the same as that received at the Earth, if the con- 
 stituents of their atmospheres be the same as that of the Earth, and 
 greater if the density be greater. So that the effective solar heat at 
 Jupiter and Saturn, notwithstanding their far greater distance from 
 the Sun, may be greater than at either the inferior planets Mercury 
 or A'^enus.] 
 
 Let us add, in concluding our notice of the atmosphere with which 
 our planet is surrounded, that by diffusing on all sides the light of 
 the Sun, it interposes a bright curtain between the celestial bodies 
 and the Earth, which, during the day, veils, as it were, the starry 
 vaidt, as the stars are not sufficiently bright in comparison to remain 
 visible. Without this diffused light the sky, instead of presenting 
 that azure tint which we all know so well, would assume the ajjpearance 
 of a black ground, on which the stars would appear and shine in broad 
 davli""ht, as they do when a solar eclipse cuts off the source of its 
 diffused light.
 
 THE EARTH. 107 
 
 V. 
 
 THE EARTH. 
 
 MOVEMENT OF ROTATION. 
 
 Apparent Diurnal Movement of the Stars and Sun — Real Rotation of the Earth — 
 The difference between Sidereal and Solar Days — The Rapidity of the Earth's 
 Rotation varies with the Latitude. 
 
 Mercury, Venus, and the Sun, the three celestial bodies the move- 
 ments of which we have studied, each turns round one of their 
 diameters ; this is a phenomenon which seems general ; and it has, 
 in fact, been observed in all the bodies near enough and large 
 enough to permit of their surfiice-markings being observed from 
 the Earth. 
 
 The rotatory movement of the Earth was established before that of 
 any of the other celestial bodies, and no one at the present time is 
 ignorant of the manner in which it daily manifests itself to us. 
 
 At an hour of the morning, which varies according to the seasons, 
 the Sun is seen to appear on the eastern horizon. By degrees it 
 rises, its disk becomes entirely visible, and mounts gradually in the 
 sky. At noon it reaches the highest point of its course ; it then 
 commences its downward course, describing in the second half of the 
 day an arc sjamnetrical to the first ; it then sets, and finally disappears 
 in the evening in the west. The rotation of the Earth is thus shown 
 during the day. 
 
 During the night the stars accomplish the same apparent move- 
 ment. The entire heavens seem then to possess a movement of 
 rotation, which always takes place from east to west round a Ime 
 of constant direction, to which astronomers give the name of Axis 
 of the World. This is no other than the axis of the Earth. 
 
 It was for some time imagined, that the heavens themselves
 
 108 
 
 THE SOl.AR SYSTEM. 
 
 actually revolved in this direction ; but in fact, it is our Eartli which 
 rotates in a contrary one, that is to say, from west to east, with a 
 uniform movement, the duration of which for each rotation is a little 
 less than four-and-twenty hours. Since the time of Copernicus and 
 Galileo, the fact of the rotation of the Earth, demonstrated beyond 
 all contradiction, has been universally admitted, as well as its annual 
 translation round the Sun ; but it is none the less true now, that there 
 is still in some minds a singular confusion, arising from the fact that 
 they cannot distinguish clearl}^ between these two movements. 
 
 The rotation of the Earth, let us repeat, is a daily or diurnal 
 movement, which is accomplished in about twenty-four hours, and 
 which produces, besides an apparent revolution of the entire celestial 
 vault, the phenomena of day and night. 
 
 Independently of this diurnal rotation, the Earth has a move- 
 ment in space, describing round the Sun, like all the other planets, 
 a nearly circular curve or orbit. This movement of translation 
 gives rise to the year and the seasons, biit it does not cause the ap- 
 parent diurnal revolution of the starry sphere, nor the succession of 
 days and nights ; it only modifies their relative length, as will be seen 
 in the sequel. 
 
 Let us return to the Earth's movement of rotation. 
 
 We have before seen, that this movement is uniform ; that is to 
 say, its angular velocity is constant. The proof of this uniformity is 
 easy, and astronomers have satisfied themselves about it by measuring 
 the length of the arcs described in the same time by different stars. 
 These arcs always measure an equal number of degrees. If we note 
 with precision the interval of time which elapses between two con- 
 secutive passages (or transits, as they arc called) of the same star 
 over the meridian of a place from night to night — between two 
 successive culminations* — we shall know the exact length of an 
 entire rotation. 
 
 It has thus been found to be about twenty-three hours fifty-six 
 minutes. 
 
 This interval of time has received the name of a sidereal dai/,'\- 
 whilst the term solar day is reserved for the interval of time which 
 
 * Astronomers call the vertical plane which passes through the north and 
 south 23oiuts of the horizon of a place, indefinitely prolonged into space, the 
 meridian of that place. When a star passes the meridian it is at the highest 
 point of its apparent diurnal course. Hence the name of culmination is given 
 to this passage (or transit). 
 
 t The sidereal day is divided, like the solar day, into twenty-four hours. 
 Sidereal and solar hours are divided into sidereal and solar minutes respectively 
 and so on : of this more presently.
 
 THE EARTH. 
 
 109 
 
 elapses between two successive passages of the Sun over tlie meridian ; 
 this second interval is about four minutes longer than the first. 
 
 Between the sohir day and the sidereal diij there is a funda- 
 mental ditference, — that of length. There is another not less impor- 
 tant ; whilst the length of the sidereal day is invariable, that of the 
 solar day varies throughout the year.* 
 
 We must linger somewhat on the fundamental fact that the 
 sidereal day is shorter than the solar one, as it is a fact which follows 
 
 IwTi 
 
 RFi^ifM¥5s«s^wi?v:?i!^ 
 
 Fi^-- 47. — Comparative leufrth of the si<lcrcal and solar day. 
 
 from the annual revolution of the Sun, and in truth is one of the 
 best proofs of its existence. This proof is rendered evident by 
 fig. 47. 
 
 * Hence it is that astronomers, witli a view of obtaining a convenient and 
 uniform measure of time, have recourse to a mean solar day, the length of which 
 is equal to the mean or average of all the apparent solar days in a year. An 
 imaginary Sun, called the " INIean Sun," is conceived to move uniformly with the 
 real Sun's mean (or average) motion, and the interval between the departure of 
 the mean Sim from a meridian, and its succeeding return to it, is the duration of 
 the mean solar day. Clocks and chronometers are adjusted to mean solar 
 time, so that a complete revolution (through twenty- four hours) of the hour-hand 
 shall be performed in exactly the same interval as the revolution of the Earth 
 on its axis_with respect to the mean Sun. 
 
 As the time deduced from observation of the true Sun is called "tru-:' 
 or "apparent" time, so the time deduced from the mean Sun, or indicated by 
 the machines which represent its motion is denominated " mean time."
 
 110 THE SOLAR SYSTEM. 
 
 We there see the Earth in two positions in its orbit, positions 
 which we will suppose separated one from the other, by an interval 
 of a sidereal day ; that is to say, by an entire rotation. 
 
 In the first of these positions — that on the left, the meridian S O 
 passes on one side throug-h the Sun ; it is noon for the parts of the 
 earth situated along this meridian in the illuminated hemisphere ; on 
 the opposite side it passes, let us say, through some particular star ; 
 it is midnight on that j)art of the Earth, situated along this meridian 
 in the dark hemisphere. 
 
 Let us imagine an entire rotation accomplished, while our planet 
 is travelling along its orbit. What will hapj^en ? This, namely, 
 that the meridian considered in its first position, after having rotated 
 round the Earth's axis, is again parallel to that position, so that 
 if the Earth had remained fixed in space, the Svm and stars would 
 reappear at the same time in the meridian ; the sidereal day would 
 have been of the same length as the solar day. 
 
 But the Earth is not so fixed, she has travelled onward to another 
 point. The star, because it is situated at an infinite distance, is 
 again found, after a complete rotation, in the meridian, w^hich, on the 
 illuminated side, no longer passes through the Sun. It is clear 
 from the figure, that the Earth must still describe a fraction of its 
 movement of rotation, in order to bring the meridian we have lettered 
 S O, again to the Sun. 
 
 Thus, the difference in the length of the diurnal rotation of the 
 Earth, and of the solar day, is explained by the annual revolution 
 of our globe round the Sun, which is thus proved geometrically. 
 
 Since the Earth has the form of a sphere, and turns with a 
 uniform angular velocity round an ideal line of invariable direction, 
 there ought to result from this movement different rates of movement 
 for tlie various points of its surface. 
 
 At the two poles this velocity is nil: but from the poles to the 
 Equator it increases constantly, as the radii of the circles described 
 by the differcait points along a meridian, — or, in other words, the 
 distance from the axis of rotation, — increase as these points are 
 nearer the Equator. 
 
 In twenty-four hours, the circle described by a part of the 
 fflobe. situated in the latitude of Paris, is entirelv traversed, as is 
 that parallel to it, in the latitude of Reikiawitz in Iceland, or, finally, 
 in the latitude of Quito on the Equator itself. These circles are of 
 very different lengths. Hence very unequal real velocities. These 
 velocities are, at Eeikiawitz 221 yards ; at Paris, 333 yards ; and
 
 THE EARTH. lU 
 
 at Quito (on the Equator), 507 yards a second ; or 450, 682 and 1038 
 miles an hour respectively. 
 
 How is it then, that, carried with such a rapidity, we do not 
 ourselves perceive our movement ? It is because the entire bidk of 
 the Earth, atmosphere, and clouds, participate in the movement.* 
 
 This constant velocity, with which all bodies situated on the 
 surface of the Earth are animated, would be the cause of the most 
 terrible and general catastrophe that could be imagined, if, by any 
 possibility, the rotation of the Earth were abruptly to cease. Such 
 an event would be the precursor of a most sweeping destruction of 
 all oro'anized beings. 
 
 But the constancy of the laws of nature permits us to contem- 
 plate such a catastrophe without fear. It is demonstrated that the 
 position of the poles of rotation on the surface of the Earth is in- 
 variable. It has also been asked whether the velocity of the Earth's 
 rotation has changed, or, which comes to the same thing, if the length 
 of the sidereal day and that of the solar day deduced from it have 
 varied within the historical period ? Laplace has replied to this 
 question, and his demonstration shows that it has not varied the 
 one hundi'edth of a second during the last two thousand years. 
 
 * Who was that ingenious inventor who, seriously or otherwise, suggested that 
 we should utilize the Earth's rotation as the most rapid mode of locomotion 
 at once the most simple and economical that could be conceived ? This was 
 to be accomplished by rising, in a balloon for instance, to a height inaccessible to 
 aerial currents. Then the balloon, remaining immovable in this calm i-egion, 
 would simply await the moment when the Earth, rotating underneath, would 
 present the place of destination to the eyes of the travellers, who would then 
 descend. A well-regulated watch, and an . exact knowledge of longitudes, would 
 thus render possible travelling from east to west, all voyages from north to 
 south, or from south to north, naturally being interdicted. This suggestion has 
 only one fault ; it supposes that the atmospheric strata do not participate in the 
 movement of rotation of the solid part of the globe. 
 
 The inventor did not remark that on the hypothesis of an immovable atmos- 
 phere, while we rotate at London with a velocity of 333 yards a second, there 
 would result a wind in the contrary direction ten times more violent than the 
 most terrible hurricane. Is not the absence of such a state of things a con- 
 vincing experimental proof of the participation of the atmospheric envelojie in 
 the general movement 'I
 
 112 
 
 IIIK SOLAR SYSTEM. 
 
 VI. 
 
 THE EAUTJI. 
 
 lii;\()J,l TlOX UOL i\J) THE SUN. 
 
 TIiu Yeai- — Uiinensions of the Eartli's Orbit — The Seasons — DiiTerence in tin; 
 Length (jf Days and Nights, according to the Seasons and Latitudes — Zones 
 and Climates. 
 
 The movement of the Earth on its axis manifests itself to us, as we 
 liave seen, by an apparent revolution of the whole heavens in the 
 spaec of a day. 
 
 By a similar illusion, the Sun seems to describe in a year, round 
 our planet, an orbit, which, in reality, is traversed by the Earth round 
 the Sun. 
 
 The exact time of this revolution, in mean solar days, is 3G5 
 days 6 hours 9 minutes 10 seconds and 75 hundredths of a second. 
 In this interval of time, the Earth sets out from one part of its orbit, 
 travelling- in a direction from right to left, or from west to east, and 
 regains the point of departure ; accomplishing' thus, without end, 
 and alwaj^s in the same manner, its movement of translation. 
 
 This orbit is not a circle, but an ellipse, of which the Sun occupies 
 one focus. The mean radius of the orbit, that is to say, the mean 
 distance of the Sun from the Earth, measures 95,298,000 miles. 
 
 The velocity of the Earth's passage along this immense curve is 
 variable, but its average rate is 33,290 yards a second, or 68,000 
 miles an hour. So that, we not only rotate every instant, describing 
 arcs round the terrestrial axis, the length of which, varying with the 
 latitude, may reach as high as 500 yards a second ; but w^e are again 
 carried through space with a velocity which exceeds 19 miles a second. 
 
 If we contemplate the dimensions of the globe and the enormous
 
 THE EARTH. 113 
 
 mass of the Earth, the imagination will be confounded in the presence 
 of the gigantic ball, which glides through space with such rapidity. 
 
 A calculation of two contemporary philosophers, Helmholtz and 
 Mayer, will perhaps give an idea of the prodigious movement which 
 impels our globe. These physicists have endeavoured to ascertain what 
 amount of heat would be developed by the abrupt stoppage of the Earth 
 in its orbit. They have found that this heat would suffice to melt the 
 entire globe, and to reduce a great portion of it to a state of vapour. 
 
 If it be true that the Earth moves thus around the Sun relatively 
 fixed, in proportion as she travels in one direction along a certain 
 portion of her orbit, the Sun itself will seem to describe an equal 
 arc in a contrary direction when we consider the arcs described 
 separately ; but if we compare, in fig. 48, the real curve described 
 by the Earth with the apparent one described by the Sun, we shall 
 
 ,pprtr<'^^____0/-^/V Rpg t ^fj iit 
 
 ^^>-.r the S5'l-' 
 
 Fig. 48. — Real Orbit of the Earth, aud apparent Orbit of the Sun. 
 
 see that the directions are the same. So that, as the proper move- 
 ment of the Sun, which causes the delay of its passage across the 
 meridian, or, what is the same thing, the inequality of the solar and 
 sidereal days, takes place from west to east, the real movement of 
 the Earth is also efiected in the same direction. It is for want of 
 comprehending this, that some authors with rather imaginative minds 
 have cried out against the fallacies of astronomers. 
 
 The Sim, then, must move each instant across the starry vault 
 of the heavens, and its centre will coincide from day to day with 
 difierent stars. During the day this displacement is not perceptible 
 when no exact measure of the position of the Sun is taken. But 
 we need only recognise that, corresponding with the displacement of
 
 114 THE SOLAR SYSTEM. 
 
 the Sun during the day, there must be an analogous movement of the 
 heavens during the night, to comprehend, that the aspect of the 
 consteUations must var}^ throughout the whole year. It is in eon- 
 sequence of the translation of the Earth, that the heavens defile 
 progressively over the horizon of any given place, or, if not the whole 
 heavens, at least that portion of them which are brought by the 
 diurnal movement above that horizon. 
 
 The length of the year, that is to say, of the interval of time which 
 elapses between two successive passages of the Earth through the 
 same point of its orbit, is about 365 ^ mean solar days. How many 
 entire rotations on its axis does our globe execute during that time ? 
 — 366| ; in other words, if the number of the solar days of the year 
 is 365 1, the number of the sidereal days is exactly greater by imity. 
 
 This is a direct consequence of the yearly revolution of the Earth, 
 combined with its diurnal movement of rotation. The same phe- 
 nomenon, which at first seems paradoxical, is produced in all the other 
 planets, whatever the nimiber of rotations accomplished during a 
 complete revolution round the Sun, and whatever the durations of 
 their sidereal and solar days. 
 
 Let us recall the fact that after an entire rotation, the Sun, which 
 at the point of departure passed the meridian at the same time as a 
 given star, lags behind about four minutes. At the following rotation 
 there is further delay, which is added to the preceding one ; and so on, 
 until the annual revolution being terminated, matters are found in 
 the same state as at the beginning. Now, if in order to return to a 
 coincidence of the Sun with the star which serves as a means of com- 
 parison, the Earth has efiected 366 rotations on its axis, the star will 
 have passed 366 times over the meridian, whilst the Sun, exactly 
 behind by one transit, will have returned to the meridian once less, 
 that is to say, only 365 times. 
 
 Let us now pass to other phenomena of great interest to the 
 inhabitants of the Earth, — phenomena which have their source in 
 the double movement of our planet. 
 
 From one day to another, the inhabitants of the same place — let 
 us rather say the inhabitants of the same latitude — see the Sun ascend 
 above the horizon to variable heights. The points in the east and 
 west, where the radiant body rises and sets, change their places ; 
 the Sun at noon attains a greater or less altitude, and the length of 
 its daily sojourn above the horizon gives to the days and nights 
 their variable and unequal lengths : hence difierent temperatures 
 and diversified climatic conditions ; hence, the Seasons. On the other 
 hand, these conditions themselves change, not only in both hemi-
 
 X 
 H 
 
 < 
 
 HI 
 Hi 
 
 O
 
 THE EARTH. 117 
 
 spheres of the Earth, but even in the same hemisphere, according to 
 the hititude of the place considered. Hence, climate, frigid zones 
 with long days and nights, temperate zones, torrid zones, and the 
 regions nearest to the equator, which have each year two summers 
 and two winters, and where the length of the day is always equal to 
 that of the night. 
 
 The astronomical reason of all these phenomena rests in the 
 simultaneous movements of the Earth. But there is a circumstance 
 which influences their succession in a dominant manner, and on 
 which we must now fix our attention. 
 
 Let us glance at Plate YI, which represents the orbit of the 
 Earth and the position of our planet in various points of it. We 
 shall see that the axis of rotation is neither perpendicular to the 
 plane in which the orbit lies, nor in this plane, but that it 
 forms with it a certain angle nearly equal to two-thirds of a right 
 angle (66° 32' 42'^) This inclination is constant during the whole 
 year, or at least varies only between very small limits ; besides this, 
 the axis always remains parallel to itself. It is the parallelism of the 
 axis which accovmts for the nearly invariable position of the celestial 
 pole above the horizon in each locality, provided we bear in mind 
 the nearly infinite distance of the stars from the Earth. 
 
 Among all the positions which the Earth occupies in its orbit, 
 there are four principal ones, diametrically opposed, in pairs, which 
 influence in a most important manner the relative lengths of day and 
 night, and the seasons; these are the two equinoxes and the two 
 solstices. 
 
 Here are the order and dates of their succession : 
 
 Towards the 21st of March, the Earth is at the first of these 
 points, called the Spring Equinox ; then comes the Summer Solstice, 
 about the 21st of June ; the Autumnal Equinox, near the 22nd 
 of September ; and, lastly, the "Winter Solstice, which gene- 
 rally falls on the 21st of December. Each of these points marks 
 the commencement of the season after which it is named. The 
 precise epochs of these four fundamental positions vary each year, 
 but within a somewhat restricted limit, as may be seen from the 
 following table : — 
 
 Commencement of the Four Seasons. 
 
 1865. 
 Spring . . . March 20, S"" IQT a.m. 
 Summer . . . June 21, 5 25 a.m. 
 Autumn . . . Sept. 22, 7 1 p.m. 
 Wester . . . Dec. 21, 1 13 p.m. 
 
 18C1. 
 
 
 March 20, 2'' 15"' 
 
 p.m. 
 
 June 21, 10 55 
 
 a.m. 
 
 Sept. 23, 1 8 
 
 a.m. 
 
 Dec. 21, 6 59 
 
 p.m .
 
 118 
 
 THE SOLAR SYSTEM. 
 
 When the Earth is at one or the other of the Equinoxes, the 
 plane of the Equator prolonged passes precisely through the centre 
 of the Sun. The two poles of the planet are then symmetricall)^ 
 placed with regard to the radiant body, and the circle of separation 
 of the illuminated hemisphere and the dark hemisphere lies in a 
 meridian. 
 
 It results from this particular position, that each part of the Earth, 
 whatever its latitude, describes half its daily journey imposed od 
 it by the Earth's rotation, in shade, and half in sunshine. Fig. 49 will 
 explain this. 
 
 
 B^^^B 
 
 
 
 
 ^H 
 
 pi 
 
 
 w^' 
 
 J 
 
 f 
 
 
 
 ^E'&A- 
 
 ^'i 
 
 ^^^jg^^'s^^ — 
 
 ■ 
 
 
 \ \ym 
 
 ^^k 
 
 H 
 
 ^K 
 
 
 ^''^\ • ^1 
 
 ^^H 
 
 ^E^^^^g 
 
 ^BB^t^iTiM^'^J 
 
 ^^ 
 
 ' 1 
 
 w»- i iflH 
 
 ^H 
 
 ^^^^S^^^^^^ 
 
 ^^^R 
 
 ■ 
 
 'J 
 
 
 1 
 
 ^^^^^^^n^^^ 
 
 I^^ISB 
 
 
 
 ^^^i 
 
 ^^^^^ 
 
 Fig. 40. — The Earth at an equinox. Einial day and night all over the world 
 
 Thus, at the time of the equinoxes, the length of the day is equal 
 to that of the night all over the world. The Sun remains twelve 
 liours above the horizon of each place, and twelve hours below it. 
 
 From the spring equinox to the summer solstice, the Earth 
 traverses the portion of its orbit, which corresponds to the months 
 of April, May, and June. Its axis remaining always parallel to itself, 
 one of its poles, — the iS'orth Pole, — is turned more and more towards 
 the Sun ; during the same period, the South Pole, on the contrary, 
 is turned more and more away from it. The day and night become 
 more and more unequal in length, and this inequality attains its 
 maximum towards the 21st of June (fig. 50). 
 
 The circle of separation of sunshine and shade having travelled 
 farllier fnmi iht' Pole, it follows that the length of the nights of the
 
 THE EAKTH. 
 
 119 
 
 northern hemisphere has continuously decreased ; the day, on the 
 contrary, increasing, and in much greater proportion as the places 
 are more distant from the equator. 
 
 The southern hemisphere has, during this period, experienced 
 inverse phenomena ; at the equator only, has the day continued to 
 be equal with the night. 
 
 From the 21st of June to the 22nd of September, the Earth 
 passes from the summer solstice to the autumnal equinox. During 
 the second period, the North Pole is turned towards the Sun, while 
 the South Pole remains plunged in darkness ; the alternations of day 
 
 Fij.'. 50.— The Eai tb nt a Solstice. Unequal day and night. 
 
 and night present, in inverse order, during the summer the same 
 phenomena as during the spring. 
 
 Thus, for six months, the regions near the North Pole have 
 continually seen the Sun above the horizon, those of the South 
 Pole have always had it below. Hence, in their icy deserts, here 
 a day of six months, there a night of six months, tempered, it is 
 true, by a continual twilight. During each twenty-four hours, in 
 consequence of the diurnal rotation, the Smi thus describes a curve, 
 which grazes the horizon, though not quite parallel to it, describing 
 a double spiral, which rises constantly until the 20th of June, and 
 afterwards descends to the beginning of autumn. 
 
 If the course of the Earth during one half of the year has been 
 well vmderstood, it will be easily seen how, during the other half,
 
 120 
 
 THE SOLAR SYSTEM. 
 
 similar phenomena will occur in symmetrically inverse order. At the 
 autumnal equinox we shall have equal days and nights throughout the 
 Earth. The autumn and winter of the northern hemisphere will be 
 the spring and summer of the southern one. The same differences in 
 the relative lengths of night and day will also be presented : the 
 only difference arising from the different length of the corresponding 
 seasons in the two hemispheres. 
 
 A word now on the inequality of seasons. 
 
 Let us once more insist upon the fact, that the orbit of the Earth 
 
 Jutumnal Equinix 
 
 Aphelion}, 
 S'/mmfr Solstice) 
 
 Fig. 51. — Orbit of the Earth. Varying length of the different Seasons. 
 
 is not a circle, but an ellipse, and that the Sun is not in its centre, as 
 would appear from Plate VI, but in one of its foci. More than this, 
 the major axis of the ecliptic — which is the astronomical term for the 
 orbit of the Earth — does not exactly pass through the solstices. In 
 fig. 51 their non-coincidence has been purposely exaggerated. 
 
 We shall immediately comprehend from an inspection of the 
 diagram that winter ought to be the shortest, and summer the 
 longest of the four seasons : the two other seasons are of inter- 
 mediate lengths, spring being the longer of the two. 
 
 This would be true, owing to the fact of the inequality of the arcs
 
 THE EARTH. 121 
 
 traversed alone, even if the Earth travelled with a constant velocity- 
 over every portion of its orbit. But the inequality is largely increased 
 from another cause. 
 
 We shall see in the sequel, that every planet moves round the 
 Sun with variable velocities, and more rapidly as it approaches the 
 common focus. The Earth, therefore, moves less quickly during the 
 summer season of the northern hemisphere than during the winter 
 season, and this again contributes to increase the difference in their 
 lengths. 
 
 The mean dvirations of the seasons, in the order in which we have 
 spoken of them, are as follows : — 
 
 Days. Days. 
 
 Spring 92-9 Autumn .... 897 
 
 Summer .... 93-6 Winter 89-0 
 
 The first two seasons, as do the two others, differ only but seven- 
 tenths of a day, that is to say, 16 hours 48 minutes. But the spring 
 exceeds the autumn by 3 days 4 hours 48 minutes, and the summer 
 is longer than the winter by 4 days 14 hours and 24 minutes. 
 
 At the extremities of the larger diameter (or " major axis," as it 
 is called) of its orbit, the Earth is nearest to and most distant from 
 the Sun. It is at its maximum distance, or ajihelion, in the first days 
 of July, and at its minimum distance, or perilieUon, a few days after the 
 winter solstice — about the 31st of December. 
 
 Thus the Sun is farther from us during the spring and summer 
 than during the autiimn and winter of the northern hemisphere, a 
 circumstance which proves that it is not to the decrease of the Sun's 
 distance that we must attribute the increase of heat, or rather of 
 temperature, of any given place on our hemisphere. 
 
 During the northern spring and summer, the Sun remains above 
 the horizon of a place longer than in autumn and winter : the length 
 of the day exceeds more and more that of the night as the solstice is 
 approached. That is the most important cause of the increase of 
 temperature during the summer months. Another cause, not to be 
 passed over, is the height of the Sun above the horizon. The diurnal 
 arc described by the great light-giver rises higher and higher from 
 the time of the spring equinox to the svmimer solstice, returning in 
 inverse order from the summer solstice to the autumnal equinox. 
 The rays that he sheds on the divers points of the northern hemi- 
 sphere traverse the atmosphere less obliquely than in winter and 
 autirmn ; and the intensity of the heat received is much greater when 
 this obliquity is less, a circumstance easily explained by the smaller
 
 122 THE SOLAR SYSTEM. 
 
 thickness of the atmospheric strata traversed. Besides, if we leave 
 the thickness of the atmosphere out of the question, the obliquity of 
 which we speak is in itself a cause why the heat received should be 
 less considerable. 
 
 The preceding explanation api:>lies to the southern hemisphere 
 during the seasons of autumn and winter, which are to it what 
 spring and summer are to us. And as the Sun is, besides, at a less 
 distance from the Earth, the intensity of the heat is greater : as also 
 in the winter seasons of the same hemisphere, the cold is more 
 intense. In the long run, however, these inequalities are compensated, 
 and the mean temperatures of the year are nearly the same, both 
 north and south of the Equator. 
 
 We speak here merely of the purely astronomical influences, 
 leaving out of the question the thousand local disturbing causes 
 which may exist or arise ; the climate of a place being a resultant 
 of them all. From this point of view, it is also easy to comprehend 
 why the maxima of heat and cold do not fall exactly at the 
 Solstices, but some little time after, in July and January. From 
 the 20th of June, the Earth, already warmed by the days of 
 spring, continues to receive from the Sun during the day more 
 heat than it radiates during the night : its temperature, therefore, 
 still increases. On the other hand, towards the 21st of December, 
 the Earth, already chilled by the long nights of the autumnal period, 
 still continues to get colder, because it loses more heat daring the 
 night than it receives during the day. 
 
 More than this, the seasons are very difierent for every point of 
 the same hemisphere. From the equator to one or the other pole, 
 we pass by imperceptible degrees from an intense heat to an ex- 
 treme cold. On the surface of the globe, five zones or climates are 
 distinguished, which succeed in the following order : 
 
 T]ic Torrid Zonr, which comprises all the regions north and south 
 of the equator, where the Sun is vertical twice a-year ; it is bounded 
 by the tropics in lat. 23° north and south. 
 
 Two Temperate Zones, which extend on either side the trojjics to a 
 latitude of 66°. To all the countries comprised in these zones, the Sun 
 never rises to the zenith, and the limit of its least meridional altitudes 
 is comprised between 66° and the horizon. 
 
 Lastly, two Frigid or Circumpolar Zones. "Within the limits of 
 these zones the Sun descends to the horizon and disappears even be- 
 neath it, here for one day, there for six months. It never rises to an 
 altitude greater than 46°, and at the pole itself the maximum altitude 
 is but half that quantity.
 
 THE EARTH. 12?^ 
 
 The superficial areas of these zones are very unequal : the torrid 
 zone emhraces -jYgths of the total surface of the terrestrial spheroid ; 
 the two temperate zones, j^^^2_t}js • anc[^ lastly, the two frigid zones, 
 j ^oths. Thus the two temperate zones, the most favourable to human 
 habitation and to the development of civilised life, comprise more than 
 half the extent of the Earth ; the frigid zones, w^hich may almost be 
 termed uninhabitable, form a very small fraction. In these quantities 
 both land and sea are included. 
 
 The various phenomena which we have just considered depend 
 directly upon the rotation of the Earth and its annual movement of 
 translation in space. The length of this rotation, or of the sidereal 
 day, the inclination and the parallelism of the axis, the duration of 
 the year, the form of the orbit and its real dimensions, are so many 
 elements which combine to produce them. If all, or some of them, 
 Avere to change, the days and the nights, the seasons and climates, 
 would change also, and the consequences which would result to the 
 conditions of life on our planet would produce, either in the long run, 
 or suddenh', the most profound modifications and most considerable 
 changes. 
 
 We have already seen that the length of the day has remained in- 
 variable during the historic period. The same may be said of the year. 
 But if the fonn of the terrestrial orbit and the inclination of the axis 
 of rotation vary imperceptibly, the periodical variability of these ele- 
 ments is confined within narrow limits, so that, except for unforeseen 
 and improbable catastrophes, the astronomical conditions of our planet 
 can be considered as invariable. The source of the light and heat, 
 and even of life, on our own globe, and on the other planets, no doubt, 
 is being gradually dried up, but calculation has shown that millions 
 of years must elapse before the gradual weakening of its rays can 
 modify perceptibly our terrestrial climates. 
 
 When we come to study the other members of our system, we shall 
 soon find the most curious variety in the astronomical — and therefore 
 climatic — conditions of each of them. Governed by identical laws, 
 they, nevertheless, will present to us the most wonderful diversities, 
 in the same manner as the organic kingdom constructed with a small 
 number of simple elements on a plan, the unity of which is, day 
 by day, becoming more evident, furnishes, nevertheless, to the in- 
 telligent admiration of man a considerable number of various 
 substances, and a still more prodigious number of genera, species, 
 and varieties.
 
 124 THE SOLAR SYSTEM. 
 
 YIL 
 
 THE MOON. 
 
 Phases of the Moon — Its Movement round the Earth — Dimensions and Distances; 
 recent measures, Eflfect of Irradiation in the Case of the Moon — Rotation — 
 Rotation and Revolution performed in the same time — The Moon's Orbit 
 always Concave to the Sun — The Moon's Rotation on its Axis. 
 
 Owing to its successive and periodical appearances and disappearances, 
 — the variety of form of its luminous portion, and the varying illu- 
 mination due to its light, the Moon is certainly the body which 
 above all others gives the greatest diversity to the aspect of our 
 nights. The white and soft light with which it inundates the land- 
 scape is the delight of all those who are sensible to the beauties of 
 nature ; poets have not failed to introduce it in their descriptions, 
 and painters in their pictures. But the absence or the presence 
 of the Moon in the starry vavdt is not less interesting to astronomers 
 than to artists — to science than to poetry. It is only from the astro- 
 nomical point of view that we shall here speak of it. 
 
 AVTien the Moon shines in the heavens, when even she shows us a 
 small part only of her illuminated side, the brightness of her light 
 effaces the smallest stars visible to the naked eye. The number of 
 stars thus rendered invisible — put out as it were, is much more con- 
 siderable as the Moon approaches the full; then the glimmer of the 
 Milky Way is lost in the diffused light of the atmosijhere, and only 
 the most brilliant stars remain perceptible to the unaided vision. 
 Moreover, as the time during which the Moon is visible, increases with 
 its brightness, it soon becomes impossible for astronomers to make 
 delicate observations, at least if they do not propose to study the 
 Moon itself, or the more brilliant stars. 
 
 Happily for observers, the Moon disappears periodically from the 
 heavens, and thus restores to the celestial vault, when the air is clear 
 and calm, all its splendour and magnificence. The great proximity 
 of the Moon to the Earth, which it incessantly accompanies in its revo- 
 lution round the Sun, makes it one of the most interesting among the 
 celestial bodies. What can be, in fact, more curious than this little
 
 THE MOON. 125 
 
 system in the vast system of the solar world ; this Earth in minia- 
 ture, perpetually executing round our globe a series of movements 
 entirely similar to those that our Earth in turn performs round the 
 Sun. Further on, when we shall see other planets, accompanied 
 also by satellites, and forming with them so many little systems, 
 we shall more easily be able to appreciate the phenomena which 
 these Satellites present to observers situated in the primary body, 
 if we in this place study those presented by our own Moon and Earth 
 in detail. 
 
 Let us begin with our satellite as it appears to the naked eye. 
 
 Two facts are known to every one ; first, that in an interval of 
 twenty-nine or thirty days, the Moon is seen under a series of 
 appearances which are called phases, which are reproduced periodi- 
 cally in the same order. Second, that it always presents the same 
 face to the Earth, in such a manner that we only see one of its 
 hemispheres ; the other half of the lunar globe remaining for ever 
 invisible to us. 
 
 Now, these two facts are proofs positive, that the Moon has two 
 motions, one of revolution round the Earth, another of rotation on 
 itself. These two movements, by a curious coincidence, are made in 
 the same interval of time. If we follow the Moon through the course 
 of one of its revolutions, we shall be convinced of the reality of these 
 movements, and of the fact, that they are both performed in the 
 same time. 
 
 We know that there is a New Moon, when our satellite is invisible, 
 both during the day and night. It then occupies in the heavens a 
 place very near the Sun, presenting to us its dark hemisphere ; for 
 this reason, and because also it is lost in the splendour of the solar 
 rays, it is invisible to us. 
 
 About four days elapse between the disappearance of the Moon 
 in the morning in the east, and its reappearance in the evening in 
 the west, a little after the setting of the Sun ; the instant of new 
 moon occurs precisely in the mid interval ; after this epoch it 
 gradually emerges from the Sun's rays. 
 
 "We see it first (Plate VII) in the form of a very slender crescent, 
 the convexity of which is turned towards the point below the horizon 
 occupied by the Sun ; at this time the obscure portion of the Moon's 
 disk is seen very distinctly. The delicate transparent tint which 
 renders it visible is known under the name of Earth-shine, or lumiere 
 cendree, and is due to the reflected light of the Earth. 
 
 Drawn along, apparently, by the diurnal movement, the body 
 soon sets below the horizon. The next day, the same appearance
 
 126 
 
 THE S()I,AK SYSTEM. 
 
 again occurs, but alreadj^ the crescent is less delicate, the luminous 
 portion larger, and the Moon somewhat farther from the Sun, setting 
 also a little later. 
 
 The fourth day after the new moon, the form and appearance of 
 our Satellite, which sets only three hours after the Sun, is that which is 
 represented in the second figure of Plate YII. The Earth-shine is still 
 very perceptible, although it diminishes more and more, to disappear 
 altogether at the following phase, which is called the Fird Quarter. 
 
 Between the seventh and eighth days of the Moon, it is presented 
 to us under the form of a semicircle, partly visible during the day, 
 as this time the diurnal movement only causes it to approach the 
 horizon some six hours after sunset. In the preceding phase, the 
 various features spread over the Moon's surface were visible. But 
 at this time, these markings are distinguished Avith great clearness 
 on the luminous half circle, more especially at the division between 
 the illuminated and the dark portion called " the terminator." 
 
 Between the first quarter and the Fidl 3Ioon, seven days again 
 elapse, during which the- form of the illuminated part approaches 
 nearer and nearer to that of a complete circle ; the Moon rises and 
 sets later and later, always turning towards the west the circular 
 portion of its disk. 
 
 Lastly it presents to us the whole of its illuminated portion, 
 about fifteen days after the new moon ; then the hour of its rising 
 is nearly that of the setting of the Sun, which in turn rises when 
 
 the Moon sets. It is midnight when 
 it attains the highest part of its course, 
 or, in astronomical language, when it 
 passes the meridian ; then the Sun 
 itself passes the lower meridian under 
 the horizon ; that is to saj', relatively 
 to the Earth, the Moon is precisely 
 opposite the Sun. 
 
 From the time of full moon to the 
 next new moon, the circular form of 
 the visible portion of the disk di- 
 minishes by degrees, and at last puts 
 on the appearance first noticed, — 
 But this time the convexity is turned 
 towards the east ; in fact, the half circle bounding the illuminated 
 portion naturally always faces the Sun. 
 
 In the mid interval which separates the full moon from the follow- 
 ing period, at the Last Quarter we get a phase like that presented at 
 
 Fig. 52.— Last phase of the Moon. 
 
 that of a slender crescent.
 
 PLATE VII. 
 
 THE PHASES OF THE MOON.
 
 THE MOON. 
 
 129 
 
 the first quarter, but inversely situated. In this second part of the 
 lunar period, or Lanation, the apparent position of the Moon in the 
 Heavens approaches nearer and nearer that of the Sun. Towards 
 the last days, it precedes its rise by very little, until it is again lost 
 in its rays, finally to disappear, and then to again appear as a new 
 moon, at the conunencement of the next lunation. The Earth-shine 
 
 Fig. 53.— Orbit of the Moon. Explanation of the Phases. 
 
 again becomes visible, after the last as before the first quarter, and 
 becomes more apparent as the visible portion of- the disk dininushes. 
 This succession of the phases of the Moon, which is constantly 
 reproduced, and always in the same manner, results evidently from the 
 movement of the Moon round the Earth. This will be easily under- 
 stood from fig. 53, and it will be there seen why the phases of suc- 
 
 K
 
 130 THE SOLAR SYSTEM. 
 
 cessive lunations are precisely the same, when the Sun, the Earth, and 
 the Moon occupy the same relative positions ; while if we referred 
 the place occupied by the Moon to the stars, in two or more similar 
 and consecutive phases, it would be seen that it does not occupy 
 the same point of the sky — that it does not even traverse the same 
 constellations ; a fact which results not only from the movement of 
 the Earth in its orbit, but from the variations of the movement 
 of the Moon in hers. In a little more than 29^ days,* the 
 Moon returns to occupy the same position with respect to the Sun 
 and Earth ; this marks the length of the lunar month or lunation. 
 
 It must be added, that this length exceeds by more than two 
 days the time of a complete revolution of the Moon in its orbit, 
 that is, of its sidereal revolution.! This difference is due to the 
 movement of the Earth round the Sun. 
 
 Via. :)i — Appaicnt dimensions i>( tliu- Xvou at its txtrciiie anil mean distances from the Eartn, 
 
 Considering the Earth as fixed, the orbit which its satellite 
 describes round it is an ellipse of which the Earth occupies one of 
 the foci. The distance, therefore, which separates the two bodies, 
 varies incessantly, and as a consequence, the apparent diameter of 
 the Moon varies also, but inversely ; a fact proved by observation, 
 and especially by micrometrical measures of its disk. 
 
 The figures here given will enable us to form a precise idea of 
 these variations. 
 
 The greatest distance of the Moon from the Earth is al)out 
 64| times the equatorial radius of our globe. When the Moon 
 is at this distance, it is said to be in aporjce. At the time of 
 p('rigee,X that is to say, its least distance, it is not further from us 
 
 * 29 days, 12 hours, 44 minutes, 3 seconds. 
 
 t The Moon's sidereal day is 27 days, 7 hours, 43 minutes, ll^ seconds long. 
 
 X Apogee, from ^o. from, and >?. the Earth. Perigee, from ^rs^J, near, and y~/i.
 
 THE MOON. 131 
 
 than 57^ of these radii, whence it results that its mean distance 
 is 60| radii, that is, nearly equal to the 400th part of the distance 
 of the Earth from the Sun, which is, as we have seen, in round 
 numbers 24,000 terrestrial radii. We must then make a chain of 
 thirty globes, equal in size to the Earth, touching- each other, and 
 in a straight line, to reach the Moon. [According to the latest 
 researches of Professor Adams, the mean distance of the centres of 
 the Earth and Moon is 238,793 miles, and this we know to be 
 correct within a very few miles.] 
 
 After we have passed the heavens under review, we shall return 
 to the interesting question of the distances of the different bodies, 
 and we shall attempt to give an idea of the methods which enable 
 us to determine them. We shall then see how, relying on very simple 
 geometrical principles, and aided by instruments of great perfection, 
 astronomers can measure the distances of some bodies near the 
 
 Fig. 55. — Difference of tlie distances of tlie Moon, at the lioriznu and at the zenitli. 
 
 Earth, infer from these measures the distances of the other members 
 of the system, and, finally, gauge the profundities of the ethereal 
 vault without quitting the movable stand - point where Nature 
 has placed us. 
 
 The mean distance which separates us from the Moon is but 
 little more than nine times the circumference of the Earth at the 
 Equator. There are many sailors who have, in their voyages, tra- 
 versed as long a distance — one that an express train would easily 
 accomplish in less than 300 days. 
 
 The apparent magnitude of the huiar disk varies, as we have seen, 
 with the distance of the Earth from the Moon : but even on the surface 
 of our globe, and at the same instant, the diameter of the disk does 
 not appear of equal magnitude to all observers. It appears smaller 
 to an observer who sees tbe Moon rising or setting at the horizon 
 than to him who sees it at the zenith.
 
 132 THE SOLAK SYSTEM. 
 
 To the former, tlic distance A L of the Moon is nearly equal 
 to the actual distance of the centres of the two bodies. To the 
 latter, on the contrary, the distance, A'L, is equal to the first 
 diminished by the terrestrial radius, or by the sixteenth part of the 
 
 total distance. 
 
 Hence it follows, that as the Moon is carried by the Earth's 
 rotation from the horizon of an}^ place to its zenith, that place is 
 actually brought nearer to it some 4000 miles. The lunar disk should, 
 therefore, appear to us larger at the zenith than at the horizon ; 
 but, singularly enough, by a pure illusion, the opposite effect obtains ; 
 at the rising and setting of the ^Moon its disk appears to us enor- 
 mous ; it seems, on the contrary, to diminish insensibly when it is 
 removed from the objects situated on the horizon and mounts the 
 stiirry sky. 
 
 We have remarked, that this is a pure illusion. To be convinced 
 of it, it is sufficient to take exact measures when the Moon is in 
 the two positions ; if this be done with rigorous exactness by means 
 of an instrument, to make the result independent of our ordinary 
 way of judging, it is entirely opposite to the appearance. How 
 is this singular phenomenon explained ? 
 
 By an error of our judgment. When the luminous disk of the 
 body is near the horizon, it seems placed beyond all the objects on 
 the surface of the Earth interposed between us and it, and therefore 
 more distant than at the zenith where nothing separates it from us. 
 Now an object which keeps the same apparent dimensions is to us, 
 by virtue of the instinctive habits of our eye, by so much greater 
 as it appears to be more distant.* 
 
 We next come to the real dimensions of the Moon. These are 
 readily determined, since we know, with the most wonderful exactness, 
 both the apparent magnitude of the Moon's diameter and our dis- 
 tance from it.f 
 
 [A recent discovery of very great interest shows us that in the 
 case of the Moon, the word " apparent" moans much more then it does 
 with regard to other celestial bodies. Indeed, its brightness causes 
 our eyes to play us false. As is well known, the crescent of the 
 new moon, by an effect of irradiation, seems part of a much larger 
 sphere than that which it has been said, time out of mind, to 
 
 * If, when the disk of the IMoon appears at the horizon with these illusory 
 dimensions, it is looked at with the naked eye through a tube, or the hands 
 placed tubewisc, the illusion disappears ; it does not seem then to exceed in size 
 the lunar disk seen at the zenith. 
 
 + The diameter of the Earth bein^j 1, that of the Moon is (V2729.
 
 THE MOOX. 133 
 
 " liukl in its anus." We now learn that the briglit portion of tlie 
 Moon, as seen in our measuring instruments as well as with the naked 
 eye, covers a larger area in the field of view of the telescope, than it 
 would do if it were not bright. This has recently been proved by 
 measuring the dark Moon. Our readers may possibly ask how this 
 has been done ? Well, we get an approach to a dark moon when we 
 observe the occultation of a star at the dark limb, and under these 
 circumstances, it has been recently found by the Astronomer E-oyal, 
 that in the main, all such occultations go to show that the limb of 
 the jyioon is not so far away from its centre, in other words, that its 
 radius is not so great as we thought. Again, in total or annulaj- 
 eclipses, we deal entirely with the dark Moon, and Mr. De La Rue's 
 exquisite photographs of the total eclipse of 1860 entirely endorse 
 the results of the twenty-five years' labour's at Greenwich. 
 
 P'ig. 56. — Comparative dimeusions of the Earth and Moun. 
 
 The Astronomer Royal's result is, that the Moon's angular dia- 
 meter hitherto received, is too large by 2". Mr. De La Rue's, that 
 it is two large by 2''* 15. And this quantity must be looked upon 
 as a "telescope-fault." 
 
 Hansen gives the mean angular semi-diameter of the Moon as 
 15' 33"*36. We must now call it 15' 31"*36 ; and its diameter, which 
 was formerly supposed to be 2160 miles, or a little moi^e than a 
 quarter of the diameter of the Earth, must be reduced by something 
 like seven miles. We shall have something else to say on this dis- 
 covery when we refer to the question of the lunar atmosphere.] 
 
 Supposing the Moon spherical, the total surface of its two liemi- 
 spheres, visible and invisible, is equal to a little less than tlie 
 thirteenth part of the surface of the terrestrial globe; that is to say.
 
 134 THE SOLAR SYSTEM. 
 
 tluit it measures 14,5(58,000 square miles. Lastly, if from its super- 
 ficial extent we pass to its volume, wc find that tlie Moon is scarcely 
 more than the forty-ninth part of that of our Earth, or 5,200,000,000 
 cubic miles. 
 
 The Moon's motion, we have before remarked, is effected along 
 
 an elliptical curve or oval, at one of 
 
 \ ^J the foci of which is the Earth. 
 
 \ } ^ Such would be, indeed, the lunar 
 
 ' ' ^--^ orbit if the Earth remained fixed in 
 
 \^ space. But it is well laiown, that. 
 
 \ 1 « li) i far from remaining at the same spot, 
 
 ' our globe itself travels round the 
 
 Sun in an orbit the mean radius of 
 \ "^Af- Q Avhich is four hundred times greater 
 
 ' \\ ""^x than that of the Moon. As the 
 
 ^l^„ \ \ Moon accompanies the Earth in its 
 
 stupendous journey, keeping the rc- 
 \ -^ lative positions necessitated by its 
 
 circum-terrestrial movement, it fol- 
 lows that the form of its real orbit 
 is much more complicated than a 
 ' |i'' ^ O simple elliptic one would be. 
 
 [Its real path consists of a series 
 of curves, or rather of an epicycloidid 
 curve, alurii/s concave to the S/ii/, 
 and intersecting the orbit of the 
 Earth twice during a lunar month.] 
 But the total departure of the 
 Moon from the Earth's orbit, does 
 not exceed the -j-^jyth part of the ra- 
 dius ; so that, if di-awn to scale on 
 a large sheet of paper, it would be 
 almost impossible to detect the de- 
 parture of the Moon's orbit from 
 that of the Earth. 
 
 If the Earth and the Moon, in- 
 stead of moving siinidtaneously 
 along their orbits, in such a Avay 
 
 Fifj- jT. — The hinai- orhit. , jt n •.• -^ 
 
 1. AmpUficcl. •>. In its relative ,li,nonsions. ^"^^ tO OCCUpy the fivC pOSltlOnS m- 
 
 dicatcd in figure 57, [which will 
 be understood to be grossly exaggerated, nor is the real orbit pre- 
 cisely represented], were simply, the first to remain at rest, and the
 
 THE .MODN. 
 
 13; 
 
 second to circulate in its orbit round our globe ; it is easily seen 
 that the appearances presented would be precisely the same, at 
 least, if we compare the positions of the two bodies with regard 
 to the Sun. 
 
 It is in this manner that a person, on the deck of a vessel in motion, 
 believes that in walking round the mast, he is mo-\^ng in a circle, 
 whilst the curve which he describes on the surface of the sea, is a 
 sinuous curve, the form of which is analogous to that of the real orbit 
 of the Moon. In reality, the path which this person traverses is more 
 
 Fig. 5S.— The curve deseiibcrl in a year, by Ibe Moon round the E^irth. 
 
 complicated still ; and to obtain its real form, we must take into 
 account his proper movement, the movement of the vessel on the sea, 
 the double movement of rotation and of revolution of the Earth itself. 
 It will be seen later on, that the Sun moves also through space, 
 drawing with it the Earth, the other planets, and their satellites, 
 whence follow, for the orbits of all those bodies, sinuous curves, the 
 degree of complexity of which varies with the nmnber of the various 
 motions with which they are animated. 
 
 We must recollect that it is the phases of the Moon whkh have
 
 136 
 
 THE SOLAR SYSTEM. 
 
 demonstrated to us its revolution round the Earth. This movement, 
 'added to the fact, that the Moon constantly presents the same hemi- 
 sphere to the Earth, proves that it turns also on itself, in a period of 
 time exactly equal to the length of its sidereal revolution, that is 
 to say, in ahout twenty-seven days and a third. 
 
 In speaking of the movement of rotation of the Moon on its axis, 
 it is right to anticipate an ohjection often made, proceeding from a 
 false idea sometimes conceived of the rotatory movement of a mov- 
 able body. " Since the Moon," it is said, " always presents the same 
 
 Fii,'. fj9. — Rotation ol a sphere, supiiosed to be at rest. 
 
 face to us, it cannot turn on itself. If it turned on an axis or pivot, 
 it ought to present us all its sides successively." Such is the objec- 
 tion simply put. 
 
 To solve this difficulty, let us examine into the phenomena. 
 What is a movement of rotation Y How is it known that a body, a 
 
 Fi^:' iHi. — Actual luuvement of rotation of tlie Mtjon iii tlio niterval of a lunation. 
 
 sphere for example, does rotate ? and how is it known when an entire 
 rotation has been completed ? Evidently, when the sphere has pre- 
 sented successively one of its sides towards every point of the space 
 which surrounds it. If we divide the entire rotation into four 
 periods, the accompanying diagram will show how the sphere would 
 be seen at the commencement of each of those periods, to an observer 
 at rest. 
 
 Now, if the sphere, durmg the exact time that it takes to effect
 
 THE :m(>()X. 137 
 
 this rotation round its axis, executes a movement of revolution round 
 the obsei'ver, whether the observer be at rest or not, it is none the 
 less evident, that the entire rotation would be effected, if the side, 
 of which the point A forms the apparent centre, is successively pre- 
 sented to all parts of space. Now, this is the case with the Moon, 
 during- a complete revolution in its orbit, as may be seen from the 
 comparison of lig'ures 59 and 60.* 
 
 We shall see further on, that it is not rigorously true to affirm 
 that the Moon always presents the same face to the Earth ; our 
 satellite, in fact, midergoes what astronomers call a HJ)r(dion, or 
 apparent swinging from east to west, and another from nortli to south. 
 
 These librations result from causes of which more anon ; it is 
 sufficient here to know that they do not modify the fundamental fact 
 of the equal duration which characterizes the two simidtaneous 
 movements of rotation and revolution of the Moon. More than this, 
 the central point of the disk is not precisely the same to observers 
 situated in different parts of the Earth. 
 
 We shall now proceed to describe the Moon as it is seen in the 
 telescope, and to inquii'e into what is known of its physical constitu- 
 tion, a question of absorbing interest from so many points of view. 
 
 *' The sphere iu tig. 59 occupies tive pusitidiis in inverse (jieler to these ol'llu' 
 Muuu iu fijir. 60. But this docs uot atiect the demoustratiou.
 
 l;{8 THK S()1,AK SYSTKM. 
 
 VIII. 
 
 THE MOON. 
 
 PHYSICAL CONSTITUTION. 
 
 The Aspect of tlie j\Ioon to the naked eye — The Seas or Maria ; Mountains — 
 Principal Mountain Chains — Volc.xnic character of the Lunar Mountains — 
 The Craters Tycho and Copernicus — Walled Plains — Annular Mountain- 
 ramparts — Craters, Peaks, and Cones — Terrestrial An dogies — Heights and 
 Dimensions of the IMountains — Bright Rays ; Centi-es from which they 
 Emanate ; Mr. Nasmyth's Explanation of them — Rilles or Furrows— Sug- 
 gested Explanations ; Recent Labours of Schmidt. 
 
 The world wliich we are about to explore, — somewhat in detail, 
 thanks to its small distance and to the great power of our modern 
 optical instruments, — though like the Earth in some general cha- 
 racters, totally differs from it in others. If an inhabitant of the 
 Earth were transported to the surface of the Moon, he would be 
 at once struck with the strangeness of the scene. The configuration 
 of the surface, every corner broken up and rugged, here circular 
 cavities, there elevated peaks ; the aspect of the heavens ; the bright 
 stars shining in the broad day ; the sharpness of the lights and 
 shades; the eternal silence which reigns in these desolate regions; 
 the extreme temperatures, now glacial, now torrid; the singular 
 life-conditions of organised beings — if it be that life is possible 
 there ; all would unite to upset the most familiar notions. 
 
 Nevertheless, Avhatever may be the contrasts between the lunar 
 world and our own globe, it Avill be seen that the variety which is 
 manifested with a marvellous richness, here, as in all the works of 
 Nature, is the efi'ect of but a small nvmiber of causes, or rather the 
 result of simple modifications of elements Avhich are really the same 
 for all celestial bodies. The simplicity of the laws which govern 
 astronomical phenomena causes the unity of plan of the whole solar 
 system to shine fortli with incomparable clearness.
 
 PLATE VIII. 
 
 THE FULL MOON
 
 I'HE MOON. 141 
 
 The full Moon in u very pure sky jillows the naked eye to dis- 
 tinguish the pi'incipal dark and bright features — Itvitui'es, the per- 
 manence of which, as we have before remarked, sliows that the same 
 foce — the same hemisphere, is always turned towards us. From east 
 to west, going northward, several large greyish spaces are dis- 
 tinguished, the uniform aspect of which contrasts with the southern 
 half of the disk, which is almost entirely covered with a multitude 
 of bright jjoints. The north-east and north-west borders of the disk 
 are terminated by whitish and bright marks, whilst the central 
 regions pai'tieipate in the general tone of the southern part. 
 
 Of old the name of "seas" was given to the large dark spots 
 which mottle the Moon's northern hemisphere and part of the 
 southern one, towards the west and east. The name is still retained, 
 although its literal meaning must not be attached to it. The lunar 
 seas are now regarded as plains, whilst the most brilliant portions 
 are principally mountainous regions. We will now briefly describe 
 both, asking thg reader to follow the description on Plate VIII, 
 which represents the full Moon as seen with the aid of a telescope 
 of small magnifying power. 
 
 [As the image of a celestial object seen in a telescope is inverted, 
 the top of the plate represents the South Pole, and the bottom the 
 North Pole, the right hand is east and the left hand west.]* 
 
 To begin with the Seas, or Marui. 
 
 Close to the western border or limb, is seen a greyish spot, of an 
 oval form, plainly visible by contrast and isolated in the brighter 
 portions : this is the Mare Cvhinm — the Sea of Crises, Between this 
 spot and the centre of the disk, a large dark space divided by a 
 kind of sharp promontory has been named the Sea of Tranquillity, 
 — Mare TranquUlifaik. It throws out towards the south two portions, 
 the largest and most western of which is the Sea of Fecundity — Marc 
 Fecunditatis, whilst the other, smaller and nearer the centre, is the 
 Sea of Nectar — 3Iare Ncetaris. 
 
 If now, leaving the Sea of Tranquillity we travel northward, we 
 Hnd the Sea of Serenity — Mare Serenifafis, which is traversed 
 throughout its length by a very bright and nearly rectilinear ray, 
 which gives to the whole spot the form of the Greek capital 
 
 * [A slight change has been made here in the translation, at the suggestion 
 of the Rev. T. VV. Webb, to accommodate the expressions "east" and "west" to 
 the general usage of selenographers, according to which the terms employed in 
 describing the relative position of objects upon the disk, inijily a revei-.^ion of E. 
 and W., compared with their situation on teri-cstrial maps, but not au iuvci-siou 
 of N. and S.]
 
 142 THE SOLAR SYSTEM. 
 
 phi <D. The Sea of Vapours — .Yr/rr Vaporum, is a prolongation 
 towards the centre of the disk of the Sea of Serenity. 
 
 Lastly, the Sea of Rains — Blare Iinhyium, of round form, the 
 largest of all those which have been named, forms the northern ter- 
 mination of the series of greyish spots to which the incorrect 
 appellation of seas is still applied. 
 
 We must now re-descend towards the east to find the Ocean of 
 Tempests — Oceamis ProceUanim, of which the outlines, not very well 
 defined, are lost towards the south in the Sea of Moisture — Hare 
 Ilajjionmi, and Sea of Clouds — Mare NiMum, at a short distance 
 from a luminous point, Avhence diverge in all directions whitish 
 rays of great length. 
 
 This last point, which may be considered as the centre of the 
 mountainous regions which surround the southern pole, is no other 
 than Tycho, one of the most important elevations of the visible 
 hemis^Dhere of the Moon. 
 
 If now, in order to observe the details of the lunar disk, we 
 employ a telescope of considerable magnifying power, we shall be 
 astonished at the prodigious multitude of small spots of annular form, 
 round or oval, which cover the entire surface. At the time of full 
 Moon, these features are not well defined, which arises from the 
 position of the visible hemisphere with regard to the Sun. 
 
 If, on the contrary, we choose for the time of observation 
 the epoch of the first or last quarter, the portions near the edge of 
 the illuminated portion of the Moon will appear eaten, into cavities, 
 surrounded by circular ramparts, throwing their shadows away from 
 the Sun, here towards the interior, there towards the exterior of 
 the cavity. More than this, along the whole line of separation of 
 the light and shadow called the Terminator the interior of the 
 annular cavities seems quite black, whilst here and there luminous 
 points show themselves detached from the illuminated portions of the 
 Moon. These spots indicate mountain-tops or ranges, which, accord- 
 ingly as we observe at the first or last quarter, receive the rays of the 
 Moon's morning sun, or the sunset rays which linger after the low- 
 lands are in shade. 
 
 Such are the mountains of the Moon. Figs. Gl and G2 jrive an 
 idea of the appearance of the mountainous regions wdth which our 
 satellite is overspread. 
 
 The chains of momitains, as distinguished from the annular moun- 
 tain-ramparts are not relatively numerous in the visible hemisphere 
 of the Moon. The greatest number is found in the northern part 
 of the disk. The Alps, the Caucasian range, and the Apennines, are
 
 THK M(K)X. 
 
 143 
 
 the most reiuarkable. This last chain separates the Seas of Serenity 
 and of Vapours from the Sea of Rains, which is surrounded as with 
 a belt of semicircular form by the three ranges we have named. 
 It is well seen in the drawing of the fidl Moon which we have 
 before given. We may also notice the Carpathian and the Oural 
 mountains, which separate the Ocean of Tempests from the Sea of 
 Rains, and the Sea of Clouds ; the Taurus mountains, Avest of the Sea 
 of Serenity ; the mountains Dorfel and Leibnitz, at the southern pole ; 
 the Pyrenees, which sej)arate the Seas of Fecundity and of Nectar ; 
 towards the west, the Altai mountains, near this last sea, which ex- 
 tends 276 miles from north to south. [The Altai mountains ajoproach 
 closely to the arc of an ellipse, the major axis of which is terminated 
 on the south by the crater Piccolomini, and on the north by the twin 
 craters Isidorus and Capella, sotith. 
 
 which are in a very disturbed 
 region. The monster craters, 
 Catherina, Cyrillus, and Theo- 
 philus, are just within the north- 
 east portion. There are two con- 
 centric crater ranges separated by 
 plains between the Altai moun- 
 tains and the Mare Nectaris.] 
 Jjastl}^, we have the Cordilleras 
 and the mountains D'Alembert, 
 near the western limb. The range 
 of the Apennines, the most con- 
 siderable* of these mountain- 
 chains, is, however, but 373 miles 
 in length. 
 
 It is impossible not to recog- 
 nise the eminently volcanic cha- 
 racter of the lunar mountains. 
 All the crust of our satellite is 
 pierced by craters which indicate 
 an innumerable series of volcanic 
 eruptions, some limited to a small 
 space, others embracing an im- 
 mense area on the surface. We are enabled by the kindness of Mr. 
 Warren De La Rue to lay before our readers in the Frontispiece, copied 
 
 * [Or rather " the most familiarly known." It is surpassed in height, ami pos- 
 sibly in extent, by the ranges (if they are not aiiiiukir ramparts in prutile) of the 
 S. andE. hmbs.— T. W. W.] 
 
 
 NORTH. 
 
 
 Key-Map to tbc 
 
 Froutispic'cc. 
 
 'g 
 
 1 — Clavius 
 
 S Oroiitius. 
 
 
 Magimis 
 
 9. h'a>st;ridts. 
 
 3. 
 
 Loii-onioiitanus. 
 
 10. Lcxell. 
 
 4. 
 
 Pictet. 
 
 U. Gaiiriciis. 
 
 5. 
 
 Saus.surc. 
 
 12. W\n-ZL:lb,iiii;r. 
 
 6. 
 
 Tyclio. 
 
 13. Hell. 
 
 7. 
 
 Nasii-eddin. 

 
 144 
 
 THE SOLAK SYSTEM. 
 
 from his largo photograph of 38 inches diamotor, an engraving of 
 Tycho, and the region lying to the west and south. We here give (fig. 
 61) an additional representation of the crateriform abysses lying to the 
 
 Fig (il.— Mountains of the Moon. View of the region to the soutli-uasc of TyeUo. (Nasniy tli. ) 
 
 south-east of the same crater, as observed and drawn by Mr. Nasmyth, 
 who is second to none in his hand-drawings of the lunar surface. The 
 circular rampart of Copernicus, one of the largest :uniiil;ir inoinilains
 
 THE MOON. 
 
 145 
 
 of the Moon, near the Carpathians, is represented in detail, in fig. 62, 
 as observed by Mr. Nasmyth. We are indebted to the kindness of 
 the late Admiral Smyth for permission to present it to our readers, it 
 forms one of the illustrations of his magnificent work, the " Speculum 
 Ilartwellianum. ' ' 
 
 The regions near Tycho are formed, as may be seen, of a number 
 of craters of various dimensions, some of which are hollowed out in the 
 form of cups or funnels, whilst the largest present the appearance 
 of circles with flat bottoms, at the centre of which rise peaks of pyra- 
 
 Fig. 62 —The Mountains of the Moon. View of Copernicus. (Nasmyth.) 
 
 midal form. One crater is situated at the centre of a circle which it 
 surpasses in altitude ; whilst at the bottom of a crater Avith very ele- 
 vated ramparts, and here and there in the winding valleys which the 
 circular walls leave between them, other small volcanic vents scarcely 
 rise above the neighbouring surface. Tlie irregular edges of all these 
 openings bear testimony to the convulsions, rents, and dislocations, 
 which the surface of our satellite underwent at the period a\ hen these 
 eruptions took place.
 
 UQ 
 
 THE SOLAR SYSTEM. 
 
 The cniter-wall of Copernicus shows numerous traces of debn'.s 
 ejected from the crater. Many other hmar mountains present, like 
 Copernicus, evident traces of stratification [or terraces, if the com- 
 mon geolog-ical meaning of "stratification" shovdd be thought to 
 imply aqueous action], doubtless owing to the deposits of successive 
 
 eruptions. 
 
 If the volcanic mountains of the Moon present great analogies 
 to the volcanoes of our Earth, they are also distinguished by very 
 marked characters. If the preceding drawings be compared with the 
 topographic view (fig. 63) of the Peak of Tenerifie and its environs, 
 
 Fig. 63.— The Peak of Tencritt'c ai,d its Eiiviu, 
 
 the differences, as well as the analogies, will be seen. Whilst the 
 craters on the Moon have enormous dimensions, — the diameter of 
 Ptolemy being 114^ miles, of Copernicus 56, and of Tycho 54, 
 — the dimensions of the terrestrial volcanoes are relatively ex- 
 tremely small. The relief of the Isle of Bourbon (fig. 64), which 
 we reproduce as constructed by a French engineer, M. L. MaiUard, 
 shows large depressions of nearly circular form, at the points A\4ierc 
 cones of eruption originally existed. It is, perhaps, to sinkings of 
 this nature that the circles of the Moon are due.* But it must be 
 
 * [The elevation, however, of the surrouuding ramparts seems to render this 
 improbable, as they would on such a supposition indicate tlie former existence 
 of cones of most disproportionate dimensions. The exterior of the lunar craters 
 seldom exhibits any approach to a vertical position. — T. W. W.]
 
 THE MOON. 
 
 147 
 
 remarked that the exterior profile of these volcanic cavities has not 
 that sharp vertical direction which on our satellite disting-uishes the 
 walled craters, the elevation of the sides of which is less on the 
 exterior than in the interior, as demonstrated by the differences in the 
 lengths of the shadows cast. The bottom of a lunar crater is generally 
 of lower level than that of the plain which surrounds it ; the contrary 
 always holds in terrestrial volcanoes. It is true that this observation 
 applies to the walled craters of great extent, rather than to the craters 
 properly so called. 
 
 If, as is believed, the generally rounded form of the lunar fea- 
 tures, including even the chains of mountains, proceeds from the 
 
 Fig. 64. — Topographical relief of the Isle of Bourbon. (Maillai'd.) 
 
 action of the interior strata against the solidified crust of the spheroid, 
 — if the walled craters are but craters of upheaval, would it not be 
 allowable to attribute the interior depression of the bottom of the 
 circular cavities to a kind of sinking of the half liquid matter ? ^ 
 
 * [Mr. Mallet, in his fourth report on Earthquake Phenomena {Reports of 
 the British Association for the Advancement of Science, 1858, p. 61), shows that 
 the Earth's surface is to a great extent divided into saucer-shaped shallow 
 depressions, bounded by flowing coast lines, generally uniting in closed curves ;
 
 148 THE SOT.AK SYSTEM. 
 
 One word now on the lieig-hts of the hmar mountains. 
 
 The hijj^hest of all are in the vicinity of the southern pole : there 
 Dorfel is found, the summit of which attains 8897 yards in altitude ; 
 the mountains Casatus and Curtius are 7078 and 7409 yards high, 
 and the annular crater Newton is 7951 yards deep. " The excavation 
 of this last is such," say IJeer and Madler, " that neither Earth 
 nor Sim is ever visible from a great part of its bottom." 
 
 In the northern regions, considerable heights are also found : 
 Calippus in the Caucasus, Huygens in the Apennines, respectively 
 attain 6193 and 6020 yards in height. The central peaks and cones 
 are nearly always much surpassed in height by the annular mountains. 
 The central cone of Tycho measures 5000 feet, and that of Eratosthenes, 
 at the extremity of the chain of the Apennines, rises to a height of 
 5250 yards above the floor of the crater. 
 
 To sum u^ ; of the 1095 heights measured by Beer and Madler, 
 39 are higher than the summit of Mont Blanc, and 6 are more than 
 6500 yards high. 
 
 Thus the vertical heights of the lunar mountains arc not less 
 astonishing than their lateral dimensions. We have ah'eady men- 
 tioned the immense walled plains of Ptolemy, Copernicus, and Tycho, 
 but among the craters, properly so called, it is not rare to find some 
 which have diameters of 100 to 120 miles. The crater of Schickard 
 is one of the most considerable on the visible hemisphere of the Moon : 
 its diameter is not less than 133 miles ; and the height of one of the 
 mountains which lies near it is 3500 yards. It is a noteworthy 
 circumstance that an observer placed at the centre of the immense 
 walled plain Schickard, would not be able to see the summit even of 
 the lofty irregular wall which surrounds it on every side. The dis- 
 tance would be so great, that the borders of the crater would lie below 
 the visible horizon. How different to the craters of our own vol- 
 
 and on p. G4 he says ; " Enough, however, has probably been stated, to indicate 
 that, viewed on the broadest scale, the surface of our globe consists, as respects 
 its solid surf^xce, of a number of saucer-like depressions, when large having also 
 convex central areas, all having plain outlines approximating to extremely irre- 
 gular ovals, or other closed curves, and hoimdcd hij mountain chains, or more 
 rounded or flat-topped ridges, or elevations of the solid sphere, greater or less ;" 
 and also on p. 61 he says, " Each great oceanic saucer, bounded by the existing 
 contin^ts and their fragmentary outliers, presents an almost continuous fringe 
 around, of mountain chains and volcanic foci." It is not a little remarkable 
 that the lunar volcanic vents are arranged similarly to those of the terrestrial, 
 cither breaking out on, or even piercing through, the walls of the smaller craters, 
 or arranged in lines across the larger lunar depicssions, not unlike the sub-oceanic 
 linear volcanic ranges of which Mr. Mallet speaks. W. R. B.l
 
 THE MOON. 149 
 
 canoes,, which, as remarked by Iliimboldt, -would at the difstauce 
 of the Moon, be scarcely visible with the telescope. 
 
 To complete this description of the Moon, which is at once 
 geological, geographical, and topographical, we must mention two 
 singular phenomena which have much puzzled astronomers. We 
 refer to the luminous bands and rilles. 
 
 In Plate YIII there are seen to start from two principal points, 
 Tycho and Copernicus, two series of luminous rays, which, traversing 
 the mountains and the neighbouring features, extend to a great dis- 
 tance from those brilliant centres. More than a hundred luminous 
 bands thus diverge from Tycho. Aristarchus, Kepler, and the Carpa- 
 thians, [and many other centres] present analogous systems, which 
 apjjear to converge, intermingle, and connect themselves together. 
 These singvilar appearances, of which no entirely satisfactory explana- 
 tion has yet been given, are only visible about the time of the full 
 moon. They disappear at the other phases ; and this seems to show that 
 they are not due to elevations, as then they would cast shadows, and 
 would be, on that accomit, clearly visible. Do they owe their origin 
 to the eruptions of the volcanoes which occupy their centre ? If this 
 be so, would they not seem to be crevices tilled subsequently with 
 reflecting and ci'ystalline substances, thus forming on the surface of 
 the Moon so many slightly luminous threads ? 
 
 [Mr. Birt has informed us, that some of these rays arc visible 
 under all illuminations ; one, which emanating from Tycho, ciosses 
 a crater on the north-east of Fracastorius, is not only distinctly 
 visible when the terminator grazes the west edge of Fracasto- 
 rius, but is even brighter as the terminator approaches it. Those 
 emanating from Tycho are evidently different in their character 
 from those emanating from Copernicus, while those from Proclus 
 form a third class. The rays from Copernicus and Kepler appear 
 to be very similar. One very bright ray, in the neighbourhood of 
 Geminus, we have found to coincide in direction with a ridge of 
 high land. 
 
 Mr. Nasmyth has been able to produce somewhat similar appear- 
 ances on a glass globe by tilling it with cold water, closing it up 
 and plunging it into warm water. This causes the enclosed cold water 
 to expand very slowly, and the globe eventually bursts, its weakest 
 point giving way and forming a centre of radiating cracks similar 
 to the fissures — if they be fissures — in the Moon.] 
 
 According to the \4ews of an eminent observer, M. Chacornac, 
 the ring- formed mountains, or craters, which form points of diver- 
 gence for these radiations, are of a relatively recent origin. At the
 
 ]50 THE SOLAR SYSTEM. 
 
 time of the eruption which produced these craters, the gaseous masses 
 escaping by the new volcanic vents, or becoming jirecipitated, swept 
 before them the pulverulent and whitish substances which covered the 
 summits of the neighbouring craters of anterior origin, or in case 
 of concentric divergence, the summit of the craters existing on the 
 same spot ; hence the long white bands which radiate from Tycho in 
 the direction of meridians having this volcano for a common pole. 
 This explanation of the singular luminous bands which radiate from 
 Tycho, Proclus, Aristarchus, Copernicus, and Euler, may, perhaps, 
 throw some light on the physical constitution of our satellite.* 
 
 The Rilles differ from the liuninous bands in that they are evidently 
 formed of two jDarallel slopes, more or less steep, leaving a sort of 
 sunken way between them. They appear bright in the full moon, and 
 in the other phases as dark lines, one of the two ridges projecting 
 its shadow on the bottom of the trench. 
 
 It was at first believed that these were ancient river-beds ; 
 but their form, often wider at the centre than at the extremities, 
 their immense breadth, which sometimes reaches 1-| miles, and still 
 more their depth, which varies between 450 and 700 yards, render 
 this hypothesis untenable. Besides which, their length is relatively 
 slight, being usually comprised between 10 and 125 miles. Lastly, 
 one circumstance which is frequently observed, and which will show 
 that it is not possible to consider them ancient river-beds, is, that 
 many of them traverse mountains, and cut through the sides of 
 high craters in such a way as to present the greatest diversity of 
 level. Some of them are widened in parts, and form oval valleys ; 
 others again present a series of small craters, joined together, f We 
 here reproduce (Plate IX) from the beautiful map of Beer andMitdler 
 two regions of the central mountainous parts of the Moon, which 
 contain some of the most curious of these ajipearances. 
 
 Beer and Madler, in their remarkable work " Der Mond," have 
 added 70 to the list, and point out, as an important fact, the con- 
 stancy of direction of the majority of thcm.+ All these facts tend to 
 
 * [The enormous length and smoothness of these rays, together with their 
 perfect imiformity of level, seem, however, to militate against any explanation 
 which has as yet been attempted. — T. W. W.] 
 
 t [They are not unfrequently met with in the interior of great walled plains, 
 a fact, perhaps, of some selenological import. — T. W. W.] 
 
 I Schroter, Pastorflf, Giniithuisen, and Lohrmann preceded the two German 
 astronomers in these interesting discoveries. 
 
 [Dr. Schmidt of Athens has been most indefatigable in this department of 
 Innar astronomy ; he has discovered no less than 278 of these curious formations
 
 Vk; 
 
 i 
 
 i ^ / 
 
 L_^ 
 
 PLATE IX. 
 
 
 Opj'^iiiiiuou 
 
 Drsrark' 
 
 ^^, ^^>^^?^<^-^ -'--<.; ->t'^'"' .>^'<' 'v'^v ^r'^x-'-y/ — »/-.i; ' 1/ 
 
 
 MlborsiJilai;' 
 
 Grave liiti Krb.:^iJ;^'jiiapirle 'iE . 
 
 LUNAR TOPOGRAPHY. 
 
 )i'ii;-.iic-i1\cr. 
 
 1. Walled PUain.s, Craters, and Rillcs. Rillcs of Abulfeda. 2. Billcs of the central regions near 
 
 the Sinus Merlii.
 
 THE MOOX. 153 
 
 show that these singular markings date from the last period of 
 geologic change on the lunar surface, and are, therefore, posterior 
 to the craters and ring-formations, as is proved by the rainnre of 
 Hyginus, which penetrates to the interior of this crater, breaking- 
 through its boundary wall. 
 
 making with previous discoveries 425, which he has arranged in classes ; tlio 
 order of discovery is as follows : — 
 
 om 1787 to 1801 Schroter discovered 
 
 11 
 
 „ 1823 „ 1827 Lohrmann . 
 
 75 
 
 „ 1823 „ 1841 Madler . 
 
 55 
 
 „ 1847 „ 1848 Kenan . 
 
 6 
 
 „ 1842 „ 1865 Schmidt 
 
 278 
 
 425 
 
 Mr. Birt has recorded an observation in which a rille appears to have been 
 diverted from its course by two craters, and the same rille, in a further part of its 
 course, is completely internipted hy another crater, as if the craters were of more 
 recent origin.] 
 
 [In connexion with Rilles, Mr. Mallet has in his report on Earthquake 
 Phenomena, p. 62, this remarkable passage : " A vast fissure (noticed by Hum- 
 boldt), and marked by an almost continuous line of volcanic rents, extends in a 
 direction nearly east and west, right across Mexico, between 18° and 19° lat. It 
 is nearly 500 miles in length. Its main direction if produced, bears upon the 
 volcanic island of Kevillegigedo, and, as Humboldt also thinks, probably extends 
 to Monna Roa in the Sandwich Islands. The Mexican extremity of this enormous 
 crevasse, probably marks the continental end of one of the great dividing ridges 
 of the sub-basins of the Pacific." It would be desirable to know the breadth of 
 this crevasse. — W. R. B.]
 
 154 J'HE SOLAR SYSTKJVI. 
 
 IX. 
 
 THE MOON. 
 
 PHYSICAL CONSTITUTION {cOllfilUied). 
 
 Absence of Air and Water on the Moon's Surfece — Has the Moon an Atmo- 
 sphere ? — Aspect of a Lunar Landscape — The Moon's Past History ; Professor 
 Frankland's Hypothesis based on Traces of Glacier-action — The Moon's Cli- 
 mate — Days, Nights, and Seasons — Extent of the Visible and Invisible 
 Portions of the Lunar Globe — Astronomy from a Lmiarian's point of view — 
 Lunar Photography ; the British Association " Moon Committee." 
 
 We have already supposed an irihabitant of the Earth landing on the 
 desolate lunar world bristling- with mountains and covered witli 
 thousands of volcanic vents. We have described him contemplating 
 with wonder this strange globe. But we ought to mention one fact, 
 which would render his sojourn much more than painful — impossible ; 
 namely, that he would not find, on the surface of the Moon, the most 
 indispensable elements to his existence, — air and water. 
 
 The Moon, indeed, it would appear, is entirely devoid of atmo- 
 sphere. 
 
 This fact seems demonstrated by the occultations of stars. "WHien, 
 by reason of the Moon's movement across the constellations, one of the 
 luminous points of the starry vault is covered by the darlc part of 
 the lunar disk, it is extinguished suddenly, without any gradual 
 diminution of its light indicating the presenc-e of a gaseous envelope. 
 This fact holds good with the smallest as with the largest stars, even 
 during the eclipses of the Moon, Avhen the terrestrial atmosphere is 
 no longer illuminated by our satellite. 
 
 If, moreover, an atmosphere, however slight its density, envel- 
 oped the lunar si^heroid, such atmosphere would refract, that is to 
 say, a star, after its real immersion behind the disk, would still remain 
 visible for an instant. In the same way, it would again become 
 visible on its emersion a little before its actual occultation had
 
 THE MOON. 155 
 
 terminated, so that the duration of the occiiltation woidd be, for 
 tAVo reasons, less than the time assigned by calcidation, and de- 
 duced from precise and mathematical knowledge of the movement 
 of the Moon. Now nothing like this has been observed. Hence, it 
 results that if the atmosphere of the Moon really exists, its density 
 is less than the 2000th part of the density of the Earth's atmosphere. 
 Such an atmosphere would be more rare than the vacuum which is 
 obtained, under the best conditions, in the most perfect air-pumps. 
 
 The only objections that can be made to the consequences di-awn 
 from the preceding fact, are, as Arago remarked, that the apparent 
 diameter of the Moon is not perhaps known with sufficient precision ; 
 and again, the singular phenomenon observed in the total eclipse 
 of the Sun in 1860, and pointed out by M. Laussedat, that the horns of 
 the solar crescent were truncated and rounded, near the Moon's limb. 
 There is also another point. It is known that the exterior edge 
 of the lunar disk forms a line unbroken in appearance, whilst 
 near the centre, the terminal ellipse, or terminator, marking the 
 separation of the light and shade, is deeply indented and irregular. 
 The cause of this difference is easily understood ; the summits of the 
 craters and peaks, situated at the edge of the disk, form a series of 
 undulations which are averaged and levelled by the effect of per- 
 spective, and prevent therefore a regular and uniform outline ; at 
 the centre of the disk, on the contrary, the irregularities are presented 
 to us in face, as in a bird's-eye view, so to speak, so that the simimits 
 illuminated by the light of the Sun stand out from the dark low^er 
 levels of the plains. But after all the uniformity of the limb is not 
 so decided that it can be argued that in an occupation of a star the 
 difference between the observed and the calculated times is, or is not 
 due to the existence of an atmosphere. 
 
 [Now, with regard to the recent discovery to which we have before 
 referred ; of the 2"-0, by which we now know that the Moon's apparent 
 diameter must be reduced, certainly a part, probably the whole, is 
 due to the irradiation of the telescopic semidiameter. But the reader 
 may perhaps attribute a part to refraction by the Moon's atmosphere. 
 If the whole were attributable to that cause, it would imply, ac- 
 cording to the Astronomer Royal, a horizontal refraction of l'*0, 
 which is only about the ^-^-^-^ part of the Earth's horizontal refraction ; 
 probably implying a tenuity of lunar atmosphere which would 
 make the atmosphere undiscoverable in any other way.] 
 
 Is it possible that there may be an atmosphere confined to the 
 bottom of the lowest plains and the deepest craters ? Nothing renders 
 probable or contradicts this hypothesis. But at all events no cloud
 
 156 THE SOLAK SYSTEM. 
 
 ever disturbs the purity of its sky ; for clouds, even of sliolit dimen- 
 sions, would be easily perceived from the Earth, and no convincing- 
 observations of any are recorded.* 
 
 In consequence of this want of atmosphere, the lunar land- 
 scapes have a very peciJiar aspect — the shadows have everywhere 
 the same blackness. At the most, the crudity of the bright and 
 luminous tints, which stand out on a nearly black sky, and of the 
 nearly black shadows, is tempered by reflexions, which are, however, 
 very numerous as the levels are so broken. Then, again, there is no 
 aerial perspective — none of those effects of light, of those cloud- tints, 
 which give our terrestrial landscapes so much charm and softness. 
 There refraction does not decompose sunshine into glorious colouring, 
 and a thousand varied tints ; the rainbow and other phenomena of the 
 same kind are unknown on the surface of the Moon. But then the 
 stars and the other celestial bodies shine in full day in the starry 
 vault. 
 
 Plate X may give an idea of the aspect of the landscape in the 
 mountainous parts of the southern hemisphere. 
 
 The absence of air on the surface of the Moon implies absence of 
 water. If there existed lakes, seas, or even rivers, the liquids forming 
 these reservoirs or currents, would be reduced to vapour by the fact 
 that they would not be maintained as such by atmospheric pressui-e. 
 But the solar heat, acting still more energetically, would develope a 
 gaseous envelope, — thick clouds of vapour. A cloud of 200 yards in 
 diameter would be easily visible. Now, as we have before said, no 
 moving object has ever been seen on the disk of the Moon. 
 
 No air and no water ! This implies, of necessity, absence of winds 
 and currents, — absence of motion everywhere — in the sky as on the 
 surface. At the most, under the influence of the alternations of heat 
 and cold, the disintegration of the rocks and the destruction of 
 equilibrium of the heavy bodies causing the fall of dehfis break the 
 monotony of the stillness and eternal silence. For sound, as it cannot 
 be communicated without an aerial medium, can only make itself 
 known by the contact of solid molecules. To an inhabitant of the 
 Earth, our light-giver by night would appear, according to the ex- 
 pression of Humboldt, but a silent and voiceless desert. 
 
 It has been said before that the large dark spots, which the first 
 
 * [After all fair deductions on tlie score of imperfection of observation or 
 precipitancy of inference, there are still residuary phenomena,— such as, for in- 
 stance, the extraordinary profusion of brilliant points which, on rare occasions, 
 diversify the Mare Crismm,—s,o difficult of interpretation, that we may judge it 
 wisest to avoid too positive an opinion. — T. W. W.]
 
 THE MOON. 159 
 
 observers took for seas, are now known to be vast plains, lower in 
 level than the valleys of the mountainous regions. One thing, 
 which, doubtless, in the first instance, increased the illusion, was, that 
 many of these spots appear of a light greyish green colour : others 
 are greenish grey, reddish, or, again, of a deep grey, like steel. The 
 absence of seas, Avaters, and — as a natural consequence — of rains, is 
 so much the more probable, as it well explains the present appearance 
 of the surface of the Moon, or, in other words, the geology of its 
 superficial strata.* " The Moon," says Humboldt, " is nearly such 
 as the Earth must have been in its primitive state, before being 
 everywhere covered, owing to the continuous action of tides and 
 cm-rents, with sedimentary beds rich in shells, gravels, and alluvium." 
 It is, necessary, however, to distinguish between the mountainous 
 regions and the regions of the plains. These latter oficr a much more 
 uniform surface, and it appears probable that it is owing to sedimentary 
 beds which are there deposited. 
 
 [Instead of seas they are most probably old sea-bottoms. 
 
 Such, then, are the results of the telescopic observations of the 
 side of our satellite turned towards us. Do we know anything about 
 the like conditions of the side turned away from us ? or, again, can 
 we dive into the past history of the Moon ? 
 
 The illustrious Hansen has held that it is quite possible that the 
 lunarians on the side away from us may possess both water and an 
 atmosphere, and that the side turned towards us may be regarded as 
 one vast mountain. Adams and Le Yerrier, however, have shown 
 that such a hypothesis is not very securely based. 
 
 Professor Frankland has perhaps provided us with some data 
 towards answering the second question. A study of the glacial epoch 
 on our OAAai globe, he asserts, renders it probable that the other 
 bodies belonging to our solar system have either already passed 
 through a similar epoch, or are destined still to encounter it. With 
 the exception of the polar ice of Mars we have hitherto obtained no 
 certain glimpse into the thermal and meteorological condition of the 
 planets ; and, indeed, the Moon is the only body whose distance is not 
 too great to prevent the visibility of comparatively minute details 
 upon her surface. Professor Frankland believes, and his belief rests 
 on a special study of the lunar surface, that our satellite has, like its 
 
 * [The long continuance of eruptive action, so distinctly marked by the suc- 
 cessive encroachment of moi-e recent craters upon the boundaries of older ones, 
 and the decrease of its euergy, equally traceable in the diminished magnitude ot 
 the results, are too evident to admit of a question. But many other features are 
 of a more equivocal character. — T. W. W.]
 
 160 THE SOL AH SYSTEM. 
 
 primary, also passed througli a g-lacial epoch, and that several, at 
 least, of the ralkys, rillcs, and strml-a of the lunar surlace, are not 
 improbably due to former glacial action. Notwithstanding the ex- 
 cellent definition of modern telescopes, it could not be expected that 
 other than the most oiffantic of the characteristic details of an ancient 
 glacier bed would be rendered visible. What then may we expect to 
 see ? Under favourable circumstances the terminal moraine of a 
 glacier attains enormous dimensions ; and, consequently, of all the 
 marks of a glacial valley this would be the one most likely to be first 
 perceived. Two such terminal moraines, one of them a double one, have 
 appeared to him to be traceable upon the Moon's surface. The first 
 is situated near the termination of that remarkable streak which com- 
 mences near the base of Tycho, and, passing under the south-eastern 
 wall of BuUialdus, into the ring of which it appears to cut, is gradually 
 lost after passing Lubiniezky. Exactly opposite this last, and ex- 
 tending nearly across the streak in question, are two ridges forming 
 the arcs of circles whose centres are not coincident, and whose external 
 curvature is towards the north. Beyond the second ridge a talus 
 slopes gradually down northwards to the general level of the lunai- 
 surface, the whole presenting an appearance reminding the observer 
 of the concentric moraines of the llhone glacier. These ridges arc 
 visible for the whole period during which that portion of the Moon's 
 surface is illuminated ; but it is only about the third day after the 
 first quarter, and at the corresponding phase of the waning Moon, 
 when the Sun's rays, falling nearly horizontally, throw the details of 
 this part of the surface into strong relief, and these appearances 
 suggest this explanation of them. The other ridge, answering to a 
 terminal moraine, occurs at the northern extremity of that magnificent 
 \'allcy which rmis past the eastern edge of Rheita. 
 
 With regard to the probability of former glacial, or even aqueous, 
 agency on the surface of the Moon, difficidties of an apparently very 
 formidable character present themselves.* There is not only now no 
 evidence whatever of the presence of water, in any one of its three 
 forms, on the lunar surfiice, but, on the contrary, all selenograpliic 
 observations tend to prove its absence. Nevertheless, the idea of 
 former aqueous agency in the Moon has received almost universal 
 acceptation. It was entertained by Gruithuisen and others. But, if 
 
 * [It may be objected to this ingenious theory that the traces of such an 
 action would be far more numerous, there being great probabihty that tliere 
 would ])e a rejrular gradation in their proportions, and an absolute certainty that 
 they would be visible in modern telescopes, even if of far less magnitude than 
 tliosc referred to.— T. W. W.l
 
 THE MOON. 161 
 
 water at one time existed on the surface of tlie Moon, Avhither has it 
 disappeared P If we assume, in accordance with the nebuhir hypo- 
 thesis, that the portions of matter composing respectively the Earth 
 and the Moon once possessed an equally elevated temperature, it 
 almost necessarily follows that the Moon, owing to the comj^arative 
 smallness of its mass, woidd cool much more rapidly than the Earth ; 
 for, whilst the volmne of the Moon is only about -J-gth, its surface is 
 nearly ^ 3th that of the Earth. This cooling- of the mass of the Moon 
 must, in accordance with all analogy, have been attended with con- 
 traction, which can scarcely be conceived as occurring without the 
 development of a cavernous structure in the interior. Mvich of this 
 cavernous structui-e would doubtless communicate, by means of 
 fissures, with the surface ; and thus there would be provided an in- 
 ternal receptacle for the ocean, from the depths of which even the 
 burning Sun of the long limar day would be totally unable to dislodge 
 more than traces of its vapour. Assuming the solid mass of the 
 Moon to contract on cooling at the same rate as granite, its refrigera- 
 tion, through only 180° Fahrenheit, would create cellular space equal 
 to nearly 14^ millions of cubic miles, which would be more than 
 sufficient to engulf the whole of the lunar oceans, supposing them to 
 bear the same proportion to the mass of the Moon as our OAvn oceans 
 bear to that of the Earth. 
 
 Now, if such be the present condition of the Moon, we can scarcely 
 avoid the conclusion that a liquid ocean can only exist upon the 
 surface of a planet so long as the latter retains a high internal 
 temperature. The Moon, then, becomes to us a prophetic picture of 
 the idtimate ftite which awaits our Earth, when, deprived of an ex- 
 ternal ocean, and of all but an annual rotation upon its axis,* it will 
 revolve round the Sun an arid and lifeless wilderness, one hemisphere 
 being exposed to the perpetual glare of the solar rays, the other 
 shrouded in eternal night.] f 
 
 The climate of our Satellite must be not less extraordinary than 
 its geology. During about fifteen days the Svm pours its rays, with- 
 out any cloudy curtain or aerial cvu-rent to temper them. To this 
 temperature, more intens.e even than that of our torrid zone, succeeds 
 an intense cold, which a night of fifteen days' length renders more 
 
 * [Mayer has recently proved that the action of the tides tends to arrest the 
 motion of the Earth upon its axis. And although it has been asserted that, since 
 the time of Hipparchus, the length of the terrestrial day has not increased by 
 the fijth part of a second, yet this fact obviously leaves untouched the con- 
 clusion to which Mayer's reasoning leads.] 
 
 + Professor Frankland, " Proc. Eoyal Institution," vol. iv. p. 17;). 
 
 M
 
 162 THE SOLAR S^SSTEM. 
 
 glacial than that of our polar winters. It is true, that during the 
 day the radiation of the solar heat into space again is not prevented. 
 We must conclude, therefore, that the climates of the various re- 
 gions of the Moon have a certain analogy with those of our Alpine 
 regions ; seeing that the depression of the temperature, and the rever- 
 beration of the intense light there, become insupportable by the 
 continuity of their action. 
 
 There are, properly speaking, no seasons on the Moon. The 
 slight inclination of its axis of rotation maintains the Sun at a nearly 
 constant inclination in each latitude. But whilst in the equatorial 
 regions the radiant body scarcely leaves the zenith ; at the middle of 
 the day, in the polar regions, it scarcely rises above the horizon. 
 The j)olar mountains enjoy perj)etual day.* 
 
 One can understand, also, that the inclination of the Sun to the 
 lunar surface, variable according to the latitudes, can never have on 
 the Moon the same importance as on the Earth ; since the rays, 
 whether luminous or calorific, are transmitted directly to the surface 
 without having to traverse atmospheric strata of unequal thicknesses. 
 
 The revolution of our Satellite is effected with variable velocity, 
 whilst its movement of rotation is uniform. Hence residts a want 
 of correspondence between the two movements ; and the Earth is 
 found sometimes to the east, sometimes -to the west, of the point of 
 space opposite to a fixed point of the surface of the Moon, considered 
 as the centre of the visible hemisphere. We thus discover regions 
 both at the eastern and western limbs, which, without this circum- 
 stance, would remain hidden to us. 
 
 Nor is this all ; the inclination of the plane of the lunar orbit, 
 added to that of its equator, to the plane of the terrestrial orbit, 
 causes the Moon to present to us sometimes the north, sometimes 
 the south pole, of its globe, and thus to uncover certain portions of 
 its polar regions which otherwise we should not see. 
 
 From these two lihrations, which is the name given to these 
 movements, it follows that of 1000 parts of the surface of the Moon, 
 569, or more than half, are visible to the Earth, whilst only 431 
 remain constantly hidden from us. 
 
 * " The Sun does not descend below the real horizon of a lunar pole, at the 
 most, to an angle greater than the inclination of the equator of the Moon ; that is 
 to say, 1" 30' ; but the smallness of the globe of our satellite is such, that at an 
 elevation of 650 yards we see 1° 30' below the true horizon. Now there exist st 
 the North Pole mountains upwards of 4000 yards in height ; consequently the 
 summit of these mountains can never be hidden from the light of the Sun." — 
 Beer and Mddler, " Fragments .lur les Corps Celestes'''
 
 THE MOON. 163 
 
 But as the dimensions of the Earth are very appreciable when 
 compared to its distance from the Moon ; it follows that an observer, 
 as he moves on the terrestrial spheroid, displaces the apparent centre 
 of the lunar disk, — or, what comes to the same thing, perceives the 
 different portions near the limbs. 
 
 The. effect of this displacement again increases the dimensions of 
 the portion of the Moon which is accessible to us, in such a manner, 
 that of 1000 parts 424 only remain definitely and absolutely hidden, 
 576 are visible to us. 
 
 From east to west, the part of the Moon which must for ever 
 remain imknown to us embraces 2780 miles ; from north to south, 
 2815 miles ; from the lat. of 40° north. To the same latitude south, 
 2690 miles. Whilst the same dimensions, calculated for the visible 
 surface, are respectively 3310, 3266, and 3390 miles, according to 
 Beer and Madler. 
 
 A complete zone, therefore, of the half of the Moon which is 
 turned away from the Earth, is accessible to the eyes of man. [So 
 much foreshortened, however, that our knowledge of great part of it 
 must always remain very defective.] 
 
 " Now, observations have not indicated," we quote these two most 
 diligent explorers of the Moon, " any essential difference between 
 those regions which form the seventh part of the lunar surface 
 generally hidden from our gaze, and those with which we are ac- 
 quainted ; the same mountainous countries and the same maria are 
 found there." Hence, it is most natural to conclude the similarity of 
 the invisible portions to those which we see. 
 
 That the part actually invisible will for ever remain imknown 
 to the Earth, follows from the searching analysis of Laplace. 
 
 To bring to an end the description of the physical particularities 
 which make the Moon a body so different from the globe which we 
 inhabit, let us see if the astronomical phenomena are the same for 
 her as for the Earth. Without examining into the interesting — 
 almost insoluble question, of the existence of living and organized 
 beings on the surface of the Satellite of our little Earth,* we shall 
 suppose an observer successively placed on each of its hemispheres. 
 
 * Others, more daring than ourselves, will doubtless cut the knot of this 
 difficulty. They will assert, with a great chance of being believed, that an or- 
 ganized being cannot live without air and water, and that the chmatic conditions 
 of the Moon are evidently opposed to such organisms ; we will not contradict 
 them. The cause of our reserve, however, is easy to understand. If, before having 
 observed any of the innumerable organisms which people the waters on our planet,
 
 164 THE SOLAR SYSTEM. 
 
 The phases of the Moon indicate, that she presents all the points of 
 her sphere to the Sun in an interval of 29,Vo ^^y^^ or as it may he 
 put, in about 709 hours ; each of these points, therefore, receives during 
 3541 hours the solar light and heat, for this is the length of the 
 Moon's day. During 354^ hours the same point is entirely deprived 
 of light and heat, this is the length of its night. From this point of 
 view there is an entire equality between the visible and the invisible 
 hemispheres. 
 
 The absence of atmosphere must give to the Imiar days a singular 
 aspect. The disk of the Sun, seen sharp and distinct, is deprived of 
 those rays which surround him to a great distance as seen from the 
 Earth. If it be true that the Sun is surrounded by an atmosphere, 
 this envelojDe should be clearly visible in the lunar sky, which ever}^- 
 where else, as we have said, remains dark, and even in broad day is 
 overspread with stars. 
 
 But the intensity of the light of the Sun and that of his direct 
 heat, are not the same at mid-day in each hemisphere of the Moon. 
 In fact, it is noon for the points of the lunar meridian which is 
 presented to us at the exact moment of full moon ; while, for the 
 other half of this meridian, our lunar antipodes, noon coincides with 
 the instant of the new moon. Now, in the first j)osition the Moon 
 is further from the Sun than in the second by double its mean 
 distance from the Earth, or by the 200th part of the distance of 
 the Sim from the Earth. So the apparent diameter of the Sun is 
 greater in the second case than in the first by about the two- 
 hundredth part. 
 
 During the nights of this latter hemisphere, the lunar observer 
 will constantly see the Earth under the form of a luminous disk, 
 14 times larger than the Moon in our 0"svn sky, and presenting 
 successively a series of phases analogous to her O'svn. The nights, 
 therefore, will never be quite dark, as in fact is indicated by the 
 Earth-shine. -At midnight, that is, at the Moon's midnight, the side 
 of which we speak — the one turned towards us, then invisible 
 
 and before having heard of their existence, any one had suddenly learned that it 
 is possible to exist, breathe, and move in water, and if he then referred to simple 
 experiment, which teaches that prolonged immersion in a hquid is fatal to all 
 the organisms known to him, even to man himself ; without doubt, the assertion 
 would cause him the greatest surprise. Such would be our surprise were it ever 
 demonstrated by facts beyond dispute, that living beings exist on the surface of 
 the Moon. Nature is so varied in its modes of action, so infinite in the manifes- 
 tations of its power, that nothing in Nature can be pronounced by man to be 
 absolutely impossil ilc.
 
 THE MOOX. 1{;5 
 
 because it is lost in the Sun's rays — will have full Ediili. The 
 light, which she then receives from the luminous disk of our planet, 
 is equal to that which would be received by ourselves, if 14 full 
 Moons equal to our o^^ti were at the same time lighting up our 
 evening sky. 
 
 On the other hand, the Earth is unknown to the lunarian ob- 
 server situated on the invisible hemisphere, and the darkness of the 
 nights there can only be imagined by bearing in mind that they 
 are tempered by no twilight, and that the only illumination received 
 by that hemisphere is star-light. Between these two regions, which 
 form together six-sevenths of the surface of the Moon, is the 
 zone, near the limb, which comprises the parts in which the Earth 
 is sometimes in view, sometimes invisible. In this zone the Earth rises 
 and sets, but its disk rises only a short distance above the horizon. 
 
 In the visible hemisphere, the phases of the Earth, the observa- 
 tion of the different features which appear and disappear in turn 
 by the eifect of rotation serve as a clock ; it is a dial, all but fixed 
 in the same point of the sky, like an immense lamp, behind which 
 the stars defile slowly along the dark sky. 
 
 As to those regions of the Moon which are invisible to the Earth, 
 as soon as the Sun has disappeared below their horizon they are 
 suddenly plunged into the deepest night. During 350 hours, an 
 astronomer, if he were transported to such a favourable sky, would 
 be able to carry on his observation of planet and star unimpeded by 
 cloud, moon, or twilight. Another difference which characterises 
 the invisible hemisphere is, that there the Sun is never eclipsed, 
 whilst in the hemisphere turned towards us solar eclipses last some- 
 times two hours. 
 
 Here, then, we may bring our notice of the details of the lunar 
 world and its singular constitution to a conclusion. The phenomena 
 to which we have just referred — Eclipses of the Sun and Moon — 
 which are invested with such absorbing interest, now demand our 
 attention. 
 
 [Before, however, parting company with the Moon, we would 
 refer the reader who would know more about her to the Rev. T. W. 
 Webb's work, " Celestial Objects for Common Telescopes." That 
 observer has for many years made the Moon his special study. A\ e 
 may also congratulate ourselves that at last, thanks to the example 
 set by Mr. Webb, Mr. Birt, and other observers, and to the advance 
 of lunar photography in the hands of Mr. De La Rue, whose latest 
 result is an exquisite photographic portrait of the Moon, 38 inches m 
 diameter, the investigation of the Moon's surface is being taken up
 
 166 
 
 THE SOLAR SYSTEM. 
 
 in earnest, and there is now a standing "Moon Committee" of tlie 
 British Association, under whose auspices good work is being done ; 
 and a map, 8 feet 4 inches in diameter, is being constructed by Mr. 
 Birt, whose perseverance in this field of investigation is worthy of 
 all praise * In spite of the general excellence of Beer and Madler's 
 map, some parts so ill represent the actual appearance of the Moon, 
 that in some minds the idea has arisen of changes actually going 
 on. This is improbable, but we must wait for a larger map and 
 for more observations before we can positively assert that 'this is 
 not the case. As has been pointed out by Mr. Grove, observations 
 of the geologically or rather selenologically recent formations will 
 from this point of view be the most hopeful and valuable. We may 
 venture to express our belief that this and similar work will be best 
 done by filling in from eye-observations a skeleton map prepared 
 from photographs of the various regions, on the largest scale possible, 
 reduced to a mean Hbration.f] 
 
 * [The labours in lunar astronomy, more especially as regards the Moon's 
 surface, of Julius Schmidt of Athens, have been very extensive. Since the year 
 1842 he has made and calculated 4000 micrometrical measures (made at the 
 observatory of Olmutz, between 1853 and 1858) of the altitudes of lunar moun- 
 tains. In addition to these he has nearly 1000 original sketches, which he is 
 now engaged in combining into a map of three feet radius.] 
 
 t [Since this was written, the Moon Committee have determined upon con- 
 structing such a skeleton map as here proposed, and zones 1° or 2° broad will be 
 distributed among different observers. We may thus hope for a complete map on 
 an adequate scale in a comparatively short space of time.]
 
 ECLIPSES OF THE SUN AND MOON. 167 
 
 X. 
 
 ECLIPSES OF THE SUN AND MOON. 
 
 General Theory of Eclipses — Eclipse of the Sun can only take place at the time 
 of New Moon — The Eclipses of the Moon happen at Opposition — Why each 
 Lunation is not accompanied by Two Eclipses. 
 
 When the moveinents of the Moon and the Earth bring these two 
 bodies in such a position, that their centres and the centre of the 
 Sun are all in the same straight line, the phenomenon which follows 
 from this particular situation of the three celestial bodies is what is 
 called an Eclipse. If it be the Moon which occupies the intermediate 
 position, it tm-ns its dark hemisphere towards the Earth ; and the 
 interposition of its black disk between us and the luminous body 
 of the Sun prevents the rays of the latter reaching us, and an 
 Eclipse of the Sun is produced. If it be the Earth which occupies 
 the mid- interval, our globe acts as a screen ; the liuiar hemisphere 
 turned towards the Sun no longer receives his rays, its disk is 
 obscured, and we have an Eclipse of the Moon. 
 
 But this way of considering the phenomena is only from our 
 own point of view. In reality, in both these cases, there is simul- 
 taneously an eclipse to each of the three bodies in question. 
 
 What happens, in fact, in the first case ? 
 
 To an observer placed on the Sun, the Moon seems projected on 
 the Earth, hiding a portion of the surface, although it is true that the 
 two superposed disks, as they are both luminous, would not permit the 
 darkened part of the surface of the terrestrial globe to be seen from 
 the Sun. An observer situated on the dark hemisphere of the Moon 
 will perceive an eclipse of the Earth, that is to say, a successive dark- 
 ness over all the regions of our globe in which the eclipse of the Sun 
 is visible. Lastly, in the case which produces an eclipse of the Moon as 
 seen from the Earth, there is also an eclipse [occultation] of the Moon
 
 168 
 
 THE SOLAR SYSTEM. 
 
 to the Sim, whilst there is an eclipse of the Sim to the lunar 
 hemisphere turned towards us. 
 
 Eclipses may be regarded and explained in another way. 
 
 Fig. 65. — General theory ot Eclipses. 
 
 The Earth and the Moon are two spherical and opaque bodies, 
 and the halves of both are constantly illuminated by the rays of the 
 Sun, whilst the other halves are in the shade. The illuminating body
 
 ECIJPSKS OF THE SUX AND MOON. 169 
 
 is itself a sphere of mucli greater dimensions. Not only, therefore, 
 have the Moon and the Earth always one of their hemispheres dark, 
 but each of these two bodies throws behind it, away from the 8mi, a 
 shadow of conical form, the length and diameter of which depend 
 on the distance and diameter of the illuminating body, and the 
 diameter of the illuminated body. 
 
 This cone of shade encloses all those parts of space where, by 
 reason of the interposition of the opaque body, no ray of light 
 from the Sun can be received. Beyond the summit of this cone 
 of pure shadow — of iiuihra — and in its prolongation, are situate all 
 tliose points of space which see a part of the Sun, vmder the form of 
 a luminous ring, bordering the obscure disk of the opaque body. 
 
 Lastly, these two regions are themselves surrounded by what 
 is called a penumhra. Every part of space situated in the penumbra 
 only receives light from one part of the Sun, the luminous disk of 
 which seems partially invaded b}^ the obscure disk of the opaque bod}'. 
 The darlaiess produced by the penumbra is so much more intense 
 as the point in question is nearer the umbra. 
 
 The Moon and the Earth, in their movements, carrj^ with them 
 tlieir cones of umbra and penumbra, and it is by projecting these 
 total and partial shadowings one on the other that they produce the 
 phenomena of eclipses. 
 
 Now, if we look at fig. 65, it will be at once seen why an 
 eclipse of the Sun, when it does happen, always takes place at the 
 moment of the new Moon, and why, on the contrary, an eclipse of the 
 Moon is only possible at the period when our satellite is in opposi- 
 tion, that is to say, at the moment of full Moon. In all the other 
 positions of our satellite, that is to say, in all the other phases of 
 the lunation, the lunar cone of shade is projected into space away 
 from the Earth, and the terrestrial cone of shade does not meet 
 the Moon. 
 
 This is confirmed by all the observations of eclipses. It does not, 
 however, follow that there is an eclipse at every full Moon, or at 
 each new Moon ; and the reason for this is not far to seek. 
 
 There would be really two eclipses in each lunar month, one of 
 the Sun, the other of the Moon, if the orbit of tlie Earth round tlie 
 Sun and the orbit of the Moon round the Earth were described in the 
 same plane. Then, at the epoch either of opposition or of con- 
 junction, the centres of the three bodies would be necessarily in a 
 straight line. 
 
 But it has been seen that this is not the case. The orbit of the 
 Moon is inclined to the jjlace of the ecliptic, so that it often
 
 170 
 
 THE SOLAR SYSTEM. 
 
 happens that, at the moment of ihe new Moon, our satellite projects 
 its cone of shade above or below the Earth. Similarly, at the period 
 of opposition, the Moon, in consequence of its position out of the 
 plane of the ecliptic, passes sometimes above, sometimes below the 
 terrestrial cone of shade. Every time that this happens of course 
 there is no eclipse. 
 
 Let us see, then, what conditions are necessary for an eclipse of 
 the Sun or the Moon. 
 
 The orbit of the Moon, we repeat, is situated in a plane which 
 makes with the plane of the terrestrial orbit a certain angle, nearly 
 constant. 
 
 It follows that half of the monthly revolution is effected above 
 this latter plane, whilst the other half is accomplished below it. 
 The Moon then passes through the ecliptic twice every lunation. 
 
 The two positions which it occupies during these passages are the 
 Nodes. One is called the ascending node, the other the descending 
 node; because they correspond, the first to the movement of the 
 Moon when it rises from the south side to the north side of the 
 ecliptic, the second to the inverse movement. 
 
 If the nodes remained invariable in their relative positions with 
 regard to the Sun, one of two things would happen ; either there would 
 be no eclipses at all, or there would be two in each lunar month. But 
 the nodes are displaced from one lunation to another ; and it is easy 
 to comprehend that an eclipse will take place every time that they 
 coincide with the phases of the full and the new Moon — with the 
 st/zi/gies, as they are called. This coincidence need not be absolute ; 
 it suffices that the nodes be so near these phases, that the size of 
 the cones of shade makes an immersion either of the Moon or of the 
 Earth possible. 
 
 Such is the first general condition of possibility of these phe- 
 nomena. There are still others which are proper to each kind of 
 eclipse, which we shall discuss in describing the two kinds of 
 eclipses separately.

 
 VL . Ill 
 
 J 
 
 
 TOTAL ECLIPSE OF THE SUN 
 
 OK JULY, 1860. 
 
 Telescopic views of the red flames , from a drawing 
 of M^ Warren del a Rue. 
 
 J'.lAckerba.uer del. 
 
 imp Becaa e t Fsris .
 
 ECLIPSES OF THE SUN. 171 
 
 XI. 
 
 ECLIPSES OF THE SUN. 
 
 Conditions of the Possibility and Visibility of Eclipses of the Sun — Total, 
 Annular, and Partial Eclipses — Path of the Moon's Shadow along the 
 Earth — Longest possible Duration of Solar Eclipses — The Corona, Red Pro- 
 minences ; they belong to the Sun ; their Shape and Height — Influence of 
 the Phenomena of Eclipses on Living Beings. 
 
 Solar eclipses are of three kinds. Some are total ; tlie dark 
 disk of tlie Moon then entirely covers the Sun. Others are partial ; 
 that is, a portion only, large or small, of the solar disk is 
 eclipsed. Lastly, there are annular eclipses, which take place when 
 the disk of the Moon is not large enough to entirely cover that of 
 the Sun, and leaves a luminous ring visible round its own body. 
 
 As the Moon is much smaller than the Smi, it will be understood 
 that it is its small relative distance which causes its disk to appear 
 of equal, and even greater dimensions than that of the Smi. This 
 distance varies by reason of the elliptical form of its orbit, and hence 
 the dimensions of the lunar disk are sometimes larger, sometimes 
 smaller than, and sometimes equal to, those of the Sun. 
 
 This is the same as saying that the cone of real shadow or mnbra, 
 projected by the new Moon towards the Earth, reaches or does not 
 reach the svirface of our globe. If it reach this surface, there is a 
 total eclipse to all the parts of the Earth which are plunged in it ; 
 a partial eclipse to all the regions contained in the penumbra. This 
 will be understood from the following figure. 
 
 If the cone of the Moon's shadow does not reach the Earth, there 
 will be an annular eclipse visible in those parts comprised in the 
 prolongation of the cone ; a partial eclipse to those which are only 
 fomid in the penmnbra. This case is represented by the next figure. 
 
 It will be seen, therefore, that the conditions of the possibility of 
 a total eclipse of the Sun are the following : —
 
 172 
 
 THE SOLAR SYSTEM. 
 
 The Moon must be in conjimction, that is, she must be new ; 
 
 8he must at the same time be near a node ; 
 
 Lastly, her distance from the Earth must be less than the length 
 of the cone of shadow projected by her into space. 
 
 The same conditions, except the last, are necessary for an 
 annular eclipse. 
 
 Fig. fi<i.— Total Eclipse of the Sun; Theory. 
 
 Those who are accustomed to I'ead in scientific journals or in 
 almanacs the announcements of eclipses, which are calculated long- 
 beforehand by astronomers, must often have noticed these words ; 
 iiiri.sib/c at London (or some other place). An eclipse of the Sun, 
 (we shall speak further on of those of the Moon) may then take 
 
 Fig. 67.— Annular Eclipse of the Sun ; Theory. 
 
 place without being visible to all parts of the Earth. A little 
 thought will convince us of this, and make it easy to account for the 
 circumstance. 
 
 First, it is evident that there will be no eclipse at those places 
 where the Sun remains invisible during its entire duration ; secondly, 
 in many places which have the Sun above their horizon, if the Moon's
 
 ECLIPSES OF THE SUN. 
 
 173 
 
 sluulow is not large enoiig'li to cover the illuminated surface, there 
 will be no eclipse. 
 
 But the Moon has a diameter nearly four times less than that of 
 the Earth. Its cone of shade, therefore, at its greatest, is much too 
 small to enshroud the whole Earth ; and near the extremities of this 
 cone its dimensions are small enough to throw on the surface of our 
 globe but a very small circle of shadow, about 50 miles in diameter. 
 An eclipse of the Sun is then only total at the same instant, in a 
 circle of these dimensions. But the rotation of the Earth and the 
 
 Fig. 08.— Tutal Eclipse of the Sun of the ISth July, 1800. Path of the shadow aiul iicuumbra 
 ou the surface of the Earth. 
 
 movement of translation of the Moon combined, cause the cone of 
 shadow to travel in reality over a very large surface, tracing a dark 
 curve on the surface of the continents and seas.* 
 
 * The length of the cone of shade projected by the Moon into space, varies 
 between 57 and 59 radii of the Earth. On the other hand, we have seen that 
 the distance between the centres of the Earth and the Moon also varies between 
 57 and 64 terrestrial radii. From the centre of the Moon to the nearest point 
 of our globe there are then from 5(5 to 63 of these radii.
 
 174 THE SOLAR SYSTEM. 
 
 The same observations apply to the penumbra. 
 
 Thus, according to the position of pLaces relatively to the Sun 
 and to the Moon, the first of these bodies may be eclipsed totally or 
 partially, or even appear only in simple contact with the obscure 
 disk of our Satellite. 
 
 The astronomical theories of the movements of the Moon and of 
 the Earth are now so perfect, that astronomers can predict these 
 phenomena with the most wonderful precision. Not only does the 
 calculation indicate the day of the eclipse ; but the exact second, the 
 time, the dimensions or phases of the phenomenon for every spot of 
 the Earth are given. Maps are generally added to these numerical 
 details, and show those parts of the Earth where the eclipse will be 
 visible. 
 
 We have drawn a map of this kind, for the total eclipse which 
 took jjlace on the 18th of Jn\j, 1860, according to the indications of 
 the " Connaissance des Temps" and the " Nautical Almanack," — 
 works published many years in advance for the benefit of astronomers 
 and na\"igators. 
 
 A curve in the form of go marks the points of the globe where the 
 eclipse commenced or ended at sunrise or sunset. Another line, which 
 cuts the first in two parts, passes through those places which only 
 saw half the eclipse, because the middle of it coincided at those 
 places, either with the rising or setting of the Sun. 
 
 One line, darker than the rest, marks the line in which the eclipse 
 was total and central. Parallel to this line, other lines which are not 
 marked on the diagram would indicate the regions where the partial 
 eclipse was visible imder smaller and smaller phases,* imtil the line is 
 reached which limits the phenomenon, passing through all the places 
 where the eclipse is reduced to the simple contact of the disks of 
 the Sun and Moon. 
 
 The black Ime of central eclipse is in reality but the path of the 
 shadow thrown by the Moon on the surface of the Earth, as the 
 complete figure represents the path of the penumbra on the same 
 surface. 
 
 The duration of an eclipse of the Sim is variable. But we must 
 distinguish carefully between the total duration of the phenomenon 
 on the whole earth, and on any given place. We here give, accord- 
 ing to the calculations of Dionis de Sejour, cited by Arago, a table 
 showing the greatest possible duration of the difierent phases : 
 
 * Astronomers formerly expressed the size of the phases by the number of 
 digits, a digit being the twelfth part of the diameter of the solar disk. Thus, if 
 the jjhase was f, digit, the Moon's limb extended to the centre of the Sun.
 
 ECLIPSES OF THE SIX. 175 
 
 
 Gi-eatest j 
 h. 
 
 ■!Ossibl 
 m. 
 
 e duration 
 s. 
 
 A total eclipse . 
 
 r At the Equator . , . 
 "[ At the latitude of Paris 
 
 4 
 3 
 
 29 
 
 26 
 
 44 
 32 
 
 Annual phase 
 
 f At the Equator . . . 
 1 At the latitude of Paris 
 
 
 12 
 9 
 
 46 
 56 
 
 Total obscurity . 
 
 r At the Equator . . . 
 1 At the latitude of Paris 
 
 
 7 
 6 
 
 58 
 10 
 
 The total eclipse of the Siui, of which the preceding figure gives 
 the path on the surface of the terrestrial globe, commenced at hour 
 3 minutes p.m., Paris mean time, and ended at 5 hours 6 minutes p.m. 
 after having lasted in the whole of its phases 5 houi's 3 minutes. At 
 Paris, where the eclipse was only partial, the duration of the phe- 
 nomenon was only 2 houi's 14 minutes. 
 
 Total eclipses of the Sim are very rare, even for the Earth in 
 general ; they are much more so for particular places. From the 
 16th until the beginning of the 19th century, there were altogether 
 nine total eclipses of the Sun, and seven annular ones. Paris, during 
 all the 18th century, only witnessed a single total eclipse, that of 
 1724. London also, as little favoured as the capital of France, has 
 not seen one since 1715. 
 
 Since 1801, seven total eclipses have been observed, those of 1806, 
 1842, 1850, 1851, 1856, 1860, and 1861. We give here those which 
 will take place before the end of the present century, with the places 
 where they will be total : 
 
 1870. 22nd December Azores, south of Spain, north of Africa, 
 ^ "^ ', '''■■^-> I i 7 S Sicily, and Turkey. 
 
 1887. 19th August . North-east of Germany, south of Russia, 
 
 Central Asia. 
 1896. 9th August . . Greenland, Siberia, and Lapland. 
 1900. 8th May . . . Spain, Algeria, Egypt, the United States. 
 
 None of them will be total at London. 
 
 The eclipses of the Sun and Moon no longer are privileged to 
 excite fear, at least among civilised nations. Instead of a super- 
 stitious terror, they inspire an interest of curiosity. Announced 
 a long time beforehand, they testify to the precision of astronomical 
 calculations ; and all are getting accustomed by degrees to admire the 
 fixed laws, order and harmony, where formerly ignorance supposed 
 but accidents, precui'sors of evil, and testimonies of the celestial anger. 
 
 As to the astronomer, he finds in them matter for researches of the 
 highest importance. Even the partial eclipses, the least interesting 
 of all, give him occasion to verify the exactitude of his tables, by the
 
 176 THE SOLAR SYSTEM. 
 
 agreement, or otherwise, between the hour predicted by calculation, 
 and the hour really observed. But it is the total eclipses, especially 
 the last ones— those of 1842, 1850, 1851,' 1858, 1860, and 1861,— 
 which have been so fertile in new and precious facts. , 
 
 We propose to give a brief description of these facts, besides 
 placing luider the eyes of the reader, in Plates XI. and XII., draw- 
 ings which represent some of the various phenomena observed. 
 Let us follow the phenomenon in its ^progressive march. 
 It is always the western border of the Smi which first receives 
 the impression of the contact of the Moon ; and consequently it is 
 the eastern border of the lunar disk which by degrees encroaches 
 on the radiant bod}^, until it covers it entirely. The eclipse is 
 necessarily .partial, therefore, before the moment when the last 
 luminous thread disappears. The total obscuration, — the totality, as 
 astronomers call it, — then connnences. At the end of some minutes 
 a fine luminous thread appears at the western edge, and the partial 
 eclipse passes, in inverse order, through the same phases as in the first 
 j)art of the phenomenon. There are then, in all, four contacts of the 
 two disks — two exterior contacts and two interior ones. 
 
 Attempts have been made to prove by the form of the horns of the 
 liuninous crescent the existence of a lunar atmosphere. Most ob- 
 servers have seen nothing. Nevertheless, the eclipse of the 18th of 
 July, 1860, furnished a curious fact on this point : one of the horns 
 of the solar crescent appeared roimded and trimcated. At the other 
 extremity a contraction was remarked, which was followed by the 
 separation of a luminous point, and of a trimcation identical with the 
 
 first. To M. Laussedat we owe 
 the communication of the photo- 
 graphic negative obtained by him : 
 the drawing (fig. 69) is an exact 
 reproduction. 
 
 [The phenomenon observed would 
 appear to be somewhat similar to the 
 peculiar notched appearance some- 
 times presented, called " Baily's 
 Beads." These, however, are con- 
 Fig. 09. -Total Eclipse ufthu Suu of the isth sidcrcd by Mr. Dc La Rue to arise 
 oft^hlsS'tSti^^cmw from atmospheric disturbance. This 
 «'='i=^t-) and the irregularity of the Moon's 
 
 limb are, doubtless, sufficient to account for the singular appearance.] 
 Some minutes before and after, but especially dm-ing the totality, 
 a, luminous appearance in the form of a halo surrounds the Smi,
 
 PLATE XII. 
 
 ECLIPSES OF THE SUN. 
 
 1. Annular Eclipse. 2. Annular Eclipse ot the lath May, 1836, showing "Daily's Beads." 3. Total 
 Eclip.se of the 28th July, 1S.51 (Dawes). 4. Eclipse oflSoS (Liais). .5. Total Eclipse of the 18th 
 .Tuly, ISOO (Feilitzsch) ik Total Eclipse of the Sth July. 1842. 
 
 N
 
 ECLIPSES OF THE SUN. - 179 
 
 and throws in every direction rays of light, separated by dark spaces. 
 In many total eclipses, independently of the regidar corona, other 
 light portions, the rays of which have directions more or less eccen- 
 tric, have been remarked irregtdarly situated on its contour. Plate XII 
 shows in detail the coronas of several total eclipses. The colour of 
 the corona which immediately siu-roimds the dark disk is sometimes 
 of a pearly or silvery white, sometimes yellowish, and even red. 
 
 The explanation generally given of this corona is, that it indicates 
 the existence of a solar atmosphere, enveloping the radiant body to 
 an enormous distance. 
 
 We now come to a phenomenon of great interest, which was 
 noticed for the first time in the total eclipse of 1842, and which has 
 since been the object of important and minute observations. 
 
 Prominences of various forms and of a reddish colour were 
 visible throughout the contoiu' of the Moon's limb, during the period 
 of totality. Some took the form of mountain-peaks, others rose 
 normally from the disk and tm^ned at right angles, others, again, 
 appeared completely detached, as floating clouds might do. Their 
 tint was sometimes of a bright red, sometimes rosy, here and there 
 varied by greenish-blue. Arago regarded the latter colour as a 
 simple efiect of contrast. 
 
 It has now been proved, beyond all question, that these protube- 
 rances belong to the Sun. If we examine with care Plate XI, the 
 two drawings which we reproduce from the interesting memoir of 
 Mr. Warren De La Eue, on the total eclipse of the Sun of the 18th 
 of July, 1860, representing these remarkable phenomena at the 
 beginning and at the end of the totality, this fact will appear evi- 
 dent. 
 
 [It may be further added, that the photographic evidence collected 
 by Mr. De La Rue in his imequalled efibrts, although perfectly 
 satisfactory, does not stand alone ; a comparison made by Father 
 Secchi and Mr. De La Eue, between the photographs obtained by 
 both, shows that at an interval of seven minutes of absolute time, 
 the prominences observed at the two stations were identical.] 
 
 As soon as the last luminous thread of light disappeared behind 
 the eastern edge of the Moon, the rose-coloured prominences were 
 seen on the contour of the limb, where the solar crescent had just 
 disappeared. On the opposite side — the western one — they were not 
 yet entirely visible ; their tops only extended beyond the obscure disk, 
 at its upper and lower parts. The Moon, advancing, hid by degrees 
 the prominences first observed, exposing to view, at the opposite 
 side, those previously covered.
 
 180 THE SOLAR SYSTEM. 
 
 The facts, then, occur absohitely as the hypothesis, now accepted 
 on all hands, requires ; namely, that the prominences do not belong- 
 to the lunar disk, and are not optical effects caused by its presence, 
 but are absolutely part and parcel of the Sun. 
 
 They were first supposed to be enormous mountains on the 
 surface of the Sun. But the form of many of the prominences, and 
 their occasional complete separation from the solar disk, soon caused 
 this hypothesis to be abandoned. All the observed facts lead to the 
 conclusion that these immense appendages, the dimensions of which 
 reach 25,000 and even 50,000 miles, in height* and length, are 
 possibly clouds, here adherent to a continuous stratum, which re- 
 poses on the Sun, here floating in an atmosphere limited by the 
 corona. 
 
 [It has been suggested that they may be solar aurora).] 
 
 The intensity of the illumination of the atmosphere naturally 
 diminishes gradually during the entire duration of a total eclipse, 
 from its commencement until the beginning of the totality, to again 
 as gradually recover its primitive intensity. This obscurity, during 
 the phase of totality, is, however, very far from being complete. 
 Thus only the brightest stars, and some of those of the second mag- 
 nitude, are seen. The planets Venus and Mercur}', Jupiter, Mars, 
 and Saturn, however, have been likewise observed. 
 
 Terrestrial objects take bj' degrees a livid hue ; they are coloured 
 with various tints, among which olive-green predominates. Orange, 
 yellow, vinous-red, and copper tints, give to the landscape a sin- 
 gular appearance, which, joined to a very perceptible lowering of 
 temperature, contributes to produce a profound impression on all 
 animated beings, 
 
 Arago thus describes the attitude of an entire population, awe- 
 impressed by the magnificent and solemn spectacle offered by the 
 total eclipse of the 8th of July, 1842. 
 
 "At Perpignan, people dangerously ill alone remained in their 
 rooms. The population from early morning covered the terraces, 
 the ramparts of the town, and the hills outside, whence they hoped to 
 see the rising of the Sun. At the citadel, we had under our eyes, 
 besides the numerous groups of citizens on the glacis, the soldiers 
 who were being reviewed in the vast court. 
 
 "The hour of the commencement of the eclipse approached. 
 Nearly twenty thousand people, with smoked glasses in hand, ex- 
 
 * The highest prominence, in the form of a peak, measured by ^h: Warren 
 T)e La Rue in 1860, was 45,000 miles in vertical height above the solar surface.
 
 ECL11'!>ES OF THE SUN. 181 
 
 aniined the radiant globe, projected on au azure sky. Scarcely, 
 armed with our powerful telescopes, had we begun to perceive a little 
 encroaclimeut on the western border of the Sun, when an immense 
 shout, mixed with a thousand diflFerent exclamations, told iis that we 
 had anticipated only by a few seconds the observations made with 
 the naked eye by twenty thousand improvised astronomers. A lively 
 curiosity, an emulation, a desire not to be beaten, seemed to have given 
 to the imarmed sight an extraordinary penetration. 
 
 " Between this moment and that which preceded the totality, we 
 remarked nothing in the behaviour of the spectators which deserves 
 relating. But, when the Sun, reduced to a narrow thread, began to 
 throw on our horizon but very feeble light, a sort of inquietude 
 seized upon every one ; each felt the desire to communicate his im- 
 pressions to those by whom he was surrounded. Hence, a dull roar 
 like that of the distant sea after a tempest. The uproar became 
 stronger in proportion as the solar crescent became thinner. The 
 crescent disappeared ; at last, darkness suddenly succeeded to light, 
 and an absolute silence marked this phase of the eclipse, as absolutely 
 as the pendulum of our astronomical clock. The phenomenon, in its 
 magnificence, triumphed over the petulance of youth, the careless 
 air which some men take for a sign of superiority, and over the noisy 
 indifierence ordinarily assumed by soldiers. A profound calm also 
 reigned in the air ; the birds ceased to sing. 
 
 " After a solemn waiting of abovit two minutes, transports of joy — 
 frenzied plaudits greeted, with the same accord, the same sj)ontaneity, 
 the reappearance of the first solar rays. . . ."* 
 
 Animals testify, by unmistakable signs and movements, the efiect 
 which eclipse-phenomena produce upon them. Vegetation even is 
 not altogether unafiected. In 1842, the leaves of certain plants were 
 shut. Diu-ing the eclipse of July, 1860, M. Laussedat, who observed 
 it in Algeria, relates this fact : — " The plants showed how rapid is 
 the action of light, which they receive as by a kind of difiused sense 
 in their corollas, for, in spite of the short duration of the totality, 
 daturas, convolvuli, poppies, and night-shades, which had been closely 
 shut, were observed to half open diu-ing the total eclipse." 
 
 The observations made during total eclipses of the Sun are ver}^ 
 important and interesting from a physical and astronomical point of 
 view, but so numerous that they woidd fill volumes. We shall 
 confine ourselves, therefore, to saying one word more on the phe- 
 nomenon of the fringes of the waves, alternately light and dark, 
 
 * " Anuuaire du Bureau des Longitudes," 1846, pp. 303-5.
 
 182 THE SOLAR SYSTEM. 
 
 whicli sweep over the Earth, in a direction perpendicular to their 
 length, and the direction of which, when carefidly measured, has been 
 found to be parallel to the tangent at the first point of interior contact. 
 These fringes are "referred by M. Faye,* to an elFect of oblique mirage, 
 produced by a difference of density in the atmospheric strata which, 
 compose the cone of the mnbra. 
 
 [It is to be hoped that the famous " Himalaya " expedition to Spain 
 in 1860, to watch the eclipse which occurred in that year, will on 
 all future similar occasions be drawn into a precedent. Mr. "Warren 
 De La Rue's Memoir, published in the " Philosophical Transactions," 
 containing a complete account of the results of all his observations, 
 and especially of his photographic ones, is at present the most valuable 
 contribution to this branch of our subject possessed by astronomers. 
 We would gladly, if space permitted, make large extracts from it.] 
 
 * This interesting phenomenon was observed with minute care by MM. Laus- 
 sedat and Mannheim, members of the commission sent by the Polytechnic School 
 to Batna (Algeria), in July, 1860. They furnish us with the first exact measures 
 of the direction and rapidity of these phenomena. The following year (during 
 the eclipse of 1861) a French officer, M. Poulain, repeated the measures, according 
 to the indications of M. Mannheim. The Monthly Notices of the Royal Astrono- 
 mical Society of London, mentioning in 1862 this last observation, have omitted 
 (we know not why) to refer to the original observation published in detail in 
 the Coni'ptes Rendus de V Academie des Sciences de Paris, and in the Annales de 
 Physique et de Chimic, in the year 1860.
 
 TOTAL AND PARTIAL ECLIPSES OF THE MOON 
 
 The colour of the eclipsed portion is due to the refraction of the Sun's Heht 
 
 bj the atmo sphere of the Earth. 
 PJ^acIcerbduer del. 
 
 Jmp.Bt 
 
 ecquei lar.s
 
 ECLIPSES OF THE MOOK. 183 
 
 XIL 
 
 ECLIPSES OF THE MOON. 
 
 Conditions of Possibility and Visibility of Eclipses of the Moon — Partial and 
 Total Eclipses — Colour of the Lunar Disk during the Phases of a Total 
 Eclipse — Periodicity and Calculation of Eclipses — Occultations of the Fixed 
 Stars and Planets. 
 
 Like the eclipses of the Sun, those of the Moon may be either partial 
 or total ; but they are never annular, the Earth's cone of shade being 
 always, at the greatest distances of our satellite from us, much more 
 considerable than the lunar disk itself. 
 
 A fundamental distinction, however, between the two phenomena 
 is, that, while an eclipse of the Sun is visible in a part only of that 
 terrestrial hemisphere which has that body above the horizon, an 
 eclipse of the Moon is visible from every part of the Earth where 
 she has not set. This is not all : it is only successively that an eclipse 
 of the Sun is observed at different stations, in proportion as the 
 lunbra and penumbra of the Moon traverse the surface of our globe, 
 but, on the contrary, the obscuration of the lunar disk begins and 
 terminates everywhere, not at the same hour, because the hour 
 varies according to the longitude of the place of observation, but 
 at the same physical instant. 
 
 The reader has already understood the reason of this essential 
 difference. In the solar eclipse, the surface of the radiant body 
 is not really darkened, but only hidden by the obscure disk of the 
 Moon, so that the interposition is an effect of perspective, varying 
 according to the respective positions of the observer, of the Moon 
 and of the Sun. The lunar eclipse is, on the contrary, produced by 
 a real fading of the Moon's light, and the darkness consequent upon 
 it is observed at the same instant everywhere where the Moon is 
 in view.
 
 184 
 
 THE SOLAK SYSTEM. 
 
 The two diagrams, figs. 70, 71, show under what conditions an 
 eclipse of the Moon is partial or total. When the Moon, in opposi- 
 tion, traverses the cone of shadow thrown by the Earth, at its 
 thickest part, the eclipse is total and central, and its duration the 
 greatest possible. The eclipse may, however, be still total, with- 
 out being central, when the orbit of the Moon traverses a sufficient 
 breadth of the cone. But if the Moon's node is too far from the 
 centre of the cone, its disk, penetrating the umbra only in part, will 
 only be incompletely darkened — the eclipse wiU be partial. 
 
 At the commencement of a total eclipse of the Moon, there is 
 
 Fig. TO. — Path of tlic Moon in the Earth's cone of shade. Total Eclipse. 
 
 fii'st noticed a marked diminution of the brightness of the disk, this 
 is due to the Moon's entering the penumbra. Then, suddenly, 
 a small patch of darkness is seen, which by degrees invades the 
 luminous parts of the disk, but the outline of the portion thus eaten 
 out is far from being as sharp as that observed in solar eclipses. 
 The form is circular, but the curvature is less decided, a fact easily 
 imagined, and confirmed by calculation, the diameter of the Earth's 
 shadow being nearly three times as great as that of the Moon 
 itself.* 
 
 r 
 
 * The mean diameter of the Earth's shadow at the distance at which Eclipses 
 occur, is about 82', whilst the lunar diameter is only 31'.
 
 ECLirSES OF THE MOON. 185 
 
 The colour of the shadow is at first a greyish bUick, which per- 
 mits us to see nothing of the part eclipsed ; but, as the shadow gaius 
 on the lunar disk, a reddish tint makes its appearance, and the 
 details of the principal spots become visible. Between the luminous 
 crescent and the ruddy centre of the shadow is observed a band of 
 greyish blue, shoAvn in Plate XIII. 
 
 From the time of totality the red becomes more intense, and is 
 soon spread over the whole of the disk. According to Beer and 
 Madler the bluish tint is of a dark grey when compared with that 
 part of the Moon illuminated by the Sun ; it seems blue, and clearer 
 than the red, if compared with the latter. 
 
 
 
 
 
 EcUi'tiu. ^n 
 
 I^^^^^^B^S^HH^b'^' 
 
 Kclii>tio. 
 
 
 
 m^^^^^^^^^K\ 
 
 vbo^^-'^^ 
 
 ^- 
 
 
 
 
 
 
 -^r"^!*^ 
 
 
 
 Fig 71.— Putli of the Moon in the FJartli's cone of shade. Pavtial EcUpsc. 
 
 Some minutes before the reappearance of the light on the opposite 
 side of the disk, the bluish tint slightly colours that side also, and 
 the phases of the eclipse are reproduced in an inverse order, until 
 the entire emersion of the Moon. 
 
 The Moon, therefore, does not always completely disappear in 
 total eclipses. The cause of this fact lies in the refraction of the 
 solar rays in traversing the lower strata of the Earth's atmosphere ; 
 they are diverted, and purple our Moon with the tints of sunset. 
 
 It sometimes happens, however, that the Moon becomes quite 
 invisible dm-ing a total eclipse ; as examples of this we may quote 
 the eclipses of 1642 and I Sin. At other times, the visibility,
 
 186 THE SOLAR SYSTEM. 
 
 without being absolutely nil, is very indistinct ; we find the ex- 
 planation of these circumstances in the state of our atmosphere 
 at the time on the periphery ol our Earth which comprises the places 
 where the Smi is rising and setting at the moment of the eclipse. 
 
 Another phenomenon, which happens, however, very rarely, ap- 
 pears contradictory to the geometric and astronomical theory of eclipses. 
 We refer to the simultaneous presence of the Sun and Moon during 
 the phenomenon. The first of these bodies setting at the moment 
 when the other rises, it would seem that the Moon, the Earth, 
 and the Smi, are no longer in a straight line. This appearance 
 again is owing to refraction. The Sim, actually already below the 
 horizon, is raised up by refraction, and remains visible to us. The 
 same thing happens to the Moon, which is not yet really risen, 
 although we see it. The eclipses of 1666, 1668, and the 19th of 
 July, 1750, may be quoted as having presented this singular cir- 
 cumstance. 
 
 "We must now bring our notices of eclipses to a conclusion, by 
 saying a w^ord on theu- periodicity. 
 
 About every eighteen years, the Earth, Moon, and Sun, again 
 occupy the same relative positions. This is a fact which the 
 ancients proved by observation long before the theory of the celestial 
 movements had demonstrated its near approach to the truth. If, 
 then, we start from the epoch of an eclipse of the Sun or Moon, 
 that is to say, from a lunar opposition or conjunction coinciding 
 Avith one of the nodes of the Moon, after eighteen years the three 
 bodies will again be found in a situation nearly identical. Hence, 
 the eclipses which succeed one another in the first period follow 
 again and in the same order during the second period. 
 
 This is the principal point of departure in the calculation of 
 eclipses ; but the approximation is too rough for modern astronomers 
 to content themselves with, and nowadays eclipses are foretold for 
 many years in advance, true to a second of time. 
 
 The Moon, in traversing its orbit roimd the Earth produces 
 again another kind of eclipse, to which the name of occultation has 
 been given. 
 
 We say that a planet or a star is occulted when it passes behind 
 the lunar disk. We have spoken of these phenomena with reference 
 to the question of the existence of an atmosphere on the surface 
 of the Moon. 
 
 Let us add that the occultations of stars are calculated like 
 eclipses, and that, as they are frequent, they have been made of 
 use to our navigators. The Moon being very near the Earth, com-
 
 ECLIPSES OF THE MOON. 187 
 
 pared with the distance of the stars and even of the planets, it 
 follows that two observers, placed in two different parts of the globe, 
 do not see it projected at the same instant on the same part of the 
 heavens. The occultation of a star, therefore, does not take place 
 to them at the same instant of time. 
 
 The starry heavens resemble, from this point of view, an imiversal 
 dial, of which the Moon is the minute hand, marking the time at 
 once in all parts of the Earth. Thanks to the tables calculated by 
 astronomers, these various hours can bo converted the one into the 
 other, and the traveller in the desert, as well as he who traverses 
 the ocean, is thus enabled to arrive at his position and to determine 
 his route.
 
 188 I'HE t^OLAK SYSJ'EM. 
 
 XIII. 
 
 THE METEORIC KINGS. 
 
 Shooting or Falling Stars — " Star-showers" — Their Numbers — Eadiant Points 
 — Position, Form, and Inclination of the JMeteoric Rings — Heights, Velocities, 
 and Weights of Shooting Stars — Luminous Meteors (Bolides), their Telescopic 
 Appearance — Meteorites ; Professor Maskelyne's Classification ; Mr. Sorby's 
 Microscopic Examination of them, and its results — Remarkable Meteorites. 
 
 Every one is familiar with shooting or falling stars. We have 
 all seen their luminous trains furrowing- the heavens during: the 
 night, like so many brilliant points suddenly detached from the 
 celestial vault. Are these appearances, now rare and isolated, now 
 numerous and periodical, due to meteors of atmospheric origin, or 
 must they he considered as manifesting the existence of bodies 
 situated in the extra-terrestrial regions ? The place which our de- 
 scription of these phenomena of the solar system occupies shows 
 pretty clearly that it is to this last conclusion that science has 
 definitely come. 
 
 The nvmiber of shooting stars is very variable according to the 
 time of the year ; hence, the distinction between sporadic meteors 
 and the showers of shooting stars which appear in the nocturnal sky 
 in large numbers, and generally periodically. During ordinary 
 nights, the mean number of shooting stars observed in an interval of 
 an hour is from four to five, according to some observers ; it is as 
 high as eight according to others.* 
 
 But at two periods of the year, about the 10th of August and 
 the 11th of November, these phenomena are much more numerous, 
 and the number of shooting stars observed in one hour is often 
 more than tenfold that seen on ordinary nights. Let us quote, for 
 the month of August, the observations of Capocci and Nobile, who, 
 
 * This hourly mean is from five to six, according to Olbers ; from four to 
 five, according to Dr. J. Schmidt ; five to seven is given by M. Coulvier-Gravier, 
 and Saigey ; and, lastly, eight by M. Quetelet.
 
 TIIK METF.OKIC KINGS. l,Sf» 
 
 in four hours, counted at Naples 1000 shooting stars (10th of August, 
 1(S;30), and those of M. Walferdin, who in an hour observed 316 
 (at Bourbonne-les-Bains, in the night of the 8th-9th of August, 
 1836). It is to this phenomenon that popular tradition formerly 
 gave the name of " St. Lawrence's tears." The luminous trains 
 being nothing else, to the ndive populations of Catholic Ireland, 
 than the burning tears of the martyr, whose feast falls on the 10th 
 of August. 
 
 The month of November has furnished still more extraordinary 
 facts, and the appearances of the 12th of November, 1799, and of 
 Ihe night of the 12th- 13th November, 1833, are well worthy ol* 
 mention. Humboldt and Bonpland, who were at Cumana on the first 
 of these dates, relate that between the hours of two and five in the 
 morning, the sky was covered with innmnerable luminous trains, 
 which incessantly traversed the celestial vault from north to south, 
 presenting the appearance of fireworks let off at an enormous height ; 
 large meteors, having sometimes an apparent diameter of one and 
 a half times that of the Moon, blending their trains with the long, 
 luminous, and phosphorescent paths of the shooting stars. In Brazil, 
 [jabrador, Greenland, Germany, and French Guj'ana, the same phe- 
 nomena were observed. 
 
 The showers of the 12th- 13th of November, 1833, were not less 
 extraordinary. " The meteors were observed," says Arago, * " along 
 the eastern coast of America, from the Gulf of Mexico as far as 
 Halifax, from nine o'clock in the evening till sunrise, and even, in 
 some places, in full day, at eight o'clock in the morning. They were 
 so numerous, and were visible in so many regions of the sky at once, 
 that in trying to count them, one could only hope to arrive at a 
 very rough approximation. An observer (Olmsted) at Boston com- 
 pared them at the moment of maximum to half the number of flakes 
 which are seen in the air during an ordinary fall of snow. When 
 the brilliancy of the display was considerably reduced, he counted 
 650 in 15 minutes, though he confined his observations to a zone 
 which was not a tenth of the visible horizon. According to him, this 
 number was but two-thirds of the total ; thus he estimates the nmnbcr 
 at 866, and in all the visible hemisphere, 8660. This last value would 
 give during each hour 34,640 shooting stars. Now, the phenomenon 
 lasted more than seven hours, and therefore the number seen at 
 Boston exceeded 240,000 ; and yet it must not be forgotten, that 
 the bases of this calculation were obtained at a moment when tlu^ 
 display was already notably on the decline." 
 
 * " Astronomie Populaire," vol. iv. p. 310.
 
 190 THE SOLAR SYSTEM. 
 
 [Mr. Newton, an American astronomer, who has given much 
 attention to this subject, finds that the average number of meteors 
 which traverse the atmosphere daily, and which are large enough to 
 be visible to the naked eye on a dark clear night, is no less than 
 7,500,000 ; and applying the same reasoning to telescopic meteors, 
 their numbers will have to be increased to 400,000,000 ! If allow- 
 ance be made for the space occupied by the Earth's atmosphere, we 
 find, that, in the mean, in each volume as large as the Earth, of 
 the space which the Earth traverses in its orbit about the Sun, 
 there are as many as 13,000 small bodies, each body such as woidd 
 furnish a shooting star visible imder favourable cii'cumstances to the 
 naked eye. If telescoj)ic meteors be counted, this number should be 
 increased at least forty- fold.] 
 
 Several less important periods have been recognised at other 
 times of the year, but they have not the same regularity as those 
 of August and November. 
 
 These last-mentioned periods also present a rise and fall in the 
 hourly number of shooting stars observed. From a maximum of 110 
 stars, in August 1848, the number was reduced to 40 in 1858, and 
 since then the numbers in the same month have regained their upward 
 march. The November shower, of old so remarkable, is now re- 
 duced to the point of being less remarkable than that observed at 
 night towards the end of October. Since 1862, however, this shower 
 is again increasing in numbers. 
 
 Most frequently the paths described by shooting stars have the 
 appearance of straight lines. The luminous trains left in the heavens 
 by their rapid movement, enable us easily to verify this fact. But 
 there are exceptions, and stars of this kind have been seen to describe 
 before disappearing, strangely curved paths. 
 
 [Some interesting conclusions on the causes of these variations 
 and of their meteorological teachings have recently been published 
 by a French observer.] 
 
 Their brilliancy is also very variable: some have surpassed in 
 apparent size the most brilliant fixed stars, and even Venus and 
 Jupiter. The colour likewise varies. 
 
 On observing a given nmnber of shooting stars, it has been found 
 that about two-thirds are white ; while yellow, reddish-yellow, and 
 green, characterise the remainder. 
 
 We noAV come to a fact of great importance, which has thrown 
 much light on the origin of these meteoric showers, and revealed 
 their cosmical nature. In observing the direction of the trajectories 
 on the celestial vault, it has been noticed that the sreatest number
 
 THE METEORIC RINGS. 
 
 191 
 
 of those observed at any one time are emitted from the same part 
 of the heavens, called the radiant point, because from it they radiate 
 in all dii'ections. 
 
 The star Mu in the constellation of the Lion (/a, Leonis), is the 
 point of the November showers, whilst Gamma in Perseus (7 Persei), 
 is the radiant point of the stars observed in the month of August. 
 No less than 56 radiant points have been shown to exist in different 
 seasons of the year. 
 
 We must infer from these facts, that shooting stars are luminous 
 bodies, the movement of which is independent of the rotation of the 
 Earth, and that they are external to our atmosphere. This conclu- 
 
 Fig. 72.— Radiant Point of Sliootiug Stars. R.A. 94°, N. Deol. 37°. 
 Radiant Point of Meteors observed at Hawkhurst, Nov. 28. (Mr. Alexander Herschel.) 
 
 sion is singularly corroborated by this other fact, that the radiant 
 points in the Lion and Perseus are precisely those towards which 
 our globe is travelling, in its annual movement round the Sun, at the 
 two epochs of November and August. 
 
 Astronomers have therefore concluded, that the appearance of 
 shooting stars is caused by the Earth's passage througli rings com- 
 posed of myriads of these bodies circulating, like the larger planets, 
 round the Sun, and the parallel movements of which seen from the 
 Earth seem to radiate towards that ]3art of the heavens api^roached 
 by our Earth. The appearance required by this theory is exactly 
 that which is presented to us. 
 
 At first it was a question, whether there existed one ring, the
 
 192 THE SOT,\K SYSTEM. 
 
 various rejyions of wliicli, sometimes richer, sometimes poorer in 
 cosmical matter, could give rise to tlie varying phenomena observed. 
 Or whether we should admit the existence of many separate rings, 
 successively traversed by the Earth. 
 
 It will be seen in fig. 73 how the periodical appearances of August 
 and November can be explained on the hypothesis of a single ring. 
 We may suppose that the plane of the meteoric ring coincides ex- 
 actly, or nearly so, with that of the ecliptic, and that the orbit of the 
 meteors is a more elongated curve, or one of greater excentricity, 
 than the terrestrial orbit. A single inspection of the figure will show 
 that the Earth should encounter a larger number of meteors, in 
 travelling from its aphelion in July to its perihelion at the end of 
 December, than in the opposite period of its revolution. 
 
 Now, if we suppose two rings inclined at different angles to 
 the plane of the ecliptic, and cutting this plane, one in August and 
 in February, the other in May and November, we can also account 
 both for the two maxima and minima of the j^ear. 
 
 [It is now generally held, that these little bodies which we are 
 now weighing and numbering are not scattered uniformly in the 
 planetary si^aces, or collected into either one or two rings, but are 
 collected into several rings — tangible orbits — round the Svm, and 
 that when our Earth in its orbit breaks through one of the rings, 
 or passes near it, its attraction overpowers that of the Sun ; and 
 causes them to impinge on our atmosphere, when, their motion being- 
 arrested and converted into heat and light, they become visible 
 to us as meteors, fireballs, or shooting stars, according to their 
 size. 
 
 Thus, it is now considered that we have one ring which furnishes 
 us with the August meteors, and another through which we pass 
 in November. We know that the position of these rings in space 
 is very different, for while the November one lies almost in the same 
 plane as that in which the Earth's annual course is performed, that 
 of the August shooting stars is considerably inclined to it, and its 
 nodes are situated at the extremities of its major axis. There are 
 also other points of difference ; for while the nodes of the August 
 ring are stationary, those of the November one have a direct proper 
 motion. A French physicist, who has in the most crucial manner 
 examined the temperature of the months of August and November 
 since 1806, has detected the fact, that in both the months there is 
 an increase of temperature about the period of the star- showers, and 
 a decrease of temperature in February and May, i.e., in the mid 
 interval between these annual showers in both months ; and this
 
 'J'HE METEOUIC KINGS. 
 
 193 
 
 he does not hesitate to ascribe to the influence of these meteoric 
 rings. 
 
 The existence of anomalies in the temperature of these four 
 months has long puzzled meteorologists, and various causes have 
 been assigned, but the curves which M. Deville has prepared enable 
 him to affirm that the temperature, which each day of those months 
 should possess by virtue of the Earth's place in the ecliptic, is 
 affected by a certain coefficient dejaending upon cosmical causes. 
 To explain this, he reproduces the theory of Erman, that the lower- 
 
 Maxiii-nixii > 
 
 ,,.-<=#tS^ 
 
 
 T^,^ 
 
 llNov..„.--^>'"' 
 
 
 
 /^ ■^>, TMaxixa-am. 
 
 f7^ 
 
 ■ ;": / 
 
 ,»„^^:^ 
 
 «;\S:fc-iic^'-;: 
 
 cBr 
 
 
 [ 
 
 
 jt 
 
 '*v^ ■• ■^' 
 
 PI 
 
 l^„,^.J 
 
 
 "''■"^■:V \ 
 
 -V 
 
 
 
 J^'ch 
 
 A^v^ 
 
 B^.J^H 
 
 
 
 
 
 Fig. 73. — Ring of meteorical bodies circulating round tlie Sun. 
 
 ing of the temperature in February and May is caused by the 
 interposition of the meteoric rings between us and the Sun ; and that 
 the increase of temperature in August and November is caused by 
 their preventing the radiation of heat from our globe, and possibly 
 by radiating towards us part of the heat they themselves receive] 
 
 The heights of a great number of shooting stars at the moment 
 of their appearance and disappearances, have been determined. 
 
 " Shooting stars," says Humboldt,* " descend nearly to the sum- 
 
 * " Cosmos," vol. iii. p. 615
 
 194 THE SOLAK SYSTEM. 
 
 mits of Chimboraco and of Aconcagua, at 8750 yards above the level 
 of tbe sea." 
 
 [Much attention has lately been given in England, America, and 
 Italy, to this subject. Mr. Herschel, who is to England what Mr. 
 Newton is to America, has recently collated the observations under- 
 taken to determine the heights of meteors. It appears that the heights 
 of shooting stars at Rome are sensibly the same as in those latitudes 
 of Northern Europe and America where they have chiefly been 
 observed ; and this height, as determined from the most trustworthy 
 observations since 1798, may be stated to be respectively 73 and 
 52 miles, at first appearance and disappearance above the surftice 
 of the Earth, with a probable error of not more than two or three 
 miles.'] 
 
 The height of a shooting star, at the two extremities of its path, 
 and the time of its flight, are elements which enable us to determine 
 the mean velocity of the body. This velocity often exceeds the 
 velocity of the translation of the Earth, which is nearly 18 miles 
 a second. Meteors have been observed which have traversed space 
 with the enormous rapidity of 43 miles, and others 50, and even 
 100 miles a second ; that is, from two to five times the velocity of 
 the Earth. 
 
 [The average velocity of shooting stars, however, in 66 instances 
 observed by Mr. Herschel, is 34*4, or in round nimibers 35 miles 
 per second.] 
 
 The tremendous velocity with which these meteors traverse the 
 celestial spaces enables us readily to understand their incandescence 
 when they enter our atmosphere ; composed of easily inflammable 
 matter, like some sulphureous metaUic combinations, the intensity 
 of the friction which they undergo in the upper strata of our atmo- 
 sphere results in a very great rise of temperature sufficient to 
 produce incandescence. 
 
 [Mr. Herschel has roughly estimated, according to the dynamical 
 theory of heat, the weight of twenty shooting stars, and found it 
 to be on an average a little more than two ounces. A similar estimate 
 of the largest meteor observed in 1863 gave two hundredweight.] 
 
 Difierences of chemical composition and of degrees of incandescence 
 also account for the diversity of colours which are observed. 
 
 In the immense number of meteors which invade the regions of 
 the air in a year, there are some perhaps that only pass through 
 its domain and continue their path in space, after after having 
 presented us with the spectacle of a transient illumination. A great 
 number, on the other hand, not only do not again leave our at-
 
 THE METEORIC RINGS. 
 
 195 
 
 mosijhere, being- vaporised therein, but, when of large size, attain 
 the very surface of the Earth. Falls of stones, ferruginous 
 masses, and dust, from the upper regions of the air, are proofs of 
 this assertion. 
 
 From shooting stars to meteors, or bolides, the transition in our 
 narrative is easy : the difference between these two orders of pheno- 
 mena is not very strongly marked. 
 
 Bolides are kiminous bodies of circular, or rather of spherical, 
 form, and of sensible apparent-diameter. Like shooting stars, they 
 appear suddenly, but generally they move more slowly, and dis- 
 appear after some seconds. Their light is ordinarily less vivid, 
 but their much more considerable apparent dimensions are sufficient 
 to compensate this difference of intensity. The illumination of the 
 landscape by the presence of a meteor sometimes approaches that 
 of moonlight. Most of them leave behind a luminous train like 
 
 Fig. 74. Appearance of a Meteor in a Telescope. (Scliniidt.) 
 
 the one figured in Plate XIV ; others explode with violence, and 
 sometimes the explosion is accompanied with reports like discharges 
 of artiller}^ 
 
 The appearance of meteors is more rare than that of shooting- 
 stars, the total nmnber of observations recorded amounting at most 
 to a thousand, reckoning those recorded by, the ancients. 
 
 A curious circTimstance, and one which helps to prove the relation- 
 ship between the shooting stars and meteors, is the fact that the 
 appearances of meteors are more frequent in August and November 
 than at other epochs of the year ; and the total number from Jidy 
 to December exceeds also that observed from December to July. 
 
 [One of the most curious observations of a meteor which have been 
 recorded, leaving that of 1783 out of the question, was recently made 
 by Dr. Schmidt, who was fortunate enough to observe a large meteor 
 in a telescope, under a magnifying power of eight times. The hrc- 
 ball was twin, and was followed by several smaller ones, following
 
 196 THE SOLAK SYSTEM. 
 
 side by side with parallel motions of translation until all were extin- 
 guished (fig. 74). This observation lends force to the supposition 
 that meteors exist in space as a crowd of bodies, revolving round 
 each other, before they enter ovir atmosphere.] 
 
 The heights of meteors from the surface of the Earth are often 
 very considerable ; they vary between 7 and 310 miles. It must 
 then be held, as remarked by Arago, that the sudden incandescence 
 of meteors is produced in regions where it was formerly supposed 
 that the strata of the terrestrial atmosphere were so rarefied that all 
 action of its elements on the matter of the shooting stars would be 
 regarded as impossible. 
 
 It has been suggested, not without some probability, that the 
 attraction of the Earth is susceptible of retaining meteors in the 
 state of permanent satellites ; and astronomical treatises quote the 
 calculations of a French astronomer, M. Petit, of Toulouse, who 
 assigns to one of these bodies a revolution romid our globe, the 
 period of which would be three hours and twenty minutes. The 
 distance of this singular companion of our Moon is 5000 miles from 
 the surface of our Earth. 
 
 We are here brought naturally to say a word of the falls of 
 meteorites — stony and ferruginous masses, which, leaving the inter- 
 planetary spaces, have at various tunes astonished our populations 
 by their unexpected fall. 
 
 [Professor Maskelyne has recently made a convenient classifica- 
 tion of meteorites into " Aerolites or Meteoric Stones ; " " Aero- 
 siderites or Meteoric /ro;? ; " and " AerosideroKtes," which includes 
 the intervening varieties. 
 
 Thinking that, unlike all terrestrial rocks, meteorites are pro- 
 bably portions of cosmical matter, which has not been acted upon 
 by water or volcanic heat, Mr. Sorby was led to study their 
 microscopical structure. He has thus been able to ascertain that 
 the material was at one time certainly in a state of fusion ; and 
 that the most remote condition, of which we have positive evi- 
 dence, was that of small, detached, melted globules, the formation 
 of which cannot be explained in a satisfactory manner, except by 
 supposing that their constituents were originally in the state of 
 vapour, as they now exist in the atmosphere of the Sun ; and, on the 
 temperature becoming lower, condensed into these "ultimate cosmical 
 particles." These afterwards collected together into larger masses, 
 which have been variously changed by subsequent metamorphic 
 action, and broken up by repeated mutual impact, and often again 
 collected together and solidified. The meteoric irons are probably
 
 < 
 
 DC 
 
 (- 
 
 
 o 
 
 I-
 

 
 THE METEORIC RINGS. 197 
 
 those portions of the metallic constituents which were separated from 
 the rest by fusion, when the metamorphism was carried to that 
 extreme point. Though at present he looks upon it as a mere hypo- 
 thesis, he ventures to suggest that there is a similar relation between 
 these ultimate cosmical globules and planets, that there is between 
 the minute drops of water in the clouds, and an ocean ; and that the 
 study of the microscopical structure of meteorites reveals to us the 
 physical history of the solar system at the most remote period of 
 which we have any evidence.] 
 
 It is now universally admitted that there is an intimate relation 
 between the phenomena of shooting stars, meteors, and meteoric 
 falls ; as the falls have been in several instances known to occur 
 after the appearance of a meteor. 
 
 On the 26th of April, 1803, at Aigle, Department of Orne, a 
 few minutes after the appearance of a large meteor, mo\ang from 
 south-east to north-west, which was also perceived at Alencon, 
 Caen, and Falaise, a frightful explosion, followed by detonations 
 similar to the noise of cannon, or the roll of musketry, proceeded 
 from a single black cloud in a very clear sky. A large quantity 
 of meteorical stones, still fuming, was found on the surface of the 
 ground, over an extent of comitry measuring not less than six miles, 
 in the direction of its greatest length. The largest of these stones 
 weighed rather less than twenty-four pounds. 
 
 More recently, in the e^'ening of the 15th of May, 1864, the 
 identity of meteorites and meteors was evidenced by the appearance, 
 explosion, and fall of a splendid meteor, which was observed over 
 a great extent of France. A globe of a brilliant light, leaving 
 behind it a whitish train, was shattered rocket-wise into numerous 
 fragments. A noise like the prolonged rumbling of thunder followed 
 the explosion at some minutes' interval, and a fall of stones which 
 took place over about two square leagues enabled the extra-telluric 
 matters of which the meteor was composed to be examined. 
 
 Aerolites and aerosiderites of the same origin, but of much more 
 considerable size, have been collected in different museums. M. 
 Daubree has permitted us to reproduce here, in figs. 75 and 76, two 
 of the most beautifid specimens of meteorites now known. The first 
 is a block of pure iron, fomid in a plain in the department of Var, which 
 weighs upwards of eleven hundredweight. It is very remarkable for 
 its crystalline structure, visible even on its exterior, but rendered still 
 more evident by a section made artificially at one of its angles. 
 
 This is one of the treasures of the mineralogical galleries of the 
 Natural History Museum of Paris; where, thanks to the zeal of
 
 198 
 
 THE SOLAR SYSTEM. 
 
 M. Daubree, the number of meteorites gathered from different points 
 of the globe is increasing dail5^ 
 
 Mineralogists and chemists have analysed these meteorites with 
 
 Fig. 75. — Mass of meteoric iron, weighing 15 cwt., loiuul in 182S by Brarrt, 
 at Caillc (Var). 
 
 Fig. 76. — Aerolite fouud at Juvenas(Ardeche), the 5th of June, 1S21. 
 
 great care, and it has been found that their composition is nearly 
 always the same, whatever difference their external aspect presents.
 
 THE METEORIC RINGS. 199 
 
 Oxygen, sulpliur, phosphorus and carhon, silicium and aluminium, 
 potassium, sodium, sulphide of iron, metallic and magnetic iron, and 
 other metals, such as nickel, cobalt, manganese, tin, copper, &c., have 
 been recognised among the substances of which meteorites are com- 
 posed. Latterly, the presence of nitrogen has been detected, besides 
 the eighteen simple bodies, of which the principal have just been 
 cited. 
 
 It is worthy of remark that all the simple bodies found 
 in meteoric stones are known in our OAvn planet. The chemical 
 combinations of these bodies do not differ from those with which 
 we are acquainted, excepting two or three, of which one, schrciherzite, 
 has been recently artificially reproduced.* 
 
 Thus, thanks to the phenomena which we have described — meteor- 
 ites, meteors, and shooting stars — the planetary spaces, which seemed 
 for ever shut out from direct investigation, have been correlated with 
 our Earth. These masses, midefiled until the time of their fall by 
 living contact, relate to us the mineralogy and chemistry of a whole 
 region of the sky. By combining the indications which they furnish 
 \^dth the marvellous revelations of spectrum analysis, man is beginning 
 to obtain precise notions on the composition of the most distant 
 celestial bodies ; and he will thus expand those ideas which the 
 laws of attraction, of light, and of heat, have already enabled him 
 to hold on their physical constitution. 
 
 * By MM. Faye and IL Deville.
 
 200 THE SOLAR SYSTEM. 
 
 XVL 
 
 MARS. 
 
 JMovemeiit of Mars round the San — Mars, in Opposition ; Conditions uecessaiy 
 for a favourable Opposition — Bright and Dark Spots — Eifects of the Transit 
 of Clouds — Colour of the Planet's Disk ; Why Mars is sometimes Red — 
 Polar Snows — Melting of the Polar Snows — Rotation — Seasons and Climate. 
 
 In pursuing our exploration of the solar world, we meet with Mars 
 after the Earth : it is the next planet in the order of distance from 
 the Sun, and therefore the first, the orbit of which encircles that 
 of the Earth, or of those bodies which are called by astronomers 
 Exterior or Superior P/aiiefs. 
 
 At successive intervals of two years, one month, and nineteen 
 days, its movement of revolution brings it in opposition with the Sun ; 
 that is to say, in a line passing respectivelj' through the centres of 
 the Sun, Earth, and the planet. Mars is then comparatively very 
 near to us, and in an extremely favourable situation for observations 
 of its disk ; indeed, excepting the Moon, there is no planetary body, 
 the physical constitution of which has been better studied. 
 
 Mars appears to the naked eye as the reddest star in the heavens,* 
 but its brightness varies considerably, on account of the variability 
 of its distance from the Earth. Occasionally its light scintillates, 
 but most frequently this does not happen ; and it is thus, like all 
 other planets, distinguished from the stars of the same apparent 
 magnitude. 
 
 If, instead of observing with the naked eye, a telescope of suf- 
 ficient magnifying power is used, the scintillation entirely disappears, 
 the luminous point takes the form of a clearly defined disk, and the 
 degree of intensity of the red colour diminishes and passes to a 
 general tint of a yellowish red. 
 
 * Beer, Madler, and Arago.
 
 Ik 
 
 PL. VI. 
 
 MARS 
 
 ] . Equatorial regions . II. Sou-tliftTid North polar regions ( .Beer and Madler.) 
 Ill, Hemispheres sdiowin^ phases ( Secchi.) 
 
 ^^^ Imo.Bfcquet Par:s 
 
 ckerb 
 
 ickeybduej

 
 MARS. 
 
 201 
 
 As is the case witli Yenus and Mercury, the light which Mars emits 
 is borrowed from the Sim ; but it is more difficult to prove this fact, 
 common to all the bodies which revolve round the tentral fire, ab- 
 solutely, because the phases of its disk are extremly small. Thev 
 exist, however. It is easy to accomit for this difference, which we 
 shall find more decided still in the planets furthest from the Sun. 
 
 When Mars and the Earth are brought, by their movements of 
 translation round the Sun, into a straight line with it (see fig. 77), but 
 
 Fig. 77. — Orbit and Phases of Mars. 
 
 in such a manner that we are placed between the Sun and the planet, 
 Mars is presented to us under the form of a completely illuminated 
 disk. It presents the same appearance when it is on the other side 
 of the Sun. In the intermediate positions, there is but little perceptible 
 change in the appearance to that presented at conjimction or oppo- 
 sition.* At certain distances, however, from these two extreme 
 
 * Let us remind the reader that a planet is in Conjimction when it is on the 
 same line as the Sun, between it and the Earth. It is in Opposition when on 
 the same line as the Sun, but on the opposite side of the Earth to the Sun.
 
 202 
 
 THE SOLAR SYSTEM. 
 
 positions,* Mcars presents to us a slight portion of its dark hemi- 
 sphere, although the luminous part is always by far the larger. 
 
 The aspect * of Mars at this time caused Sir J. Herschel to apply 
 the term gibbous. The apparent form of the planet then is that of 
 the Moon, two or three days before or after fidl. 
 
 HoAvever slight this phase, it suffices to prove, as we have before 
 said, that Mars is not self-luminous, and we shall see that it is thus 
 with all the other j)lanets. 
 
 If we consider the orbit which Mars describes round the Sun, 
 we shall readily see how it is that this planet is most favour- 
 ably situated for observations of the physical particularities of its 
 sui'face. 
 
 The two inferior planets, Mercury and Venus, oscillating at small 
 distances round the Sun, are often hid in its rays ; besides, dui-ing 
 
 B^^^fcfo'^ 
 
 ' ^ 
 
 \ 
 
 •a 
 
 i^t^^tsl^^^^ 
 
 ■ 
 
 
 
 
 >-1 
 
 
 ^^IMM 
 
 
 ;JHH| 
 
 
 L... 
 
 rliU 
 
 
 
 H 
 
 Fig. 7S. — Apparcut dimensions of Mars at its mean and extreme distances from the Eartli. 
 
 their periods of visibility, they show us a considerable portion of their 
 dark sides. It is not thus with Mars, which is only lost once in a 
 revolution in the solar rays, and is almost without phases. We have 
 before said that when a superior planet is in opposition with the 
 Sun, its distance from the Earth is least. At this epoch, indeed, this 
 distance is measured by the difference between the distances of the 
 planet and the Earth from the Sun. 
 
 Let us look into this statement a little closer with regard to Mars. 
 
 Like all planets. Mars describes an orbit which is not circular, 
 so that its distance from the focus of the system varies continually. 
 xVt its greatest distance from the Sun, Mars is 159,000,000 miles 
 
 * At the quadratures, or in the positioif in which hncs drawn to the Earth 
 and Sun form the greatest possible angle.
 
 MAKS. 203 
 
 removed ; at its niinimuiii distance, 132,000,000 ; its mean distance 
 being 145,000,000 miles. Thus the difference in the distance from 
 the Sun at perihelion and aphelion amounts to 27,000,000 miles 
 These nmnbers indicate an orbit of considerable ellipticity. 
 
 From this follow also enormous differences in the distance of 
 IVIars from the Earth ; in their various relative positions ; whilst the 
 planet is sometimes distant from us 256,000,000 miles, in its most 
 favourable opposition it is not more than 35,000,000 miles away from 
 us — a distance seven times less than the first. 
 
 It will not be astonishing, then, in glancing at figure 78, to 
 find such great differences between the apparent dimensions of the 
 disk of Mars, seen from the Earth at its extreme and mean dis- 
 tances , 
 
 [We may, with advantage, pursue this subject a little further. 
 In the case of an infcn'or planet, if we suppose, bearing the elliptic 
 form of the orbit in mind, the perihelion of the Earth to coincide in 
 direction — or, as astronomers put it, to be in the same heliocentric 
 longitude — as the aphelion of the planet, it will be obvious that the 
 conjunctions which happen in this part of the orbits of both will bring 
 the bodies nearer together than will the conjimctions which happen 
 elsewhere. Similarly, if we suppose the aphelion of the Earth to 
 coincide with the perihelion of a superior planet — let us say of Mars 
 — it will be obvious that the oppositions whicli happen in that part of 
 the orbit will be the most favourable for observation. It happens, 
 however, that these points are in no case coincident.* 
 
 The perihelion of the Earth is situated in 99° of heliocentric 
 longitude, its aphelion is therefore situated in long. 279° (=99°+ 180°). 
 The perihelion of Mars is in long. 332°. The oppositions, therefore, 
 which occur near that part of his orbit are looked forward to with 
 the greatest interest and utilized to the utmost by astronomers — 
 the oppositions of 1830 and 1862 to wit.] 
 
 Mars traverses its orbit with varying velocities. Its mean rate 
 of motion is upwards of 54,000 miles an hour, or about 16 miles a 
 second. 
 
 The apparent diameter of Mars is less than that of Venus ; this 
 is due to two causes ; firstly, Venus approaches nearer the Earth, and, 
 secondly, the diameter of that planet is larger in reality than that of 
 Mars ; it exceeds it by about three-fourths. The diameter of Mars is 
 
 * [The orbit of the Earth, however, is so nearly circular, that practically 
 notliing is lost- Were it as elliptical as that of Mars, this consideration would be 
 of great importance.]
 
 204 THE SOLAR SYSTEM. 
 
 4113 miles, that is, a little less than half that of the Earth, which 
 is nearly 8000 miles. 
 
 Lastly, whilst the surface of Mars is scarcely more than a quarter 
 of the Earth's surface, its volmne does not exceed the seventh part. 
 Nevertheless, it is more than double that of Mercury, and about seven 
 times that of the Moon. 
 
 We now arrive at a subject of extreme interest — the physical 
 constitution of the planet. 
 
 To observe Mars, we must choose the most favourable epoch — that 
 of opposition ; and, if possible, of an opposition when the planet, as 
 we have before explained, is at its greatest possible proximity to the 
 Earth. We must also furnish ourselves with a powerful telescope, 
 one, if possible, driven by clockwork, so that it exactly keeps pace 
 with the planet in its westward course. 
 
 Fig. 79, — Mars aud the Earth ; comparative dimensions. 
 
 Let us turn our instrument on the reddish limiinous point of 
 light, on a clear night, when the air is calm and charged with 
 moisture, and the height of the planet above the horizon is the 
 greatest possible, so that we shall have the least possible thickness 
 of atmosphere to penetrate. It will be then nearly midnight, since 
 it is towards that hour that all planets culminate — that is to say, pass 
 the meridian — at opposition. 
 
 The disk of the planet will appear of a nearly circular form, per- 
 fectly well defined, and overspread with light and dark spots, which 
 differ considerably in tone and colour — tone especially. The brighter 
 portions, excepting in two points nearly diametrically opposed, are 
 sometimes of a reddish tint, whilst the dark spots, as some hold, by 
 the effect of contrast, as others hold, absolutely, seem of a blue or 
 greenish grey. Throughout its circmnference the disk is more
 
 MAK8. 
 
 205 
 
 luminous than the oontral part ; the dark spots also arc effaced and 
 disapi)ear at the limb. 
 
 Lastly, at two points, of which mention has been before made, 
 which are not situated at the extremities of a diameter, two spots of 
 unequal extent and of extreme whiteness, which contrasts with the 
 reddish parts, shine with a very particular brightness.* These two 
 spots mark near the poles of Mars. [Both arc visible when the planet 
 is observed at the time of its solstices ; at others, only the one tipped 
 down towards the Earth can be seen.] 
 
 All the appearances on the surface of the planet may be divided 
 into permanent and variable ones. The permanence of the features, 
 that is to say, the constancy of their principal shapes, and of their 
 relative situations, has been proved by numerous and minute obser- 
 
 Fig. SO. — Views of Mars at two liours' interval. (Warren Do La Rue.) 
 
 vations — a matter more difficult than miofht be imag-ined at a first 
 examination. Indeed, as the observation of the spots shows, the planet 
 has a movement of rotation effected in about 24^ hours. Hence it 
 follows that in a few hours the aspect of the disk changes : of this 
 we may gain an idea by examining figures 80 and 81. Besides, 
 when, in consequence of the movement of rotation, a spot approaches 
 the edge, it disappears before having attained it. Tliis disappearance 
 
 * " The colour of the polar spots " (we quote Beer and Miidlcr) " were, every 
 time the planet was distinctly seen, always of a bright and pure white, in no way 
 similar to the colour of the other parts of the planet. In 1837 it liappcned once 
 that Mars, during the observations, was completely obscured by a cloud, with the 
 exception of the polar spot, which remained distinctly visible to tlic view." — 
 Fragments sur les Corps Celestes. 
 
 Arago estimated that the brightness of tlic [)olar spots is more tlian double 
 that of the other bviuht spots, on the edge of tlie disk.
 
 206 THE SOLAK SYSTEM. 
 
 is owing, doubtless, to the atmosphere of Mars, seen in these points 
 under a great obliquity, and the brightness of which effaces the 
 darker tint of the spot. 
 
 Lastly, the orbit of Mars does not coincide with the ecliptic : the 
 two planes form a slight angle (1° 51'). But if to this is joined the 
 much greater inclination of the axis of rotation, we shall readily see 
 why, at successive opposition, Mars does not present to the Earth the 
 same portions of its sui'face. Hence changes produced by perspective, 
 so much more decided as a spherical surftice is in question. 
 
 The variability, often very rapid, which is observed in the form 
 of the features which overspread the disk, has suggested the opinion 
 that these phenomena are owing to the interposition of cloudy masses 
 in the planet's atmosphere above the general level of land and sea. 
 Mr. Lockyer, who carefully followed the changing features of Mars 
 during the opposition of 1862, thus writes in his Memoir* on the 
 
 Fig. SI.— Rotation of Mars. Movement of the sjiots observed ilunug the opposition 
 of 1S30. (I3ccr and Miidler.) 
 
 planet : " Although the complete fixity of the main features of the 
 planet has been placed beyond all doubt, daily — nay, hourly — 
 changes in the detail and in the tones of the different parts of the 
 planet, both light and dark, occur. These changes are, I doubt not, 
 caused by the transit of clouds over the different features." The 
 drawings which accompany this Memoir seem to fully justify this 
 opinion. 
 
 It is generally held that the reddish and bright spots of Mars are 
 the solid parts of the siu"face on the continents, whilst the dark bluish 
 spots form the liquid parts on the seas. This distinction is founded 
 on the unequal reflexion of the light by the land and the water. 
 According to Mr. Lockyer, if we admit that the darkest spots in- 
 
 * " Memoirs of the Royal Astronomical Society," vol. xxxii. p. 183.
 
 MAUS. 207 
 
 dicate water, the darkest among them are those portions which arc 
 most hind- locked. 
 
 AVhence comes the reddish colouring, which characterises the 
 bright parts of the disk ? If Mars were self-luminous, this tint would 
 doubtless be attributed to the very nature of its light ; but it only 
 reflects to us the white light of the Sun ; it is evident, therefore, that 
 the colour is imparted by the planet or its atmosphere. Several 
 hypotheses have been suggested on this subject. Some have attributed 
 the red tint of the continents to the nature of the soil, composed of red 
 sandstone. Others, among them Lambert, have thought that the 
 colour of the vegetation, instead of being green, as it is on our 
 Earth, is red on Mars. This explanation is not an impossible one ; 
 but, if it be true, there shoidd be variations in the intensity of the 
 tint on each of the hemispheres of the planet corresponding to the 
 seasons ; the tint should diminish during winter, to reappear in 
 spring, and to attain its maximmn in the sununer. 
 
 It has also been proposed to explain the colour of the spots by the 
 refraction of the rays of the Sun through the atmosphere of Mars. 
 Arago has refuted this hj^Dothesis by the simple remark, that at the* 
 borders of the planet the redness should be more decided than in the 
 central portions, since the luminous rays traverse a greater thickness 
 of atmosphere, and traverse it more obliquely, in the regions near the 
 limb, where the contrary effect is observed. Let us add, that this 
 h;^']^)othesis does not explain why the red tint is not general. The 
 ruddy light of Mars cannot, therefore, in this manner be assimilated 
 to our twilight hues. 
 
 [But the question is altered if we take the existence of clouds into 
 consideration. Observations of the planet in 1862 suggested to Mr. 
 Lockyer that the colour might depend upon the cloudy state of the 
 planet, and the spectroscope substantiates this hypothesis. In 
 1862 the planet was clearer of clouds, and more ruddy than in 
 186-1. The suggested explanation is that, when Mars is clouded, 
 the light reflected by the clouds midergoes less absorption than 
 that reflected by the planet itself; and on one occasion the spec- 
 troscope indicated this increased absorption by revealing the fact 
 that the sunlight was reflected to us minus a large portion of the 
 blue rays.] 
 
 We must now occupy ourselves with the polar snows. We have 
 seen that they are distinguished from the other features by their 
 brilliant white light ; they are equally distinct from the rest by reason 
 of the variation in their dimensions. In proportion as the white spot 
 on one of the poles diminishes, the other increases ; the minimum of
 
 208 THE SOLAR SYSTEM. 
 
 both always corresponding witli the summer, and the maximnm with 
 the winter of the hemisjjhere in which it is situated. Thus, during 
 the opposition of 1830, the southern snow zone was seen to diminish 
 by degrees, and its outline to recede till the time which corresponds, 
 for that hemisjjhere of Mars, to the middle of the month of July on 
 our northern hemisphere ; from this moment it increased again. 
 (Beer and Madler.) In 1837 similar diminutions were observed in 
 the dimensions of the spot of the northern pole. At the same time, 
 the snowy regions of the southern pole had a considerable extension. 
 Now these variations corresponded equally to the summer season of 
 the northern hemisphere, and to the winter of the southern hemi- 
 sphere of Mars. 
 
 Thus from the Earth we can watch the formation of the polar 
 ice, and the fall and thaw of the snows on the surface of a neigh- 
 bouring planet ; in a word, all the vicissitudes of heat and cold which 
 distinguish the seasons of winter and spring, autumn and winter. 
 The succession of these changes is now so well established, that 
 astronomers can predict approximatively the form, relative size, and 
 ■'position of the northern and southern snow-zones. 
 
 We have said that the two white spots are not of the same extent 
 either during their respective winters or summers. The snowy cap 
 of the southern hemisphere varies within much greater limits than 
 that of the opposite pole : it is much more extensive during the 
 winter season, and it diminishes during the summer to such an 
 extent that it does not occupy more than the fifth part of the super- 
 ficies of the snowy spot of the northern pole. This difference is easily 
 explained by the great inclination of the axis of the planet to the 
 plane of its orbit, and by the fact that the southern pole is turned 
 towards the Sun, when Mars is nearly at its smallest distance from 
 the focus of light and heat. The summer time, on the other hand, 
 of the northern hemisphere, occurs at the epoch of its greatest distance. 
 The quantities of heat received by the globe of Mars, at these two 
 opposite points of its orbit, vary in the ratio of seven to five.* 
 
 In truth, these differences of temperature are partly compensated 
 in the course of a revolution ; but the extremes of heat and cold are 
 still very decided. 
 
 We have seen that Mars presents the most curious analogies with 
 the Earth ; and it is probable that to the inhabitants of Venus out 
 
 * At the mean distance of Mars from the Sun, the disk of this last body is 
 but ili^iths of that presented to us, or less than half ; but at its shortest distance, 
 the Sun's apparent diameter is about three-fourths ; the a[>pai'ent surface of its 
 disk is then a little more than half that which is presented to us.
 
 MARS. 
 
 209 
 
 planet presents the same appearances tbat Mars does to us. Ijike the 
 polos of Mars, the poles of the Earth are covered with snow and ice : 
 it is also our Southern Pole which is the most frost-bound, and for the 
 same astronomical reasons, by the congelation of the aqueous vapour. 
 Lastly, the points of greatest cold on Mars, as on the Earth, do not 
 coincide exactly with the poles of rotation. This excentricity is very 
 evident in the views of Mars given in fig. 80. 
 
 If snow falls in Mars, it is because water is there evaporated by 
 lieat ; hence, the water must spread on the surface under the form of 
 clouds, which condense sometimes in a liquid state in the form of rain, 
 
 Fig. 82.— Incliuatiou of the axis of votatiou. Mars at one of its solstices. 
 
 sometimes as snowy crystallizations. Thus Mars certainly possesses 
 an atmosphere of aqueous vapour. 
 
 But we see too distinctly the permanent spots of the disk, not to 
 be certain of the existence of an atmosphere analogous to our own, 
 the pressure of which, by counterbalancing the expansion of the 
 aqueous vapour, prevents it from usurping all the surface. U e have 
 already said that the more luminous borders of the disk allow us to 
 infer the existence of a cloud-bearing atmosphere, which effiices by its 
 brightness the dark spots when the rotation brings them towards the 
 limb. 
 
 The meteorology of Mars is, then, to a great extent known. Tt 
 
 p
 
 210 THE SOLAR SYSTEM. 
 
 presents, we repeat, the greatest analogies with the meteorology of 
 our Earth. But at the same time notable differences distinguish 
 them. As Professor Phillips has remarked, the considerable periodical 
 exchange of moisture which is made between the two hemispheres, 
 especially between the two poles, nmst give rise to hurricanes and 
 storms, of the violence of which we can form no idea ; while the 
 melting of the snows over such large areas must produce terrible 
 periodical inundations. 
 
 "We have seen that Mars turns on itself in about 24^ hours.* 
 Thus the duration of its movement of rotation exceeds that of our 
 sidereal day by 41 minutes. 
 
 Mars accompKshes an entire revolution roimd the Sun in 687 of 
 our terrestrial days. But the year of Mars only contains 669f of its 
 own sidereal days; and as the nimiber of the solar days — we have 
 explained this for the Earth — is always less by one than that of the 
 rotations, the year of Mars is in reality composed of 668-| of its own 
 solar days, which gives, for the duration of one of these days, 24 hours, 
 39 minutes, 35 seconds. 
 
 Thus a whole day of Mars exceeds one of our days by 39 minutes 
 35 seconds. The difference is not very perceptible. 
 
 Besides, the inclination of the axis of rotation to the plane of the 
 ecliptic is nearly the same as that of the axis of the Earth, f Tt follows 
 that, in the course of a year. Mars presents its various regions to the 
 Sun, nearly like our globe so that the length of the days and the 
 nights, in the different latitudes, is distributed in the same manner. 
 The extreme zones, torrid and frigid, are a little more extended, 
 proportionally, which consequently reduces the surface of the temperate 
 zones. But it must not be forgotten that this is a favourable circmn- 
 stance, at least for the tropical regions, since the solar light and heat 
 arrive at the planet with an intensity much less than on our globe. 
 
 Between Mars and the Earth, however, there is an important dis- 
 tinction, and it lies in the difference between the lengths of the Torres- . 
 trial and Martial seasons. In the northern hemisphere of the planet, 
 the GG8 days of its year are divided as follows : 
 
 Spring lasts . . . . 191 days 8 hours. 
 
 Summer ... . 181 „ „ 
 
 Autumn 149 „ 8 „ 
 
 And Winter .... 147 „ „ 
 
 But the summer seasons of the northern hemisphere are the 
 
 ■^ 24 hours, 37 minutes, 2*62 seconds. (Kaiser.) 
 t 61° 9' for Mars ; 06° 33' for the Earth. 
 
 I
 
 MARS. 211 
 
 winter seasons of the southern hemisphere, whence it follows that 
 the spring and summer, taken together, last 76 days longer in the 
 northern hemisphere than in the southern one. 
 
 The glohe of Mars is not exactly spherical ; it is flattened at the 
 poles, and it bulges, like our Earth, at the equator. But the measure- 
 ment of this flattening presents considerable difiicvdties, which 
 most of the observers attribute to errors of measurement in- 
 duced by the irradiation caused by the polar spots. Arago, who 
 made a series of measures of the two diameters with great care, 
 concluded that the latter is shorter than the former by the thirtieth 
 part of its value. Herschel, in 1784, found the same quantity, -Jg^ ; 
 whilst more recent measures appear to reduce it to the third of the 
 value measured by Arago. M. Kaiser (of Leyden) gives -^^-^ for 
 the flattening, as measured during the opposition of 1862. Sup- 
 posing that the planet was fluid in the first instance, the figures 
 which precede are too great to be in accordance with the laws of 
 hydrostatics, which govern the configuration of the celestial bodies. 
 But it is to be regretted that the uncertainty of the measures is 
 here, perhaps, the only cause of this apparent anomaly, and we 
 hope that, at the next most favourable oppositions, astronomers will 
 arrive at more precise data on this point. 
 
 It remains for us, before we complete this monograph of Mars, 
 to speak of its density, which is very near that of the Earth ; * 
 of its mass, which is rather more than an eighth of the terrestrial 
 mass ; and, lastly, of the force of gravity by which the bodies are 
 retained on its surface. This last is half that which is observed 
 on the surface of the Earth, whence we may conclude that the organi- 
 zation of the li^ang bodies which people Mars difiers notably from 
 that with which we are familiar. 
 
 It may be seen, also, that the conditions of temperature to which 
 these beings are subjected are very variable, and that the solar 
 illumination varies very largely. But before we can draw from these 
 facts positive conclusions, the constitution and density of the atmo- 
 sphere, an element so important in the physiology of the celestial 
 bodies, must be known. 
 
 Mars has no satellites. Its nights are therefore completely dark, 
 if indeed they are not lit up by aurorae and long lingering twilights. 
 
 At all events, this is not a great privation, to judge by the 
 imperfect manner in which our Moon acquits itself of its function 
 of torch-bearer to our Earth. 
 
 * 0-948, that of the Earth beiuj? ] .
 
 212 THE SOLAR SYSTEM. 
 
 XV. 
 
 THE MINOR PLANETS. 
 
 Considerable number of Celestial Bodies circulating round the Sun between 
 Mars and Jupiter — Bode's Law — Olbers' hypothesis — Interlacing of their 
 Orbits — Some Details on the principal Planets of the group, Juno, Pallas, 
 Ceres, and Vesta — How to discover a New Planet. 
 
 Thk number of the known planets in the solar system, sixty-four 
 years ago, was only seven, among which was counted the large planet 
 Uranus, di.scovcrerl by Sir "W". Ilerschel. At the present time 
 thi.s number is increased to 92, so that, without reckoning the new 
 comets and the recently discovered satellites, the solar system has 
 been increased by (S5 bodies. It is true that, witli tlie exception of 
 Neptune, which forms part of the group of large planets, all tlicsc. 
 bodies are of extreme smallness, and, taken separately, do not even 
 equal in size the satellites of the principal planets. 
 
 Hence they have been named A.sfcroid.s, Minor Planets, and Tele- 
 scopic Planets. 
 
 In spite of their smallness, they form a very interesting group, 
 which gives a new appearance to the solar system, at the same time 
 that it throws a fresh light on the problem of its formation and develop- 
 ment. The 84 telescopic planets now known — the number increases 
 every year — are all situated between Mars and Jupiter; the orbits 
 which they describe round the Sun are so near one another, and so 
 interlaced, that a contemporary astronomer, M. D'Arrest, deduced 
 from this circumstance the evident proof of a common origin. 
 
 " One fact," he says, " seems above all to confirm the idea of 
 an intimate relation between all the minor planets ; it is, that, if 
 their orbits are figured under the form of material rings, these rings 
 Avill be found so entangled, that it would be possible, by means of 
 one among them taken at hazard, to lift up all the rest."
 
 THE MINOR PLANETS. 213 
 
 At the time wlieu these lines were Avrittcn, only 14 asteroids 
 were known; since then, 70 more newly discovered phmets have 
 been fovmd to occupy the mid-interval. The comparison of D'Arrest, 
 and the inlerence that ho di^aws from it, are therefore so much 
 the more strengthened. 
 
 Before the discovery of the minor planets, astronomers, in com- 
 paring the intervals which separated the known planets from the 
 8mi, noticed the relative considerable distance between the two 
 pLanets, Jupiter and Mars. The imagination of Kepler, which led 
 the ilhistrious disciple of Tycho into theoretical views of extreme 
 hardiness, placed an midiscovered planet in the vacant space ; and 
 this hypothesis seemed corroborated by a discovery made by an 
 astronomer of the eighteenth century — Titius, who detected a sin- 
 gular connexion, since knoA\ii mider the name of Bode's Law, between 
 the successive distances of the planets. 
 
 This connexion was as follows. If we write down the following 
 series of numbers — 
 
 3 6 12 24 48 9G 
 
 and add 4 to each of them, we shall have another series — 
 
 4 7 10 16 28 52 100 
 
 Now, the terms of this series, with the exception of the tifth — 
 28, — very nearly represent the relative distances of the planets known 
 in Titius' time : — 
 
 Mercury, Venus, Earth, Mars, — Jupiter, Saturn. 
 
 After this empirical law was announced, the discovery of Uranus 
 in 1781, extended the series, and it was foimd that the distance of the 
 new planet was precisely that represented by the eighth term, 196, of 
 the regidarly formed series. Hence, to conclude the existence of a 
 planet, which should fill the blank existing between Jupiter and 
 Mars, was natural. "Baron de Zach," says M. Lespiaidt, in his 
 excellent monograph of the asteroids, " went so far as to publish 
 beforehand, in the Berlin Abnanac, the elements of the supposed 
 planet, and he organized an association of astronomers to search 
 for this body. The Zodiac was divided into twenty-four zones, each 
 
 * " Memoirs of tlie Physical ami Natural Sciences of Bordeaux," vol. ii. 
 p. 171.
 
 214 THE SOLAR SYSTEM. 
 
 of which was confided to the special surveillance of one of the mem- 
 bers of the Society. The discovery was made, but certainly not in the 
 manner contemplated. 
 
 In fact, on the 1st of January, 1801, at Palermo, Piazzi in- 
 augurated the nineteenth century by the discovery of Ceres, thus filling- 
 the gap indicated by Titius and Bode, of which, truth to tell, he 
 thought very little. Singvdarly enough, however, Ceres precisely 
 occupied the vacant number 28, which expressed the distance of the 
 new planet from the Sun, the distance of the Earth being represented 
 by 10. Fifteen months after, a second planet, Pallas, was added to 
 the list, and this greatly disturbed the views of the prophets of the 
 first discovery. 
 
 The able astronomer, Olbers, who had discovered Pallas, then 
 hit upon an ingenious theory. He considered the two new bodies 
 were fragments of a planet which had been destroyed. Now, the 
 laws of mechanics indicated that after such a catastrojjhe, whatever 
 might be the cause, the fragments, in whatever directions they might 
 be thrown, ought to lie at the same mean distance from the focus of 
 their movements, the Sun ; and should pass, moreover, at each of 
 their revolutions, through the point of space in which the catastrophe 
 took place. 
 
 PaUas and Ceres very nearly fulfilled these conditions, and it 
 was the same with the third planet discovered, Juno, which was 
 supposed to be a third fragment of the hj'pothetical planet. 
 
 The researches were continued under the influence of these views ; 
 and lastly, Olbers himself, in 1807, discovered Vesta. But, curious 
 contradiction, this discovery, which it was expected would definitely 
 consolidate an ingenious and otherwise rational theory, on the contrary, 
 shook it to its foimdations. The distance, and other elements of the 
 orbit of Vesta, presented serious differences both with this theory 
 and Bode's law ; and both have since received their coup-de-grace.^ 
 
 In fact, since 1845, the epoch of the discovery of the fifth aste- 
 roid, the nmnber of these bodies has rapidly increased, and we 
 have every reason for believing that it will continue to do so. 
 
 * The planet Neptune, the last of the known planets of the solar system in 
 the order of distance, is far from satisfying the empirical formula of Titius. Its 
 distance, which should be represented by the number 388, is in reality only .300. 
 Let us add, as noticed by others, that the first number of the series, that which 
 corresponds to Mercury, is not formed in a regular manner. Instead of it 
 should be I'o, which, by adding 4, would become 5-5, whilst the true distance of 
 I\lcrcury is 3-87. 
 
 It seems, however, well to retain, as an aid to the memory, the scries we owe 
 to Titius ; it is, moreover, intimately connected with tlie history of Astronomy.
 
 THE MINOR PLANETS. 215 
 
 In the actual state of discovery, the 84 small planets form a zone, 
 almost entirely confined to that half of the interval between Mars 
 and Jupiter nearest to Mars. One only, Maximiliana, which is con- 
 sequently the furthest from the Sun, is found to be nearer to Jupiter 
 than to Mars. The breadth of the zone is upwards of 100,000,000 
 miles ; * but throughout this interval the planets are very irregularly 
 distributed, since 64 are situated in the half of the zone nearest 
 Mars, and 18 only in the other half. It follows, from these nimi- 
 bers, that the 64 minor planets nearest the Sun are only separated 
 from each other, on an average, by 990,000 miles, or less than 
 four times the distance of the Moon from the Earth. 
 
 Flora and Maximiliana are the names of the two extreme planets ; 
 the first is at a mean distance of 210,000,000 miles, the second at 
 323,000,000, so that the middle of the zone is 266,000,000 from the 
 central body. The distance of the Earth from the Sun being re- 
 presented by 10, this last distance would be represented by 28, 
 — the term of Bode's series, which at first pointed out the gap ; but 
 the uneven distribution of the minor planets much reduces the value 
 of tliis coincidence. 
 
 We have seen that, if averaged, the orbits of these bodies lie 
 near together. If we compare them one by one from this point of 
 view, we shall find the real distance, in some cases, to be much smaller. 
 
 The orbits of Egeria and Astrea are separated by a mean interval 
 of 50,000 miles ; those of Emydice and Clytie 30,000 ; lastly, Leto 
 and Bellona are only 26,000 miles apart. But it must be well un- 
 derstood that these numbers do not apply to the planets themselves, 
 first, because, at a given epoch, they are found in very difierent 
 directions, and also because their orbits are more or less elongated, 
 and the planes in which they move are very diversely inclined. 
 
 The forms of the orbits are far from being circular. The least 
 elongated of all, that of Freia, is proportionately much more elliptical 
 than the orbits of the Earth, Neptune, or Venus, which are the 
 nearest to the circular form among the orbits described by the bodies 
 of our solar system. The most elongated is the orbit of Polyhymnia, 
 of which the major axis surpasses the minor axis by one - third 
 of its length, which causes between its greatest and least distances 
 from the Sun a difierence of 184,800,000 miles. Fig. 83 shows 
 the form and relative size of these two orbits, comj)ared with each 
 other and with that of the Earth. 
 
 The planes in which the telescopic planets move are very di- 
 
 * This breadth is increased to 248,000,000 miles, if \vc take the extreme 
 distances into account.
 
 216 
 
 THE SOLAR SYSTEM. 
 
 versely inclined to each other. In comparing them with the 
 plane of the Earth's orbit, it is found that some among them, 
 those of Massilia and Angelina, for example, nearly coincide with 
 it; whilst the orbit of PaUas, as may be seen in Plate I., rises at an 
 angle of 34°, that is to say, nearly | ths of a right angle.* 
 
 It now remains to us to terminate this general sketch, by saying 
 a word on the times occupied in their revolutions round the Sun. 
 
 Fre,ii 
 
 Pig. 83.— Orbits of Proia and Polyliymnia, compared togetbcr, and with that of thu ICarth 
 
 These periods are comprised between 1193 and 2310 mean solar days, 
 that is to say, between 3 years, 3 months, and 7 days, and G years, 
 3 months, and 28 days, which mark the length of the years of Flora 
 and Maximiliana respectively. It happens, as it does also with the 
 mean distances, that some of the asteroids perform their journey round 
 the Sun in times almost equal. In the case of Egeria and Astrea the 
 difference is not more than half a day ; for Eurydice and Clytie a 
 
 * These considerable inclinations have caused the name of ultra-zodiacal 
 planets to be given to the asteroids ; as a great number of them, in consequence 
 of their inclination arc observed out of the zone in wliich the principal planets 
 move.
 
 THE MINOll PLANETS. 217 
 
 quarter of a day ; and, lastly, Leto and Bellona accomplish their 
 revolutions, one in 1G88"295 days, the other in 1688-546, that is 
 to say, with a difference of about six hours and two minutes only. 
 This is a direct consequence of one of the laws of Kepler, which con- 
 nects the time of revolution and mean distances of the planets of 
 the system. 
 
 We will now pass under review some of the principal bodies 
 of this group, and see whether we have yet discovered any facts 
 relating to their dimensions and physical constitution. 
 
 Vesta is the most brilliant of the entire family. It is visible to 
 the naked eye in a very clear sky, and its light, a palish yellow, 
 is whiter than those of the three planets discovered before it. 
 It takes three years and eight months to accomplish its entire 
 revolution round the Sun, iit a mean distance of 223,000,000 miles. 
 As its orbit is relatively but little elongated, there is only a differ- 
 ence of 4,000,000 miles between its perihelion and aphelion. Its 
 real diameter, measured by Madler, is about 300 miles, not the 
 twenty-fifth part of the diameter of the Earth, so that the surface 
 of our globe comprises nearly 700 times that of Vesta. Here, 
 then, is a planet, the entire surface of which contains but the nintli 
 part of the European continent. The volume of the Earth is nearly 
 18,000 times that of Vesta. 
 
 Juno has the aspect of a star of the eighth magnitude, and is, 
 consequently, invisible to the naked eye. Its colour is reddish, and 
 its light is subject to variations, which are not less remarkable than 
 the rapidity with which they are accomplished. This phenomenon 
 is not pecu.Kar to Juno ; it is observed in Vesta — which sometimes 
 becomes very bright, — in Ceres, and in many other of the minor 
 planets. Several hj^otheses have been suggested to explain this 
 fact. Some suppose that the different faces of these small bodies 
 do not reflect the solar light with the same intensity ; that some 
 are formed of crystalline facets, or even have a light of their own. 
 Others believe that the small planets are irregularly formed, pre- 
 senting to us consequently sometimes very extensive, and at others 
 very limited, sui'faces. Whichever hypothesis we admit, both take 
 for granted a rotation. Perhaps, in studying with care the j^eriods 
 of these variations, we may learn the durations of these rotatory 
 movements. M. Goldschmidt, who ranks ahnost highest among 
 living astronomers in this branch of research, has already made some 
 interesting observations with this object in view.
 
 218 THE SOLAR SYSTEM. 
 
 Juno recedes from the Sun, at aphelion, to a distance of nearly 
 800,000,000 miles ; at perihelion, it approaches within 180,000,000 
 miles ; hence its mean distance is about 240,000,000, and there is 
 a difference of 120,000,000 miles between its extreme distances. Its 
 orbit is far from having a circular form. 
 
 Madler estimated the diameter of the planet at 360 miles ; which 
 is thus 22 times less than that of the Earth, and its surface is a little 
 more extensive than that of Yesta. It travels over its orbit in 1592^ 
 days, or in 4 terrestrial years and 4 months. 
 
 Ceres, the 56th of the group in the order of distance, and, as 
 has been seen, the first in the order of discovery, appears as a 
 reddish star, the brightness of which is intermediate between that 
 of Juno and Yesta. 
 
 An illustrious observer, Schroter, thought he detected in the 
 vaporous appearance of its disk the proof of the existence of a very 
 extensive atmosphere. The same seemed to hold good for Pallas, 
 and he concluded that these two planets were suiTounded with a 
 gaseous envelope of 500 miles in thickness. Since his time it has 
 been found that these appearances were due to the imperfection of his 
 telescope. 
 
 Ceres revolves round the Sun in 1680| days, at a mean 
 distance of 260,000,000 miles. But, at its minimum distance, 
 it is nearer by 42,000,000 miles than at its greatest distance. 
 The heat and light received from the Sun by these bodies, the dis- 
 tance of which varies in such considerable proportions, vary also 
 between rather wide limits. But as nothing is known of the phy- 
 sical constitution of Ceres or of the condition of its surface, it is im- 
 possible to draw certain conclusions from these data relative to the 
 actual variations in the planet's temperature. 
 
 The diameter of Ceres has been measured several times. But the 
 results are not concordant : whilst it is 450 miles according to Schroter, 
 it is only 160 according to Sir "NY. Herschel, and Argelandcr valued 
 it at 220. If we adopt this last number, we find that the surface 
 of Ceres is only the ISOOth part of that of the terrestrial globe, so 
 that 46,000 globes as large as the planet would be required to equal 
 the volume of the Earth. 
 
 We now pass to Pallas, which revolves round the Sun in 16831 
 days, in an orbit nearly as elongated as that of Juno, greatly 
 inclined to the plane of our ecliptic, and at a mean distance of 
 260,000,000 miles. At its aphelion, Pallas is 320,000,000 miles away
 
 THE MINOR PLANETS. 
 
 219 
 
 from the Sim, whilst at its least distance it is scarcely 200,000,000. 
 At the time of its nearest approach to the Earth, Pallas has the aspect 
 of a star of the seventh magnitude, of a beautiful yellow colour. 
 Its diameter has been estimated at GOO miles.* It is the most import- 
 ant of all the smaller planets, 
 although its diameter is 
 13 times, its surfoce 168 
 times, and its volume 2177 
 times less than that of our 
 Earth. All these numbers, 
 it will be miderstood, are 
 merely approximate, and we 
 give them principally in 
 order that a clear idea may 
 be formed of the relative 
 importance of all the ce- 
 lestial bodies of our system. 
 clearer still. 
 
 Fig. 84. — Comparative dimensions of tlio Eai'th and 
 Juno, Ceres, Pallas, and Vesta. 
 
 Fig. 84 should make this jjoint 
 
 The four planets of which we have just given some details are 
 among the most important of the group. The smallness of nearly 
 all the others is such, that it is not possible to measure their diameters ; 
 as they apjDcar in a telescope merely as luminous points. It is pro- 
 bable that the least of these microscopic bodies have diameters which 
 do not reach many score miles, so that their surface is less than that 
 of one of our English counties. M. Lespiault, from whom this com- 
 parison is borrowed, adds that a good walker could easily in a day 
 make a tour of many of these miniature worlds. 
 
 How long; shall we so on makino- discoveries of fresh bodies in 
 
 o a o 
 
 this zone between Mars and Jupiter ? This is a difficult question 
 to solve, but it is probable that we are now acquainted, if not with 
 the largest of the minor planets, at all events with all those most 
 easily visible from the Earth. The discovery of others will, therefore, 
 become more and more difficult, and the extension of their number is 
 partly subordinate to the use of larger instruments in the research, 
 and more detailed celestial maps. At all events, M. Leverrier, from 
 mathematical considerations, has assigned to the total mass of the 
 bodies which compose the ring such a limit, that, if we suppose them 
 to possess a density equal to that of our own globe, those already 
 discovered form only the tuVo^I^ P^^'^ of it. This woidd make the 
 
 * Laiaont.
 
 220 
 
 THE SOLAR SYSTEM. 
 
 number of the minor planets about 150,000. But, admitting tbat this 
 nmnber may be excessive, and in reducing it to the tenth of its value, 
 this swarm of celestial bodies will still be counted by thousands. 
 
 We have heard so often during the last twenty years of the dis- 
 coveries of new asteroids, that some of our readers may be interested 
 to know the way in which these discoveries are made. Let us begin 
 by stating that it is not chance that presides over these researches. 
 From the discovery of Piazzi do^^^l to our own time, it is only b}' 
 special and systematic examination that our knowlege of the solar 
 system has been increased in such an astonishing manner. 
 
 It is not, as we have already said, by its aspect, that a planet 
 is distinguished in the midst of the starry vault from the multitude of 
 luminous points which surround it ; and this remark applies especially 
 to these small bodies, the diameter of which is insensible. It is 
 by its proper motion, — by its progressive displacement, that it is 
 recognised. How, then, can this be detected ? By using very de- 
 tailed celestial maps, containing all the verj^ small stars, and inces- 
 santly watching the regions mapped for the appearance of new ones. 
 Such is the first sine qua non for such a research, and the astro- 
 nomer who undertakes the construction of celestial maps, executed 
 with the necessary detail and precision, is of necessity the fellow- 
 labourer of him who actually discovers the planets. Let us add, 
 that often these two collahomtcurs arc one and the same person. 
 
 It is not necessary to explore the entire sky. It is sufficient to 
 examine the regions nearest the ecliptic, because, as the orbit of u 
 planet must, necessarily, twice in each revolution, pass through the 
 plane of the orbit of the Earth, it is enough to look out for the body at 
 one or other of these nodal passages. 
 
 Fig. 85 reproduces, on a reduced scale, one of the maps con- 
 structed by a distinguished observer, M. Chacornac, to whom 
 astronomy owes, besides numerous observations of difierent kinds, 
 the discovery of eight telescopic planets. 
 
 This map includes all stars doAvn to the thirteenth magnitude. 
 Furnished with a map of this kind, and a telescope powerful enough 
 to show all the stars marked on it, the observer who intends to 
 devote himself to the search after small planets will proceed in the 
 follo^\'ing manner : — 
 
 He will place in the field of view of his telescope six spider lines 
 at right angles to each other, and all of them the same distance apart, 
 in such a manner that several squares will be formed, embracing just 
 as mucli of the heavens as do those shoA\n in the map. He will then
 
 THE MIXOR PLANETS. 
 
 221 
 
 direct his telescope to the region of the sky he wishes to examine, 
 represented by the map, so as to be able to compare successively 
 each square with the corresponding portion of the sky. 
 
 He can then assure himself if the numbers and positions of the 
 stars mapped, and the stars observed, are identical. If he observe 
 in the field of view a luminous point which is not marked in the map, 
 it is evident that this can only arise from two causes, if the map be well 
 made. It may be that the new body is a star of variable brightness, and 
 
 Fig. 80. — Ecliptic chart, from M. Cliacorijac's "Star Atlas. '' 
 
 that it was not visible at the time the map was made ; or indeed that 
 it is a planet. It then becomes necessary to distinguish between these 
 two possible cases. We must examine whether the new body remains 
 invariably fixed at the same point, or, on the other hand, if it changes 
 its position with regard to the neighbouring stars. The proper motion 
 is generally so sensible, that in the course of one evening the change 
 of position may be detected. In this last case a new j^lanet, or perhaps 
 a comet, has been discovered.
 
 222 
 
 THE SOLAR SYSTEM. 
 
 Fig. 86, whicli represents, on the left the map itself, on the right 
 the field of view of the instrmnent, will be sufiicient to give an idea of 
 the manner in which this result is attained. The stars shown in both 
 are the same, and in the field of view of the telescope is seen the new 
 
 Fig. 80 — Discovci-y of a small pianct by hh m- .ii i:,lij.ti.- C'liarts. 
 
 body, which, absent from the map, by its successive positions indi- 
 cates the presence of a bod}^ belonging to our solar system. 
 
 It is scarcely necessary to add after this that these researches 
 are not only laborious, but demand the greatest patience and watch- 
 fulness.* 
 
 * In 18G1 nine minor planets were discovered, in 1802 five, and in 180.3-4 
 six only. If we suppose the watch kept constant, there is here exhibited a 
 decrease, resulting apparently from a downward march, which may be explained 
 by the fact that the minor planets actually discovered, being the largest of the 
 group, the others more and more escape observation. The hypothesis of a largo 
 number of asteroids is not shaken.
 
 juriTER. 223 
 
 XVI. 
 
 JUPITEE. 
 
 Distances of Jupiter from the Earth and Sun — Eeal and apparent Dimensions — 
 Movement of Eotation ; Days and Nights — Years and Seasons — Dark and 
 Light Belts on its Disk ; Atmosphere — Satellites, their ]\Iovements and 
 Distances — Eclipses of the Satellites — Their real Dimensions. 
 
 From that region of space where we have just seen the smallest 
 members of our system circulating in their orbits, we pass without 
 transition to the largest planet — the colossal Jupiter. 
 
 To the naked eye, Jupiter appears as a star of the first magnitude, 
 the brightness of which, variable with its distance from the Earth, is 
 sometimes, when the Moon is absent, sufiicient to throw a shadow. 
 Its light is constant, and scintillates but rarely. But if, to examine it, 
 a rather powerful telescope is used, the point expands into a well- 
 defined disk, and is generally seen to be accompanied by three or 
 four little points of light, which oscillate in short periods of time 
 round the central planet : these are the satellites of Jupiter. 
 
 Venus, Mercury, and Mars,, as we have seen, are without 
 satellites; the Earth has only one. Jupiter, with its four moons, 
 which the powerfid attraction of its bulk compels to revolve round 
 him, exhibits to us therefore a small system analogous to the solar 
 one of which it forms part and which it reproduces on a smaller scale. 
 
 To arrive in our journey from the Sun as far as the Jordan 
 system, we must pass over a distance which exceeds five times the 
 mean distance of the Sun from the Earth, or, in the mean, nearly 
 500,000,000 ndles. But the orbit described by Jupiter round the 
 Sun difters from the circular form more than does the Earth's. Its 
 distance, therefore, is more variable, and while at perihelion it 
 reaches 472,000,000 miles, at its greatest distance it is not loss than
 
 224 THE SOT-AR SYSTEM. 
 
 520,000,000 miles from the Sun, the difference being therefore 
 48,000,000 miles. 
 
 Jupiter, therefore, as seen from the Sun, presents an apparent 
 diameter sometimes greater, sometimes less, than its mean one ; and of 
 course the same phenomenon is seen by observers situated on the 
 Earth, but in a much greater proportion. Fig. 87 will give an idea 
 of the variations of size which the disk of Jupiter, at the times of 
 its mean and extreme distances from the Earth, presents to us. 
 
 The reason of this difference between the apparent diameters of 
 the disk is easily explained. The orbit of Jupiter, like that of Mars, 
 encircles the terrestrial one, and the motions of the two bodies in 
 their respective orbits brings them, once in every 13 months, in the 
 
 Fig. 87. — Apparent diuiciisioiis nf JiijiUcr at its mean aud extreme distances 
 tnmi the Earth. 
 
 same straight line with the Sun, and on the same side of it ; Jupiter 
 is then in oppoaifion, and its distance from the Earth is measured by 
 the difference of the distances of the two bodies from the Sun. In a 
 similar period the two planets are still in a straight line with regard 
 to the Sun, but on opposite sides of it. This is the conjunction of 
 Jupiter, and the distance of the two planets is found by adding their 
 respective distances from the Sun. These distances themselves are 
 sometimes smaller and sometimes greater that at others, and, there- 
 fore, the same thing happens with regard to those which separate 
 the Earth from Jupiter at the time of opposition and conjunction. 
 
 At its greatest distance from the Earth, Jupiter is 617,000,000 
 miles from us ; at opposition it may be within 375,000,000 miles ; but 
 in the mean, the distance of Jupiter at conjunction with the Sun is 
 501,000,000 miles, and at opposition, 400,000,000 miles, the difference 
 being the diameter of the Earth's orbij.
 
 JUPITER. 225 
 
 From the preceding numbers we may perceive the iromense de- 
 velopment of the orbit described by this member of our planetary 
 system. Thus, to traverse this path, it requires 12 years. This gives 
 a mean rate of upwards of 700,000 miles a day, or nearly 30,000 
 miles an hour. 
 
 The movements with which we are acquainted on the Earth can 
 give us no idea of such a mass travelling eternally through the depths 
 of space with a velocity 80 times greater than that of a caimon-ball. 
 
 We look upon the volume of the Earth as immense when we 
 compare it to the objects which we are in the habit of seeing about 
 us ; but how much more stupendous is the size of Jupiter, which is 
 
 Fig. S8. — Comparative dimensions of Jupiter and the Earth. 
 
 1400 times larger than our globe ! This value has been deduced 
 from the real diameter of the planet, .which has in turn been deduced 
 from its apparent size and real distance. 
 
 Fig. 88 gives an idea of the comparative dimensions of the Earth 
 and the planet which we are about to describe. The diameter of 
 Jupiter is nearly 11 times greater than that of the Earth, being 89,000 
 miles. Seen at the distance of the Moon, this immense globe would 
 appear to us with a diameter 34 1 times larger than that of our 
 satellite, and its disk would embrace, on the celestial vault, 1200 
 times the space that the full moon occujDies. 
 
 The form of the globe of Jupiter is not that of a perfect sphere : 
 it is an ellipsoid, flattened, like the Earth, at the poles of rotation. 
 But whilst the polar compression of the terrestrial spheroid is but about 
 j^oth, that of the globe of this immense planet is -^^th, so that there is
 
 226 
 
 THE SOLAR SYSTEM. 
 
 between the polar diameter— the smallest, and the equatorial diameter, 
 a diflPerence of 4900 miles, which gives for the flattening of each 
 pole 2450 miles. 
 
 This elliptical form is very perceptible in the telescope ; it is per- 
 ceived at once, without any measurement. The drawings which 
 accompany our description convey a good idea of this flattened form. 
 
 If it be true, as physical experiments and geological facts tend to 
 show, that the planets are bodies the primitive state of which was 
 fluid, the elliptical form of their meridians is but a consequence of 
 their rotation. The flattening of a sphere, therefore, gives rise to the 
 idea of its rotation round an axis which passes through its centre. 
 
 Yenus, the Earth, Mars, have movements of rotation — is it the 
 same with Jupiter ? It is, and the velocity of its movement, taken in 
 
 Fig. 89.— Rotation of Jupiter. Apparent displacement of two spots in 37 minutes 15 seconds. 
 
 connexion with its small density, explains at once the extent of the 
 flattening to Avhich we refer, which has been carefully measured. 
 
 Very early* observations of the planet demonstrated the rotatory 
 movement of Jupiter. This was accomplished by observing the 
 movement of the spots on its surface. "We give two views of the 
 planet, after Beer and Madler, at an interval of 37 minutes 15 seconds, 
 which clearly show the apparent displacement of the two dark spots 
 produced by this movement. 
 
 This immense globe revolves on itself in about 10 hours (9 
 hours 55 minutes, 26 seconds.) A point situated on the equator 
 of Jupiter travels, therefore, by virtue of this movement, eight miles 
 a second, or 27 times as rapidly as one situated on our equator. 
 
 * Cassini, in 1665, first measured the time in which this rotation is accom- 
 phshed. After him we must mention Sir W, Herschel, Airy, Beer, and 
 Madler,
 
 JUPITEK. 227 
 
 The rotation of Jupiter of course produces the phenomena of day 
 and nig-ht on the planet. But as the axis of rotation is very little 
 inclined to the plane of the orbit,* there is but little difference in 
 their length, the maximum of which is five hours, for the greater por- 
 tion of the surface, throughout the length of the planet's long year. 
 Two very narrow zones, situated at the two poles, comprise those 
 regions of the planet where the day and night exceed the time 
 of rotation. At the poles themselves, the Sun is visible for nearly 
 six years, and remains set afterwards for a like period. 
 
 The seasons are also very slightly varied on Jupiter ; at least at 
 any given place. Summer reigns during the whole year in the 
 zones nearest to the equator, whilst the temperate regions rejoice in 
 a perpetual spring, those which surround the poles being subject 
 to a continual winter. Nothing is known as to the real climatic or 
 
 Pig. 90. — Iiiclinatiou of the axis of Jupiter to the plane of its orbit. 
 
 meteorological conditions of these seasons. At Jupiter's distance 
 from the Sun, the light and heat of that radiant body only possess 
 but a small fraction of their intensity at the distance of the Earth ; 
 but this diminution, consequent upon the increasing distance, may be 
 compensated by physical conditions, such as a greater density of the 
 atmosphere, a higher calorific or luminous capacity of the matters 
 composing the soil. Does the globe of Jupiter still possess an internal 
 heat considerable enough to raise the temperature of the crust to 
 an extent sufficient to make up for the relative feebleness of the 
 solar heat ? These are questions on which science is still silent. 
 
 Juj)iter's year is, as we have said, equal to about twelve of ours : 
 to be exact, the length of his year is 4232-jfiff days, or 11 years, 10 
 
 * The angle of inclination is nearly a right angle — it is about 87^
 
 228 THE SOLAR SYSTEM. 
 
 months, 14 days, 19 hours. It follows, therefore, that, measured in 
 sidereal days of the planet, the year of Jupiter comprises 10,478 rota- 
 tions, or 10,477 Jovian sidereal days. From these numbers, it is 
 easily found that between the sidereal and solar day of Jupiter 
 there is scarcely three seconds difference — that is, three of our 
 seconds. 
 
 Seen from the Earth, Jupiter does not present perceptible phases ; 
 its great distance, and the fact that its orbit is so far removed from 
 our own and away from the Sxm, render a reason for this, which of 
 course does not in any way affect what we have said concerning the 
 planet's days and nights. We possess decisive proofs that the planet 
 does not shine by its own light ; and wc may remember that Mars, 
 at a less distance from the Earth and Sun, presents only small indi- 
 cations of phases. 
 
 We must now look upon the planet from a physical point of view. 
 What we know of the planet's physical constitution has been derived 
 from observations of the belts or patches of different shades which 
 girdle the planet in a direction parallel to its equator. The draw- 
 ings we have already given indicate these appearances, but to give 
 our readers the best idea possible, we reproduce, in Plate XVI., a 
 magnificent drawing by jMr. De La Hue. 
 
 Broad greyish belts stretch across the disk, north and south of 
 the equator, and between these a brighter [often rose-coloured] space 
 marks the equatorial regions. On either side of the principal belts 
 approaching the polar regions, other narrower bands are seen, 
 sometimes dark, sometimes light. The brightness of the disk is 
 decidedly more feeble at the poles. 
 
 With a low magnifying power these belts seem perfectly straight, 
 but under better ojDtical conditions it is easy to see numerous irregu- 
 larities and transverse markings, vandyking and crossing the more 
 visible features in various directions, in the middle even of the bands. 
 One important circumstance is, that the dark bands do not reach the 
 borders of the disk. 
 
 Independently of the bright and dark belts, spots of various forms 
 are seen ; and it is by the observation of these points that the 
 time of rotation has been determined. The spots and belts vary 
 besides in fonn and position. On several occasions one or other of 
 the two large dark belts has entirely disappeared. This happened 
 to the northern belt in 1834 and 1835. 
 
 It is, then, considered certain that these phenomena are atmo- 
 spheric, and the parallelism of the strata of clouds is very naturally
 
 . .TTPFTF.K. 231 
 
 expluinod by the clirectioii and velocity of the rotation. The equa- 
 torial regions of Jupiter are doubtless regions of great aerial currents, 
 analogous to the trade-winds of our planet — with this difference, 
 however, as remarked by Arago, that the direction in wliich the 
 cloud-belts move is ojjposite to that of our own trade- winds. 
 
 The variability of position of the irregular spots indicates a proper 
 motion ; according to Beer and Madler, the rapidity of their dis- 
 placement is about 100 miles a day, — the velocity of a light wind on 
 our Earth. We have, therefore, no reason for supposing the existence 
 of violent tempests and hurricanes, which were at first imagined. 
 We may hold, on the contrary, that the Jovian meteorological phe- 
 nomena are produced very calmly. The long year of the planet, 
 the slight and gradual variations of its seasons, the no doubt con- 
 siderable density of its atmosphere, the force of gravity at its surf\ice, 
 are so many facts which tend to produce a great atmospheric stability. 
 
 The mass of Jupiter equals 338 times that of our globe, whilst its 
 
 Fig. 91. — Jupiter and its Four Satellites. 
 
 volume, as we have seen, exceeds the Earth's nearly 1400 times. 
 This gives, for the matter of which it is formed, a mean densitj^ less 
 than a quarter of the terrestrial density. It is a third more than 
 that of water : it is easy to conjecture that the strata forming the 
 sm-fiice have, at most, the density of water. Is the surface of Jupiter, 
 then, in a liquid state ? Here observations fail us. 
 
 Four luminous points — four small stars — unceasingly accompany 
 Jupiter in its twelve - yearly revolution. They are easily observed 
 with small telescopes. 
 
 From hour to hour their positions vary, and they seem to oscillate 
 from one side to the other of the disk, in paths nearly parallel to the 
 direction of the belts, that is to say, to the equator of Jupiter. These 
 are its moons or satellites. They are besides frequently seen to dis- 
 appear, one, two, and even three at a time. It sometimes, nideed, 
 even happens that not one of the fom- is visible. Jupiter then 
 appears alone, deprived of its companions. This state of thmgs was
 
 232 
 
 THE SOLAR SYSTEM. 
 
 observed by Mr. Dawes on the 27th of September, 1843. But it only 
 happens very rarely. 
 
 Taking these satellites in the order of their distances, the times 
 of their revolutions are as follows : 
 
 First satellite (lo) . . . 
 Second „ (Europa) . 
 Third „ (Ganymede) 
 Fourth „ (Callisto) . 
 
 1 day, 18 hours, 28 minutes. 
 3 „ 13 „ 14 „ 
 7 „ 3 „ 43 „ 
 16 „ 16 „ 32 „ 
 
 In comparing these times with that of the revolution of the Moon, 
 it is seen that the movements of the satellites of Jupiter are much 
 
 Fig. 02.— The Jovian system. Orbits of the Satellites. 
 
 more rapid than that of our Moon. This rapidity is the more 
 marked, as their distances from the planet, and therefore the lengths 
 of their orbits, are more considerable than in the case of our satellite. 
 Measured from the centre of the planet, the mean distances of these 
 satellites are, of lo, 278,000 miles ; of Europa, 443,000 ; of Gany- 
 mede, 707,000 ; and of CaUisto, 1,243,000. These distances are, of 
 course, the radii of their orbits. 
 
 The orbits of the two first satellites are nearly circular ; those of 
 file other two are more elongated. But on the scale on which figure
 
 JUPITER. 233 
 
 92 presents the system of Jupiter to us, these elongations arc im- 
 perceptible. 
 
 The total space occupied by this interesting system measures 
 nearly 2^ millions of miles in diameter. 
 
 The study of the other phenomena observable in the Jovian 
 system presents great interest. Among these phenomena, we must 
 especially mention eclipses. 
 
 Jupiter, like all celestial bodies not self-luminous, casts into space, 
 in the direction opposite to the Sun, a cone of shade, the axis of which 
 is always situated in the plane of the orbit, and the length of which 
 is proportionate to the dimensions of the planet and its distance from 
 the Sun. Now, the three first satellites revolve round Jupiter in 
 planes but little inclined to the planet's orbit, so that at each revo- 
 lution they pass through the cone of shade ; thereby causing, to them- 
 selves an eclipse of the Siui, and to Jupiter an eclipse of its satellites. 
 From the Earth, we can distinctly see the disappearance or immersion 
 of the satellites in the planet's shadow, as also their reappearances or 
 emersiom. 
 
 The fourth satellite also undergoes eclipse, but, on account of the 
 much greater inclination of the plane of its orbit, these eclipses 
 are less frequent. Sometimes it grazes, as it were, the limit of the 
 cone of shade, and the small loss of light which the satellite under- 
 goes shows us that it is but partially eclipsed. 
 
 The nights of Jupiter, then, are illuminated by four moons, whicli 
 are to be seen, sometimes singly, sometimes together, above the 
 horizon, and which may present at the same time all the varying 
 phases of our single satellite. The nearest appears to the inhabitants 
 of the planet with dimensions nearly equal to those of the Moon seen 
 from the Earth. It is always eclipsed when full, that is about every 
 42 hours, or four of Jupiter's days. 
 
 The second and third satellites in the order of distance, seen 
 from Jupiter, appear of equal apparent diameter — a little more than 
 half of that of our Moon : their eclipses take place, for the second, 
 every 85 hours, — 8 J of Jupiter's days; and, for the third, at 
 successive intervals of 171 hours, or 17^ of the planet's days. 
 
 Jupiter's moons, at each of their revolutions, pass also between 
 the planet and the Sun. At the time of each new Moon, then, as 
 their distances are such that the cone of shade which they throw 
 behind them reaches the surface of Jupiter, they give rise, on the 
 regions they pass over, to eclipses of the Sun, either partial or total. 
 These phenomena are also visible from the Earth, and Plate XVI.
 
 234 
 
 THE SOLAK SYSTEM. 
 
 represents Jupiter at the moment when the shadow of a satellite 
 appears on the disk in the form of a small black dot, whilst the 
 satellite itself is seen, a small bright circle, on the greyish belts of 
 the planet. The three first satellites are never subject to simul- 
 taneous eclipses ; this follows from a law of their motions and rela- 
 tive distances, discovered by Laj)lace. 
 
 This consequence is only applicable to eclipses properly so called, 
 
 Oil'i! 
 
 
 ^artlt 
 
 Fig. 03.— Eclipses and ii.is&ifrcs of the Siitullitcs olMiipitcr, seen fi-oni the E;irtli. 
 
 that is to say, to the passages of tlic satellites through the cone of 
 the planet's shadow. But to an observer placed on the Earth, a 
 satellite may disappear witliout undergoing an eclipse ; it may pass 
 behind the disk of the planet and be occulted. 
 
 Lastly, as mentioned above, it may happen tliat during the 
 disappearance of the three satellites, the fourth is between the Earth 
 and the planet. Then the planet equally appears solitary and de- 
 prived of its companions. 
 
 Figure 93 will render clear the various positions Avliich tlic
 
 JUPITER. 235 
 
 satellites may occupy with reference to the Earth. One of them 
 in this figure is represented eclipsed, the other is seen projected on 
 the disk, on which also its shadow is thrown ; a third is hidden by the 
 planet, and the fourth is entirely visible. 
 
 We have seen what are the apparent dimensions of the four 
 satellites, as seen from Jupiter, compared to the apparent size of our 
 Moon. But we must not confound the apparent with the real 
 diameters. It follows, from the measures made by astronomers, that 
 the diameter of the first satellite is 2440 miles ; of the second, 21D2 ; 
 of the third, 3579 ; and of the fourth, 3062 miles. So the third and 
 fourth in the order of distance are the first and second in order of 
 magnitude ; one only is less than our Moon ; taken together, they 
 would form a body 9^ times larger than it, or about one-fifth of the 
 Aolume of the Earth. 
 
 Lastly, the volmne of the largest exceeds by two-thirds the 
 
 Fig. P4.— Dimensions of the S.atellitcs of Jupiter compai-ed with tliose of the Eartli 
 rind the Moon. 
 
 volume of the planet Mercury. Here, then, we have a secondary 
 body larger than a primary one of the first order, and far surpassing 
 in size those which circidate between Mars and Jupiter. 
 
 Sir W. Herschel studied with great care the variations of bright- 
 ness of each satellite, and found that they occurred in each period 
 of revolution. He hence imagined that this variability was owing 
 to the nature of the faces which eacli of these bodies successively 
 presents to the Earth. It follows from these observations that, as 
 in the case o'f the Moon, the same lace is always turned towards the 
 primary : this of course would render the times of rotation and 
 revolution equal. The disks of the third and fourth satellite present 
 spots which are represented in the preceding drawing (fig. 94). 
 
 There is another point. These moons can be distinguisluHl uot
 
 236 THE SOLAR SYSTEM. 
 
 only by their dimensions and the brightness of their light, but also 
 by their colour. According to Beer and Madler, the first and 
 second satellites have a bluish tint, especially when compared with 
 the third, the light of which is yellow. Some difference in colour 
 is certain, although contrast may go for something. The light of 
 the fourth satellite is bluish,* like that of the first two. 
 
 [Bringing our Mars observations to bear, we might almost be 
 justified in supposing these varying colours to be caused by different 
 distributions of land and water.] 
 
 The united masses of Jupiter and its four satellites are the 
 joL-th part of the mass of the Sun ; that of Jupiter alone is 
 6000 times the mass of its satellites, or, as we have said, 338 
 times the mass of our Earth. Lastly, Jupiter exceeds in mass 
 all the other bodies of the solar system, the Sun excepted, by 2^ 
 times. 
 
 f* The light, of the fourth satellite is reddish, decidedly.— W. R. 1).]
 
 SATURN. 237 
 
 XVll. 
 
 SATURN. 
 
 Its Exceptional Pliysical Constitution — Distances of Saturn from the Sun auJ 
 from the Earth — Apparent and real Dimensions — Movement of llotation and 
 Polar Compression — Days, Nights, and Seasons — Rings ; Movement of Rota- 
 tion — Satellites — Celestial Phenomena to an Inhabitant of the Planet. 
 
 If Jupiter be the largest planet of the solar system, Saturn is by far 
 the most gorgeously attended among the secondary systems of which 
 that system itself is composed. 
 
 Not by four only, but by even eight satellites, is the central planet 
 encircled ; and if these eight moons in their revolutions do not give 
 rise to eclipses as frequently as do those of Jupiter, the inhabitants of 
 Saturn possess a much stranger spectacle, one, as far as we know, 
 iinique in the planetary system ; we allude to the wondrous ring- 
 system which surrounds the planet at some distance from its equator, 
 and revolves eternally round it. Thus, then, we see that the further 
 we go in our exploration of the solar system, the more have we to 
 admire the wonderful variety in the constitution of the bodies which 
 people it. Now we have to deal with isolated planets, such as Mercury, 
 Venus, and Mars ; now, with such a group of celestial pigmies as 
 the telescopic planets ; and, again, with matter more finely divided still, 
 like the Zodiacal light, and the shooting stars. Then we find the* 
 Earth accompanied by a single Moon in its annual revolution round 
 the common focus ; and, lastly, the group of large planets, which 
 are not only distinguished by their enormous dimensions, but by 
 the number of secondary bodies maintained in their sphere of 
 attraction, which with their primaries form real systems in minia- 
 ture. 
 
 Up to this point, however, whatever may have been the variety 
 of the elements of each planet, there has been a common point of
 
 288 THE 80LAK SYSTEM. 
 
 resemblance — the form of eacli lias been a regular spheroid. Nor 
 have the revelations of the telescojje taught ns that the planets 
 which we have already described are surrounded with anything save 
 the satellites we have described. 
 
 [The first peep at Saturn, however, infinitely extends our mental 
 horizon ; besides eight satellites, it is surrounded by a system of 
 rings, some shining with a golden light, others transparent ; and 
 it may possibly be, that the Zodiacal light, and the meteoric and 
 asteroidal rings, may be to the 8mi what Saturn's rings are to 
 Saturn — an innumerable company of satellites, as the sands on all 
 shores for multitude.] 
 
 Before, however, describing the rings of Saturn in detail, as 
 they deserve, we must chronicle the principal astronomical data of 
 the planet itself. 
 
 The mean distance of Saturn from the Sun exceeds 9| times 
 that of the Earth, a distance expressed by the . enormous num- 
 ber of 909,000,00U miles, — not far from double the distance of 
 Jupiter. Seen from such a distance, the solar disk is reduced to the 
 hmidredth of its apparent size to us ; and it is in this projiortion 
 that the intensity of the light and heat is reduced, unless there be 
 some comijensating power in its atmosphere. 
 
 The orbit of Saturn is not circular ; it has, like that of the other 
 planets, the form of an ellipse, of which the Sun docs not occupy the 
 centre, but the focus. The planet is therefore sometimes nearer to, 
 and sometimes more distant from the radiant body. At its perihelion 
 and aphelion respectively, these distances are 858,000,000 and 
 960,000,000 miles. There is, therefore, between the extreme dis- 
 tances a difference of some 100,000,000 miles. From these numbers 
 it is easy to deduce the length of the path described by Saturn in 
 his long year of 10,760 of our days, or 29 years 167 days. Saturn 
 travels along this orbit with a mean velocity of 529,000 miles a-day, 
 or 22,000 miles an hour.* 
 
 If Saturn, by reason of the elliptical form of its orbit, approaches 
 more or less to the focus of the solar system, it is easy to understand 
 that its distances from the Earth must vary still more, according 
 to the relative positions of the two planets and the Sun, It is at 
 opposition that they are nearest ; at conjunction, on the contrary, 
 their distance is much more considerable. These two periods occur 
 
 * If this velocity be compared with those that we have given for the planets 
 lying between the Sun and Saturn, we shall see that the movement of the 
 planets in their orbits is slower as the distance increases. This is a direct 
 consequence of a law of Kepler's, of which more anon.
 
 SATURN. 239 
 
 at interviils of 878 days, or a little more than a year. But the 
 maxiinum or mininium distances themselves vary, from one period to 
 another, because each of the two planets, according to the point of 
 its orbit which it occupies, is itself more or less removed from the 8un. 
 The distance of Saturn from the Earth may vary between 1,057,000,000 
 and 761,000,000 miles. This difference of 296,000,000 miles pro- 
 duces, as may be imagined, a corresponding variation in the appa- 
 rent dimensions of the planet : fig. 95 shows between what limits that 
 variation lies. 
 
 Nevertheless, as seen from the Earth, Saturn always appears 
 under the aspect of a star of the first magnitude, which our best 
 telescopes present to us under the form of a spheroidal globe, sur- 
 rounded, as we have seen, with a ring brighter than itself. 
 
 Let us continue to consider but the nucleus of this singular 
 system. Its distance being kno^vn, it is easy to deduce its real 
 dimensions : these show Saturn to be the second of the principal 
 planets, as far as its size is concerned. 
 
 Fig. 95. — Apparent dimensions of Saturn at its extreme and mean dist.uiccs from the Earth, 
 showing also the different appearances presented by its ring-system. 
 
 As it turns rapidly on one of its diameters, it is much flattened 
 at the poles of rotation, so that it is necessary, in giving its dimen- 
 sions, to distinguish between the axis, or polar diameter, and that 
 of its equator. While the latter measures 9^ times more than that 
 of the Earth's mean diameter, or 75,100 miles, the former is only 
 8^ times greater, or 68,270 miles. The difference of 6830 miles 
 represents a flattening of ^i^th, that of the Earth being j^ o-th, or 
 26 miles. 
 
 To make a tour of this immense globe, taking the shortest way, 
 its inhabitants would have to travel nearly 214,000 miles passing- 
 through the poles, or 236,000 along the equator. 
 
 These distances are less than on Jupiter, but they arc more than 
 nine times greater than those of our globe. These dimensions give 
 a surface of 16,655,000,000 square miles, and a volume more than 
 776 times greater than that of our Earth. 
 
 But the mass of this enormous spheroid is far from being com- 
 parable with its volume — at least, if we compare this with that
 
 240 
 
 THE SOLAR SYSTEM. 
 
 of the Earth : it is but a little more than 100 times greater* (102-683). 
 This indicates, on the supposition that it is equally dense throughout, 
 that it is composed of matter seven times lighter than the materials 
 of our Earth, and consequently less dense than water ; [in fact, about 
 the density of oak or sulphuric ether.] 
 
 ■ The rotatory movement of Saturn has been determined by 
 observations of the dark bands which cross the disk in a direction 
 parallel to its equator ; the inequalities of these bands, by their 
 periodical return, have enabled astronomers to calculate the time of 
 rotation, which is 10 hours, 29 minutes, 17 seconds. 
 
 Fig. 96. — Saturn and the Earth ; Comparative dimensions. 
 
 Here, then, we find one of the largest planets with a period of 
 rotation less than half those of Mercury, Venus, Mars, or the Earth. 
 Day and night succeed each other on the average at intervals of five 
 hours, but the length of the year, which comprises 24,631 complete 
 rotations, or 24,630 solar days of Saturn, causes the seasons to modify 
 the lengths of day and night but very slowly. 
 
 As to the seasons themselves, they are much more varied than 
 on Jupiter, since, owing to the considerable inclination of the axis 
 to the plane of the orbit, f Satiu'n presents to the Sun sometimes 
 
 * That is the j^th part of the mass of the Sun. 
 [t This inclination is G;i° 10' 32".J
 
 PLATE XVII. 
 
 SATURN. 
 
 From the observations of Bond, Struve, and Warren De La Rue, 
 
 November 1852, and March ISriG.
 
 SATURN. 243 
 
 one, and sometimes the other, of its poles of rotation. For the same 
 place on its sm-face, the altitude of the Sun above the horizon is still 
 more variable than on the Earth ; but if we wish to form an idea of 
 the change of temperature due to this cause, it is important to remark 
 that the altitude of which we sj)eak varies thirty times less rapidly 
 than with us. Each of Saturn's seasons lasts more than seven of our 
 years, and there is nearly fifteen years' interval between the 
 autumn and spring equinoxes, and between the summer and winter 
 solstices. 
 
 But we should have but an incomplete idea of the phenomena 
 presented by the days, nights, and seasons of Saturn, if we did not 
 take into account the modifications produced in these elements by 
 the existence of the annular appendages by which this magnificent 
 planet is surrounded, and by the presence on the horizon of the eight 
 satellites which escort it in its long revolution of thirty years. The 
 drawings given in Plate XVII show Saturn as it was observed, at 
 an interval of nearly three years, in two points of its orbit, distant 
 enough to modify perceptibly its position relatively to the Earth and 
 the Sim ; all the details of the disk and of the rings perceived by 
 the most powerful instruments are faithfully reproduced. 
 
 At the time of the discovery of this strange system, telescopes 
 had just been invented. The imperfections of these instruments 
 threw Galileo, says Arago, " into great perplexity." A letter to 
 the Grand Duke of Tuscany informs us that Saturn seemed to him 
 tn'corj)s. " When I observe Saturn," he remarks, " with a glass of 
 a power of more than thirty times, the central body seems the largest ; 
 the two others, situated one on the east, the other on the west, and 
 on a line which does not coincide with the direction of the zodiac, 
 seem to touch it. They are like two supporters, who help old 
 Saturn on his way, and always remain at his side. "With a glass 
 of smaller magnifying power, the planet appears elongated and of the 
 form of an olive." 
 
 Saturn subsequently appeared to the illustrious astronomer per- 
 fectly round. He regarded his preceding observations as optical 
 divisions, and in his disappointment exclaimed, " Can it be possible 
 some demon has mocked me ? " This is the first record we have of 
 the disappearance of the rings, of which more presently. Huyghens 
 subsequently observed these appendages of Saturn, and he first 
 gave the explanation which the theory of the planet's motion and 
 the employment of more powerful instruments have definitely con- 
 firmed. 
 
 [Most encouraging is the chapter of the history of Modern Astro-
 
 244 THE SOI;AR SYSTEM. 
 
 nomy whicli tells us how eye and mind have bridged over the tre- 
 mendous gap which separates us from the planet. We have seen 
 by degrees a ring evolved out of the triform planet, and the great 
 division in the ring and the irregularities on it, brought to light. 
 Enceladus, and coy Mimas, faintest of twinklers, are caught by 
 Herschel's giant mirrors, and he, too, first among men realises the 
 wonderful tenuity of the ring along which he saw those satellites 
 travelling, " like pearls strung on a silver thread." Then Bond comes 
 on the field, and furnishes evidence to show that we must multiply 
 the number of separate rings we know not how many fold. And 
 here we reach the golden age of Saturnian discovery, when Bond, 
 with the giant refractor of Cambridge, U.S., and Dawes, with his 
 eagle gaze and 6i- inches Munich glass, first beheld that wonderful 
 dark semi-transjjarent ring which still remains one of the wonders of 
 our system. But the end is not yet ; ere summer on the southern 
 surface of the ring fades into autumn, Otto Struve in turn comes 
 upon the field, detects, as Dawes had previously done, a division even 
 in the dark ring, and measures it Avhile it is invisible to Lassell's mirror 
 — a proof, if one were needed, of the enormous superiority possessed 
 by refractors in such inquiries. Then we approach 1861, when the 
 ring-plane again passes through the Earth, and Otto Struve and Wray 
 observe those curious nebulous appearances, of which more anon.] 
 
 We know indeed, now, that surrounding Saturn, and nearly in 
 the plane of its equator, is extended a system of rings which may be 
 l)roadl3^ divided into three, of unequal breadths : of these the thickness 
 is relatively very small. The exterior ring, the one furthest from the 
 planet, is separated from the intermediate one by a very distinct 
 break, whilst the interior ring, that nearest to Saturn, seems joined 
 on to the second. Their brightnesses are very different : the inter- 
 mediate ring, the most binlliant of the three, is more luminous than 
 the globe of Saturn ; the exterior ring is of a greyish tint, nearly of 
 the same shade as the dark bands of the disk. Both of these are 
 opaque, and throw on Saturn a very distinct shadow. The interior 
 ring, on the contrary, is dusky, and almost of a purple tinge, and 
 transparent ; it stands out on the globe of Saturn as a dark band, 
 through which the luminous disk is readily seen, [and. without dis- 
 tortion.] 
 
 [Let us dwell for a moment on this transparent ring, the phy- 
 sical features of which are perhaps less remarkable than the fact that 
 it was not discovered till 1850, and had been entirely overlooked, if it 
 existed, till then, not only by all ordinary observers, but by Herschel's 
 great telescopes. When Bond and Dawes discovered it, it was bv no
 
 SATURN. 245 
 
 means easy of observation, but now It may be seen in a four-mcli 
 acbromatic. Another remarkable fact is the probable increase in width 
 since the time of its discovery, which we shall see subsequently to have 
 an Important bearing on one of the h}^3otheses suggested to account 
 for the entire appendage.] 
 
 In order that a clear Idea may be obtained of the positions and 
 breadths of the rings, we give In fig. 97 a view of the system, such 
 
 Fig. 97. — Bird's-ej-e View of Saturn u.ud its riiig-system. 
 
 as would be obtained by an observer placed above the plane of the 
 rings in the prolongation of the planet's polar axis. 
 
 The exterior diameter of the outer ring is 173,500 miles, and Its 
 Inner diameter 153,500 miles; Its breadth, therefore. Is 10,000 miles. 
 These dimensions, for the middle ring, are respectively 150,000, 
 113,400, and 18,300 miles. The distance which separates these tAVO 
 rings Is 1750 miles. The dark ring joins the middle (or bright) 
 ring. The space between Its interior edge and the surface of the 
 planet is 10,150 miles ; Its breadth, therefore. Is 9000 miles. The
 
 246 THE SOLAR SYSTEM. 
 
 entire breadth of the ring-system, therefore, is 39,050 miles. Its 
 thickness is probably not more than 100 miles. 
 
 Can then, such a material system, whether solid or liquid, sustain 
 itself, without point of contact or support, in a constant — or nearly 
 constant— position with regard to the planet ? And, if so, how do its 
 different parts resist the " pull " which the attraction of Saturn ex- 
 ercises on each of them ? It would seem that this immense bridge 
 ought by degrees to break up, and then — catastrophe far beyond 
 anything the face of heaven has yet presented to man's eye — be 
 precipitated in unutterable and headlong fall upon the surface of 
 the planet. 
 
 Laplace first considered this problem. He showed that its equi- 
 librium could not be possible and stable, unless the section of the 
 ring, of elliptical form, presented in several points inequalities of 
 breadth or curvature. Observation has shoAvn that these conditions 
 exist, as the centre of gravity of the ring does not coincide with that 
 of the planet, and slow oscillations in their relative positions take 
 place. Moreover, he showed that there was another essential con- 
 dition — the ring ought to rotate in its plane with a velocity of little 
 more than ten hours. Herschel imagined that he had also detected 
 this ■ rotation, which thus agreed with the result of calculation. His 
 observations, made in 1790, gave a period of rotation of 10 hom-s, 
 32 minutes. 
 
 [Since this time, however, Laplace's investigation has been shown 
 to be insufficient, and Pierce and Maxwell have in turn demonstrated 
 that the rings are not solid, and are not liquid ; and their non-solidity 
 appears to be shown, not only by the variable traces of divisions in the 
 ring and the appearance — may we not almost say the birth ? — of the 
 dark ring, but by the possible increase in the breadth of the ring- 
 sj^stem. The least favourable measure of the width of the ring in 
 Huyghens' time gives 23,667 miles ; Herschel found it 26,297. The 
 most modern recorded measurements give 28,300, so that if we accept 
 these measurements, the present annual increase in the breadth of the 
 ring is 29 miles. 
 
 Of what, then, are the rings composed ? It is now held by some 
 that they are composed of Satellites, and it has been pointed out that 
 on this hypothesis, — 
 
 " The temporary divisions and mottled stripes are easily ex- 
 plained. It is conceivable, for instance, that the streams of sat- 
 ellites forming the rings might be temporarily separated along arcs of 
 greater or less length by narrow strips altogether clear of satellites, 
 or in which satellites might be but sparsely distributed. Divisions
 
 SATURN. 
 
 247 
 
 of the former kind would appear as dark lines, while those of the 
 latter kind would present precisely that mottled appearance seen in 
 the dusky or ash-coloured stripes. The transparency of the dark 
 inner ring is easily understood if we consider the satellites to be 
 sparsely scattered throughout that formation. The fact that this 
 ring has only become ^dsible of late years no longer presents an in- 
 superable difficulty, for it is readily conceivable that the satellites 
 forming the dark ring have originally belonged to the inner bright 
 ring, whence collisions or disturbing attractions have but lately pro- 
 pelled or drawn them. The gradual spreading out of the rings is 
 explicable when the system is supposed to consist of satellites only 
 connected by their mutual attractions ; while the thinness of the 
 system is obviously a necessary consequence of such a formation, 
 
 Fig. 98.— Satuni, Dec. 26, 1861, atlS'' SO-". OV'ray.) 
 Luminous appendages of the ring. The satclUtes are also seen apparently on the ring. 
 
 for the attraction of Saturn's bulging equatorial regions would compel 
 each satellite to travel near the plane of Saturn's equator."* 
 
 The elliptical shading on the inner bright ring at the ends of the 
 apjDarent longer axis of the dark ring, which is rej)resented in our 
 figures, and has been a sore puzzle to our observers, also finds a 
 possible explanation : — 
 
 " We have only to imagine that the satellites are strewn more 
 densely near the outer edges of the bright rings, and especially of 
 the inner bright ring, and that this density of distribution gradually 
 diminishes inwards. For instance, we may conclude that along the 
 inner edge of the inner bright ring the satellites are so sparsely strewn 
 that, at the extremities of the apparent longer axis of that edge, the 
 dark background of the shj becomes visibte through the gajis between the 
 satellitesJ" 
 
 Mr. Dawes attributes this shading to the overlapping of the 
 
 * Saturn and its S>/slem, p. 118. This volume forms the most complete 
 monograph of the planet yet published.
 
 248 
 
 THE SOLAR SYSTEM. 
 
 dark ring, which may be thicker than the inner edge of the bright 
 ring. 
 
 Otto Struve and Wray also noticed, in 1861 and 1862, curious 
 appendages, like clouds of a less intense light, lying on the ring, 
 differing much in colour from the ordinary colour of the rings — not 
 
 Fig. 99.— Saturn, Jan 
 
 Luminous appeudage of the ring, (Wray.) 
 
 yellow, but more of a livid colour, bro\VTi and blue. These appearances, 
 on the hypothesis to which we refer, are supposed to be due to the 
 satellites drawn out of the plane of the ring by the attraction of 
 Saturn's outer satellite.] 
 
 Fig. 100.- Explanation of the Phases of Saturn's rings. Periodical disappearances of the rings. 
 
 In its movement round the Sun, the axis of Saturn, like that of 
 the other planets, remains parallel to itself. The axis of motion of the.
 
 SATURN. 249 
 
 rings is also constant, and, as their inclination to the plane of the 
 planet's orbit is considerable,* it follows that the Sun sometimes 
 illuminates one of the faces of the system, sometimes the other. In 
 two diametrically opposed positions in Saturn's orbit, the plane of the 
 rings is directed to the Sun, and consequently their edge only re- 
 ceives its light. This takes place at the epoch of Saturn's equinoxes. 
 What, then, are the appearances presented to us Earth-dwellers ? 
 Evidently that the rings, by an effect of perspective easily gathered 
 from fig. 100, appear sometimes more and sometimes less open ; 
 and that during one-half of the planet's year, the part of the ring 
 between us and the planet is apparently projected on the northern 
 hemisphere ; and during the other half, the " dip" is in an inverse 
 
 Fig, lul.— Saturn, Nov. 22, lS-18 (Bond) Luminous points visibl<; near the period of 
 the disappearance of the ring. 
 
 direction, and the ring is seen to cover a part of the south hemi- 
 sphere. At two particular periods the ring, being only illuminated 
 at the edge, disappears entirely, except in the most powerful instru- 
 ments, which then show a light luminous line near the j)rolongation of 
 Saturn's equator. 
 
 We give two drawings which represent Saturn in this particular 
 position. The first (fig. 101) shows the planet as it was observed by 
 Professor Bond, in November, 1848. The other (fig. 102), which 
 we also owe to the same astronomer, gives the explanation of the 
 luminous points recorded in fig. 101. 
 
 * [The inclination of the Planet's orbit to the plane of the 
 
 Ecliptic is 2" 29' 26" 
 
 The inclination of the plane of the rings to the same plane is 28 10 22j
 
 250 THE SOLAR SYSTEM. 
 
 [He supposes them due (as the Earth was not in the plane of the 
 ring) to the light reflected from the edges of the difl'erent rings, 
 which, near the epoch of the passage of the ring-plane through the 
 Sun, received the Sun's light.] 
 
 Besides this cause for the disappearance of the rings, which is 
 independent of the position of the Earth in its orbit, and depends 
 only upon the passage of the plane of the rings through the Sun, there 
 is another, which depends upon our Earth, passing through the 
 plane of the ring. An observer, situated on our globe, would then, 
 equally, only see the edge of the ring ; he could observe neither the 
 upper nor lower surface, but merely, near the time of passage, the 
 lummous points to which we have referred. 
 
 Fig. 102 —Explanation of the brilliant points observed near the epoch of tlie disappearance 
 
 of the ring. 
 
 There is still another cause for the disappearance of the ring, 
 and this occurs when the Earth is on one side of the plane of the 
 rings and the Sim is on the other. At such times obviously, as the 
 dark surface is turned towards us, we cannot see it. 
 
 Such appears to us Saturn, at its enormous distance from the 
 Earth. We have said that it is the richest of the systems, or worlds 
 in miniature, which surround the Sun. It is distiaguished from 
 all the others, not only by its wondrous rings, which bear wit- 
 ness, perhaps, of the method of formation of our planetary worlds, 
 but in addition by eight satellites, the incessant revolution of 
 which round the central globe adds to the variety of its celestial 
 phenomena.
 
 SATURN. 
 
 251 
 
 We give below the names of the eight moons of Satui-n, with 
 their distances from the centre of the planet, and the time of their 
 revolutions in terrestrial mean solar days. 
 
 
 Distance from Saturn s 
 centre iu miles. 
 
 Time of sidereal revolutions 
 d. h. ni. s. 
 
 Mimas . . 
 
 . . 119,725 . 
 
 . . 
 
 22 
 
 37 23 
 
 Enceladus . 
 
 . 153,630 . 
 
 . . 1 
 
 8 
 
 53 7 
 
 Tethys . . 
 
 . . 190,225 . 
 
 . . 1 
 
 21 
 
 18 26 
 
 Dione . . 
 
 . . 243,670 . 
 
 . . 2 
 
 17 
 
 41 9 
 
 Rhea . . . 
 
 . . 340,320 . 
 
 . . 4 
 
 12 
 
 25 11 
 
 Titan . . . 
 
 . . 788,915 . 
 
 . . 15 
 
 22 
 
 41 25 
 
 Hyperion . 
 
 . . 954,160 . 
 
 . . 21 
 
 7 
 
 7 41 
 
 Japetus . . 
 
 . . 2,292,790 . 
 
 . . 79 
 
 7 
 
 54 40 
 
 The fii'st four satellites are all nearer to Saturn than the Moon 
 is to the Earth. Mimas is, moreover, but 82,000 miles from Saturn's 
 
 Hg. 103. — Saturn and its satellites. 
 (Sir J. Herschcl.) 
 
 surface, and Dione about 206,000 ; Mimas' distances from the edge 
 of the ring being but about 31,000 miles. On the other hand, Japetus 
 is nearly ten times more distant from Saturn than we are from oui- 
 satellite, so that the diameter of the Saturnian system measures nearly 
 4,500,000 miles. 
 
 Figure 104 shows the system of the orbits of the satellites, sup- 
 posing their planes coincident with the plane of Saturn's orbit. 
 These curves are not circidar ; but their excentricity is not accurately 
 known, and the elliptical form would not be perceptible, on the small 
 scale we have adopted. 
 
 We see, by the times of revolution, that the movements of 
 the satellites are extremely rapid ; their phases, therefore, must vary 
 rapidly to the inhabitants of the planet. Mimas passes from new 
 to full moon in less than twelve hours, a little more than Saturn's day. 
 In one or two days, the four following moons present the same 
 succession of appearances. Japetus alone accomplishes its entire 
 revolution in a longer time than our lunar month. The two interior
 
 252 
 
 THE SOLAR SYSTEM. 
 
 satellites and Hyperion are very difficult to observe, and require 
 experienced observers, provided with the most powerful instruments. 
 The remaining five satellites, however, are well seen with careful 
 watching in a five-foot achromatic. The diameter of Titan, the largest 
 satellite, has been measured. It is about the sixteenth part of that 
 of Saturn. It is, therefore, more than half the diameter of the Earth. 
 
 Fig. 104 —Bird's-eye View of the Orbits of Saturn's satellites. 
 
 Thus, as in the case of Jupiter's satellites, one of the secondary 
 bodies of this marvellous S3^steni exceeds in size such planets as 
 Mercury and Mars ; its volume is about nine times that of our Moon. 
 
 We have before referred to the days, nights, and seasons in 
 this planet. It will be readily understood, by referring to what we 
 have said of these matters in the case of the Earth, that similar varia- 
 tions must also take place on this planet in a given place, in the course 
 of the year, and at the same moment, in different latitudes. 
 
 At the two poles, and throughout the polar zones, these varia- 
 tions attain their maximum. During fifteen of our years the Sun docs
 
 SATURN. 253 
 
 not leave the nortli pole, and a night of the same length envelopes 
 the soutli pole of Saturn ; the reverse phenomena occur during the 
 fifteen following years. Doubtless, an intense cold is the consequence 
 of this prolonged privation of the rays of light and heat. To this 
 long winter, and the ice and snow with which the polar regions are 
 c\oubtless covered, may perhaps be attributed the whitish zone which 
 has been remarked round the poles ; but at such a distance physical 
 facts elude us, and we must rest content with hypotheses. 
 
 The atmosphere of Saturn is doubtless very dense, especially near 
 the equatorial regions ; the bright belts with which the disk is girdled 
 are probably produced by the reflexion of light from immense cloud- 
 masses, which the rapidity of the movement of rotation incessantly 
 accumulates. The darker belts possibly, as we remarked in the case 
 of Jupiter, indicate a more serene atmosphere. 
 
 Let us imagine ourselves on the globe of Saturn. Thence let us 
 gaze on the appearances of the celestial vaidt during the day and 
 night. 
 
 If we start from either pole, in advancing as far as 63° of 
 Saturnicentric latitude, we shall traverse those regions of the hemi- 
 sphere where the ring is never visible ; only the satellites appear 
 above the horizon, and present to the spectator the varied aspect of 
 their phases. 
 
 Leaving this latitude, the ring-system begins to be visible. But 
 it is only during the two seasons of spring and summer that the 
 surface of the rings, turned towards the hemisphere where we are 
 placed, receives the rays of the Sun, and lights vip, by reflexion, the 
 planet's nights. During the day, their arcs send forth but feeble 
 light, analogous possibly to that reflected by our Moon when visible 
 in broad day. 
 
 The foi-m and extent of the immense luminous arches vary, more- 
 over, according to the latitude ; starting from 63°, and advancing 
 towards the equator, they rise higher and higher above the horizon. 
 We first see a part of the exterior ring, then the ring in its entirety. 
 At the mean latitude of 45°, the two first rings are observed. In 
 proportion as we descend towards the equatorial regions, the entire 
 system becomes visible, but at the same time, the visual rays having 
 a more oblique direction, the rings diminish in apparent breadth, con- 
 tinually, however, rising more and more above the horizon. At the 
 Equator, they are only visible by their interior edge. This edge is 
 then presented as an immense luminous band, stretched from east to 
 west, passing through the zenith.
 
 254 
 
 THE SOLAR SYSTEM. 
 
 To give an idea of the magnificent spectacle which the starry vault 
 presents during the nights of the summer season, we have sketched, 
 according to the laws of perspective, the appearance of the rings, from 
 a latitude comprised between 25° and 30°. These two ideal views 
 represent the ring at midnight, the one a little after the equinox, the 
 other at the beginning of simimer, towards the period of the solstice. 
 
 In the first of these Saturnian landscapes (fig. 105), the ring-sys- 
 tem is seen forming an immense arch, interrupted by a large space at 
 the summit. The sky is visible through the division, which separates 
 
 Fig. 105. — The Rings seen from Saturn at a latitude of about 28°. Ideal View, at midnight, 
 towards tlie period of tlio Saturnian solstices. 
 
 the two principal rings, and it again appears below the arch. The 
 interruption at the summit is produced by the shadow cast by 
 Saturn, and is only distinguished from the sky by the absence of 
 stars. It is possible, however, that this eclipsed portion of the 
 rings may be sometimes rendered visible by the refraction of the 
 solar rays by the atmosphere of the planet. The eclipsed band 
 may take a coloured tint, analogous to the reddish colour of the 
 Moon during total eclipses. 
 
 The second ideal landscape (fig. 106) allows us to see the exterior 
 ring in its entirety. At the solstices, the shadow of the planet is 
 thrown only on the interior rings. We must remark, also, that at 
 the difierent hours of the night the position of the shadow is not the
 
 SATURN. 
 
 255 
 
 same. It only occupies the middle of the arc at midnight. It hence 
 follows that, after sunset, the western part of the ring first ap- 
 pears ; by degrees, as the night advances, the western arc dimin- 
 ishes, and the other portion appears at the east ; until, at midnight, 
 the lengths of the two arcs are equal. From midnight, the western 
 portion still diminishes, and at last disappears, whilst the eastern 
 arc increases in length. When we add to the strange beauty of this 
 spectacle the presence of the satellites, presenting different phases, 
 some full, others new, others gibbous, or crescent, an idea will be 
 formed of the variety of aspects of the Saturnian nights. 
 
 Fig. 106.— The Riuss seen from Saturn at a latitude of about "S°. Ideal View, tnkeu at 
 midnight, between the Satumian equinoxes and the solstices. 
 
 During the winter seasons, the rings present their dark sides, and 
 are only visible during the night, negatively ; that is to say, by the 
 absence of the stars on the celestial vault which they eclipse. Never- 
 theless, towards the morning and evening, they may possibly reflect 
 the light they receive from the illuminated part of the planet ; at the 
 east and west they show doubtless a slight glimmer, similar to the 
 Earth-shine of our Moon, or, again, to the Zodiacal Light. 
 
 But if the winter nights are deprived of the light of the rings, 
 the days of the same season present, on the other hand, the most 
 curious phenomena. As, by reason of the diurnal rotation, the Sun 
 moves apparently along circular arcs, sometimes more, sometimes less
 
 256 
 
 THE SOLAR SYSTEM. 
 
 elevated above the horizon, the god of day, being compelled thus to 
 pass behind the rings, undergoes long and frequent eclipses. The 
 duration of these phenomena is shorter than was at first supposed, 
 because, as the apparent path of the Sun is not parallel to the arcs of 
 the rings, he, though eclipsed at rising, reappears under the ring, to 
 again disappear before sunset. 
 
 [In latitude 40°, we have morning and evening eclipses for more 
 than a year, gradually extending until the Sun is eclipsed during the 
 whole day, and these total eclipses continue for nearly 7 years, eclipses 
 of one kind or another taking place for 8 years 292 '8 days. Mr. 
 
 Fig. 107.- — Idcul view ofa pliase of .■^aturii. 
 
 Proctor, in the book from which we have already quoted, re- 
 marks : — " If we remember that latitude 40° on Saturn corresponds 
 with the latitude of Madrid on our Earth, it will be seen how largely 
 the rings must influence the conditions of habitability of Saturn's 
 globe, considered with reference to the wants of beings constituted 
 like the inhabitants of our Earth."]' 
 
 Nearer the equator and poles solar eclipses are still very frequent, 
 but the period of time during which they last is gradually reduced. 
 Judging of the loss of light by the intensity of the shade thrown on 
 the planet, the apparent night produced by those eclipses is, doubt- 
 less, very decided, although atmospheric refraction would prevent them
 
 SATURN. 257 
 
 from being absolute. If we could watch the various celestial pheno- 
 mena from the rings, the appearance of the sky would be very diffe- 
 rent : if we supposed ourselves located over the edge of the ring, we 
 should have a long night of fifteen years succeeding a day of the same 
 length. 
 
 During the period of illumination of each side of the rings, the 
 Sun is eclipsed every 10^ hours. These eclipses, due to the interpo- 
 sition of Saturn's disk, produce partial nights, the duration of which 
 varies between IJ and 2 hours over a large surface of the ring. These 
 are the phenomena which caused the interruption of the luminous arc 
 seen from Saturn, as represented at two different epochs in our two 
 ideal views. 
 
 Fig. lOS. — The globe of Saturn, seen from the ring. 
 
 But for nearly fifteen years each side of the rings is entirely de- 
 prived of the light of the Sun. This long night is partly compensated 
 by the light reflected by the illuminated hemisphere of the planet, 
 or at least by the part of that hemisphere visible from the ring. 
 During every period of 10^ hours the immense globe appears under 
 various phases. It is first a luminous point, which rises from the 
 horizon, taking more and more the form of a half crescent (fig. 107), 
 but much less curved than that of the Moon. After 5^^ hours it is 
 nearly a half circle, which embraces the eighth part of the whole 
 celestial vault, the surface of this half circle is thus more than 20,000
 
 258 THE SOLAR SYSTEM. 
 
 times that of the hmar disk (fig. 108). On this disk is perceived a 
 dark zone, divided by a bright line : it is the shadow projected by 
 the ring- on the planet. Bright and dark belts, and, doubtless, many 
 other physical details that we cannot see at our enormous distance 
 from Saturn, distinguish the various parts of this immense disk. 
 
 The more we leave the inner ring, the more does the visible por- 
 tion of the planet increase ; but its apparent dimensions diminish, on 
 the other hand, with the distance, always, however, remaining con- 
 siderable. Figs. 107 and 108 will give an idea of the aspect of 
 Saturn, seen from a point on the middle ring, at an interval of about 
 3 hours.* 
 
 It remains for us to point out, in terminating our review of 
 Saturn's phenomena, and of the celestial phenomena presented to the 
 Saturnians, the numerous eclipses produced by the eight satellites, both 
 wlien they pass over the solar disk and when they themselves plunge 
 into the shadow thrown by the planet. These phenomena can be 
 watched from the Earth in powerfvd instruments. The last occasion 
 took place in 1862, when Mr. Dawes and Mr. Lockyer were enabled 
 to observe the shadow of Titan traversing the planet's disk, the 
 satellite itself on one occasion grazing the planet's lower limb. [Mr. 
 Dawes also witnessed an eclipse of Titan — a unique observation, as far 
 as we know.] 
 
 * [In these two ideal views, as in the two preceding ones, M. Guillcmin has 
 been compelled, naturally enough, to appeal in his foregrounds, to our terrestrial 
 prejudices. Of course, the rights of the different hypotheses referred to in the 
 text are " strictly reserved."]
 
 URANUS. 259 
 
 XVIII. 
 
 URANUS. 
 
 Discovery of Uranus in the last Century — Form and Dimensions of its Orbit — 
 Its apparent and real Dimensions — Its Satellites; Inclinations of their 
 Orbits, and Directions of their Movements. 
 
 The Solar System, as known to the ancients, comprised all those 
 celestial bodies the movements and physical constitution of which we 
 have just studied, with the exception of the telescopic planets and the 
 satellites of Saturn and Jupiter. A century ago, the number of the 
 planets remained the same as for ages past, and the confines of the sys- 
 tem did not extend beyond Saturn. It was reserved for one of the most 
 illustrious observers of modern times, Sir William Herschel, to double 
 the radius of the sphere which embraces the bodies subject to the 
 attraction of the Sun, by the discovery of a new planet — Uranus, 
 
 It was on the 13th of March, 1781, between ten and eleven o'clock 
 at night, that Herschel, employed in exploring with his telescope 
 the constellation of the Twins, observed a star, the disk of which 
 attracted his attention. Perceiving, after a few nights of observation, 
 that the new body moved, he first took it for a comet. His observa- 
 vations, when submitted to calcxdation, soon showed that he had 
 discovered a body which was at such a great distance from the Sun, 
 and the orbit of which was so circular, that it was impossible long to 
 hesitate as to its real character : it was a planet. 
 
 Uranus, usually — but this depends upon its distance from the 
 Earth — shines as a star of the sixth magnitude. It is, therefore, 
 sometimes visible to the naked eye. This insignificant size and 
 brightness, however, are merely relative, and are caused by the im- 
 mense distance of the planet from the Sun, and, therefore, from the 
 Earth, and also by the feeble intensity of the light received from 
 the first-named body. But if it be examined with a telescope of 
 high magnifying power, the circular form of its disk appears vnth. 
 clearness, and its apparent diameter may be measured.
 
 260 
 
 THE SOLAR SYSTEM. 
 
 The orbit described by Uranus round the Sun surrounds the orbit 
 of the Earth at so great a distance, that it is impossible to perceive on 
 its disk any appearance of phases. It has the appearance of always 
 turning its bright side towards us. 
 
 The orbit is not a perfect circle, but, like those of the other 
 planets, is eUiptical ; so that, during the whole course of its revolu- 
 tion, which lasts about 84 years— more exactly, 30,686^85 days— 
 the distance of Franus from the Sun constantly varies between 
 1,743,000,000 and 1,913,000,000 miles : there is thus a difference 
 of 170,000,000 miles. 
 
 Its distance from the Earth varies even more, being greatest when 
 the two planets are on opposite sides of the Sun, and, of course, least 
 when they are on the same side. In the former case, Uranus is in con- 
 junction, and its mean distance from the Earth exceeds 1,923,000,000 
 miles, whilst at opposition the mean distance is 1,733,000,000. 
 
 Its apparent diameter, seen from the Earth, then varies in a way 
 which mav thus be exhibited : — 
 
 Fig. 109.— Apiiarent dimensions of Uranus .it its mean and extreme distances 
 from tho Earth. 
 
 From the distance of Uranus and its apparent size, its real di- 
 mensions have been deduced : it is a spherical body, 82 times larger 
 than our Earth, the diameter of our planet being 4^ times less than 
 
 that of Uranus (4-344). The real 
 diameter, therefore, is 34,500 miles. 
 Fig. 110 shows the comj)arative di- 
 mensions of these two bodies. 
 
 Astronomers are not agreed as to 
 whether Uranus is perfectly spheri- 
 cal or flattened at its poles of I'ota- 
 tion. Sir William Ilerschel asserted 
 the latter, and Madler found some 
 y«Jars ago a flattening of ^^gth, which 
 is suggestive of a rapid rotation ; but other astronomers, Otto Struve 
 among them, have not been able to detect any j)orceptible flattening. 
 This, however, does not, perhaps, militate against the observations 
 of Madler and Ilerschel ; for if, as Arago remarks,* Ave assume that 
 
 Fig. 110. — Comparative size of Urauus and 
 the Earth. 
 
 Popular Astronomy," vol. iv. p. 493.
 
 URANUS. 
 
 261 
 
 the equator of Uranus is situate nearly in the plane of the orbits 
 of its satellites, this will explain how, at different epochs, observers 
 have arrived at different results. The axis of rotation of the planet 
 vvoidd, on this supposition, nearly coincide with the plane of the 
 orbit of the Earth. If the axis be turned towards our globe, the 
 
 Fig. 111. — DitterencG between the apparent fomis of a flattened glolje, 
 seen in two different positions. 
 
 ellipsoid will seem to us circular ; if, on the other hand, it is at right 
 angles with us, the polar compression will become visible. 
 
 The preceding figure explains both the difference of position, 
 and the change of appearance resulting therefrom. 
 
 Fig. 112. — System uf the Satellites of Uranus; relative dimensions r.f the Orbit.?.
 
 262 THE SOLAR SYSTEM. 
 
 Uranus, like Saturn, is the centre of a little system, comprising, 
 besides the principal planet, four— it was formerly thought eight* 
 —moons or satellites, revolving in planes nearly perpendicular to the 
 plane of the planet's orbit. These bodies, whose revolutions are 
 accomplished, the nearest in 2 days, and the most distant in about 
 13 days, possibly compensate, in some degree, by their reflected light, 
 during the nights of the planet, the feeble intensity of the daylight. 
 The Sun is visible at Uranus as a small disk, the superficial extent 
 of which is 370 times less than that seen from o\ir globe. The heat 
 received from it, too, is 370 times less than with us. 
 
 We have shown in fig. 112 the relative dimensions of the orbits of 
 the satellites, as they would be seen if we could obtain a bird's-eye 
 
 view of the plane in 
 which they revolve. We 
 have already mentioned 
 the fact, that their move- 
 ments are performed in a 
 direction nearly perpen- 
 dicular to the plane in 
 which the planet re- 
 volves round the Sun. 
 Another peculiarity, and 
 this is found nowhere 
 else throughout the solar 
 system, further distin- 
 guishes Uranus ; the di- 
 rection of these move- 
 ments is retrograde, that 
 is to say, it is contrary 
 to that of all the other 
 known movements of sa- 
 tellites and planets. But 
 
 Fig. 113. — luclinatiou of the planes of the orbits of tlic this aUOmalv probablv 
 
 satellites to the orbit of Ui'anus. , „ . 
 
 results irom the very 
 
 great inclination of their orbits, shown in fig. 113, 
 
 The first satellite is but 128,000 miles, or about half the distance 
 
 of our Moon, from the planet. The most distant of the four of 
 
 which we have certain knowledge is 392,000 miles. Of these four, 
 
 the two nearest, Ariel and Umhriel, were discovered by Lassell and 
 
 Otto Struve respectively; the two remaining ones, Titania and Oberon 
 
 by Sir W, Herschel. [Mr. LasseU has recently stated, as the result 
 
 * [That the number of Satellites is limited to four, has been established by 
 Mr. Lassell quite recently — indeed since the diagram was engraved.]
 
 URANUS. 2(>{ 
 
 of a long'-continiied series of observations, that the existence of the 
 other four is, to say the least, extremely problematical.] 
 
 Observations of the variations of the quantity of light reflected 
 by these enormously distant bodies — and these observations, we need 
 scarcely say, are extremely difficidt — have led to the inference that 
 they possess movements of rotation, an idea strengthened by other 
 planetary analogies : it is, however, as yet by no means certain. 
 Nor have we yet observed their eclipses by the shadow of the planet, 
 or those of the Sun which doubtless sometimes take place when they 
 pass between the planet and that body. We may, however, infer 
 these phenomena as well as phases ; and the simultaneous presence or 
 absence of these bodies from the Uranian sky doubtless affords a 
 great variety in the appearances presented to the inhabitants — if such 
 exist — of the mid- planet of our system. 
 
 Observations have at present given us no information on the 
 physical constitution of Uranus. No feature is visible on the disk 
 at such a distance. Astronomical calcidations can only tell us of its 
 mass, which is fifteen times that of the Earth ; taking this and its 
 volume into account, we find, for the density of the matter of 
 which it is composed, a value equal to the -^'^th of that of our Earth : 
 the density of Uranus, therefore, is a little more than that of ice. 
 
 On the surface of Uranus, the force of gravity is ^^th greater 
 than on the surface of our Earth, so that the phenomena of equi- 
 libriimi and movement are about the same as with us — with this 
 difference, that the surface of the planet ma)/ be much less solid.
 
 264 THE ?>OLAK SYSTEM. 
 
 XIX. 
 
 NEPTUNE. 
 
 Discovery of Neptuue — The Method of Discovery — Distance — Apparent ami 
 real Dimensions — Volume and Density — Satellite of Neptune. 
 
 At a mean distance from the Sun of 2,862,000,000 miles, that is to 
 say, more than thirty times the radius of the Earth's path, the most 
 distant of the known phmets of the system circidates in its orbit. 
 The nearly circular orbit which it describes round the common centre 
 is so great, that it requires nearly 165 years to accomplish its revo- 
 lution. 
 
 This planet is Neptune. Scarcely eighteen years separate us from 
 the time when it was first distinguished as a planet ; so that Ave have 
 merely yet seen it traverse the ninth part of its orbit. The recent 
 date of its discovery, and the immense distance of the planet from tlie 
 Earth, are sufficient reasons for the few data we possess regarding 
 it. But it is surrounded wdth another kind of interest to compensate 
 for this insufiiciency — we refer to the method, unique in the annals 
 of astronomy, which served as a basis for its discovery. 
 
 We have seen that among the known bodies which compose the 
 solar system, eight only were distinguished by the ancients from 
 the multitude of briUiant points which spangle the starry vaidt : the 
 dimensions of some, — the Sim, Moon, and Earth ; the proper motion 
 of others among the constellations, — Mercury, Venus, Mars, Jupiter, 
 and Saturn, were the characters which early led to their being classed 
 by themselves as wanderers. 
 
 Later, the telescope enlarged man's field of view, and permitted 
 astronomers to add to these eight bodies a considerable nimiber of 
 new ones. Uranus, the Asteroids, the Satellites of Jupiter, Saturn, 
 and Uranus, were ranked successively among the Solar family. But 
 what was the method employed to discover all these celestial bodies ?
 
 NEPTUNE. 265 
 
 \n attentive and minute survey of every part of the starry sky, the 
 comparison of celestial maps with the field of view of powerful optical 
 instrimients, the happy discovery of a change of place of a luminous 
 point. But in all this there was no prevision founded on theory, no 
 preconceived notion on the future discovery, which, indeed, in all 
 cases has been due to the persevering zeal of observers and to happy 
 chances. 
 
 The method to which the discovery of Neptune is owing was 
 entirely a difierent one. 
 
 We shall speak subsequently on the principles of the movements of 
 celestial bodies round their foci of revolution, as they act and react 
 on each other in such a manner as to disturb the regularity of their 
 movements ; on the observed perturbations, and on the manner 
 in which the perturbations observed are connected with the laws 
 which govern them. Now, among these perturbations, there was one 
 which utterly defied explanation on any known theory, and which 
 astronomers had in vain attempted to ascribe to the action of one 
 of the known bodies. The tables constructed for the planet Uranus 
 did not at all agree with the observations, and the motion of this 
 body was evidently disturbed by some unknown body. This body 
 was, nevertheless, for some time suspected by Bouvard, Hansen, and 
 many other astronomers, who held that the perturbations were due 
 to an undiscovered planet, beyond the orbit of Uranus. But the 
 complete solution of the problem was accomplished independently 
 by an Englishman, — Professor Adams, and a Frenchman, — M. 
 Leverrier, both of whom now take rank among the foremost living 
 astronomers. 
 
 [Wlien we come to consider the problem in all its grandeur, we 
 need not be surprised that two minds, who felt themselves competent 
 to its solution, should have independently undertaken it. As far 
 back as July, 1841, we find Mr. Adams determined to investigate the 
 irregularities of Uranus ; early in September, 18-16, the new planet 
 had fairly been grappled. We find Sir John Herschel remarking : 
 "We see it as Colvmibus saw America from the shores of Spain. 
 Its movements have been felt trembling along the far-reaching line of 
 our analysis with a certainty hardly inferior to ocular demonstration." 
 
 On the 29th July, 1846, the Equatorial at Cambridge was first 
 employed to search for the planet in the place theoretically assigned 
 to it by Mr. Adams. M. Leverrier's theoretical researches were 
 published on the 31st August, and his letter to the Berlin astro- 
 nomers pointing out where he expected it would be found, was 
 received on the 23rd September, his theoretical place and Mr. 
 Adams's being not \° apart.' There, thanks to the Berlin star-
 
 266 
 
 THE SOLAR SYSTEM. 
 
 maps, which the English astronomers had not received, Dr. Galle 
 fomid the planet the same evening. 
 
 We need not now refer to the imfortunate, though perhaps 
 necessary, discussion as to the comparative merits of these two astro- 
 nomers, which almost clouded the brilliancy of their discovery. Let 
 us rather look upon the work of each as a stupendous triimiph of 
 intellect, and the result to which the labours of both have led us as 
 one which for ever sets a seal on the theory of universal gravitation.] 
 In the words of Arago, " Such a discovery is one of the most 
 brilliant manifestations of the exactitude of the system of modern 
 qstronomers. It will encourage our most eminent geometers to seek 
 with fresh ardour the eternal truths which remain hidden, as Pliny 
 expresses himself, in the majesty of theories." 
 
 Neptune is invisible to the naked eye. In telescopes, it has the 
 aspect of a star of the eighth magnitude. Its apparent movement 
 is extremely slow ; but, as the orbit which it describes round the Sun 
 is so immense, its real velocity is nevertheless considerable ; it 
 is about 12,400 miles an hour. 
 
 Like all other planets, it is sometimes nearer and sometimes further 
 
 from the Earth. At the time 
 of conjunction it is distant from 
 us, on the average, 2,958,000,000 
 miles, whilst its minimum dis- 
 tance at opposition is less by 
 218,000,000 miles. The appa- 
 rent dimensions vary in inverse ratio, their limits are shown in 
 fig. 114.* 
 
 The real dimensions are somewhat considerable, and in virtue of 
 
 them Neptime is the third planet of 
 the system. Its diameter is 4*72 timea 
 greater than the diameter of the Earth, 
 or 37,000 miles. The surface of the 
 globe of Neptime is more than 22 
 times that of the Earth, and its volume 
 is nearly 105 times. 
 
 If we turn to fig. 2, page 19, we 
 shall see to what small dimensions the 
 apparent diameter of the Sun is re- 
 duced, as seen from Neptune. The in- 
 tensity of the heat and light received by that planet is but little 
 
 Fig. 114. — Apparent dimcusioiis of the disk of Nep- 
 tune at its iiieau and extreme distances from 
 the Earth. 
 
 Fig. ll.j,— Neptune and the Earth : 
 comparative dimensions. 
 
 * This disk has not yet presented any perceptible trace of flattening, neither 
 can any spot be distinguished on it, so that the time of its rotation remain? 
 unknown.
 
 NEPTUNE. 
 
 267 
 
 more, at that enormous distance, than the thousandth part of that 
 received hy us. But as nothing is known of its physical and atmo- 
 spheric conditions, or of its rotation, nothing can be determined on 
 the climatic conditions of the planet. 
 
 At a distance nearly equal to that of the Moon from the Earth, 
 that is to say, about 225,000 miles, a satellite revolving roimd 
 Neptune, in a very circular orbit, in 5 days, 21 hours, 8 
 minutes,* this has enabled astronomers to calculate the mass of 
 
 "' satellite 
 
 Fig. 116. — Satellite of Neptune. 
 
 the primary. It is equal to about the tt^cjo^^^ V^^^ ^^ ^^® mass of 
 the Sun, or to 21 times that of the Earth. Hence, the density of 
 the matter of which Neptime consists is less than the fourth of that 
 of the Earth, or nearly equal to the density of nitric acid, and a 
 little less than that of sea-water. From this point of view Jupiter 
 is the planet most analogous with this body, whilst the force of 
 gravity at its surface is about the same as on Saturn and Uranus. 
 
 * Observers have also imagined that Neptune is surrounded by a ring^; but it 
 is now certain that this appearance, which was also suspected in Uranus, must be 
 considered an optical illusion.
 
 BOOK THE THIRD. 
 
 COMETS. 
 
 The name of " Comet" for the most part gives rise to the idea of a body 
 of curious form, accompanied witli a luminous train, traveUing ca- 
 priciously through space, appearing- suddenly and disappearing in like 
 manner, and at once astonishing by its strange aspect both learned 
 and vulgar. This manner of distinguishing comets from other 
 celestial bodies is no longer strictly accordant with the discoveries 
 of science, which has succeeded in discovering the laws of their 
 movements, and in assigning to them their true place in astronomical 
 classification. 
 
 It is now proved that most of the observed comets, if not all, 
 form part of the solar system, and that, if they are distinguished 
 from the principal and secondary planets, it is by characteristics en- 
 tirely diiferent from those which are ordinarily assigned to them. 
 
 Let us see what these are.
 
 270 THE SOLAR SYSTEM. 
 
 I. 
 
 Aspect of Comets — Head ; Nucleus ; Tails, simple and multiple — How Comets 
 are distinguished from the other Bodies of the Solar System — Forms and 
 IncHnations of their Orbits ; Direction of their Movements. 
 
 If we refer to tlie ethnology of the word, " comet " signifies a hairy 
 body. Most frequently, indeed, a comet appears as a star, the nucleus 
 of which is surrounded with a nebulosity more or less brilliant, to 
 which ancient astronomers gave the name of hair. 
 
 Independently of this nebulosity, the body is frequently accom- 
 panied by a train, the length of which varies in each comet, or in 
 the same comet at different times : this luminous train is called the 
 taiL The form of the head and its apparent and real dimensions, 
 and the form and dimensions of the tail, are extremely variable, and, 
 indeed, comets have been seen with two or even several tails. 
 
 But the nebulous aureola and luminous nucleus which generally 
 form the head of the comet, and the single or multiple train with 
 which the head is accompanied, cannot be considered absolutely as 
 specific characters, seeing that bodies without these characteristics 
 would be required to be ranged in a dift'erent category. 
 
 There exist, in fact, some comets deprived both of tail and nu- 
 cleus ; such is the one represented in the right hand di^awing of 
 fig. 117, which consists, as we see, of a simple rounded nebulosity.* 
 Others, like that represented in the first drawing in the same 
 figure, possess a nucleus surrounded with a nebulosity, but no tail. 
 
 Nor is the nebulous character of the head always constant ; 
 comets have been observed which have presented the apj)earance of 
 stars, with which, indeed, they have been confounded. The astro- 
 
 * We shall describe further on the nebulous appearances entirely distinct 
 from comets ; these are the nebulce, properly so called. The difference between 
 them is, that whereas the nebulae retain a fixed position, the comets move more or 
 less rapidly across the sky.
 
 COMETS. 271 
 
 nomical history of the last century oifers u striking example. When 
 Sir W. Herschel, by the aid of his powerful instruments, discovered in 
 the distant regions of the solar system the planet which now bears the 
 
 Fig. 117. — 1. Tiiillcss Comet. 2. Head without tail or miclcus. 
 
 name of Uranus, he first mistook this body for a comet. Still, there 
 was no trace of nebulosity to mislead the illustrious astronomer of 
 
 Fig. 118.— Comet of 1744 (ChOseanx's Comet), wilii multiple tails. 
 
 Slough. His opinion was founded upon a roiigh determination of 
 its orbit. 
 
 But it is right to say tliat, among the ninnerous comets observed
 
 272 
 
 THE SOLAR SYSTEIM. 
 
 lip to the present time, either with the naked eye, or by means of 
 telescopes, the majority are distinguished by a nebulosity surromiding 
 the nucleus, and a great number, especially of the most brilliant 
 ones, possess a luminous train or tail. With others, the tail, dis- 
 played fan-like, is divided into man}^ branches, as if the body had 
 in reality several distinct tails. Plate XYIII and fig. 118, give 
 an idea of the varied forms of those cometary appendages. 
 
 This diversity of asjDcct will, perhaps, some day, enable astrono- 
 mers to class comets into genera, species, and varieties, and will 
 doubtless facilitate the perfection of the theory of the phenomena 
 which these bodies present, which is still so obscure. 
 
 Fig. 110. — Form of coiiiotnry orbits. 
 
 Comets, as we have said, form part of our solar system. Like 
 the planets, they revolve round the Sun, traversing with very vari- 
 able velocities extremely elongated orbits ; the form of the cometary 
 orbits furnishes us with the first of their specific characters. 
 
 Whilst the planets now known move in nearly circular, closed 
 curves, and thus remain continually visible, if not to the naked 
 eye, at least with the aid of telescoiDes, most of the comets revolve 
 round the Sun either in extremely elongated ellipses, or in infinite
 
 PLATE XVIII. 
 
 FORMS OF COMETS. 
 
 1. Coinot nf 1,'(T7. (Cornelius Gemma). 2, Comer r.l'KigO. (J C. fitnrni.) 
 3, Comot ot irfiH,
 
 COMETS. 275 
 
 curves, or ut least in curves which appear so. Hence it results that 
 comets are observable only in a very limited portion of their paths, 
 that is, when they approach nearest to the Sun and Earth. Moreover, 
 as the period of their revolutions increases with the departure of their 
 orbits from a closed curve, it has only been possible to determine 
 the return of a very small number of these solar satellites. There 
 are some, which, judging by what we know, will never revisit our 
 system. 
 
 In lig. 119 are represented the tliree kinds of orbits described by 
 comets. 
 
 The first, of oval form, having the Sun for its focus, is the 
 cllipxv. It is a closed or re-entering curve. Although elongated, 
 it is clear that the body that traverses it must return periodically, 
 to the Sun, at epochs more or less distant. 
 
 The second is of a form very analogous to the ellipse, but it is 
 distinguished from it by the fact that its two brandies constantly 
 get further apart, and therefore never join. This is ihe pavahohi ; but it 
 is quite possible that those comets the orbits of which appear para- 
 bolic really describe extremely elongated ellipses, and that this 
 form is taken for the parabolic one during the period of visibility 
 of the body, in consequence of this similarity ; but on this hypothe- 
 sis, the period of revolution, necessary to give rise to this confusion, 
 must be so great, that a return can never be proved : still, strictly 
 speaking, a return may take place. 
 
 It is another matter, however, when the comet describes the 
 third kind of orbit, to which geometers give the name of hypcrhola. 
 The two branches of the liijpcrhola not only are infinite, but they are 
 distinguished essentially from the ellipse, as the branches depart 
 much more from the re-entering form which characterises an ellipse, 
 with which form no portion of the hyperbola can be confounded. 
 
 Several comets move in orbits of this kind, so that, after having once 
 formed part of our «olar system, they go away for ever, seeking perhaps 
 in the depths of the heavens another Sun, which they will after- 
 wards abandon as they do our own. Among the elliptic cometary 
 orbits now known, that which the nearest approaches the circle is 
 much more elongated than the planetary orbit which departs from it 
 most. In fig. 120 are given, on the one hand, the most excentric of 
 the planetary orbits, and, on the other, the least excentric cometary 
 one, so that this difference ma}' be appreciated by the eye. 
 
 Thus comets are distinguished from planets, in the first place, by 
 the extreme elongation of the curves which they describe round the 
 Sun.
 
 276 
 
 THE SOLAR SYSTEM. 
 
 There are two other characters which are not less important 
 than this: these are, first, that the inclinations of their orbits, 
 instead of being contained, like those of the planetary orbits, within 
 small limits, take every possible value. Hence comets traverse the 
 starry vault in every direction, different in this from the other bodies 
 of the solar system, the apparent paths of which never vary much 
 from the narrow zone known imder the name of the Zodiac. 
 
 Lastly, the direction of movement is sometimes from west to 
 east, sometimes in the contrary direction, or, if we recall the 
 signification of these words, sometimes direct, sometimes retrograde. 
 
 Fig. 120.— Comparison of the cxcoutvicity of the planetary and comctary orbits 
 
 x^ow the fundamental fact should be ever present in our memory, 
 that all the planets move in the same direction, that is to say, from 
 right to left, or from west to east, to an observer placed on the 
 northern side of the plane of the ecliptic. 
 
 Such are the essential differences which render comets a peculiar 
 family of celestial bodies, and a most interesting one in the double 
 point of view of their movements and physical constitution ; indeed, 
 they give to the solar group, already so varied, an incomparable 
 richness.
 
 COMETS. 277 
 
 II. 
 
 Periodic Comets — Halley's Comet; its return in 1759 and in 1835 — Encke's 
 Comet of Short Period ; Acceleration of its Movement — Division of Gambart's 
 Comet — Elements of the Principal Periodical Comets, 
 
 In spite of the oft-renewed protests of astronomers, a singular re- 
 proacli is often laiinclied against them. When a comet, visible to 
 the naked eye, appears in the sky attracting notice on all sides by 
 its brightness perchance, or the length of its tail, a nimiber of people 
 are astonished at the carelessness or ignorance of astronomers in 
 having failed to predict it. We shall, therefore, now show how it 
 happens that they are miable to predict, except in a few instances, the 
 approach of a comet, as they do the position of a planet or the 
 phenomena of eclipses. 
 
 All comets, as we have seen, have the Sun for the focus of their 
 movements, and all describe a curve roimd it — an orbit the concavity 
 of which is always turned towards the Sun. But, as we have also 
 stated, most of the cometary orbits are so elongated, that they appear 
 to be parabolic, the branches of which are infinite ; others, again, are 
 hyperbolic. What must we expect, then, in the case of comets which 
 describe such orbits ? Either they will never return to us, the 
 immense distance to which they travel from the Sun perhaps carry- 
 ing them into the sphere of attraction of some other system ; or if 
 they do return, it will be at an enormous interval of time, perhaps 
 after a lapse of thousands of centuries. 
 
 Thus most of the comets observed visit the celestial regions 
 occupied by our world for the first time, or, if they have already been 
 with us, their visit happened at periods so remote from ours that no 
 human observation has been handed down to us, even if man then 
 existed on the Earth. On these hypotheses the impossibility of a 
 scientific prediction is evident : we must observe a comet and ascertain 
 the elements of its motion, before we are able to predict its return. 
 A certain number of comets, it is true, move in closed orbits — in
 
 278 THE SOLAR SYSTEM. 
 
 ellipses. Among these we distinguish the comets of short period from 
 those the revolutions of which occupy centuries— the anterior obser- 
 vations of which are unknown, or so confused, that it is impossible 
 to base any calculations on them. Of these, however, science has also 
 predicted the return, if not on a given day, at all events between certain 
 limits. But in the case of the periodical comets of short period, their 
 movements are known with a precision which permits the return to 
 be easily announced, and we can predict for any given day and hour 
 the various places they will occupy in the starry vault. 
 Let us enter somewhat into detail. 
 
 The first of these comets, the periodicity of which has been well 
 established, both by observation and calculation, bears the name of 
 Halley, an English astronomer of the seventeenth century. It is to 
 him that we owe, in fact, the identification of the comet of 1682 with 
 those of 1531 and 1607, and the prediction of its return at the end 
 of 1758, or the beginning of 1759. The event justified the prediction. 
 This Avas not all : at this latter period cometary astronomy was elevated 
 at once to a state of perfection comparable with that of the other 
 branches of the science. A French geometer, Clairaut, calculated the 
 effect of perturbations of the two large planets, Jupiter and Saturn, 
 in the vicinity of which the body was expected to pass, on the path 
 of the announced comet. He assigned a delay of 618 days ; 100 
 due to the action of Saturn, 518 to that of Jupiter. 
 
 The return of the body to its perihelion was predicted, therefore, 
 to occur in the middle of April 1759, with an error of a month, 
 more or less, the uncertainty arising from the neglect of some terms 
 in the calculation which Clairaut, pressed for time, omitted. It ac- 
 tually returned to perihelion on the 13th of March. 
 
 Since then, in 1835, Halley 's comet reappeared in our regions of 
 the sky, but this time its passage was predicted with such precision, 
 that there was only three days' difference between calculation and 
 observation. 
 
 The form of the orbit of Halley 's comet is shown in fig. 121, 
 which also gives those of the other comets of short period at present 
 known in our system. This orbit, too elongated to be represented in its 
 entire development, is shown in Plate I, where it is seen that at its 
 most distant point from the Sun, it reaches beyond the orbit even of 
 Neptune. The comet requires more than 76 years — 27,866 days — to 
 traverse this immense curve. TVe also see that in consequence of one ji 
 
 of those characteristics which especially distinguishes such a body " 
 
 from the planets of the solar system— the elliptical form of its 
 
 I
 
 PLATE XIX. 
 
 H ALLEY'S COMET. (Sir J. Herschel.) 
 
 1. VieT? of the comet in Ophiucluis with the naked eye, Oct. 22, 1S35. 
 
 2. The same viewed with a telescope of 7 feet local leiipth. 
 
 3. 4, 5, 6. Details of the head of the comet, October, 1835— February, lS3ii
 
 COME'l'S. 
 
 281 
 
 oi-ljit^-Halley's comet sometimes is nearer to the Sun than Venus, 
 within, indeed, a distance which does not exceed 56,000,000 miles, 
 and sometimes it recedes from the focus of heat and light to a distance 
 60 times more removed — a distance exceeding 3,200,000,000 miles. 
 
 These enormous variations in distance would lead us to suppose 
 most astonishing differences in the quantity of light and heat received 
 by the comet from the Sun. And, in fact, the intensity of these 
 physical agents varies in the ratio of 3000 to 1, or, as it may be put, 
 
 Fig. 1-Jl.— Orbits of the periodic Comets. 
 
 the Sun's light and heat arrive at the comet with a force 3000 times 
 more considerable at perihelion than at aphelion. 
 
 Halley's comet moves from east to west in a plane inclined to the 
 orbit of the Earth the fifth part of a right angle. In Plate XIX are 
 represented the various appearances assumed by it in 1835, both in 
 its general aspect and m the portion of it surrounding the nucleus. 
 
 Following the order of disco\'ery. W'v mu«t next describe Encke s 
 comet.
 
 282 THE SOLAR SYSTEM. 
 
 Invisible to the naked eye, it appears in the telescope under the 
 form of a nebulous mass, nearly spherical, and without either tail or 
 nucleus. It is a singular fact that the head of this comet varies both 
 in form and dimensions at the same time, and it is at its nearest 
 approach to the Sun that its volume is smallest. 
 
 Of all the comets, the periodical return of which has been demon- 
 strated, this comet accomplishes its revolution round the Sun in the 
 shortest space of time, which in the mean is 1205 days, or a little less 
 than 3^ years. It moves from west to east in an orbit such that its 
 perihelion and aphelion distances are respectively 32,000,000 and 
 387,000,000 miles. 
 
 Here, then, is a body which, at each of its revolutions, penetrates 
 within the orbit of Mercury, and at its greatest distance from the 
 Sun surpasses the orbits of the asteroids, and almost reaches that of 
 Jupiter. Since 1818, the time of its discovery, all its returns, to the 
 number of fourteen, have been regularly observed ; but, singular cir- 
 cumstance, the period of its revolution is continually diminishing ; so 
 that, if this progressive diminution always follows the same rate, the 
 time when the comet, continually describing a spiral, will be plunged 
 into the incandescent mass of the Sun can be calcidatcd. This con- 
 tinued approach has been attributed to the existence of a resisting 
 medium in the regions of space.* 
 
 Encke's comet is also specially designated by the appellation of 
 tlie comet of 8hoH iK'riod. 
 
 Among the other comets, of which both calculation and ob- 
 
 * It appears at first paradoxical to say that a resistance to a movement can pro- 
 duce an acceleration in the time of the successive revolutions : the first tendency of 
 the mind is to see, on the contrary, a cause of slackening ; but with a little reflec- 
 tion it is easy to convince oneself of the exactitude of the first explanation, or, 
 at least, of its probability. We have shown, in another work (" Les Mondes," 
 XIX. Causerie), that acceleration, combined with the third law of Kepler, and 
 the theory of universal gravitation, is a direct consequence of such a resistance. 
 The explanation of this would here be premature ; we must refer to the third 
 part of the pi-esent work, in which an exposition of astronomical laws is given. 
 
 Is it possible that the nebulous ring which forms the zodiacal light can be 
 the medium which accelerates the period of Encke's comet ? Or, again, may not 
 the same effect be attributed to the perturbations to which the body is 
 subjected at its periodical passages through the regions of shooting stars or the 
 telescopic planets ? All these questions are still extremely problematical, and it 
 will be understood that this is not the place to discuss. the various degrees of 
 probability of each of them. We may, however, remark that M. Faye, in at- 
 tributing to the solar heat a repulsive force, has suggested a theory of the 
 physical constitution of comets which accounts at once for the form of the 
 appendices of these bodies, and for the acceleration of the period which observation 
 has demonstrated for Encke's comet.
 
 COMKTS. 
 
 283 
 
 scrvation have coTifirmcd the periodicity, bearing the names of 
 Garabart or Biela, Faye, De Yico, Brorsen, and others, the first only 
 requires a special mention. The latter are all telescopic, and do not 
 offer any particular interest in their phj^sical aspect. 
 
 This is not the case, however, with Gambart's comet. Dis- 
 covered in 1826, its' first reappearance occurred in the autumn of 
 1832, and much excitement was caused by the somewhat premature 
 announcement that it must in its passage meet the Earth. More 
 precise calculations demonstrated, before the event, that the comet 
 would cross our orbit a month before our globe would reach the 
 point of passage, and thus contact was impossible. 
 
 But the alarm had been sounded. The imagination was excited, 
 
 Fig. 122. — Subdivision of Gambart's Comet. (Struve.) 
 
 and the idea of the end of the world — of our little world — occupied 
 numerous minds. Even among those who placed confidence in the 
 precision of astronomical calculations, there were some who at least 
 feared a derangement of our orbit. Doubtless, to them, an orbit 
 was something material — a metallic circle, for example. " As if," 
 says Arago, in relating this curious notion, " the form of the para- 
 bolic path in which a bomb after leaving a mortar, traverses sjjace, 
 was dependent on the number and positions of the paths which other 
 bombs had formerly described in the same region." 
 
 Further on we will say a few words on this question, which 
 some day or another may largely interest the inhabitants of our 
 globe — we allude to the danger and the probability of a comet's 
 contact with the Earth.
 
 284 
 
 THE SOLAK SYSTEM. 
 
 If Gambart's comet did not justify the fears that were conceived, 
 it was itself subjected a little later to a strange transformation — 
 it subdivided itself into two. In 1846, it appeared under the 
 form of two comets, of unequal size, which gradually separate more 
 and more. In 1 852 the two comets reaj)peared travelling together, 
 but the distance between the two nuclei, which had reached 150,000 
 miles in 1846, then amomited to 1,240,000 miles. 
 
 Astronomical annals have before recorded similar transforma- 
 tions; but as they related to comets which have not reappeared, 
 authorities hesitated to believe them. Gambart's comet, however, 
 leaves no doubt on the fact. 
 
 We here give some data on the short-period comets to which 
 we have referred : — 
 
 Tlmu of 
 Comets. revolution 
 
 ill years. 
 
 Encke's 3-29 . 
 
 DeVico's .... 5'46 , 
 
 Winnecke's .... 5'54 . 
 
 .Brorsen's .... 5'58 , 
 
 Biela's (or Gambart's) 6'61 
 
 B' Arrest's .... 6-84 . 
 
 Faye's 7'44 , 
 
 M6chain's . . . . 13'60 
 
 Halley's 76' 78 
 
 DistiUicos from tho Suu. 
 A)ihelioii. Perihelion. 
 
 Miluo. Miles. 
 
 387,000,000 
 475,000,000 
 
 537,000,000 
 585,000,000 
 
 32,000,000 
 110,000,000 
 
 64,000,000 
 82,000,000 
 
 603,000,000 192,000,000 
 3,200,000,00t» 56,000,000 
 
 Time of next 
 return. 
 
 1866, October. 
 1866 (?). 
 
 1869, Juno. 
 1868, May. 
 1866, January. 
 
 1870, October. 
 1866, Feb. 
 
 1871, October 
 1910 (0- 
 
 All these have their direction of movement the same as that of 
 the movement of the planets, that is, from west to east. Among 
 the periodical comets, already mentioned, that of Halley is the only 
 exception. [Besides the comets included in the preceding table, 
 others with periods of about seventy years, have been discovered 
 by Wesphal (1852) ; Pons (1812) ; De Vico (1846) ; Olbers (1815) ; 
 Brorsen (1847).] 
 
 We will content ourselves with the preceding details of the 
 astronomical short period of the periodical comets. It remains for 
 us to give some details on those comets the return of which is either 
 very remote or has not been determined.
 
 (;()METs. 285 
 
 III. 
 
 Comets of Long Period — Large Comets visible to the Naked Eye — Physical 
 Coustitutiou of Comets ; Mass, Density — Nature of their Light — Danger 
 which might result from the Contact of a Comet. 
 
 Must we accept literally the comparison of Kepler, who aiRrmed 
 that comets are scattered throughout the heavens with as much 
 profusion as fishes in the ocean ? Arago, adopting the hypothesis 
 of an equal distribution of comets in every region of the solar 
 system, and basing his calculations on the number of comets between 
 the Sun and Mercury, estimated the number of these bodies which 
 traverse the solar system within its kno-mi limits, that is to say, 
 within the orbit of Neptune, at 17,500,000.* 
 
 Whatever we may think of these hypotheses, observation proves, 
 from year to year, that the number of comets is really considerable. 
 Leaving mere reappearances out of the question, new ones are con- 
 stantly found to arrive from the depths of space, describing 
 round the Sun orbits which testify to the attractive power of 
 that radiant body, and, for the most part, going away for- 
 centuries, to return again from afar after their immense revolu- 
 tions. 
 
 During the two or three centuries in which comets have been 
 observed with care, more than 200 have been recorded. Adding 
 them to those noted in ancient annals, we must reckon them at 
 five or six hundred, among which there are only about forty of 
 which we have been enabled to determine the period of revolu- 
 tion. 
 
 Of this number, five complete their revolutions in periods which 
 vary between sixty-nine and seventy-five years. But what shall we 
 say of those which take thousands of years to accomplish their circuit, 
 
 * As early as 176.5 Lambert, basing his calculations on other data, regarded 
 .'>()( ),()00, 000 as a very moderate estimate of those within the orbit of Saturn.
 
 28G THE SOLAK SYSTEM. 
 
 of the famous comet of 1680, the perihelion point of which was 
 so near the Sun, that Newton vahied its temperature while passing- 
 through that part of its orbit at 2000 times the heat of red-hot 
 iron ? Its period is about 8814 years. But there are some longer 
 still ; and the period of the comet of July, 184-1, has been estimated 
 at not less than 100,000 years. If the calculation is exact, here 
 is a comet the return of which will be observed by the astro- 
 nomers of the year 101,844 ! At a mid date of this immense 
 period, it will be travelling in space at a distance not less than 4000 
 times that of the Earth from the Sun. 
 
 The velocity of comets, diminishing like that of the planets 
 as their distances from the Sun increase, varies between very large 
 limits, and at their greatest distance from the central body it is 
 extremely small ; thus the comet of 1080 scarcely traverses, at its 
 ajihelion, more than three yards a second ! 
 
 Among the numerous comets observed, there are very few that 
 are visible to the naked eye, and a still less number which strike 
 ordinary observers by their large dimensions and the brightness of 
 their light. It is these, nevertheless, which possess the greatest 
 interest, by reason of the peculiar plienomena ])resented by their t;iils 
 and nuclei — phenomena which throw great light on their physical 
 constitution. 
 
 Among tlie most remarkable comets of by-gone centuries must 
 be mentioned the large comet of 1500, wliich the Italians sur- 
 named // Si(/)ior A-sfoiu' ; the comet of Charles the Fifth, of 150(), 
 which, according to astronomical calculations having already ap- 
 peared in 1264, ought to have made its reappearance about 1860, 
 and has not been again seen ; that of 1686, the bright nucleus of whicli 
 shone as a star of the first magnitude ; the comet of 1744, witli 
 several tails; and that of 176!), which is represented, as given in 
 the drawings of the time, in I'late XVIII and fig. 118. 
 
 The portion of the nineteenth century already elapsed has been rich 
 in brilliant comets, visible to the naked eye. We liere reproduce 
 some of the most remarkable; first, the large comet of 1811, the 
 appearance of which made an extraordinary sensation. It will not 
 again return for thirty centuries. The head measured 1 12,000 miles 
 in diameter, whilst the diameter of the luminous nucleus was little 
 more than 400 miles. The tail, of prodigious dimensions, attained a 
 length of 112,000,000 miles. 
 
 The great comet of 184-3 was one of the most brilliant ever 
 observed. Not only the nucleus, but a portion of the tail, was visible
 
 PLATE XX. 
 
 DONATI'S COMET. 
 
 1. 2-1 September, 1S5S. 2. 26 Soiitombcr, ISOS. (G. T. Boud.)
 
 COMETS. 
 
 289 
 
 ill full day. The tail was besides very remarkable for its length, and 
 still more for the uniformity of its breadth. This is, of all known 
 comets, that which is the nearest to the Sun. At the time of its 
 shortest distance from the centre of our system, the nucleus was 
 not more than 470,000 miles from the centre of the Sun, and con- 
 sequently only 30,000 miles from its sui-face. 
 
 In these latter years three comets, visible to the naked eye, have 
 been the object of the most interesting observations. The most 
 brilliant of all, Donati's comet, made its appearance in 1858. Per- 
 ceived at Florence, for the first time on the 2nd of June, by the 
 
 Fig. 123.— Great Comet of ISll, from a drawing Vij- Admiral Smj-th, in the 
 " Speculum nartwellianum." 
 
 astronomer whose name it bears, it became visible to the naked eye 
 towards the first days of September, and was soon distinguished 
 among the northern constellations by the brightness of its brilliant 
 nucleus, and the magnificent development of its tail. 
 
 Those persons who were witness to the splendid spectacle ofiered by 
 the nights when this beautiful body was visible, will be able to recog- 
 nise and follow, in Plates XX, XXI, XXII, and XXIII, the aspect 
 of the comet at diiferent epochs, and its path across the starry vaidt. 
 
 In 1861 and in 1862 two other comets were also visible, although 
 inferior in brilliancy to that of 1858. There will be found further 
 
 u
 
 290 
 
 THE SOJ.AK SYSTEM. 
 
 on (fig-s. 124 and 125, and Plate XXIII), detailed representations 
 of the head and nebidous envelopes of these bodies— details extremely 
 interesting from a physical point of view. 
 
 The problems connected with the study of the physical consti- 
 tution of' comets are numerous, and of extreme difficulty. It may 
 be asked, in the first place. What is the nature of the matter which 
 composes them? or whether this matter be entirely gaseous? or, 
 ao-ain, if the nuclei enclose liquid or even solid particles, and if so 
 what is their bulk, and their density ; if the tail is of the same 
 nature as the head or nucleus ; or by virtue of what influence these 
 sino-ular appendages are formed, wliich abnost unnoticeable when 
 the comet is far from the Sim, are developed as it approaches it, to 
 
 Fig. 124. — ConiL't of isir.'. Fonii.s :iiid ixisitiims of tlie luiniiioiis jets, on August 23, 
 at one o'clock iii tlic moriiiug, ainl ut uiuu in the evening. 
 
 diminish, and finally disappear again in the more distant half of its 
 oi-bit ? 
 
 Next comes the question of the light wliich renders the comets 
 visible in space. Do comets shine with their own light ? do they 
 borrow their light from the Sun ? or do they send us ra}'s proceeding 
 from both these sources ? Again, can anything plausible be con- 
 jectured on their temperature, or on the changes induced upon this 
 element by the prodigious variations of distance which are the con- 
 sequence of the extreme elongations of their orbits ? 
 
 Lastly, what is the cause of the modifications to which these 
 strange bodies are subjected, not only from one revolution to another, 
 but under our very ej^es, during the short interval of a single appear- 
 ance ? Not only is the tail fonned, developed, diminished, and again 
 absorbed, but the envelope of the nucleus is subject to ihe most
 
 PLATE XXII. 
 
 DONATIS COMET. 
 
 1. Oct. .3, 18.38. 2. Oct.-"), 18.58. (G. V. Bond.)
 
 COMETS. 
 
 293 
 
 curious transformations. If we look at the drawings (figs. 124 and 
 120) of the comet of 1862, drawings which represent the head 
 of the body at intervals of a day at the most, we shall be as- 
 tonished at the rapidity of the changes of position and foiTn of the 
 luminous jets which successively were emitted from the nuclevLS, in 
 a direction nearly always opposed to that of the tail. In an interval 
 of seventeen days, the able observer to whom we owe the conunimica- 
 tion of these drawings, M. Chacornac, was able to distinguish the for- 
 mation of thirteen of these jets, similar to jets of steam, and alternately 
 directed towards the 8un and to the east of it, that is to say, in a 
 
 Fig. \-2a — C'iriiet of 18o2. Aspect ut' tlie lit^d or the C'^mct. at nine in the evening, 
 the 23rd August, and the 24th August at the same hour, 
 
 direction opposite to the movement of the comet. After each of these 
 emissions, the nebulous matter, accumulated at the end of the jet, 
 seemed driven back by a repulsive force emanating from the Smi, 
 and then flowed in the direction of the tail. These phenomena 
 would seem to confirm the hypothesis of M. Faye, to which we have 
 before alluded, Avhich attributes to the Sun, independently of an 
 attracting force by virtue of its mass, a repulsive power by virtue 
 of its heat. By means of this h^'[)othesis, M. Roche has been 
 enabled to accomit for the variation in form of the nucleus and 
 envelopes. 
 
 ["We may here remark that these last have recently been specially 
 the object of a searching inquiry by the lamented. Professor Bond, 
 in Ills most admirable memoir on the comet of 1848. These enve-
 
 294 THE SOLAK SYSTEiM. 
 
 lopes, however, must nof be confounded with the Umhulhmg, or outer 
 faint veil, which may extend for some distance around the head. 
 They were observed to regularly expand outward from the 
 nucleus, and the history of no less than seven of them has been 
 recovered.] 
 
 To what forces are these strange phenomena due ? 
 
 To these questions of great interest, which, it must be admitted, 
 are still very obscure, may be added others which at different times 
 have been privileged to captivate the attention of the public. We 
 have seen that Gambart's periodical comet was expected in 1832, 
 to come in contact with the Earth. What Avould have resulted from 
 jsuch an event ? 
 
 A century ago, f<afrin.s still considered comets to be bodies, the 
 impact of which on our globe, or witli another planet, would entail 
 the most frightful consequences. 
 
 " When the movement of the comets is considered," says Lam- 
 bert, in his Lett res Cosmohgiques, " and we reflect on the laws of 
 gravity, it will be readil}^ pei'ceived that their approach to the 
 Earth might there cause the most woeful events, bring back the 
 universal deluge, or make it perish in a deluge of fire, shatter it 
 into small dust, or at least turn it from its orbit, drive away its 
 Moon, or, still worse, the Earth itself outside the orbit of Saturn, and 
 inflict upon us a winter several centuries long, which neither men 
 nor animals would be able to bear. The tails even of comets would 
 not be unimportant phenomena, if the comets in taking their 
 departure left them cither in whole or in part in our atmo- 
 sphere." 
 
 Maupertuis, at the same time, had already described in nearly 
 the same manner the catastrophes which the fear of the Earth's 
 contact with a comet had led astronomers to imagine. Only, by 
 the side of possible inconveniences, he enumerated the advan- 
 tages that might be derived from the distant influence of these 
 bodies, such as the changes of the seasons into a perpetual spring, 
 the acquisition of new moons, or of a ring like that of Saturn. He 
 then adds : " However dangerous might be the shock of a comet, it 
 might be so slight, that it would only do damage at that part of the 
 Earth where it actually struck ; perhaps even we might cry quits 
 if while one kingdom were devastated, the rest of the Earth were to 
 enjoy the rarities which a body which came from so far might bring 
 to it. Perhaps we should be very surprised to find that the debris 
 of these masses that we despised were formed of gold or diamonds ; 
 but who would be the most astonished, we, or the comet-dwellers,
 
 COMETS. 297 
 
 who would be cast on our Earth ? What strange beings each would 
 find the other!"* 
 
 At the present day astronomers have abandoned these fears. 
 Not only, according to them, is the probability of a shock so 
 slight, that it is not worth while to trouble ourselves about such 
 an event ; but, again, the mass of comets appears such a small frac- 
 tion of the mass of the terrestrial globe, that the shock would be 
 quite imperceptible. 
 
 This way of looking at the matter rests on considerations and on 
 facts which render it very probable. In 1770 a comet was seen to 
 traverse the system of Jupiter, without inducing the smallest pertur- 
 bation in the movement of the satellites, whilst the nebulous body 
 itself was so much disturbed that its entire orbit was changed. 
 
 [Then, again, we have good reason to believe that we actually 
 passed through the tail of the comet of 1861, and the only effect 
 observed was a peculiar phosphorescent mist.] 
 
 But would it be the same with all comets ? In our opinion, 
 it is at least prudent not to generalise too hastily. If comets exist, 
 the nebulosity of which seems entirely gaseous, and so transparent 
 that small stars remain visible through them, there are others, the 
 nucleus of which is doubtless very dense, since their light has been 
 strong enough to be perceptible in full day, even in the vicinity of 
 the Sun. The mass of Donati's comet has been valued by MM. 
 Faye and Roche at about the seven-hundredth part of the bulk of the 
 Earth. 
 
 " That is," says M. Faye, " the weight of a sea of 40,000 square 
 miles 109 yards deep ; and it must be owaied that a like mass, 
 animated with a considerable velocity, might well produce by its 
 shock with the Earth very perceptible effects." 
 
 Of the heat pecidiar to the comets, and of the nature of the light 
 that they emit, very little is yet known. [Spectrum observations 
 have been made, but their discordance renders it necessary to wait 
 for others.] Doubtless, in the vicinity of the Sun, the action 
 of the high temperature of the radiant body cannot fail to be felt 
 on the exterior strata of the cometary nuclei ; and it is thus that 
 the formation of the luminous jets which, becoming detached from 
 the central mass and acted upon by some miknown force, give rise 
 to the tail, may be accoimted for. 
 
 On the other hand, it seems proved that the light of the comets 
 
 * " Lcttre sur la Comete. CEuvrcs de M. de Maupertuis," p. 203. Dresden, 
 ] 752.
 
 298 
 
 THE sol, AH SYS'IKM. 
 
 is, in part at least, borrowed from the Sun. But may tliey not 
 also possess besides a lig-lit of their own y and, on this last hypo- 
 thesis, is this brightness owing to a kind of phosphorescence, or 
 to the state of incandescence of the nucleus ? Truly, if the nuclei 
 of comets be incandescent, the smallness of their mass would eli- 
 minate from the danger of their contact with the Earth only one 
 element of destruction ; the temperature of the terrestrial atmo- 
 sphere would be raised to an elevation inimical to the existence 
 of organised beings ; and we should only escape the danger of a 
 mechanical shock to run into a not less frightful one of being calcined 
 in a many days' passage through an immense furnace. 
 
 If we enlarge on these considerations, which are merely h}^o- 
 thetical, it is not with the intention of reviving the fears or super- 
 stitious terrors of another age. AVe but wish to show to what con- 
 jectures science is still reduced on the problem, so interesting from 
 so many points of view, of the physical constitution of comets.
 
 OKNERAI, SURVEY. . 'i!)!) 
 
 JKNEPtAL SURVEY OF THE SOLAR SYSTEM. 
 
 We have now terminated our description of the various phenomena 
 presented by the Solar System. 
 
 We have reviewed successively all the bodies which compose it, 
 from the immense central-body — the fountain-head of heat and light 
 — to the most distant planets which its powerful attraction maintains 
 in their orbits, and to those vagabond bodies, the comets, some of 
 which perhaps visit but once those regions of the sky in which the 
 movements of our system take place. 
 
 We are about to quit the system of which our Earth forms 
 part — a system so prodigiously vast, when the dimensions are com- 
 pared either with the most gigantic constructions of man, or even 
 with the terrestrial globe itself, the magnitude of which reduces man 
 to nothingness. We shall now launch out into space, far, very far 
 beyond Neptune, to such distances that the Earth, the planets, the 
 Sun itself even, when looked back upon, would but appear as lu- 
 minous points, and the whole solar system woidd dwdndle down to 
 a single speck of light. 
 
 There, we shall find myriads of other Suns, other worlds, of which 
 the physical constitution, distances, and movements, must also be 
 studied. But before undertaking this immense voyage in the infi- 
 nite, let us sum up in a few general remarks the more striking fea- 
 tures of the Solar System, which will constantly serve us for com- 
 parison \^dth the other systems with which we shall have to deal. 
 
 We have seen how the different celestial bodies which revolve 
 round the Sun are grouped. In describing each of them we have 
 given their real dimensions, both absolute, and compared with those 
 of our Earth. Plate XXIY contains all these comparative dimensions
 
 300 . THE SOLAR SY.STKM. 
 
 grouped together, whence we may gather by a coup d'ceil how much 
 the volume of the Sun preponderates over that of all the planets 
 and their satellites put together. Calculation shows indeed that the 
 solar globe itself contains 600 times the united vohmies of all these 
 bodies. Its mass is still more considerable ; and if the Sun were 
 placed in one of the scales of a celestial balance, 750 times the 
 weight of all the planetary masses must be placed in the other to 
 equal it. 
 
 We have from the commencement divided the planets into three 
 principal groups ; that of the planets of average size, that of the 
 asteroids or telescopic planets, and that of the larger phmets. A 
 fact which renders this division more striking is, that the celestial 
 bodies, of wliieli each group is forui(>d, uot only present a similarity 
 in size, while the distances of all from the Sun seem to obey a law, 
 but other physical analogies seem to indicate that they form so 
 many natural families, the members of which have perhaps a common 
 origin. 
 
 Thus Mercury, Venus, the Earth, and Mars, have a movement 
 of rotation, the time of which is nearly equal ; and, except in the case 
 of Mercury,* their density is very similar, and the polar flattening 
 is either very slight or imperceptible. 
 
 With regard to the inclinations of their axes to the planes of 
 their orbits, a condition of things which has an overpowering in- 
 fluence on the seasons in each i3lanet, the four smaller planets of 
 which we speak must [if we accej^t the old observations] be divided 
 into two sub-groups. Mercury and Venus in one category. Mars and 
 the Earth in the other. 
 
 We know very little of the physical constitution of the minor 
 planets ; but, besides the fact that they are all accumulated in one 
 narrow zone, and are all of small dimensions, they possess a family 
 likeness in the great excentricity of their orbits, and the generally 
 very great inclinations of the planes in which they revolve round the 
 Sun. 
 
 We come now to the four larger planets, Jupiter, Saturn, Uranus, 
 and Neptune ; at once we are struck with their much more rapid 
 rotation, which we should have predicted from the considerable 
 polar flattening of the tAvo flrst bodies. With regard to the two 
 other planets, Uranus and Neptune, we are still in the dark on 
 these points. Their density is at most but a quarter of that of the 
 
 * According to Enckc, the density of Mercury is really much less than that 
 at present adopted ; it is not very diflfercnt from the density of the Earth.
 
 PLATE XXIV. 
 
 COMPARATIVE DIMENSIONS OF THE SUN, THE PLANETS AND THEIR 
 
 SATELLITES.
 
 GENERAL SUllVEY. 308 
 
 smaller planets, and this is almost the case for all the members of the 
 group. 
 
 But the other elements do not offer such close analogies. The 
 inclination of the axis, small in the case of Jupiter, is larger in the 
 case of Saturn, and probably excessive in Uranus. 
 
 But another point of resemblance is, that all these larger planets 
 have a great number of satellites, whilst the Earth alone, of all the 
 planets of the system, is accompanied by a single moon. 
 
 The question as to the habitability of the other planets of the 
 system has been much agitated. It has been asked if only the 
 Earth's surface is embellished by the productions of animal and 
 vegetable life, if it alone is inhabited and governed by intelligent 
 and sensible beings. 
 
 Astronomy can only indirectly touch on these interesting ques- 
 tions, the solution of which will, doubtless, long remain beyond 
 us, although we have seen with what minute care science collects 
 together all the elements of the problem, all the data which observa- 
 tion can furnish on the meteorological and physical conditions 
 belonging to each member of the solar system. 
 
 Doubtless, if we reason by the analogies which are permitted 
 us, there are strong probabilities that most of the planets and their 
 moons are inhabited. But what is the organisation of the vege- 
 table and animal kingdoms which people them ? Of this it is 
 difficult to form an idea, in the actual state of our knowledge. 
 
 But is it not probable that the ages of the planets are very 
 different, and that, if we suppose that they all must pass through 
 the same geological phases, these phases will be far from being the 
 same at the same epochs ?
 
 PAUT THE SECOND. 
 
 THE SIDEREAL SYSTEM.
 
 f 
 
 ANDROMEDA PERSEUS 
 Aldol 
 
 AURIGA 
 Cap ell a 
 
 The CAMELOPARD The lYNX 
 
 PL . VII. 
 
 The LION 
 
 CASSIOPEIA 
 
 Pole Star 
 
 T..SWAK CZP»„S SM«XMAK B«CO ,. C^Vwl .OOT,. 
 
 THE HEAVENS FROM THE HORIZON OF PARIS. ( NoHh S.de; 
 
 Talten at midnidlit Dec. 20. 
 
 E. Gudlemm etH. Clerget del. 
 
 Imp^Becquet a Paris .
 
 BOOK THE FIRST. 
 
 THE STARS. 
 
 Let us imagine a sphere, having the Sun for its centre, the ideal 
 surface of which lies at a distance of thirty times the mean radius 
 of the Earth's orbit ; this sphere will comprise in its vast extent all 
 the celestial bodies, the comets excepted, which periodically efl'ect 
 their revolutions round the Sun, and of which we have described 
 the movements and physical constitution. 
 
 Do other planets exist more distant still than Neptune ? and do 
 the comets of long period which, after having shone once in our 
 regions, bury themselves in depths exceeding many thousands of 
 times the distance of the Sun from the Earth, really belong to our 
 system ? 
 
 These are questions which at present cannot be answered, and 
 for the solution of which we must wait, perhaps for centmies. We 
 may, therefore, be allowed to regard the sphere which we have 
 just imagined, as fixing an approximate limit to the dimensions of 
 the Solar System. 
 
 Let us, however, in thought triple the radius of this sphere ; 
 let lis give it a radius of a hundi-ed radii of our orbit, that is, a 
 radius of some 9,500,000,000 miles, — an enormous distance, which 
 the imagination can with difficvdty grasp, and which a ray of light 
 would reqmre more than eleven days to traverse, in spite of its 
 extraordinary velocity of 192,000 miles a second ! 
 
 Nevertheless, we shall soon see that this immense line is but 
 a point, when we compare it with the dimensions of that portion 
 of the universe which our sight is able to grasp. The nearest of 
 the innumerable systems which people that universe would be
 
 808 THE SIDEREAL SYSTEM. 
 
 removed from the then confines of the solar system to a distance two 
 thousand times greater than the radius of our imaginary sphere. 
 We could scarcely hope, therefore, that it would ever be possible, 
 even with the aid of the most powerful telescopes, to make out the 
 physical peculiarities of celestial bodies so immensely distant. But, 
 thanks to some ingenious apijliances and to methods of an extreme 
 delicacy, the latest investigations have furnished observers with a 
 rich series of interesting phenomena. 
 
 The constitution even of the visible imiverse has thus by degrees 
 been revealed : the distribution of the various bodies, their group- 
 ings and movements, the intensity and colour of their light, and a 
 thousand other interesting facts, are so many points, the positive 
 knowledge of which now surrounds sidereal astronomy with the 
 highest interest. 
 
 We are, therefore, about to consider the heavens as a whole 
 and in detail. The knowledge we have acquired of the system to 
 which the Earth belongs will be a great help to us in this study, 
 as it will continually afford us points of comparison to reason by 
 analogy on other systems.
 
 THE STARS. 309 
 
 THE STARS. 
 
 Scintillation of the Stars — Apparent Fixity of their relative Distances — Number 
 of Stars visible to the Naked Eye — Approximate Number of Stars visible in 
 Telescopes. 
 
 No sight, as we said at the beginning of this work, is at once so 
 awe-inspiring and so grand as that of the heavens on a beautiful 
 night. If care be taken to choose as a stand-point for observation 
 an open place, such as a plain or the summit of a hill on land, or, 
 again, the open sea ; and if the atmosphere, somewhat charged with 
 dew, j)ossesses all its transparency and purity, we shall see thousands 
 of luminous points twinkling in all directions, accomplishing slowly 
 and together their silent march. The contrast of the obscurity which 
 reigns on the sm-face of the Earth with the brightness of that re- 
 splendent vault, gives an indefinite depth to the celestial ocean that 
 deepens over our heads. But let us here leave the magnificence 
 of the spectacle, to study it in its most minute details. 
 
 Let us commence with the appearances. A character common 
 to all the stars is an incessant and very rapid change of brightness, 
 which has received the name of sciniiUation. This is accompanied 
 by variations of colour equally rapid, due to the same cause as the 
 successive disappearances and reappearances. All stars scintillate, 
 whatever may be their brilliancy, at least in our temperate regions. 
 But the intensity of this luminous movement is not the same in all, 
 and it varies, moreover, both with the degree of purity of the 
 sky, the elevation of the stars above the horizon, and the temperature 
 of the night. 
 
 According to Arago, scintillation is due to the difference of ve- 
 locity of the various coloured rays traversing the unequally warni, 
 unequally dense, unequally humid atmospheric strata. Thus, in
 
 310 THE SIDEREAL SYSTEM. 
 
 tropical regions, where the atmospheric strata are more homoge- 
 neous, scintillation is rarely observed in stars the elevation of which 
 above the horizon is more than 15°, or the sixth of the distance 
 of the horizon from the zenith. " This circumstance," says Hum- 
 boldt, " gives to the celestial vault of these countries a particularly 
 calm and soft character." 
 
 As to the planets, they scintillate little or not at all ; it is rare 
 that traces of this phenomenon are observed in Saturn or Jupiter, 
 but it is more perceptible in Mars, Venus, and Mercury. This 
 difference suffices, in our climates, to afford to those who are not 
 very familiar with the configuration of the celestial groups the first 
 means of distinguishing a planet from a star. 
 
 Another specific character of the stars is, that their diameters 
 are without appreciable dimensions. To the naked eye, this dis- 
 tinction woidd be insufficient, since, the Moon and the Sun excepted, 
 the most considerable planets have not sensible diameters. But, 
 while the magnifying power of optical instruments shows us the 
 principal planets under the form of clearly defined disks, the most 
 powerful glasses only show a star as a limiinous point. The dis- 
 tance which sej)arates us from these bodies is so great, that there is 
 nothing to astonish us in such a result. 
 
 WoUaston affirms that the apparent diameter of the most bril- 
 liant star in the heavens, Sirius, is not more than the fiftieth part 
 of a second of arc. But let us hasten to say that this result still 
 leaves a good margin as to the real dimensions of the star, since, at 
 the distance of Sirius, an apparent diameter of this size would re- 
 present a real diameter of 11,000,000 miles ; that is, twelve times 
 the diameter of our Sun. 
 
 Let us add, lastly, that the absence of appreciable apparent di- 
 mensions does not suffice to distinguish absolutely the stars from 
 the planets, since a certain number of the latter, as we have before 
 seen, appear in telescopes only as simple limiinous points. Let us 
 come, then, to a permanent specific character, the knowledge of 
 which will always prevent us from confounding a star with one of 
 the known or unknowTi bodies which form part of our solar group. 
 This characteristic is as follows : — 
 
 The stars, properly so called, preserve among themselves — nearly 
 enough for our present purpose — the same relative distances. They 
 form, then, on the celestial vault apparent groups, the configuration 
 of which is nearly invariable. Centuries must elapse to show a 
 change of form, unless we employ extremely delicate measures.
 
 THE STARS. 311 
 
 A planet, on the contrary, moves rapidly across these groups, to 
 such a degree that, in the interval of a night, or at most of a few 
 nights, this displacement is very perceptible ; hence the old denomi- 
 nation oijixed stars, in opposition to the wandering ones, or planets. 
 
 "We must be careful, however, to guard against assigning to this 
 word a rigidity which it does not possess, for we shall shortly see 
 that the stars really move with a velocity not inferior to that which 
 animates the members of our system. Their immense distance is 
 the only cause of their apparent immobility, which vanishes when 
 precise observations, embracing a sufficient interval of time — some 
 years, for example — are made. 
 
 A fact which strikes every one is the great diversity of brightness 
 in the stars which peoj)le the heavens. All degrees of intensity are 
 remarked, from the resplendent light of Sirius to the scarcely per- 
 ceptible glimmer of those hardly visible to the naked eye. 
 
 Whence arises this difference of brightness ? This question we 
 cannot answer for any star in particular ; but it is easy to imagine 
 that it may result from various circumstances, such as their less or 
 greater distance, the real and various dimensions of the bodies, and, 
 lastly, the intrinsic brightness of the light peculiar to each. How- 
 ever this may be, astronomers, without regard to the unknown causes 
 which may influence the intensity of the stellar light, have divided 
 stars into classes or magnitudes ; and when we speak of a star of the 
 first, second, or fifth magnitude, it is understood that this way of 
 speaking refers only to the apparent brightness, and that nothing 
 is affirmed either as to real dimensions or distance, or even intrinsic 
 brightness.* 
 
 Besides, as the stars, arranged in the order of their brightness, 
 would form a progression decreasing by imperceptible degrees, the 
 classes adopted are themselves conventional and arbitrary. The first 
 six magnitudes compt-ise all stars visible to the naked eye. But 
 the use of the most powerful telescopes brings to view stars of feebler 
 light, descending to the sixteenth and seventeenth magnitudes. In 
 truth, the progression has no inferior limit : it extends more and 
 more in proportion as the progress of the optician's art increases the 
 penetrating power of our instruments. 
 
 * What we have said of a particular star is not rigorously true when the 
 whole of the stars are considered. The calculus of probabilities enables us, 
 in this case, to deduce from the brightness of the stars of a certain size some 
 inferences on their mean distance. We shall return to this point.
 
 312 
 
 THE SIDEREAL SYSTEM. 
 
 To gain an idea of the respective intensities of the light emitted 
 by the stars of the first six magnitudes, following the scale adopted 
 by astronomers, the following drawing shoidd be inspected ; in it the 
 stars are figured by disks, the surfaces of which are in proportion to 
 their brilliancy. 
 
 Fig. 120. — Relative brightness of the stars of the first six magnitudes. 
 
 But, we repeat, it must not be thought that the stars ranked in 
 the same class are, on that account, of the same brightness.* Thus 
 the light of Sirius is estimated at four times that of the star Alpha 
 (or a) Centauri ; but both, nevertheless, are included by astronomers 
 in the number of the stars of the first magnitude. 
 
 We here give the names of the twenty most brilliant stars of the 
 two hemispheres, which it is usual to consider as forming the first 
 class. They are here arranged in the order of their brightness : — 
 
 1. Sirius. 
 
 2. Eta (»)) Argtis.t 
 
 3. Canopus. 
 
 4. Alpha (a) Centauri. 
 
 5. Arcturus. 
 
 6. Rigel. 
 
 7. Capella. 
 
 8. Vega. 
 
 9. Procyon. 
 10. Betelgeuse.J 
 
 11. Achernar. 
 
 12. Aldebaran. 
 
 13. Beta (/S) Centauri. 
 
 14. Alpha (a) Crucis. 
 
 15. Antares. 
 
 16. Atair. 
 
 17. Spica. 
 
 18. Fomalhaut. 
 
 19. Beta (A) Crucis. 
 
 20. Pollux. 
 
 Lastly, Regulus, a bright star in the constellation of the Lion, is 
 also ranked by some astronomers in the first magnitude, while others 
 only admit in this class the first seventeen stars in the above list. 
 These divergencies are of no importance. 
 
 * [Astronomers not only class the stars in magnitudes, but tabulate them in 
 the order of their brightness in each constellation, the principal stars being de- 
 noted by the letters of the Greek alphabet. A star is described by a Greek 
 letter, followed by the name of the constellation in Latin : thus «, or Alpha Cen- 
 tauri, denotes the brightest star in the constellation of the Centaur. /3 Lyrte is 
 the second brightest star in the Lyre, and so on.] 
 
 t It will be seen, subsequently, that the brightness of this star undergoes 
 astonishing changes. 
 
 X The brightness of this star is variable ; it has recently descended to the 
 sixth magnitude.
 
 ^ THE STAKS. 313 
 
 In proportion as tbe scale of brilliancy or magnitude is descended, 
 the number of the stars contained in each class rapidly increases. 
 The number of second-magnitude stars in the entire heavens is about 
 65; of the third, about 200; of the fifth, 1100; and of the sixth 
 magnitude, 3200. Adding these numbers together, we obtain a few 
 over 5000 stars of the first six magnitudes, and these comprise very 
 nearly all those that can be seen with the naked eye. 
 
 The smallness of this number nearly always astonishes those who 
 have not tried to form an exact estimate of the number of stars which 
 shine in the celestial vault on the most favourable nights. 
 
 The aspect of the multitude ofsparkling points which are scattered 
 over the sky makes us disposed to believe that they are innumerable, 
 and to be counted, if not by millions, at all events by hundreds of 
 thousands. This is, nevertheless, an illusion. All observers who 
 have taken the trouble to make an exact enumeration of the stars 
 visible to the naked eye, have arrived at a maximum of 3000 as the 
 mean number which can be observed in every part of the heavens, 
 visible at the same time, at the same place ; this, of course, is but 
 half of the entire heavens. 
 
 Argelander has published an exact catalogue of the stars visible 
 on the horizon of Berlin during the course of the year. This cata- 
 logue comprises 3256 stars.* According to Humboldt, there are 
 4146 visible on the horizon of Paris in the whole course of the year ; 
 and as this number increases in proportion as we approach the 
 Equator, that is to sa}^ in proportion as the double movement of the 
 earth unfolds to us during a year a more extensive portion of the 
 heavens, 4638 stars are already visible to the naked eye on the 
 horizon of Alexandria. 
 
 We repeat, the maximum number is comprised between 5000 and 
 6000 stars for the entire heavens, including those seen by the most 
 piercing and most accustomed eyes in the best nights for observation. 
 When the atmosphere is lit up by the Moon, or by twilight, or, as 
 
 * M. Heis (of Munster) affirms that his sight is so penetrating that he can 
 perceive with the naked eye 2000 more stars than those catalogued by Argelan- 
 der in his Uranometria Nova. On the other hand, there are many eyes which 
 distinguish at most stars of the fifth magnitude, and do not see any of those of 
 the sixth. 
 
 The degree of visibility of the stars to the naked eye depends also on the 
 state of the atmosphere, ou its degree of purity, and on the altitude of the place. 
 Londoners, to be assured of these differences, have only to compare the sparkling 
 sky of the country with that which they see through the haze which almost 
 constantly envelopes their city.
 
 314 
 
 THE SIDEKEAL SYSTEM. 
 
 happens in the great centres of population, by the illumination of the 
 houses and streets, the lowest magnitude stars are effaced altogether, 
 and the number of those visible is consequently much more limited. 
 We may add, in conclusion, that the more decided the scintillation, 
 the more easy is it to distinguish very faint stars. 
 
 A word now on the number of stars that can be seen with the 
 help of the telescope. Here we shall find the numbers which our 
 
 Fig, 127. — A part of tuc constellation of the Twins, as seen through a telescope.* 
 
 imagination had erroneously led us to believe are visible to the 
 naked eye. 
 
 According to the illustrious Director of the Observatory of Bonn, 
 — Argelander — the seventh magnitude comj)rises nearly 13,000 
 stars ; the eighth, 40,000 ; and, lastly, the ninth, 142,000. The cal- 
 culations of Struve ffive the total number of stars visible in the 
 
 * This drawing is tlie reproduction, on a small scale, of one of the maps of 
 the beautiful Ecliptic Atlas pubUshed by M. Chacornac.
 
 THE STARS. 315 
 
 entire heavens by the aid of Sir William Herschel's 20-feet 
 reflector, as more than 20,000,000. But, without doubt, these ap- 
 proximate numbers are much below the real ones. It will be seen, 
 besides, that the richness of the different parts of the heavens in stars 
 is very imequal. The bright zone known under the name of the 
 Milky "Way alone contains, according to Herschel, 18,000,000.* 
 
 Nothing is more curious than to examine, both with the naked 
 eye, and by the aid of a telescope, the same part of 
 the sky. There, when the eye scarcely distinguishes 
 a few scattered stars, the telescope reveals thou- 
 sands. The two figures (127 and 128) will enable 
 those of our readers, who do not possess a telescope, 
 to judge of the surprise experienced by those who Fig. r2s— The Kimc 
 
 1 . 1 . 1 , . part of the constella- 
 
 make this observation. tion of the Twins 
 
 These drawings represent the same part of the (Gemini) seen with 
 
 . . naked ej'e. 
 
 constellation of the Twins. The naked eye is able 
 to see six stars. Now the same celestial region, seen by the aid of 
 a refractor of six-inches aperture, contains 3205 stars, varying from 
 the third to the thirteenth magnitudes. It appears as a perfect mass 
 of luminous points : and were we to apply to the same region in- 
 struments still more powerful, the eye would then discover at depths, 
 80 to speak, infinite, stars of all the smaller magnitudes. 
 
 * M. Chacoraac considers this estimate as even less than the number of 
 stars comprised between the first and thirteenth magnitudes. " For my part," he 
 remai'ks, " according to Sir William Herschel's gauges, and those of the Ecliptic 
 Charts, I estimate at 77,000,000 the number of stars comprised in the first 
 thirteen magnitudes, if we take the mean indicated in the preface to the Cata- 
 logue of Bessel's Zones, reduced by Weiss." What would the number become if 
 we added to these already prodigious estimates, all the stars of which the various 
 star-clusters now known are composed ?
 
 I 
 
 316 THE SIDEREAL SYSTEM. 
 
 II. 
 
 THE CONSTELLATIONS. 
 
 General Survey of the Starry Heavens — Constellations visible on the Horizon 
 of London — Northern Circumpolar Zone. 
 
 Before studying one by one the plienomena which the starry 
 heavens present to us — before penetrating, so to speak, to the heart 
 of the visible universe, to grasp its marvellous structure, and to 
 embrace in thought its tremendous extent, it is well to familiarise 
 ourselves with the groups of stars such as they are presented to the 
 eye of an inhabitant of the Earth. The movements with which the 
 so-called fixed stars are endowed, are effected, as we have before 
 said, with extreme slowness ; it follows, therefore, that the artificial 
 groups or constellations preserve for a long period the same con- 
 figurations. This constancy of form, joined to the difference of 
 brightness of the principal stars, wiU enable us to extricate ourselves 
 from the apparent chaos produced by so many luminous points 
 scattered on all sides on the celestial vault. When we shall pos- 
 sess, in a manner, a mental map of the sky, we shall be able to 
 follow with more interest the particular features which distinguish 
 its various regions, which are as varied in reality as they are at first 
 uniform in appearance. 
 
 In order to make this survey of the heavens we must choose 
 a station. Let it be London. As our globe, by virtue of its diurnal 
 movement, completes an entire rotation on its axis in about twenty- 
 four hours, it follows that the portions of the celestial vaidt, visible 
 at our station, will completely defile before us during that time. 
 Twenty-four hours, then, would suffice us to make our survey, if the 
 illumination of the atmosphere did not efflice the stars daring the day. 
 The succession of day and night, in fact, allows us only to see a 
 portion of the visible stars in a given place at tlie same time of the 
 vear.
 
 THE CONSTELLATIONS. 317 
 
 Fortunately, however, owing to the movement of the Earth 
 in its orbit, this difficulty disappears. In consequence of this move- 
 ment, each night shows us fresh stars, whilst those first visible, 
 disappear. In the course of a year, then, the Earth presents a dark 
 hemisphere to every part of the sky — to all those parts at least 
 which are visible on the horizon of our station. 
 
 Lastly, it must not be lost sight of, that even then one entire 
 part of the celestial vault will ever remain invisible to us Londoners. 
 Let us recall what is the effect of the diurnal movement of rotation 
 on the aspect of the heavens in any given place, like London the 
 station we have chosen. A point situated at a certain height above 
 its horizon, and towards the north and on the meridian, remains 
 immovable. It is one of the poles. Starting from this point in our 
 survey, the stars seem to describe, from rising to setting, larger and 
 larger circles, in proportion as they are situated further from the 
 pole. As long as the lower arcs of these circles do not touch the 
 horizon, the stars situated on them do not rise or set, and therefore 
 remain constantly visible ; these are the northern circumpolar 
 stars. 
 
 Beyond these, however, the circles described plimge partly be- 
 low the horizon ; they increase as far as the Equator, on the other 
 (the south) side of which the stars describe shorter and shorter arcs. 
 The last ones in our survey scarcely rise above our horizon, and, 
 when they do, shortly set and disappear. 
 
 It follows, then, that there is a zone of stars which never rises 
 above the horizon of London, and which remains for ever invisible to 
 all places of the Earth having the same latitude. These stars are 
 those which surround the southern pole of the heavens, and which 
 an observer would become acquainted with by degrees, in approach- 
 ing the equatorial regions of the Earth.* 
 
 The whole of the heavens, then, in our middle latitudes, may be 
 
 * By virtue of the two movements of the Earth, and of its spherical form, 
 the portion of the celestial sphere visible in any part of the globe varies with the 
 latitude of the place. 
 
 At the Equator the whole sky, both northern and southern, passes before our 
 view at night, during the entire year. The two poles lie on the horizon, of which 
 they mark the north and south points ; the celestial equator crosses the sky 
 from east to west, passing through the zenith. 
 
 In proportion as we travel from the Equator towards one or other pole, the 
 portion [of the visible sky diminishes even down to the half, for at the poles 
 themselves only one-half of the heavens, north or south, according to the pole, 
 is seen. The celestial equator then forms the horizon, and the celestial pole is 
 in the zenith. 
 
 [A little thought will show us that, as seen from the poles, the stars never set,
 
 318 THE SIDEREAL SYSTEM. 
 
 considered as forming three zones, the first always visible at night 
 when the sky is clear, whatever may be the time of year, the second 
 visible in part only on any given night, the third always invisible. 
 Let us successively pass these three zones under review. Let 
 us occupy ourselves first with that which is always in sight when 
 the sky is clear, on all the points of the Earth which have the 
 same northern latitude as London. From the mouths of the Thames 
 and Ehine, as far as the southern extremity of Kamtschatka, passing 
 by Antwerp, Cassel, Central Poland, Orenbourg, Southern Asiatic 
 Eussia, and Northern China, in the Old World, and the Aleutian 
 Islands, Queen Charlotte's Island, and the southern parts of British 
 j^orth America in the New, all the inhabitants of the parallel of 
 which we speak view the same spectacle during the whole year; 
 the hour only, at which the various constellations are on the meridian 
 of the various places, difiers. All the constellations comprised in 
 this zone of circumpolar stars are represented in Plate XXV. Let 
 us try to recognise them. 
 
 Suppose it midnight, at the end of autumn, near the 20th of 
 December; it is the night of the winter solstice. Let us look towards 
 the north. Let us imagine a circle which, toiiching the horizon at 
 the north point, extends somewhat beyond the zenith.* The centre 
 of this ideal circle will be found a little above a point nearly equi- 
 distant between the zenith and the horizon ; it is the Northern Pole 
 of the Heavens. Near this point is seen a rather brilliant star of 
 the second magnitude ; it is named the Pole Star. As it is im- 
 portant to know how to recognise this star, the position of which 
 remains nearly invariable during the whole course of the year, we 
 will show how this may be done. 
 
 If we examine the right-hand portion of Plate XXY, and fig. 129, 
 we shall find a group of seven stars, six of the second magnitude, 
 one of the fourth. It composes a constellation of the northern 
 heavens known for ages under the name of the Great Bear. Let 
 us scan well this part of the sky, whence we shall soon make 
 many alignments to help us in our survey of the starry heavens. 
 The seven stars of which it is composed may be divided into two 
 groups, the first of which towards the upper part, forms a quad- 
 
 they perpetually describe circles parallel to the horizon. At the Equator, all rise 
 and set every day, the movements of the equatorial stars being vertical. In mid 
 latitudes, the paths of the equatorial stars are intermediate between these two 
 main directions.] 
 
 * The zenith is the point of the heavens situated vertically above the head of 
 the observer.
 
 THE CONSTELLATIONS. 319 
 
 rilateral, whicli is called the hodij of the Bear, whilst the three 
 lower stars form the tail. The two extreme stars of the body- 
 are called the Pointers* Six of the seven principal stars of this 
 constellation are of nearly equal brilliancy, and of the second 
 magnitude. But it is easy to perceive, with the naked eye, that the 
 star in the body of the Bear nearest to the tail is inferior in 
 brilliancy to the others; it is now, indeed, only a fourth -mag- 
 nitude star, although in the seventeenth century it was as bright as 
 its neighbours. 
 
 The star in the middle of the tail (or shaft, if we think of 
 Charles' Wain), is accompanied, on the left, by a very small star 
 called Alcor, easily enough distinguished by an ordinary eye.f The 
 
 1 
 
 • 
 
 ' 'C'i 
 
 ^ 
 
 ^^ : ■ ■ 
 
 
 
 • 
 
 • 
 
 
 • 
 
 
 '.O 
 
 
 
 A 
 
 P^^^^^l^ 
 
 
 
 
 
 
 Polaris 
 
 
 
 K^W?''^ 
 
 
 c 
 
 I ^H E y ^ 
 
 Blm^^M'M^'^- 
 
 > 
 
 . il 
 
 Fig. 129.^The sky of the horizon of liOiidou. Northern circumpolar constellations. 
 
 naked eye perceives 138 stars in the Great Bear, amongst which, 
 besides the principal seven, are eight stars of the third magnitude, 
 and six of the fourth ; the others belong to the two last orders 
 of brightness perceptible to the unassisted vision. From the Great 
 Bear let us return to the Pole Star. 
 
 To do this, let us prolong the straight line which joins the 
 pointers, so called because they point to it, carrjang our eye along 
 this line towards the centre of the portion of the sky in our sight 
 when looking north. At a distance of about five times the space 
 
 * The Groat Bear has also been called " Charles' Wain." 
 t Humboldt affirmed tliat Alcor could lie but rarely seen with the naked eye 
 ill Europe. Let our readers judge of its present brightness for themselves.
 
 320 THE SIDEREAL SYSTEM. 
 
 which separates these two stars, we shall find the Pole Star. The 
 Pole Star plays an important part in the northern heavens, since, 
 being very near the pole, it is, so to speak, one of the pivots of the 
 ideal axis round which the Earth executes its real diurnal rotation, 
 and the heavens their apparent one in the opposite direction. It 
 follows, therefore, that it appears immovable, and always preserves 
 the same elevation above the horizon, while the other stars de- 
 scribe round it circles of unequal size. Thus the Great Bear, 
 situated to the east of the pole at the time we have chosen for the 
 commencement of our inspection, mounts towards the zenith as the 
 night advances. Towards six o'clock in the morning, it will be 
 above, or to the south, of the Pole Star, whilst at six in the evening 
 it will occupy a position diametrically opposite, below the pole and 
 near the horizon. 
 
 As all the stars participate in this movement, it is clear, that, 
 as their relative positions do not change, the figures of the groups 
 remain always the same. This must be well borne in mind. To 
 the west of the Pole Star, at the same height above the horizon 
 as the Great Bear, and at nearly the same distance from the pole, 
 is seen another group of six stars, of which two are of the second 
 magnitude, three of the third, and one of the fourth. This is 
 the constellation Cassiopea,* which contains sixty-seven stars visible 
 to the naked eye. The six which we have mentioned form a kind 
 of reversed chair, or a bad W, the figure of which, once thoroughly 
 caught, renders this constellation easy to recognise. 
 
 Between the Great Bear and Cassiopea is the Littt,e Bear, of 
 which the Pole Star is the most brilliant star. Of the twenty-seven 
 stars visible to the naked eye w^hich compose it, there are seven which 
 form a figure having a great resemblance to the seven stars of the 
 Great Bear, but arranged in an inverse order : the four intermediate 
 stars are seen with difiicidty. 
 
 Below the Little Bear a series of stars forms a sinuous line 
 prolonged nearly to the pointers, and terminated at one extremity by 
 a group of four stars arranged in the form of a trapezium. This is 
 the Dragon, which, among 130 stars visible to the naked eye, con- 
 tains one only of the second magnitude and nine of the third. 
 
 Cepheus, the Giraffe, and the Lynx, are three other constella- 
 tions near the pole : the first between the Little Bear and Cassiopea ; 
 the second opposite the Dragon ; the third on the same side as the 
 
 * By drawing a line from the middle star of the Great Bear (the least bril- 
 liant of the seven), to the Pole Star, and prolonging it to a nearly equal distance, 
 the star Beta ()3) Cassiopese is reached.
 
 THE CONSTELLATIONS. 321 
 
 second. Neither one nor the other offers anything very remarkable, 
 especially the Giraffe and the Lynx, all the stars in which constella- 
 tions are at most of the fourth magnitude. 
 
 Among all the stars which, on the horizon of London, never set, 
 the brightest is a star of the first magnitude, known under the name 
 of Capella, in the constellation of Auriga. 
 
 About the 20th of December, at midnight, Capella is near the 
 zenith, as represented in Plate XXV. This remarkable star can be 
 found by prolonging the line which joins the two stars of the 
 quadrilateral of the Great Bear, nearest the pole. Auriga, which 
 contains sixty-nine stars visible to the naked eye, comprises, besides 
 Capella, one star of the second magnitude, and three others between 
 the third and fourth. 
 
 In the number of the constellations visible, at least partly, during 
 the whole year, the stars of which, as they surround the pole, have, 
 as we have seen, received the name of Circumpolar stars, must be 
 ranked Perseus, situated near Auriga. It occupies, at the time we 
 have chosen, a western position, relatively to this latter constellation, 
 above Cassiopea. Of eighty-one stars visible to the naked eye, one is 
 of the second magnitude, six are of brightness superior to the fourth. 
 Among these latter is Algol, noted for its variable light, alternatively 
 passing from the second to the fourth magnitudes. We shall speak, 
 further on, at some length on this singular star. 
 
 Before continuing our description of the celestial vault, and of the 
 groups into which the stars have been formed, we will say one word 
 on the aspect of the northern circumpolar zone. 
 
 We have supposed, to describe it, that the time of observation was 
 midnight, on the 20th of December. But it is easy to find, by the 
 help of Plate XXY, the actual appearance and the positions of the 
 various constellations for any hour of the night or any night of the 
 year. We know that the entire rotation of the diurnal movement is 
 effected in twenty-four sidereal hours. In six hours, therefore, a 
 quarter of the total movement is accomplished, and it therefore 
 follows that a constellation — such as Cassiopea, for example — which 
 at midnight is to the left of the pole, was above it at six o'clock in 
 the evening, and will be found below it, near the horizon, at six in 
 the morning. 
 
 Hence, if the Plate is turned round so as to have its top and sides 
 at the bottom by turns, to show on the horizon, as it were, each of its 
 four sides, the successive positions of the stars of the circumpolar 
 zone for the following hours will be by turns represented : —
 
 ;322 THE SIDEREAL SYSTEM. 
 
 / Plate, as it stands, represents the stars as seen at midnight. 
 
 i With its right-hand side at bottom at 6 p.m. 
 
 20th December ^.^j^ .^^ ^^^^ ^^ ^^^^^^ at noon.* 
 
 ( With its left-hand side at bottom at 6 a.m. 
 
 By placing the Plate in intermediate positions, we may represent 
 progressively the rotation of the starry vault at all the hours of the 
 night between those that we have indicated. 
 
 From one day to another this aspect will change, each star occu- 
 pying earlier the position of the preceding nights. The rate of this 
 yearly progression is six hours in three months. Consequently, if 
 we ao-ain take in the same order the four positions before indicated, 
 they will correspond to the following days of the year, and hours of 
 the day : — 
 
 Plate as it stands . . . 
 Right-hand side at bottom 
 Top side at bottom . . . 
 Left-hand side at bottom . 
 
 22iid March. 
 
 •2lst Juuo. 
 
 •22ud Scptoinbcr 
 
 6 p.m. 
 
 Noon 
 
 6 a.m. 
 
 Noon 
 
 6 a.m. 
 
 Midnight. 
 
 6 a.m. 
 
 Midnight . 
 
 6 p.m. 
 
 Midnight . 
 
 6 p.m. 
 
 Noon. 
 
 * At this time, as we all know, the stars are "put out" by the superior light of 
 day. Still the constellations and stars occupy the positions indicated on the 
 Plate. This may be proved by using a telescope of suflicient power, in which the 
 various .stars are rendered visible, down even to the small magnitudes, according 
 to the power of the instrument.
 
 i
 
 THE CONSTELLATIONS. 323 
 
 JIL 
 
 THE CONSTELLATIONS (Continued) 
 
 Our Survey (coutiuued), Equatorial and Southern Constellations visible on 
 
 Horizon of London. 
 
 Let us return to the stars visible at midnight on the 20th of 
 December. 
 
 Plate XXVI represents the southern part of the starry vault, 
 seen as it appears if we turn our back to the circiunpolar stars, a 
 description of which occupied our last chapter. 
 
 This immense zone very nearly embraces half the horizon from 
 east to west, passing by the south, and extending in altitude to 
 the zenith. It comprises the most beautiful constellations and the 
 most brilliant stars in the heavens. It is divided obliquely by the 
 Milky Way. 
 
 Orion occupies nearly the middle view. This magnificent con- 
 stellation forms a quadrilateral, higher than it is broad, in the centre 
 of which three stars of the second magnitude are arranged in a 
 straight line. 
 
 Two of the stars of the quadrilateral, named Betelgeuse and 
 Rigel, are of the first magnitude. Betelgeuse is remarkable for the 
 reddish tint of its light. Among the 115 stars visible to the naked 
 eye, besides the two most brilliant, are included four of the second 
 magnitude, and five between the second and the fourth. 
 
 In prolonging towards the north-west the line formed by the 
 three stars in the belt of Orion — the name given to them — the eye 
 perceives a red star of the first magnitude : this is Aldebaran, the 
 most beautiful star of the constellation of the Bull. Aldebaran is in 
 the midst of a group of small stars named the Hijndes. A little 
 farther, in the same direction, will be found the Pleiades, so easy to
 
 324 
 
 THE SIDEREAL SYSTEM. 
 
 recognise in the heavens by reason of the six stars visible to the 
 naked eye, which compose this interesting group. The Bull contains 
 121 stars visible to the naked eye below the second magnitude. 
 
 If now we prolong towards the south-east of Orion the line which 
 has found for us Aldebaran on the north west, we perceive, near the 
 edge of the Milky Way, the constellation of the Great Dog, which 
 includes Sirius, the most brilliant star in the two hemispheres, re- 
 markable on account of its scintillation and by its dazzKng whiteness. 
 
 Towards the west, and nearly at the same height as Betelgeuse, 
 shines Proojon, on the other side of the Milky Way. This is a star 
 of the first mao-nitude, and the most brilliant one in the constellation 
 
 Fig. 130.— The heavens on the liorizon of London. Equatorial zone. Orion. 
 
 of the Ltttle Dog. Betelgeuse, Sirius, and Procyon form a triangle, 
 the three sides of which are nearly of the same apparent length 
 (fig. 130). This circumstance enables us easily to recognise these stars. 
 
 Above Procyon, and towards the zenith. Castor and Polhix point 
 out the Twins, which include, besides these two stars of the first and 
 second magnitudes, fifty-one stars visible to the naked eye. Towards 
 the west, and by the side of the Pleiades, lies the constellation of ihe 
 Ham, and, a little below, those of the Whale and Eridanus, neither 
 of which, in those parts visible to us in London, contains any stars of 
 the first magnitude. 
 
 But while we are enumerating and contemplating this brilliant 
 portion of the heavens, the stars defile across it, set, and disappear in 
 the west, whilst others rise in the east, revealing new constellations.
 
 THE {'ONSTELLATIOXS. 
 
 325 
 
 Before passing these under review, we raay mention that the 
 southern horizon, represented in Plate XXVI, presents the same 
 aspect at the epochs and hours mentioned below : 
 
 Midnight 20th of December. 
 
 Six o'clock in the evening . 22nd of March. 
 
 Noon 20th of June. 
 
 Six o'clock in the morning . 22nd of September. 
 
 From the 20th of December, the time of the winter solstice, to 
 the 22nd of March — the vernal equinox — by reason of the Earth's 
 journeying along its orbit, the part of the heavens opposed to the 
 Sun, and therefore visible at night, changes progressively. By this 
 
 Fig. 131.— The heavens on the horizon of Loudon. Equatorial zone. The Lion. 
 
 movement from west to east, we gradually see new eastern constel- 
 lations at the same hours of the night. 
 
 Thus, by the 22nd March, at midnight, the aspect of the southern 
 starry vault' has almost entirely changed, and in place of Orion, which 
 is then setting, the Lion occupies the centre. The sky then, looking 
 south, presents the appearance represented in Plate XXVII. The 
 Milky Way is inclined to the west, and cuts the horizon towards 
 the north. 
 
 The principal stars of the Lion form a kind of trapezium, the 
 western side forming a half circle like a sickle. It is at the lower 
 extremity of the handle of this instrument that Regulus, a star of 
 the first magnitude, which is also named the Lion's Heart {Cor Leonia) 
 shines. Denehola is the star situated at the other extremity of the
 
 326 THE SIDEKEAL SYSTEM. 
 
 trapezium. Of the seventy-five stars visible to the naked eye in this 
 constellation, without counting Regulus, there are three of the second 
 magnitude and five of the third. Three stars of the first order shine 
 with Regulus in the heavens visible to us, at the time we have chosen. 
 In the south-west, Procyon is not yet set. , Then, at the same altitude 
 as this star, but to the east of the Lion, is Spica in the Virgin, 
 which will soon ascend the meridian, and, lastly, Arcturus, the most 
 brilliant star in the constellation Bootes. S^jiea, ArcUvrus, and 
 Benehola form the summits of a triangle, the sides of which are 
 nearly equal, and of which the line which joins the two latter stars 
 forms the base, nearly parallel to the horizon (fig. 131). 
 
 The Virgin and Bootes are, with the Lion, the most important 
 constellations in view. The first contains a hundred, and the 
 second eighty-five stars visible to the naked eye, amongst which 
 sixteen exceed in brilliancy stars of the fourth magnitude. Between 
 the Lion and Bootes, a cluster of stars lying very near together is 
 perceived : this is Berenice's Hair. To the east of Arcturus, six 
 stars, arranged in a half circle, the most brilliant of which is named 
 Alpheta, form the Northern Crown, below which are found the 
 Head of the Serpent and Ophiuchus. Lying round Spica, and 
 a little below, towards the horizon, are distinguished the Balance, 
 the Crow, and the Cup. The two first constellations only contain 
 a few stars of the second magnitude. Lastly, in the mist on the 
 horiaon appear a few stars of the Scorpion and the Centaur, con- 
 stellations which we shall again meet in our survey of the celestial 
 zone which surrounds the southern pole. 
 
 To conclude our examination of the constellations visible on the 
 22nd of March, at midnight, we must notice the Hunting Dogs, 
 above Berenice's Hair ; the Little Lion, above the Lion ; the 
 Crab, to the west of Regidus ; and, lastly, close to the horizon and 
 the Milky Way, the Water Snake, where brilliantly shines Cor 
 Hydrce, a variable star of the second magnitude, and the Unicorn, 
 below Procyon. 
 
 The zone which we have just described occupies, on the horizon 
 of London, the same position at the following epochs and hours : — 
 
 March 22nd Midnight. 
 
 June 20th Six o'clock in the evening.* 
 
 September 22nd Noon. 
 
 December 20th Six o'clock in the morning. 
 
 * We must here make, with reference to the 20th of June, the same remark 
 for six o'clock in the evening in summer as we have already done for midday : 
 the brightness of the atmosphere renders the stars invisible.
 
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 THE CONSTF,I,l,ATIONS. 327 
 
 On the 20th of June, at midnight, another part ol' the equatorial 
 zone passes before our eyes. Looking south, the aspect of the sky 
 will be as represented in Plate XXVIII. 
 
 The constellations of the Northern CrowTi and Bootes, the Ser- 
 pent, the Balance, and the Virgin, which, on the 22nd of March, 
 occupied the eastern part of the starry vault, are now at the west. 
 Arcturus is situated vertically above Spica. The Milky Way, now 
 divided into two large branches, rises obliquely from the southern 
 horizon towards the north-east. 
 
 Three stars of the first magnitude shine at unequal heights in 
 three diiFerent constellations. These are, going from west to east, 
 Antares, or the Heart of the Scorpion, which scarcely rises above 
 
 G R E A' T , -B 
 
 P H A C 
 
 L-I T Ti E LION 
 
 HAIR . ^ •._^ 
 
 Fig. 132.— Tlie heavens of the horizon of London. Equatorial zone. 
 
 the horizon near the Milky Way. Afterwards comes Vega, in the 
 Lyre, which is nearly in the zenith, and, lastly, at a mid height, 
 Afair, in the Eagle. 
 
 A few words on the constellations now in view. 
 
 We have first, to the west of the Northern Crown, nearly in the 
 zenith, Hercules, which, in a total number of 155 stars visible 
 to the naked eye, includes only two approaching the second mag- 
 nitude, and ten between the third and fourth. It is towards a 
 point in this constellation, as we shall see anon, that our Sun 
 is actually travelling, carrying with him all his system of planets, 
 satellites, and comets. 
 
 To the east of Hercules is the Lyre, where we have already
 
 328 THE SIDEREAL SYSTEM. 
 
 noticed the brilliant and white Vega, easily recognised by the four 
 stars which form below it a little parallelogram. 
 
 Still going towards the west, to the left of the Lyre, the constel- 
 lation of the Swan is noticed : this traverses the MHky Way, and its 
 most briUiant star, Alpha Cygni, is between the first and second 
 magnitudes. This star forms, with four others of the third mag- 
 nitude, a large cross, which at this hour is inclined to the horizon, 
 and serves to distinguish the constellation to which it belongs. 
 
 Alpha Cygni forms also, with Atair and Yega, a large isosceles 
 triangle, that is, a triangle two sides of which are nearly equal. 
 In the Swan is found a small star which, though scarcely visible to 
 the naked eye, is celebrated in astronomical annals as being the 
 
 Fig. 133.— The heavens of the horizon of London. The Ljtc, Swan, and Eagle. 
 
 first the distance of which has been measured. The Swan contains 
 145 stars perceptible to the unaided sight. 
 
 The Fox, the Aruow, the Dolphin, between the Lyre, the Swan, 
 and the Eagle, contain no remarkable star. 
 
 Near the horizon towards the east are perceived the constella- 
 tions of the Waterbearer and of the Goat ; then, partly in the 
 Milky Way, the Archer. Here we again meet the stars of the 
 Scorpion, amongst which is Antares, which will soon disappear under 
 the horizon, with the four stars with which it forms a sort of fan. 
 
 Above the Scorpion, Ophiuchus and the Serpent are entirely 
 visible. Four stars of the second magnitude, and seventeen between 
 the second and the fourth, are met with in these constellations.
 
 THE CONSTELLATIONS. 329 
 
 And here we finish our survey of the equatorial zone of stars 
 visible at midnight at the summer solstice. This zone presents the 
 same appearance at the four following epochs : — 
 
 June 20th Midnight. 
 
 September 22nd Six o'clock in the evening. 
 
 December 20th Noon. 
 
 March 22nd Six o'clock in the morning. 
 
 It remains for us, in order to finish our description of the stars 
 visible above the horizon of London, to pass in review the constella- 
 tions of the equatorial zone, as they appear at midnight at the 
 autumnal equinox. 
 
 Fig. 134.— The heavens of the horizon of London. Equatorial zone. 
 
 If at that time — the 22nd of September, at midnight,— we turn 
 our eyes towards the south, we embrace in our view all the region of 
 the sky which extends from the west to the east as far as the zenith. 
 Plate XXIX reproduces the aspect of the celestial vault at this hour 
 and time of the year. 
 
 In the west appears Atair, in the Eagle, and higher up the 
 Swan ; at the east, the Pleiades, and the Bull, in which constella- 
 tion shines Aldebaran. Orion, already partly visible, will soon 
 mount above the horizon. We have surveyed, between December 
 and September, three quarters of the sky, which have defiled before 
 our eyes, or rather the whole starry vault, if we include the stars 
 now visible.
 
 330 THE SIDEREAl SYSTEM. 
 
 Towards the middle of the heavens, a little nearer the zenith than 
 the horizon, Kes a large square of four stars, three of which are 
 of the second and one of the third magnitudes. Close to this, and 
 on the eastern side, are three other stars of the second magnitude, 
 about the same distances apart ; these make of the square which we 
 have mentioned a much more extended figure, having a great 
 resemblance to the group of the seven principal stars of the Great 
 Bear. Of these seven stars, three belong to the constellation of 
 Pegastjs, three to Andromeda, and, lastly, the most eastern one is 
 no other than Algol, the variable star in Perseus. 
 
 Andromeda and Pegasus contain between them 101 stars 
 visible to the naked eye, amongst which twelve only exceed the 
 fourth magnitude. 
 
 Between the square of Pegasus and the Bull we meet with two 
 constellations, the Fishes and the Ram : this latter contains only 
 two rather brilliant stars, situated at nearly equal distances from 
 the Pleiades and the two eastern stars of the square of Pegasus. 
 Below the Fishes and the Ram is the "Whai.e, some of the stars of 
 which are below the horizon. 
 
 Among ninety-eight stars visible to the naked eye in this con- 
 stellation sis are between the second and fourth magnitudes, and 
 two of the second. 
 
 Among the first, one is very remarkable on account of the 
 periodical variations of its brightness, which sometimes cause it to 
 appear as a star of the fourth magnitude, and sometimes efface it 
 sufficiently to render it invisible ; this is Mira (the marvellous), or 
 CO Ceti. 
 
 To the west of this constellation, we again find the "Waterbearer 
 and the Goat, then, quite to the south, and touching the horizon, 
 the stars of the Southern Fish, amongst which we may distinguish, 
 if the atmosphere be pure, and terrestrial objects do not intervene, 
 Fomalhant, a beautifid star of the first magnitude. 
 
 The zone, which we have passed under review, offers the same 
 aspect at the following epoghs and hours : 
 
 September 22nd Midnight. 
 
 December 20th Six o'clock in the evening. 
 
 March 22nd Noon. 
 
 June 20th Sii o'clock in the morning. 
 
 Let us add, in terminating this rapid review of the southern part 
 of the sky, visible in London, that the four Plates which have helped 
 us to recognise the different constellations may also be used at other
 
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 THE CONSTELLATIONS. 331 
 
 epochs of tte year, and at other hours of the night. Only, the stars 
 still preserving the same relative positions, will be diversely inclined 
 to the horizon. The more the hour is advanced beyond midnight, 
 then the more will the western stars have disappeared, while more 
 new stars at the east will be seen. This constant change, which 
 results from the diurnal movement of the Earth, will be produced in 
 the same manner, if we pass from one day to the other, or from one 
 month to the following one, so that at the same hour of the night 
 the stars successively visible in the same part of the sky are situate 
 in constellations more and more eastern. 
 
 We have already said that this second apparent movement of the 
 starry vault is due to the translation of the Earth in its orbit. This 
 is effected with great slowness. Thus, for example, the displacement, 
 which requires six hours of diurnal rotation, demands three whole 
 months of annual revolution.
 
 .332 THE SIDEREAL SYSTEM. 
 
 IV. 
 
 THE CONSTELLATIONS (Continued). 
 
 Our Survey of the Starry Heavens (continued) — Southern Circumpolar Zone — 
 Stars invisible at London. 
 
 From tlie Northern hemispliere of the Earth, where we have been 
 placed until now to observe the starry vault, let us transport our- 
 selves to the Southern one. Let us choose a place, the distance of 
 which from the Equator is precisely the same as that of our first post, 
 that is to say, one situated on the parallel which passes through the 
 antipodes of London. Let us suppose ourselves, for example, placed 
 on a part of the coast of Patagonia, near to ihe south point of South 
 America. There, all the stars forming the Northern circumpolar 
 zone, which on the parallel of Paris never set, would be constantly 
 invisible. Looking away from the Equator, that is to say, towards 
 the North, we shall see pass before us, from one end of the year to 
 the other, all the constellations of the equatorial zone which we 
 have just described. 
 
 But the stars will here be found arranged in an entirely inverse 
 order, at least relatively to the horizon ; so that the two stars of the 
 great quadrilateral of Orion, which in London formed the base, 
 appear as the upper side ; Sirius, which appears in the North- 
 ern hemisphere to the left and below Orion, will be here found to 
 the right, and higher on the horizon. This change of aspect is easily 
 explained by the complete change of the observer's position. But 
 if we look southwards, we shall be able to observe a number of 
 stars unknown to the terrestrial zone which extends from the paral- 
 lel of London as far as the northern pole. These are the constel- 
 lations which surround the southern pole of the heavens, and which 
 never set on our new horizon. If, then, we pass in review this zone, 
 we shall have terminated onr description of the whole celestial vault.
 
 The SHIP 
 Ela 
 
 PL.Al. 
 
 DORALUS ERTDAl^US ThePHENIX 
 
 fanopus Madellanic douds Acliernar 
 
 SOUTHERN CROSS ALTAR 
 
 WOLK CENTAUR TRIAU GLE 
 
 PEACOCK SOUTHEHN FISH 
 
 HYDRA mniATJ CRANE 
 
 THE SOUTHERN HEAVENS 
 
 SIARS INVISIBLE ABOVE THE HORIZON OE PARIS SEEN AT MIDNIGHT 
 on the southern coast of Paia^oma Becemher 20 . 
 
 i'.'. GuiUemm stil. CJer^ei del. 
 
 /mp Becmiel a Psris.
 
 THE CONSTELLATIONS. 
 
 333 
 
 Let us choose the 20th of December, the period of the winter 
 solstice,— the commencement of the warm season in the southern 
 hemisphere. It is midnight, and the perfectly pure atmosphere 
 permits us to contemplate the sky in all its splendour. Plate XXX 
 represents the position of the constellations as viewed from our new 
 stand-point, at this hour and time of the year. The Milky Way, 
 ramified into diverse branches, rises slightly inclined on the horizon, 
 on the left, that is, the eastern side. But what at first most strikes 
 us in the celestial picture spread out before us is the multitude of 
 brilliant stars which follow the course of the Milky Way as far as 
 the zenith, and, passing over our heads, go behind us, to rejoin Sirius, 
 Procyon, Aldebaran, nearly on the northern horizon. 
 
 Fig. 135.— Stars iuvisiWe at London. Southeru circmnpolar zone. The Southern Cross 
 
 Let us begin by the constellations which compose this glorious 
 girdle. 
 
 Nearly at the height of the pole, four stars, one of which is of 
 the first, and two of the second magnitude, form an elongated figure 
 lying parallel to the horizon. These are the principal stars of the 
 Southern Cross— the pole-star of the South. 
 
 Below the most brilliant star of the Cross, and between two 
 branches of the Milky Way, two stars of the first magnitude point 
 out the great constellation of the Centaur, in which the eye perceives 
 five stars of the second magnitude. The Centaur extends to the east 
 and north of the Cross, which it nearly entirely surrounds. We 
 shall, in a future chapter, have occasion to speak of the brightest
 
 334 
 
 THE SIDEREAL SYSTEM. 
 
 star of this constellation, which is not only remarkable for its light ; 
 we shall see that it forms a system of two suns, revolving one round 
 ihe other ; and also that it is the nearest to us among the stars the 
 distances of which have been measured. 
 
 Below the Centaur, and near the horizon, appear a great number 
 of stars of the third and fourth magnitudes, which form the constel- 
 lation of the Wolf. One detached branch of the Milky "Way tra- 
 verses the Wolf, and is lost in the Scorpion, of which only a few 
 stars have risen at this hour above the horizon. 
 
 The Altar and the Southern Triangle, which lie along the 
 Milky Way in looking towards the pole, and in which there is 
 
 Fiff. 186.— stars invisible at London. So'ithem circnmpolar zone. Argo. 
 
 nothing remarkable to notice, bring us back, above the Cross, to 
 the magnificent constellation of the Ship, or Argo. Here a mul- 
 titude of bright stars, arranged roxmd the pole, give to this region 
 of the sky an incomparable splendour. Canopiis, looked upon in 
 old times as the most brilliant star in the celestial vault after 
 Sirius, is at this hour close to the zenith and nearly in the meridian ; 
 whilst above Alpha Criicis, Eta Argus astonishes us by its unusual 
 magnificence. This singular star, which, varying in brilliancy at 
 different times for some two hundred years, has at length reached 
 and surpassed the brilliancy of Canopus, and has.banishcd it to the 
 third rank in the scale of the stars of the first magnitude. 
 
 From the Ship we pass, without meeting any remarkable con-
 
 THE CONSTEIJ.ATIONS. 
 
 335 
 
 stcllution, by the Flying Fish, Doradus, and the Reticule, and 
 we arrive at Eridanus, of which we have already noticed the part 
 visible at London. It is at the extremity of this constellation, 
 nearest to the pole, that Achernar, a beautiful star of the first mag- 
 nitude, shines. To the right of Achernar, three stars, one of the 
 second, the two others of the third order, form the Phcenix, below 
 which, returning to the horizon and to the meridian, are found 
 Toucan, the Crane, the Indian, and the Peacock. Two stars of the 
 second magnitude, and some of the third, distinguish these constel- 
 lations, the position of which can be exactly recognised in figs. 
 135 and 137. 
 
 In this enumeration of the circumpolar constellations of the 
 
 Fig. 137. — Stars invisible at London. Southern circumpolar zone. 
 
 South, we have said nothing of the stars situated at the Pole itself. 
 The reason is simple ; there are none deserving mention, and, with 
 the exception of the star /5 Hi/dnp, none approach the third mag- 
 nitude. 
 
 There is not, then, in the southern sky, any star analogous to 
 Pohiris in the northern heavens. But we have already seen that 
 this poverty of the polar regions is singularly compensated by the 
 number and brightness of the stars which entirely surround the zone 
 which we have just described. 
 
 Let us add^ that outside the ^lilky Way, and in the vicinity of 
 the least brilliant parts of the zone, appear two objects which give 
 to the starry vault a ver}^ singular aspect ; they are two whitish
 
 336 THE SIDEREAL SYSTEM. 
 
 clouds of unequal size, which seem, at first sight, detached portions 
 of the Milky Way itself ; these are the Great and Little Clouds, 
 which astronomers still designate under the popular name of Ma- 
 gellanic Clouds. "We shall subsequently describe in detail these 
 curious appearances. 
 
 If we turn to Plate XXXI, so as to place at the bottom each of its 
 sides, the aspect of the sky will be represented, for all points of 
 the terrestrial parallel which have the same latitude as the antipodes 
 of London. It will here suffice, to obtain these different positions, 
 to refer to what we have said about Plate XXVII, always taking care 
 to transpose the words right and left. 
 
 Now, that the appearance of the sky is known to us, we will 
 examine into it a little more closely, and study in detail these 
 thousands of fires, these suns and groups of suns, which the telescope 
 multiplies with such an astonishing profusion. 
 
 (
 
 DISTANCES OF STARS. 
 
 V. 
 
 DISTANCES OF THE STARS. 
 
 Distances of some Stars from the Earth — Time required by Light to reach the 
 nearest Stars — A rough sketch of the Dimensions of the Visible Universe. 
 
 The stars are sirns. 
 
 Each of those luminous points, which the unassisted sight reveals 
 to us by thousands on the vault of heaven, which the telescope shows 
 by millions in the depths of space, shines with its own light. Each 
 star is a focus from which, doubtless, bodies analogous to the planets 
 of our own system, and forming with their central svm a system 
 similar to ours, receive light and heat. The stupendous con- 
 ception, which affirms the whole visible universe to consist of an 
 almost infinite midtitude of suns, is no longer a gratuitous hypo- 
 thesis, or a simple conjecture ; it is one of the most firmly established 
 truths of astronomy. 
 
 The data which science now possesses relating to the immense 
 distances of the stars, of those even nearest to the Sun, place the 
 fundamental fact, that each star is a light- source, and does not 
 shine with radiance borrowed from the Sun, beyond all doubt. 
 
 We will in this place go over the evidence on which this asser- 
 tion depends. We shall by-and-by endeavour to give an idea of 
 the methods which have furnished it. 
 
 As long as we were dealing with our own system, we found it 
 possible to express the various distances by taking the diameter 
 of our own globe as a standard measure — as a unit. Thus we 
 found the mean distance from the Earth to the Sim to be about 
 12,000 Earth-diameters, or 95,000,000 miles ; and the vast dis- 
 tances which separate the Sun from the planets, situated on the 
 confines of the solar system, were expressed in the same manner. 
 
 z
 
 338 THE SIDEREAL SYSTEM. 
 
 But when, breaking the bounds of our solar system, astronomers 
 wished to measure and to express the distances of the stars, even of the 
 nearest among them, they soon found that the former unit, or sound- 
 ing line, vanished into a point, compared with the immensity to 
 which they had to apply it. 
 
 Nay, even the radius of the terrestrial orbit itself, a measuring 
 rod of some 95,000,000 miles in length — a distance which a cannon- 
 ball would require twelve years to traverse — was soon found insuffi- 
 cient, and still remains so for a great many stellar distances ; but 
 the improvements continually effected in the methods of observation, 
 and in the measuring instrimients themselves, have at length enabled 
 some of our most celebrated astronomers to measure approximately 
 the distances of some few stars, and to tell us how many radii of the 
 Earth's orbit they are removed from us. 
 
 The first result obtained was in the case of a star, nearly in- 
 visible to the naked eye, situated in the constellation of the Swan, 
 and marked 61 in the star-maps and catalogues.* The distance 
 of this star, the first in the order of discovery, the second in the 
 scale of magnitude, is nearly three times as great as that of one 
 of the brightest stars of the heavens, Alpha (a) Centauri^ which 
 according to our present knowledge is the nearest to us of all the 
 stars. 
 
 Alpha Centauri is distant from us more than 200,000 times the mean 
 distance of the 8un from the Earth— more than 19,000,000,000,000 
 miles. The most powerful imagination in vain tries to picture this 
 fearful distance ; in vain the mind would heap line upon line, number 
 upon number, to bridge the immensity of this abyss. Let us see if, 
 by some other means, by images, or comparisons, we can — though 
 certainly not with the precision which attaches to numbers — appeal 
 to our senses to comprehend this fact. 
 
 Every one knows with what wonderful rapidity light travels ; the 
 light-waves are propagated at the rate of 192,000 miles a second. 
 Now, a very simple calculation will show that a light-ray, leaving 
 a Centauri, will not reach our eye till the end of three years and 
 seven months. When, on the surface of our Earth, on this grain of 
 sand belonging to the system governed by our Sim, we endeavour 
 to picture to ourselves a long distance — a hundred or a thousand 
 miles, for instance— it is with difficulty we can form an idea of it. 
 We can only represent it well to ourselves by associating with the 
 
 * The fame of this first aud important determination is due to the illustrious 
 astronomer Bessel. Peters, the two Struves, Henderson, Maclear, Schliiter, and 
 Wichmann have also distinguished themselves in these researches.
 
 DISTANCES OF STARS. 339 
 
 sense of sight the perception of time : we ask ourselves, for ex- 
 ample, how many hours or days are necessary to accomplish the 
 distance. What is, then, this distance of 192,000 miles* which 
 light traverses in a second ? This distance is an abyss to onr 
 imagination. 
 
 But, lastly, supposing we coidd grasp, as in a bird's-eye view, 
 this distance, ab-eady so considerable, let us associate it with the 
 short duration of a second ; and then let us imagine that a single 
 day of twenty-four hours contains 86,400 such intervals ; and let us 
 stay to contemj^late the enormous distance to which the luminous 
 ray woidd arrive after a day's journey — it will have plunged into 
 space to a depth seven times greater than the distance of Neptune. 
 Still, according to what we have just stated, it woidd not have 
 accomplished the thousandth part of its route ; it must continue its 
 course for 1300 days with the same tremendous velocity, journey- 
 ino- ever on during three entire vears before it attains the nearest 
 star — that brilliant sun of the southern heavens, a Centauri. Such, 
 in every direction, are the dimensions of the space devoid of stars 
 which surrounds our solar system. 
 
 And, nevertheless, the stars nearest to us only are here in question. 
 From a Lyrae, from the sparkling Sirius, light requires more than 
 twenty years to reach us ; from the Pole Star, half a century 
 is needed. Lastly, to traverse the space which separates Ca- 
 pella from the world on which we live, or, as it may be slated, 
 425,980,000,000,000 miles, 72 years, or a man's whole lifetime, 
 would be required. 
 
 Shall we endeavour to obtain, from another point of view, an 
 idea of these distances ? Suppose a spectator placed at one of the 
 extremities of the line which joins our Sun to a Centauri. At this 
 
 * The velocity of light here given, as we have said before, will possibly require 
 modification. Some remarkable experiments, based on a method both exact and 
 ingenious [the apphcation of Wheatstone's rotating-mirror], have led M. Leon 
 Foacault to a much reduced value. The velocity of the light is, according to 
 him, about 184,000 miles a second ; and his modification entails a corresponding 
 reduction in the number which expresses the distance from the Earth to the Sun. 
 
 Until a complete discussion of this question shall have established the correct- 
 ness of these new values, and until they shall be generally accepted, we have pre- 
 ferred to retain the old ones, though there is little doubt that the new ones are 
 nearer the truth. The inconvenience, if there be any, is reduced by the fact that 
 the relative distances between the various bodies of the solar system remain intact, 
 as also those which give the distances of the stars expressed in radii of the 
 terrestrial orbit. The time which the light from these bodies takes to reach us 
 is also, of course, vinaltered.
 
 ;J40 THE SIDEREAL SYSTEM. 
 
 point, the entire radius of the Earth's orbit would be hidden by a 
 thread of a -'- inch in diameter, held at a distance of 650 feet 
 from the eye'; that is, a line 95,000,000 miles in length, looked at 
 broadside on at this distance, would appear but as an imperceptible 
 
 point. 
 
 We give below a table of the principal distances already deter- 
 mined expressed in radii of the Earth's orbit ; they can be converted 
 into miles by multiplying them by 95,000,000 miles, the length of 
 that orbit. 
 
 We give also the number of years required by light to travel the 
 different distances : 
 
 Radii of Earth's orbit. Years. 
 
 «Centauri 211,.330 
 
 61 Cygni 550,920 
 
 Vega 1,.330,700 
 
 Sirius 1,375,000 
 
 ' Urste Majoris 1,550,800 
 
 Arcturus 1,622,800 
 
 Polaris 3,078,600 
 
 Capella 4,484,000 
 
 3-6 
 9-4 
 21-0 
 22-0 
 25-0 
 26-0 
 50-0 
 72-0 
 
 Other smaller distances are also known, but with less precision ; 
 nearly all are still greater than those here given. None are less 
 than the distance of « Centauri. 
 
 Thus, if we imagine a sphere having for its centre the Sun, and 
 for its radius 200,000 times the mean distance of the Sun 
 from the Earth, none of the innumerable stars which we see shining 
 during our nights will be comprised within it. And, nevertheless, 
 the volimie of this ideal sphere contains 275,000,000,000 times the 
 entire volume of our planetary sphere — the radius of which stretches 
 from the Sun to Neptune. The comets have here full scope to ac- 
 complish their most excentric revolutions, and to describe ellipses 
 bordering on the parabola. 
 
 If we now imagine our Sun plunged in space, to the distance 
 of the nearest star, and calculate, according to the laws of optics, 
 what will be the reduction of its light, we find that it will but put 
 on the brightness of a star of the second magnitude, that it will shine 
 with the brilliancy of the Pole Star, and the principal stars in the 
 constellation of the Great Bear. 
 
 Is it now understood how impossible it is that the stars can shine 
 hj reflected light ? At the distance at which the nearest of them 
 are from the Sun, they receive from the focus of our world a light 
 the intensity of which, as we have just shown, does not exceed that
 
 DISTANCES OF STARS. '341 
 
 of a star of the second magnitude. If each star were a dark body, 
 the light wliich it woiikl receive from our Sun woukl be at most 
 equal to that received by us on our darkest nights, when but 
 a single star pierces through a thick stratum of clouds. And 
 even this feeble glimmer would require to again traverse the im- 
 mense abyss which sej)arates the star from the Earth, before it 
 reached us, twinkling and brilliant as we see it. We might then 
 affirm, on this ground alone, which supplies most incontestable 
 evidence, the astronomical truth, which we announced at the begin- 
 ning of this chapter. [But there is much more convincing proof to 
 which we shall refer anon.] 
 
 The stars then, are suns. Each of them is a focus of light and 
 heat, and probably the centre of a system which comprises, like ours, 
 planets, satellites, and comets. Each star, in fact, may represent a 
 system. 
 
 The distances of some stars being approximately known, is it 
 possible to deduce from them their real dimensions, as has been done 
 in the case of the planets and the Sun ? It is not, and for a simple 
 I'eason : the apparent diameter of the most brilliant stars is so small 
 that it defies all measurement. The finest spider's web, placed at the 
 focus of an optical instrxmient, entrrely hides the disk of these bodies. 
 AVhen, by the movement of the Moon across the constellations, the 
 limb of our satellite reaches a star, the occultation is instantaneous. 
 The extinction of the light, instead of being gradual, is sudden and 
 complete. This fact is not extraordinary, when we consider that the 
 diameter of the Sim, removed to the distance of the nearest star, 
 would not measure a hundredth of a second of arc, — an angular 
 quantity so small that it is entirely inappreciable. 
 
 But if we suppose that the intrinsic intensity of the light be the 
 same, for Sirius, for example, as for the Sun of our system, we shall 
 arrive at pretty clear, if only conjectural, views on the dimensions 
 of this magnificent star. On this hypothesis, the diameter of Sirius 
 would be fifteen times that of our Sun ; so that, even in granting to 
 its light an intrinsic brightness triple that of the Sun, the dimen- 
 sions of Sirivis would still be five times greater, and its volume would 
 be 125 times that of the Sun. 
 
 Doubtless these numbers are below the reality ; doubtless, also, 
 in the multitude of worlds, so distant and so difierent from ours, the 
 most varied dimensions distinguish the central bodies and the spheres 
 in which their direct action is felt. 
 
 So much for our first sketch of the dimensions of the visible 
 universe. We shall return to this interesting subject, when we
 
 342 
 
 THE SIDEREAL SYSTEM. 
 
 describe the structure of this vast ensemble, such as the most recent 
 investigations in sidereal astronomy present it to us. 
 
 We shall also consider not only isolated stars, but systems of 
 suns, and the series of groups forming clusters more and more 
 numerous, and more and more extensive.
 
 MOVEMENTS OF STARS. 343 
 
 YL 
 
 MOVEMENTS OF STARS. 
 
 Stars uot Immovable in Space — Measure of their proper Motions : Velocities of 
 some of them — Translation of the Solar System through Space. 
 
 It was for a long- time believed that the stars preserved invariably 
 their relative positions ; that they and the Sun also were immov- 
 able in space. Hence, the term fixed stars, which has so long been 
 assigned to them, in opposition to the wandering ones, or 2^/anets. 
 Modern astronomical observations, rendered much more precise by 
 the perfection of the instruments now employed, has at length ex- 
 ploded the idea of the immovability of the stars. 
 
 Movement is the common law of all bodies. In our solar 
 system, the planets and their satellites are endowed, as we have 
 seen, both with a movement of rotation round their centres, and 
 with a movement of revolution round their common focvis. As 
 to the Sun, it is now known that he also turns on his axis in about 
 twenty-five days ; and, lastly, comets likewise possess rapid move- 
 ments, which carry them to great distances beyond the limits of 
 the planetary world. 
 
 More than this, the Sun himself moves through space, and draws 
 with him all his numerous train, and yet the distances and relative 
 positions of the difierent stellar bodies undergo no apparent change. 
 Member of a vaster system, and one still unknown, he describes in 
 thousands — in millions — of centuries, perhaps, his immense orbit. 
 
 The same thing holds with all the other suns or stars ; the move- 
 ments of a great number among them have been demonstrated, and 
 already even we possess some knowledge of the direction and velocity 
 of these movements. 
 
 Let us endeavour to show, by the aid of a familiar comparison, 
 how it is possible to assure ourselves of these facts. 
 
 Let us suppose ourselves immovable in the centre of an extensive
 
 344 THE SIDEREAL SYSTEM. 
 
 plain, crossed by roads and railways in various directions, on which 
 pedestrians, carriages, and trains, are travelling with varying ve- 
 locities. If these moving bodies are near us, they appear to move 
 with great relative rapidity. But the more distant they are, the 
 more their apparent velocity will diminish, until, when on the 
 horizon, they appear to move with a slowness, which nearly ap- 
 proaches a state of rest ; at this moment, if we examine them with 
 a telescope, their apparent velocity will again recover somewhat 
 of the rate it had lost, but only to vanish again in proportion as 
 the distance becomes more considerable. 
 
 It is thus with the proper movements of the stars : at first com- 
 pletely imperceptible, they have at length been revealed to astro- 
 nomers furnished with powerful instruments, and provided, moreover, 
 with measuring apparatus of infinite delicacy. It has thus been 
 shown that many stars are displaced with unequal velocities and in 
 different directions. But we must not be mistaken in the magnitude 
 of these movements, or think we can detect them in a single obser- 
 vation ; it requires, indeed, the patient observations of years to 
 establish them. 
 
 Let us quote some examples. 
 
 The brightest star of Bootes, Arcturus, requires a whole century 
 to traverse only the eighth part of the diameter of the Moon, 
 a Centauri, in the same interval of time, is displaced a quantity 
 measured by the fifth of this diameter. Many others move more 
 slowly still. The most rapid movements are those of the star 61 
 Cj^gni, the distance of which has, as we have seen, been measured, 
 and of two stars of the southern heavens, one in the constellation of 
 the Indian, the other in the Ship. 
 
 Nevertheless, these three bodies would each require more than 
 300 years to move across the starry vault a distance equal to the 
 Moon's diameter. 
 
 Of course it is only here a question of apparent velocity. 
 To determine the real velocity, the distances of the stars of which 
 the proper motion is measured must be known ; now, this element 
 is known — at least for some among them. 
 
 It has thus been found that Arcturus moves through space 
 with a velocity not less than 197,000 miles an hour, or 54 miles a 
 second. We give a table of some velocities* which have been 
 determined : — 
 
 *■ These velocities are possibly still greater, since the paths in space may be 
 inclined, whereas their projections are here in question.
 
 MOVEMKNTS OF STARS. .'345 
 
 Miles a second- 
 
 Arcturus 54 
 
 61 Cygui 40 
 
 Capella ;3() 
 
 Sirius 14 
 
 a Centauri 13 
 
 Vega 13 
 
 Polaris 1^ 
 
 Thus these stars, which were believed to be fixed, are in perpetual 
 motion ; nay, the velocity of some of these distant worlds much 
 exceeds that of the planetary bodies, which varies, as we have seen, 
 between three and thirty miles a second. The Earth, which moves 
 in its orbit with such prodigious rapidity, travels three times more 
 slowly than Arcturus, 
 
 How have we arrived at the knowledge of the fact that the 
 solar system itself in its entirety moves through space ? Another 
 familiar comparison will help us to answer this question. Let us 
 place ourselves again in the centre of an extensive plain, bordered 
 at the horizon on every side with rows of trees differently grouped. 
 So long as we ourselves are at rest, these objects keep the same 
 relative positions and distances. But if we move in any one direc- 
 tion, what happens ? As we walk, the trees in front of us open out 
 — are gradually separated ; while behind us, on the contrary, they will 
 gradually get nearer together, will close up ; whilst on either side they 
 will seem to recede in a direction contrary to our movement ; these 
 are, it is clear, merely effects of perspective. But between all these 
 apparent movements in various directions, and the direction of 
 our walk, there is an intimate connexion, the study of which, 
 if we were not conscious of our movement, would enable us to 
 detect it. 
 
 Now the immense expanse of the heavens is our plain, and the 
 trees on the horizon are the stars and the constellations, and the 
 traveller whom we have imagined to walk in a given direction is the 
 Sun and its system. 
 
 There are, however, between our supposition and the reality 
 
 differences which somewhat complicate the problem. The stars, as 
 
 we have just seen, have a real movement of their own, and there are 
 
 other apparent movements, owing to the movement of revolution of 
 
 the Earth, and the combination of this movement with the velocity 
 
 of light. It has, therefore, been necessary to unravel these 
 
 complicated movements, and to sift out the real from the apparent 
 ones.
 
 346 
 
 THE SIDEREAL SYSTEM. 
 
 If to these difficulties we add those which result from the ex- 
 treme delicacy of the measurements required, and of the variation of 
 the measuring instruments themselves, an idea will be formed of the 
 sagacity, patience, and genius which have been necessary to arrive at 
 such magnificent conclusions.* 
 
 Towards what portion of the sky, then, are we travelling ? Accord- 
 in o- to the most recent calculations, the Sun is advancing towards a 
 point situated in the constellation Hercules f with such velocity 
 that in a year it traverses more than once and a half the radius 
 of the terrestrial orbit, or 153,000,000 miles — about 4 miles a 
 second ! 
 
 The movement of the Sun takes place, possibly, round a centre 
 still unknown to us. The present opinion of astronomers is in favour 
 
 Pig. lob. — Point (if the heavens towards which the solar system is travelling iu its 
 
 of I'C volution. 
 
 of the Pleiades being the centre of this movement, but precise know- 
 ledge on this point is difficult to arrive at. 
 
 If the stars move unequally and in different directions, — if the Sun 
 progress towards a certain point in the heavens, how will this even- 
 tually show itself in the aspect of the starry vault ? By a continual 
 change, which will ultimately give to the constellations groupings 
 vastly differing from those under which they are at present seen. 
 " The Southern Cross," says Humboldt, " will not always keep its 
 characteristic form, for its four stars travel in different directions and 
 
 * The astronomers who have attempted, discussed, and solved this beautiful 
 problem, are, among others, Sir W. Herschel, Argelander, 0. Stnive, Madler, and 
 Peters. 
 
 t On the straight line which joins the two stars ■« and f^ of this constehation, 
 at a quarter of the separating distance from the first.
 
 MOVEMENTS OF STARS. 347 
 
 with unequal velocities. At the present time it is not known how 
 many myriads of years must elapse until its entire dislocation." We 
 may, then, rest quiet, and study the sky as it is, without fearing pre- 
 sent confusion : let us leave to our descendants of the year 9000 to 
 determine the position which the star of the Hunting Dogs, known as 
 No. 1830 Groombridge, will then occupy. It may possibly be 
 found in Berenice's Hair!
 
 348 THE SIDEREAL SYSTEM. 
 
 VII. 
 
 DOUBLE AND MULTIPLE STARS. 
 
 Distinction between Optical and Physical Doubles — Characteristics of the latter 
 — Movements of Revolution of Double Stars — Multiple Systems. 
 
 There is, in the vicinity of Vega, the brightest star in the constella- 
 tion of the Lyre, a small star which appears elongated to some 
 possessed of very keen eyesight, and this appearance suggests that it 
 may really be composed of two luminous points ; indeed, it is only 
 necessary to examine it with an opera-glass to see that it really con- 
 sists of two stars, separated by an interval equal to about the ninth 
 part of the apparent diameter of the Moon.* 
 
 Here, then, we have an example of a coarse and easily divided 
 double star, which a keen eye or an opera-glass of small magnifying 
 power is sufl&cient to separate into its components. But this is not 
 all ; if we employ an instrument of considerable optical jjower to 
 examine each of the two stars of which the coarse double is 
 composed, we find • that each component itself consists of two stars 
 so near together that the intervals separating them are not more 
 than the -^^th part of the total distance of the couples themselves,! 
 so that we have here a douhle-douhle-s,iA\\ A star which appears 
 single to the naked eye becomes quadruple when examined with a 
 powerful telescope. 
 
 A century ago, only about twenty double stars were known ; now, 
 however, we possess catalogues of more than 6000.;^ Now is the 
 
 * 3' 27". The star is Epsilon (s) Lyrse. 
 
 t Struve. 
 
 + Kirch, Bradley, Flamsteed, Tobie and Christian ]\Iayer, Sir "^V. Herschel, 
 in the last century ; the two Struves, Bessel, Argelander, Encke and Gall, Preuss 
 and Madler, and Sir John Herschel, in the first half of the present one, have 
 assisted in the discovery of these pairs, now so numerous and so interesting.
 
 DOUBLE AXl) MULTIPLE STARS. 
 
 349 
 
 union of two suns in a small space of the starry vault to be looked upon 
 as purely accidental, or must we rather consider it to indicate a real 
 physical connexion of the two bodies — a real system? 
 
 On the first supposition, the proximity of the two stars to each 
 other would be attributed to an eflfect of perspective ; the stars them- 
 selves, though widely differing in their distance from us, lying in the 
 same line of sight. In the second case, the two suns are at nearly 
 equal distances, and their apparent connexion proceeds from the 
 relative smallness of the interval which separates them. 
 
 Hence a sifting of double stars into optical and physical pairs. 
 As soon as the number of double stars began to increase, it was 
 thought extremely probable that groupings of this kind might not 
 all be owing to the effects of perspective ; and the existence of real 
 systems of suns was suggested, before even observation had directly 
 confirmed it ; this suggestion has since been abundantly justified. 
 
 Out of a total number of 6000 double stars known at the present 
 
 Fig. 139.— The trapezium of Orion. (Sir J. HerscheL) 
 
 time, 650 have been demonstrated to be physically connected sys- 
 tems — two suns, turning round a common centre of gravity. There 
 are still more complicated groups — systems of three, or four, or even 
 more suns. In the constellation of Orion, near the centre of the 
 glorious nebula which we shall soon describe, there is a system where 
 the unaided sight only distinguishes a luminous point. With the 
 help of a powerful telescope, however, this point is divided into four 
 stars ; these can be seen in a small telescope, in the form of a 
 trapezium ; but when a telescope of 5 or 6-inches aperture is used, 
 two of the stars in the trapezium are themselves seen to be accom- 
 panied by two other very small stars, forming altogether a group 
 of six suns (fig. 139). "Probably," says Humboldt, "the sextuple 
 star, 6 Orionis (generally called the "trapezium of Orion"), 
 constitutes a real system, for the five smaller stars have the same 
 proper motion as the principal one." We may add, that Mr.
 
 350 THE SIDEREAL SYSTEM. 
 
 Lassell has discovered a seventh star in this remarkable system, so 
 that 6 Orionis is a septuple star. An attentive study of this group, 
 on which the attention of astronomers is fixed, as it forms such 
 an admirable test object for their instruments, will eventually 
 show us what truth there is in Humboldt's statement ; the various 
 stars will be seen to progress in their orbits, and science will be 
 enriched with a new fact well worthy the attention of geometers : 
 — the reciprocal and simultaneous movement of seven suns. 
 
 What magnificence, what variety is there in the constitution of 
 the sidereal universe ! Our solar system places before us the grand 
 spectacle of a central star, surrounded with more than a hundred 
 planetary bodies and thousands of comets, harmoniously executing 
 their eternal evolutions round the focus of their heat, and light, and 
 life. 
 
 In the unfathomable space which surrounds our system, have been 
 revealed to us, at prodigious distances, millions of stars, which are so 
 many sims, surrounded doubtless, for the most part, with a cortege of 
 planets like our own. And more than this, among these myriads of 
 systems, we become acquainted with some which present to us the 
 more marvellous association still of suns grouped by twos, and 
 threes, and fours, moving round each other in the same manner as 
 with us the planets move round their common centre. 
 
 Not only is the division of double stars into optical and physical 
 couples not arbitrary — founded as it is on precise observations — but 
 it has furnished valuable data for the solution of several most im- 
 portant problems in stellar astronomy. A word on this subject. 
 We can at once recognise that the two components of a double 
 star form a real or physical system, when the movement of revo- 
 lution of one round the other is observed. Thus, the satellite of 
 Castor,* and those of the stars, jj Cassiopea), ]) 8erpentai'ii, ^ Ursa) 
 Majoris, have completed an entire revolution since the epoch (1780) 
 of the first observations. 
 
 The physical couples are again distinguished by another cha- 
 racter, — a common proper motion ; that is to say, when this is in 
 the same direction and extent, it is extremely probable that we 
 are dealing with a veritable system, although their movement of 
 revolution is so slow that we cannot detect it. As to the optically 
 
 * Castor is a binary system to which, according to Struve, doubtless belongs 
 a third star, which participates in the proper movement of the two others. 
 Here then are two suns accompanied with a third sun fifteen times more distant 
 from the first than is the second.
 
 DOUHLK AM) !\iri,TIPI,E STARS. 351 
 
 double stars, they are distinguished by the opposite characteristics ; 
 in other words, no movement of revolution can be detected in them, 
 and the proper motion of one is not participated in by the other. 
 Such is the case with the optical couples formed by the companions 
 of Vega, Atair, Pollux, and Aldebaran. 
 
 If the double stars of the first kind — the physically connected 
 ones — have increased man's knowledge of the constitution of the Uni- 
 verse, by showing the identity of the laws which govern the stellar 
 worlds with those of the movements of the planets, the optical double 
 stars have furnished, as we shall see in a future chapter, the means 
 of measuring distances, and thus of sounding the depths of the 
 heavens. 
 
 We will now j)roceed to give some details of the principal 
 double stars, the movements of which have been observed and the 
 orbits calculated. 
 
 There exists, in the constellation of the Great Bear, very near 
 that of the Lion, a star designated in the catalogues by the Greek 
 letter ^, known as a double star since 1782. The two compo- 
 nents of this system are, one of the fourth, the other of the iifth 
 magnitude. The movement of revolution of the second romid the 
 first * having been detected, a French astronomer, Savary, deter- 
 mined by calcidation the elements of the orbit. The period of 
 revolution is sixty-one years, whence it follo^vs that, since the dis- 
 covery of the system, the orbit has been entirely traversed, and that 
 one-third of the second period is already completed. 
 
 The elliptical or oval form of the orbit of this binary is very 
 decided ; its excenti"icity is comparable to the orbits of our periodical 
 comets, since, even among the telescopic planets, there is no orbit 
 which differs so much from a circle. But among the double stars 
 there are some, the orbits of which are still more elongated. Such 
 is that of a Centauri, the period of revolution of which exceeds 
 seventy-eight years. 
 
 We may also cite the following periods of double stars which 
 have been determined : — 
 
 ? Herculis 36 years. 
 
 Z Cancri 59 
 
 fi Coronse Borealis .... 66 
 
 2i Ophiuchi 92 
 
 y Virginis 150 
 
 61 Cygni 452 
 
 * Or, rather, the movement of each star round the comraon centre of gravity 
 of the system.
 
 352 THE SIDEREAL SYSTEM. 
 
 There is, as is seen by this table, great variety in the periods, 
 the latter surjjassing the first by twelve times. But it is probable 
 that some still more divergent will be found. In Berenice's Hair, 
 and in the Lion, there are two pairs, the first of which has a period 
 of less than fourteen years, whilst the second completes its orbital 
 movement in twelve centuries.* 
 
 If we have been able to determine the form of the paths described 
 by these pairs of suns, and the duration of their periodical movements, 
 we are still — to sj)eak generally — far from knowing the absolute di- 
 mensions of the orbits ; to determine these we must, of course, know 
 the distances of the stars from us. We know this, however, in the 
 case of a Centauri and 61 Cygni. 
 
 The mean distance from each other of the two stars which com- 
 pose the second of these systems, is not less than 1,019,000,000 miles. 
 Compared to the distances of the planets from the Sun, this distance 
 is comprised between those of Saturn and Uranus. The orbit of 
 the comj^anion of 61 Cygni has a mean radius of about forty- 
 five times the distance of the Sun from the Earth, or more than 
 4,275,000,000 miles. Let us bear in mind, that such dimensions 
 are quite lost to the unaided sight ; so immense is the distance of 
 these stars, that a powerful telescope only can divide them. 
 
 That astronomy has arrived at such a point of perfection, as to 
 be able to calculate the elements of such distant systems, is indeed 
 an admirable result, and a proof of the power of calculation, when 
 supported by observations worthy of confidence. But this is not 
 all ; it is now demonstrated, that the laws which regulate the stellar 
 sj^stems are identical with those which govern the bodies of our 
 own system ; we have thence been able to form an approximate esti- 
 mate of the masses of these bodies. Thus, it has been found, that 
 61 Cj'gni — that small star scarcely visible to the naked eye — weighs 
 more than a third of our Sun. 
 
 Quite recently, the exactitude of these theoretical deductions 
 has received a brilliant confirmation. Every one knows Sirius, 
 the brightest star of the heavens. While studying with minute 
 care the proper movement of this magnificent sun, the illustrious 
 Bessel — one of the greatest astronomers and geometers of the 
 century — suspected the existence of a satellite, the mass of which, 
 acting on the central star, produced variations in its movement. 
 Was this satellite a dark body analogous to our planets, or a second- 
 ary sun, +he light of which is lost in the dazzling rays of Sirius ? 
 
 * Both these periods are however uncertain.
 
 DOUBLE AM) MULTIPLE STARS. 353 
 
 On this point nothing was known ; other astronomers attempted the 
 same pi»oblem, and one of them, M. Peters, calcuhited for the un7 
 known orbit a period of fifty years. Such was the state of things 
 when an American optician, Mr. Alvan Clark, on the 31st of 
 January, 1862, in turning a new and powerful telescope on Sirius, 
 discovered the satellite, the cause of the observed perturbations. 
 Since that time, it has been again seen by other astronomers : * 
 and it now remains to verify by observation the orbit and period 
 calculated before its discovery. 
 
 When a branch of science, scarcely known two centuries ago, 
 and cultivated steadily less than a hundred years, arrives at such 
 results, what may we not hope for the future progress of sidereal 
 astronomy ? 
 
 Doubtless, many points will long remain in the domain of con- 
 jecture. But, without overstepjDing probabilities, it will gradually 
 be more and more possible to form a correct idea, both of the unity 
 of the laws which govern the celestial bodies, and of the infinite 
 variety of the phenomena which they ofier to man's observation. 
 
 We may thus liken the innumerable suns scattered over the 
 heavens to the central body of our own system. Doubtless, round 
 each revolve other bodies, some like our planets, others, perhaps, 
 gaseous, like our comets. The phenomena of day and night, and of 
 the seasons, again occur in those secondary worlds, rendered invisi- 
 ble by their immense distance. By carrying ourselves back to the 
 phenomena of our planetary system, we can conceive those eternally 
 going on in the worlds of which we speak. 
 
 But how much more varied still must be the phenomena in those 
 systems composed of two or three, or even more suns, w4th their 
 varying lights and heats, sometimes combined, and sometimes ex- 
 perienced in succession. Let us imagine ourselves, for example, on 
 one of the planets of the triple sun 4/ Cassiopeas ; the movements 
 of rotation and revolution of such a planet, combined with the 
 movements of revolution of the three light-giving bodies, would 
 bring on its horizon sometimes one, sometimes the other of the 
 suns of the system, and sometimes, also, two or three at a time. 
 To periods of day and night would succeed periods of continuous 
 day ; and the temperature and seasons would vary also by reason 
 of ever new conditions. To these we must add the varieties of 
 colour which characterize the lights of the component stars of the 
 
 * The Rev. W. R. Dawes in England, MM. Chacornac and Goldschmidt at 
 Paris. Mr. Lassell at Malta, Father Secchi at Rome. 
 
 A A
 
 354 THE SIDEREAL SYSTEM. 
 
 system,— varieties which would produce on the planet sometimes red 
 days, sometimes green, or blue ones, or even days iUuminated by a 
 light compounded of these three colours, in varying proportions ; an 
 idea will thus be formed of the odd effects of light, and singidar con- 
 trasts which objects must present according to the hour of the day 
 and the time of the year. 
 
 That brings us naturally to say a few w ords on the colour of the 
 stars in the simple or multiple systems.
 
 COI.OLRKl) STAIIS. 855 
 
 VIII. 
 
 COLOURED STARS. 
 
 Variety of Colours presented by the Stars — Colours of Single Stars — Colours of 
 Double and Multiple Stars — Variations observed in Colour; presumed 
 Causes of their Changes. 
 
 The rapid variations of briglituess, which a star presents to the 
 naked eye, are ordinarily accompanied with instantaneous changes of 
 colour ; and to these two phenomena combined has been given the 
 name of "scintillation." It is, however, known that these changes do 
 not take place in the star, but are caused by our atmosphere, through 
 which the luminous waves reach our eye. 
 
 But, independently of these apparent and ever-changing tints, 
 the stars possess real and constant colours, arising from real differ- 
 ences in the nature of the light which they emit. 
 
 This we can all see for ourselves. If we observe some of the 
 most brilliant stars in the heavens, it will be remarked, that the light 
 of Sirius, of Yega, of Regulus, and of Spica, are perfectly white, 
 whilst Betelgeuse, the brightest star in Orion, and Aldebaran show 
 a decided red tint. 
 
 The Greek astronomers, as remarked by Arago, only recognised 
 red and white stars. Now, however, that this branch of observation 
 is carefully cidtivated, all colours, all the tints of the rainbow, have 
 been detected, in the light of different stars.* 
 
 * Observations of this nature are very deUcate : and although the use of tele- 
 scopes renders them more certain, as the star is deprived of nearly all its scintil- 
 lation, they are still subject to errors, proceeding both from the personality of 
 the observer and the peculiarities of his instrument. We are surprised tliat 
 there has not yet been instituted a precise mode of observation, in employing, 
 for example, a chromatic scale, the degrees of which would serve for terms of 
 comparison with the coloured lights of the stai's.' 
 
 ' Since the publication of the first edition of this work, we Lave received from Admiral Smj'th a 
 memoir by that celebrated observer, who proposes a chromatic scale precisely of the kind referred to. 
 The title of this memoir, published in Loudon in 18(34, is "Sidereal Chromatics, or the Colours of 
 Multiple Stars."'
 
 356 THE SIDEREAL SYSTEM. 
 
 Among the single stars of reddish tint, we may quote Arcturus, 
 Antares, and a star in the Whale, the famous Mira Ceti, which we 
 shall again soon meet with among the variable stars. Procyon, Capella, 
 and Polaris are yellow. The light of Castor is green, and that of 
 a Lyra) is of a decided blue tint. Nevertheless, white is undoubtedly 
 the colour of the great majority of stars. 
 
 These diverse and permanent tints can only be attributed to real 
 differences in the nature of the light emitted by each Sun. If the 
 hypothesis of a photosphere, or incandescent gaseous envelope, now 
 admitted by many of our astronomers in the case of the Sun, be ex- 
 tended to the physical constitution of the stars, it suffices to suppose a 
 different chemical composition in the photospheres of these bodies, 
 or different absorbing processes going on in their atmospheres, to 
 explain the differences of colour. Doubtless, also, the degree of the 
 temperature of the incandescent media will go for something in 
 influencing the phenomena. 
 
 It is in the double and multiple stars, that the colour of the 
 light is presented with all its brightness and richness. The greatest 
 variety distinguishes the colours of the components of these systems, 
 already so remarkable from so many other points of view. 
 
 The illustrious and laborious astronomer of Dorpat and Poulkowa, 
 M. W. Struve, who has consecrated thirteen years of watching to 
 the examination of 120,000 stars, amongst which he has found more 
 than 3000 double stars, thus writes on this subject : — 
 
 " The attentive observation of the bright double stars teaches us, 
 that, besides those which are white, all the colours of the spectrum are 
 to be met with ; also, when the principal star is not white, its light 
 borders on the red side of the siaectrum, whilst that of its satellite 
 offers the bluish tint of the opposite end. Nevertheless, this law is not 
 without exception ; on the contrary, the most general case is that 
 the two stars have the same colours ; I find, indeed, among 596 bright 
 double stars : — 
 
 375, the two componcuts of which have the same colour and the 
 
 same intensity : 
 101 of the same colour, with a different intensity ; 
 120 of totally different colours. 
 
 Among the stars of the same colour, the most numerous are the 
 white, and of the 476 stars of this kind, I have found 
 
 295 in which the two components are white ; 
 118 „ they are yellow or reddish ; 
 63 „ they are bluish."
 
 PL XTI 
 
 COLOURED STARS 
 
 1 Group surrounding kappa of the Cross. - 2. Kappa in Pegasus. - 3. 61 of the Swan. 
 _ 4 Delta in the Serpent. - 5. Gamma in Andromeda. - 6. Eta in Cassiopea. - 7. A double 
 star in the Ship - 8. 32 of the River Eridanus. - 9. Sigma in Cassiopea. - 10. Beta in 
 the swan. - 11. Gamma in the Lion, - 12. Alpha in Hercules. - 13. Eta in Perseus.

 
 COLOURED STARS. 357 
 
 It was at first believed that the blue colour was a simple efiect of 
 contrast, owing to the feebleness of the light of the smaller star, 
 compared to the yellow and more brilliant light of the principal one. 
 But if this optical illusion be sometimes met with, observation shows 
 that it is accidental, and that blue stars do really exist. Indeed 
 Struve has as often met with a blue satellite to a white star, as to 
 one of a decided yellow. Besides, couples are mentioned of which 
 the components are both. blue. Such are the double stars, d Ser- 
 pentis, and 59 Andromedae. Lastly, there is in the Sovithern heavens 
 a group composed of a multitude of stars, which are all blue. 
 
 All possible shades, we have before said, are met with in the 
 coloured double stars. White is found mixed with light or dark 
 red, purple, ruby, and vermilion. Here we have a green star with 
 a deep blood-red companion, there an orange primary accompanied 
 by a purple or indigo blue satellite. The triple star, y Andromedae, 
 is formed of an orange-red sun, accompanied with two others, the 
 light of which is of an emerald green colour. [If indeed, one be 
 not blue, and the other yellow, the green residting from the close 
 juxtaposition of the two.] Two stars, the distances and period of 
 whose revolutions we have already cited, 61 Cygni and a Centauri, 
 have each for their components two orange-yellow sims. Plate XXXI 
 will give an idea of these associations of colours, which will, perhaps, 
 furnish us later with some facts bearing on the constitution of the 
 sidereal world. We have given, according to Sir John Herschel, an 
 extremely remarkable group, situated in the Southern Cross, near the 
 star Kappa. It is composed of 110 stars, of which seven only exceed 
 the tenth magnitude. Among the principal ones, two are red and 
 ruddy, one is of a greenish blue, two are green, and three others are 
 of a pale green. " The stars which compose it, seen in a telescope of 
 diameter large enough to enable the colours to be distinguished, 
 have the effect," says Herschel, "of a casket of variously coloured 
 precious stones."* 
 
 We have already remarked, that it is necessary to distinguish 
 between the real and constant colours of the stars, and the instan- 
 taneous and oft-renewed variations due to scintillation. Nevertheless, 
 the constancy of the real colour is not absolute. 
 
 It seems at length to be an undoubted fact, that certain stars do 
 change colour. Sirius is the first example of this. The ancients re- 
 presented it as a red star, while, at present, this sun is distinguished 
 by its brilliant whiteness. 
 
 * " Astronomical Observations at the Cape of Good Hope," p. 17.
 
 358 THE SIDEREAL SYSTEM. 
 
 Two double stars, one of the Lion, tlie other of the Dolphin, 
 noted as white by Herschel, are now composed of primaries of 
 golden-yellow, accompanied by a reddish-green star in the first pair, 
 and a bluish-green one in the second.* 
 
 But after all, this variation of colour will seem less astonishing 
 when we see how much the brightness of the light of the stars itself 
 is subjected to variations. 
 
 The cause of the colours of the stars, and of the changes of tint 
 they undergo, is still, as we have before remarked, not entirely 
 accounted for. " It must be left to time and careful observation," 
 says Arago, " to teach us if the green or blue stars are not suns, in 
 process of decay, if the different tints of these bodies do not in- 
 dicate that combustion is operating upon them at dijBPerent degrees." 
 
 All that can be at present said with certainty, is, that the celes- 
 tial spaces, far from presenting to us immutability and immobility, 
 are the theatre of incessant movement and continuous transformation. 
 The study of variable stars, and of new or temporary stars, which 
 have suddenly appeared to disappear as suddenly, will again furnish 
 us with decisive proof of a truth that has taken us so long to 
 learn. 
 
 * This variation does not seem to be explained by the difference of the 
 instruments used, since the mirrors of Herschel's telescope gave rather a reddish 
 tint to all objects ; and it was Struve, who first established their colour with the 
 large Poulkowa refractor. 

 
 VARIABLE STAKS. 359 
 
 IX. 
 
 VARIABLE STARS. 
 
 Periodical Changes of Brilliancy of Mira Ceti, and Algol in Persens — Other Va- 
 riable Stars — Explanation of these Changes — Hypothesis of the Rotation 
 of Stars. 
 
 There is in the constellation of the Whale a star marked on the 
 maps by the Greek letter o (Omicron), which astronomers know also 
 under the Latin name of Mira (the marvellous). This star has 
 been long remarked on account of the periodical variations of its 
 brightness. During each interval of eleven months it passes through 
 the following phases. 
 
 During fifteen days it attains and preserves its maximum bright- 
 ness, which is equal to that of a star of the second magnitude. Its 
 light afterwards decreases during three months, until it becomes 
 completely invisible, not to the naked eye only, but even to our 
 telescopes.* 
 
 It remains in this state during five whole months ; after which 
 it reappears, its light increasing in a continuous manner during 
 three other months. Its cycle of variability is then ended, and it 
 attains again its maximmn brightness to pass a second time through 
 the same phases. These singular variations have been known since 
 the end of the sixteenth century ; but the exact measure of the 
 period was only effected a century later. At the present time it is 
 known with great precision, and is valued at 331 days, 15 hours, and 
 7 minutes. 
 
 In truth, irregularities have been discovered in the period of 
 Mira ; but these irregidarities also are subjected to a periodicity 
 
 * We are surprised that observations of this singular star have not been pur- 
 sued with the most powerful instruments during the period of invisibility. Jt is 
 only known that it is then below the eleventh magnitude.
 
 3G0 
 
 THE SIDEREAL SYSTEM. 
 
 which renders the phenomenon still more interesting. The greatest 
 brilliancy does not always rank it in the same magnitude. Some- 
 times it scarcely exceeds the fourth, whilst at certain epochs (in 
 1799, for example,*) its light was almost as brilliant as that of a 
 star of the first magnitude, and it was scarcely inferior to Al- 
 debaran. 
 
 Mira is not the only example of the periodical change of bright- 
 ness of stellar light ; and the duration of the variations is not always 
 
 so long as in it. Algol, in the head 
 of Medusa, in the constellation of 
 Perseus (fig. 140), is at least as in- 
 teresting as Mira, but its period is 
 much shorter, and it is never in- 
 visible, even to the naked eye. A 
 star of the second magnitude during 
 two days and thirteen and a half 
 hours, it suddenly decreases, and 
 in three hours and a half descends 
 to the fourth magnitude. Then its 
 brightness regains the ascendant, 
 and at the end of a fresh interval 
 of three hours and a half, attains 
 its maximum. All these changes 
 are effected in less than three days, 
 or, more exactly, in 2 days, 21 hours, 49 minutes. 
 
 Among the variable stars with long periods, Betelgeuse, one of 
 the four stars of the great trapezium of Orion, may also be men- 
 tioned, the period of this star is nearly 200 days. There is a star 
 in the Swan, the variations of which are effected in 406 days. 
 Three of the seven stars of the Great Bear vary in periods imper- 
 fectly known, but they certainly embrace several years. 
 
 In the number of variable stars with short periods, d Cephei 
 is distinguished by the regularity of its changes of brightness, which 
 last 5 days 8 hours 40 seconds. This star has been observed 
 since 1784. 
 
 Lastlj', there is a great number of stars, the variability of which 
 IS proved without their periods having yet been determined, either 
 because these periods are irregular, or because the time they occupy 
 is very considerable. 
 
 The preceding examples will sufiice to give an idea of the 
 
 Fig. 140. — Variable star Algol in Perseus. 
 
 * The 6th of November, as cited in Humboldt's " Cosmos."
 
 VARIABLE STARS. 361 
 
 interest attached to these singular phenomena, the cause of which, 
 although suspected, is still unknown. The periodicity even of the 
 changes observed indicates that the variations of brightness are 
 possibly produced by a movement of rotation of the variable itself, 
 or by the movement of revolution of a dark or opaque body round 
 the luminous body. 
 
 On the hypothesis of the rotation of the variable stars, it has 
 been held that the different sides of the body vary in luminosity, and 
 even, in certain cases, are completely dark.* Spots of large dimen- 
 sions, analogous to solar spots, and encroaching on a part of the 
 surface of these suns during long intervals of time, have been sug- 
 gested to account for the phenomena. 
 
 On the other hand, if each star be considered the focus of the 
 movements of dark bodies similar to our planets, an hypothesis 
 which is far from being completely improbable, it must happen to 
 a certain number of them, that the planes of the orbits of these 
 secondary bodies, if prolonged, would pass through our system. 
 In this case, at each revolution, there would be an eclipse to our eyes 
 of the central body, a partial or total eclipse, according to the dimen- 
 sions and the respective distances of the dark satellite angl its sun. 
 Many satellites of unequal periods would then explain the different 
 phases of variability. 
 
 Another explanation of the variability of certain stars has 
 been suggested by the fact, that, during the minimum of bright- 
 ness, some of these bodies have appeared surrounded with a kind of 
 mist. This is, that the variability is owing to the interposition of 
 nebulous masses travelling through space, and which, not being self- 
 luminous would veil, or even quite extinguish, the stars in question. 
 
 [The question of variable stars, one of the most puzzling in the 
 whole domain of astronomy, has recently engaged the attention 
 of Mr. Balfour Stewart, who has done so much good work on the 
 Sun, — which by the way, is doubtless a variable star. He remarks, 
 "We are entitled to conclude, that, in our owti system, the approach 
 of a planet to the Sun is favourable to luminosity, and especially 
 in that portion of the Sun which is next the planet. Let us take 
 variable stars. The hypothesis which, without being physically 
 probable, gives yet the best formal explanation of the phenomenon 
 
 * The idea of Maupertuis, that among the suns there are, doubtless, some of 
 which the forms differ from a sphere, and which are presented to us, by reason 
 of a movement of rotation, sometimes in section, sometimes in plan, scarcely 
 seems in accordance with the principles of mechanics, which account for the 
 figures of celestial bodies.
 
 362 THE SIDEREAL SYSTEM. 
 
 there represented, is that which assumes rotation on an axis, while 
 it is supposed that the body of the star is not equally luminous on 
 every part of its surface. Now, if instead of this, we suppose such 
 a star to have a large planet revolving round it at a small distance, 
 then, according to our hypothesis, that portion of the star which is 
 near the planet, will be more luminous than that which is more 
 remote; and this state of things will revolve round as the planet 
 itself revolves, presenting to a distant spectator an appearance of 
 variation, with a period equal to that of the planet. Let us now 
 suppose the planet to have a very elliptical orbit ; then for a long 
 period of time it will be at a distance from its primary, while, for 
 a comparatively short period, it will be very near. We should, 
 therefore, expect a long period of darkness, and a comparatively short 
 one of intense light, precisely what we have in temporary stars." So 
 that, according to Mr. Stewart, the phenomena of variable stars 
 depend upon the formal law, that the approach of the heavenly 
 bodies produces light in the same way as the approach of atoms 
 produces light ; an offshoot of the fact, that heat and light are modes 
 of motion.] 
 
 Among the variable stars, [as if in proof of Mr. Stewart's hypo- 
 thesis,] binary couples are noticed. Such is 7 Virginis, of which we 
 have had occasion to cite the movement of revolution. The two 
 stars which compose it have changed in brightness, and the most 
 brilliant has become inferior to the other, at the end of some years. 
 The variable star, a Cassiopea?, is also a double star; according 
 to Struve there are many others. "That which is especially of 
 great importance," remarks that eminent astronomer, " is, that it 
 can be demonstrated from this variability of double stars, that they 
 move round an axis of rotation, and that, in consequence, we have 
 found a fresh analogy between these systems of many suns and our 
 planetary system." On the h3T)othesis of dark satellites, it will be 
 seen that, if the analogy seized upon is different, it is not less 
 curious. 
 
 According to Mr. Hind, the colour of a great number of variable 
 stars is red ; but that is not an essential characteristic ; if Mira is 
 of a red colour, the light of Algol is white.* 
 
 * [We are enabled, by the kindness of Messrs. Knott and Baxendell, to add 
 here a woodcut (fig. 140a), which will give the reader an idea of the method 
 adopted by our observers in this class of observations. 
 
 By successively contracting the aperture of the telescope, stars of all mag- 
 nitudes can be made to disappear. Finding by experiment with what aperture 
 stars of known magnitude became just extinguished, the aperture at which a
 
 VARIABLE STARS. 
 
 36;^ 
 
 As we advance in the study of the stellar world, the apparent 
 uniformity of the heavens, in which the indifferent spectator at first 
 only sees a multitude of luminous points always the same, always 
 immovable, gives place to a most rich and varied picture. The 
 number of phenomena of which we are the witnesses is only equalled 
 by the moulds of time and space in which they are cast. 
 
 We have seen, in the solar system, the most wonderful order 
 governing the combination of the movements of the bodies which 
 compose it, and the simplicity of the means by which the most 
 astonishing differences are everywhere produced. In the sidereal 
 world, the same harmony governs the suns even, the changes of 
 which, as we have seen, are subjected to laws and regulated periods. 
 
 It must not be thought, however, that it is necessarily thus with 
 all celestial phenomena, and that regularity is the characteristic 
 sign of the movements or transformations of stars. We are about 
 to describe some phenomena which bear, for the most part, the ap- 
 pearance of sudden catastrophes, or which, when thev occurred 
 gradually, were rapid enough for observers to register all their 
 phases. These sudden changes will doubtless strike the imao-ina- 
 tion ; but our reason will none the more look upon them as pro- 
 digies, habituated as it is to see everything subjected to laws : 
 Omnia reguniur numero, pondere et mensurd / 
 
 VIII. hours. 
 
 star so disappears becomes an index of its magnitude. Preparatory, there- 
 fore, to commencing operations on a variable star, the observer furnishes 
 himself with a chart of the surrounding 
 stars, with a selected list of conveniently 
 situated comparison stars, whose magni- 
 tude is carefully measured in the manner 
 we have indicated. The " comparison stars " 
 are lettered for convenience of reference. 
 The obseiver then compares the variable 
 with those stars on the list which differ 
 least from it in brightness, and carefully 
 estimates the difference in tenths of a 
 magnitude. He thus obtains several inde- 
 pendent values for the magnitude of the 
 variable at the date of observation. The 
 mean of these is adopted for the night- 
 The successive observations are laid down 
 on cross-ruled paper, the dates of obser- 
 vation forming the abscissae, and the mean 
 magnitudes the co-ordinates. Through the 
 points thus obtained a curve is laid down ; 
 this is the light-cicrve, and from it the dates of maximum and minimum 
 brightness are determined.] 
 
 o 2 
 
 i m. 
 
 28 111. 
 
 30 ni 
 
 ,_( 
 
 
 + 
 
 
 ^B 
 
 ■ 
 
 b 
 
 
 B| 
 
 ■ 
 
 + 
 b 
 
 
 1 
 
 I 
 
 Fiff- 140a. — Variable Star Oiai-t 
 U Cancri. (Kuott.)
 
 364 'J'HE SIDEREAL SYSTEM. 
 
 X. 
 
 TEMPORARY STARS. 
 
 New Stars — Temporary Star of 1572 — Lost Stars — Explanation of these Sudden 
 Appearances and Disappearances. 
 
 " One night," writes Tj^cho Brabe, " as I was examining as usual 
 the celestial vault, the aspect of which is so familiar to me, I saw, 
 with unspeakable astonishment, near the zenith, in Cassiopea, a 
 star of extraordinary brightness. Struck with surprise, I could 
 scarcely believe my eyes. To convince myself that there was no 
 illusion, and to obtain the testimony of other persons, I called the 
 workmen occupied in my laboratory, and I asked them, as well as 
 all the passers-by, if they saw, as I did, the star which had so 
 suddenly made its appearance. I learnt later, that in Germany 
 the coach-drivers and others of the people had acquainted the astro- 
 nomers of a strange appearance in the sky, and thereby furnished 
 occasion for a renewal of the accustomed railing against scientific 
 men." 
 
 It was in the course of November, 1572, that this strange appari- 
 tion took place. 
 
 The new star observed by Tycho had none of the appearances 
 of a comet ; no nebulous head, no tail accompanied it ; it, moreover, 
 remained completely immovable in the same point of the heavens 
 during the seventeen months that it was visible. It twinkled in an 
 extraordinary manner, and at first its brightness exceeded that of 
 Vega, Sirius, and even Jupiter at its smallest distance from the 
 Earth. " It could only be compared," says Tycho, " to that of Yenus 
 in quadrature." It also remained \^sible in the day, at noon, when 
 the sky was clear. But, by degrees, its light diminished in intensity. 
 
 In January 1573, it was already less brilliant than Jupiter;
 
 TEMPORARY STARS. 
 
 365 
 
 from the month of April it passed from the first to the second 
 magnitude ; after this it rapidly decreased, and disappeared at last in 
 March 1574. 
 
 Not only was this extraordinary star variable in brightness, but 
 even its colour was subjected to rapid changes ; first, white during 
 the first two months, the period of its greatest brilliancy ; afterwards 
 it passed to yellow, then to red. Tycho then compared it to Mars, 
 to Betelgeuse, and especially to Aldebaran. Lastly, in the spring 
 of 1573, the red colour reappeared, and remained until the end of 
 its visibility. 
 
 Several similar appearances have been noticed in more remote 
 times in various regions of the sky ; two of them are especially 
 
 Fig. 141. — New and temporary star of 1572, in Cassiopea. 
 
 interesting. They were observed in 945 and in 1264, between 
 Cepheus and Cassiopea, nearly in the same position as that taken up 
 by the Pilgrim, the name given to the star of 1572. If this identity 
 were actually established, temporary stars would then be shown 
 to be no other than periodical variable stars, a conclusion at which 
 we have before hinted ; and the only difierence between them would 
 arise from the inequality of the cycle of variability, and of the 
 intensity of the variations. 
 
 [From a careful reduction of the places recorded by Tycho Brahe, 
 Argelander has arrived at the folloA\ing figures, as giving its position 
 for 1865 :— 
 
 19 57-7 
 
 Decl. 
 
 o / " 
 
 + 63 23 55-4 
 
 taking up the theory that this temporary star is refilly a long- 
 period variable, he has been inquiring whether any suspicious star
 
 366: THE SIDEREAL SYSTEM. 
 
 exists in or near the above place. He finds that D' Arrest has 
 observed a star of the 10| magnitude, in the following position :— 
 
 R.A. Decl. 
 
 h 111 s o , 
 
 4 19 30 .... +63 22-y 
 
 Many years ago, observers sought in vain for a star in this position, 
 and, it may be, that D'Arrest's star may be the great temporary star 
 of 1572 slowly recovering its light. Nor are the intervals between 
 the dates we have mentioned widely diiferent ; the mean of them 
 gives 1885 as the epoch of its next maximum.] 
 
 Since the observation of Tycho Brahe, many temporary stars have 
 been seen in the constellations of Serpentarius and Cygnus.* 
 
 But the most brilliant of all of these — that of 1604, which was, 
 however, inferior to that of 1572 — was especially remarkable by its 
 vivid scintillation ; it disappeared like the first, leaving no trace 
 behind. 
 
 Among these stars some have been recorded which, after having 
 varied in brightness, have remained visible, preserving permanently 
 their last phase of brightness. 
 
 Lastly, some stars, the first appearance of which was not observed, 
 have disappeared. Hence, the names temporary stars, new stars, and 
 lost stars, given to these three kinds of stars respectively. 
 
 To what causes must these truly extraordinary phenomena be 
 ascribed ? If wc suppose these stars to be variable, it is still difficidt 
 to explain these quick changes of brightness, these nearly sudden 
 appearances of bodies, which at once attain their greatest brightness. 
 
 It has been attempted to account for them, by supposing them to 
 be endowed with very rapid movements ; but, of all the hypotheses, 
 this is evidently the most imjDrobable. Arago, examining this ques- 
 tion, f shows, that to pass from the first to the second magnitude by 
 a simple change of distance, a star would require six years, in the 
 case of a star travelling with the velocity of light, or 192,000 miles 
 an hour. Now, the star of 1572 underwent this change in a month, 
 and we must suppose for it a velocity 72 times greater, that is to say, 
 a velocity 200,000 times greater than that of any known star. 
 
 On the other hand, if these phenomena are explained by some 
 
 * Most of the new or temporary stars have made their appearance either in 
 or near the Milky Way. Tycho hence conchidcd that these bodies were formed 
 of the matter of which this great nebula was then thought to consist ; but 
 this opinion, at present, is inadmissible, since it is now known that the Milky 
 Way is entirely composed of distinct stars. 
 
 t " Annuaire du Bureau des Longitudes, 1842," p. 327.
 
 TEMPORARY STARS. 367 
 
 stupendous process of combustiou — some sudden conflagrations taking 
 place on the surfaces of bodies until then obscure, by progressive 
 extinction inducing first a decrease of brightness, and afterwards dis- 
 appearance — such catastrophes are well adapted to strike our imagin- 
 ation, and to destroy the ancient idea of the immutability of the 
 heavens. 
 
 Perhaps electric and magnetic powers play some part in the 
 production of these gigantic coups de theatre. Humboldt seems in- 
 clined towards this idea. He protests against the hypothesis of 
 destruction, of the actual combustion of the stars which have dis- 
 appeared. "That which we see no more," he says, "has not 
 necessarily ceased to exist The eternal play of apparent cre- 
 ation and apparent destruction does not prove an annihilation of 
 matter ; it is a pure transition towards new forms, determined by 
 the action of new forces. Some stars which have become obscure, 
 may again suddenly become luminous, by the renewal of the same 
 conditions which, in the first instance, developed their light." 
 
 It is, perhaps, yet more difficult to imagine these variations due 
 to movements of rotation. The various faces must, indeed, be sup- 
 posed to be of a prodigiously unequal brightness ; and even in that 
 case, the sudden appearances could scarcely be accounted for, at- 
 taining, as they do, at once, the maximum intensity. The changes 
 of colour would be likewise inexplicable on this hypothesis. 
 
 Lastly, some astronomers attribute these appearances and dis- 
 appearances to the movement of nebulous masses, not self-luminous ; 
 a kind of cosmical cloud, interposed between the star and ovir system, 
 might produce an eclipse, and this eclipse might cease when the 
 ' clouds ' had entirely passed. Thus, 
 lost stars, as well as new and tempo- 
 rary stars, would be at once ex- 
 plained. 
 
 It is difficult to say which is 
 the most probable of these hypo- 
 theses. The truth is, that, although 
 the phenomena which have sug- 
 gested them are facts, — authentic 
 facts, — we are yet quite at a loss to 
 assign a cause for them. 
 
 We will bring this chapter to 
 a close with a description of the v\^. 112— vniiabic star, Eta Argus. 
 
 most astonishing of all the pheno- 
 mena of this kind, — namely, the variations of the star »j Argus ;
 
 368 
 
 THE SIDEREAL SYSTEM. 
 
 a singular star, which can be classed neither among the temporary 
 nor among the variable stars. 
 
 Towards the end of the seventeenth century, this star was only 
 of the fourth magnitude ; less than a century after, in 1751, it 
 attained the second. Sixty years later, it again descended to its 
 first brightness, increasing anew until the year 1826. From that 
 epoch, it has passed through the most astonishing phases, oscillating 
 between the first and second magnitudes, sometimes equal to 
 a Crucis, then to a Centauri ; surpassing Canopus, and approaching 
 lastly to Sirius. The raj)idity of these changes, their unequal 
 periods, the long duration of this state of variability, the im- 
 possibility of finding a law more or less regular, all contribute to 
 make this beautiful star one of the most curious objects of the sky. 
 
 Let us think for a moment on the actual phenomena, which give 
 rise to such metaniorijhoses. Let us reflect on the vicissitudes 
 necessarily undergone by the planets which move round such a 
 strange sun, arising from the variations in the intensity of its light 
 and heat, and on the stupendous changes which are the necessary 
 conseqiience. Perhaps our Sun has been, or will be one day, the 
 scene of like variations, which are only, after all, the manifestations 
 of the eternally active forces which govern all systems.* 
 
 * A contemporary astronomer, Mr. F. Abbott, who has followed the variations 
 of n Argfts until now, informs us that after having, in 1843, attained the bril- 
 liancy of Sirius, it diminished progressively, passing through all the orders of 
 intermediate magnitudes between the first and sixth. In 18G3 it was no 
 longer visible to the naked eye.
 
 STAK-GROU]*S. 369 
 
 XL 
 
 STAR-GROUPS. 
 
 Natuial Groupings of Stars — Groups Visible to the Naked Eye — The Pleiades — 
 The Hyades — Prsesepe — The Group in Perseus. 
 
 Are the stars that are visible to the naked eye spread orderless 
 on the celestial vault ? or is there not between those, apparently most 
 closely connected, some real or physical connexion which requires 
 us to rank them in natural groups ? 
 
 These questions have been already partly solved, by what is 
 known of the double and multiple star-systems. Soon, exploring the 
 regions of the sky visible by means of the telescope, we shall have 
 to pass in review a multitude of stellar associations, in which suns 
 are found so compact and so numerous, and the form of the groups 
 so regular, that it is impossible to deny their reciprocal dependence. 
 
 But long before the discovery of these islands, these archipelagos 
 of worlds, scattered with such astonishing profusion over the infinite, 
 the naked eye had already distinguished a certain number of groups, 
 the stars composing which were so near together that it was im- 
 possible to doubt their physical connexion. 
 
 Such, for example, is the group of the Pleiades. Such, again, 
 are the groups known under the names of the Hyades, of Proosepe, 
 and of Berenice's Hair. All are visible to the naked eye, and good 
 eyes distinguish without difficulty the principal stars of the first- 
 named groups. The Pleiades (fig. 143) are situated in the con- 
 stellation of the Bull, which we can distinguish so easily, to the 
 north-west of Orion and Aldebaran. 
 
 Of about eighty stars, which form the group of the Pleiades, six are 
 visible without the help of telescopes. Formerly, the Latin poet tells 
 us, seven were counted, which may be held to prove that one of them 
 is variable, and has diminished in brightness, or else has disappeared. 
 
 B B
 
 370 
 
 THE SIDEREAL SYSTEM. 
 
 [But the power of different eyes in distinguishing stars in a group 
 of this kind, varies extremely, and Ovid's remark, — 
 
 " Qua3 septem dici, sex tamen esse solent," 
 
 i 
 
 Fi!<. l-):i.— Tho Piui;vlcs. (Ilaraiiig.) 
 
 although it still ordinarily applies, must not be insisted on too 
 strongly. One member of the family of the Astronomer Royal 
 
 i'lg. 144. — Tlitj i'leiiiaes as seen witli tile liakeU eye. 
 
 habitually sees seven stars, and on rarer occasions twelve — those 
 shown in the accompanying diagram (fig. 144).]
 
 STAR-GROUPS. 
 
 871 
 
 The most brilliant, Alcyone, is of the third niagnitude ; Electru 
 and Atlas are of the fourth ; Merope, Mai'a, and Taygete, of the fifth. 
 Three others again have received particular names, although they are 
 below the limit of ordinary vision ; these are Pleione, Celeno, and 
 Asterojie, from the sixth to the eighth magnitude. All the others 
 are only visible by the aid of a telescope ; but with an ordinary glass 
 it is possible to distinguish a large number. The Pleiades* are 
 known under the name of the Hen-coop, doubtless because Alcyone 
 appears in the group as a hen surrounded with her chickens. 
 
 The Hyades, which are near the Pleiades, form a less numerous 
 and more scattered group. The bright light of Aldebaran, which is, 
 
 
 m 
 
 m 
 
 rm 
 
 T^^^^^^^^J 
 
 
 1 
 
 1 
 
 ^'J^jl_ 
 
 fete •>''"'. 1 
 
 Y^^^^S'^^yf.^"' 
 
 '''.'■!''! jT- 
 
 ^|i^ 
 
 
 
 Il^^^5r^ 
 
 s 
 
 
 1 
 
 
 I^^^^^K;)" " 
 
 ^' ■■■■'■■' 
 
 
 m 
 
 BB 
 
 
 fes.v; 
 
 'V^5 
 
 ;pj 
 
 
 Fig. 14a.— The llyades. (Harding.) 
 
 as is known, of the first magnitude, renders them more difficult to 
 distinguish with the naked eye. 
 
 They appear in the rainy season. Hence their name of Hyades, 
 from the Greek word which signifies " to rain." 
 
 The connexion of the stars which compose this grouj) is not so 
 striking as in the case of the Pleiades. Nevertheless, it seems dif- 
 ficult to admit that they are quite independent of each other's 
 
 * The ancient poets also called them Hespevides or Atlantides. The name 
 of Pleiades is stated to have been derived from -rXilv, which signifies " to navigate :" 
 because, according to Lalande, in the spring and near the epoch when they rise 
 with the Sun, the season for navigating the Mediterranean commences. Others 
 ■say that these stars were dreaded by mariners, on account of the rains and 
 storms which seemed to rise with them, which they attribute to their influ- 
 ence.
 
 372 
 
 THE SIDEREAL SYSTEM. 
 
 attraction. In examining- the position of these two groups in the 
 vicinity of the Milky Way, and observing that both are situated 
 in the prolongation of a branch of the great zone, we are almost 
 entitled to consider them as two clusters of stars, belonging to the 
 Immense stellar stratum which surrounds us, and in the midst of 
 which it will be seen that the Sun himself is placed. 
 
 In Berenice's Hair, most of the stars are visible to the naked eye, 
 and are perfectly distinguished in the sky, a little to the east of the 
 Lion. No very brilliant star in the Aacinity inconveniences the eye 
 by effacing their light. 
 
 The next group is situated in the Crab, and is known vmder the 
 
 Fifjc. 14G — rrpescpe in Cancer. 
 
 uanie of Prcesepe ; it is visible to the unassisted sight ; but it is 
 impossible to distinguish the separate stars without the help of a 
 telescope. Nevertheless, an instrument of moderate power easily 
 separates them, and they then take the aspect represented in fig. 146. 
 
 The groups which we have just described form a transition be- 
 tween the stars scattered over the celestial vault, and the more con- 
 densed clusters, the undefined aspect of which caused them formerly 
 to be designated under the general name of Nebuhe. [This desig- 
 nation, however, is now much more limited, as we shall see by- 
 aud-bye.] 
 
 Doubtless, if we could place ourselves in space, and contemplate 
 from a sufficiently distant stand-point, the whole of the stars which 
 appear to us isolated, Ave should see them condensed into one or
 
 s'i'AR-fjnoui's. 373 
 
 several distinct groups, analogous to those of the Pleiades ; whilst, 
 were wc to penetrate into the midst of one of these compact clusters, 
 we should see the stars of which it is formed, separated, and scattered 
 over the celestial vault in such a way as to give it the aspect of our 
 own heavens.
 
 374 THE SIDEREAL SYSTEM. 
 
 XII. 
 
 STAR -CLUSTERS. 
 
 Clusters of Stars of Globular or Spherical Form — Enormous Number of Stars in 
 certain Clusters — Clusters in Perseus, Centaurus, Toucan, Aquarius, &c. 
 — Curious Forms of some Clusters. 
 
 Among star-clusters, a very small number, as we have before 
 remarked, are bright enough or considerable enough to be visible 
 to the naked eye. In all of these, the stars are so close together, 
 that it is impossible not to recognise in them real stellar groups, — 
 real companionship, — real systems of Suns. 
 
 [Of this class, the cluster in Perseus is at once a striking example, 
 and one of the most glorious objects in the heavens. Let the render 
 search for its faint glimmer in the Milk)' Way, between the bright 
 stars in Cassiopea and Perseus, and turn even a small hand telescope 
 upon it, the sight will well repay him ; but if a six-inch or a larger 
 aperture can be used, he will never forget the glorious picture 
 unfolded before him.] 
 
 Their generally rounded form gives them a cometary aspect, and 
 observers, not completely familiar with the divers regions of the sky, 
 may easily be, and indeed sometimes are, mistaken in their nature ; 
 although the permanence of their form, andespecially of their position, 
 is a characteristic whicli sliould suffice to distinguish them from 
 comets. 
 
 There are also some clusters, although these are not numerous, 
 the contours of which are very irregidar ; in these, the number 
 of the stars is generally much smaller than in the clusters of globular 
 form, and their distribution is also verj'^ different. If we look at the 
 figures (1, 2, 3,) in Plate XXXII, we shall be struck with the re- 
 markable condensation of the luminous points at the centre. This 
 condensation is easily explained, if we suppose that the real form of 
 the cluster is nearly that of a spherical globe. Then, even on the
 
 PLATE XXXTI. 
 
 STAR -CLUSTERS. 
 
 1. lu the Balance. 2. In Hercules. 3. In Capricorn. 4. In Aquarius. 5. In the Serpent. 
 6. In GeiTiini. (Sir J. Herschel.)
 
 
 Ii
 
 STAK-CLUSTEKS. 
 
 377 
 
 ]i\^othesis that the stars are equally distributed, it will be under- 
 .stood, that, as the visual ray traverses its centre througliout all the 
 extent of its diameter, and as in approaching the borders, it traverses 
 smaller and smaller portions, the laws of perspective will account 
 for the apparent collection of the luminous points at the centre. 
 
 But the increase of brightness from the border to the centre is 
 often more rapid than the hypothesis of an equal distribution of the 
 stars in the interior will sanction. It has been held, therefore, that, 
 besides the apparent or purely optical condensation, there exists a 
 real condensation, which is produced, doubtless, by the influence 
 of the central forces, resulting from the separate attractions of each 
 of the Sims which compose these systems. 
 
 •' ■ ■.'.•': ■"• . 
 
 .v;v-; 
 
 ;Sv--S';=W"-^ 
 
 ' ■ ' ."'•'-■■• ' ] 
 
 • ■ •• ' : ;'-::r'''.;i^;^ 
 
 
 
 
 
 W^^ 
 
 ^^^Vv 
 
 II 
 
 
 
 w^^^ 
 
 -1 
 
 ivfj.'jV/.;.";'; 
 
 
 • ■. ' V • • ." ; •'• V ■'-.'iy'-'' 
 
 ;.;.;/!-\v:^^\\-.-'^- ■:■:• • 
 
 ■ ■■' •''•:.> 
 
 
 
 
 
 
 
 
 Fig. 147.— Star cluster near si Centauri. (Sir J. Herschel J 
 
 "How can these isolated systems," says Humboldt,* "be main- 
 tained ? How can the suns, which crowd at the interior of these 
 systems, accomplish their revolutions freely and without clashing ? " 
 
 These questions, which apply to all clusters, are the most difficult 
 of all the problems of celestial mechanics. But it must not be 
 forgotten that these stellar aggregations are situated at great 
 distances, and that their particles, so to speak, which seem to us so 
 near one another, have between them intervals perhaps as consider- 
 able as the distance of our Sun from the nearest star. Their move- 
 ments are, therefore, doubtless effected with all freedom, through 
 spaces as vast as the general equilibrium necessitates, and with a 
 relative slowness proportionate to the dimensions of their orbits. 
 
 * " Cosmo.s.'' vol. iii. p. 153.
 
 378 
 
 THE SIDKREAL SYSTEM. 
 
 The nimiber of stars contained in clusters of a globular form is 
 often prodigious. 
 
 We have seen that the cluster of the Southern Cross (Plate 
 XXXI), so curious on account of the varied colours of its components, 
 onl}^ contains 110 stars, but Herschel has calculated that many 
 clustei'S contain 5000 collected in a space, the apparent dimensions 
 of which are scarcely the tenth part of the surface of the lunar disk. 
 
 ►Sufh is the cluster situated between the two stars >j and ^ 
 Ilerciilis (Plate XXXII, 2), one of the most magnificent in our 
 northern heavens on fine nights ; this cluster is visible to the naked 
 eye as a luminous sj^ot of rounded form ; in the telescope, it is re- 
 
 Fi},'. US. ^Cluster in A(|U.irius. (Lord Rossc.) 
 
 solved into a multitude of stars, and preserves its globular appearance, 
 but is fringed on the borders with several threads of outlying stars. 
 
 Ihe cluster near u Centauri (fig. 147) is also visible to the naked 
 eye, and shines as a star of the fourth or fifth magnitude ; it is re- 
 solved, by very powerful instruments, into a multitude of stars 
 greatly condensed towards the centre, the light of which varies 
 between the thirteenth and fifteenth magnitudes. 
 
 The beautiful cluster in Aquarius, which Sir J. Herschel's 
 drawing exhibits as fine luminous dust (Plate XXXII, 4), when 
 examined through the Earl of Rosse's powerful reflector appeared 
 (fig. 148) like a magnificent globular cluster, entirely separated into stars.
 
 STAR-CI.rSTERS. 
 
 379 
 
 But the most beautiful specimen of this kind is, without doubt, 
 the splendid cluster in Toucan, quite visible to the naked eye in the 
 vicinity of the smaller Magellanic Cloud, in a region of the southern 
 sky entirely void of stars. The condensation at the centre of this 
 cluster is extremely decided ; there are three perfectly distinct gra- 
 dations, and the orange red colour of the central agglomeration 
 contrasts wonderfully with the white light of the concentric envel- 
 opes. 
 
 The clusters of spherical form are ordinarily the richest, and the 
 telescope has the least difficulty in analyzing them into stars. 
 
 Fij?- 140.— Cluster in T. 
 
 (Sir J. Hcrschul ) 
 
 Nevertheless, among the others there are some, the resolution of 
 which, until lately impossible, has been accomplished by the use of 
 telescopes of the greatest optical power. Such is the oval nebula in 
 Andromeda, of which more anon. 
 
 Here are some curiously formed clusters (fig. loO), in which 
 every indication of concentration has disappeared. The drawing 
 which represents the cluster in the Twins (Plate XXXII, 6) shows 
 it to be intermediate between the irregular groups, and the clusters 
 of decidedly spherical form which Ave have passed under review. At 
 the summit of a kind of pyramid — the form of this singular cluster —
 
 380 
 
 THE 81DEREAL SYSTIOI. 
 
 the luminous points seem to press towards a preponderant mass. In 
 the clusters in figure 150, nothing sinrilar is seen. 
 
 The clusters are not equally distributed over the heavens ; they 
 are most numerous in the Milky Way, and in the two Magellanic 
 Clouds. The region richest in globidar clusters is situated in the 
 southern hemisphere, and forms an important portion of the Milky 
 Way, comprised between the constellations of the Wolf, Altar, Scor- 
 pion, Southern Cross, and Sagittarius. 
 
 In describing the most beautiful of the star-clusters, that of 
 Toucan, we remarked that the central part is rose-colour, siuTounded 
 
 Fig 150. — Clusters of .-<iiigiilar forms (Sir J. Ilcnschel.) 
 1. Cluster ill Scorpio. 2. Cluster iu the AlUr. 
 
 with a white concentric border. The cluster being entirely resolved 
 into stars, this coloration evidently belongs to each of the com- 
 ponents ; a fact which will not surprise us, after that which has been 
 seen of the simple and double coloured stars. 
 
 The cluster in the Southern Cross (Plate XXXI), which, as we 
 have seen, is formed of a great number of white stars, interspersed 
 with some red, green, and blue stars, appears as a white cluster. On 
 the other hand, we have quoted a cluster of the southern skj^ entirely 
 composed of blue stars. 
 
 The colour of these star-clusters is then easily explained by tlic 
 predominant colour of the stars of which they are composed.
 
 IHK .MILKY WAV. 381 
 
 XIII. 
 
 THE MILKY WAY 
 
 fTeueiul Aspect of the Milky Way — Its Coui^c through the Northcm and 
 Southern Constelhitions — llesolvability into Stars and Star-Ckisters — Im- 
 penetrability of certain regions of the ^lilky Way. 
 
 With the exception of the Magellanic Clouds, of which more anon, 
 and a few star-clusters, all the star-groupings which we have yet 
 reviewed are invisible to the naked eye. Their extremely small 
 apparent dimensions contribute to this result, bearing in mind the 
 prodigious distances at which they lie from the solar world, — dis- 
 tances which so considerably weaken the brightness of the component 
 stars. 
 
 It is not thus with the Milky Way. The light of this immense 
 zone is, one might almost say, bright ; its extent, which embraces 
 the entire circumference of the starry vault, and its breadth, are so 
 considerable, that it is readily distinguished at the first coup cVceil, 
 whenever the apparent movement of the heavens brings it above the 
 horizon. 
 
 This last circumstance occurs, it is true, every night of the year 
 and in all latitudes; but the Milky Way is much better visible when 
 it rises to a great height, and to see it best we must, therefore, choose 
 certain epochs of the year or certain hours of the night. 
 
 The general appearance of the Milky Way is that of a long 
 nebulous train, which follows very nearly the circumference of a 
 grand circle of the celestial vault. First of all, it may be remarked, 
 that it is divided into two principal branches throughout nearly half 
 its entire length. Its breadth is very variable ; sometimes it contracts 
 so as to occupy no more than six to eight times the Imiar diameter, 
 at others it spreads out to an extent four times as great. 
 
 Before stating what is known of the composition and structure
 
 382 THE SIDEREAL SYSTEM. 
 
 of this immense congregation of stars, let us describe it as a whole, 
 noting the principal constellations which it traverses in both hemi- 
 spheres. We will avail ourselves for this purpose of tfhe two 
 l^lates XXXIII and XXXIV, which show it as it is seen in a small 
 telescope, with the variations in form and brilliancy which its differ- 
 ent ramifications present. 
 
 The northern half of the Milky Way extends from the constel- 
 lations of the Eagle and the 8erjDent to the Unicorn, at the altitude 
 of, and near, the belt of Orion. Divided into two branches from the 
 Equator as far as the Swan, it passes by Atair, and traverses the 
 Arrow and the Fox, besides the coTistellations before named. Near 
 the Swan a dark opening is observed in it, a kind of gap through 
 which the sight plunges into the distant regions of the sky beyond 
 the regions occupied by this zone. One branch is directed towards 
 the Little Bear and Cepheus, and it is in this part that it approaches 
 nearest to the northern pole of the heavens. It afterwards bears 
 awa}'^ under the form of a single and narrow branch, which traverses 
 Cassiopea, passes by the Waggoner very near Capella, borders 
 the eastern portion of the Twins, and of the Little Dog, and 
 the southern portion of Orion. Before arriving at this point, a 
 branch leaves the main portion in Perseus, and stretches as far as 
 the Pleiades, where it is lost. 
 
 The northern portion of the Milky Way presents the greatest 
 intensity in the Eagle and in the Swan ; in Perseus and near the 
 Unicorn, it is the least luminous. 
 
 Let us now follow its course through the southern hemisphere. 
 After having crossed the Equator and passed Sirius, it enters the 
 Ship, gradually increasing in brightness ; it is then divided into 
 several branches which extend fan-like over a large area, and dis- 
 appear all at once, reappearing further on in the same constellation. 
 
 These branches are again united in the Centaur and Southern 
 Cross, at a point where the breadth of the Milky Way is at its 
 minimum. Here is the famous Coal- Sack, a dark gaj) in the form 
 of a pear ; surrounded on every side by the nebulous zone, the eye 
 can only perceive in it one or two stars. 
 
 Very near a Centauri, the Galaxy is divided anew into two 
 principal branches, with numerous ramifications, and the bifurcation 
 continues throvigh the Wolf, the Altar, the Scorpion, and Sagit- 
 tarius, as far as the Serpent. Then the two branches again cross 
 tlie Eqviator and rejoin the northern part of the Milky Way, at the 
 point whore our description began. 
 
 In this immense course, which embraces, as we have said, a com-
 
 c c
 
 THE MILKY WAY. 387 
 
 l)letc great circle of the celestial vault, the glimmer of the star- 
 cloud is extremely variable. We have seen that the brightest 
 part of the northern Milky Way is that which traverses the Eagle 
 and the Swan. In the southern hemisphere, the part comprised 
 between the Ship and the Altar is still more remarkable. But, 
 as Himiboldt has observed, there is a circumstance which still 
 more increases the magnificence of the Milky Way in the southern 
 hemisphere ; this is the vicinity of a long zone of very brilliant stars, 
 which we have already remarked in reviewing the constellations, — a 
 zone which begins at Sirius in Canis Major, traverses the Ship 
 and the beautiful stars of the Cross, the Centaur and the Scorpion. 
 According to an English observer. Captain Jacob, the rising of this 
 portion of the heavens is heralded by a general illumination of 
 the sky, so decided that he compares it to the light of the new moon. 
 
 When the Milky Way is examined by the help of telescopes, it 
 is resolved into a multitude of stars very near together, but very irre- 
 gularly arranged. Star-clusters of irregular form are especially very 
 numerous ; but globular clusters are only found in the brightest 
 portion of the southern zone. "If some regions," says Humboldt, 
 "present large spaces where the light is uniformly spread, there 
 are others where spaces, shining with very bright light alter- 
 nating with others poor in stars, cover the sky with an irre- 
 gularly luminous network. We find, also, even in the interior 
 of the Milky Way, dark portions where it is impossible to discover 
 a single star even of the eighteenth or twentieth magnitudes. At the 
 sight of these absolutely void regions, it is impossible not to believe, 
 that the visual ray has really penetrated into space, traversing the 
 entire thickness of the stellar stratum which surrounds us."* 
 
 In many parts, this nebulous zone has been so completely resolved 
 that the stars appear projected on a black ground, absolutely de- 
 prived of all nebulosity. But in other regions, a whitish glimmer 
 is still perceived, behind the stars, w^hich shows that in these direc- 
 tions the Milky Way is really impenetrable. 
 
 We shall by-and-by examine, what is, in all probability, the 
 real form of the Milky Way, and what inferences may be drawn 
 from it as to the general structure of the visible Universe. 
 
 * Humboldt, "Cosmos," vol. iii. p. 159.
 
 388 THE SIDEKEAL SYSTEM. 
 
 XIV. 
 
 PHYSICAL AND CHEMICAL CONSTITUTION 
 OF THE STARS. 
 
 Stars arc Suns. This is the last verdict of Science on the constitu- 
 tion of these so prodigiously distant bodies. For a long time the 
 fact has been dawning on us, and already men of science and genius 
 have based on this idea brilliant researches on the general structure 
 of the Universe. 
 
 But how could we hope to be able to pass beyond the domain 
 of conjecture in this matter ? Were we to imagine that optical 
 instruments, refractors, and reflectors, the construction of which 
 is already so perfect, would acquire by new progress a power 
 superior to that they now possess, and penetrate to depths of sj)ace 
 a thousand times more considerable than those they already reach, 
 what would result ? 
 
 That many of the suns nearest to us might then be scrutinized, at 
 200, 600, or 1000 times the distance of our Sun: this would be a 
 step certainly not to be despised, but at most we should only be able 
 to estimate their real dimensions by the measure of their apparent 
 diameters, which might then possibly become sensible. 
 
 Fortunately for us, this unexpected, if indeed not impossible, 
 perfection of optical instruments, is not requisite. Thanks to an 
 admirable method of analysis, which enables us to affirm by ob- 
 servation of a Imninous spectrum the presence or absence of certain 
 substances in the light-source, — in a word, thanks to spectrum 
 analysis, we are now able to say, that such and such a metal, as 
 iron, copper, or mercury, exists in a certain star ; that another 
 contains sodium or manganese. Already have we discovered, in 
 Sirius, Aldebaran, a Orionis, Yega, and others, the presence of 
 manv substances known in our world, and of others with which we 
 
 I
 
 CONSTITUTION OF THE STARS. 389 
 
 are not acquainted ; and this new branch of astronomy promises the 
 most interesting and abundant harvest. 
 
 In presence of such astonishing conquests of Science, we do not 
 know truly which to admire the most, the magnificent chain of 
 natural phenomena which enables us to conclude from one fact, 
 actual or present, another fact past or future, of which the theatre 
 is, as it were, at an infinite distance ; or the power of penetration 
 of the human mind which patiently seizes each link in the chain of 
 facts, and connects the most distant and the most invisible with those 
 which are at our very doors. 
 
 [Let us endeavour to give an idea of this branch of research, and 
 of the progress already made. "We have already referred, in our 
 Chapter on the Sun, to the solar spectrum, which was familiar to 
 man's gaze in the rainbow, that child of showers and rain-drops, 
 long before Philosophy claimed it or utilised its teachings. What 
 nature does by means of a rain-drop, physicists accomplish by means 
 of a prism ; and the first teaching of the prism was, that a beam of 
 light is not a single thing, but a bundle of things, called rays, each 
 with its own special mission, as if each had a master of its own, 
 and had a difierent tale to tell or note to sing. And so it has. 
 Let all the rays in a sunbeam sing in chorus, and the chord which 
 falls on our eye, as sound would fall on our ear, is ichite. Now, let 
 the beam be sent through the prism, and let the latter work its spell ; 
 the chord has vanished. In place of it we find each ray with a 
 coloured note, and we may liken the glorious coloured band, which 
 we call the solar spectrum, to the key-board of an organ ; each ray 
 a note, each variation in colour a variation in pitch ; and as there 
 are sounds in natm-e which we cannot hear, so there are rays in 
 the sunbeam too subtle for our eyes. 
 
 But observe the spectrum of the sunbeam more closely ; there 
 are gaps, which we may liken to silent notes. How is this ? Let 
 us try an experiment ; let us light a match, or anything which 
 burns white, and observe its spectrum. It is continuous, that is, 
 from reddest red, through the whole gamut of colour, to the visible 
 limit of the violet, each ray accomplishes its special mission, tells 
 its tale and sings its song. There are no silent notes, no dark 
 lines breaking up the band. 
 
 Let us try another experiment. Let us burn something which 
 does not burn white, some of the metals will answer our purpose. 
 We see at once by the brilliant colours that fall upon our eye from 
 the vivid flame, that a difierent chord is struck ; but let the prism 
 work again its spell, and tell us the notes.
 
 390 THE SIDEREAL SYSTEM. 
 
 This time we shall find, not only that the spectrum is not con- 
 tinuous, but that the chord consists perchance of only two, three, 
 four, or more single notes, as if on an organ, instead of striking down 
 all the keys, we but sounded one or two notes in the base, tenor, 
 or treble. 
 
 Again, let us try still another experiment. Let us so arrange 
 our prism, that while a sunbeam is decomposed by its upper portion, 
 a beam proceeding from such a light- source as sodium, iron, nickel, 
 copper, or zinc, may be decomposed by the lower one. We shall 
 find in each case, that when the bright lines of which the spectrum of 
 the metals consists flash before our eyes, they will occupy absolutely 
 the same positions in the lower spectrum as some of the dark hands, the 
 silent notes, do, in the upper solar one. 
 
 Here, then, is the germ of Kirchhoff''s discovery, on which his 
 hypothesis of the physical constitution of the Sun is based ; here is 
 the secret of the recent additions to our knowledge of the stars, for 
 stars are suns, and Nature's laws are the same for all. 
 
 Vapours of metals and gases absorb those rays which the same metals 
 and gases themsdres emit. 
 
 We are now in a position to inquire, what has become of those 
 rays, which the dark lines in the solar spectrum tell us are wanting — 
 those rays which were arrested in their path, and prevented from 
 bearing their message to us. Before they left the regions of our 
 incandescent Sun, they were arrested by those particular metallic vapoiirs 
 in his atmosphere, with which they beat in unison; and our assertion, that 
 this and that metal exists in a state of vapour in the Sun's atmosphere, 
 is based upon their non-arrival ; for so marked, various, and constant 
 are the positions of the bright bands in the spectra we can observe 
 here, and so entirely do they correspond with certain dark bands of 
 the spectrum of the Sun, that it can be affirmed, that the chances 
 against the hvpothesis being right are something like 300,000,000 
 to 1. 
 
 So much for the Sun. Fraunhofer was the first to apply this 
 method to the stars ; and we have lately reaped a rich harvest of 
 facts, in the actual mapping down of the spectra of several of the 
 brightest stars, and the examination more or less cursory of a 
 very large nvunber. In all, the plan of structure has been found 
 to be the same ; in all we find an atmosphere sifting out the 
 rays, which beat in unison witb the metallic and gaseous vapours 
 which it contains, and sending to us the residuum, a broken spectrum 
 abounding in dark spaces. But how eloquent is silence sometimes ! 
 Wh.0 would think, that in those gaps would lie the secret of the
 
 CONSTITUTION OF THE STARS. 391 
 
 physical constitution of distant worlds, and detailed information as to 
 their constituent materials ! 
 
 Let us see what Dr. Miller and Mr. Huggins, two of the latest 
 labourers in the field, can tell us. 
 
 Take the spectrum of Aldebaran, for instance ; the coincidence of 
 the bright bands of light given out by sodium, magnesium, hydrogen, 
 calcium, iron, bismuth, tellurium, antimony, and mercury, with dark 
 lines in the solar spectrum, has been proved, seven other elements 
 being tried and rejected. In Betelgeuse, the coincidence of sodium, 
 magnesium, calcium, iron, and bismuth, has been proved. 
 
 The seventy or eighty lines already measured and mapped in 
 each of these stars, represent some of the stronger only of the nu- 
 merous lines which are seen in their spectra. Already we are 
 beginning to think, that in the spectra of the stars the chemist is 
 introduced to many new elements. 
 
 It has been mentioned as a very suggestive fact, that the lines of 
 hydrogen corresponding with C and F of the solar siDoctrum are 
 wanting in the spectra of a Orionis and /S Pegasi, and in these two 
 stars only, out of more than fifty stars examined. 
 
 /3 Pegasi contains sodium, magnesium, perhaps barium. 
 Sirius „ sodium, magnesium, iron, hydrogen. 
 
 Vega „ sodium, magnesium, iron. 
 
 Pollux „ sodium, magnesium, iron. 
 
 How forcibly are we here reminded of that gigantic query of the 
 immortal Newton, " Are not the Sun and stars great earths vehe- 
 mently hot ? " for surely a Orionis, with its atmosphere containing 
 five of our elements, and Aldebaran, with nine, cannot be vastly dif- 
 ferent in constitution from our Sun, the atmosphere of which con- 
 tains ten — possibly fourteen — according to our present knowledge.* 
 
 We have also, as has been pointed out by the observers we 
 have named, pretty certain proof of the idea which has long 
 been floating in many minds as to the cause of the colours of 
 the stars, though their variability in colour, which has lately been 
 so strongly insisted upon, is still to be explained. They re- 
 
 * ELEMENTS IN THE SUN. 
 
 Sodium. 
 
 Copper. 
 
 Cobalt, douhtftil. 
 
 Iron. 
 
 Zinc. 
 
 Strontium „ 
 
 Hydrogen. 
 
 Calcium. 
 
 Cadmium „ 
 
 Magnesium. 
 
 Chromium. 
 
 Potassium, probably not. 
 
 Barium. 
 
 Nickel. 
 
 
 The above according to KirchhofF, except Hydrogen.
 
 392 THE SIDEREAL SYSTEM. 
 
 mark: — "As spectrum- analysis shows that certain of the laws 
 of terrestrial physics obtain in the Sun and stars, there can be 
 little doubt that the immediate source of solar and stellar light must 
 be solid or liquid matter maintained in an intensely incandescent 
 state, as the result of an exceedingly high temperature. For it is 
 from such a source alone that we can produce light, even in a feeble 
 degree comparable with that of the Sun. As the continuous spectrum 
 of the light from incandescent solid and liquid bodies appears to 
 be connected with the state of solidity or liquidity, and not with the 
 chemical nature of the body, it is highly probable that the light, 
 when first emitted from the photospheres of the Sun and stars, should 
 be in all cases identical, the differences of colour depending upon the 
 differences of const if iif ion of tJie in resting atmosphere, and these again 
 intimately connected with the chemical constitution of the stars. 
 The light of the stars will vary in consequence of the loss of differ- 
 ent rays. For, in proportion as the dark lines occur more largely, 
 or are more intense, in particular parts of the spectrum, so will those 
 colours be weaker, and the colours of the other refrangibilities will 
 equally predominate." 
 
 This, however, is but one of the sides of the inquiry. We are now 
 furnished with many others. Thus, for instance, we must for the 
 fu'ture look upon a Orionis and /3 Pegasi as worlds without hj'drogen ! 
 while, probably, the atmosphere of Sirius is more charged with 
 vapours than is that of our Sun. 
 
 These observations, as a whole, show that the stars differ from 
 each other and from our Sun, only by the lower order of differences 
 of special modification, and not by the more important differences 
 of distinct plans of structure. There is, therefore, a i^robability that 
 they fulfil an analogous purpose ; and are, like our Sun, surrounded 
 with planets, which, by their attraction, they uphold, and, by their 
 radiation, illuminate and energize. It is remarkable that the ele- 
 ments most widely diffused through the host of stars are some of 
 those most closely connected with the constitution of the living or- 
 ganisms of our globe, including hydrogen, sodimn, magnesium, and 
 iron.]
 
 ROOK I'HE SECOND 
 
 THE NEBDLiE. 
 
 [Onp: of the most iini)orttmt discoveries of modern times has been 
 that whicli lius furnisliod cvideiuie of ii fact, long- ago conjectured Ly 
 the master minds among- us, namely, that Nebuhv are something- dif- 
 ferent from masses of stars, and that their cloiid-like appearance is to 
 be ascribed to something besides their possible distances, and the still 
 comparatively small optical means one can bring to bear upon them. 
 The discovery is still so recent, that there has not yet been time to sort 
 out the real from the apparent nebuhe. But we are, at all events, 
 justified for the purpose of our present sketch, in accepting- as nchuhe 
 everything hitherto classed as such, although it is nearly certain that 
 the powerful means of differentiation which spectrum-analysis has now 
 placed at our command will place many of them in the category of 
 distant star-clusters, if, indeed, it does not in time indicate a tran- 
 sitional state.] 
 
 If we examine the space in Andromeda, which sej)aratcs the 
 square of Pegasus from Cassiopea, we shall readily perceive, a little 
 below the line which joins these two constellations, a luminous mass — 
 a little whitish cloud of elongated form, in which the eye cannot 
 distingviish any stars. 
 
 If we employ a telescope even of great power, the form becomes 
 more defined, and the oval seems more decided, but the soft and pale 
 glimmer of tliis little celestial cloud retains its nebulous appearance, 
 and there is still no trace of a star. 
 
 This is a nebula, well known under the name of the Great 
 Nebula in Andromeda.* 
 
 * Simon Marius, or Mayer, observed and described this nebula in 1612. It 
 was the first which attracted the serious attention of astronomers. Forty-four 
 years later, Huyghens discovered the great nebula which surrounds the sextuple
 
 '')0 I 'nii.; sri)i',i{KAi, svs'i'KM. 
 
 ^riic cchiHi.iiil Hpiiccs iir(! Hlmwii wifli ii mull il iidc orHi'iniliir ()l)j('(!f,H, 
 vnricd in (liinciiHiun, I)iI<^1i1ii('hm, iiiid (oiiii. All linve r(;(;(!iv(!(l, on 
 ii,(',(t()iin(. of i]\(' clondy ii|)|)(';ii'iiii('(! wliicli ihc.y offer at first Hif^lil-, 
 lii(^ n;ini(! oC Nclnild'. A v<uy liniilcd niindx'j- iii'(! vi.sil)l(! to jjio 
 n)d<(id vy(\ u (u'rcuniHlaiico cxpliiineHl by ilu! HnialhuisH of tlu^ir np- 
 piirdiif, dinxniHions, Ok; focblciK^Hs ol'lJKiir li^lif, and in Hoinc caHOH the 
 vicinily oT rcliiiivcly bii/^lit HtarH. In IIk; <,(;1oh(;oi)0 they appear by 
 tliouHundH; irion! llwin TiOOO uw. now known ; and thin number in- 
 creaHeH in ])ro|)ortion jih Mio diflrn^nt 7'(^«>-ionH of th(^ sky are explored 
 wiih rnorc! jxjwcwCm! in.slrunicnfH. 
 
 ^riic (jucHtion, What are th(! n(!bu]:er' has Ion j2^ been aHked. Were 
 liu^y iin'i^lonieraiioiiH of difl'uHed matter? eele.stiid biminouH eh)uds ?" 
 or wvAv, they ^Tonj)H of eondenHed hUxth, wliieli extreme dislanee 
 r(Midei-ed separately inviHible r' 
 
 Wluwi Htndyin/^' tluMiatnral ^^ronps ofHtiirw, KU(!li as tlu; PlcuadeH, 
 W(M-eniaik<'(l (liat. some eytis only distin^ni,sh a, (ronfn.sed glimmer. 
 To micli, the I'hn'adeH put. on the ap|)earance of a nc^bula — a eir- 
 cumstancc! i-cproduced in tlm <'as(! of a. great, inany clustxirH, which, 
 where tlu^ best, eyes only distinguish an ill-delined luminous mass, 
 are traiisformod, as we hav(! hihmi, by teleHeopew info a nndtitude of 
 (list inet. stai's. 
 
 lienee, in the old elassifieation, the fii'st. class of nebuhe (iomprised 
 the Htar-eluHtors. AstronomerH gave tliiH name to uU nebulosities, 
 vvhi(!h teh^Hcopes (entirely separafx^d into stars, 
 
 A second elasH eomprised those partially separated into stellar 
 j)()ints, but in which some portion resisted n^solufion. 
 
 Jjastly come the nebuLc, properly ho calh^l, in wliicli the most 
 ])owerful telescopes distinguished no stars. 
 
 I>ut. this classilication was held to be (piite relative, and depending 
 entirely on llie uplicid jxiwer ol" the instrument, the sight, of flu; 
 observer, an<l purity of the sky at the time of obsei'vat ion. 
 
 L'riiis was tru(> in the nuiin, and still r(>mains so ; but, as wc^ shall 
 see by-and-by, we now I'ccognise in I lie nebul;e pro[»er a distinct 
 ]>liyslca1 cnnstitul ion. | 
 
 il(>fore comnieiiring our detailed d esc lipt ion of the nebula), lot US 
 say a woi'd on thiur distribution ovei' the starry vault. This is very 
 une(pial in tlie northern h(>misphere, and in thos(> parts of the southern 
 one visible in the northern temperate zone. 
 
 star fi Orionis ; since tliai pci'iod, iind ospccia.IIy siiico the cud of tlic eighteenth 
 cHMitury, tlio cat.iil();.;ucs of iicUulio have boon enriched with nuiuerous obscrvii- 
 tioim, and a comploto branch of a,stron<jiny lias boon dovelopod. 
 
 I
 
 TIIK NI'liri.K. 
 
 nor, 
 
 'I'lic ^^rcjilcHl, iiiimlx'i' IS luiiiid in ;i /one wlilcli nc^in'ccly ('iiil>i'ii('(iH 
 thr (m';^Ii11i jKiil of lli(^ licnvciiH. 'I'lio (ioiihIcIIiiI ioiiM <»!' I li(^ Lioii, t Ik* 
 (nciii I'x'iir, flic (lii'iid'c, iiiul \\w l)r'!ij>'(>li, ili<tH(M(r HooIch, |{('r('iii(M;'H 
 ll.ilr, :iii(l (lie 1 1 iiiil iii;^' I )o;^',s, Itiil |)i'iiH'i|):illy i]n' Vir^'hi, I'oriri IIiih 
 /Diic, vvliicli cxIcikIs iiH (itr iis IJk; iiiiddNi of I lid ( Jriiljuir ; il is kiiuwii 
 iiiidcr ihv, iiiiiiK! ol' l]i(i iifhiiloiis rcf/ioii of Viiyo. 
 
 N(^!irly ill, (lid (i|)|)(»sild |)()l(! ol' (li(( sky, iiiiolJicr ii-}^'<^'loMi('rii(i()ii of 
 licbiilii! (iiiihi-iiccs vXiidroiiKulii, I'co'uhiih, jiikI llio P'lHlidH, mid (ixIcikIh 
 lowci- tliiiii lli(^ lirsl-iijinidd coiislc^Il.-iiion, iiilo llio Hoiiihcni licjivdiis. 
 
 Il, is iH)l,(nv()i'l,liy ili;i,l, ilid I'c^^-IoiiH ii('iii'(^si lli(! Milky W.'iy Jird fli(( 
 |)o()i'(wi 111 nebula!, vvliilsl, (lu; two ricJic^Hi r(!^i<)iiH Ho id llio two poles 
 of Ijiiii ^TCiii bell, In wliicli ilie hIiii'h lire ho iiiiirK^roiiH mid coiideiiHed. 
 TIm' iKibiilie !ii'(! iiioiMi iiiiiroriiily sprc'id over tlic /one vvliieli Hiir- 
 roiilids I lie Soiilli I'ole; lli<'y iii'<' I't (lie HiiiiK^ time iiiiieli lesH 
 minieroiiH. ()ii lli<: oilier limid, iJiere ii,r(! (,wo iiiiif^iiilieciil, i(!^ioiiH 
 I lier'(!, wlii(;li alone eonlain ncm'ly 400 nebuluj mid Htur-cluHterH.
 
 ^V.)^^ THE SIDEREAL SYSTEM. 
 
 I. 
 
 NEBULA OF REGULAR FORM. 
 
 Circular, Elliptical, Annular, and Spiral Nebulae — Annular Nebula in Lyra — 
 Spiral Nebulae in Canes Venatici, Virgo, and Leo. 
 
 The forms, apparent dimensions, and intensity of the light of 
 nebula), are extremely varied. The very different distances, doubt- 
 less, by which they are removed from us, have something to do with 
 these appearances ; but it is probable also that their real structure 
 and dimensions, and the state and temperature of the matter of which 
 they are formed, also influence their apparent characters. In the 
 present transitional state of our knowledge, all classification is purely 
 arbitrary, and it will be understood that its only object is to infuse 
 a little order into our inventory. We shall then be guided in our 
 description by the apparent forms assumed by the nebidae ; and we 
 will begin with the nebida) of regular shape. 
 
 The round, globidar, or spherical form is very frequent. It may 
 possibly be foimd that, in many cases, the nebula) which affect these 
 appearances are nothing else than star-clusters ; their immense dis- 
 tances, or the extreme smallness of the stars which compose them, 
 may prevent our distinguishing separately the luminous points, 
 which, even in the most jjowerful telescopes, only present a confused 
 phosphorescent gliimncr. Great probability is lent to this ^lypo- 
 thesis, as we have before hinted, by the fact, that each new triimiph 
 of optical skill results in a resolution of some nebulae, before irre- 
 ducible, and helps us at the same time to discover new nebula), at 
 greater depths of space. 
 
 Fig. 151 gives some examples of circular and oval nebulae, chosen 
 from a numerous collection of similar objects. The perfectly roimded 
 form of some is seen to pass, by imperceptible gradations, to the
 
 NEBUT,.^ OF REGT'l.AK FORM. 
 
 397 
 
 most elongated ellipses, at last approximating to a straight line. 
 Near the centre of* some of these nebulae a marked condensation 
 of light is also noticed, which indicates an analogy with the spherical 
 star-clusters. In some globular nebulae, the brightness does not in- 
 crease in a continuous manner from the circumference to the centre ; 
 the gradation is replaced by concentrical strata, analogous to tliosc 
 which we noticed in the cluster in Toucan. This circumstance affords 
 another point of resemblance between the resolved globidar clusters, 
 and the nebulae of the same form. 
 
 Fig. 151.— Nebulae of circular and oval forms. (Sir J. Herschel.) 
 
 The oval form probably belongs to very flattened condensations 
 presented to us edgewise, the degree of flattening being attributed 
 either to their real form, or to an inclination more or less decided 
 towards the region of the sky which our system occupies. Among 
 the nebulae of round or oval form, there are only a few which 
 present another very peculiar and curious structure ; we refer to 
 the annular or perforated nebula;. 
 
 One very interesting examjDle is situated in the constellation of
 
 398 
 
 THE SIDEREAL SYSTEM. 
 
 the Lyre, not far from Yega, between the two stars (3 and 7 of that 
 constellation. A nebulous ring of oval form surrounds a darker 
 space, the pale uniformly spread glimmer of which resembles a '* light 
 gauze " stretched across the ring. Such is the appearance which this 
 singular object at first presented (fig. 153, 1). 
 
 Lord Rosse's telescoiDe has since partially resolved the ring into 
 luminous points, and has shown parallel lines in the opening, the 
 exterior borders are also stellated with fringes (fig. 152, 2). 
 
 We reproduce here, from the drawings of Sir J. Ilerschel, 
 two other annular nebidae, one oval, the other romid. The first 
 (fig. 152, 3), which is very similar to the nebula in Lyra, is situated 
 
 Fit;. IJ-- — Annular Nebuliu, 
 
 1. In Lyra (Sir J. Hcrschul). 2. The same (Lord Rosse). 3. Annular nebula iu Cygiius. 4. In 
 
 Ophiuchus. 5. In Scorino. 6. Nebula near y Andromedae. 
 
 between the constellations of the Swan and the Fox; the second, 
 (fig. 152, 4) in Ophiuchus. 
 
 The oval form of the ring is already decided in the nebula 
 numbered 5, which presents, moreover, a singularity which we shall 
 again soon meet with; two stars are situated on the ring, at the 
 extremities of its smallest diameter. But, in an annular nebvda 
 (fig. 153, 6), near the beautiful triple star y Andromedao, the ring is 
 excessively elongated, and two stars are there also symmetrically 
 placed, only this time it is at the extremity of the major axis of 
 the ellipse. 
 
 This regularity in the forms of a great number of ncbida:" is 
 doubtless apparent only. It partly disappears when they are
 
 NEBULA OF KEGULAK FORM. 399 
 
 examined with very powerful instruments ; that is to say, when 
 brought nearer to us tho}^ reveal the details of their structm-e. Then 
 the large masses of light not being preponderant, the form loses its 
 symmetry, as may be seen in the two drawings which represent the 
 annular nebula in Lyra (fig. 152, 1, 2). 
 
 h'v^. 153. — Conical or comotary nebula;.— 1. lu Endanus (Sir J. Hcrscliel). ■.;. In the Unicorn (Lord 
 Rosse). 3. lu the Great Bear (Sir J. Herschel.) 
 
 Again, always bearing in mind that the classification which we 
 have adopted is an arbitrary one, we may rank among the regular 
 nebulae those which affect the conical, or parabolic form, similar to 
 that of some comets. "VVe give here (fig. 153) three examples of these 
 nebulas, the form of which is analogous to some star-clusters, for 
 
 Fig. IG4.— Nebula in Argo. (Sir J, Herschcl.) 
 
 example, the cluster numbered 6 in Plate XXXII, which shows 
 the same luminous concentration at the apex. 
 
 Here, again, is a nebula (fig. 154), which by its widening form 
 approaches the cometary nelnila, but which seems to suggest a( the
 
 400 THE SIDERKAl, SYSTEM. 
 
 same time, by its singular outline, the first approach to a spiral 
 
 nebula. 
 
 In all the nebulae which we have examined, the regularity of 
 form is manifested by a symmetry, such that each object is divided 
 into two equal parts by its axis. But it is important to insist on 
 the fact that this regularity often disappears when optical instru- 
 ments of greater illuminating power show the different portions Avith 
 more clearness. It is often surprising to see a nebula thus trans- 
 formed to the eye in a most complete manner. In no example is this 
 chana-e of form so decided as in the nebula in Canes Yenatici. 
 
 Let us look at the following figure. 
 
 We see, at the centre of a ring, double throughout half its con- 
 tour, a bright, globular nebula, accompanied by a smaller nebulosity 
 
 Fig. 166. — Nebula in Caue.-> Vunatici. (Sir J. llc-isclicl ) 
 
 of round form situated outside the ring and at some distance 
 away. It was under this form that it was first seen and drawn by 
 Sir J. Herschel (fig. 155). 
 
 Observed later by Lord Rosse, with the help of his magnificent 
 telescope, the same nebula was presented under a form of wonderful 
 strangeness (fig. 156). Brilliant spirals, unequally luminous, and 
 overstrcAvn with a multitude of stars, diverge from the centre, and 
 become separated one from the other more and more as they recede 
 from it, at last being lost in a direction common to all. The 
 exterior filaments of this prodigious spiral of stars join the smaller 
 exterior globular nebula, which at first appeared isolated from the 
 ring. 
 
 Lastly, according to the most recent observations of M. Cha-
 
 NKBTTT,.^ OF REGUT,AR FORM. 401 
 
 rornac, this latter nebula itself affects the spiral form, its contours 
 beino- connected with the spirals of the principal nebula. 
 
 The imagination remains confused in presence of such a grand 
 spectacle. It loses itself in endeavouring to calculate the total 
 
 Fig. 156.— Spiral form of the nebula iu Caucs Venatici. (LorU Russe ) 
 
 dimensions of this immense system by assuming a probable distance 
 for the atoms of this star-cluster. "VVe are startled at the depth of 
 the ab^'sses into -which the human gaze plunges. What strange forces 
 have produced this hurricane of matter — perhaps of suns ? Is th(> 
 spiral the original form of those gaseous nuttters, the condensation of 
 
 T) 1)
 
 402 
 
 THE SIDEREAL SYSTEM. 
 
 which may give, or lias given, birth to each individual of this 
 
 gigantic association ? 
 
 These are questions which the mind puts to itself, the complete 
 solution of which will doubtless demand many centuries. Shall 
 we ever be able to recognise in these groups variations of form, dis- 
 tinct from those due to the varying power of the various instruments 
 and the difference of sight of observers ? In a word, shaU we 
 ever be able to prove the movements of the constituent parts of the 
 nebulae ? 
 
 Fig 157.— Spiral nebula in Virgo. (Lord Rosso.) 
 
 The spiraloid form is not confined to the nebula wc have de- 
 scribed. It is quite as clearly defined in the nebula in Virgo 
 represented in fig. 157. The luminous branches of this spiral, four 
 in number, are clearly separated by dark intervals, and divided 
 besides by darker spirals, which indicate strings of matter less 
 condensed. All diverge from a central nucleus where a much more 
 decided light indicates a powerful concentration. 
 
 The number of nebul;i3 in which the spiraloid form is observed 
 was at first rather small. But in propoition as the sky is explored
 
 NEiiLL.E OF REULLAK EOUM. 
 
 403 
 
 1)V more powerful iii.strunieiit.s the iimuber has increased. In the 
 important memoir, published by Lord Itosse in 18G1,* we have 
 noted forty spiral nebidaj, and thirty more in which this form is 
 suspected. 
 
 In a nebula of the northern heavens, situated on the confines 
 of the Great Bear and of Bootes, the centre is like a large globulur 
 nebula with a very marked condensation, whence radiate branches 
 arranged in the form of spirals. In several points of these branches 
 other centres of condensation are noticed. Sir J. Herschel had classed 
 this among the nebuhu of rounded globular form, doubtless because 
 
 Fig. loS— Spiral uebuUi. (Lord nosse.) 1. Of tlie Lion. '2. Of Pegasus. 
 
 the central nebulosity was the only one revealed by his telescope. 
 Some few stars are ' scattered here and there on the ground which 
 it occupies. In the two nebula; in fig. 158, which are situated, the 
 first in the Lion, the second in Pegasus, the spiraloid form is less 
 decided. The spirals approach nearer to an elliptical form and are 
 enveloped one in the other. 
 
 * "On the Construction of Specula of Six-feet Aperture; ami a Selection 
 from the Observations of Nebulaj made with them." — PhU. Trans. 1802.
 
 404 
 
 THE SIDEREAL SYSTEM. 
 
 IL 
 
 NEBULA OF IRREGULAR FORM. 
 
 Large Nebulous Masses affecting no Symmetrical Form — Diversity of Aspect with 
 Instruments employed — Nebulae in the Constellations of Andromeda, the 
 Lion, Fox, Sobieski's Shield and the Bull — Great Irregular Nebute of Orion 
 and Arjfo. 
 
 All the nebulae we have just described are distinguished by a regu- 
 larity and a symmetry of form which, joined to a condensation of the 
 light either in one central point, or along converging curves, indicate 
 some bond linking together all the constituent portions of each. 
 It is impossible not to recognise in them so many systems, althougli 
 
 Fig. 1'j9.— Nebula in Andromeda. 
 
 as yet we are ignorant of their precise nature. And it is possible 
 that a number amongst them are not yet decomposed into stars, owing 
 to the immensity of their distance, or, what comes to the same 
 thing, the insufficient power of our telescopes.
 
 NEBUL.E OF IRREGULAR FORM. 
 
 405 
 
 Besides these regular aggregations, there are other large nebulous 
 masses which affect the most various forms, completely removed from 
 all symmetrical appearance. But such is the variety, such the 
 richness of these systems, that we can pass from the nebulse of 
 spherical form to others, the most bizarre and irregular, by every 
 imaginable gradation. 
 
 Let us examine the glimmer of lengthened oval form represented 
 in fig. 159. 
 
 Fig. 160.— Nebula iu Andromeda. (G. P. Bond.) 
 
 The condensation of light noticed at its centre makes it resemble, 
 according to the expression of the first observer — Simon Marius, 
 " the flame of a candle seen through a leaf of transparent horn." 
 It is the nebula of Andromeda, which we have already mentioned. 
 Its symmetrical form, which caused it to be once classed among the 
 regular nebula?, has disappeared in the powerful refractor of Cam- 
 bridge, U.S. (fig. 160). The nebulous masses which compose it 
 have been found to be separated by two long canals, and it has been
 
 400 
 
 THE SIDEUKAI. SYSTF,:M. 
 
 partly resolved into stars. r)oiid has counted more than 1500. The 
 general i)riinitivo form is still recognised at the centre of the 
 
 Fig. ir.l.- Elliptiial .iiiinilar iiclnil.i of tlic Lion. (Sir .J llcrsrhel ) 
 
 nebula, but it is singularly altered, and instead of one central point 
 of luminous condensation, several excentricallj' situated may be 
 
 noticed. • 
 
 Another nebula of elli})- 
 tical form sitiiated in the 
 constellation Leo, and repre- 
 sented in the drawing (fig. 
 151 7), as seen by Sir J. Iler- 
 schel, has been observed in a 
 
 different form (fig. 101) in 
 Lord Ilosse's telescope ; the 
 central nucleus is composed of 
 envelopes which take an annu- 
 lar spiral form, and the ex- 
 tremities of the oval are marked 
 by luminous strifr placed on 
 each side of the axis, like the 
 
 Fig. 102.— Umiib-bcll nebula. (Sir J. Hev.schcl.) 
 
 vertebral column oi a fish. 
 Lastly, another remarkable example of these optical transform- 
 ations, purely apparent, as they only depend on the power of the 
 
 I
 
 XF.Hri,.T, OF IHKK/;T'r,A]{ Vi)UM. 
 
 407 
 
 instruments, is furnished by a nebula situated in the eonstollation 
 Vulpecula. Sir J. Hersehel, to whom we owe the tirst drawing of 
 this nebida (fig-. 162) gave it the name of the "Dumb-bell." 
 
 Two luminous masses, symmetriealh- plaeed and bound together 
 by a rather short neck, the whole surrounded with a light nebulous 
 envelope of oval form, gave it a very marked appearance of regu- 
 larity- This aspect was, however, modified by Lord Rosse's tele- 
 
 Fig. Ui3 — Duinb-bcjU nebvila. (Lord Rosse.) 
 
 scope of three-feet aperture, and the nebulous masses showed a 
 decided tendency to resolvability. Later still, with the six-feet 
 telescope, numerous stars were observed standing out, however, on 
 a nebulous ground. The general aspect retains its primitive shape, 
 less regular, but striking nevertheless (fig. 163.) 
 
 The irregidar ncbulse are sometimes presented under the most 
 fantastic forms. Sometimes they affect long vaporous trains, which 
 here and tliere send out their branches ; sometimes these shapes are
 
 408 
 
 THE SlDEKliAL SYSTEM. 
 
 bizarre in the extreme. Such is the nebula in Sobieski's Shield. 
 An ellijjtical portion is terminated b}^ two appendages, one of which 
 is nearly rectilinear, and gives it the form of the Greek capital 
 letter Omega (n). In the middle of one of the angles (fig. 1G4), 
 two luminous condensations are remarked similar to the spherical, 
 or globular clusters. 
 
 A form, still more singular, is that of the nebula in Taurus, 
 which, viewed in instruments of slight illuminating power, appears 
 like a regular oval. As seen in Lord Rosse's large telescope, it re- 
 sembles (fig. 165) a gigantic lobster, the antennae and claws of which 
 are figured by long strings of stars. 
 
 In the centre of one of the two Magellanic Clouds, which we have 
 
 Fig. U14.— Nebula in Sobieski s Sliiclil. (Sir J. Uerschcl.; 
 
 already referred to as among the most beautiful objects of the south- 
 ern sky (we shall describe them further on in some detail), is a nebula 
 the complex form of which serves as a transitional point for us to 
 pass to the large irregular nebulae. It is the nebula of Doradus 
 (Plate XXXY). The central part is composed of three bright 
 annular masses ; the two smallest are circular ; the largest, of the 
 form of a pear, is surrounded with much paler appendages, studded 
 with a great number of small stars. 
 
 Imperceptibly we arrive at the large nebulae, the shapeless 
 forms of which resemble clouds disturbed and torn by the tempest. 
 But, here again, we find in the glimmers of these distant agglo- 
 merations, indications of resolution into stars, [or into something, of 
 Avhich more anon.]
 
 PLATE XXXV 
 
 NEBUL/E IN DORADUS AND SURROUNDING ETA ARGUS. 
 
 (Sir J. Heischel.)
 
 NKP.UL.T; of IKRKOUI.AK T-'omi. 
 
 411 
 
 Human language has no expressions capable of rendering- the 
 sentiments of admiration, of profound stupefaction, into which the 
 thought is plunged, when, thanks to the marvellous power of our 
 telescopes, our sight penetrates the distant strata of the sky, in which 
 these unearthly objects shine. 
 
 The largest, nebula) of irregular form are found in the vicinity 
 of the Milky Way, among the most brilliant constellations of the 
 
 starry sky. 
 
 Let us o-ive some details of the two most interesting. 
 
 Fig. 105.— Nclnila iu Taunis (Crab nebula.) (Lord Rf .s.su.) 
 
 One, situated in Orion, surrounds the magnificent sextuple star 
 d, which we have described when speaking of the systems of 
 audtiple Smis. The other surroimds a star equaUy noteworthy, 
 yi ArgCis, so remarkable on account of its quick and capricious 
 variations of brightness. The drawings, which we here give of these 
 two great nebula3 (Plates XXXV and XXXVI), according to 
 two illustrious observers, Sir J. Herschcl and 0. V. Bond,
 
 412 THE SIDEKEAL SYSTEM. 
 
 relieve us of a description wliicli would necessarily be vague and 
 incomplete.* 
 
 Since Huygbens discovered the nebula of Orion in 1656, this 
 magnificent object has been observed with a constantly increasing 
 care, and the diiferent regions, more or less luminous, which compose 
 it have been described in every detail. By degrees, the stars which 
 overspread the expanse have been recognised as more nimierous; 
 and astronomers have arrived at the conviction that it is resolvable. 
 
 Sir J. Herschel compares the brightest portion to the head of 
 a monstrous animal, the mouth of which is open, and the nose 
 of which is in the form of a trunk. Hence, its name, the Fish- 
 mouth Nebula. It is at the edge of the opening, in a space free 
 from nebula, that the four brightest of the components of 6 are 
 to be found ; around, but principally above, the trapezium formed by 
 these four stars is a luminous region with a mottled appearance, 
 which Lord Eosse and Bond have partly resolved. 
 
 This reoion is remarkable on account not only of the brilliancy 
 of its light, but also of the numerous centres where this light is con- 
 densed, and each of which appears to form a stellar cluster. The 
 rectansrular form of the whole is also worthy of attention. The 
 nebulous masses surrounding it, the light of which is much fainter 
 than that of the central region, are lost gradually ; according to 
 Bond, they assume a spiral form, as indicated in the drawing ex- 
 ecuted by that astronomer. 
 
 [It has now been placed beyond all doubt that the changes of 
 form have taken place in the course of our most modern observa- 
 tions, f] 
 
 According to Sir J. Herschel, the great nobida of Orion occupies 
 a space on the sky, the apparent dimensions of Avhich have the same 
 extent as the lunar disk. He seems to believe that it is attached 
 
 * " It is almost impossible to recoucile the two drawings of Bond and Her- 
 .schel, without admitting the supposition of a considerable change to which 
 the most luminous region has been subjected during the interval elapsed 
 between the epochs of the two observers." — Liajjounov, " Observations of the 
 Great Nebula of Orion, made at CazanP 
 
 t " The observations as to the distribution and brightness of the nebulous 
 ma,tter do not imply any change of form, but many tiuctuations in the bright- 
 ness of the different parts. The general impression which I have received from 
 these observations is, that the central part of the nebula is found in a state of 
 continual agitation, like the surface of the sea." — 0. Struve, Observations at Poid- 
 kowa, " Memoirs of the Academ?/ of Sciences of Saint Petersburg, 1862." 
 
 [Since this was written, M. Otto Struve, Father Secchi, and many observers 
 in this country, have completely established the fact that a change of form has 
 taken place.]
 
 z 
 o 
 
 O -g 
 
 Li. a 
 O „■
 
 NEUUL-Ti OF IKHKGULAR FORM. 415 
 
 to the Milky Way — that it is perhaps the prolongation of the bruneh 
 which leaves the main trunk in Perseus, and extends itself towards 
 the Pleiades and xVldebaran. 
 
 [This, hoAvever, is now no longer probable.] 
 
 The nebulosity which surrounds jj Argus (Plate XXXV), like 
 that of Orion, does not present any symmetry in its form or in its 
 outlines. It is situated in the Milky Way, in the midst of a region so 
 rich in stars, that more than 1200 have been counted in the area 
 occupied by the nebula. The stars, however, do not seem to form 
 part of the nebulosity, but rather appear to be simply projected 
 on it. 
 
 Towards the centre of the nebula, and close to the star vj, an 
 opening of a lengthened and rounded form is noticed, which leaves 
 in view the dark gromid of the sky, [This has been named by Mr. 
 Abbott, a careful observer, " the Crooked Billet." 
 
 The evidences of change in this nebula are even more decided 
 than in that of Orion. This object indeed may supply a link of the 
 greatest importance, for we read that the objects of which it is 
 composed (not stars) " are now of a larger character, and more re- 
 fulgent than nebidous matter in general."*] 
 
 [* Mr. Abbott, in " Monthly Notices of the Eoyal Astronomical Society," 
 April, 1853, p. 193.]
 
 410 
 
 THE SIDERKAI. SYSTT'.M. 
 
 ]II. 
 
 PLANETARY NEBUL.E AND NEBULOUS STARS. 
 
 Planetary Nebula; — Variation ol' A!S[)ect— Nebulous Starx. 
 
 The name of Flaiiofdri/ Nchithr has beon o-ivcn to those the form of 
 which is that of a disk uiiifoniily huiiiuous, an a])peaiaiu'e whicli 
 causes theni to resemble a spherical body slig-htly illuminated by 
 borrowed light, — in a word, a phinet. In lig. 166 are represente<l 
 a few of these nebuhe of circular form. 
 
 That which distinguishes those nebulae from those we have ])ve- 
 
 FIlt. lOli.—I'laiictaiy nebulae. (Sir J. nerschcl.) 
 1. In Ursa Major. 2 lii Pisces. 3. In Aiidroiucda. 
 
 viously described is the equality of their light and the absence of 
 all luminous condensation at the centre. 
 
 It is only on the very borders of the nebulous disk, that a slight 
 diminution in the intensity of the light can be perceived. It 
 cannot be held that they are extremely di.stant clusters of stars of 
 spherical or ellipsoidal form, since, as Ave have seen, even on the 
 supposition of an equal distribution in space of the components of
 
 PLANETARY NEBUL.E AND NEBULOUS STARS. 
 
 417 
 
 the group, perspective alone would give an apparent condensation 
 towards the centre. Again, are any of them by any possi- 
 bility veritable clusters of flattened form presented to us with their 
 circular surfaces perpendicular to our line of sight ? Or, as remarked 
 by Sir J, Herschel, are the stars of these nebuke in the form of a 
 hollow spherical shell ? 
 
 [These questions, so long asked, are, as we shall see, now partially 
 answered.] 
 
 The planetary nebida in Ursa Major, the light of which is 
 uniformly distributed in Sir J. Herschel' s drawing (fig. 166), has 
 been seen under quite another aspect through the large telescope 
 of Lord Rosse. The disk is changed into a double luminous 
 crown, surrounded with a fringed border, two points appearing 
 
 Fig. 107.— Planetary Nebute. (Lord Rosso.)— 1. In Ursa Major. 2. In Andromeda. 
 
 at the centre of the nebulosity which have every aspect of stars 
 (fig. 167). 
 
 Another example of these changes is furnished us by the plane- 
 tary nebula near ■/. Andromedac, which, perfectly round in the 
 drawing of Herschel (fig. 166, 2), appears under the form of a 
 luminous ring in that of Lord Rosse (fig. 163). 
 
 Let us finish our list, so marvellously rich in various forms of 
 nebulce, by mentioning those which have received the name of 
 " nebulous stars." 
 
 These are no other than nebvda), sometimes circular, sometimes 
 oval, sometimes annular, but always regular ; in the interior of which 
 appear one or several stars standing out distinctly from the ne- 
 bulosity, and being moreover symmetrically placed. 
 
 If the nebida be circular, the star occupies the centre ; in the case 
 of an elliptical form, two stars are seen placed as if they were the- 
 
 E. E
 
 418 
 
 THE SIDEKEAL SYSTEM. 
 
 two foci of the curve. One is figured in tig. 168, 5, where three 
 stars are regularly disposed at the angles of an equilateral triangle, 
 whilst another very elongated nebula has two stars, placed outside 
 the extremities of the greatest diameter. Here, as in the planetary 
 
 1 
 
 2 3 
 
 4 
 
 mm- 
 
 5 6 
 
 Fifr- KiS. — Nebulous stirs. (Sir J. Herschel ) 
 1. In Cygnus. 2 In Perseus. .■?. In tlie Centaur. 4. In Sagittarius. 5. In Auriga. 
 6. In Aiidroniedn. 
 
 nebulsB, very powerful telescopes enable us to see, instead of a disk 
 
 feebly but equally illuminated, forms which are much more irregular, 
 
 and in which the light is distributed 
 in a much more unequal manner. 
 
 Such are the nebulae represented 
 in fig. 169, taken from the original 
 drawings of Lord Rosse. It has also 
 been asked if we may not see, in these 
 nebulous stars, suns enveloped with 
 an atmosphere of considerable dimen- 
 sions, rendered visible at these enor- 
 mous distances by the light of the 
 stellar foci. This opinion is certainly 
 not deprived of probability, although 
 again we may consider nebulous stars 
 as clusters of a multitude of very 
 small ones, having at their centre a 
 
 sun, single, double, or even multiple, of which the great brilliancy 
 
 suffices to explain its particular visibility. 
 
 Fig. 109. — Nebulous stirs. (Loi-d Rosse.) 
 1. In Gemini. 2. In Argo.
 
 PLANETARY NEBULA AND NEBULOUS STARS. 419 
 
 Sir J. Herschel describes a planetary nebula, the light of which 
 is about equal to that of a star of the sixth or seventh magnitude, 
 its diameter about 12", its disk slightly elliptical, with a sharp, 
 clear, and well-defined outline, " having exactly the appearance of a 
 planet, with the exception only of its colour, which is a fine and 
 full blue verging somewhat on the green."* The same astro- 
 nomer describes three other nebulae, the colour of which is a clear 
 sky-blue. 
 
 As these latter nebidae are all planetary nebulae, if the hypothesis 
 of a diffused matter be admitted, it must be supposed that its light 
 possesses a particidar colour. 
 
 * " Outlines of Astronomy," 8th Edition, p. 645.
 
 420 
 
 THE SIDEREAL SYSTEM, 
 
 IV. 
 
 DOUBLE AND MULTIPLE NEBULA. 
 
 Groups of Nebulse — Probability of a Physical Connexion between the Compo- 
 nents — Multiple Nebuloe in the larger Magellanic Cloud. 
 
 We have noticed nebulae accompanied by systems of double or mul- 
 tiple stars, placed in a manner so sjTnmetrical in the midst of the 
 nebulosity that it is impossible to doubt the existence of a real 
 
 Fig. 170. — Double and multiple nebute. — (Sir J. Herschel.) 1 and 4. In Virgo. 2 and 5. In 
 Coma Berenices. 3. In Aquarius. 6. In the Nubecula Major (Great Cloud of Magellan). 
 
 connexion between the stars and the nebulee. Evidently these are 
 physical groups of a special constitution. 
 
 There exist also groups of nebulce analogous to groups of stars, 
 that is to say, the components are physically, and not merely
 
 DOUBLE AND MULTIPLE NEBULA. 421 
 
 optically connected. We again find in these interesting associations 
 the same varieties of aspect and form as in the simple nebulae. 
 
 Some appear formed of two globular clusters, in which the central 
 condensation indicates not only a spherical figure, but probably also 
 the existence of real centres of attraction ; examples of this are 
 seen in fig. 170. Sometimes the components appear entirely 
 separate and distinct, sometimes they encroach one on the other; 
 but whether these appearances are optical only, or whether there 
 be a real physical connexion, we know not. 
 
 Sometimes, again, one of the components is round or globular, 
 whilst the other takes an elongated elliptical form. The nebida 
 represented in fig, 171 is composed of two rounded masses, terminated 
 by brilliant appendages, enveloped by a nebulosity common to 
 
 Fig. 171.— Double uebula. (Lord Bosse.) 
 
 both, the whole surrounded by light luminous arcs similar to frag- 
 ments of a nebulous ring. 
 
 The number of the nebulous centres is often very considerable. 
 Sometimes it is as high as seven, as in the multiple nebulae observed 
 by Sir J, Herschel, of which we reproduce a curious specimen (fig. 
 170, 6), The group, in question, is one of the numerous clusters 
 which form the largest of the two clouds of Magellan. We may gather 
 from this circumstance that the connexion of these seven nebulae is 
 purely optical, if the general nebulosity which envelopes them all 
 does not indicate a physical union. 
 
 For the rest, the connexion of the components in the multiple 
 nebidsB will not, doubtless, be demonstrated in the same manner as 
 we have seen that of the systems of the double stars. In these 
 latter systems, the movement of revolution of one of the sims 
 round the other can be studied, because their distance, hoAvever 
 stupendous, renders this movement observable in a limited number 
 of vcars.
 
 422 THE SIDEREAL SYSTEM. 
 
 On the otlier hand, the multiple nebulaD are supposed to be 
 banished to such infinite depths in the abysses of the heavens, that 
 any movement would remain imperceptible. Thousands of years— 
 thousands of centuries, perhaps— would be necessary lor us to become 
 sure of any change in the position of the whole. Our telescopes 
 will in vain increase their power, and the sight become more pene- 
 trating. We cannot anticipate time. Compared to the life of the 
 worlds, our life is but a second, as our entire system is but a point 
 in the expanse of the infinite.
 
 MAGKI.I.AMIC Cl.OUlKS. 423 
 
 V. 
 
 MAGELLANIC CLOUDS. 
 
 Position of the Two Magellanic Clouds in the Southern Sky — Structure of tlie 
 Little and of the Great Cloud — Star-Clusters ; Isolated Stars and Nebulae 
 which they contain. 
 
 When we look on the region of the celestial vault which surrounds 
 the South Pole, we cannot help being struck with the contrast pre- 
 sented by the small quantit}^ of stars which it contains, with the 
 brilliant zone which borders on the Milky Way, from Orion and 
 Argo, to the Centaur, passing by the Southern Cross. One solitary 
 star of the first magnitude, Achernar, more distant from the Pole 
 than are the beautiful stars of the Centaur and of the Cross, shines 
 in this part of the sky. 
 
 But even this circumstance renders the singular aspect of the 
 two nebulous spots, which seem two detached pieces of the great 
 galactic zone, still more striking. These half-stellar, half-nebuloua 
 . systems, unequal in magnitude and brightness, but easily seen with 
 the naked eye in a clear moonless night, are situated, one, the 
 larger and more brilliant, between the Pole and Canopus, in the 
 constellation of Doradus ; the other, the smaller, and less brilliant, 
 ordinarily invisible during the full moon, in Hydrus, betAveen 
 Achernar and the Pole. 
 
 Both are known by astronomers and navigators under the name 
 of "Cape Clouds," or, again, "Magellanic Clouds." And, to dis- 
 tinguish them, we have again the Great Cloud {Nuheada Major) and 
 the Small Cloud [Nuhecnla Iliuor). In figs. 172 and 173 the general 
 form of these two nebuloo is represented. 
 
 The Clouds of Magellan are distinguished from all other nebulse 
 which we have as yet described by their great apparent di- 
 mensions, and by their physical structure ; this last character dis-
 
 424 
 
 THK SIDEREAL SYSTEM. 
 
 tinguishes them from most of the branches and offshoots of the Milky 
 Way, with which we may also add they do not appear connected in 
 any way. 
 
 
 Fig. 172— Magellanic Clouds. The Small Cloud. 
 
 The Great Cloud extends over a space which embraces not less than 
 forty-two square degrees — about two hundred times the apparent 
 
 Fig. 173.— Magellanic Clouds. The Great Cloud. 
 
 surface of the lunar disk. The Small Cloud occupies an extent four 
 times less than ihe other ; according to Humboldt, it is surrounded
 
 MAGELLANIC Cf.OUDS. 
 
 425 
 
 " with a kind of desert," where, it is true, shines the magnificent 
 steUar cluster of Toucan, of which mention has been before made. 
 If the exterior aspect of these two remarkable nebulae and their 
 situation in a celestial region poor in stars give to the southern 
 sky a peculiar appearance, their real structui'e makes them one 
 of the wonders of the heavens. Examined by the aid of a powerful 
 telescope by Sir J. Herschel, during his stay at the Cape of Good 
 Hope, they were both decomposed in a manner of which fig. 174, 
 which represents a portion of the Great Cloud, gives an idea. 
 
 We have first a great number of single stars, the brightness of 
 
 Fig. 174.— Clouds of Magellan. A portion of the Great Cloud. (Sir J. Ilcvschcl.) 
 
 which varies between the fifth and eleventh magnitudes : then star- 
 clusters, some of irregular form, others— and the largest number— 
 taking a globular, spherical, or oval shape; lastly, nebulsc, some 
 separate, others grouped in two, three &c., most of them rounded 
 and regular. Some of them, known under the name of Nebulae of 
 Doradus, already described and represented in Plate XXXIII, are 
 situated in the Great Cloud. "This nebula," says Humboldt, ''scarcely 
 occupies the ^i^th part of the area of the Cloud ; and Sir J. Herschel 
 has already measured in this space the position of 105 stars, of 
 the fourteenth, fifteenth, and sixteenth magnitudes, standing out on
 
 426 THK SIDEREAL SYSTEM. 
 
 a nebulous background of unbroken and uniform brilliancy, which 
 has resisted the most powerful telescopes." 
 
 The double and multiple Nebulae are also much more numerous 
 here than in the other zones of the heavens, richest in objects of 
 this nature. 
 
 Thus, we repeat, the constitution of these irregular Nebulae 
 appears quite different from that of the Milky Way, from which we 
 may also add they are some distance removed. They are dis- 
 tinguished also from other known Nebulae, and seem like miniatures 
 of the entire heavens. 
 
 A word now on the structure of each of the two Clouds. In the 
 Great Cloud Herschel has counted 582 single stars, amongst which 
 one only is of the fifth magnitude ; six others are of the order im- 
 mediately inferior, and would doubtless be visible to the naked eye 
 if their light were not effaced by the general glare. Then come 
 291 Nebulae and 46 star-clusters, forming so many distinct groups. 
 
 In the Small Cloud, the single stars are proportionally more 
 numerous, since 200 have been counted, amongst which three are of 
 the sixth magnitude, whilst it only includes thirty- seven NebtJae and 
 seven star-clusters. These immense aggregations, the elements of 
 which are themselves swarms of suns, remind us of the largest, in 
 appearance at least, of all the clusters which the eye contemplates in 
 the depths of the sky — the Milky Way.
 
 PHYSICAL CONSTITUTION OF THE NEBUI-^:. 42'i 
 
 VI. 
 
 PHYSICAL CONSTITUTION OF NEBULiE. 
 
 All the Nebulae, scattered throughout the depths of the sky, were 
 but lately considered to be so many agglomerations of stars, differing 
 only from star-clusters by their general form, and the grouping of 
 the components. But it has often been thought that, among these 
 celestial clouds, there were some, at all events, composed of diffused 
 vaporous matter, or at least formed by the accumulation of bright 
 corpuscules, of great relative tenuity, and as such, possessing no 
 analogy with the other celestial bodies — with suns. 
 
 The hypothesis of a nebulous matter endowed with its own light 
 and scattered in immense masses over the expanse of the infinite, 
 was proposed originally by astronomers whose instruments were 
 unable to resolve these cosmical clouds. The large Nebula) especially, 
 like that which surrounds the star S Orionis, of irregular form, 
 lent great probability to this hypothesis. But the resolution of the 
 globular Nebulae, one by one, at length resulted in the idea of a real 
 nebulosity being confined to the irregidar Nebulae. 
 
 But later still, modern observations, made with instruments of 
 great power, by degrees showed an apparent identity of composition 
 with the stellar clusters in a great number of these last Nebulae. 
 Thousands of little stars appeared, where before a phosphorescent 
 milky glimmer, according to the expression of astronomers, of an in- 
 definable and characteristic aspect, was noticed. The Nebulae in 
 Andromeda and Orion, in which observers had remarked no sus- 
 picion of stars or stellar sparkling, indicative of probable resolution, 
 have recently been stated to be resolved, at least in part ; and as a 
 consequence, the hypothesis of a difiiised, nebulous matter lost ground 
 in proportion as our means of observation were increased. But still
 
 428 THK SIDEREAL SYSTEM. 
 
 it was asked, Must it be quite abandoned ? The existence of matter 
 of this kind is not incompatible with what is known of the 
 physical constitution of the celestial bodies. Comets, with their 
 vaporous nuclei, Avhich sho\v various degrees of condensation, their 
 envelojjes, and tails, so diffused that stars are perceived through 
 them, and their small masses, show that this existence is possible and 
 real. The agglomeration, of whatever nature it be, which produces 
 the zodiacal light, also supports the hypothesis of nebulous matter. 
 
 Not long ago, however, in addition to the analogies in colour, 
 distribution, and above all, in physical connexion, which the Nebulae 
 present with stars, both single and united in couples, a new analogy 
 was discovered. We refer to the variability of their light, which, 
 paradoxical as it may seem, seemed to render any analogy between 
 them, as far as their physical constitution was concerned, impossible. 
 Of two Nebulae, both situated in the constellation Taurus, the first, 
 near a star of the tenth magnitude of variable brightness, presented 
 variations which appeared to correspond with those of the stars,* 
 and has since finally disappeared. The second Nebula, situated near 
 ^, after having gradually increased in brightness during more than 
 three months, f also disappeared. 
 
 Some analogous phenomena had been already recorded by Sir 
 W. Herschel. Two stars, surrounded with circular Nebulae in 1774, 
 presented no traces of these envelopes in 1811. Arago has described 
 another fact, bearing on the same kind of transformation ; " Lacaille," 
 he remarks, in a note to his Biography of Sir W. Herschel, " during 
 his stay at the Cape, saw in Argo five small stars in the middle of a 
 Nebula, of which Mr. Dunlop, with much better instruments, could 
 not see the slightest trace in 1825." 
 
 Lastly, as we have before seen, it is impossible to reconcile the 
 observations and the drawings of the Nebula of Orion, made by 
 many contemporary astronomers, without being obliged to admit 
 that it has undergone real changes in brightness and in the outlines 
 of its different regions. 
 
 The variability, the disappearance even, of a star, is explained 
 by the aid of more or less satisfactory hypotheses. This, however, 
 is not the case with a Nebula, if we admit that it is composed of 
 distinct stars. 
 
 * D' Arrest, Hind, Chacornac. 
 
 t Observed by M. Chacornac. The disappearance was not decidedly proved 
 for more than six years after the maximum of brilliancy. It would be interesting 
 to know whether a gradual decrease succeeded the phases of increase, or whether 
 the disappearance was sudden.
 
 PHYSICAL CONSTITUTION OF THE NEJJLL.E. 429 
 
 [Was then the hypothesis of a nebulous matter correct after 
 all, seeing that variations of brightness, progressive or even sudden 
 extinction of light, might be comprehensible in masses of this kind 't 
 
 This question, thanks again to spectrum-analysis, we can now 
 answer in a decided affirmative. 
 
 On August 29, 1864, Mr. Huggins, whose observations* on stellar 
 spectra we have before referred to, directed his telescope, armed 
 with the spectrum apparatus, to the planetary Nebula in Draco. At 
 first he suspected that some derangement of the instrument had taken 
 place, for no spectrum was seen, but only a short line of light 
 perpendicular to the direction of dispersion. He found that the 
 light of this Nebula, unlike any other ex- terrestrial light which had 
 yet been subjected to prismatic analysis, was not composed of light, 
 of different refrangibilities, as we saw that of the Sun and stars to be, 
 and it therefore could not form a spectrum. A great part of the 
 light from this Nebula is monochromatic and was seen in the spectro- 
 scope as a bright line. A more careful examination showed another 
 line narrower and much fainter, a little more refrangible than the 
 brightest line, and separated from it by a dark interval. Beyond 
 this again, at about three times the distance of the second line, a 
 third exceedingly faint line was seen. 
 
 The strongest line coincides in position with the brightest of the 
 air lines. This line is due to nitrogen, and occurs in the solar spec- 
 trum about midway between h and F. The faintest of the lines of 
 the Nebula coincides with the line of hydrogen corresponding to the 
 line F in the solar spectrum. The other bright line was a little less 
 refrangible than the strong line of barium. 
 
 Here, then, we have three little lines for ever disposing of the 
 notion that Nebulsc may be clusters of stars. How trumpet-tongued 
 does such a fact speak of the resources of modern science ! 
 
 An object-glass collects a beam of light w^hich for ever without 
 such aid would have bathed the Earth invisibly to mortal ej'e ; the 
 beam is passed through a prism, and in a moment we know that 
 we have no longer to do with glowing Suns enveloped in atmo- 
 spheres enforcing tribute from the rays which pass through them, 
 but with something deprived of an atmosphere, and that something 
 a glowing mass of gas. 
 
 Mr. Huggins has not been idle since his discovery, and has ob- 
 served a large number of nebulae with the most interesting results. 
 
 Not only must we discard the notion — a very pardonable one 
 
 [* Philosophical Transactions, 1864. Part II. p. 437.]
 
 4''30 THE SIDEREAL SYSTEM. 
 
 wlien we consider how it came to be held — that the glorious cluster 
 in Perseus, or that somewhat more typical one in Hercules, may be 
 taken as an exemplar of all our nebulao, could we bring sufficient 
 optical power to bear ujDon them : but the conclusion is obvious, that 
 the detection in a nebula of minute closely associated points of light, 
 which have hitherto been considered as a certain indication of a 
 stellar constitution, can no longer be accepted as a proof that the 
 object consists of true stars. These luminous points, in some nebulae 
 at least, must be regarded as themselves gaseous bodies, denser 
 portions, probably, of the great nebulous mass, since they exhibit a 
 constitution identical with the fainter and outlying parts which have 
 not been resolved. The nebulae are thus shown by the prism to 
 be enormous gaseous systems, and it appears probable that their 
 apparent permanence of general form is maintained by the continual 
 motions of the denser portions which the telescope reveals as lucid 
 points. 
 
 More than this, the proper motion of nebulae has not yet been 
 inquired into, because everybody, looking upon them as irresolvable 
 star-clusters, thought them infinitely remote. Noav, however, that 
 we know that they are not clusters of stars, properly so called, it is 
 possible that they may be much nearer to us than we imagine. 
 
 The conclusions to which Mr. Huggins has been led, by his obser- 
 vations, are, curiously enough, the very opposite to those which specu- 
 lation would have predicted. Speculation would have looked upon 
 nebulaB as sun-germs — as composed of the very matter of which 
 Faye has so recently stated the interior of our own Sun to be still 
 composed. The faint glimmer of one of those eloquent lines here, 
 three there, four elsewhere, a faint, continuous spectrum with 
 bright lines in one place, and a well-defined continuous spectrmn 
 in another, would, taking the relatively insignificant optical means 
 employed into consideration, have been held to bridge over the gap 
 between star and nebula as successfully as we have now bridged over 
 that which once separated sun and star.]
 
 BOOK THE THIRD. 
 
 STRDCTUEE OF THE HEAVENS. 
 
 I. 
 
 OUR OWN UNIVERSE— THE MILKY WAY. 
 
 Real Form of the Stellar Stratum which composes the Milky Way — Position of 
 the Sua in the Interior of this Stratum — General idta of its Dimensions. 
 
 As we have before stated, the Milky "Way extends across the heavens, 
 following nearly the circumference of a great circle of the starry sphere, 
 the irregularities of its form, and the inequalities of its breadth in 
 different portions, not being taken into account. It divides the 
 celestial vault into tAvo portions, not quite of the same extent, the 
 smaller of the two being that which contains the constellations of 
 Pisces, Cetus, in short, those near the vernal equinoxial point. It 
 follows, therefore, that the Milk}^ Way includes the region occupied 
 by our Sun. But what is the true form of this prodigious assemblage 
 of stars, which, according to Sir "W. Herschel's estimate, deduced 
 from a considerable number of " gauges " of the heavens, contains 
 certainly not less than 18,000,000 stars? The small breadth of the 
 zone, compared with its other dimensions, shows that it is formed of a 
 stratum of suns, distributed irregularly' and comprised between two 
 nearly parallel planes, which give the whole the figure of a flattened 
 millstone, the rim of which is split into two portions throughout one 
 half of its circumference. 
 
 It is nearly at the centre of this gigantic collection of stars,
 
 432 
 
 THE SIDEREAL SYSTEM. 
 
 about half way between its two surfaces, and near the region where 
 the separation of the zone into two strata occurs, that, lost in this 
 vortex of worlds, our little Solar System lies. The dimensions of the 
 centre of this system— the Sun, which appeared to us at first so great, 
 but which a second look at the stellar universe, showed to be those of 
 a star of the second or third order — are now found to represent but 
 an atom of the luminous sand of the Milky Way. 
 
 The position of the Sun in the zone explains the general aspect 
 of the whole firmament, and shows, besides, that all the stars so 
 universally and singidarly distributed, and apparently so distant from 
 those portions of the Milky AVay itself which give rise to the ap- 
 pearance, probably form a part of it. 
 
 Indeed, when from the point where we are situated, we look in 
 
 
 
 ■ 
 
 I 
 
 1 
 
 ■ 
 
 
 JSmma^mi 
 
 S 
 
 HB 
 
 
 
 1 
 
 
 
 1 
 
 
 ^t_-LlIl£l_'^ 
 
 @i 
 
 ^^^H 
 
 H 
 
 ^H 
 
 
 
 ■ 
 
 1 
 
 1 
 
 ■ 
 
 
 ^^^^^^^^ 
 
 1^ 
 
 ^^MH 
 
 
 
 m 
 
 1 
 
 1 
 
 1 
 
 
 H^B^^^^^^^P 
 
 m 
 
 Q 
 
 ^^^^^^^^ 
 
 ' 
 
 %^' ' 
 
 
 
 ' 
 
 
 
 1 
 
 1 
 
 1 
 
 1 
 
 1 
 
 
 ^^^^^^^^ 
 
 ' *-•>. '■■ 
 
 1 
 
 Fig. 175. — Suction of the Milky Way ; position of the Sun in it. 
 
 the direction of the length of the stellar stratum, we meet with, so 
 to speak, indefinite " files " of stars and clusters of stars, which give 
 to the Milky Way its maximimi density, and brightness. If, on the 
 contrary, the sight be allowed to travel in directions more and more 
 inclined, the visual ray traverses strata continually decreasing in 
 thickness, and the density should decrease with great rapidity. 
 Lastly, in the direction perpendicular to the thickness of the stratum, 
 the stars should appear dispersed, as they really do in those parts of 
 the heavens apparently most distant from the great nebulous zone. 
 " Just as we see," says Sir J. Herschcl, in his " Outlines of Astro- 
 nomy," " a slight haze in the atmosphere thickening into a decided 
 fog-bank near the horizon, by . the rapid increase of the mere 
 length of the visual ray." 
 
 Figure 175, which represents according to Herschel's hypothesis, 
 the Milkj^ Way, in a section perpendicular to its thickness, and
 
 OUR OWN UNIVERSE THE MILKY WAY. 4'^li 
 
 along its greatest diameter, which passes through the Sun, renders the 
 exj)lanation we have given easy. 
 
 With the help of this conception, we may again refer to the rapid 
 decrease in the number of stars in those regions which, on both 
 sides the Milky Way, extend as far as the two poles of the great 
 circle which the galaxy traces on the face of the heavens. 
 
 The poles of the Milky Way are situated, the north pole near Coma 
 Berenices, the south pole in the constellation of Cetus. When, from 
 one or other of these points, we advance progressively towards the 
 Milky Way, the mean number of the stars increases, at first very 
 slowly, then, in the vicinity of the galactic plane, with very great 
 rapidity, so that it is about thirty times greater in this plane than 
 in the galactic polar regions. 
 
 Until now we have but obtained a general idea of the form of 
 the Milky Way, and of the position which the Sun occupies in the 
 midst of it. To complete our account of what is known of its struc- 
 ture, we must here attempt to give some idea of its real dimensions. 
 
 Comparing the photometric brightness of the stars' of the difier- 
 ent orders of magnitude, with the order of probable distances. Sir 
 AV. Herschel arrived at the most astonishing conclusions on the 
 dimensions of the Milky Way. 
 
 The stars visible to the eye comj)rise, it is known, the first six 
 orders of magnitude. The illustrious astronomer of Slough estab- 
 lished that, in the mean, those of the sixth order, that is to say, the 
 smallest stars visible to the naked eye, are twelve times more distant 
 than the stars of the first magnitude. Starting thence, and calcu- 
 lating the space-penetrating power of his telescopes, he arrived at 
 this inference, that he coidd observe in the depths of the heavens 
 stars situated at a distance 2300 times greater than the mean dis- 
 tance of the stars of the first magnitude. And Herschel recognised, 
 moreover, that the visible extent of the Milky Way, in some regions, 
 increased with the power of the instrument brought to bear upon 
 it, and that even his large 40-feet telescope could not reach the 
 limits of this star- zone, which he therefore declared unfathomable. 
 
 Now, when we recall the stupendous distance between the nearest 
 star to oiu" world — such a distance, that light takes years to traverse 
 it — we shall recognise this wonderful fact, namely, that the Milky 
 Way in the direction of the most distant regions accessible to our 
 view, can only be completely traversed by a light-ray, in a period 
 of time upwards of ten thousand years. Thus, when, applying the 
 eye to the eye-piece of the largest astronomical instrmncnts, we ob- 
 serve, on the not qiute dark background of the sky, feeble luminous 
 
 F F
 
 434 THE SIDEREAL SYSTEM. 
 
 jjoints, we receive on our retina the impression of an undulatory 
 movement, which was set in motion ten thousand years ago, by the 
 incandescent mass of suns like ours, which form, as he does, part of 
 the same sidereal group. 
 
 Calculating the thickness of the Milky Way from its ajDparent 
 breadth, Ilerschel arrives at the result, that its thickness is about 
 eighty times greater than the distance of the stars of the first magni- 
 tude. Thus, the stellar stratum greatly surj)asses in this direction 
 even the sj)ace-penetrating power of the human eye. Whence it 
 follows, that, as we have before stated, " Not only our Sun, but all 
 the stars, that we can see with the naked eye, are deeply plunged in 
 the Milky Way, and form an integral portion of it."* 
 
 * Struve, " Etudes d'Astronomie Stellaire." 

 
 OTHER UNIVERSES. 435 
 
 11. 
 
 OTHER UNIVERSES. 
 
 " It is extremely probable," remarked Sir W. Herschel, in a memoir 
 in 1818, "that some of tbe nebulas of come tar y form, many of the 
 stellar nebulse, and a considerable number of nebulous stars, are 
 merely clusters of stars, banished in space to such depths that the 
 penetrating power of the telescope has not yet been able to resolve 
 them." 
 
 This opinion, we have seen, has now become a certainty, thanks 
 to the power of oui' modern instruments. 
 
 The stellar clusters and nebulae are, then, the most distant of 
 celestial objects which the eye can reach ; the accumulation, in 
 a small space, of a multitude of luminous points, allows them only 
 to be distinguished as a whole. The astronomer whose words we 
 have quoted estimated the distance of the 75th cluster of Messier' s 
 catalogue at more than 700 times that of the stars of the first 
 magnitude. It is not visible to the naked eye, but it would become 
 so if its distance were reduced to a quarter. If we suppose it re- 
 moved to five times its actual distance, that is to say, to 3500 times 
 the distance of Sirius, the large Herschelian telescope of 40-feet 
 focus would still show it, but only an irresolvable nebula. It is, 
 then, extremely probable that, among the many nebulae, indecom- 
 posable into stars, beyond the Milky Way, in the depths of the 
 heavens, many are as distant as that of which we speak. Doubtless, 
 many are still more so. Now, to reach us, light-rays must have 
 left stars, situated at such a distance, more than 700,000 years 
 ago. When we reflect on the immensity of such a time, which em- 
 braces thousands of centuries, and on the extraordinary velocity of the 
 luminous movement in the bosom of the ether, thought is utteily 
 confounded in the contemplation of such abysses, the extent of which 
 measures, not indeed the dimensions of the Heavens — thev are
 
 436 THE SlUEllEAL SYSTEM. 
 
 inexpressible — but those portions of them wbicb surround us, and 
 of wliicli Astronomy has studied the structure. 
 
 We can now represent the Heavens, even in their majestic whole. 
 In the depths of limitless space, exist numerous assemblages of stars, 
 like so many archipelagoes in an infinite ocean. Each of these 
 Universes is itself formed of a multitude of clusters, in which the 
 suns are o-rouped like so many systems, the condensation of which 
 is more decided than in the structure generally. 
 
 Suns are the individuals of these associations of worlds. But 
 here again is found the tendency to form groups ; and double and 
 multiple stars present to us simple systems of two or three suns 
 aravitatino: one roamd the other. 
 
 Here, then, would end what we could know of the structure of 
 the Universes, if we did not ourselves form part of one of the most 
 simple of these solar systems ; if the study of the planetary system, 
 and of its varied organization, did not teach us what part each of these 
 millions of celestial bodies may play in journepng through space, 
 continually radiating afar their rays of heat and light. 
 
 Each of these elementary groups may itself be subdivided into 
 smaller groups, — into systems of bodies which gravitate round a 
 central body, presenting the wonderful spectacle of a system in min- 
 iature. Who knows besides what the study of each of the suns 
 which people the expanse might reveal, if it were given to us to 
 penetrate into the sphere of their action, and to observe the phe- 
 nomena of which this sphere is the scene ? But, if imagination has 
 a right to form conjectures on this subject, it is not so with science; 
 the severe methods of which reject, without condemning them, hy- 
 potheses not based on facts and observation, and inferences drawn 
 from facts b}^ rigorous reasoning. 
 
 Here ends the purely descriptive part of our task, the object of 
 which has been to give a picture of the phenomena of the Heavens 
 according to actual astronomical knowledge. We may be mistaken, 
 but we hope that more than one reader will wish to penetrate deeper 
 still, and will not be sorry to comprehend, as much as it is possible 
 without previous scientific preparation, the laws which regulate 
 celestial movements, and explain the most complex phenomena. 
 These laws are at once simple and sublime, and are an 
 eternal honour to their discoverers, and a monument of the power of 
 the human mind.
 
 PART THE THIRD. 
 
 THE LAWS OF ASTRONOMY. 
 
 METHODS AND INSTRUMENTS EMPLOYED BY 
 ASTRONOMERS.
 
 BOOK THE FIRST. 
 
 THE LAWS OF ASTRONOMY, 
 
 The marvellous panorama presented by tlie visible Universe has now 
 passed before our eyes ; the solar system, eacb component of wliicli 
 we have explored, has shown us in detail what kind of bodies are 
 those suns with which infinite space is strewn ; and what may be 
 the conditions of those other bodies, not self-luminous, which circulate 
 round them, what their motions, dimensions, and physical consti- 
 tution. The sidereal world has in turn revealed to us its mag- 
 nificence, in its groupings and gigantic assemblages of suns, and we 
 have been enabled to form an idea of the structure and unutterable 
 dimensions of the Universe, or rather of that part of it rendered 
 accessible by the telescope. 
 
 Were we to stop here, therefore, the object we had in view in 
 this description of the physical constitution of the Heavens would 
 have been accomplished as far as the limits of this volume would 
 permit. The results of modern investigations have been passed under 
 review, and we have dwelt upon them sufiiciently to indicate the 
 interest and importance which attach to them. 
 
 And yet, hitherto, we have left in the background, or trenched 
 on but lightly, that part of the subject which makes astronomy, 
 regarded from an intellectual point of view, the most exact, the most 
 admirable, the most sublime of all the natural sciences. We refer 
 to the laws of the motions of the celestial bodies, and to the formulae, 
 so simple in appearance to us, which have demanded so much mental 
 labour, time, and genius for their discovery, — precious conquests of 
 the mind, which have enabled man to penetrate even into the very
 
 442 THE LAWS OF ASTRONOMY. 
 
 heart of celestial phenomena ; to discover their causes and relation- 
 ships, and permit us now-a-days to predict the return of the pheno- 
 mena and to calculate the variations with incomparable precision. 
 
 Thanks to these laws, the motions of the celestial bodies, their 
 distances, dimensions, and even weights, have been traced, calculated, 
 and valued. The relative positions of the bodies of the solar system 
 — planets and satellites, and even comets — positions so extremely 
 variable, influenced by so many causes, can be assigned long before- 
 hand, and thus furnish to the other sciences, and even to practical 
 men — our sailors, to wit, — most important data. 
 
 It is not in a popular treatise on the physical phenomena of the 
 Heavens, such as the present, that an account of these laws, rigorous 
 in its treatment, will be expected ; to give this, it would be necessary 
 to call to our aid the mathematical sciences, the language of which, 
 though so clear to those who have made it their special study, is, 
 nevertheless, purely enigmatical to the uninitiated. 
 
 But it must not be imagined from this remark, that it is im- 
 possible for our readers to gain an idea of these laws, or that they 
 are confined in a sanctuary where the vulgar can enter not. To 
 those whom a rigorous mathematical demonstration would avail 
 nothing, a clear and well - defined exposition, and apt, if even 
 familiar, comparisons, are often sufficient to enable the mind to 
 see the law and the drift of the method. And for such who love to 
 render a reason, and to take nothing upon trust, this present Third 
 Part is written. 
 
 The laws of planetary motion, as announced by Kepler, and of 
 gravity discovered by Galileo, and extended by Newton to the 
 heavens ; the secondary phenomena which result from these fund- 
 amental laws, such as planetary perturbations and the tides ; the 
 magnificent hypothesis, by means of which Laplace has explained 
 the origin and formation of our system, will occupy the First Book. 
 These will be followed by an account of the methods which have 
 been employed by our philosophers to measure the distances of the 
 Moon, Sun, and Stars ; and we hope our account will enable those who 
 still suspect the possibility of such measures, to be convinced of the 
 solidity of the methods employed. Lastlj', we shall bring the volimie 
 to a close by a description of the principal instrimients employed by 
 astronomers, and of one of those edifices, held by some to be shrouded 
 in mystery, where, in the silent night, so many men, devoted to 
 science, have explored and still explore the depths of heaven.
 
 KEPLER S LAWS, 443 
 
 L 
 
 KEPLER'S LAWS. 
 
 The Planets describe Ellipses round the Sun — Law of Areas — Connexion 
 between^ ^the time of Eevolution and the Mean Distances of the Planets 
 from the Sun. 
 
 Copernicus, by his diacovery of the movement of the Earth and 
 Planets round the Sun, laid the foundations of modern astronomy. 
 Galileo strengthened the building by basing the system upon new 
 proofs. But the real form of the Earth's orbit, and that of the other 
 planets, and the velocity with which they moved in the various 
 portions of those orbits, and their relative distances from the central 
 body, remained still unknown for some time, although the deter- 
 mination of these problems was indispensable for the future progress 
 of the science. Eor this, however, we had not long to wait. Thanks 
 to the genius and the perseverance of Kepler, in less than a century 
 these different problems were completely solved. Taking, as the 
 basis of his researches, the observations of his master, Tycho Brahe, 
 this great man, after seventeen years of unflagging toil, discovered 
 three laws to which posterity has attached his name. We will now 
 endeavour to give an idea of these laws, which will complete what 
 we have written on the Solar System. 
 
 We know that a planet in moving round the Sun describes a 
 continuous curving line, each point of which Kes in an ideal plane, 
 which passes through the centre of the Sun. Such an orbit is named 
 in geometry " a plane curve." Now, what is the form of this curve, 
 and what is the exact position occupied by the Sun in this plane ? 
 Kepler's first law answers these two questions. 
 
 The orbit of each planet is an oval curve — an ellipse. How, then, 
 can we regularly define an ellipse ?
 
 444 
 
 THE LAWS OF ASTROXO]MY. 
 
 or 
 
 Take a thread, the extremities of which are attached to two nails 
 pins; press these nails, or pins, into a sheet of paper, or a 
 board, or any plain surface on which the curve in question may be 
 traced ; but take care that the thread is longer than the distance 
 between the two fixed points. This done, by the aid of a pencil 
 stretch the thread till it is tight, in such a manner that the point of 
 the pencil can travel over the surface destined to receive its trace. 
 Then let the pencil move along the thread, the latter being 
 always tightly stretched, and the point will trace part of a curve, 
 which can be easily completed by afterwards placing the thread 
 and the pencil on the other side of the line which joins the fixed 
 
 Fig. 176. 
 
 points. Fig. 176 shows how this may be done, and gives also the 
 form of the curve obtained. 
 
 Such is the line which in geometry is termed an ellipse. 
 
 The two points, at which the extremities of the thread are fixed, 
 have received the name of/oei, and the two portions of thread, Avhich 
 connect these foci with each point of the ellipse, are called the radii 
 reafores of this point.* 
 
 It is easy to see that this curve is elongated in the direction of 
 the line which joins the foci. The line a a is called the mqfor axis 
 of the ellipse, the middle point of the major axis is the centre of 
 the curve. 
 
 * As the length of the thread remains constant, the sum of the 7-adu vcctores 
 is the same for all points of the ellipse. This property serves to define this 
 curve.
 
 Kepler's laws, 445 
 
 If, still retfiining the same foci, we describe otlier ellipses with 
 shorter threads, we shall obtain figures more elongated. The con- 
 trary will happen if we use threads of greater length. In the 
 latter case, the ellipses will gradually approach the form of the 
 circle ; they will never, however, absolutely reach the circular form. 
 
 Lastly, if with a thread of the same length we increase or decrease 
 the distance between the foci, the same difierences of form will be 
 obtained. In this case, the length of the major axis will remain 
 the same, but the more the foci are separated, the more oval will 
 become the curve ; contrariwise, the nearer they are together the 
 nearer the figure wiU resemble a circle, finally becoming one when 
 the foci are situated in a single point. 
 
 We shall now be able to understand Kepler's first law. 
 
 Each planet describes round the Sim an orbit of elliptic form, and 
 the centre of the Sun always occupies one of the foci. 
 
 We have already seen, that the dimensions of the orbits described 
 by the planets difier among themselves, and that the ellipticity of 
 these orbits is far from being the same for all. Some orbits are nearly 
 circular, as for instance, those of the Earth, of !f^eptime, and especially 
 of Venus. Others are more elongated in shape ; those of Mercury and 
 of the Asteroids which lie between Jupiter and Mars. Lastly, the 
 comets of our system have the most elongated orbits, and, among 
 them, that of Halley's is the most decided. 
 
 It evidently follows, from Kepler's first law, that the distance 
 of a planet from the Sun varies continually during its revolution, 
 and takes all possible values between the extreme limits, which cor- 
 respond to the two* positions occupied by the planet at the two extre- 
 mities of the major axis of the orbit. 
 
 Is, then, the velocity of a planet's motion always the same in the 
 difierent parts of its orbit ? No. The movement is by so much the 
 more rapid, as the planet is nearer the Sim. Kepler's second law 
 shows us how this velocity varies. 
 
 Let us take a planet in difierent positions in its orbit, and let us 
 mark off" on the orbit arcs described by a planet in the same time, 
 
 We have said that the velocity varies ; this evidently is the same 
 as saying, that the paths described in equal times are of unequal 
 length ; so that the difficulty consists in finding some connexion 
 between these constant variations in length. Let us insert the 
 planet's radii rectores, in each of the positions chosen ; we shall by 
 these means form as many triangles as there are arcs under 
 consideration. Now, the surfaces or areas of these triangles, of
 
 446 
 
 THE LAWS OF ASTRONOMY. 
 
 which the bases are formed by the arcs described in equal times, 
 are always equal. And therefore, if the length of time be doubled, 
 tripled, &c., the areas of the triangles will be doubled, tripled, &c. 
 
 Kepler, therefore, thus announced his second law. 
 
 The areas, described or ^xmed over by the radii vector es of a planet 
 round the solar focus, are proportionate to the time taken in describing 
 
 them. 
 
 Now, it clearly foUows from this second law, that the arcs 
 described in equal times are smaller as the planet recedes from the 
 Sun, and become greater as the Sun is approached. The triangles 
 gain in breadth what they lose in length, and their areas remain 
 constant. In other words, the planet moves faster the nearer it is 
 to the Sun. 
 
 Kepler's first two laws apply not only to the orbits of the planets, 
 
 Fig. 17^ 
 
 but to those of their satellites. Thus, the cui'ves described by the 
 Moon round the Earth considered fixed, is an ellipse, and our globe 
 occupies one of the foci. More than this, the velocity of our satellite 
 is such, that, if we divide its orbit into lengths passed over in equal 
 times, all the triangles formed by the radii vectores of the Moon in 
 its difierent positions will have a surface of similar extent. 
 
 "We now come to Kepler's third law, that which cost him much 
 more labour. More abstract than the first two, though equally 
 simple in its enunciation, it is of the last importance for a proper 
 comprehension of the subject, and merits in every way our attention. 
 
 The first two laws deal with each planet considered by itself, 
 
 * In planetary orbits one focus only is considered — that in which the Sun is 
 placed. In each position of the planet, therefore, there is but a single radius 
 vector.
 
 KEPLER S LAWS. 
 
 447 
 
 and would hold good if the system were only composed of two 
 bodies, the Sun and a planet. The third law estabUshes a relationship 
 between every planet in the system. 
 
 "We must here again call attention to the fundamental fact, that 
 the mean distances of the different planets from the Sun contin- 
 ually increase from Mercury to Neptune, and the same thing holds 
 good for their revolutions round the Sun. But what relationship 
 exists between the length of these periods and the distances, or, in 
 other words, between the periods of revolution and the major axes of 
 -the orbits ? Such is the problem resolved by the third discovery 
 of Tycho Brahe's disciple. 
 
 We will write, in two separate columns, the periods of the revo- 
 lutions of the principal planets, mean days, and double their mean 
 distances from the Sun in thousandths of double the mean distance 
 of the Earth, as follows : — 
 
 Mercury 
 
 Periods of Revolution. ] 
 Days. 
 
 87-97 
 
 Jouble mean Distances from the 
 Sun, or Major Axes. 
 
 387-1 
 
 Venus . 
 
 224-70 
 
 723-3 
 
 The Earth 
 
 365-26 
 
 1000-0 
 
 Mars . 
 
 686-98 
 
 1523-7 
 
 Jupiter 
 Saturn . 
 
 4332-58 
 10759-22 
 
 5202-8 
 9538-8 
 
 Uranus 
 
 30686-82 
 
 19182-7 
 
 Neptune 
 
 60126-72 
 
 30040-0 
 
 Let now the periods of revolution be multiplied by themselves. 
 To multiply a niimber by itself, is to form what is called its square. 
 This first very simple operation will then give the square of the period 
 of the revolutions of the planets ; this will form another new column. 
 Let us pass to the second ; midtiply each number which represents 
 the major axis of the orbits by itself, this will give the squares of 
 these axes. Now multiply each of these squares, not by itself, but 
 by the figures in the column which represent the major axis, this 
 will give the cubes of the major axis, and we shall have a second new 
 column. This done, let us compare two squares in the first column, 
 and two corresponding cubes in the second one. Divide one square 
 by the other, this will give us their ratio ; divide in like manner 
 the two cubes, and let us compare the quotients.* 
 
 We shall find them equal. And this will happen whichever two 
 
 * Let us take Venus and Jupiter for examples. The squares of the times 
 are for Venus 50490-0900, and for Jupiter 18771249-4564, The cubes of the 
 major axis are, for Venus 378,391,648, for Jupiter, 140,835,258,325. Divide 
 one square by the other, the quotient is 372. Divide one cube by the other, the
 
 448 THE LAWS OF ASTRONOMY. 
 
 planets we take. Kepler's third law, therefore, is enunciated as 
 follows : — 
 
 The squares of the times of revolution of the planets round the Sun 
 are proportional to the cubes of their major axes. 
 
 Thus, we need only know the time of revolution of the planets, 
 to deduce their major axes, and, as a consequence, their mean distances 
 from the Sim. And if we know the absolute value of one of these, 
 we know the absolute value of all. Thus, the knowledge of the 
 relatice distances of the different bodies of the system depends 
 upon the knowledge of one only, that of the Earth, for example. 
 Further on, we shall endeavour to give an idea of the method which 
 enables us to investigate, how many radii of the Earth, or how many 
 miles, bridge over the distance which separates us from the centre of 
 our system. We may also add, that Kepler's third law applies to 
 the satellites of any given planet ; that is, it has been found to hold 
 in the cases of the satellites of Jupiter, Saturn, and Neptune. 
 
 quotient is still 372. These quotients would change if we took other planets 
 for examples, but they would still be equal to each other, and it is this equality 
 which forms the subject of Kepler's third law.
 
 UNIVERSAL GRAVITATION. 449 
 
 11. 
 
 UNIVERSAL GRAVITATION. 
 
 Gravity on the Surface of the Earth — Law of the Diminution of the Force of 
 Gravity with increased Distance — The Fall of the Moon towards the Earth — 
 Gravitation is universal — How the Sun and Planets are weighed. 
 
 Everything visible and tangible, or, more strictly, every tbing exist- 
 ing in a solid, liquid, or gaseous state, witb wbicb we are acquainted 
 on our planet, is subjected to the law of gravity, or, in other words, 
 has weight. What, then, do such expressions as "weighty," or 
 "heavy bodies," and "weight" mean? This, namely, that every 
 portion of matter left to itself, either in the atmosphere or in vacuo, 
 falls in the direction of the vertical of the place on which it falls. 
 That if the body be sustained and remains in equilibrium, or in repose 
 on a surface, it still exercises a force — a pressure on whatever hin- 
 ders it from falling lower, a force, of which it is easy to convince 
 ourselves by noting the effort made by the hand, when it forms the 
 supporting surface.* 
 
 Experience proves that the direction of this force, known imder 
 the name of " gravity," lies always in a vertical line ; that is to say, 
 in a line perpendicular to the horizon, or to the surface of water at 
 rest. But as the Earth is sensibly spherical, the verticals of the 
 various places all tend towards the interior of the sphere, very nearly 
 to the actual centre itself. 
 
 We owe to Galileo the study of the laws of gravity ; those which 
 come into play in the fall of bodies on the surface of our globe. 
 Since the time of this great man, it has been discovered that gravity 
 is a force inherent to the matter even of which the terrestrial 
 
 * If the body in question be sustained by a spring, the constant tension of 
 this spring also affords evident proof of the constancy of the force of gravity. 
 
 G G
 
 450 THE LAWS OF ASTRONOMY. 
 
 globe is composed ; it is known, that the energy with which it is 
 exercised depends on the distance of the body which is influenced, 
 so that the energy increases when the distance diminishes, and de- 
 creases, on the contrary, when the distance augments. 
 
 For example, the flattening of the two poles of the terrestrial 
 globe, or, what amounts to the same thing, the swelling of the 
 spheroid towards the equatorial regions, causes the distance from the 
 surface to the centre of the globe to increase continually as the 
 equator is approached. It should therefore follow, that the attraction 
 of the Earth on heavy bodies is exercised with much greater in- 
 tensity at the poles than at the equator. This fact is abimdantly 
 proved by observation. 
 
 The law which regulates this diminution of the force of gravity, 
 when the distance of the heav}^ body from the centre of the Earth 
 increases, is as follows : — 
 
 To understand the law well in its simplicity, let us imagine a 
 heavy body placed on the surface of the Earth, and consequently 
 distant from the centre by the length of the Earth's radius, or in 
 round nmnbers 4000 miles. Let us place it twice, three times, four 
 times . . . ten times further away. The action of gravity on this 
 body will be four times less at 8000 miles — that is to say, at the 
 second position ; nine times less at the following position, sixteen 
 times, ... a hundred times less at the consecutive distances ; in 
 such a manner, that when the distances increase, following the num- 
 bers 1, 2, 3, 4, 5, . . . 10, &c., the force of gravity diminishes in the 
 proportion of the squares of these same nimibers, or becomes 
 1, 4, 9, 16, 25 . . . 100 times less, and so on. 
 
 The force of gravity is measured by the space fallen through 
 during the first second of the body's fall. So that, if experiment 
 shows that a body requires a second to fall from a height of sixteen 
 feet to the surface of the Earth, when it is removed to a distance 
 double that of the terrestrial radius, it will not travel more than four 
 inches during the first second of its fall ; at a distance sixty times 
 as great as the radius of the Earth, it would not fall more than the 
 •^^oth part of an inch. 
 
 This number gives precisely the measure of the diminution of 
 the energy of terrestrial gravity on a heavy body situate in sjDace 
 at the mean distance of the Moon. 
 
 If, then, the Earth exercises its action on bodies situated at what- 
 ever distances in space, it ought to act on the Moon, and its action 
 should be precisely equal to that which we have just calculated. 
 Such is the question which the genius of Newton put to him, and
 
 UNIVERSAL GRAVITATION. 451 
 
 which he solved, when he showed that the Moon, in moving in 
 its curvilinear orbit, falls towards our Earth that very quantity in 
 a second. It is this incessant fall, combined with the centrifugal 
 movement, which, if left to itself, would impel the Moon into space, 
 which produces the elliptical movement of our satellite in her orbit. 
 
 Such is the bold generalization which served as a point of de- 
 parture to the great geometer whom we have just named. 
 
 He went further ; he penetrated more profoundly into the secret 
 of the sublime mechanics which rule the celestial bodies. He extended 
 to all the bodies of our solar system this law, which is sometimes called 
 "the law of attraction," but more correctl}^, "the law of gravitation." 
 
 Newton showed, that if the planets move round the Sun, describ- 
 ing elliptical curves, according to the laws the discovery of which 
 is due to Kepler, it is because that they are submitted to a con- 
 stant force, located, as it were, in the Sun, — a force the direction of 
 which is that of a radius vector, or a right line which joins the 
 planet and the common focus. He showed, also, that all the circum- 
 stances of the movements of the planets are well explained by 
 supposing that the force of gravitation is gravity itself, exercised 
 by the Sun on the planets in the inverse ratio of the squares of their 
 distances. 
 
 Thus, the same force, which precipitates on to the surface of the 
 Earth bodies abandoned to themselves, is that which maintains the 
 Moon in its orbit. It is a force of similar nature, exercised by the 
 preponderant body of the system — the Sun — which also maintains 
 the planets and the comets in their elliptical orbits, and prevents 
 them from losing themselves in space, following the impulse with 
 which they are animated, and thus breaking up our system. 
 
 By what series of reasonings, ideas, calculations, and verifica- 
 tions, Newton arrived at this great discovery, we cannot in this 
 place narrate. Nevertheless, it is as well to know, that Kepler's 
 second law relative to the equality of the areas formed the start- 
 point for his demonstration of the tendency of the unknown force to 
 act towards the Sun ; as he found that it necessarily acted in the 
 direction of a radius vector. 
 
 The third law of Kepler, combined with the second, led Newton 
 to another inference, namely, that the force varies in the inverse 
 ratio of the distances. Lastly, he showed that the elliptical form of 
 the planetary orbits follows from the very law of the variation of the 
 force in question. The nature of the substances, of which the various 
 planets are composed, is quite independent of the mode of action 
 of gravity, so that the mass of the Sun wovdd act with an equal
 
 452 THE LAWS OF ASTROXOMY. 
 
 energy on a unit of tlie mass of all the planets, if they were all 
 placed at the same distance from the common centre. 
 
 But as, by virtue of a universal principle of mechanics, every 
 action of one material body on another necessarily supposes a re- 
 action, that is to say, an action equal and in a contrary direction, 
 it follows, that if the Earth and the other bodies of the solar system 
 gravitate towards the Sun, the Sun also gravitates towards each of 
 them. The same laws rule in each secondary world, composed of a 
 central j)lanet and its satellite. 
 
 Modern investigations in the field of sidereal astronomy have 
 extended these laws to the systems composed of two or many suns, 
 and the force thus shown to be diffused everywhere in space, has 
 taken the legitimate name of unk-ersal gravitafion ; " all the mole- 
 cules of matter gravitate towards each other in the ratio of 
 their masses, and reciprocally as the squares of their mutual dis- 
 tances." 
 
 We will here terminate these considerations — which will be con- 
 sidered abstract ones perhaps, but which it is impossible to pass by 
 in silence in an astronomical work — with a word on one of the truths 
 of that science which is so daring, not to say venturesome, in its 
 attacks upon nature. We refer to the statistics given in astronomical 
 treatises on the mass or weight of the difierent celestial bodies. Is 
 it possible to know the weight of a star, — of the Sun, for instance ? 
 
 We must first well understand what that means. This is not a 
 question of minute quantities ; and if we have expressed in billions of 
 tons the weight of the Sun, it has been for the purpose of placing in 
 relief the immensity of the Sun's mass, or that of the other members 
 of the system. 
 
 Astronomers take a unit of mass or of weight in connexion with 
 the quantities which they would measure. They take for the purpose 
 of comparison, either the mass of the Sim or the mass of our globe. 
 So that the question is in some measure transformed into an- 
 other : — 
 
 How many times is the mass of the Sun greater than the mass of 
 the Earth ? 
 
 If it were possible to place our globe and the Sun successively in 
 presence of the same bod}-, and then to measure the force with which 
 each of the two bodies would act on the third at the same distance, 
 the problem would be solved. For example, we shoidd determine 
 the space travelled by the body in a second of time towards the 
 Earth, then the space travelled in the same time by the body towards 
 the Sun. These two distances, expressed in numbers by means of the
 
 UNIVERSAL GRAVITATION. 453 
 
 same imit, would evidently give the ratio of the masses of the Sun 
 aud of the Earth respectively. 
 
 Well, at the surface of our globe, experiment tells us that a heavy 
 body traverses during the first second of its fall sixteen feet ; * and, 
 as, according to Newton's theory, the attraction of a sphere acts 
 on external bodies as if the entire mass of the sphere were con- 
 centrated at its centre, we can and must consider the heavy body 
 falling on the surface of the terrestrial globe, as situated at a dis- 
 tance from the centre of attraction equal to the radius of the Earth. 
 Let us bear this in mind. 
 
 The mass of the Earth, then, acting on a body situated at a dis- 
 tance of 4000 miles, causes it to fall 16 feet in one second. On 
 the other hand, the Earth itself gravitates towards the Sun ; the 
 orbit which it thus describes in a year shows how much it falls 
 towards the Sun during the first second of fall. The distance is 
 found to be "0099 feet. But we must bring this measure of the 
 attractive energy of the Sun to what it would be at a distance from 
 its centre equal to 4000 miles, or to the terrestrial radius, a distance 
 23,984 times smaller than the Sun's actual distance. 
 
 The law, by which Newton found that the intensity of gravitation 
 varies, indicates that the preceding number must be midtiplied by 
 the square of 23,984. Efiecting this operation, this second result is 
 arrived at : 
 
 The mass of the Sun, acting on a body situated at a distance of 
 4000 miles from its centre, causes it to travel, in the first second, 
 6,708,763 feet, or 1075 mHes. 
 
 We can now compare the mass of the Sun with that of the Earth, 
 since we know the actions of these two masses, on a body situated 
 at the same distance from theii' centres ; and it is clear, that the mass 
 of the Sun is by so much greater than that of the Earth, as the 
 number 5,708,763 is greater than 16. Dividing, we find in roimd 
 numbers 355,000. 
 
 We must have, then, 355,000 globes of the same weight as ours 
 to balance the Sun. 
 
 To solve this problem, it has been necessary to know the velocity 
 of fall of a heavy body on to the planet. This element is directly 
 observable on the surface of the Earth. In planets which have 
 
 * [This, of course, varies at different distances from the equator, for a reason 
 we have akeady stated. The distance is more correctly as follows :— 
 
 16 feet 0|^ inches at the Equator. 
 
 16 „ 1 „ at London. 
 
 16 „ li „ at Spitzbergen.]
 
 454 THE LAWS OF ASTRONOMY. 
 
 satellites, this velocity is deduced from the movements of these 
 secondciiy bodies in their orbits. In the case of the planets without 
 satellites, it is not possible to calculate in this manner the force of 
 gravity on them. But by studying the influence of their masses on 
 the other planets, and the perturbations which they cause in their 
 movements, we have arrived at data equally precise in the case of 
 all the masses of the bodies of the solar world, compared either to the 
 mass of the Sun, or to that of our globe.
 
 PRECESSION OF EQUINOXES. 455 
 
 HI. 
 
 Precession of the Equinoxes — Nutation — Planetary Perturbation. 
 
 The rotation of tlie Eartli on its axis produces day ; its translation 
 round the Sun gives the year. But, in the same manner as we have 
 distinguished two kinds of day, the one sidereal, the invariable dura- 
 tion of which is due to the movement of rotation, the other solar, 
 which varies in length in the course of a terrestrial revolution, in the 
 same manner also astronomers distinguish two years — the tropical 
 and the sidereal. 
 
 If we consider the time, which elapses between two successive 
 passages of the centre of the Earth to the same equinox, the spring 
 equinox, for example, we have what is called the tropical year, the 
 length of which, expressed in mean days, is 365 "242264 days. If, 
 instead of thus defining the year, we take the time which the Earth 
 requires to return to the point of its orbit, in which the Sun appears 
 to coincide with the same point of the heavens — with the same star — 
 we have the sidereal year, the duration of which, expressed in mean 
 days, is 365'2563835 days. The sidereal year exceeds, then, the 
 tropical year by about 20 minutes 20 seconds. 
 
 Whence comes this difference, and how can we explain it by the 
 movement of the Earth in its orbit? Let us remember that the 
 equinox occurs when the plane of the terrestrial equator passes pre- 
 cisely through the centre of the Sun. If this plane remained in- 
 variably parallel to itself, its line of intersection with the plane of 
 the ecliptic would keep likewise the same parallelism ; and it would 
 be always at the same point of the orbit of the Earth that the suc- 
 cessive equinoxes would take place. There would not be, then, any 
 difierences between the length of the tropical and sidereal year. The 
 length of the latter being the greater, shows that the equinoxial 
 point has fallen back, so that the Earth arrives earlier at this point
 
 456 THE LAWS OF ASTRONOMY. 
 
 than it would have done if it had remained immovable. Hence 
 the name oi precession of the equinoxes given to this phenomenon. 
 
 What follows from this fact ? That when the Earth occupies the 
 same positions in its orbit year by year, the Sun corresponds with 
 stars more and more to the east, so that, little by Kttle, and pro- 
 gressively, the aspect of the constellations seen at the same seasons 
 is changed. 
 
 Let us analyse still more the phenomenon in question. To say that 
 the equinox falls back or retrogrades is the same as saying, that the 
 plane of the equator has varied in position ; and as the axis of the 
 Earth is always perpendicular to this plane, it follows that this axis 
 has not remained rigorously parallel to itself. "We know, indeed, that 
 it varies in direction, still, however, preserving the same angle with 
 the eclijDtic, in such a way as to describe an entire cone in an interval 
 of about 25,870 years ; so that at the end of this period, the equi- 
 nox, having accomplished an entire revolution on the terrestrial 
 orbit, returns to occupy its initial position. 
 
 The terrestrial axis, in executing this slow movement on the 
 sm-face of the starry vault, describes a complete circle. The celestial 
 poles, therefore, are incessantly variable, so that the fixity which we 
 ascribed to them in our description of the heavens is quite relative. 
 In fact, the northern pole, now quite near the Pole Star, is stiU 
 approaching it. This diminution of angular distance will continue 
 imtil the year 2120, when they will not be more than half a degree 
 apart. This epoch passed, the pole will recede from Polaris, will pass 
 from the Little Bear to Cejjheus, then over the borders of the Swan. 
 In 12,000 years, the bright star nearest to the north pole will be 
 Vega in Lyra, which will then play the part of Pole Star ; Canopus, 
 ia the southern sk}', will be equally foimd in the vicinity of the 
 other pole. 
 
 The phenomenon of the precession of the equinoxes, discovered two 
 thousand years ago by Hipparchus, has during the last century been 
 ascribed to its true cause, of which we will speak a word further on. 
 
 Let US now mention another movement of the axis of the Earth, 
 executed simidtaneously with that which we have just described. Its 
 period is much shorter, since it is only 18| years. 
 
 The conical movement of the axis of the Earth, which produces 
 the precession of the equinoxes, and which is efiected in about 26,000 
 years, changes progressively the dii"ection of this axis, without, how- 
 ever, modifying its incKnation to the j)lane of the ecliptic. In truth, 
 however, this inclination does vary by reason of another movement, 
 which causes the axis to osciQate dming each period of ISf years
 
 PRECESSIOK OF EQUINOXE!?. 457 
 
 arouiid the mean position it would occupy, were it influenced only by 
 the movement of precession. The name of nutation has been given 
 to this oscillation — this " nodding" of the axis of om' globe, which 
 gives rise to slight changes, sometimes greater, sometimes less, in the 
 obliquit}^ of the ecliptic* 
 
 All these movements, both those of rotation and translation round 
 the Sun, and those of nutation and precession, are efiected simulta- 
 neously by the Earth. The motion of our globe has often been com- 
 pared, and with justice, to that of a top which, while turning on itself 
 with great rapidity, and tracing on the surface which supports it a 
 line which may be likened to its orbit, undergoes also a balancing of 
 its axis of figure or rotation, analogous to the oscillations of the 
 Earth. There is this difference, that the various movements of the 
 the Earth are accomplished with mathematical regidarity, in periods 
 relatively very long, and according to laws which allow us each 
 instant to assign its true position in space. 
 
 Ha\dng described the phenomena, let us indicate briefly how they 
 are connected with the great law of the Solar System — with universal 
 gravitation. If the Earth were rigorously spherical, the direction of 
 its axis of rotation woidd remain always the same, and would preserve 
 indefinitely the parallelism of which we have before spoken. The 
 action of gravity of the other celestial bodies would not change this 
 direction, if we suppose, as observation shows, that the terrestrial 
 poles occupy an invariable position on the globe. But it is kno\\Ti 
 that the Earth is not a sphere, it is swollen at the Equator ; it is 
 like a perfect sphere, covered with padding, the thickness of which 
 decreases from the equator to the poles, giving rise to a section 
 resembling an ellipse. At the poles the thickness of the pad is 
 nil. 
 
 Now, it has been proved that the action of the mass of the Sun 
 on this " padding " is the cause of the continuous retrograde move- 
 ment of the equinoxial points, which produces a corresponding ad- 
 vance of the successive equinoxes. In the same manner, the action 
 of the mass of the Moon on the same padding produces an analogous, 
 but much more rapid action ; that of the nutation of the Earth. f 
 
 This is stiU another kind of influence which afiects the movement 
 
 * The maximum of these changes does not reach 10" of arc. 
 
 t We have ah'eady seen, that an astronomer of Alexandria, Hipparchus, first 
 discovered the precession of the equinoxes. It is to Bradley (1647), that the dis- 
 covery of nutation is due. Lastly, the glory of binding firmly these two phenomena 
 to the Newtonian theory of gi-avitation was reserved for D'Alembert. Laplace 
 has since perfected this beautiful hypothesis.
 
 458 THE LAWS OF ASTRONOMY. 
 
 of the Earth, and which is also a consequence of the law of gravi- 
 tation. This proceeds from the combined actions of the masses of 
 the other planets on the mass of our globe. As the actions of which 
 we speak are reciprocal, what we say of the Earth in this matter is 
 applicable to any other planet ; but to dAvell on such abstract and 
 complex considerations as these woidd be to go beyond the purpose 
 of this work. We will, therefore, confine ourselves to pointing out 
 its extreme importance. 
 
 Kepler's laws, Avhich we have announced and explained, and from 
 which Newton deduced the law of gravitation, are only rigorously 
 true when we consider a single planet and the Sun. But as the 
 masses of the other planets also act on this planet, each following the 
 general law, there follows a series of modifications which periodically 
 alter its movement. The inclination, the direction of the major axis, 
 the eccentricit}' of the orbit, are elements which especially Yarj, in a 
 manner changing at once the position and form of the orbit of the 
 planet. These alterations which, very far from contradicting the 
 law of gra^-itation, most brilliantly cenfirm it, are known in astro- 
 nomy under the name of " planetary perturbations." "We have referred 
 to their great importance, not only because they enable us to calcu- 
 late with precision the future position of the celestial bodies of our 
 system, but again, because they will serve — and the discovery of 
 Neptune is a proof of our remark — to complete the knowledge which 
 we possess of the Solar System. 
 
 In our next chapter we shall discuss the action of the combined 
 forces of the Sun and Moon on the liquid part of the surface of the 
 terrestrial globe, and we shall see manifested, in a manner visible to 
 all, and in extremely short periods, the forces, the perturbations, to 
 which we have referred.
 
 THE TIDES. 459 
 
 IV. 
 
 THE TIDES. 
 
 Pheuomeua of the Ebb and Flow, High and Low-water — Epochs of Spring-tides 
 — Coincidence of the Plieuomena with the Positions of the Moon and Sun — 
 Theory of the Tides deduced from the Law of Gravitation — Combined 
 Actions of the Sun and Moon. 
 
 If we were to compare the sea to au iiimieuse being which lives, 
 moves, and breathes, it is iu the tempest we should see its anger, 
 and in calms its sleeping hoiu's, whilst the periodical movements of 
 the tides woidd typify its regidar and constant respiration. But 
 these are poetical fancies on which we do not care to insist. These 
 great phenomena of nature offer an interest so real that they re- 
 quire no more embellishment. The true explanation of the tides, 
 moreover, the connexion of the cause which produces them with the 
 great theory of universal gravitation, are quite recent conquests of 
 science. It is scarcely a century since they were first submitted to 
 calculation. They still offer, therefore, to many the attraction of 
 novelty. 
 
 Every one knows that twice a-day, at an interval of about 12 
 hours and 25 minutes, the shores of the ocean present us with the 
 spectacle of the flow of the tide : the tide by degrees rises, gaining on 
 the beach, which it covers to a greater and greater height, and after 
 six hours swelling attains its maximum. It is a beautiful sight to 
 see the agitated waves which come with increasing fury, to beat 
 the pebbles and the foot of the rocky shore, throwing their salt spray 
 high into the air. 
 
 Scarcely is the instant of high-ivafer or flood- fide attained, than 
 the ^fJow or rise of the water ceases ; the descent commences, and the 
 ebb succeeds to the^^o^r. The sea then leaves the beach which it 
 covered, and by degrees re-descends to its point of departure ; we
 
 460 THE LAWS OF ASTRONOMY. 
 
 have, then, hw-water or ehh-tide. Then begins another rising tide, 
 followed by an ebb, and so on. 
 
 It must be understood, that the instant of low water is not at 
 the mid-interval which se]3arates two consecutive flood-tides, the flow 
 being of much shorter length than the ebb, or, in other words, the 
 sea takes longer to go down than to rise. This difference varies 
 according to the ports ; thus, it is 16 minutes only at Brest, and at 
 Havre 2 hours and 16 minutes. Such, in the main, is the phenomenon 
 of the tides. 
 
 If we were confined to the observation of this periodicity of the 
 movements of the sea, science would not have penetrated very pro- 
 foimdly into the mystery of their causes ; it could not predict, as it 
 now does correctly, the height of the tides at the different ports, and 
 the precise times of high water, and thus afford valuable information 
 to navigators. 
 
 Before commencing our explanation of the causes, we will con- 
 form om'selves to the natural course of science, and look more closely 
 into the facts. 
 
 Between two consecutive flood-tides we have, as we have stated, 12 
 hours and 25 minutes. It follows, therefore, that from one day to 
 another, high-water is 50 minutes behind. Thus, the daily jDeriod 
 of the phenomenon is exactly equal to the lunar day, the length of 
 which is also 24 hours and 50 minutes, on the average. In other 
 words, the successive retardations of high-water are presented by 
 the successive transits of the Moon over the meridian. If, then, we 
 note the hour of high water in a port, it will be easy to predict the 
 hour for another da3^ Sailors, profiting by this fact, make their 
 arrangements accordingly, as they require to enter or leave the port 
 on that day. 
 
 Let us also notice this ; 50 minutes of retardation in one day 
 produce in about 14 days and three-quarters a total retardation of 
 12 hours ; and a retardation of 24 hours, or one day, in 29 days 
 and a half; that is to say, in the period of a lunation. The hours 
 of the tides are, therefore, the same every 15 days, with this 
 difference, that the morning tide becomes the evening one, and re- 
 ciprocally. At the end of a lunar month the hour becomes iden- 
 tically the same. 
 
 The facts which we have already stated deal only with the times 
 of high-water and their variations. Let us now occupy ourselves 
 with the height of the tide. 
 
 This height is itself very variable for the same sea and the same 
 port ; but here again is presented a remarkable periodicity, which
 
 THE TIDES. 461 
 
 shows that the phenomenon is connected with the relative positions 
 of the Sun, Moon, and Earth. 
 
 Near the new and the fidl moon the flood-tide attains its maxi- 
 mmn, whilst the corresponding low-water descends to its lowest 
 jioint. These are the "spring-tides," or the " fides of f//e si/zijgics." 
 Their height, then, decreases more and more, to the time of the 
 first and last quarter of the Moon. We have, then, the ne(ip-tides 
 or tides of the quadratures. Then, starting from these two periods, 
 the height of the tide again increases till the next syzygies, that is, 
 until the Moon is again in conjunction or opposition. 
 
 But the highest, like the lowest tide, does not really fall on the 
 same day as the Imiar phase ; in every part of the ocean, there is a 
 difierence of 36 hours or a day and a half. It is, then, the third tide 
 which follows the full and the new Moon, which is the highest; 
 the lowest tide, which follows the quadratures, is also the third. 
 
 These remarkable coincidences between the times, the periods of 
 high-water, and the positions of the Moon and the Sun with respect 
 to the Earth, have given rise for some time to the supposition that the 
 cause of the phenomenon resides in these two bodies. " Causa" 
 says Pliny, " in Sole Lnndque." But of what nature is their influ- 
 ence ? This is a problem which it has been given to modern science 
 to solve. Descartes first dared to draw the veil and sovmd the mys- 
 tery ; and if this great philosopher did not succeed in his attempt, 
 it was on account of his preconceived ideas on the system of the 
 world. The honour still remains to him of having dared. But let 
 us pursue the study of facts. 
 
 The height of the tides again varies with the declinations of the 
 Moon and Sun, it is by so much greater as the two bodies are 
 nearer the equator. Twice a-year, towards the 21st of March and 
 the 22nd of September, the Sun is actually in the Equator. If, at 
 the same time, the Moon is near the same plane, the tides which 
 occur then are the highest of alL These are the equinoxial spring- 
 tides, because the Earth is then at the spring, or autiunnal equinox. 
 
 On the other hand, the smallest tides take place towards the 
 solstices ; if the Moon attains its smallest or its greatest meridional 
 height at the same time as the Sun. 
 
 Lastly, the distances of the Moon and Sun from the Earth have 
 also their influence on the height of the tides. Other things being 
 equal, the height of a tide is by so much greater as the two bodies 
 are nearer the Earth. Thus, the tides of the winter solstice are 
 higher than those of the summer one. 
 
 Such are the general circumstances which characterise the
 
 462 THE LAWS OF ASTRONOMY. 
 
 periodical movements of the sea. But it must not be forgotten, that 
 they are not the only ones ; the force and direction of the winds, the 
 configuration and direction of the coasts, the depth and extent of 
 the seas, — circumstances which depend upon position and time — are 
 so many multiplied influences which singularly complicate the tides. 
 Thus, every one is aware that isolated seas, like the Caspian, or those 
 of small extent, and commimicating with the ocean by narrow straits, 
 like the Black and Mediterranean Seas, have but imperceptible tides.* 
 The opposite coasts of the Atlantic, which face each other, west 
 and east, have very unequal tides. It is the same with the eastern 
 coasts of Asia, which have strong tides, whilst at the other side of 
 the Pacific, and in the Oceanic Archipelago, the flow, which is very 
 regular, attains but little height. 
 
 But sj)eaking only of the European ports, the intensity of the 
 phenomena is extremely variable, even in neighbouring places. Let 
 us take an example ; according to the calculations of the tides for 
 the year 1864,t the highest tide was that which followed a day and 
 a half after the full moon of the 15th September, a little before the 
 autumnal equinox ; it occurred on the 17th. 
 
 The calculated height of this tide was at Brest, 12 feet ; at 
 Granville, 23| ; 10^ feet at Cherbourg ; 14 feet at Havre. These 
 numbers, very difierent for the neighbouring ports, show only the 
 height above the mean level of the sea, that is, the level the water 
 would take if there were no tides. They must be doubled, if we wish 
 to have the height of the flood-tide, above the level of low- water, for 
 the same day. Thus, at the ports of Granville and St. Malo the 
 waters rose on the date mentioned to a total height of about 46 feet. 
 If the wind favom^s such a tide as this, and increases its violence and 
 its height, great disasters may be feared. 
 
 There is a vast difierence between the tides of the western coast 
 of Europe and those of the isles of the Southern Sea, which scarcely^ 
 rise to a height of 20 inches. But there are some still more ter- 
 rible ; and amongst them we will content ourselves with quoting 
 those of the Bay of Fundj^ in New Caledonia, which rise, it is said, 
 to a height of nearly 100 feet. 
 
 The cause of these difierences in height is greatly owing to local 
 circumstances. Thus, the ports of the Channel are subject to strong 
 tides, because the moment of the waters meets with an obstacle in 
 
 * According to the observatious of the able and regretted G. Ainic, who 
 studied the undidation of the tides at Algiers during two years, the height of 
 the luni-solar tide in that port is nearly 3| inches on the day of syzygies. 
 
 t By MM. Laugicr and Mathieu, Annuaire du Bureau des Longitudes.
 
 THE TIDES. 463 
 
 the narrowing of the coasts, and the further the gulf is penetrated, 
 the higher is the tide. 
 
 The tide is felt in great rivers, to a distance depending upon 
 their size and depth. At the moment of high-water, the waters of 
 the river flow back, re-ascending their course, but the transmission 
 of this river-tide is progressively retarded. 
 
 Hence follow the curious phenomena, known in France under 
 the names of mascaret and harre [such as the "bore" in the river 
 Severn.] 
 
 We will now speak of the causes of the tides. 
 
 It is the combined actions of the Moon and Sun on the liquid 
 mass with which our globe is three-quarters surrounded, which pro- 
 duce the alternate movements of the ebb and flow. 
 
 We have seen, that if two bodies, such as the Earth and the Moon, 
 are present, the molecules of both have a mutual tendency, known 
 under the name of " gravitation," the intensity of which varies di- 
 rectly as the masses, and inversely as the square of the distance. 
 Let us now see how this action is exercised by the Moon on the 
 liqiud molecules of a sea. 
 
 The Earth having the form of a spheroid, the liquid stratum 
 which covers it would have a form exactly similar, and continually 
 the same — except accidental variations due to meteorological causes 
 — if the Moon and Sun did not exist. 
 
 Let us consider the Moon at a given moment. Let us connect 
 its centre with the centre of the Earth by an ideal line ; this line 
 will meet the surface of the globe in two points diametrically oppo- 
 site. The one, nearest the Moon, will be the place on the Earth, at 
 which the Moon is in the zenith. The opposite point will have the 
 Moon at the nadir, every place of the Earth which has the same lati- 
 tude as the first, will see the Moon on the meridian at that instant. 
 
 The attraction of the Moon on the nearest liquid molecules partly 
 counterbalances the attraction of the Earth ; it lessens their gravity 
 in the vertical direction. The molecules, which their fluidity and 
 independence separate from the surface to the solid part of the 
 Earth, rise by virtue of that attraction. The same thing happens, 
 but in a less degree, with the neighbouring molecules in the hemi- 
 sphere turned towards the Moon, the attraction being slighter as 
 these molecules are situated further from the point which lies at the 
 summit of the hemisphere turned towards the Moon. 
 
 Hence it follows, that the liquid sheet, with which this hemi- 
 sphere is covered, is swelled up towards the Moon, and, instead of
 
 464 
 
 THE LAWS OF ASTRONOMY. 
 
 keeping its spherical form, takes — though not of course in exact 
 proportion — the form of an egg. There is high water at the top, low 
 water at every place which has the Moon on the horizon. If the 
 Earth had no movement of rotation, this tide would be permanent, 
 and the waters woidd thus remain in equilibrium, or at least would 
 follow the movement of revolution of the Moon ; the tides would 
 have no other periods than the limation. But the Earth in its 
 rotation presents all its surface to the Moon, so that the wave follows 
 the parallel which corresponds to the position of our satellite. 
 
 So far we have explained the high and low tide for the hemi- 
 sphere turned towards the Moon ; but how is it that the waters 
 are also swelled up at the same instant on the opposite hemi- 
 sphere ? 
 
 Fig ITS.— Attraction of the Moou on the waters of the sea. Simple lun.av tide. 
 
 This is easily accounted for. The limar attraction makes itself 
 felt on all the molecules which compose the Earth and sea ; but its 
 energy is much more slight, as these molecules are more distant. 
 If this action were exercised on every point with equal intensity, 
 there would follow a total displacement towards the Moon, but no 
 change of form. The inequality of attraction causes the most dis- 
 tant molecules to remain behind; their gravity towards the Enrth 
 is diminished, and aU the liquid strata on the hemisphere opposite 
 the Moon take precisely the same form as those that are in front. 
 [In a word, on one side, the water is puUed from the Earth, on the 
 other the Earth is pulled from the water.] 
 
 This problem, when submitted to mathematical analysis, indicates
 
 THE TIDES. 
 
 465 
 
 for tlie general form of the surface of the ocean, that of an ellipsoid, 
 swollen in the direction of the diameter of the Earth which when 
 prolonged passes through the Moon at every instant. 
 
 There is then high-watgr whenever the Moon transits either 
 the upper or lower meridian, 
 that is to say, every 12 hours 
 and 25 minutes, and low- 
 water every time that it is 
 at the horizon of a place ; that 
 is to say, at periods of equal 
 duration. 
 
 But it is not the Moon 
 alone which acts, there is also 
 a tide produced by the attrac- 
 tion of the Sun. The enor- 
 mous bulk of that body would 
 produce immense movements 
 of the waters, if its distance, 
 four hundred times greater 
 than that of the Moon, did 
 not counterbalance the attrac- 
 tive force due to its mass. 
 The solar tides, although 
 much smaller than the lunar 
 ones, sometimes increase them, 
 at others neutralize them. 
 
 They increase them when 
 the two bodies are on the 
 same line with the Earth, 
 which occurs at the syzygies 
 — at new and full moon (fig. 
 179). The actions of the two 
 bodies neutralise each other, 
 when the Moon is at a right 
 angle to the Sun, and, in that 
 case, the resulting tide is a 
 minimum (fig. 180). 
 
 Calculation shows that the hmi-solar action is much more intense 
 when the bodies are nearer the equator . hence, the great equi- 
 noxial tides. Lastly, the action varies in the inverse ratio of the 
 cubes of their distances, it is, therefore, clear that the tides are 
 higher when the Moon and Sun are nearer the Earth. 
 
 H H 
 
 Fig. 179.— Combiued action ot the Moon and the Sun ou 
 the waters of the sea. Luni-solar tide of the syzygies.
 
 466 
 
 THE LAWS OF ASTRONOMY. 
 
 
 Such in the main is the principle of the theory of the tides. 
 These daily and irresistible movements are subjected to immutable 
 laws ; they are, by reason of the density of the water of the sea, — 
 
 Fig. 180. — Neutralizing action of the Moon and Sun on the waters cf the sea. 
 Luni-solar tides of the quadratures. 
 
 a density inferior to that of the solid nucleus which that water 
 covers — confined within narrow limits. Natural laws suffice to " put 
 a curb on the fury of the waves."
 
 ORIGIN AXn FORMATION OF THE SOLAR SYSTEM. 467 
 
 V. 
 
 OEIGIN AND FORMATION OF THE SOLAR SYSTEM. 
 
 liiiplace's Hypothesis of the Origin and Formation of the Solar System — 
 Primitive Nebula— Luminous Nucleus — Formation of Planets and Satellites 
 — Direction of the Movements of Rotation and Revolution. 
 
 The hmnan mind seems so organised that it attaches itself with more 
 obstinacy and perseverance to the pursuit of those questions Avhich 
 it is impossible to solve than to those which are more accessible. 
 At the risk of a kind of intellectual rertigo, it loves to lean over the 
 cliffs of those abysses of thought, at the bottom of which lie in con- 
 fusion the solutions of so many grave problems, the origin and the 
 end of all things, the essence of the first cause, and many other 
 questions which are rather in the domain of metaphysics than of 
 science. 
 
 This tendency towards the abstract is, so to speak, irresistible. 
 It is not sufficient for us to fathom, with the telescope, the depths 
 of infinite space, where the eye sees succeeding each other without 
 end suns and clusters of suns ; Ave still Avish to know if this pro- 
 gression has an end, a limit. We cannot believe in nothing, and 
 our mind is lost in the contemplation of an indefinite chain of 
 being. 
 
 By a like curiosity, we attemjit to remount the course of time, 
 and to picture to ourselves the first origin of things. We almost 
 know what is the actual state of the Universe. The discovery of the 
 most general laws authorises us to predict the future state of the 
 celestial bodies, at least in our system. We, therefore, try to know 
 what it is Avhich has given them birth, and lacking the positive 
 knowledge, which it is so difficult to acquire in such matters, we 
 attach ourselves to the traditions which have been in vogue from the 
 first ages of humanity.
 
 468 THE LAWS OF ASTRONOMY. 
 
 Will there ever be any certain notions on this subject? We 
 know not. But we sliall not be sorry to know wbat are actually 
 the most probable conjectures deduced from those sciences which 
 deserve in the highest degree to be called jMsitive. 
 
 Geology teaches us that the Earth, at its origin, existed in a 
 fluid state. Formed from an immense agglomeration of gaseous 
 matter, endowed with an excessive temperature, condensed at its 
 centre, it has slowly cooled, then formed a liquid shell enveloped 
 with a high and thick atmosphere. Then, in consequence of the 
 gradual loss of heat, the superficial strata by degrees solidified, until 
 a certain state of general equilibrium has given it the dimensions 
 and form which it now possesses. 
 
 Among the many witnesses, which testify to this ancient history 
 of the Earth, there are two which still remain, and which we can 
 now question. These are, on the one hand, the increasing tem- 
 perature of the strata as we descend, which compels us to consider 
 the interior nucleus of the Earth as being still in an incandescent 
 state ; volcanic eruptions are an additional proof in support of this 
 hypothesis. On the other hand, in the form of the terrestrial globe, 
 in its flattening in the direction of its axis of rotation, and the 
 swelling out of the equatorial portion, lies the mechanical proof of 
 the fluid primitive state. 
 
 Such are the most certain data which we possess on the ancient 
 history of the Earth, the different evolutions of which can be fol- 
 lowed. It is not easy, however, to assign certain ej^ochs to the 
 various phases of this development, but, in such a case, probabilities 
 suffice, and all agree in gi"v^ng to our planet an age, the antiquity 
 of which is counted by some hundreds of thousands of years. ' 
 
 Is the Earth, then, the only planet of the solar system to which 
 we must assign such an origin ? Here precise data fail us, and 
 it is to analogy that we must appeal for an answer. We have said 
 that facts are wanting. We mistake ; there is one which is of great 
 weight ; it is the fact of a common flattening, which is certain in 
 Mars, Jupiter, and Saturn, and which the difficulty of measurement 
 only has prevented us from proving in the other planets of the Solar 
 System. It is then extremely probable that, at the origin, the whole 
 Solar System was formed from an agglomeration of matter in a 
 gaseous state, which by degrees was transformed into distinct 
 bodies, under the influence of a cooling going on during thousands 
 of centuries. We thus arrive at the hypothesis formulated by 
 one of the greatest sons of modern science, Laplace, who has thus 
 attempted to account for most of the phenomena of planetary astro-
 
 ORIGIN AND FORMATION OF THE SOLAR SYSTEM. 469 
 
 nomy. Wo shall try to describe in a few words this theory of the 
 origin of the bodies which compose our system. 
 
 If we go back in thought to an epoch distant from our own 
 by a considerable series of centuries, the whole Solar System, or, 
 more exactly, all the matter which now forms the different groups, 
 existed in a purely gaseous state, or, as it may be put, under the 
 form of an immense Nebula, extraordinarily diffused, presenting no 
 indication of condensation. In such a condition, the molecules of 
 the nebidosity were so distant one from the other, that the repidsive 
 force with which they are endowed entirely annulled the attractive 
 force by virtue of which, gravitating one round the other, they would 
 tend to form groups. But centuries elapsed ; the nebulosity by degrees 
 cooled by incessant radiation into space ; the action of the repulsive 
 force diminished, and that of attraction was exercised more and more ; 
 it condensed and formed one or many centres in various parts of 
 the nebulosity. 
 
 The solar Nebula ought then to have presented at last the 
 aspect of a luminous nucleus enveloped to a great distance by a kind 
 of gaseous atmosphere, in form nearly spherical. Such appear to us 
 in space the nebulous stars ; we have seen, indeed, that astronomers 
 consider these last systems as irreducible into stars, or as simple, 
 double, or multiple suns, surrounded with a real nebulosity, either 
 self-limunous, or illuminated by the central body. At this period 
 of its formation, the Sun existed alone, the planets and their satel- 
 lites remained undeveloped in the atmosphere. 
 
 But the entire mass was endowed with a movement of rotation, 
 which forced in the same direction either the molecules of the 
 nucleus, or those of the nebidosity. At a given moment, the limits 
 of this latter depended upon the distance at which the cen- 
 trifugal force due to rotation was in equilibrium with the central 
 force of gravitation. These limits changed, and approached the 
 centre, under the influence of a continual cooling, which induced 
 in consequence a diminution of volume in the Nebula. Hence, the 
 abandonment of a zone of condensed vapour, at the distance of the 
 first limits. 
 
 By degrees, the celestial atmosphere abandoned a series of zones 
 of vapour nearer and nearer the centre, all being nearly in the plane 
 of the general equator, that is to say, that in which, in consequence 
 of the velocity of the rotatory movement, the centrifugal force was 
 naturally preponderant.
 
 470 THE LAWS OF ASTRONOMY. 
 
 These are the zones which have given birth to the planets, or to 
 the groups of planets and asteroids. 
 
 For it to have been otherwise, for the zones detached from the 
 general nebulosity to have kept the form of rings concentric with the 
 Sun, there must have been a perfect equilibrium continuing to exist 
 between the dijBPerent molecules composing these rings. But, ac- 
 cording to Laplace's expression, the chances were greatly against 
 this. The rings di-\dded, and the most considerable debris, attaching 
 and incorporating the rest, again fonned centres or nebulous nuclei. 
 It is important to remark, that each of them must have been ani- 
 mated with two simultaneous movements, one of rotation round its 
 own centre, the other of translation round the common centre. 
 Moreover, as these two movements were but the continuation of the 
 general anterior movement, their direction remained the same as 
 that of the rotation of all the system, or of the solar nucleus. The 
 planets once formed, we can understand perfectly how these smaller 
 Nebulae, similar to the larger one, produced the birth of new bodies 
 gravitating and revolving round each of them ; such is the origin 
 of the satellites. 
 
 Laplace next explained why the satellites formed no more new 
 satellites, and why these secondary bodies present the same side 
 to the planet around which they gravitate ; it is that their small 
 distances giving to the attraction of their primary a preponderating 
 influence, the satellites themselves, when still in a fluid state, were 
 swollen up tide-like towards the planet ; and from their rotatory 
 movement followed a time of rotation nearly identical with that 
 of their movement of revolution. After a certain number of revolu- 
 tions, these periods become rigorously equal. 
 
 Such is, in a few words, the magnificent theory which Laplace 
 has presented to the scientific world, with a reserve which testifies 
 to the profound respect which this great genius accorded to the truths 
 demonstrated with all the rigour of science. It must be acknow- 
 ledged that it is in perfect accord with the laws of general mechanics, 
 and with the facts of both astronomical and physical observation. 
 Without extending this subject further, it is impossible not to be 
 struck with the agreement which the system of Saturn presents 
 with the concejDtion of the illustrious geometer ; Laplace insists with 
 reason on this point. 
 
 "The regular distribution of the mass of Saturn's rings around 
 its centre, and in the plane of its equator, follows naturally from 
 this hj^othesis, and without it it must rest without explanation;
 
 ORIGIN AND FORMATION OF THE SOLAR SYSTEM. 471 
 
 these rings * appear to me to be ever present proofs of the primitive 
 extension of the atmosphere of Saturn, and of its successive con- 
 tractions." 
 
 * Some very curious physical experiments, imagined by M . Plateau, account 
 in the most satisfactory way for the phenomena which we have just described ; 
 they appear to us well adapted to dissipate the obscurity, which a description of 
 such an abstract conception would naturally leave in the minds of some of our 
 readers. 
 
 These experiments consist essentially in freeing a fluid mass from the action 
 of gravity, in such a manner that all its parts may be merely acted upon by their 
 mutual attraction ; and in imparting afterwards to this mass a movement of 
 rotation more and more rapid. To do this, M. Plateau places a quantity of oil in a 
 glass vessel, filled with a mixture of water and alcohol, the lower strata of which 
 are less dense than the oil, whilst the upper strata are lighter. The mass of oil 
 descends in the mixture as far as the stratum of the same density, where it 
 remains, taking the form of a sphere. 
 
 In this state, the mass of oil is freed from the action of gravity, and the 
 form which it takes is due simply to the mutual attraction of its molecules. 
 
 Next, by the help of a metallic disk introduced with care into the sphere 
 of oil, and a stem which passes through its centre and communicates with 3, 
 handle, M. Plateau imparts to the system a progressive movement of rotation. 
 
 When this movement is slow, the sphere is transformed into a spheroid, 
 swelled at the Equator, flattened at the poles, under the action of the centrifugal 
 force, which developes the movement. The phenomenon accounts then per- 
 fectly for the form of the planets. 
 
 If the movement becomes more rapid, the flattening becomes more consider- 
 able ; the spheroid at last becomes indented at its poles, spreading out more and 
 more in the horizontal direction, until the oil, entirely leaving the disk, is formed 
 into a circular ring. At this moment, the phenomenon at once explains both 
 the zones detached at the origin of the solar mass, and the rings of Satui-n. 
 
 Lastly, if the rotatory movement, rendered more rapid, is continued with a disk 
 of a diameter sufficiently large, the centrifugal force, in driving the particles 
 of the surrounding medium towards the ring, soon separates it into several 
 isolated masses, which form themselves into individual spheres ; each of which 
 preserves for a certain time, a movement of rotation of its own in the same 
 direction as the ring. 
 
 This last phase of the phenomenon offers a striking analogy with that of the 
 formation of the centres of condensation which, on Laplace's hypothesis, are the 
 origin of the planets of our system.
 
 BOOK THE SECOND. 
 
 METHODS AND INSTRUMENTS EMPLOYED BY 
 ASTRONOMERS. 
 
 I. 
 
 CELESTIAL MEASUREMENTS. 
 
 General Idea of the Problem by which the Distance of Inaccessible Objects is 
 determined — Solution of this problem on the Earth's Surface — Distance of the 
 Earth from the Moon — Solar Parallax, Distance of the Sun from the Earth — 
 Stellar Parallax, Distance of the Stars. 
 
 We are now about to discuss one of those problems, the solution 
 of which leaves so many doubts, and gives rise to so much incre- 
 dulity in the minds of those unacquainted with mathematical science 
 and methods : we refer to the determination of the distances which 
 separate us from the various celestial bodies. 
 
 In enunciating the problem generally, we shall put in evidence 
 the essential difficulty, the cause of the incredulity to which we have 
 referred, and which we must attempt to remove. The problem is as 
 follows : — 
 
 To measure hij means of a conveniently chosen unit the distance of a 
 risible but inaccessible ^jo/;;^. 
 
 The difficulty lies in the circumstance that the object in question 
 is inaccessible. If we speak of measuring a line on the surface of 
 the Earth, the possibility of the operation is at once recognised. 
 Without being in the secret of the methods employed, — methods 
 often very long, very laborious, and very delicate, — it is assimilated 
 vaguely to direct measurement of a small distance by means of a chain
 
 474 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 or cord ; and no one makes a difficulty in admitting, errors excepted, 
 the results of surveys of tlie surface of ovir globe. 
 
 But how can we ever know the length of the straight line, which 
 joins the eye and an object situated in space, out of our reach — the 
 Sun or Moon, for instance? This is the question raised by most 
 persons, when they hear astronomers affirm that the Moon is 240,000 
 miles from the Earth. 
 
 We shall, therefore, show that this problem is one of no real 
 difficulty ; the necessary operations are, theoretically, very simple, and 
 
 Fig. ISl. — Measure of the distance of an inaccessible object. 
 
 it is in the practical carrying out of them, that the real difficulty — the 
 impossibility, where the thing is impossible — lies. 
 
 We will proceed from the known to the unknown, from the 
 simple to the complex, and we will commence with the problem of 
 the distance of an inaccessible point, situated on the Earth's surface. 
 We shall see that in the main the solution of this case is the same as 
 that of the most difficult ones, and applies equally to the determina- 
 tion of the distances of the heavenly bodies. 
 
 We will suppose ourselves in a level field or meadow. We see 
 on the horizon the top of a tower, from which we are separated by 
 some obstacle, such as a river. We want to know the distance of this
 
 CELESTIAL MEASUREMENTS. 475 
 
 tower from our stand-point without actually measuring or stepping 
 the distance, without, in fact, crossing the water. We shall proceed 
 as follows : — 
 
 At C, our standing-point, we plant a stick ; at B, in the meadow, 
 w^e plant another, at a distance which must not he too small compared 
 Avith the distance of the tower. We now measure accurately by 
 means of a Gunter's chain, or tape, the straight line which joins 
 B and C. Let it be, for example, 468*7 yards. 
 
 This is the Base Line of our operations. 
 
 Now, by means of a theodolite placed successively at C and B, we 
 observe the tower at each station, and the instrument will give us the 
 angle formed by the visual ray with our base-line, that is to say, the 
 angles at the base of the triangle ABC. What do we now know ? 
 First, the exact length of the line B C, measured on the ground itself, 
 directly, and secondly, two angles ; A C B, which we will suppose to 
 be equal to 80° 29', and ABC equal to 75°. We shall find these 
 data quite sufficient to lay down a similar triangle on paper, on any 
 scale that we may choose, in such a manner, that, by means of a 
 properly divided measure, we may read off the number of yards in 
 the side C A of the triangle. We shall find it 1085 yards, nearly. 
 
 The distance sought, therefore, has been found, and the problem 
 is solved. 
 
 The precision of the result will depend upon two things ; first, 
 the degree of exactness of the measurement of the base ; secondly, 
 that of the two angles. This double precision itself depends upon 
 the perfection of the measuring instruments and the skill of the 
 observer. Nor must we forget another important consideration. 
 The choice of the base, both as to its position and length, has a great 
 influence on the result. If it be too small relatively to the distance 
 measured, the form of the triangle is very elongated, and a small 
 error in the measure of either angle may cause a large error in the 
 result. In terrestrial measurements we can, of course, always choose 
 our base ; in celestial ones, on the contrary, this is not the case, and 
 a difficulty often practically occurs in this way, with which theory 
 has nothing to do. 
 
 We now arrive at the application of what we have said, and will 
 begin with the most simple case, that of the distance of the Moon. 
 
 Two astronomers arrange to observe in two difierent parts of the 
 globe. One chooses Dantzig, the other the Cape of Good Hope. 
 We wiU suppose the two stations for greater simplicity situated on 
 the same meridian, so that the time is the same at both stations at the 
 same absolute instant.
 
 476 
 
 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 They agree to observe the Moon simultaneously, that is, on the 
 same day (or night) at the same hour. These stations, A and B (fig. 
 182) being known, the difierence of the latitudes is known, this is 
 the angle A T B formed at the centre of the Earth by the verticals 
 of the two stations. 
 
 These are the data of the problem. What we have to find is the 
 length of the distance L T, or the straight line which joins the centre 
 of the Moon with the centre of the Earth, at the time the observation 
 is made. 
 
 The first observer, by the aid of a special instrument, measures 
 the angle Z A L, the zenith distance of the Moon's centre. The 
 second observer at the Cape does the same for the angle Z' B L. 
 This is all that need be done. We can now construct on paper a 
 figure similar to the four-sided figure, L A T B. The angle T is 
 
 Fig. 182. — Measure of the distance of the Moon. 
 
 known ; the lines Z A and Z B are two nearly equal radii of the 
 terrestrial sphere ; and the direction of the lines A L and B L is given 
 by the observations. AVhen once this four-sided figure is laid down, 
 we only have to connect the points T and L, and to find its length, 
 taking the Earth's radius as the unit of measurement.* 
 
 "VVe have thus found that the mean distance of the Moon is about 
 60 radii of the Earth. 
 
 We now pass on to the distance of the Sun, and of the divers 
 planets of the Solar System ; and we will commence by two remarks, 
 which will simpKfy oiu' subsequent explanation. 
 
 * In this example, as in the other, the graphic construction on paper — an 
 exceeding rough method — is not the one actually employed. The real solution 
 is accomplished by a more or less complicated, but sure, mathematical calcu- 
 lation. This calculation admits of a precision which is only limited by the 
 accuracy of the preliminary observations.
 
 CELESTIAL MEASUREMENTS. 477 
 
 If wc refer to the first general problem, of the distance of an 
 inaccessible object, we shall understand, if wc look at fig. 181, that 
 the accurate measure of the two angles at the base tells us at once 
 the angle at the apex ; or the angle formed by the two straight lines, 
 which join the tower and the extremities of the base. [As the sum 
 of the three angles in any triangle is a constant quantity = 2 right 
 angles.] 
 
 This angle is called the paraUax of the tower, and the object of 
 all the problems, dealing with celestial distances, is to determine the 
 amount of this parallax. Thus, for the distance of the Moon, what 
 we seek to know is the angle which the base A B (fig. 182) would 
 subtend at the centre of the Moon, or more generally, the angle sub- 
 tended by the diameter or radius of the Earth at the Moon. 
 
 In the case of the Sun, the problem may thus be stated. Under 
 what angle would the diameter of the Earth appear at the centre of 
 the Sun ? or, in astronomical language, A^^lat is the Sun's parallax ? 
 
 The second remark is as follows : — Kepler, by the discovery of 
 his laws, enabled us to determine not the absolute, but the relative 
 distances of the planets from the Sun. In such a manner that 
 although he was unable to express the absolute distance by means of a 
 conmion unit, in miles for example, the relative dimensions of the 
 orbits were so known, that he could say, for instance, that the mean 
 distance of Jupiter from the Sun was O-f^ that of the Earth, the dis- 
 tance of Venus from the Sun was -jVo'V ^^^^ ^^ ^^^® Earth, and so on. 
 
 So that, when once the distance of any one planet from the Sun 
 is determined, by Kepler's laws we can deduce those of all the rest.- 
 
 The planet whose distance we have therefore endeavoured to 
 determine is naturally the Earth. How then have we attempted 
 it? 
 
 [Here we at once approach a new step in measures, and touch 
 upon one of the noblest problems of Astronomy. We have seen how 
 it is possible to measure distances on the Earth ; and hoAV two 
 observers, iising their distance on the Earth as a base-line, can 
 determine the distance of the Moon. But the measure of the Moon's 
 distance in no way helps us to get at that of the Sun. The latter 
 is entirely a different operation, and on the correctness with which 
 we can accomplish it depend, " every measure in astronomy be3'ond 
 the Moon, the distance and dimensions of the Sun, and every 
 planet and satellite, and the distances of those stars whose paral- 
 laxes are approximately known."* 
 
 [* The AstrouoDier Royal in Monthly Notices, vol. xvii. p. 209.]
 
 478 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 The yaliie of the Sun's distance at present received has been 
 deduced from the transits of Yenus in 1761 and 1769, and as these 
 transits afford the most satisfactory means (although, as we have seen, 
 note page 2, there are others) of determining it, we will endeavour 
 to give an idea of the method employed. 
 
 We have seen that when Yenus crosses the Sun's disk, during its 
 transit, it appears as a round bkck spot. Let us suppose two obser- 
 vers placed at two different stations on the Earth, properly chosen for 
 observations of the phenomenon, one at a station A in the Northern 
 hemisphere, another at a station B in the Southern one. When 
 Yenus is exactly between the Sun and the Earth, the observer at 
 A will see her projected on the Sun at a certain point which we will 
 call Yo, the Southern observer at B will, from his lower station, see 
 the planet — which we will call Y — projected higher on the disk at a 
 
 Fig. 183 — Measure of the Sun's distance by the transit of Venus. 
 
 point which we will call Y,. Now, the angle which we require to 
 know, in order to determine the Sun's distance, is A Yj B, and the 
 proportion of the measured angle Y,, A Yj to the desired angle A Y, B 
 is as Yi Y to A Y, or as 72 : 28, very nearly. So that, clearly, all 
 depends upon finding the value of the angle Y^ A Yj. Now, how can 
 this be done ? • 
 
 If the distance betAveen the two stations is sufficiently great, 
 the planet will not appear to enter on the Sun's disk at the same 
 absolute moment at the two stations, and therefore the paths or 
 the * chords ' traversed, will be different. Speaking generally, the 
 chords Avill be of unequal length, so that the time of transit at 
 one station will be different from the time of transit at the other. 
 This difference will enable us to determine the difference in the 
 length of the chords described b}' the planet, and consequently their
 
 CELESTIAL MEASUREMENTS. 479 
 
 respective positions on the solar disk, and the amount of their 
 separation. Now, this separation is the angle Yj A Yo required. 
 Having this, we can compute the value of A Yj B, and infer from it 
 the Sun's distance. In fact, if A B were situated at the extremities of 
 a diameter of the Earth, we should know the angle which it would sub- 
 tend at the Sun ; in other words, we should know the Sun's parallax. 
 
 But this is on the supposition, that the Earth has no motion of 
 rotation ; let us introduce this consideration, and see not only how it 
 modifies the result, but also with what anxious foresight astronomers 
 prepare for such phenomena, and why it was requisite in 1769, and will 
 be again necessary in 1882, to go so far from home to observe them. 
 
 Let us take the transit of 1882 ; we already know the instant and 
 place (true perhaps to a second of time and arc) at which the planet 
 will enter and leave the solar disk— in other words, we know exactly 
 how the Earth will be hanging in space as seen from the Sun — how 
 much the south pole will be tipped up — how the axis will exactly lie 
 — and how the Earth will be situated at the moments of ingress 
 and egress. Now if we draw two planes cutting the centre of the 
 Earth, tangent to those parts of the Sun's limb at which the jjlanet 
 will enter and leave the solar disk, we shall recognize in a moment 
 that some parts of the Earth will see the planet enter the disk sooner 
 than others. Some parts, on the other hand, will see it leave the 
 disk later — in other words, according to the position of a jjlace with 
 reference to the plane of which we have spoken, both ingress and 
 egress will be accelerated or retarded as the case may be. 
 
 Now, if we can find a place where both the ingress will be ac- 
 celerated and the egress retarded, and another where the ingress is 
 retarded and the egress is accelerated, we shall get what we want, 
 the greatest difierence in the duration of the transit, — the greatest 
 difference in the length of the chord, of which we have before spoken. 
 
 " Selecting, then (we quote from a paper by Mr. Airy in the 
 " Monthly Notices "), the parts of the Earth at which the duration of 
 transit would be shortest, it is seen at once that in the seaboard of 
 the United States of America, the ingress is retarded by a quantity 
 represented by 0"95, and the egress is accelerated by a quantity 
 which, in the mean, is 0"83 nearly ; so that the whole shortening 
 is represented by 1*78 (the geometrical possible maximmn being 
 2'00). That locality, therefore, is very favouj-able. 
 
 " Selecting, secondly, the parts of the Earth at which the duration 
 of transit would be longest, it will be found that the choice is more 
 limited, and the practical difficulties rather greater. For the accel- 
 eration of ingress at 2^ Greenwich mean time, the obscrving-station
 
 480 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 ouo-ht to be on the right side of the diagram ; and for the retardation |:ti 
 
 Illaminated side 
 of Earth 
 at Jn^rejs 
 
 Fig. 184.— Transit of Veuus, 1SS2. 
 
 of egress at 8'' Greenwich mean time, it ought to be on the left side
 
 CELESTIAL MEASUREMENTS. 481 
 
 of the diagram. It is impossible to satisfy these conditions, except 
 by a station on the Antarctic Continent. From this and other con- 
 siderations, it has been found that the place must be in 7^ East 
 longitude nearly. Such a position can be found between Sabrina 
 Land and Repulse Bay. Here the whole lengthening of transit 
 would be rej)resented by 1-61 ; a very large amoimt (the geometrical 
 possible maximum being 2-00). Combining this with the observ- 
 ations at Bermuda, the whole difference of durations would be 
 represented by 3--il (the geometrical maximiun being 4*00). This 
 point near Sabrina Land is, in fact, the only one which is suitable 
 for the observation." 
 
 This method, we see, is somewhat more complicated than that by 
 which the Moon's distance, or that of the inaccessible object, was 
 determined ; but they all depend upon the same principle. 
 
 We have given, however, but the spirit of the method, omitting 
 all the difficulties met with in practice, and all the consequent com- 
 plications in the calculations.* 
 
 We must next endeavour to show by what methods we have been 
 able to determine the distances of the stars, situated beyond our 
 system, or at least of some of them. 
 
 The method of triangulation is still the one adopted. But the 
 base is no longer either the radius or diameter of the Earth. 
 Already we know that the angle under which our Earth would be 
 seen at the Sun is extremely small ; and it has requii^ed all the 
 precision of our modern astronomical data on the planetary move- 
 ments to obtain a positive result. But the distance of the stars is 
 so considerable, that it is useless to attempt to use a base on the 
 surface of the Earth. 
 
 It is, therefore, necessary to choose a base elsewhere, and also 
 some other unit of measiu'ement. Astronomers at once thought of the 
 distance which separates the Earth from the Sun, even before this 
 distance was determined ; so that the problem was to determine how 
 
 * The reader niay perhaps ask why the Sun's parallax is not determined 
 directly by a simple triangulation, as in the other problems we have noticed. 
 The reason is, that the base of the triangle, at its maximum, cannot exceed the 
 dimensions of the Earth's diameter. Now the distance of the Sun is so great 
 that, compared to this base, errors of observation would assume a considerable 
 importance, compared to the extremely small angle to be measured. [For in- 
 stance, an error, which in the case of the Moon would throw us out only 100 miles, 
 would in the case of the Sun amount to 16,000,000.] This difficulty, therefore, 
 has to be got over by utilising the transits of Venus— a method due to the 
 illustrious Halley, and the most effi€acioVtS of those at present adopted. 
 
 I I
 
 482 
 
 METHODS \yi> INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 manj^ times the Sun's distance was contained in the distance of any 
 given star. 
 
 Let us see in what manner we have been able to use this immense 
 base-line, which, as we know, contains some 24,000 terrestrial radii. 
 Let us again take a familiar comparison for an example, and imagine 
 an observer placed in the centre of an extensive plain. Before him, on 
 the horizon, is a tower, the top of which appears to be of a certain 
 height above the general level of the plain. Now, it is evident that 
 this ajjparent height depends upon the distance of the observer from 
 it, and that the height will increase as he approaches the object, 
 and decrease as he recedes from it. Let us glance at fig. 185. When 
 the observer is at B, the visual ray, B S, has caused the summit of 
 
 Fig. 1S5. — Apparent variation in the height of a tower at different distances. 
 
 the tower to stand out on the background of clouds at b. If he 
 move from B to A, approaching the tower, the new visual ray, A S, 
 will form a greater angle with the surface of the plain than the first, 
 and the top of the structure Avdl have gradually elevated itself from 
 /) to a. By how much ? By an angular quantity exactly equal to 
 that imder which an eye placed at S would see the base A B, that is 
 to say, the line the length of which measures the observer's dis- 
 placement. 
 
 Well, the horizontal plain represents to us the plane of the ter- 
 restrial orbit ; the summit of the tower is a star, the distance of which 
 we seek ; its height above the plain is what is called by astronomers 
 the star's latitude ; and the distance traversed, A B, will be that
 
 CELESTIAL MEASUREMENTS. 483 
 
 which the Earth accomplishes in six months, a distance of some 190 
 millions of miles. The displacement, h a, is, in fact, the parallax 
 of the star referred to. the diameter of the Earth's orbit; it is double 
 the parallax of the star if the radius of that orbit — the distance of 
 the Earth from the Sun — be taken as unit. 
 
 The question, therefore, consists in determining whether the 
 latitude of the star is sensibly augmented, when the Earth has 
 passed from the first to the second position, and the precise value of 
 this augmentation, if there be any. 
 
 A large number of most delicate observations at first showed no 
 appreciable variation in latitude ; in a word, it was impossible to 
 detect any change, even of a second of arc, in a star's place. So 
 that the visual angle under which form one of these stars a distance 
 of some 190,000,000 miles is seen, — is almost nil. 
 
 Now, in order that any given length, a yard for instance, 
 viewed in front, may be reduced so that it will subtend an angle of 
 one second only, it must be removed from our eye to 206,000 times 
 its own distance. 
 
 It results, therefore, from this, that the stars are removed from 
 us at least 206,000 times the distance of the Sun from the Earth — 
 206,000 times 190,000,000 miles. Let us imagine in space a 
 sphere, having the Earth for centre, and this tremendous distance 
 for radius ; it is perfectly certain that not a single star coidd lie 
 within it. 
 
 However interesting this first datum may be, it is only a negative 
 one. But astronomers were not discouraged. They increased the 
 perfection of their methods, and suggested a second still more deli- 
 cate than the first. "We will endeavour to give an idea of it. 
 
 Let us return to our observer. We will suppose that he has 
 not been able to detect any appreciable increase in the apparent 
 height of the tower, in consequence of the extreme smallness of 
 the displacement compared with the distance of the object observed. 
 Nevertheless, this increase, however small it may be, is a reality. 
 How then can he detect it ? In this manner. 
 
 Instead of only looking at the top of the tower, he will compare 
 its position with a neighbouring point — neighbouring at least in 
 appearance — and will then commence his approach. One of two 
 things must happen, either the two points are at the same distance 
 from the eye, or one is further ofi" than the other. 
 
 In the first case, the variation in the height \d\\ bo the same for 
 both, and the method will not succeed. In the second case, the top 
 of the tower rising higher than the poiut with which it is compared,
 
 484 METHODS AND INSTRUMENTS EMPLOYED BY ASTRONOMERS. 
 
 which we will suppose the more distant, their reciprocal distances 
 will vary. Now, on the one hand, it is much more easy to measure 
 the variation where it is confined within smalji limits, than where it 
 becomes relatively a considerable quantity. On the other hand, the 
 small apparent movements due to difierent causes, and the inevi- 
 table errors of observation and instrument, as they afiect in a like 
 manner both points observed, may all be neglected. Such is, shortly, 
 the second method employed by astronomers, the success of which 
 has enabled us to determine, with a great exactitude, our distance 
 from some of the stars. 
 
 Comparing with the greatest care, and for several years in suc- 
 cession, the apparent position of several couples of neighbouring 
 stars, [one of which, by virtue of its proper motion, we know to be 
 nearer to us than the other,] we have been able to determine the visual 
 angle, which the diameter of the Earth's orbit subtends at the 
 nearest. "We have already dealt with the results of this method of 
 observation. 
 
 Such are, in their most elementary form, the methods employed 
 by astronomers in measuring celestial distances. If, by means of the 
 foregoing explanations, we have been enabled to convince our readers 
 of the certainty of the results, and to dispel the doubts which some 
 among them may have entertained on the possibility of the solution 
 of this problem, our end is gained. But it must be fairly stated that, 
 if the spirit or principle of the methods be easy to comprehend, the 
 practical working out of them is extremely difficult ; all the resources 
 of the mathematical sciences, all the most precise astronomical know- 
 ledge, so patiently acciunidated during so many centuries, all the 
 precision of our measuring instruments, have been indispensable for 
 arriving at their exact solution. We have said nothing of the talent 
 of observation, the sagacity, and sometimes the genius, of the philo- 
 sophers who have employed them.
 
 ASTRONOMICAL INSTRUMENTS. 485 
 
 IL 
 
 ASTRONOMICAL INSTRUMENTS. 
 
 VISIT TO AN OBSERVATORY. 
 
 Instruments for obtaining Magnified Images of Celestial Objects — The Astro- 
 nomical Telescope — Newtonian, Herscbelian, Gregorian, and Silvered-Glass 
 Reflectors — Instruments used in Observatories : Transit Circle, Equatorial. 
 
 The surprise and admiration excited by a description of the mar- 
 vels which astronomers have discovered in the depths of the heavens 
 are always accompanied by a strong desire to see for oneself. 
 Hence arises a very pardonable curiosity to know more of the in- 
 struments, by means of which the circle of our knowledge of these 
 magnificent phenomena is being continually widened. Telescopes, 
 both refractors and reflectors, are eagerly sought after ; but those 
 most frequently met with are ordinarily so small, that when we 
 compare them with the large instruments now used in observatories 
 the sentiment of curiosity is rather over-excited than satisfied. 
 
 We have mentioned Observatories — Temples of the most sub- 
 lime of the sciences which in the eyes of the profane, that is, of 
 a large majority of the public, are looked upon as mysterious sanc- 
 tuaries where, in the silent night and away from the busy hum of 
 men, philosophers are in intimate communication with the innu- 
 merable worlds which people the Universe. How many there are 
 among us, — we speak of those interested in science, — anxious to 
 inspect one of them if even cursorily. In order, therefore, to do what 
 we can to satisfy this wish, we give in this last chapter a short 
 description of the principal instruments to be ftrund in them. 
 
 We may divide astronomical instruments into three distinct 
 classes : — 
 
 Those which serve to increase the power of the human eye, or, in
 
 486 
 
 ASTRONOMICAL INSTRUMENTS. 
 
 other words, to lessen distances. Such are Telescopes, divided into 
 refracting and reflecting telescopes, or, as they are called. Refractors 
 and Reflectors ; 
 
 Those which have for their object the measurement of angles, 
 and by means of which we determine the positions of the stars ; 
 Divided Circles and Micrometers are the principal instruments of this 
 class, and they are always used in conjunction with telescopes ; 
 
 Lastly, those which enable us to estimate time with all the 
 precision requisite in astronomical calculations [that is, to the tenth 
 part of a second] ; such are Astronomical Clocks and Chronometers. 
 
 "We must limit ourselves here to the first class, those which, by 
 giving us a magnified image of the object, bring it apparently nearer 
 to us, and thus assist our sight. By reason of its etj^mology 
 {rnXs, far, and cxo'TtiTv, to see), the term Telescope is applied to all 
 those instruments which fulfil this condition, whatever be their 
 construction. 
 
 In Refractors, the light is made to pass through a combination 
 
 Fig. ISG. — Theoretical Section of the Astronomical Telescope. 
 
 of lenses, called the Object-glass, and is refracted, or bent, to the 
 focus. In Heflectors, the rays are received on a mirror, or Specidum, 
 and are reflected to the focus. This is the fundamental distinction 
 between the two classes : in both the aerial image formed at the 
 focus is examined in the same manner. 
 
 The following description will enable us to xmderstand the spe- 
 cific character, which we have thus defined in such a general manner. 
 
 The Astronomical Telescope is a refractor foi'med, as we have seen, 
 of two systems of lenses, one held in position by a cylindrical tube ; 
 the one turned towards the object is termed the Object-glass, and on it 
 falls the beam of rays emitted by the object viewed ; this is grasj)ed 
 by the object-glass, tnd made to converge, at a certain distance 
 behind it, to a spot called the focus ; where it forms an image of the 
 object observed. This image, a b in the figure, is examined by the 
 aid of a magnifier, in precisel}-^ the same manner as a naturalist 
 
 f1
 
 THE TELESCOPE. 487 
 
 examines an insect or a plant. The eye, in looking at the image 
 of the object by means of this lens — which again may be and ge- 
 nerally is a combination of lenses — observes it magnified, and can 
 examine its details. Hence it is called an cije-piece. 
 
 Such is in principle the construction of an Astronomical Telescope. 
 It must be observed that it is not the object itself which is observed 
 by means of the eye-piece but its image, and it is the image alone 
 which is magnified. 
 
 [A word now as to the poicer of the telescope, and, first, as to its 
 Ulmninating iioicer. The aperture of the object-glass, that is to sa}', 
 its diameter, being larger than that of the pupil of our eye, its sur- 
 face can collect more rays than our pupil ; if this surface be a thousand 
 times greater than that of our pupil, it collects a thousand times 
 more light, and consequently the image which it forms at its focus 
 is a thousand times brighter than the image throTVT^i by the lens 
 of our eye on to our retina. 
 
 Having this image at the focus, the magnifying power of the tele- 
 scope comes into play. This, in the general opinion, is the most im- 
 portant element of power. It varies with the eye-piece employed, the 
 ratio of the focal length of the object-glass to that of the eye-piece 
 giving its exact amount. Bearing in mind that what an astronomer 
 wants is a good clear image of the object observed, we shall at once 
 recognise that magnifying power depends upon the perfection of the 
 image thrown by the object-glass and upon the illuminating power. 
 If the object-glass does not perform its part properly, a slight mag- 
 nification blurs the image, and the telescope is useless. Hence 
 manj^ large telescopes are inferior to much smaller ones in the 
 matter of magnifying jsower, although their illuminating power 
 is so much greater. Hence, again, the immeasurable superiority 
 of refractors over reflectors in this particular ; for, although by 
 virtue of their illuminating power they are admirably adapted for 
 observations of nebuloD, where the best definition of the image is 
 required, they are found sadly wanting. 
 
 There is another matter to be mentioned. In the case of stars, 
 — which, by reason of their immense distance appear as points, — no 
 increase in the size of the disk, except the one mentioned further 
 on, follows the application of higher magnifiers ; with planets this is 
 difierent ; each increase of power increases the size of the image, and 
 therefore decreases its brilKancy, as the light is spread over a larger 
 area. Hence the magnifying power of a good telescope is always 
 much higher for stars than for planets, although at the best, it is 
 always limited by the state of the air at the time of observation.]
 
 488 
 
 ASTRONOMICAL INSTRUMENTS. 
 
 If the material of which the object-glasses are composed is equally 
 pure, and their definition equally fine, those with the largest aper- 
 tures possess the greatest magnifying power. 
 
 To return to the object-glass. It is composed generally of two 
 lenses, in juxtaposition, or nearly so, one bi-convex, the crown, the 
 other, bi-concave, the flint. This combination is required to destroy 
 the chromatic aberration, which, without it, would surround the image 
 with a halo of coloured light, and destroy the purity of the image. 
 The eye-piece also is composed generally of two or more separate 
 lenses, the object of which is to reduce the distortion of the image 
 as seen through a single lens, and to increase the fleld of view. 
 
 In fig. 186 is given a section or interior view of an astronomical 
 telescope, similar to the one figured in fig. 187. 
 
 [Among the most remarkable and powerful refractors of the pre- 
 sent day, we may mention that at Chicago, of 18^-inches aperture, 
 the work of the celebrated American optician, Alvan Clark, and those 
 at Poulkowa and Cambridge, U.S., each of 15 inches aperture, from 
 
 Fig. 1S7.— Section of the Astrouomical Telescope. 
 
 the atelier of Merz, the successor of the celebrated Fraunhofer. 
 While we write, our English opticians, Messrs. Cooke and Sons, are 
 moimting an object-glass which they have just completed, of the 
 enormous aperture of 25 inches, which at one bound surpasses almost 
 our most sanguine hopes, and restores England to the place it held 
 in the optical art in the time of Dollond.] 
 
 We now come to the Reflector, which, as we have seen, differs 
 from the refractor in having a concave mirror to reflect light, in- 
 stead of an object-glass to refract it. The mirror requires to 
 be ground and polished with the most consummate care and skill 
 as its surface is not spherical, as are those of object-glasses, but 
 parabohc. 
 
 The arrangement of the mirror and the eye-piece is of course 
 different from that adopted in the refractor, as the mirror is opaque, 
 and its concavity must be turned towards the sky. We give three 
 sections of reflectors of different constructions as designed by their 
 inventors, Newton, Gregory, and Herschel.
 
 THE TELESCOPE. 
 
 489 
 
 In the first of these instruments (fig. 188), the luminous rays 
 after reflection from the principal speculum M, are again reflected 
 from a smaller one tii, inclined at an angle of 45°, in such a manner 
 as to throw the beam to the side of the _tube, to which is fixed 
 the eye-piece, which performs, as we have said, the functions of a 
 
 Fig. 188.— Newtonian Reflector. 
 
 magnifier. Thus, in Newton's construction, the observer is placed 
 sideways, at a right angle to the direction of the rays which enter 
 the telescope. 
 
 In the Gregorian form (fig. 189), the great specidimi is pierced 
 at its centre and the aperture holds a tube containing the eye-piece ; 
 the small mirror is placed in front of the large one, its reflecting 
 surface opposite to it and perfectly parallel. There is, therefore, 
 
 Fig. 189.— Gregorian Reflector. 
 
 a double reflection, as in the Newtonian form, but the eye of the 
 observer is directed to the object viewed. This double reflexion 
 naturally much enfeebles the light. 
 
 The " front-view" reflector, — Herschel's form (fig. 190), — has not 
 this disadvantage ; there is but one mirror M, inclined at the bottom 
 of the tube in such a manner as to throw the image to the lower edge 
 of the end of the tube turned towards the object. This arrangement
 
 490 
 
 ASTRONOMICAL IXSTRItMF.?>'TS. 
 
 is only good in the case of large specula, because the observer, who 
 is compelled to turn his back to the object observed, cuts off part 
 of the beam of light by his head. 
 
 In reflectors the loss of light by reflexion is much greater than 
 that caused by absorjDtion in refractors ; so that with equal apertures 
 the illuminating power, and therefore the magnif^dng power, of re- 
 flectors are very much less. 
 
 Some years ago, a skilful physicist, M. Leon Foucault, who is so 
 well known from his delicate experiments on the velocity of light 
 and his invention of the gyroscoj)e, suggested the construction of 
 glass mirrors, coated with an exceeding thin film of silver, chemi- 
 cally dejiosited, an arrangement which would much reduce the 
 price of telescopes and would render their polishing extremely easy. 
 We rei3roduce here (Plate XXXVIII) the magnificent instrument 
 he has constructed for the Observatory of Paris (it has subsequently 
 
 Fig. 190.— "Front View" Reflector. 
 
 been removed to Marseilles.) This reflector is constructed on the 
 Newtonian principle. 
 
 Among the remarkable reflectors at present in use, we must 
 mention that constructed by the Earl of Rosse, and erected at 
 Parsonstown, in Ireland. This colossal instrument is of 60 feet 
 focal length, and the mirror is 6 feet in aperture. We have seen 
 what good use the illustrious constructor of this instrumental mar- 
 vel has made of it in discovering new nebuloD, which had resisted 
 all feebler efforts. The instrument cost 12,000/. Plate XXXVII 
 gives, according to the Speculum IlartwelUanum, views both of the 
 telescope and of the structure which supports it and permits its 
 proper handling. 
 
 Nowadays, when the heavens are explored in all parts by tried 
 observers, provided with the most perfect instruments, it becomes 
 more and more difiicult to add to our knowledge of the physical 
 constitution of our system and of the other sidereal systems. While 
 the telescope was young, very small instruments were sufiicient
 
 PLATE XXXVIII. 
 
 GREAT SILVER ON GLASS REFLECTOR. 
 
 Constructed by M. Foucault.
 
 THE TELESCOPE. 493 
 
 to secure tlie most glorious discoveries. Galileo saw the satellites 
 of Jupiter by means of a telescope, which magnified seven times ; 
 and never used one which magnified more than thirty-two times. 
 Let us add, lest amateurs should be discouraged, that a small tele- 
 scope, of less than three inches apertiu-e, and with magnifying powers 
 varying from 60 to 300 times, is sufficient to enable one to pursue 
 useful investigations. M. Goldschmidt discovered fourteen minor 
 planets with a telescope such as we have described ; and he has 
 seen the satellite of Sirius with it also. 
 
 [We are glad to know that in England the number of medium- 
 sized refractors, by which so much good work has been done, is raj)idly 
 increasing. The illustrious Secchi, we think, has too hastily con- 
 demned small telescopes ; and — bearing in mind the double-star work 
 done by the Rev. W. R. Dawes with a small telescope, and the maps 
 .of the Moon and Mars we owe to the observations of Beer and Madler, 
 who used a smaller instrument still, — the increase in the use of even 
 small telescopes is a subject for much congratulation. 
 
 In this climate of ours, which by the way is not so bad, astro- 
 nomically speaking, as some Anglophobes would make it, a 6-inch 
 glass is doubtless the size which will be found the most constantly 
 useful ; larger aperture being frequently not only useless, but hurtful. 
 Still, 4| or 3| inches are apertures by all means to be encouraged ; 
 and by object-glasses of these sizes, made of course by the best 
 makers, views of the Sun, Moon, planets, and double stars, may be 
 obtained, sufficiently striking to set many seriously to work, as 
 amateur observers, and with a prospect of doing good, useful work. 
 
 Thus in the matter of double stars, a telescope of 2 inches aper- 
 ture, with powers varying from 60 to 100, will show the following 
 stars double : 
 
 Polaris. y Arietis. « Geminorum. 
 
 a, Piscium. ? Herculis. y Leonis, 
 
 //■ Draconis. i Ursse Majoris. i Cassiopeaj. 
 
 A 4-inch aperture, powers 80-120, reveals the duplicity of 
 
 /S Orionis. «■ Lyrte. § Geminorum. 
 
 £ Hydrse. % Ursse Majoris. «■ Cassiopece. 
 
 i Bootis. y Ceti. s Draconis. 
 t Leonis. 
 
 And a 6-inch, powers 240-300, 
 
 £ Arietis. /^ Ophiuchi. ' Equulei. 
 
 3 Cygni. 20 Draconis. ? Herculis. 
 
 32 Orionis. » Geminorum.
 
 494 ASTROXOMICAL INSTRUMENTS. 
 
 The testing- of a good glass refers to two different qualities 
 wliicli it shoidd possess. Its quality, as to material and the fineness 
 of its polish, should be such that the maximiun of light shall be 
 transmitted. Its quality, as to the curves, shoidd be such that the 
 rays passing through every part of its area shall converge absolutely 
 to the same point, with a chromatic aberration not absolutely nil, 
 but sufiicient to surroimd objects with a fiiint dark blue light. 
 
 The convenient altitude at which Orion cubuinates in these 
 latitudes, renders it particularly eligible for observation ; and during 
 the first months of the year, our readers who would test their 
 telescopes, will do well not to lose the opportunity of trying the 
 progressively difficult tests, both of illuminating and separating power, 
 afforded by its various double and multiple systems, which are col- 
 lected together in such a circumscribed region of the heavens, that 
 no extensive movement of their instruments — an important point in 
 extreme cases — will be necessary. 
 
 Beginning with ^, the upper of the three stars which form the 
 belt, the two components will be visible in almost any instrum^ent 
 which may be used for seeing them, being of the second and seventh 
 magnitudes, and well separated. The companion to /3, though of 
 the same magnitude as that to h, is much more difiicult to observe, 
 in consequence of its proximity to its bright primary, a first magni- 
 tude star. Quaint old Kitchener, in his work on telescopes, mentions 
 that the companion to Eigel has been seen wdth an object-glass of 
 2|-inch aperture ; it shoidd be seen, at all events, with a 3-inch. 
 ^, the bottom star in the belt, is a capital test both of the dividing 
 and space-penetrating powers, as the two bright stars of the second 
 and sixth magnitudes, of A^'hich the close double is composed, are but 
 just over 21" apart, according to Secchi's last measurements. The 
 small star below, which the late Admiral Smyth, in his charming book, 
 '* The Celestial Cycle," mentions as a test for his object-glass of 5*9 
 inches in diameter, is now plainly to be seen in a 3f . The colours of 
 this pair have been variously stated, Struve dubbing the sixth mag- 
 nitude — which, by the way, was missed altogether by Sir John 
 Herschel — " olivaceasubrubicunda." 
 
 That either our modern opticians contrive to admit more light 
 by means of a superior polish imparted to the surfaces of the object- 
 glass, or that the stars themselves are becoming brighter, is again 
 evidenced by the point of light, preceding one of the brightest stars 
 in the system, composing e. This little twinkler is now always to 
 be seen in a 3|-inch, while the same authority we have before quoted 
 — Admiral Smyth — speaks of it as being of very difficult vision in
 
 THE TELESCOPE. 495 
 
 liis instrument of inucli larger dimensions. In this very beautiful 
 compound system, there are no less than seven principal stars ; and 
 there are several other faint ones in the field. The upper very faint 
 companion of X is a delicate test for a 3| inch, which aperture, how- 
 ever, will readily divide the closer double of the principal stars, which 
 are about 5" apart. 
 
 These objects, with the exception of ^, have been given more to 
 test the space-penetrating than the dividing power ; the telescope's 
 action on 52 Orionis will at once decide this latter quality. This 
 star, just visible to the naked eye on a fine night, to the right of a 
 line joining a and B, is a very close double. The components, of the 
 sixth magnitude, are separated by less than two seconds of arc, and 
 the glass which shows a good wide black dmsion between them, free 
 from all stray light, the spurious disks being perfectly roimd, and not 
 too large, is by no means to be despised. 
 
 The " spurious disk," which a fixed star presents, as seen in the 
 telescope, is an effect which results from the passage of the light 
 through the object-glass ; and it is this appearance which necessitates 
 the use of the largest apertures in the observation of close double 
 stars, as the size of the star's disk varies, roughly speaking, in the 
 inverse ratio of the aperture of the object-glass. 
 
 Then, again, we have a capital test object in the great " Fish- 
 mouth " Nebula, by far the most glorious of its class in the Northern 
 hemisphere, and surpassed only by that surrounding the variable 
 star J] Argus in the Southern. And although, of course, the beauty 
 and vastness of this stupendously remote nebula increase with the 
 increased power of the instrument brought to bear upon it, a large 
 aperture is not needed to render it a most impressive and awe- 
 inspiring object to the beholder. In an ordinary 5-foot achromatic, 
 many of its details are to be seen under favourable atmospheric 
 conditions. Those who are desirous of studying its appearance, as 
 seen in the most powerful telescopes, are referred to the plate in 
 Sir John Herschel's " Results of Astronomical Observations at the 
 Cape of Good Hope," in which all its features are admirably de- 
 lineated, and the positions of 150 stars, which surround & in the area 
 occupied by the Nebula, laid down. 
 
 This star to which we wish to call especial attention, is situate 
 (see fig. 139) opposite the bottom of the "fauces," the name given to 
 the indentation which gives rise to the appearance of the " fish's 
 mouth." This object, which, as we have seen, has been designated 
 the " trapezium," from the figure formed by its principal components, 
 consists, in fact, of six stars, the fifth and sixth {y and a'), being
 
 496 ASTRONOMICAL INSTRUMENTS. 
 
 excessively faint. Our previous remark, relative to tlie increased 
 brightness of the stars, applies here with great force ; for the fifth 
 escaped the gaze of the elder Herschel, armed with his powerful in- 
 struments, and was not discovered till 1826, by Struve, who, in his 
 turn, missed the sixth star, which, as well as the fifth, has been seen 
 in modern achromatics of such small size as to make all comparison 
 with the giant telescopes used by these astronomers ridiculous. 
 
 Sir John Herschel has rated y' and a! of the twelfth and fom'teenth 
 magnitudes — the latter requires a high power to observe it, by reason 
 of its proximity to a. Both these stars have been seen in an ordinary 
 5-foot achromatic, by Cooke, of 3|-inches aperture, a fact speaking 
 volumes for the perfection of surface and polish attained by our 
 modern opticians. 
 
 Observations should always be commenced -svith the lowest power, 
 gradually increasing it imtil the limit of the aperture, or of the 
 atmospheric condition at the time, is reached : the former being taken 
 as eqxial to the number of hundredths of inches which the diameter 
 of the object-glass contains. Thus, a 3|-inch object-glass, if really 
 good, should bear a power of 375 on double stars where light is no 
 object ; the planets, the Moon, &c., will be best observed with a much 
 lower power. 
 
 It is always more or less dangerous to look at the Sun directly 
 with a telescope of any apertm'e above two inches, as the dark 
 glasses, without which the observer would be at once blinded, are 
 apt to melt and crack from the concentrated heat. We must, how- 
 ever, except the cases in which a Dawes' solar eye-piece is employed, 
 its smaller field of view, and consequently reduced beam admitted to 
 the eye, obviate the objections attaching to direct Adsion. 
 
 A diagonal reflector, however, which reflects an extremely small 
 per-centage of light to the eye, and by reason of its prismatic form 
 refracts the rest away from the telescope, afibrds a very handy 
 method of solar observation. When this is used, it is possible even 
 to light a cigar at the focus, while the Sun is observed in the most 
 satisfactory manner by the rays intercepted by the reflector. 
 
 Care should be taken that the object-glass is properly adjusted. 
 This may be done by observing the image of a large star out of 
 focus. If the light be not equally distributed over the image, or the 
 difii-action rings are not circular, the screws of the cell should be 
 carefully loosened, and that part of the cell towards which the rings 
 are thro-svn very gently tapped -vsath wood, until perfectly equal illu- 
 mination is arrived at. This, however, should only be done in extreme 
 cases ; it is here especially desirable that we should let well alone.]
 
 K K
 
 f
 
 THE OBSERVATORY AT PARTS. 
 
 499 
 
 We will now describe some of the most important work carried 
 on in Observatories. One of the first rooms we enter, — we suppose 
 ourselves in the Paris Observatory, — is that in which the meridian 
 instruments are placed. Here at once our attention is riveted by- 
 telescopes, divided circles, and clocks, that is to say, by instruments 
 which amplify our vision, measure angles and positions, and mea- 
 sure and divide the time. 
 
 Three telescopes, of which one is fixed to the centre of a large 
 circle, attached to a wall at the end of the room, and of which the 
 nearest is the most modern and the most powerful, have all the 
 same allotted task, that of showing us with precision the moment 
 at which the stars pass through the plane of the meridian, and of 
 measuring their angular distances from the zenith, whence their 
 position with regard to the celestial equator can be deduced. 
 
 The first instrimient is styled a mural circle, the others are transit 
 instruments. 
 
 In all these instruments the telescoj)es are arranged to turn 
 freely on their axes, placed horizontally in a direction east and west, 
 or perpendicularly to the plane of the meridian. The axis of each 
 telescope, therefore, never leaves this plane, and as the daily movement 
 of the Earth brings by turns all the stars on the meridian, it is always 
 possible to observe the exact instant of the transit, or passage, of 
 one of them through this plane. To render this observation more 
 easy, the transit instruments are provided with a series of fine 
 spider webs placed at the focus ; these are shown 
 in fig. 191. The higher the power employed, 
 the more rapidly does the star move across the 
 field of view, and the more necessary does it 
 become to notice the exact instant at which the 
 star passes behind the difierent " wires," as they 
 are called, the error of observation being di- 
 minished by taking the mean of the times of the 
 transits across all of them. 
 
 We see in Plate XXXIX, by the side of the meridian instrmnents, 
 two astronomical clocks regidated to sidereal time and beating side- 
 real seconds. The noise of the beat is sufiicient to enable the 
 observer to follow the time, this he estimates to within a tenth of a 
 second, in such a manner as to know most exactly the instant of 
 transit when it takes place between two successive beats. 
 
 Visitors will scarcely care to regulate their watches by the time 
 kept by these clocks, which, as we have before said, is sidereal^ or 
 
 Fig. 191.
 
 500 ASTRONOMICAL IXSTRIMENTS. 
 
 star-time, reckoning throughout twenty-four hours, the transit of the 
 first point of Aries being the start-point. 
 
 The mural circle consists of a metallic circle divided into degrees 
 and fractions of a degree, and placed in the meridian. A movable 
 telescope attached to its centre allows us to observe a star at the 
 moment of transit ; the direction of the telescope shows the angular 
 distance between the actual jjosition of the star and the zenith ; 
 hence we can conclude its declination, the angular distance of the 
 star from the celestial equator. 
 
 The mural circle, therefore, serves also as a transit circle ; re- 
 ciprocally we now attach divided circles to transit instrimients by 
 means of which the zenith distance is measured. The magnificent 
 transits, both on the one model, with which botli Greenwich and 
 Paris are now endoAved, perform both these important functions. 
 
 ! From the transit-room we must now pass to the dome, under 
 which is placed the large equatorial of 15 inches aperture. 
 
 As the meridian instruments enable us to observe the stars onh' 
 for a few moments as they are passing the meridian, it becomes 
 necessary to have large telescopes to follow them through those 
 regions of the heavens through which they are carried by the diurnal 
 movement. 
 
 This desideratum is accomplished by means of the equatorial. 
 
 As we see on Phite XL, the telescope is fixed on an axis, on which 
 it can move up and doAim as it were, keeping the same right as- 
 cension ; this axis is attached to another, parallel to the axis of the 
 Earth ; and when this last axis is in motion, the telescope can change 
 its right ascension. It is, therefore, also free to move in every 
 direction by a combination of these two movements. The other 
 most important parts of this instrument are as follow : firstly, the 
 telescope ; then a di\ddcd circle, at right angles to the declination 
 axis, this measures the declination or the angle distance of a star 
 from the celestial pole a second circle at right angles to the polar 
 axis, and therefore in the plane of the equator, which measures 
 right ascension. 
 
 There is also a clock-work movement, which carries the instru- 
 ment in one flirection as fast as the diurnal movement of the Earth 
 is carrying it in the other. From this residts that, if the telescope 
 be directed toAvards a celestial object, such object can be kept in 
 the field of view for hours. Tliis affords a great facility for obser- 
 vations of the planetary disks, sun-spots, tlie heads of comets, &c. 
 
 The equatorial may also be used for determining celestial po-
 
 PLATE XL. 
 
 GREAT EQUATORIAL OF THE PARIS OBSERVATORY.
 
 I 
 
 i
 
 ASTRONOMICAL INSTKUMENTS. 503 
 
 sitioiis, the divided circles, whicli enable us to find stars and other 
 objects in the day time, being used for this pm-pose. 
 
 We would gladly have entered into some particulars of the way 
 in which the measurement of angles is accomplished, and of the 
 instruments used, of the precision at which astronomers have arrived, 
 thanks to the ingenious methods and the progress of mechanical and 
 optical art. We should then have referred to micrometers, divided 
 circles, heliometers and other instruments employed in observations. 
 But our description of the instruments used by astronomers is 
 already long. We must, therefore, refer to special treatises those 
 who are desirous of entering more into detail in these matters. 
 Our object will have been sufficiently gained if in exciting curi- 
 osity we have succeeded in giving the desire to study a science so 
 capable of elevating the mind, and of afibrding it the purest and 
 noblest enjoyments.
 
 APPENDIX. 
 
 BY THE EDITOR.
 
 T A B L E S. 
 
 I. ABBREVIATIONS AND SYMBOLS ...... r)07 
 
 IT. ELEMENTS OK THE PLANETS ...... .50R 
 
 IlL ELEMENTS OK THE SATELLITES ...... .509 
 
 IV. THE SUN . . . . . . . . . .510 
 
 "V". ADDITIONAL ELKMH;NTS OK THE MOON .... .510 
 
 VI. SATUKN's KINGS . . . . . . . .511 
 
 VII. TIME . . . . . . . . . .511 
 
 VIII. VAKIAHLE stars . . . . . . . .512 
 
 IX. CELESTIAL OBJECTS . . . . . . .515 
 
 N. TEST OBJECTS . . . . . . . . 52.J
 
 EXPLANATION OF ASTRONOMICAL SYMLOL!^ AND 
 ABBREVIATIONS. 
 
 
 
 ^Signs of f//e Zodiac. 
 
 
 
 
 (». 
 
 fyi Aries 
 
 
 \) 
 
 VI. 
 
 :C^ Libra . 
 
 
 180 
 
 I. 
 
 8 Taurus . 
 
 . 
 
 30 
 
 VII. 
 
 nX Scorpio 
 
 
 210 
 
 IL 
 
 n Gemini . 
 
 
 ()0 
 
 VIII. 
 
 f Sagittal 
 
 ius . 
 
 240 
 
 III. 
 
 SB Cancer . 
 
 
 90 
 
 IX. 
 
 yf Capricorn us . 
 
 270 
 
 IV. 
 
 SI Leo 
 
 
 120 
 
 X. 
 
 £K? Aquarius 
 
 300 
 
 V. 
 
 Ttji Virgo . 
 
 
 L50 
 
 XL 
 
 X Pisces . 
 
 
 330 
 
 
 
 
 The Sun. C The Moon. 
 
 
 
 
 
 Major Planets. 
 
 
 
 
 
 § Mercury. 
 
 
 $ Mars. 
 
 
 \§. Uranus. 
 
 
 
 5 Venus. 
 
 
 7|. Jupiter. 
 
 
 ^ Neptune. 
 
 
 © 01 
 
 • J The Earth. 
 
 
 \} Saturn. 
 
 
 
 
 
 
 
 
 Minor Planets. 
 
 
 
 © 
 
 Ceres. 
 
 © 
 
 Euphrosyne. 
 
 
 © 
 
 Danae. 
 
 
 © 
 
 Pallas. 
 
 ® 
 
 Pomona. 
 
 
 © 
 
 Erato. 
 
 
 © 
 
 Juno. 
 
 © 
 
 Polyhymnia. 
 
 
 © 
 
 Ausonia. 
 
 
 © 
 
 Vesta. 
 
 © 
 
 Circe. 
 
 
 © 
 
 Angelina. 
 
 
 © 
 
 Astra^a. 
 
 ® 
 
 Leucothea. 
 
 
 © 
 
 Maximiliaua 
 
 
 © 
 
 Hebe. 
 
 @ 
 
 Atalanta. 
 
 
 © 
 
 Maia. 
 
 
 © 
 
 Iris. 
 
 , @ 
 
 Fides. 
 
 
 © 
 
 Asia. 
 
 
 © 
 
 Flora. 
 
 © 
 
 Leda. 
 
 
 © 
 
 Leto. 
 
 
 © 
 
 Metis. 
 
 © 
 
 Lc^titia. 
 
 
 © 
 
 Hesperia. 
 
 
 © 
 
 Hygeia. 
 
 © 
 
 Harmonia. 
 
 
 © 
 
 Panopea. 
 
 
 © 
 
 Parthenope. 
 
 © 
 
 Daphne. 
 
 
 @ 
 
 Niobe. 
 
 
 (iB) 
 
 Victoria. 
 
 © 
 
 Isis. 
 
 
 © 
 
 Feronia. 
 
 
 © 
 
 Egeria. 
 
 © 
 
 Ariadne. 
 
 
 © 
 
 Clytie. 
 
 
 © 
 
 Irene. 
 
 © 
 
 Nysa. 
 
 
 © 
 
 Galatea 
 
 
 @ 
 
 Eunomia. 
 
 © 
 
 Eugenia. 
 
 
 © 
 
 Eurydice. 
 
 
 © 
 
 Psyche. ' 
 
 © 
 
 Hestia. 
 
 
 © 
 
 Freia. 
 
 
 © 
 
 Thetis. 
 
 © 
 
 Aglaia. 
 
 
 © 
 
 Friga. 
 
 
 ® 
 
 MeljDomene. 
 
 © 
 
 Doris. 
 
 
 © 
 
 Diana. 
 
 
 © 
 
 For tun a. 
 
 © 
 
 Pales. 
 
 
 © 
 
 Eurynome. 
 
 
 © 
 
 Massilia. 
 
 © 
 
 Virginia. 
 
 
 © 
 
 Sappho. 
 
 
 © 
 
 Lutetia. 
 
 © 
 
 Nemausa 
 
 
 © 
 
 Terpsichore. 
 
 
 © 
 
 Calliope. 
 
 © 
 
 Europa. 
 
 
 © 
 
 Alcmene. 
 
 
 © 
 
 Thalia. 
 
 © 
 
 Calypso. 
 
 
 © 
 
 Beatrix. 
 
 
 
 
 Themis. 
 
 @ 
 
 Alexandra. 
 
 
 © 
 
 Clio. 
 
 
 © 
 
 Phocea. 
 
 © 
 
 Pandora. 
 
 
 © 
 
 lo. 
 
 
 © 
 
 Proserpine. 
 
 © 
 
 Melete. 
 
 
 © 
 
 Semelo. 
 
 
 © 
 
 Euterpe. 
 
 © 
 
 Mnemosyne. 
 
 
 © 
 
 Sylvia. 
 
 
 © 
 
 Bellona. 
 
 @ 
 
 Concordia. 
 
 
 © 
 
 Thi.sbe. 
 
 
 © 
 
 Amphitritc. 
 
 ©■ 
 
 Olympia. 
 
 
 ©1 
 
 iN()t vol luuii 
 
 •d.. 
 
 © 
 
 Urania. 
 Conjunctiun. 
 
 © 
 
 Echo. 
 
 
 ©1 
 
 
 
 (5 
 
 
 " Hours. 
 
 
 n 
 
 Quadrature. 
 
 
 
 '" Minutes of Time. 
 ^ Seconds of Time. 
 
 
 (? 
 
 Opposition. 
 
 
 
 ° Degrees. 
 
 
 
 S5 
 
 Ascending Node. 
 
 
 
 ' Minutes of Arc. 
 
 
 (S 
 
 Descending Node. 
 
 
 
 " Second 
 
 s of Arc. 

 
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 51U 
 
 APPENDIX. 
 
 IV.— THE SUN. 
 
 Equatorial liorizontal parallax 
 Mean distance from the Earth 
 
 Time of rotation 
 
 Old value. 
 8"-577G 
 
 New value. 
 8"-9l59 
 
 Diameter in miles 
 
 Inclination of axi.s to plane of ecliptic 
 
 Longitude of Node 
 
 Mass 
 
 Density 
 
 Volume . . 
 
 Force of gravity at Equator 
 
 Apparent diameter as seen from the Earth, maximum 
 „ „ ., minimum 
 
 Earth's as 1 ^ 
 
 05,274,000 .. 01,078,000 
 
 Variable Avith the latitude. The rotation in 
 24 hours of mean solar time is expressed by 
 the formula, 865' ± 165' sin J I 
 
 888,646 .. 853,380 
 
 82° 45') „ ,„ 
 
 .« ,^ f for 1850 
 73 40 J 
 
 354,936 . . 310,047 
 
 0-250 
 
 1,415,225 .. 1,200,100 
 
 28-7 .. 27-0 
 
 .. 32 30"-41 
 
 ..31 32-0 
 
 v.— ADDITIONAL ELEMENTS OF THE MOON. 
 
 Mean Horizontal Parallax 
 
 Mean Angular Telescopic Semidiameter 
 
 Ascending Node of Orbit . . 
 Mean Synodic Period 
 Time of Kotation . . 
 
 Inclination of Axis to the Ecliptic 
 Longitude of Pole . . 
 Daily Geocentric Motion . . 
 Mean Pievolution of Nodes 
 
 „ „ Apogee or Apsides 
 
 Density-, Earth as 1 
 Volume, „ 
 
 Force of Gravity at Surface, Earth as 1 
 Bodies fall in One Second 
 
 . = 57 2-70 
 15 33-30 
 
 O / // 
 
 13 53 17 
 29'530588715 days 
 
 27-3216C141H „ 
 
 o / // 
 1 30 10-H 
 
 2 
 
 13 10 35 
 
 0793d-39108 
 
 3232-57343 
 
 . = 0-50054 
 
 = 0-02012 
 
 2-0 feet
 
 APPENDIX. 
 
 511 
 
 VI.— SATURN'S RINGS. 
 
 Longitude of ascending node of ring on the ecliptic 
 Inclination of rings' plane to the ecliptic 
 Annnal precession of the vernal equinox of Saturn's northern 
 Complete revolution of either equinox in 
 
 Exterior diameter of outer ring 
 Interior „ „ 
 
 Exterior diameter of inner ring 
 Interior ,, ,, 
 
 Interior diameter of the dark ring 
 Breadth of outer bright ring . . 
 
 „ of the division between the rings 
 „ of inner bright ring . . 
 ,, of the dark ring 
 „ of the system of bright rings 
 ,, of the entire system of rings. . 
 Space between the planet and dark ring 
 
 107° 
 
 43' 28"- 93 
 
 28 
 
 10 21-95 
 
 3rn hemisphere 
 
 3-145 
 
 412 
 
 080 years 
 
 Miles. 
 
 Old value. 
 
 New value. 
 
 173.500 
 
 166,920 
 
 153,500 
 
 147,670 
 
 150,000 
 
 144,310 
 
 113,400 
 
 109,100 
 
 95,4,00 
 
 91,780 
 
 10,000 
 
 9,625 
 
 1,750 
 
 1,680 
 
 18,300 
 
 17,605 
 
 9,000 
 
 8,660 
 
 30,050 
 
 28,910 
 
 39,050 
 
 37,570' 
 
 10,150 
 
 9,760 
 
 VII.— TIME. 
 
 I.— THE YEAR. 
 
 The mean sidereal year . . 
 
 The mean solar or tropical year . . 
 
 The mean anomalistic year 
 
 Meaa solar days. 
 365d 6'' 9"' 9»-6 
 365 5 48 46-054440 
 365 6 13 49-3 
 
 II.— THE MONTH. 
 
 Lunar or Synodic Month 
 Tropical Month 
 Sidereal ,, 
 Anomalistic ,, 
 Nodical „ 
 
 ,. 39 
 
 12 
 
 44 2-84 
 
 .. 27 
 
 7 
 
 43 4-71 
 
 .. 27 
 
 7 
 
 43 11-54 
 
 .. 27 
 
 13 
 
 18 37-40 
 
 .. 27 
 
 5 
 
 5 35-60 
 
 The lunar month is the same as the lunation or synodic month, and is the time 
 which elapses between two consecutive new or full moons, or in which the moon 
 returns to the same position relatively to the Earth and Sun. 
 
 The periodic month or synodic month is the revolution with respect to the movable 
 equinox. 
 
 The sidereal month is the interval between two successive conjunctions with the same 
 fixed star. 
 
 The anomalistic month is the time in which the Moon returns to the same point 
 (for example, the perigee or apogee) of her movable elliptic orbit. 
 
 The nodical month is the time in which the Moon accomplishes a revolution with 
 respect to her nodes, the line of which is also movable. 
 
 III.— THE DAY. 
 
 The Appaeent Solae Day, or interval between two 
 
 transits of the Sun over the meridian . . vjii-iable. 
 
 The Mean Solae Day, or interval between two 
 
 transits of the mean Sun over the meridian 24 
 (Astronomers reckon this day from noon to noon, through the 24 hours.) 
 
 The Sidereal Day 23 56 4-09 
 
 The Mean Lunar Day 24 54
 
 512 
 
 THE SOLAR SYSTEM. 
 
 VIII.— CATALOGUE OF KNOWN VAKIABLE STARS* 
 
 star. 
 
 R A. 
 
 1870. 
 
 Decl. 
 
 1870. 
 
 Period. 
 
 Change of 
 Magnitude 
 
 Authority. 
 
 
 h m s 
 
 o / 
 
 Days. 
 
 from 
 
 to 
 
 
 E Andromedse . . 
 
 17 12 
 
 + 37 51-3 
 
 .. 
 
 6 
 
 
 Argelander 
 
 +E Cassiopese 
 
 17 30 
 
 + 03 25-5 
 
 
 
 .. 
 
 Tycho Brahe 
 
 T Piscimn 
 
 25 10 
 
 + 13 52-9 
 
 143± 
 
 9.5 
 
 11 
 
 E. Luther 
 
 a CassiopeJB 
 
 33 9 
 
 + 55 49-4 
 
 79-1 
 
 
 
 2.5 
 
 Birt 
 
 U Piscium 
 
 37 3r5 
 
 + 6 35-3 
 
 
 9 
 
 12 <§ 
 
 Hind 
 
 S Cassiopeje 
 
 1 10 8 
 
 " + 71 55-6 
 
 
 
 13 < 
 
 Argelander 
 
 S Piscium 
 
 1 10 46 
 
 + 8 13-7 
 
 13m.± 
 
 9 
 
 13 < 
 
 Hind 
 
 R Piscium 
 
 1 23 5G 
 
 + 2 12-6 
 
 340 
 
 7 
 
 9.5 
 
 Hind 
 
 V Piscium 
 
 1 47 30 
 
 + 8 8-5 
 
 
 
 
 
 
 Argelander 
 
 — Arietis 
 
 1 57 30 
 
 + 11 54-1 
 
 
 
 
 C. H. Peters 
 
 Pi Arietis 
 
 2 8 44 
 
 -i- 24 27-1 
 
 186 
 
 8 
 
 12< 
 
 Argelander 
 
 Ceti . . 
 
 2 12 47 
 
 - 3 34-1 
 
 3.31-330 
 
 2 
 
 12 < 
 
 D. Fabricius 
 
 5 Persei 
 
 2 50 51 
 
 + .38 20-1 
 
 33 
 
 4 
 
 
 Schmidt 
 
 /S Persei 
 
 2 b'.) 43 
 
 + 40 27-2 
 
 2-80727 
 
 2.5 
 
 4 
 
 Montanari 
 
 R Persei 
 
 3 21 J7 
 
 + .35 13-2 
 
 419 
 
 8-0 
 
 13< 
 
 Schonfeld 
 
 X Tauri 
 
 3 53 29 
 
 + 12 7-3 
 
 3-9.52 
 
 4 
 
 4.5 
 
 Baxendell 
 
 U Tauri 
 
 4 14 15 
 
 ■t- 19 30-2 
 
 
 
 
 10.4 
 
 Baxendell 
 
 T Tauri 
 
 4 14 25 
 
 + 19 13-5 
 
 
 0.7 
 
 13.5 < 
 
 Hind 
 
 R Tauri 
 
 4 21 10 
 
 + 9 52-2 
 
 327 
 
 8 
 
 ]3.5< 
 
 Hind 
 
 S Tauri 
 
 4 22 5 
 
 + 9 39'4 
 
 375 
 
 10 
 
 13 < 
 
 Oudemans 
 
 R Ononis 
 
 4 51 55 
 
 + 7 55-7 
 
 378 
 
 
 
 12.5< 
 
 Hind 
 
 e Auriga; 
 
 4 52 38 
 
 + 43 .37-7 
 
 250 
 
 3.5 
 
 4.5 
 
 Heis 
 
 JR Leporis 
 
 4 53 41 
 
 - 15 0-2 
 
 400± 
 
 7 
 
 
 Schmidt 
 
 R Aurigffi 
 
 5 G 48 
 
 + 53 20-2 
 
 
 
 .. 
 
 Argelander 
 
 a Orionis 
 
 5 48 8 
 
 + 7 22-8 
 
 100+ 
 
 1 
 
 1.5 
 
 J. Herschel 
 
 R Monoeerotis . . 
 
 32 4 
 
 + 8 50-9 
 
 
 10 
 
 13 
 
 Schmidt 
 
 X Geminorum . . 
 
 50 24 
 
 + 20 45-5 
 
 10-16 
 
 3.8 
 
 4.5 
 
 Schmidt 
 
 R Geminorum . . 
 
 59 32 
 
 + 22 54-1 
 
 370 
 
 7.3 
 
 11 
 
 Hind 
 
 R Canis Minoris 
 
 7 1 32 
 
 + 10 13-n 
 
 320 
 
 8 
 
 10 
 
 Argelander 
 
 S Canis Minoris 
 
 7 25 39 
 
 + 8 3.0-0 
 
 335 
 
 7.5 
 
 13 < 
 
 Hind 
 
 S Geminorum . . 
 
 7 .35 14 
 
 + 23 45-2 
 
 294-07 
 
 9.2 
 
 13. 5< 
 
 Hind 
 
 T Geminorum . . 
 
 7 41 30 
 
 + 24 3-3 
 
 288-04 
 
 9.5 
 
 13.5 < 
 
 Hind 
 
 U Geminorum . . 
 
 7 47 23 
 
 + 22 20-5 
 
 97 
 
 
 
 13.5 < 
 
 Hind 
 
 * This catalogue has been recently communicated (in a more complete form) to 
 the Royal Astronomical Society by Mr. G. F. Chambers, to whom, and to .Mr. G. Knott 
 and Mr. Baxendell, the Editor is indebted for a careful revision and additional in- 
 formation. 
 
 t This is the celebrated temporary star observed by Tycho Brahe. 
 
 \ This is Hind's celebrated " crimson star." It well deserves all that has lieen said 
 of it, for it is a truly remarkable object. In 1855 Schmidt reported it to be gaining light 
 and losing colour; when I last saw it, however, on .Tanuaiy 4, IH05, its colour was ns 
 deep a ruby as well could be. Its magnitude gauged by Dawes's method was Oi. 
 
 § The symbol signifies that the star's minimum magnitude fell below that given ; 
 how much is not known.
 
 VARIABLE STARS. 
 
 513 
 
 Star. 
 
 
 R.A. 
 1870. 
 
 Decl. 
 1870. 
 
 Period. 
 
 Change of 
 
 Magnitude. 
 
 Authority. 
 
 
 b 
 
 m s 
 
 o 
 
 , 
 
 Days. 
 
 from 
 
 to 
 
 
 R Cancri 
 
 8 
 
 9 24 
 
 + 12 
 
 7-4 
 
 359 
 
 6 
 
 10< 
 
 Schwerd 
 
 U Cancri 
 
 8 
 
 28 19 
 
 + 19 
 
 20-5 
 
 306 
 
 9 
 
 13. 5< 
 
 Chacoraac 
 
 S Cancri 
 
 8 
 
 36 30 
 
 + 19 
 
 30-0 
 
 9-48 
 
 8 
 
 10.5 
 
 Hind 
 
 S Hydrte 
 
 8 
 
 46 47 
 
 + 3 
 
 33-5 
 
 256 
 
 8.5 
 
 13.5 
 
 Hind 
 
 T Cancri 
 
 8 
 
 49 14 
 
 + 20 
 
 20-7 
 
 45 5± 
 
 9.5 
 
 12 
 
 Hind 
 
 T Hydrae 
 
 8 
 
 49 20 
 
 - 8 
 
 38-7 
 
 292or326± 6.5 
 
 10.5 
 
 Hind 
 
 a. Hydrae 
 
 9 
 
 21 12 
 
 - 8 
 
 5-9 
 
 55 
 
 2.5 
 
 3 
 
 J. Herschel 
 
 E Leonis Minoris 
 
 9 
 
 37 46 
 
 + 35 
 
 6-5 
 
 1 year ■±_ 
 
 7 
 
 11 
 
 Schonfeld 
 
 R Leonis 
 
 9 
 
 40 34 
 
 + 12 
 
 1-8 
 
 312-57 
 
 5 
 
 11.5 
 
 Koch 
 
 R UrsaeMajoris 
 
 10 
 
 35 25 
 
 + 69 
 
 27-4 
 
 301-90 
 
 7 
 
 13 
 
 Pogson 
 
 n Argus 
 
 10 40 2 
 
 - 59 
 
 0-1 
 
 40 years 
 
 1 
 
 4 
 
 Burchell 
 
 S Leonis 
 
 11 
 
 4 7 
 
 + 6 
 
 10-1 
 
 192 
 
 9 
 
 13 < 
 
 Chacomac 
 
 Leonis 
 
 U 
 
 31 46 
 
 + 4 
 
 5-5 
 
 
 10 
 
 14 
 
 C. H. Peters 
 
 R ComseBerenicis 
 
 11 
 
 57 35 
 
 + 19 
 
 .30-3 
 
 1 year ± 
 
 8 
 
 13 < 
 
 Schonfeld 
 
 T Virginis 
 
 12 
 
 7 56 
 
 — 5 
 
 18-7 
 
 337 
 
 8 
 
 13 < 
 
 Boguslawski 
 
 T Ursfe Majoris 
 
 12 
 
 30 28 
 
 + 60 
 
 12-2 
 
 257 
 
 6.7 
 
 13< 
 
 Argelander 
 
 R Virginis 
 
 12 
 
 31 54 
 
 + 7 
 
 42-2 
 
 146 
 
 6.5 
 
 11< 
 
 Harding 
 
 S Ursse Majoris 
 
 12 
 
 38 15 
 
 + 61 
 
 48-3 
 
 222-6 
 
 7.5 
 
 12 
 
 Pogson 
 
 U Virginis 
 
 12 
 
 44 30 
 
 + 6 
 
 15-7 
 
 212 
 
 7.5 
 
 12< 
 
 Harding 
 
 V Virginis 
 
 13 
 
 21 6 
 
 - 2 
 
 29-7 
 
 252 
 
 7 
 
 
 Goldschmidt 
 
 R {v) Hydraj . . 
 
 13 
 
 22 37 
 
 - 22 
 
 36-4 
 
 447-8 
 
 4 
 
 10< 
 
 J. P.Maraldi 
 
 S Virginis 
 
 13 
 
 26 13 
 
 - 6 
 
 31-4 
 
 380-11 
 
 
 
 11 
 
 Hind 
 
 T Bootis 
 
 14 
 
 8 
 
 + 19 
 
 40-5 
 
 
 9.7 
 
 14< 
 
 Baxendell 
 
 S Bootis 
 
 14 
 
 18 32 
 
 + 54 
 
 24-2 
 
 
 8 
 
 12 
 
 Argelander 
 
 R Camelopardi . . 
 
 14 
 
 27 35 
 
 + 84 
 
 25-2 
 
 265 
 
 7 
 
 13 
 
 "Winnecke 
 
 R Bootis 
 
 14 
 
 31 27 
 
 + 27 
 
 18-1 
 
 196 
 
 8 
 
 12 
 
 Argelander 
 
 U Bootis 
 
 14 34 4^; 
 
 + 28 
 
 1-4 
 
 
 9.5 
 
 13 
 
 Baxendell 
 
 S Serpentis 
 
 15 
 
 15 34 
 
 + 14 47-0 
 
 359 
 
 8 
 
 10 < 
 
 Hai'ding 
 
 S Coronse 
 
 15 
 
 16 6 
 
 + 31 
 
 50-2 
 
 
 6.5 
 
 
 Hencke 
 
 E Coronse 
 
 15 
 
 43 13 
 
 + 28 
 
 33-5 
 
 350 
 
 6.2 
 
 13 < 
 
 Pigott 
 
 S LibrjE 
 
 15 
 
 43 14 
 
 - 8 
 
 2-4 
 
 6-98 
 
 
 
 Schmidt 
 
 R Seri^entis 
 
 15 44 44 
 
 + 15 
 
 31-8 
 
 352 
 
 0.5 
 
 10 < 
 
 Harding 
 
 R Librae 
 
 15 
 
 40 15 
 
 - 15 
 
 50-8 
 
 722 
 
 9 
 
 13. 5< 
 
 Pogson 
 
 R Herculis 
 
 16 
 
 23 
 
 + 18 
 
 43-4 
 
 310 
 
 8.5 
 
 13.5 
 
 Argelander 
 
 T Scorpii 
 
 16 
 
 9 18 
 
 - 22 
 
 39-0 
 
 
 7 
 
 13 < 
 
 Auwers 
 
 R Scorpii 
 
 16 
 
 9 54 
 
 - 22 
 
 37-3 
 
 648 
 
 9 
 
 ]4< 
 
 Ch acorn ac 
 
 S Scorpii 
 
 10 
 
 9 56 
 
 - 22 
 
 34-2 
 
 364 
 
 9 
 
 13 < 
 
 Ch acorn ac 
 
 U Scorpii 
 
 16 
 
 14 59 
 
 - 17 
 
 34-5 
 
 
 9.5 
 
 13.5 
 
 Pogson 
 
 U Herculis 
 
 16 
 
 20 3 
 
 + 19 
 
 11-4 
 
 
 7 
 
 13 
 
 Hencke 
 
 30 Herculis 
 
 16 
 
 24 22 
 
 + 42 
 
 lO-l 
 
 106 
 
 5 
 
 6 
 
 Baxendell 
 
 T Ophiuchi 
 
 16 
 
 26 18 
 
 - 15 
 
 51-2 
 
 
 10.5 
 
 13< 
 
 Pogson 
 
 .S Ophiuchi 
 
 16 
 
 26 47 
 
 - 16 
 
 53-1 
 
 229-3 
 
 9.3 
 
 13.5< 
 
 Pogson 
 
 S Herculis 
 
 10 
 
 45 59 
 
 + 15 
 
 9-7 
 
 303 
 
 7.5 
 
 12.5 
 
 Schonfeld 
 
 *Nova Ophiuchi 
 
 16 
 
 52 13 
 
 - 12 
 
 41-4 
 
 
 4.5 
 
 13.5 < 
 
 Hind 
 
 * This is Hind's well-known star which suddenly blazed out in Opliiiichiis in the 
 spring of 1848, concerning which see Mdnthbj Notices K.A.S. vol. viii. p. 140. 
 
 L L
 
 514 
 
 
 APPENDIX. 
 
 
 
 
 Star. 
 
 R.A. 
 
 1870. 
 
 Decl. 
 1870. 
 
 Period. 
 
 Change ol 
 Magnitude. 
 
 Authority. 
 
 
 h m s 
 
 o / 
 
 Days. 
 
 from 
 
 to 
 
 
 R Ophiuchi 
 
 17 18 
 
 - 15 55-0 
 
 304-0 
 
 8 
 
 13.6< 
 
 Pogson 
 
 u Herculis 
 
 17 8 43 
 
 + 14 32-4 
 
 88-5 
 
 3.1 
 
 3.9 
 
 W. Herschel 
 
 Nova Ophiuchi 
 
 17 22 51 
 
 - 21 22-1 
 
 
 
 
 D. Fabricius 
 
 T Herculis 
 
 18 4 11 
 
 + 31 0-0 
 
 104-7 
 
 7.9 
 
 13< 
 
 Argelander 
 
 T Serpeutis 
 
 18 22 28 
 
 + 6 13-0 
 
 310 
 
 10.5 
 
 14< 
 
 Baxendell 
 
 R Scuti Sobieskii 
 
 18 40 33 
 
 - 5 60-5 
 
 71-75 
 
 5 
 
 9 
 
 Pigott 
 
 g, Lyrse 
 
 18 45 17 
 
 + 33 12-7 
 
 12-900 
 
 3.5 
 
 4.5 
 
 Goodricke 
 
 R(13)Ly/8e .. 
 
 18 51 23 
 
 + 43 46-6 
 
 46 
 
 4.2 
 
 4.6 
 
 Baxendell 
 
 R Aquilae 
 
 19 7 
 
 + 8 2-1 
 
 351-5 
 
 6.5 
 
 
 Argelander 
 
 T Sagittarii 
 
 19 8 43 
 
 - 17 11-0 
 
 
 8.5 
 
 12< 
 
 Pogson 
 
 R Sagittarii 
 
 19 9 4 
 
 - 19 32-0 
 
 465 
 
 8 
 
 13< 
 
 Pogson 
 
 S Sagittaiii 
 
 19 11 49 
 
 - 19 15-6 
 
 .. 
 
 10.5 
 
 
 Pogson 
 
 R Cygni 
 
 19 33 20 
 
 + 49 54-5 
 
 416-72 
 
 8 
 
 u< 
 
 Pogson 
 
 *Vulpeculai 
 
 19 42 14 
 
 + 26 59-8 
 
 
 
 
 Anthelm 
 
 S Vulpeculae 
 
 19 43 4 
 
 + 20 57-9 
 
 67-9 
 
 8.8 
 
 9.8 
 
 Rogerson 
 
 X Cygni 
 
 19 45 34 
 
 + 32 35-2 
 
 400-00 
 
 5 
 
 13 < 
 
 G. Kirch 
 
 ■/, AquilsB 
 
 19 45 51 
 
 + 40-4 
 
 7-1703 
 
 3.6 
 
 4.4 
 
 Pigott 
 
 S Cygni 
 
 20 2 47 
 
 + 57 30-7 
 
 324 
 
 9 
 
 13< 
 
 Argelander 
 
 R Capricorni 
 
 20 4 1 
 
 - 14 39-2 
 
 
 9.5 
 
 13.5 
 
 Hind 
 
 S AquiliB 
 
 20 5 39 
 
 + 15 14-3 
 
 124 + 
 
 8.9 
 
 11.3 
 
 Baxendell 
 
 R Sagiltai 
 
 20 8 8 
 
 + 16 20-0 
 
 70-88 
 
 8.3 
 
 10.3 
 
 Baxendell 
 
 R Delphini 
 
 20 8 39 
 
 + 8 41-4 
 
 
 9 
 
 12< 
 
 Hencke 
 
 P {:J4) Cygni . . 
 
 20 13 
 
 + 37 37-8 
 
 ] 8 years ■±_ 
 
 3 
 
 6< 
 
 Jansen 
 
 R (-^4) Cephei . . 
 
 20 23 41 
 
 + 88 44-0 
 
 73 years ±_ 
 
 5 
 
 11 
 
 Pogson 
 
 S Delphini 
 
 20 37 5 
 
 + 16 37-4 
 
 284 
 
 8 
 
 13-5 
 
 Baxendell 
 
 T Delphini 
 
 20 39 20 
 
 + 15 55-7 
 
 332 
 
 8.6 
 
 12 
 
 Baxendell 
 
 U Capricorni 
 
 20 40 54 
 
 + 15 15-6 
 
 420 
 
 11 
 
 13.5< 
 
 Pogson 
 
 T Aquarii 
 
 20 43 
 
 - 5 37-6 
 
 197 
 
 7.8 
 
 
 
 Goldschmidt 
 
 R Vulpeculae 
 
 20 58 36 
 
 + 23 18-4 
 
 138-6 
 
 8 
 
 13.5 
 
 Argelander 
 
 T Capricorni 
 
 21 14 50 
 
 - 15 42-6 
 
 274 
 
 9 
 
 l-t< 
 
 Hind 
 
 S Cephei 
 
 21 36 47 
 
 + 78 2-3 
 
 470 
 
 8.9 
 
 11.12 
 
 Winnecke 
 
 fji. Cephei 
 
 21 39 31 
 
 + 58 11-1 
 
 5 or 6 years 
 
 4 
 
 
 
 W. Herschel 
 
 T Pegasi 
 
 22 2 33 
 
 + 11 54-2 
 
 
 10 
 
 13 < 
 
 Hind 
 
 - Aquarii . .' 
 
 22 22 31 
 
 - 10 360 
 
 4f years 
 
 7.8 
 
 
 Riiniker 
 
 3 Cephei 
 
 - 22 24 21 
 
 + 57 45-0 
 
 5-3064 
 
 3.7 
 
 4.8 
 
 Goodricke 
 
 S Aquarii 
 
 i:-2 50 8 
 
 - 21 2-1 
 
 279-3 
 
 8 
 
 IK 
 
 Argelander 
 
 /3 Pegasi 
 
 22 57 28 
 
 + 27 22-7 
 
 31-5 or 43-4 
 
 3 
 
 2.5 
 
 Schmidt 
 
 R Pegasi 
 
 23 7 
 
 + 9 50-6 
 
 578 
 
 8.5 
 
 13.5 
 
 Hind 
 
 R Aquarii 
 
 23 37 5 
 
 - 10 0-3 
 
 354 or 388-5 
 
 7 
 
 io<- 
 
 Harding 
 
 R Cassiopeaj 
 
 23 51 40 
 
 + .50 30-9 
 
 434-81 
 
 
 
 14 < 
 
 Pogson 
 
 * A celebrated " temporary star."
 
 A CATALOGUE OF CELESTIAL OBJECTS. 
 
 The following Catalogue is designed to furnish a series of objects available 
 for achromatic telescopes of about three inches' aperture and four feet focal 
 length. With very few exceptions, those visible in England have actually 
 been examined with an instrument of this size ; the remainder have been 
 selected with care from various sources, but chiefly from Sir J. Herschel's 
 " Cape Observations." It is believed that this is the first systematic abridg- 
 ment of the catalogues contained in that work yet attempted. By adopting 
 the plan of combining northern and southern objects, the lists which follow 
 have been made available for use in either hemisjjhere. Speaking generally, 
 double stars are characteristic of the northern heavens ; remarkable clusters and 
 nebulse, of the southern. Nearly all the celebrated aggregations of stars are 
 situated south of the celestial equator, whilst important doubles are rather 
 scarce there. 
 
 PART L— DOUBLE, TRIPLE, AND MULTIPLE STARS. 
 
 As a general rule, no stars are inserted which are less than 2" or more 
 than 60" apart. Also, as a general rule, no principal star is included which 
 is less than the 6Jth magnitude, and no secondary one which is less than 
 the 7^th ; but in some special cases these limitations have been disregarded, 
 as in regions where objects are sparsely scattered and an adequate number 
 fulfilling the requisite conditions could not be obtained. Many stars, double 
 when examined by small telescopes, appear triple and quadruple in larger 
 ones : reckoned under the latter head they would be inappropriate for this 
 list, but not so, regarded in the more elementary form. The magnitudes 
 are chiefly from Smyth and Webb; the distances from Secchi and Dembowski. 
 Secchi's epoch is generally 1857 ±; Dembowski's, 1863 ±- Where their 
 catalogues failed, recourse was had to Smyth as a rule. In double-star 
 nomenclature A denotes the largest star, and B, C, &c., the smaller ones 
 in succession. 
 
 Star. 
 
 R.A. isro. 
 
 h m 
 
 2o 34 
 
 Dccl. 1S70. 
 
 Mag. 
 
 Distance 
 
 /3 Tourani 
 
 - 63 40-4 
 
 both 5 
 
 28 
 
 -?r Andromedae . . 
 
 20 5G 
 
 + 33 0-2 
 
 4A and 9 
 
 3G 
 
 n Cassiopeas . . 
 
 41 9 
 
 + 57 7-8 
 
 4 and 1\ 
 
 6-9 
 
 700-year binary 
 For this Cataloi^ne the Editor is indebted to the kinlncss of Mr. G. F. Chambers.
 
 516 
 
 
 
 APPENDIX. 
 
 
 , i 
 
 star. 
 
 R.A. 1870. 
 
 Decl. 1S70. 
 
 Mag. Distance. Notes. i . 
 
 65 Pisciiim 
 
 h m s 
 
 42 53 
 
 / 
 
 + 27 0-2 
 
 5i and 6 
 
 C^ s - 
 
 ■v^' Piscium 
 
 . 58 
 
 42 
 
 + 20 46-6 
 
 both fi\ 
 
 30 '. 
 
 a Ursse Mill oris 
 
 1 11 
 
 17 
 
 + 88 37-0 
 
 2i and 9^ 
 
 18-4 \ 
 
 37 Ceti 
 
 . 1 7 
 
 49 
 
 - 8 37-5 
 
 6 and 1\ 
 
 51 V, 
 
 Eridani 
 
 1 34 
 
 52 
 
 - 56 51-3 
 
 both 6i 
 
 3-7 .^■ 
 
 y Arietis 
 
 . 1 46 
 
 23 
 
 + 18 39-5 
 
 4^ and 5 
 
 8-8 
 
 X Arietis 
 
 1 50 
 
 41 
 
 + 22 57-7 
 
 51 and 8 
 
 37 
 
 a, Piscium 
 
 1 55 
 
 18 
 
 + 2 8-1 
 
 5 and 6 
 
 3-3 
 
 y Andromedae . . 
 
 1 55 
 
 55 
 
 + 41 42-4 
 
 3^ and b\ 
 
 10-3 ; B also double 
 
 59 Andromedae 
 
 2 3 
 
 4 
 
 + 38 25-5 
 
 6 and 7 
 
 16 
 
 1 Trianguli 
 
 2 4 49 
 
 + 29 41-6 
 
 5^ and 7 
 
 3-5 ' 
 
 72 P II. Cassiopeee . 
 
 2 18 
 
 22 
 
 + 66 49-0 
 
 4i, 7, and 9 
 
 l-8and7"-8 
 
 u Foniacis 
 
 2 28 
 
 8 
 
 - 28 48-3 
 
 6^ and 8 
 
 10 • 
 
 30 Arietis 
 
 . 2 29 
 
 26 
 
 + 24 4-9 
 
 6 and 7 
 
 38 ' 
 
 12 Persei 
 
 2 34 
 
 3 
 
 + 39 38-7 
 
 6 and 7^ 
 
 22-9 
 
 y Ceti 
 
 2 36 
 
 34 
 
 + 2 41-3 
 
 3 and 7 
 
 2-7 
 
 n Persei . . 
 
 2 41 
 
 13 
 
 + 55 21-2 
 
 5 andSi 
 
 28 
 
 41 Arietis 
 
 2 42 
 
 19 
 
 + 26 43-5 
 
 3,13,11,9 
 
 15, 38", and 125" (aU 
 from A) 
 
 220 P 11. Persei 
 
 2 51 
 
 36 
 
 + 51 50-0 
 
 6 and 8 
 
 12-5 
 
 i Eridani 
 
 2 53 
 
 20 
 
 - 40 49-6 
 
 4^ and 5i 
 
 8-8 
 
 a, Eridani 
 
 3 G 
 
 32 
 
 - 29 30-3 
 
 4 and 7 
 
 5 
 
 ./' Eridani 
 
 3 43 
 
 48 
 
 - 38 1-1 
 
 5 and 5^ 
 
 9 
 
 32 Eridani 
 
 . 3 47 
 
 50 
 
 - 3 20-3 
 
 5 and 7 
 
 6-8 
 
 £ Persei 
 
 3 49 
 
 7 
 
 + 39 37-8 
 
 3^ and 9 
 
 8-4 
 
 y;^ Tauri 
 
 . 4 14 40 
 
 + 25 19-2 
 
 6 and 8 
 
 19-4 
 
 •^ Horologii . . 
 
 4 15 
 
 10 
 
 - 44 34-8 
 
 6| and 8 
 
 70 
 
 T Tauri 
 
 4 34 
 
 26 
 
 + 22 42-1 
 
 5 and 8^ 
 
 68 
 
 / Pictoris 
 
 4 48 
 
 1 
 
 - 53 41-1 
 
 6 and 7 
 
 12 
 
 14 Aurigfe 
 
 5 6 
 
 56 
 
 + 32 32-2 
 
 5 and 7§ 
 
 14-6 
 
 /3 Orionis 
 
 5 8 
 
 17 
 
 - 8 21-2 
 
 1 and 9 
 
 9-5 
 
 170 0- Leporis .. 
 
 . 5 13 
 
 33 
 
 - 18 39-4 
 
 7 and7X 
 
 39 
 
 23 Orionis 
 
 5 15 
 
 59 
 
 + 3 25-1 
 
 5 and 7 
 
 32 
 
 118 Tauri 
 
 5 21 
 
 15 
 
 + 25 2-6 
 
 7 and 7^ 
 
 5-1 
 
 S Orionis 
 
 5 25 
 
 22 
 
 - 23-8 
 
 2 and 7 
 
 53-3 (Secchi, 80") 
 
 X Orionis 
 
 5 27 
 
 58 
 
 + 9 50-8 
 
 4 and 6 
 
 4-5 
 
 I Orionis 
 
 5 29 
 
 3 
 
 — 5 59-8 
 
 3i, 8|, 11 
 
 11-2, 50" 
 
 0- Orionis 
 
 5 32 
 
 13 
 
 - 2 40-5 
 
 4, 8, and 7 
 
 12, 42" 
 
 Z, Orionis 
 
 5 34 
 
 1] 
 
 - 2 0-8 
 
 3, 6^, and 10 
 
 2-4, 56" (Secchi, 
 114") 
 
 y Leporis 
 
 5 39 
 
 3 
 
 - 22 29-2 
 
 4 and 6^ 
 
 93 
 
 41 Aurigae 
 
 . 6 1 
 
 38 
 
 + 48 44-1 
 
 7 and7i 
 
 7-8 
 
 11 Monocerotis 
 
 G 22 
 
 31 
 
 - 6 57-0 
 
 6^, 7, and 8 
 
 7-2, 9"-6 (7 and H, 
 2"- 5) 
 
 12 Lyncis 
 
 6 34 44 
 
 + 59 34-2 
 
 6, 6i, and 1\ 
 
 1 -7, 8"-7 
 
 2193B.A.C.ArgoNa\ 
 
 ds 6 35 
 
 9 
 
 - 48 6-8 
 
 h\ and 8 
 
 13
 
 A CATALOGUE OF CELESTIAL OBJECTS. 
 
 517 
 
 Star. 
 
 B.A. 1870. 
 
 h m a 
 
 6 37 20 
 
 Decl. 1870. 
 
 Mag. Distance. Notes. 
 
 958 2 Lyncis . . 
 
 - 55 53-5 
 
 both 6 
 
 5-1 
 
 38 Geminorum 
 
 6 
 
 47 18 
 
 + 13 20-6 
 
 5i and 8 
 
 6 
 
 301 P VI. Lyncis 
 
 6 
 
 55 19 
 
 + 52 57-1 
 
 6 and Q\ 
 
 3-4 
 
 2336 B.A.C. Argo Navis 
 
 7 
 
 1 16 
 
 - 58 59-1 
 
 64 and 1\ 
 
 2 
 
 y Volantis 
 
 7 
 
 9 50 
 
 - 70 17-3 
 
 5 and 7 
 
 12 
 
 19 Lyncis , .. 
 
 7 
 
 12 14 
 
 + 55 31-6 
 
 7, 8, and 8 
 
 15 and 215" 
 
 a Geminorum . . 
 
 7 
 
 26 18 
 
 + 32 10-4 
 
 3 and 3^ 
 
 5-4 
 
 175 P VIL Argo Navis 
 
 7 
 
 38 30 
 
 - 26 30-4 
 
 both 6i 
 
 9-8 
 
 2 Argo Navis . . 
 
 7 
 
 39 30 
 
 - 14 22-5 
 
 7 and 7^ 
 
 17 
 
 Z, Cancri 
 
 8 
 
 4 45 
 
 + 18 2-4 
 
 6, 7, and 7| 
 
 0-7 and 5-4 
 
 y Argus . . . . 
 
 8 
 
 5 31 
 
 - 46 56-3 
 
 2 and 6 
 
 41 
 
 (p^ Cancri 
 
 8 
 
 18 55 
 
 + 27 21-6 
 
 6 and 6^ 
 
 4-7 
 
 lOSPVIILHydrse .. 
 
 8 
 
 28 56 
 
 + 7 4-5 
 
 6 and 7 
 
 10-5 
 
 124 P VIII. Cancri . . 
 
 8 
 
 32 22 
 
 + 20 0-2 
 
 7, 7^, and 6| 
 
 45 and 90" 
 
 3073 B.A.C. Argo Navis 
 
 8 
 
 53 47 
 
 - 58 43-7 
 
 6 and 7 
 
 40 
 
 38 Lyncis 
 
 9 
 
 10 44 
 
 + 37 21-2 
 
 4 and 7^ 
 
 2-8 
 
 y Leonis 
 
 10 
 
 12 47 
 
 + 20 30-1 
 
 2 and 4 
 
 2-8 
 
 3613 B.A.C. Argo Navis 
 
 10 
 
 26 23 
 
 - 44 24-1 
 
 both 7 
 
 14 
 
 1474 2 Hydrse . . 
 
 10 41 10 
 
 - 14 34-5 
 
 7, 7, and 8 
 
 71 and 6 '-7 
 
 54 Leonis 
 
 10 
 
 48 34 
 
 + 25 26-6 
 
 4^ and 7 
 
 6-3 
 
 1 Ursffi Majoris 
 
 11 
 
 11 15 
 
 + 32 16-0 
 
 4 and 5^ 
 
 2-5 
 
 i Leonis 
 
 11 
 
 17 8 
 
 + 11 14-9 
 
 4 and 7^ 
 
 2-5 
 
 1 7 Crateris 
 
 11 
 
 25 49 
 
 - 28 32-9 
 
 54 and 7 
 
 10 
 
 90 Leonis 
 
 11 27 57 
 
 + 17 31-0 
 
 6, 7i, and 9a 
 
 3-2 and 63 
 
 65 Ursfe Majoris 
 
 11 
 
 48 19 
 
 + 47 12-0 
 
 7, ^, and 7 
 
 3-8 and 63" 
 
 2 Comae Berenices 
 
 11 
 
 57 37 
 
 + 22 11-1 
 
 6 and 1\ 
 
 3-7 
 
 4115 B.A.C. Centauri . . 
 
 12 
 
 7 15 
 
 - 45 0-1 
 
 b\ and 7 
 
 4 
 
 a, Crucis 
 
 12 
 
 19 23 
 
 - 62 22-7 
 
 2, 2, and 5 
 
 5, 90" [quintuple] 
 
 17 Comse Berenices . . 
 
 12 
 
 22 27 
 
 + 26 38-0 
 
 5^ and G\ 
 
 
 S Corvi 
 
 12 
 
 23 8 
 
 - 15 47-4 
 
 3 and 8^ 
 
 2J-1 
 
 y Crucis 
 
 12 
 
 23 58 
 
 - 56 23-0 
 
 2 and 5 
 
 120 
 
 24 Comse Berenices . . 
 
 12 
 
 28 36 
 
 + 19 5-5 
 
 5i and 7 
 
 20-4 
 
 y Virginis 
 
 12 
 
 35 5 
 
 - 44-2 
 
 both 4 
 
 4-0 (1863) 
 
 231 P XII. Camelopardi 
 
 12 
 
 48 17 
 
 + 84 7-2 
 
 6 and 6i 
 
 22 
 
 12 Canum Venaticorum 
 
 12 
 
 49 57 
 
 + 39 1-1 
 
 24 and 6f 
 
 20-1 
 
 54 Virginis 
 
 13 
 
 6 30 
 
 - 18 8-1 
 
 7 and 1\ 
 
 5-7 
 
 Z, Ursse Majoris 
 
 13 
 
 18 40 
 
 + 55 36-3 
 
 3 and 5 
 
 14-4 ; Alcor, mag. 5 is 
 distant 1 1^' 
 
 /Hydrse 
 
 13 
 
 29 35 
 
 - 25 49-7 
 
 6 and 7 
 
 10 
 
 3 Centauri 
 
 13 
 
 44 20 
 
 - 32 20-9 
 
 6 and 7 
 
 9 
 
 / Bootis 
 
 14 
 
 11 'M 
 
 + 51 58-0 
 
 4i and 8 
 
 38 
 
 4749 B.A.C. Centauri.. 
 
 14 
 
 13 20 
 
 - .57 51-8 
 
 5^,8, and 11 
 
 9-6 and 3,^ 
 
 69 P XIV. Bootis 
 
 14 
 
 16 .59 
 
 + 9 2-4 
 
 6 and 7|- 
 
 6-2 
 
 a, Centauri 
 
 14 
 
 30 58 
 
 - 60 17-9 
 
 1 and 2 
 
 15-5 (1838) 
 
 •T Bootis 
 
 14 
 
 34 36 
 
 + 16 58-6 
 
 3i and 6 
 
 6-0 
 
 54 Hydrpe 
 
 14 
 
 38 30 
 
 - 24 .53-3 
 
 5i and 7i 
 
 9-8
 
 518 
 
 
 APPENDIX 
 
 
 
 star. 
 
 R.A. 1870. 
 
 Decl. 1870. 
 
 Mag. 
 
 Distance. Notes. 
 
 i Bootis 
 
 h ni s 
 
 14 39 18 
 
 / 
 
 + 27 37-4 
 
 3 and 7 
 
 2'o 
 
 39 Bootis 
 
 14 45 17 
 
 + 49 15-2 
 
 5i and 6^ 
 
 3-6 
 
 1 Bootis 
 
 14 45 22 
 
 + 19 38-5 
 
 3i and 6i 
 
 5-6 
 
 212 P XIV.Librse .. 
 
 14 49 51 
 
 - 20 48-4 
 
 6 and 1\ 
 
 12 
 
 44 Bootis 
 
 14 59 31 
 
 + 48 9-7 
 
 5 and 6 
 
 4-5 
 
 K Lupi 
 
 15 2 55 
 
 — 48 14-4 
 
 5i and 6^ 
 
 27 
 
 f/. Lupi 
 
 15 9 30 
 
 — 47 23-7 
 
 6, 7, and 6 
 
 20 and 2"A 
 
 ^ Bootis 
 
 15 19 35 
 
 + 37 .50-0 
 
 4 and 8 
 
 108 ; B also double 
 
 S Sei-pentis 
 
 15 28 36 
 
 + 10 58-5 
 
 3 and 5 
 
 3-2 
 
 X CoroDffi 
 
 15 34 29 
 
 + 37 3-6 
 
 5 and 
 
 6-2 
 
 I Lupi 
 
 15 48 35 
 
 - 33 34-9 
 
 both 0^ 
 
 10 
 
 51 Librae 
 
 15 57 13 
 
 - 11 0-8 
 
 4i and 7^ 
 
 7-2 ; A also double 
 
 /3 Scorpii 
 
 15 57 52 
 
 - 19 20-8 
 
 2 and 5^ 
 
 13-6 
 
 * Herculis 
 
 10 2 12 
 
 + 17 23-7 
 
 5^ and 7 
 
 30-4 
 
 » Scorpii 
 
 10 4 26 
 
 - 19 7-2 
 
 4 and 7 
 
 40 ; B also double 
 
 5435 B.A.C. Scorpii . . 
 
 10 11 19 
 
 - 30 35-4 
 
 7 and 7^ 
 
 27 
 
 fl- Scorpii 
 
 10 13 17 
 
 - 25 16-8 
 
 4 and 9^ 
 
 20-4 
 
 J Ophiuchi 
 
 16 17 47 
 
 - 23 8-7 
 
 5 and7i 
 
 3-4 
 
 17 Draconis . . 
 
 10 33 10 
 
 + 53 11-2 
 
 6,6|,andC 
 
 3-7 and 90" 
 
 ^ Draconis 
 
 17 2 39 
 
 + 54 38-7 
 
 4 and 4^ 
 
 2-6 
 
 30 Ophiuchi . . 
 
 17 7 20 
 
 - 26 23-9 
 
 4i and 0^ 
 
 4-4 
 
 a Herculis 
 
 17 8 43 
 
 + 14 32-2 
 
 3i and 5^ 
 
 4-7 
 
 39 Ophiuchi . . 
 
 17 10 5 
 
 - 24 8-5 
 
 5^ and 7^ 
 
 10-8 
 
 f Herculis 
 
 17 19 12 
 
 + 37 16-1 
 
 4 andSJ 
 
 3 -8 
 
 » Draconis 
 
 17 29 35 
 
 + 55 16-4 
 
 both 5 
 
 62 
 
 •i/-' Draconis . . 
 
 17 44 14 
 
 + 72 12-9 
 
 5^ and 6 
 
 31 
 
 67 Ophiuchi . . 
 
 17 54 8 
 
 + 2 56-3 
 
 4 and 8^ 
 
 55 • 
 
 95 Herculis 
 
 17 55 59 
 
 + 21 35-9 
 
 5^ and 6 
 
 6-2 
 
 70 Ophiuchi . . 
 
 17 58 .52 
 
 + 2 32-5 
 
 4J and 7 
 
 5-0 
 
 100 Herculis . . 
 
 18 2 34 
 
 + 26 4-8 
 
 both 7 
 
 14-1 
 
 40 Draconis , . 
 
 18 9 46 
 
 + 79 58-8 
 
 5^ and 6 
 
 20 
 
 K Coronse Australis . . 
 
 18 24 24 
 
 + 38 48-9 
 
 7 and 7^ 
 
 22 
 
 t Lyrrc 
 
 IH 40 
 
 + 39 32-0 
 
 5, 6^ and 5, 5 
 
 ^ 3-0and2"-5;disfanR 
 A A 207" 
 
 ? LyriR 
 
 18 40 17 
 
 + 37 28-2 
 
 5 and 5^ 
 
 44 
 
 j3 LjTa? 
 
 18 45 15 
 
 + 33 12-7 I 
 
 3-5 {i.e. var.) 
 8, 8^, and 9 
 
 1 46, 60", an<l 71" 
 
 6 Serpentis 
 
 18 49 45 
 
 + 4 1-9 
 
 4J and 5 
 
 21-7 
 
 y Corona; Australis . . 
 
 18 57 38 
 
 - 37 14-7 
 
 both 6 
 
 2^ 
 
 15 Aquila; 
 
 18 58 6 
 
 - 4 13-4 
 
 6 and 7^ 
 
 35 
 
 /3' Sagittarii . . 
 
 19 13 17 
 
 - 44 42-0 
 
 5 and 7^ 
 
 
 /3 Cygni 
 
 19 25 28 
 
 + 27 41-3 
 
 3 and 7 
 
 34-4 
 
 10 Cygni 
 
 19 38 22 
 
 + 50 13-4 
 
 6^ and 7 
 
 37 
 
 57 Aquilaj 
 
 19 47 35 
 
 - 8 33-8 
 
 6i and 7 
 
 35 
 
 320 P XIX. Vulpeculffi 
 
 19 47 39 
 
 + 20 0-0 
 
 both 7 
 
 43 
 
 26 P XX. .\ntinoi . . 
 
 20 5 57 
 
 + 28-8 
 
 0^ and 7 
 
 3-3 
 
 '■I
 
 CLUSTERS AND NEBULA. 
 
 519 
 
 Star. 
 
 R.A. 1870. 
 
 Decl. 1870. 
 
 Mag. 
 
 Distance. Note.'?. 
 
 01 Cygni 
 
 .. 20 9 32 
 
 + 46 20-9 
 
 4, 7A, and 5^ 
 
 107 and 338" 
 
 a Capricorni . . 
 
 . 20 10 50 
 
 - 12 56-7 
 
 3 and 4 
 
 373 
 
 X Cepbei 
 
 .. 20 13 15 
 
 + 77 19-1 
 
 4i and 8a 
 
 7-2 
 
 )S Capicorni 
 
 . . 20 13 42 
 
 - 15 11-4 
 
 31 and 7 
 
 205 
 
 Capricorni 
 
 .. 20 22 26 
 
 - 19 0-6 
 
 6 and 7 
 
 22-0 
 
 y Delpliini 
 
 . . 20 40 38 
 
 + 15 39-6 
 
 4 and 7 
 
 11-7 
 
 7191 B.A.C. Pavonis 
 
 . . 20 40 45 
 
 - 62 54-5 
 
 both 6J 
 
 3-2 
 
 s Equulei 
 
 . . 20 52 35 
 
 + 3 47-9 
 
 5 and 7^ 
 
 10-6 ; A also double 
 
 X Equulei 
 
 .. 20 55 48 
 
 + 6 40-2 
 
 6 and 6| 
 
 2-8 
 
 61 Cygni 
 
 .. 21 52 
 
 + 38 5-1 
 
 5i and 6 
 
 18-3 
 
 1 Pegasi 
 
 . 21 16 4 
 
 + 19 14-9 
 
 4 and 9 
 
 36 
 
 /S Cephei 
 
 . 21 26 55 
 
 + 69 59-6 
 
 3 and 8 
 
 14 
 
 248 P XXI. Cephei 
 
 . 21 34 55 
 
 + 56 54-1 
 
 6, 8i, and 8^ 
 
 12 and 20" 
 
 l» Cygni 
 
 . 21 38 18 
 
 + 28 9-5 
 
 5, 6, and 1^ 
 
 4-6 and 217" 
 
 1 Cephei 
 
 . 22 1 
 
 + 63 59-6 
 
 5 and 7 
 
 5-8 
 
 11 P XXII. Cephei . 
 
 . 22 4 12 
 
 + 58 39-4 
 
 6 and 6|- 
 
 21 
 
 33 Pegasi 
 
 . 22 17 24 
 
 + 20 11-5 
 
 6^, 9, and 8 
 
 2-2 and 60" 
 
 53 Aquarii 
 
 . 23 19 29 
 
 - 17 24-0 
 
 both 6i 
 
 8-5 
 
 ? Aquarii 
 
 . 22 22 7 
 
 - 41-1 
 
 4 and4A 
 
 3-4 
 
 I Cephei 
 
 . . 22 24 20 
 
 + 57 45-0 
 
 4i and 7 
 
 41 
 
 8 Lacertse 
 
 . 22 30 5 
 
 + 38 57-7 
 
 6^,6^,11,10 
 
 two nearest, 23" 
 
 y Piscis Australis 
 
 . 22 45 18 
 
 - 33 33-7 
 
 5 and 8 
 
 2i 
 
 8046 B.A.C. Gruis 
 
 . 22 59 43 
 
 - 51 23-2 
 
 7 and 1\ 
 
 8-0 
 
 107 Aquarii 
 
 . 23 39 15 
 
 - 19 24-1 
 
 6 and 7^ 
 
 5-6 
 
 e Cassiopete . . 
 
 . 23 52 24 
 
 + 55 1-8 
 
 6 and 8 
 
 3-1 
 
 PART IL— CLUSTERS AND NEBULA. 
 
 Many clusters and nebulae are visible in small telescopes, which cannot 
 fairly be grasped by such instruments. The largest and brightest only have 
 been selected for insertion in this list ; and it may as well be stated at 
 the outset, that many of them will be found disappointing with apertures 
 below 5 inches. Abundant light and (generally) low magnifiers are essential 
 requisites for the satisfactory examination of all kinds of clusters and 
 nebulae. 
 
 The numbers preceding the letter H refer to Sir J. Herschel's great 
 catalogue of 1864, from which nearly all the places are derived. 
 
 Name. 
 47 Toucani 
 
 31 M. Andromedse 
 
 R.A. 1870. 
 h m s 
 18 14 
 
 Decl. 1870. 
 
 -72 48-2 
 
 Notes. 
 
 35 36 +40 33-5 
 
 Fine globular cluster. 
 
 "The great nebula in Andromeda. 
 Visible to the naked eye. In small 
 telescopes, an elongated eUipse.
 
 520 
 
 APPENDIX. 
 
 Name. 
 
 78 ]^ VIII. Cassiopeas 
 Nubecula Minor 
 193 H. Toucani 
 
 ] 03 M. Cassiopese . . 
 
 33 M. Trianguli 
 31 ^ VI. Cassiopese 
 
 33 and 34 ]@[ VI. Persei 
 
 34 M. Persei . . 
 731 H.Eridani 
 
 « Tauri 
 
 y Tauri 
 
 10(31 H. Columbfe . . 
 38 M. Auriga3 
 Nubecula Major 
 
 1 M. Tauri . . 
 
 36 M. Aurigse 
 
 1181 H. Doradus . . 
 
 42 M, Orionis 
 
 1184 H. Orionis 
 30 Doradus . . 
 
 37 M. Aurigaj 
 
 35 M. Gerainorum . . 
 
 24 § VIII. Orionis . . C, 
 
 41 M. Canis Majoris 
 
 50 M. Monocerotis . . 6 
 
 38 y VIII. Argo Na\ns 7 
 40 M. Argo Navis . . 7 
 1593 H. Argo Navis . . 7 
 1619 H. Argo Navis . . 7 
 1636 H. Argo Navis . . 8 
 44 31. Cancri . . , . 8 
 07 M. Cancri . . . . 8 
 
 R.A. 1870. 
 
 li m s 
 
 35 54 
 47 24 
 
 57 49 
 
 Decl. 1870. 
 o / 
 
 + 61 4-6 
 -74 6-3 
 -71 32-7 
 
 Notes. 
 
 1 24 39 +60 1-0 
 
 1 26 30 
 
 1 37 10 
 
 2 9 57 
 
 2 33 41 
 
 3 28 41 
 
 + 29 59-3 
 + 60 35-4 
 + 56 32-9 
 + 42 13-2 
 -36 34-5 
 
 3 39 45 +23 42-] 
 
 4 12 22 +15 18-7 
 
 5 9 49 
 5 19 57 
 5 24 
 
 -40 I] -6 
 + 35 43-0 
 -69 35-6 
 
 5 26 40 +21 55-3 
 
 5 27 43 
 
 5 28 32 
 
 5 28 53 
 
 5 29 6 
 5 39 36 
 5 43 47 
 
 + 34 3-0 
 
 -66 19-8 
 
 - 5 28-6 
 
 - 4 26-4 
 -69 lO-O 
 + 32 30 6 
 
 6 49 -24 20-7 
 
 1 7 +13 58-3 
 
 41 26 
 56 41 
 
 30 38 
 35 52 
 47 40 
 56 11 
 6 39 
 32 44 
 44 7 
 
 -20 36-6 
 
 - 8 9-6 
 -14 11-8 
 
 — 14 31-2 
 -38 12-7 
 -60 30-8 
 -48 52-8 
 + 20 25-3 
 + 12 17-2 
 
 Loose cluster of small stars, -^" from 
 y to X. 
 
 Visible to the naked eye. 
 
 A highly-condensed cluster, 4' in dia- 
 meter. 
 
 j A fine field, though hardly to be called 
 ( a cluster. 
 
 I A very large, ill-defined, hazy mass of 
 I nebulous matter. 
 
 A fine field. 
 
 ( " The cluster in the sword-handle of 
 ( Perseus :" a truly gorgeous group. 
 
 A fine group of rather large stars. 
 
 ( An oval, and possibly spiral nebula : 
 I major axis, 3'. 
 
 ( The Pleiades. « Tauri is a star of mag. 
 \ 3, otherwise called Alcyone. 
 
 ( The Hyades : a scattered group of rather 
 ( large stars. 
 
 Bright globular cluster, 3' in diameter. 
 
 Cruciform cluster. 
 
 Visible to the naked eye. 
 
 The " Crab Nebula :" beyond 3 inches 
 even as a mere neb., irrespective of its 
 specialty. 
 
 A large, but rather faint cluster, with a 
 double of mags. 8 and 9; dist. 12", in 
 the field ; 2° f. a 5th mag. 
 
 Large and bright oval nebula. 
 
 " The great nebula in Orion :" the most 
 magnificent object in the sidereal hea- 
 vens. 
 
 A brilliant field ; 1° N. of I 
 
 Very large and irregular nebula. 
 
 Compact cluster. 
 
 A loose cluster of small stars perceptible 
 to the naked eye ; f ths from Castor to ?. 
 
 Loose cluster in the form of a trapezium, 
 containing a pair of mags. 7^ and 8^; 
 2-4" apart; 1° S. of v. 
 
 Large scattered cluster, 4° below Sirius. 
 
 J Cluster; rather more than ^rdfrom Si- 
 ( rius to Procyon, 
 
 Fine group of good-sized stars. 
 
 ( Cluster of mags. 8-13, ^° in diameter; 
 I in a very rich neighbourhood. 
 
 Superb cluster, 20' in diameter. 
 
 f Cluster of 200 or more stai-s, visible to 
 ( the naked eye. 
 
 Large loose cluster, fully 20' in diameter. 
 
 j Prffisepe; a fine, but scattered group, 
 ( reqmring a very low power. 
 
 Ordinarv loose cluster, of some size.
 
 A CATALOGUE OF CELESTIAL OBJECTS. 
 
 521 
 
 Name. 
 
 R.A. 1870. 
 
 Decl. 1870. 
 
 
 h m s 
 
 O / 
 
 1881 H. Argo Navis 
 
 9 30 29 
 
 -46 21-6 1 
 
 2007 H. Argo Navis 
 
 9 58 30 
 
 —59 29-7 
 
 » Argus 
 
 10 40 
 
 -58 59-9 
 
 2308 H. Argo Navis . . 
 
 11 59 
 
 -57 58-2 
 
 X Crucis 
 
 12 45 57 
 
 -59 38-6 1 
 
 u Centauri 
 
 13 18 59 
 
 -46 38-0 
 
 51 M. Canum Venat. 
 
 13 34 21 
 
 + 47 51-8 1 
 
 3 M. Canum Venat. 
 
 12 36 8 
 
 + 29 1-8 
 
 5 M. Librae . . 
 
 15 11 57 
 
 + 2 34-6 
 
 80 M. Scorpii 
 
 16 9 17 
 
 + 22 39-1 
 
 13M.Herculis 
 
 16 37 3 
 
 + 36 42-5 1 
 
 10 M. Opliiucbi 
 
 16 50 19 
 
 - 3 53-1 
 
 19 M. Ophiuchi 
 
 16 54 36 
 
 -26 4-1 
 
 92 M. Herculis 
 
 17 13 15 
 
 + 43 15-8 1 
 
 4311 H. Arse . . 
 
 17 30 6 
 
 -53 35-5 
 
 14 M. Ophiuchi 
 
 17 30 46 
 
 - 3 9-8 
 
 6012 B.A.C. Ophiuchi 
 
 17 39 35 
 
 + 5 45-2 1 
 
 23 M. Ophiuchi 
 
 17 49 16 
 
 -18 59-9 
 
 8 M. Sagittarii 
 
 17 55 54 
 
 -24 21-3 1 
 
 24 M. Clypei Sob. .. 
 
 18 10 49 
 
 -18 27-9 
 
 16 M. ClypeiSob. .. 
 
 18 11 31 
 
 -13 49-8 1 
 
 18 M. Clypei Sob. . . 
 
 18 12 19 
 
 -17 10-9 
 
 17 M. Clypei Sob. . . 
 
 18 13 8 
 
 -16 13-4 1 
 
 22 M. Sagittai-ii 
 
 18 28 29 
 
 -24 0-0 
 
 11 M. Antinoi 
 
 18 44 9 
 
 - 6 25-5 ] 
 
 57 M, Lyrse . . 
 
 18 48 42 
 
 + 32 51-9 \ 
 
 27 M. Vulpeculffi . . 
 
 19 53 55 
 
 + 22 21-9 1 
 
 15M. Pegasi 
 
 21 23 39 
 
 + 11 35-2 
 
 2 M. Aquarii 
 
 21 26 43 
 
 - 1 24-0 j 
 
 30 M. Capriconii- . . 
 
 21 33 
 
 -23 45-3 { 
 
 Notes. 
 
 Large rich cluster, upwards of 1° in 
 diameter. 
 
 Large loose ckister. 
 
 A very large and remarkable nebula. 
 
 Large scattered cluster. 
 
 Rich, loose cluster, containing many co- 
 loured stars 
 
 Fine globular cluster. 
 
 The celebrated spiral nebula : a very or- 
 dinary object in common telescopes. 
 
 Large and rather bright compact cluster, 
 rather more than \ from a, Canum to 
 Arcturus. 
 
 Sjilendid compressed cluster. 
 
 Globular cluster. 
 
 Magnificent cluster, visible to the naked 
 
 eye. 
 
 Moderate-sized roundish nebula. 
 
 Large and bright cluster. 
 
 Fine bright nebula, with a very concen- 
 trated centre. 
 
 Globular cluster. 
 
 Large cometary nebula. 
 
 Large group of bright stars closely 
 N. f. /3. 
 
 Eich and prominent group. 
 
 Fine iiTegular cluster: a pretty low 
 power field. 
 
 Large diffused group of bright stars. 
 
 Eather large cluster, with several con- 
 spicuous stars. 
 
 Very rich field. 
 
 " The Horse-shoe Nebula:" plainly vi 
 sible as an elongated nebula in a 
 small glass. 
 
 Large globular cluster. 
 
 Loose cluster, resembling a flock of wild 
 ducks ; an 8th mag. in the cl., and 2 
 of the same preceding it. 
 
 Annular nebula, midway between /J 
 and y. 
 
 " The Dumb-bell Nebula :" tolerably 
 bright. 
 
 Bright resolvable cluster. 
 
 Large globular cluster of small stars ; a 
 nebula in small telescopes. 
 
 Bright iiTegular cluster, easily found; u 
 little N.p. 41, a 5th mag. star.
 
 522 
 
 APPENDIX. 
 
 PART III.— MISCELLANEOUS OBJECTS. 
 
 The follo-wiug list contains objects not within the scope of the two 
 foregoing sections ; to wit, coloured and variable stars of tolerable mag- 
 nitude and short period. 
 
 Name. 
 
 K.A. 1870. 
 
 Decl 
 
 1870. 
 
 Mag. Notes. 
 
 Hi in Sculptor 
 
 h 111 s 
 1 21 1 
 
 
 
 -33 
 
 13-4 
 
 fi Beautiful orange-red star. i 
 
 ^ in Cetus . . 
 
 2 7 
 
 + 49-4 
 
 7 Pieddisb star. ] ' 
 
 a Ceti 
 
 2 12 10 
 
 - 3 
 
 33-9 
 
 1 Max. mag. 2 ; generally invisible at ■ 
 var.-j minimum. Period, 331<i. Epoch of 
 L max. 1805, Mar.23 ; 1800, ¥eb. 1 7. 
 
 a, Ceti 
 
 2 55 28 
 
 + 3 
 
 34-7 
 
 Qi^ f Fine orange star, ysixh. a blue com- 
 "^ \ panion. 
 
 jS Persei 
 
 2 59 41 
 
 + 40 
 
 17-2 
 
 var. Max. mag. 2; rain. 4; period,2d 20|''. 
 
 ^ in Auriga 
 
 4 43 22 
 
 + 28 
 
 18-2 
 
 8 Bed star. 
 
 R Leporis . . 
 
 4 53 41 
 
 -15 
 
 1-0 
 
 ^^ f Max. mag. 7 ; min. and period un- 
 ■ ( known : an intense crimson. 
 
 jf; in Pictor 
 
 5 39 37 
 
 -46 
 
 30-9 
 
 8 Vivid red star. 
 
 j|c in Gemini 
 
 6 18 
 
 + 14 47-5 
 
 7 Golden-yellow star. 
 
 2139 B.A.C. AurigiB 
 
 C 27 33 
 
 + 38 
 
 31-7 
 
 Pale orange star. 
 
 if. in Canis Major 
 
 7 17 38 
 
 -25 
 
 30-7 
 
 7 Red star. 
 
 E Leonis 
 
 9 40 34 
 
 + 12 
 
 1-8 
 
 ^^j, (Max. mag. 5; min. 10; period,324'': 
 ■ ( a i-uby star. 
 
 2874 Brisb. Antlire 
 
 10 11 
 
 -34 40-9 
 
 7 Scarlet star. 
 
 3C30 B.A.C. Antliai 
 
 10 29 26 
 
 -38 
 
 56-5 
 
 6| Extreme orange star. 
 
 5|e in Crater 
 
 10 54 20 
 
 -17 
 
 37-5 
 
 Q ( Intense scarlet star ; follows a. 42A" 
 
 ^ 1 andl'S. : 
 
 -if. in Criix . . 
 
 12 39 49 
 
 -58 
 
 57-0 
 
 <j, (Intense blood-red star; in the field 
 * 1 with /S Crucis, a white star. 
 
 ■^ in Bootes 
 
 14 18 12 
 
 + 26 
 
 17-9 
 
 7^ Vivid red star. 
 
 /s Librre 
 
 15 10 
 
 — 8 
 
 54-2 
 
 2^ Beautiful pale-green star. 
 
 s[s in Apus . . 
 
 15 11 50 
 
 — 75 
 
 27-5 
 
 7 Very high red star. 
 
 a. Scoi-pii 
 
 10 21 25 
 
 -20 
 
 8-5 
 
 1 Fiery-red star. 
 
 •Jif. in Ophiuchns . . 
 
 17 51 31 
 
 + 2 
 
 38-8 
 
 7 A Very fine orange star. 
 
 ^ in Sagittarius . . 
 
 1!) 26 51 
 
 -10 
 
 38-5 
 
 7 Deep scarlet star. 
 
 « Aquilffi 
 
 19 45 51 
 
 + 
 
 40-4 
 
 f Max. mag. 3-6; min. 4-4; period 
 >ai-| 7di8h. 
 
 :|s in Sagittarius . . 
 
 19 58 58 
 
 -27 
 
 35-7 
 
 7 J Fine ruby star. 
 
 ■3f in Capricornus 
 
 20 9 28 
 
 -21 
 
 43-9 
 
 Oi Pure ruby stai-. 
 
 :if. in Indus 
 
 21 12 23 
 
 -70 10-9 
 
 Ruby-orange star. 
 
 * in Cygnus 
 
 21 38 58 
 
 + 37 
 
 101 
 
 8 J Extremely intense ruby star. 
 
 (tt Cephei 
 
 21 39 31 
 
 + 58 
 
 11-1 
 
 . ( Max. mag. 3 ; min. ; period, 5 or 0, 
 ■ 1 years : " very fine deep garnet." 
 
 S Cephei 
 
 22 24 20 
 
 + 57 
 
 45-0 
 
 ) Max. mag. 3*7; min. 4-8; period, 
 ^^'\ 5-30J.
 
 TEST OBJECTS. 
 
 523 
 
 X.— TEST OBJECTS. 
 
 This table is reprinted from the late Admiral Smyth's " Cycle of Celestial Objects." 
 The position of the stars for 1865 are taken from Mr. Darby's "Astronomical Observer," 
 and their distances have, where possible, been corrected by the latest measurements of 
 Dawes and Secchi. 
 
 NEARLY EQUAL 
 
 DOUBLE 
 
 STARS 
 
 
 
 
 Name. 
 
 R.A. 1865. 
 
 Decl. 1865. 
 
 Magnitude. 
 
 Distance 
 
 y Delphini . . 
 
 20 40 
 
 26 
 
 N. 15° 38 
 
 33" 
 
 a4 
 
 6 5i 
 
 12''0 
 
 y Arietis . . 
 
 1 
 
 46 
 
 6 
 
 N. 18 38 
 
 
 
 ai- 
 
 !, ft 5 
 
 8-9 
 
 a, Geminorum 
 
 7 
 
 25 
 
 29 
 
 N. 32 17 
 
 
 
 «3, 
 
 bH 
 
 5-6 
 
 y Virginis (dist. 1844) 
 
 12 
 
 34 
 
 50 
 
 S. 42 
 
 34 
 
 a 4, 
 
 bi 
 
 4-37 
 
 fi Cygni 
 
 21 
 
 38 
 
 4 
 
 N. 28 8 
 
 10 
 
 «5, 
 
 bC) 
 
 4-6 
 
 44 Bootis 
 
 14 
 
 59 
 
 21 
 
 N. 48 10 
 
 51 
 
 a 5, 
 
 bQ 
 
 4-5 
 
 38 Piscium . . 
 
 
 
 10 
 
 26 
 
 N. 8 7 
 
 33 
 
 a 7^ 
 
 ~,b8 
 
 4-2 
 
 g Herculis . . 
 
 17 
 
 19 
 
 2 
 
 N. 37 16 
 
 23 
 
 a 4, 
 
 6 51 
 
 3-8 
 
 a, Piscium .. 
 
 1 
 
 55 
 
 3 
 
 N. 2 6 
 
 39 
 
 «5, 
 
 6 
 
 3-7 
 
 Z Aquarii . . 
 
 22 
 
 22 
 
 10 
 
 S. 42 
 
 37 
 
 «4, 
 
 bii 
 
 3-5 
 
 y Leonis . . 
 
 10 
 
 12 
 
 30 
 
 N. 20 31 
 
 35 
 
 a 2, 
 
 64 
 
 3-2 
 
 1 UrsaB Majoris . . 
 
 11 
 
 10 
 
 59 
 
 N. 32 17 
 
 39 
 
 a 4, 
 
 bbi 
 
 3-0 
 
 £* Lyrse 
 
 18 
 
 39 
 
 51 
 
 N. 39 31 43 
 
 a 5, 
 
 bQi 
 
 3-0 
 
 ft, Draconis . . 
 
 17 
 
 2 
 
 33 
 
 N. 54 39 
 
 7 
 
 ai 
 
 64| 
 
 2-7 
 
 11 Monocerotis 
 
 6 
 
 22 
 
 16 
 
 S. 6 56 
 
 52 
 
 a 7, 
 
 68 
 
 2-7 
 
 s2 Lyrse 
 
 18 
 
 39 
 
 51 
 
 N. 39 41 
 
 43 
 
 a 5, 
 
 6 5i 
 
 2-6 
 
 0- Coronse Borealis 
 
 16 
 
 9 
 
 37 
 
 N. 34 12 
 
 12 
 
 rt6, 
 
 6 6A 
 
 2-3 
 
 Z Cancri 
 
 8 
 
 4 
 
 23 
 
 N. 18 3 
 
 14 
 
 a 6, 
 
 67 
 
 1-63 
 
 36 Andromeda3 
 
 47 
 
 43 
 
 N. 22 55 
 
 53 
 
 a 6, 
 
 67 
 
 1-07 
 
 » Coronffi Borealis.. 
 
 15 
 
 17 
 
 37 
 
 N. 30 46 
 
 44 
 
 a 6, 
 
 6 6i 
 
 1-2 
 
 ir Aquilfe . . 
 
 19 
 
 42 
 
 20 
 
 N. 11 28 
 
 58 
 
 a 6, 
 
 6 7 
 
 1-4 
 
 % Bootis 
 
 14 
 
 45 
 
 9 
 
 N. 19 39 
 
 48 
 
 «4 
 
 6 4i 
 
 1-0 
 
 £ Arietis 
 
 2 
 
 51 
 
 29 
 
 N. 20 47 
 
 57 
 
 a 5 
 
 6 6i 
 
 0-9 
 
 y^ Andrumedffi 
 
 1 
 
 55 
 
 37 
 
 N. 41 40 
 
 56 
 
 a5- 
 
 t,bQ 
 
 0-6 
 
 jK* Bootis 
 
 15 
 
 19 
 
 25 
 
 N. 37 49 
 
 18 
 
 a8 
 
 6 8i 
 
 0-5 
 
 £ Equulei .. 
 
 21 
 
 52 
 
 20 
 
 N. 3 46 
 
 46 
 
 ab- 
 
 \,b7i 
 
 0-5 
 
 20 Draconis . . 
 
 16 
 
 55 
 
 45 
 
 N. 65 14 41 
 
 a 7 
 
 6 7J 
 
 0-4 
 
 UNEQUAL STARS. 
 
 /3 Cygni 
 
 19 25 17 
 
 N. 27 40 42 
 
 «3, 6 7 
 
 34-4 
 
 Z Ursse Majoris 
 
 13 18 28 
 
 N. 55 37 45 
 
 a 3, 6 5 
 
 14-4 
 
 35 Piscium . . 
 
 8 1 
 
 N. 8 4 15 
 
 a 6, 6 8 
 
 11-9 
 
 y Andromeda? 
 
 i 55 37 
 
 N. 41 40 56 
 
 a 'Si, b 5i 
 
 10-3
 
 524 
 
 Name. 
 
 I Bootis 
 
 I Cephei . . 
 
 a Herculis . . 
 
 I Hydrse . . 
 
 < Trianguli 
 
 S Serpen tis. . 
 
 £ Draconis . , 
 
 a Cassiopese 
 
 £ Bootis 
 
 y Ceti 
 
 / Leonis 
 
 ? Ononis . . 
 35 Cassiopese 
 
 Z, Herculis . . 
 
 X Ophiuchi . . 
 
 32 Orionis . . 
 
 4 Aquarii .. 
 
 y Cor. Borealis 
 
 
 APPENDIX. 
 
 
 
 
 
 
 R.A. 1865. 
 li m s 
 14 45 9 
 
 Decl. 
 o 
 
 N. 19 
 
 1865. 
 39 48 
 
 Magnitude. 
 a 34, h 6i 
 
 DistanC' 
 60 
 
 21 
 
 59 
 
 52 
 
 N. 63 
 
 58 
 
 13 
 
 a 5, 
 
 hi 
 
 5-8 
 
 17 
 
 8 
 
 29 
 
 N. 14 
 
 32 
 
 36 
 
 a3i 
 
 h 5i 
 
 4-5 
 
 8 
 
 39 
 
 38 
 
 N. 6 
 
 54 
 
 53 
 
 a 4, 
 
 J8i 
 
 3-6 
 
 2 
 
 4 
 
 32 
 
 N. 29 
 
 40 
 
 10 
 
 a 5i, 
 
 h 7 
 
 3-5 
 
 . . 15 28 
 
 21 
 
 N. 10 59 
 
 32 
 
 «3, 
 
 65 
 
 3-3 
 
 19 
 
 48 
 
 45 
 
 N. 69 
 
 55 
 
 25 
 
 a h\ 
 
 6 9A 
 
 3-1 
 
 23 
 
 52 
 
 10 
 
 N. 54 
 
 52 
 
 47 
 
 a 6, 
 
 68 
 
 31 
 
 14 
 
 39 
 
 5 
 
 N. 27 
 
 38 
 
 40 
 
 a 3, 
 
 &7 
 
 2-9 
 
 2 
 
 36 
 
 19 
 
 N. 2 
 
 40 
 
 1 
 
 a 3, 
 
 h 7 
 
 2-9 
 
 11 
 
 16 
 
 53 
 
 N. 11 
 
 16 
 
 36 
 
 a 4, 
 
 h 1\ 
 
 2-5 
 
 5 
 
 33 
 
 56 
 
 S. 2 
 
 1 
 
 1 
 
 a 3, 
 
 6 0i 
 
 2-4 
 
 2 
 
 17 
 
 58 
 
 N. 66 
 
 47 
 
 37 
 
 a 4i, 
 
 6 7 
 
 2-1 
 
 IG 
 
 36 
 
 12 
 
 N. 31 
 
 50 
 
 41 
 
 a 3, 
 
 66 
 
 06 
 
 .. 16 
 
 24 
 
 6 
 
 N. 2 
 
 16 
 
 53 
 
 a 4, 
 
 66 
 
 1-3 
 
 5 
 
 23 
 
 33 
 
 N. 5 
 
 40 
 
 40 
 
 a 5, 
 
 6 7 
 
 1-4 
 
 20 
 
 44 
 
 16 
 
 S. 6 
 
 7 
 
 45 
 
 a 6, 
 
 68 
 
 0-3 
 
 15 
 
 37 
 
 4 
 
 N. 20 
 
 43 
 
 30 
 
 a 0, 
 
 67 
 
 0-3 
 
 VERY UNEQUAL STARS. 
 
 a Lyras 
 42 Piscium . . 
 
 5 Equulei . . 
 7 Camelopardi 
 
 Polaris . . 
 41 Arietis 
 
 6 Persei 
 
 X Geminorum 
 
 n Cassiopeae 
 
 /3 Orionis . . 
 
 <p Piscium . . 
 
 S Geminorum 
 34 Piscium . . 
 
 » Ceti 
 
 « Geminorum 
 84 Ceti 
 17 Lyrse 
 11 Cancri 
 
 y Crateris .. 
 
 S Cygni 
 
 18 32 20 
 
 N.38 
 
 39 
 
 15 
 
 a 1, 
 
 6 11 
 
 43-4 
 
 15 26 
 
 N. 12 
 
 43 
 
 56 
 
 a 7, 
 
 6 13 
 
 35-0 
 
 21 7 55 
 
 N. 9 
 
 27 
 
 46 
 
 a^\ 
 
 611 
 
 28-2 
 
 4 46 27 
 
 N. .53 
 
 32 
 
 
 
 a 5, 
 
 6 13 
 
 27-0 
 
 1- 8 54 
 
 N. 88 
 
 35 26 
 
 a2i 
 
 6 9^ 
 
 18-6 
 
 2 42 1 
 
 N. 26 
 
 42 
 
 17 
 
 a 3, 
 
 6 13 
 
 15-0 
 
 2 34 48 
 
 N. 48 
 
 39 
 
 27 
 
 «4, 
 
 6 13 
 
 15-0 
 
 7 10 20 
 
 N. 16 
 
 47 
 
 3 
 
 aii 
 
 6 12 
 
 10-3 
 
 40 52 
 
 N. 57 
 
 6 
 
 
 
 «4, 
 
 6 7^ 
 
 9-7 
 
 5 8 3 
 
 S. 8 
 
 21 
 
 35 
 
 al. 
 
 69 
 
 9-5 
 
 1 6 25 
 
 N.23 
 
 52 
 
 7 
 
 a 6, 
 
 6 13 
 
 8-0 
 
 7 12 4 
 
 X. 22 
 
 13 
 
 47 
 
 a3i 
 
 69 
 
 7-1 
 
 3 10 
 
 N, 10 22 
 
 57 
 
 n'G, 
 
 6 13-J- 
 
 7-0 
 
 2 28 47 
 
 N. 5 
 
 
 
 11 
 
 fl44 
 
 6 15 
 
 6-0 
 
 7 36 20 
 
 N. 24 
 
 43 
 
 9 
 
 «4, 
 
 6 10 
 
 6-0 
 
 2 -34 11 
 
 S. 1 
 
 16 
 
 9 
 
 a 6, 
 
 6 14 
 
 5-0 
 
 19 2 19 
 
 N. 35 
 
 17 
 
 24 
 
 fl6, 
 
 6 11 
 
 3-6 
 
 8 34 
 
 N. 27 
 
 51 
 
 56 
 
 " 7, 
 
 6 12 
 
 3-2 
 
 11 18 9 
 
 S. 16 
 
 56 
 
 30 
 
 «4, 
 
 6 14 
 
 3-0 
 
 19 40 45 
 
 N. 44 
 
 48 
 
 8 
 
 «3i 
 
 69 
 
 1-67 
 
 c<^^ 
 
 V' 
 
 D 
 
 B 
 
 Loiidou : Strakgeways and Waldex, Pri.st>jis, 28 Castle Street. Leicester Sq.
 
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