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LIBRARY 
 
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 UNIVERSITY OF, CALIFORNIA. 
 
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THE 
 LIBRARY OF USEFUL STORIES 
 
FIG. i. THE PLANET SATURN. 
 
THE STORY 
 OF THE SOLAR SYSTEM 
 
 BY 
 
 GEORGE F. CHAMBERS, F. R. A. S. 
 
 OF THE INNER TEMPLE, BARRISTbR-AT-LAW 
 
 AUTHOR OF THE STORY OF THE STARS 
 
 WITH 
 
 ^ OF THE 
 
 UNIVERSITY 
 
 OF 
 
 NEW YORK 
 
 D. APPLETON AND COMPANY 
 1907 
 
COPYRIGHT, 1895, 
 BY D. APPLETON AND COMPANY, 
 
PREFACE. 
 
 HAVING in my " Story of the Stars " told of 
 far distant suns, many of them probably with 
 planets revolving around them, I have in the 
 present volume, which is a companion to the 
 former one, to treat of the Sun in particular our 
 Sun as we may call him and the body of attend- 
 ants which own his sway by revolving round him. 
 The attendants are the planets, commonly so 
 called, together with a certain number of comets. 
 I shall deal with all these objects rather from a 
 descriptive and practical than from a speculative 
 or essay point of view,' and with special reference 
 to the convenience and opportunities of persons 
 possessing, or having access to, what may be 
 called popular telescopes telescopes say of from 
 two to four inches of aperture, and costing any 
 sum between ^10 and $o. There is much 
 pleasure and profit to be got out of telescopes of 
 this type, always presuming that they are used by 
 persons possessed of patience and perseverance. 
 It is a very great mistake, though an extremely 
 common one, to suppose that unless a man can 
 command a big telescope he can cjo no useful 
 
 193715 
 
4 PREFACE. 
 
 work, and derive no pleasure from his work. To 
 all such croakers I always point as a moral the 
 achievements of Hermann Goldschmidt, who from 
 an attic window at Fontenay-aux-Roses near 
 Paris, with a telescope of only 2\ inches aperture, 
 discovered no fewer than 14 minor planets. 
 
 As this volume is intended for general read- 
 ing, rather than for educational or technical pur- 
 poses, I have kept statistical details and numer- 
 ical expressions within very narrow limits, mere 
 figures being always more or less unattractive. 
 
 John Richard Green, in the Preface to his 
 book on The Making of England, writes as fol- 
 lows : "I may add, in explanation of the re- 
 appearance of a few passages . . . which my 
 readers may have seen before, that v here I had 
 little or nothing to add or to change, I have pre- 
 ferred to insert a passage from previous work, 
 with the requisite connections and re c erences, to 
 the affectation of rewriting such a passage for the 
 mere sake of giving it an air of novelty." I will 
 venture to adopt this thought as my own, and to 
 apply it to the repetition, here and there, of ideas 
 and phrases which are already to be found in my 
 Handbook of Astronomy. 
 
 G. F. C. 
 
 NORTHFIELD GRANGE, 
 
 EASTBOURNE, 1895. 
 
CONTENTS. 
 
 CHAPTER PAGE 
 
 I. INTRODUCTORY STATEMENT .... 7 
 
 II. THE SUN 18 
 
 III. MERCURY . . . ; ',. 'V . . . . .57 
 
 IV. VENUS . . . . . . . . .61 
 
 V. THE EARTH. . . . . . .69 
 
 VI. THE MOON . . '.-*. . . . 89 
 .VII. MARS . . . . * . . .100 
 VIII. THE MINOR PLANETS. . . . . . no 
 IX. JUPITER . .. , ". Y . . . . 115 
 
 X. SATURN . >v' s ;Y . ^ .122 
 
 XI. URANUS . . , . . . . . 138 
 
 *XII. NEPTUNE 143 
 
 XIII. COMETS 150 
 
 APPENDIX TABLES OF THE SOLAR SYSTEM . . 182 
 
 GENERAL INDEX . . . . . . . . 185 
 
LIST OF ILLUSTRATIONS. 
 
 FIGURE PAGE 
 
 1. The Planet Saturn Frontispiece 
 
 2. Inclination of Planetary Orbits ..... 9 
 
 3. Comparative Sizes of Major Planets . . . II 
 
 4. Comparative Size of the Sun as seen from the Planets 
 
 Named . . - . . . . . .17 
 
 5. Ordinary Sun-spots, June 22, 1885 .... 22 
 
 6. Change of Form in Sun-spots Owing to the Sun's 
 
 Rotation ........ 29 
 
 7. Sun-spots seen as a Notch ..... 37 
 
 8. The Sun Totally Eclipsed, July 18, 1860 ... 56 
 
 9. Venus, Dec. 23, 1885 64 
 
 10. Venus Near Conjunction as a Thin Crescent . . 65 
 
 ir. Mare Crisium (Lick Observatory photographs) . 90 
 
 12. Four Views of Mars (Barnard) ..... 101 
 
 13. Mars, Aug. 27, 1892 (Guyot) 107 
 
 14. Jupiter, Nov. 27, 1857 (Dawes) . . . .116 
 
 15. Saturn, 1889 11:3 
 
 16. General View of the Phases of Saturn's Rings . . 126 
 
 17. Phases of Saturn's Rings at Specified Dates . . 129 
 
 18. Saturn with Titan and its Shadow .... 137 
 
 19. Telescopic Comet with a Nucleus .... 154 
 
 20. Comet seen in Daylight, Sept., 1882 . . . 155 
 
 21. Quenisset's Comet, July 9, 1893 .... 156 
 
 22. Holmes's Comet, the Head on Nov. 9, 1892 (Den- 
 
 ning) 159 
 
 23. Holmes's Comet, the Head on Nov. 16, 1892 (Den- 
 
 ning) ... 159 
 
 24. Comet III. of 1862, on Aug. 22, showing Jet of 
 
 Luminous Matter (Challis) 160 
 
 25. Sawerthal's Comet, June 4, 1888 (Charlois) . . 165 
 
 26. Biela's Comet, 1846 169 
 
 27. The Great Comet of 1811 177 
 
 28. The Great Comet of 1882 179 
 
 6 
 
THE 
 STORY OF THE SOLAR SYSTEM. 
 
 CHAPTER I. 
 
 INTRODUCTORY STATEMENT. 
 
 BY the term " Solar System " it is to be under- 
 stood that an Astronomer, speaking from the 
 standpoint of an inhabitant of the Earth, wishes 
 to refer to that object, the Sun, which is to him 
 the material and visible centre of life and heat 
 and control, and also to those bodies dependent 
 on the Sun which circulate round it at various 
 distances, deriving their light and heat from the 
 Sun, and known as planets and comets. The 
 statement just made may be regarded as a general 
 truth, but as the strictest accuracy on scientific 
 matters is of the utmost importance, a trivial 
 reservation must perhaps be put upon the fore- 
 going broad assertion. There is some reason for 
 thinking that possibly one of the planets (Jupiter) 
 possesses a little inherent light of its own which 
 is not borrowed from the Sun ; whilst of the 
 comets it must certainly be said that, as a rule, 
 they shine with intrinsic, not borrowed light. Re- 
 specting these reservations more hereafter. 
 
 The planets are divided into " primary " and 
 "secondary." By a " primary " planet we mean 
 one which directly circulates round the Sun ; by 
 7 
 
8 THE STORY OF THE SOLAR SYSTEM. 
 
 a " secondary " planet we mean one which in the 
 first instance circulates round a primary planet, 
 and therefore only in a secondary sense circulates 
 round the Sun. The planets are also " major " 
 or " minor " ; this, however, is only a distinction 
 of size. 
 
 The secondary planets are usually termed 
 " satellites," or, very often, in popular language, 
 " moons," because they own allegiance to their 
 respective primaries just as our Moon the Moon 
 does to the Earth. But the use of the term 
 "moon " is inconvenient, and it is better to stick 
 to "satellite." 
 
 There is yet another method of classifying the 
 planets which has its advantages. They are some- 
 times divided into "inferior" and "superior." 
 The "inferior" planets are those which travel 
 round the Sun in orbits which are inside the 
 Earths orbit; the " superior " planets are those 
 whose orbits are outside the Earth. 
 
 The following is an enumeration of the major 
 planets in the order of their distances, reckoning 
 from the Sun outwards : 
 
 1. Mercury. 
 
 2. Venus. 
 
 3. The Earth. 
 
 4. Mars. 
 
 5. Jupiter. 
 
 6. Saturn. 
 
 7. Uranus. 
 
 8. Neptune. 
 
 All the above are major planets and also primary 
 planets. In between Nos. 4 and 5 circulate the 
 " Minor " planets, an ever-increasing body, now 
 more than 400 in number, but all, except one or 
 perhaps two, invisible to the naked eye. 
 
INTRODUCTORY STATEMENT. 
 
 The " Inferior " 
 planets it will be 
 seen from the 
 above table com- 
 prise Mercury and 
 Venus, whilst the 
 " Superior "planets 
 are Mars and all 
 those beyond. 
 
 Great differ- 
 ences exist in the 
 inclinations of the 
 orbits of the differ- 
 ent planets to the 
 plane of the eclip- 
 tic, a fact which is 
 better shown by a 
 diagram than by a 
 table of mere fig- 
 ures. The orbit of 
 Uranus is indeed 
 so much inclined 
 that its motion is 
 really retrograde 
 compared with the 
 general run of the _ 
 planets : and the 
 same remark ap- 
 plies, though much 
 more forcibly, to 
 the case of Nep- 
 tune. 
 
 The actual 
 
 movements of the 
 planets round the 
 Sun are extremely 
 
10 THE STORY OF THE SOLAR SYSTEM. 
 
 simple, for they do nought else but go on, and on, 
 and on, incessantly, always in the same direction, 
 and almost, though not quite, at a uniform pace, 
 though in orbits very variously inclined to the 
 plane of the ecliptic. But an element of extreme 
 complication is introduced into their apparent 
 movements by reason of the fact that we are 
 obliged to study the planets from one of their own 
 number, which is itself always in motion. 
 
 If the Earth itself were a fixture, the study of 
 the movements of the planets would be a com- 
 paratively easy matter, whilst to an observer on 
 the Sun it would be a supremely easy matter. 
 
 Greatly as the planets differ among themselves 
 in their sizes, distances from the Sun, and physical 
 peculiarities, they have certain things in common, 
 and it will be well to make this matter clear be- 
 fore we go into more recondite topics. For in- 
 stance, not only do they move incessantly round 
 the Sun in the same direction at a nearly uniform 
 pace, but the planes of their orbits are very little 
 inclined to the common plane of reference, the 
 ecliptic, or to one another.* The direction of 
 motion of the planets as viewed from the north 
 side of the ecliptic is contrary to the motion of 
 the hands of a watch. Their orbits, unlike the 
 orbits of comets, are nearly circular, that is, they 
 are only very slightly oval. Agreeably to the 
 principles of what is known as the Law of Uni- 
 versal Gravitation, the speed with which they 
 move in their orbits is greatest in those parts 
 
 * The remark in the text applies to all the major planets 
 and to a large number of the minor planets, but certain of the 
 minor planets travel in orbits which are considerably inclined 
 to the ecliptic, and therefore to ajl the other planets. 
 
INTRODUCTORY STATEMENT. II 
 
 which lie nearest the Sun, and least in those parts 
 which are most remote from the Sun ; in other 
 
 FIG. 3. Comparative Sizes of the Major Planets. 
 
 words, they move quickest in Perihelion and 
 slowest in Aphelion. 
 
 The physical peculiarities which the planets 
 have in common include the following points : 
 they are opaque bodies, and shine by reflecting 
 light which they receive from the Sun. Probably 
 
12 THE STORY OF THE SOLAR SYSTEM. 
 
 all of them are endued with an axial rotation, 
 hence their inhabitants, if there are any, have the 
 alternation of day and night, like the inhabitants 
 of the Earth, but the duration of their days, 
 measured in absolute terrestrial hours, will in 
 most cases differ materially from the days and 
 nights with which we are familiar. 
 
 I stated on a previous page that, owing to the 
 circumstances in which we find ourselves on the 
 Earth, the apparent and real movements of the 
 planets are widely different. It would be beyond 
 the scope of this little work to go into these 
 differences in any considerable detail; suffice it 
 then to indicate only a few general points. In 
 the first place, an important distinction exists be- 
 tween the visible movements of the inferior and 
 superior planets. The inferior planets, Mercury 
 and Venus, lying as they do within the orbit of 
 the Earth, are much restricted in their movements 
 in the sky. We can never see them except when 
 they are more or less near to the rising (or risen) 
 or setting (or set) Sun. The extreme angular 
 distance from the Sun in the sky to which Mer- 
 cury can attain is but 27, and therefore we can 
 never observe it otherwise than in sunliglit or 
 twilight, for it never rises more than i hours 
 before sunrise nor sets later than i hours after 
 sunset. Of course between these limits the planet 
 is above the horizon all the time that the Sun is 
 above the horizon, but except in very large tele- 
 scopes is not usually to be detected during the 
 day-time. These remarks regarding Mercury 
 apply likewise in principle to Venus; only the 
 orbit of Venus being larger than the orbit of 
 Mercury, and Venus itself being larger in size 
 than Mercury, the application of these principles 
 
INTRODUCTORY STATEMENT. 13 
 
 leads to somewhat different results. The greatest 
 possible distance of Venus may be 47 instead of 
 Mercury's 27. Venus is therefore somewhat 
 more emancipated from the effects of twilight. 
 The body of Venus being also very much larger 
 and brighter than the body of Mercury, it may be 
 more often and more easily detected in broad day- 
 light. 
 
 It follows from the foregoing statement that 
 the inferior planets can never be seen in those 
 regions of the heavens which are, as it is technic- 
 ally called, in " Opposition " to the Sun ; that is, 
 which are on the meridian at midnight whilst the 
 Sun is on the meridian in its midday splendour to 
 places on the opposite side of the Earth. On the 
 other hand, the two inferior planets on stated, 
 though rare, occasions exhibit to a terrestrial 
 spectator certain phenomena of great interest and 
 importance in which no superior planet can ever 
 take part. I am here referring to the " Transits " 
 of Mercury and Venus across the Sun. If these 
 planets and the Earth all revolved round the Sun 
 exactly in the plane of the ecliptic, transits of 
 these planets would be perpetually recurring after 
 even intervals of only a few months; but the fact 
 that the orbit of Mercury is inclined 7, and that 
 of Venus about 3^, to the ecliptic, involves such 
 complications that transits of Mercury only occur 
 at unequal intervals of several years, whilst, in 
 extreme cases, more than a century may elapse 
 between two successive transits of Venus. For a 
 transit of an inferior planet over the Sun to take 
 place, the Earth and the planet and the Sun must 
 be exactly in the same straight line, reckoned both 
 vertically and horizontally. Twice in every revo- 
 lution round the Sun an inferior planet is verti- 
 
14 THE STORY OF THE SOLAR SYSTEM. 
 
 cally in the same straight line with the Earth and 
 the Sun ; and it is said to be in " inferior conjunc- 
 tion " when the planet comes between the Earth 
 and the Sun ; and in " superior conjunction " when 
 the planet is on the further side of the Sun, the 
 Sun intervening between the Earth and the planet. 
 But for all three to be horizontally in the same 
 straight line is quite another matter. It is the 
 orbital inclinations of Mercury and Venus which 
 enable them, so to speak, to dodge an observer 
 who is on the lookout to see them pass exactly in 
 front of the Sun, or to disappear behind the Sun ; 
 and so it comes about that a favourable combina- 
 tion of circumstances which is rare is needed be- 
 fore either of the aforesaid planets can be seen 
 as round black spots passing in front of the Sun. 
 A passage of either of these planets behind the 
 Sun could never be seen by human eye, because 
 of the overpowering brilliancy of the Sun's rays, 
 even though an Astronomer might know by his 
 calculations the exact moment that the planet 
 was going to pass behind the Sun. 
 
 When an inferior planet attains its greatest 
 angular distance from the Sun, as we see it (which 
 I have already stated to be about 27 in the case 
 of Mercury and 47 in the case of Venus), such 
 planet is said to be at its " greatest elongation," 
 " east " or " west," as the case may be. At east- 
 ern elongation or indeed whenever the planet is 
 east of the Sun, it is, to use a familiar phrase, an 
 "evening star"; on the other hand, at western 
 elongation, or whenever it is on the western side 
 of the Sun, it is known as a " morning star." 
 
 If the movements of an inferior planet are fol- 
 lowed sufficiently long by the aid of a star map, it 
 will be seen that sometimes it appears to be pro- 
 
INTRODUCTORY STATEMENT. 15 
 
 ceeding in a forward direction through the signs 
 of the zodiac; then for a while it will seem to 
 stand still ; then at another time it will apparently 
 go backwards, or possess a retrograde motion. 
 All these peculiarities have their originating cause 
 in the motion of the Earth itself, for the absolute 
 movement of the planet never varies, being always 
 in the same direction, that is, forwards in the order 
 of the signs. 
 
 Turning now to the superior planets, we have 
 to face an altogether different succession of cir- 
 cumstances. A superior planet is not, as it were, 
 chained to the Sun so as to be unable to escape 
 beyond the limits of morning or evening twilight ; 
 it may have any angular distance from the Sun 
 up to 180, reaching which point it approaches 
 the Sun on the opposite side, step by step, until 
 it again comes into conjunction with the Sun. 
 As applied to a superior planet, the term "con- 
 junction " means the absolute moment when the 
 Earth and the Sun and the planet are in the same 
 straight line, the Sun being in the middle. In 
 such a case, to us on the Earth the planet is lost 
 in the Sun's rays, whilst to a spectator on the 
 planet the Earth would appear similarly lost in 
 the Sun's rays, as the Earth would be at that stage 
 of her orbit which we, speaking of our inferior 
 planets, call superior conjunction. 
 
 For a clear comprehension of all the various 
 matters which we have just been speaking of, 
 a careful study of diagrams of a geometrical 
 character, or better still, of models, would be 
 necessary. 
 
 Something must now be said about the phases 
 of the planets. Mercury and Venus, in regard to 
 their orbital motions, stand very much on the 
 2 
 
1 6 THE STORY OF THE SOLAR SYSTEM. 
 
 same footing with respect to the inhabitants of 
 the Earth as the Moon does, and accordingly both 
 those planets in their periodical circuits round 
 the Sun exhibit the same succession of phases as 
 the Moon does. In the case, however, of the 
 superior planets things are otherwise. Two only 
 of them, Mars and Jupiter, are sufficiently near 
 the Earth to exhibit any phase at "all. When they 
 are in quadrature (/. *., 90 from the Sun on either 
 side) there is a slight loss of light to be noticed 
 along one limb. In other words, the disc of each 
 ceases for a short time, and to a slight extent, to 
 be truly circular; it becomes what is known as 
 "gibbous." This occasional feature of Mars may 
 be fairly conspicuous, or, at least, noticeable; but 
 in the case of Jupiter it will be less obvious un- 
 less a telescope of some size is employed. 
 
 If the major planets are arbitrarily ranged in 
 two groups, Mercury, Venus, the Earth and Mars 
 being taken as an interior group, comparatively 
 near the Sun ; whilst Jupiter, Saturn, Uranus and 
 Neptune are regarded as an exterior group, being 
 at a great distance from the Sun, it will be found 
 that some important physical differences exist 
 between the two groups. 
 
 Of the interior planets, the Earth and Mars 
 alone have satellites, and between them make up 
 a total of only three. The exterior planets, on 
 the other hand, all have satellites, the total num- 
 ber being certainly seventeen, and possibly eight- 
 een. In detail, Jupiter has four, Saturn eight, 
 Uranus four, and Neptune one, and perhaps two. 
 These facts may be regarded as an instance of 
 the beneficence of the Creator of the Universe if 
 we consider that the satellites of these remoter 
 planets are so numerous, in order that by their 
 
INTRODUCTORY STATEMENT. ' 17 
 
 numbers they may do something to make up for 
 the small amount of light which, owing to their 
 distance from the Sun, their primaries receive. 
 Then again, the average density of the first group 
 
 FIG. 4. Comparative size of the Sun as seen from the Planets 
 named. 
 
 of planets greatly exceeds the average density of 
 the second group in the -approximate ratio of 5 
 to i. Finally, there is reason to believe that 
 a marked difference exists in the axial rotations 
 of the planets forming the two groups. We do 
 not know the precise figures for all the exterior 
 planets, but the knowledge which we do possess 
 seems to imply that the average length of the day 
 in the case of the interior planets is about twenty- 
 four hours, but that in the case of the exterior 
 planets it is no more than about ten hours. These 
 figures can, however, only be presented as pos- 
 sibly true, because observations on the rotation 
 
1 8 THE STORY OF THE SOLAR SYSTEM. 
 
 periods of Mercury and Venus on the one hand, 
 and of Uranus and Neptune on the other, are at- 
 tended with so much difficulty that the recorded 
 results are of doubtful trustworthiness. It is, 
 however, reasonable to presume that the actual 
 size of the respective planets has more to do with 
 the matter than their distances from the Sun. 
 
 I think that the foregoing summary respecting 
 the planets collectively embraces as many points 
 as are likely to be of interest to the generality 
 of readers; we will therefore pass on to consider 
 somewhat in detail the several constituent mem- 
 bers of the solar system, beginning with the Sun. 
 
 CHAPTER II. 
 
 THE SUN. 
 
 THERE was once a book published, the title of 
 which was " The Sun, Ruler, Fire, Light and Life 
 of the Planetary System." The title was by no 
 means a bad one, for without doubt the Sun may 
 fairly be said to represent practically all the ideas 
 conveyed by the designations quoted. 
 
 There is certainly no one body in creation 
 which is so emphatically pre-eminent as the Sun. 
 Whether or no there are stars which are suns 
 centres of systems serving in their degree the 
 purposes served by our Sun, I need not now pause 
 to enquire, though I think the idea is a very 
 probable one ; but of those celestial objects with 
 which our Earth has a direct relationship, beyond 
 doubt the Sun is unquestionably entitled to the 
 foremost place. It is, as it were, the pivot on 
 
THE SUN. 19 
 
 which the Earth and all the various bodies com- 
 prising the Solar System revolve in their annual 
 progress. It is our source of light and heat, and 
 therefore may be called the great agent by which 
 an Almighty Providence wills to sustain animal 
 and vegetable life. The consideration of all the 
 complicated questions which arise out of these 
 functions of the Sun belongs to the domain of 
 Physics rather than that of Astronomy ; still these 
 matters are of such momentous interest that an 
 allusion to them must be made, for they ought 
 not to be lost sight of by the student of Astron- 
 omy. Half a century ago the actual state of 
 our knowledge respecting the Sun might without 
 difficulty be brought within the compass of a single 
 chapter in any book on Astronomy, but so enor- 
 mous has been the development of knowledge 
 respecting the Sun of late years, that it is no 
 longer a question of getting the materials prop- 
 erly into one chapter, but it is a matter of a 
 whole volume being devoted to the Sun, or even, 
 as in the case of Secchi, of two large octavo vol- 
 umes of 500 pages each being required to cover 
 the whole ground exhaustively. The reader will 
 therefore easily understand that in the space at 
 my disposal in this little work nothing but a pass- 
 ing glimpse can possibly be obtained of this great 
 subject. It is great not only in regard to the vast 
 array of purely astronomical facts which are at a 
 writer's command, but also on account of the ex- 
 tensive ramifications which the subject has into 
 the domains of chemistry, photography, optics 
 and cognate sciences. I shall therefore endeavour 
 to limit myself generally to what an amateur can 
 see for himself with a small telescope, and can 
 readily understand, rather than attempt to say a 
 
20 THE STORY OF THE SOLAR SYSTEM. 
 
 little something about everything, and fail in the 
 effort. 
 
 The mean distance of the Earth from the Sun 
 may be taken to be about 93 millions of miles, 
 and this distance is employed by Astronomers as 
 the unit by which most other long celestial dis- 
 tances are reckoned. The true diameter of the 
 Sun is about 866,000 miles. The surface area ex- 
 ceeds that of the Earth 11,946 times, and the vol- 
 ume is 1,305,000 times greater. The mass or 
 weight of the Sun is 332,000 times that of the 
 Earth, or about 700 times that of all the planets 
 put together. Bulk for bulk the Sun is much 
 lighter than the Earth : whilst a cubic foot of the 
 Earth on an average weighs rather more than 5 
 times as much as a cubic foot of water, a cubic 
 foot of Sun is only about 3^ times the weight of 
 the same bulk of water. This consideration of 
 the comparative lightness of the Sun (though in 
 his day the Sun was thought to be lighter than it 
 is now supposed to be) led Sir J. Herschel to in- 
 fer that an intense heat prevails in its interior, 
 independent it may be of its surface heat, so to 
 speak, of which alone we are directly cognizant 
 by the evidence of our senses. 
 
 The Sun is a sphere, and is surrounded by an 
 extensive but attenuated envelope, or rather series 
 of envelopes, which taken together bear some 
 analogy to the atmosphere surrounding the Earth. 
 These envelopes, which we shall have to consider 
 more in detail presently, throw out rays of light 
 and heat to the confines of the Solar System, 
 though as to the conditions and circumstances 
 under which that light and heat are generated we 
 are entirely ignorant. Of the potency of the 
 Sun's rays we can form but a feeble conception, 
 
THE SUN. 21 
 
 for the amount received by the Earth is, it has 
 been calculated, but one 23oo-millionth of the 
 whole. Our annual share would, it is supposed, 
 be sufficient to melt a layer of ice spread uniform- 
 ly over the Earth to a depth of 100 feet, or to 
 heat an ocean of fresh water 60 feet deep from 
 freezing point to boiling point. The illuminating 
 power of the Sun has to be expressed in language 
 of similar profundity. Thus it has been calcu- 
 lated to equal that which would be afforded by 
 5563 wax candles concentrated at a distance of 
 one foot from the observer. Again, it has been 
 concluded that no fewer than half a million of 
 full moons shining all at once would be required 
 to make up a mass of light equal to that of the 
 Sun. I present all these conclusions to the reader 
 as they are furnished by various physicists who 
 have investigated such matters, but it is rather 
 uncertain as to how much reliance can safely be 
 placed on such calculations in detail. 
 
 To an amateur possessed of a small telescope, 
 the Sun offers (when the weather is above the 
 English average of recent years) a very great 
 and constant variety of matters for studious scru- 
 tiny in its so-called " spots." To the naked eye, 
 or even on a hasty telescopic glance, the Sun 
 presents the appearance of a uniform disc of yel- 
 lowish white colour, though often a little atten- 
 tion will soon result in the discovery of a few, or 
 it may be many, little black, or blackish patches, 
 scattered here and there over the disc seemingly 
 without order or method. We shall presently 
 find out, however, that this last-named suggestion 
 is wholly inaccurate. Though commonly called 
 " spots," these dark appearances are not simple 
 spots, as the word might imply, for around the 
 
22 THE STORY OF THE SOLAR SYSTEM. 
 
 rather black patch which constitutes generally 
 the main feature of the spot there is almost 
 
 invariably a 
 fringe of paler 
 tint ; whilst 
 within the 
 confines of the 
 black patch 
 which first 
 catches the 
 eye there is 
 often a nucle- 
 us or inner 
 portion of far 
 more intense 
 depth of 
 
 FIG. 5. Ordinary Sun-spot, June 22, 1885. shade. The in- 
 nermost and 
 
 darkest portion being termed the nucleus, the or- 
 dinary black portion is known as umbra, whilst 
 the encompassing fringe is the penumbra. It 
 is not always the case that each individual um- 
 bra has a penumbra all to itself, for several spots 
 are occasionally included in one common pe- 
 numbra. And it may further be remarked that 
 cases of an umbra without a penumbra and the 
 contrary are on record, though these may be 
 termed exceptional, often having relation to ma- 
 terial organic changes either just commencing or 
 just coming to a conclusion. A marked contrast 
 subsists in all cases between the luminosity of the 
 penumbra and that of the general surface of the 
 Sun contiguous. Towards its exterior edge the 
 penumbra is usually darker than at its inner edge, 
 where it comes in contact with the umbra. The 
 outline of the penumbra is usually very irregular, 
 
THE SUN. 23 
 
 but the umbra, especially in the larger spots, is 
 often of regular form (comparatively speaking of 
 course) and the nucleus (or nuclei) of the umbra 
 still more noticeably partakes of a compactness 
 of outline. 
 
 Spots are for the most part confined to a zone 
 extending 35 or so on each side of the solar 
 equator ; and they are neither permanent in their 
 form nor stationary in their position. In their 
 want of permanence, they are subject, apparently, 
 to no definite laws, for they frequently appear 
 and disappear with great suddenness. 
 
 Their motions are evidently of a two-fold na- 
 ture ; the Sun itself rotates on its axis, and the 
 spots collectively participate in this movement of 
 rotation ; but over and above this it has been 
 conclusively proved that sometimes a spot has a 
 proper motion of translation of its own independ- 
 ently of the motion which it has in consequence 
 of the Sun's axial rotation. Curiously enough, 
 spots are very rare immediately under the Sun's 
 equator. It is in the zone extending from 8 to 
 20 North or South, as the case may be, that they 
 are most abundant ; or, to be more precise still, 
 their favourite latitude seems to be 17 or 18. 
 They are often more numerous and of a greater 
 general size in the northern hemisphere, to which 
 it may be added that the zone between 11 and 
 15 North is particularly noted for large and 
 enduring spots. A gregarious tendency is often 
 very obvious, and where the groups are very 
 straggling an imaginary line joining the extreme 
 ends of the group will generally be found more 
 or less parallel to the solar equator ; and not only 
 so, but extending a long way, or sometimes almost 
 entirely, across the whole of the visible disg, 
 
24 THE STORY OF THE SOLAR SYSTEM. 
 
 With respect to the foregoing matters Sir John 
 Herschel remarked : u These circumstances . . . 
 point evidently to physical peculiarities in certain 
 parts of the Sun's body more favourable than in 
 others to the production of the spots, on the one 
 hand ; and on the other ? to a general influence of 
 its rotation on its axis aL a determining cause in 
 their distribution and arrangement, and would 
 appear indicative of a system of movements in 
 the fluids which constitute its luminous surface; 
 bearing no remote analogy to our trade-winds 
 from whatever cause arising." More often than 
 not when a main spot has a train of minor spots 
 as followers that train will be found extending 
 eastwards from the east side of the spot, rather 
 than in any other direction. 
 
 Spots remain visible for very diverse lengths 
 of time, from the extreme of a few minutes up to 
 a few months; but a few days up to, say, one 
 month, may, in a general way, be suggested as 
 their ordinary limits of endurance. As the Sun 
 rotates on its axis in 25} days, and as the spots 
 may be said to be, practically speaking, fixed or 
 nearly so with respect to the Sun's body, no spot 
 can remain continuously visible for more than 
 about \2\ days, being half the duration of the 
 Sun's axial rotation. 
 
 With regard to their size, spots vary as much 
 as they do in their duration. The majority of 
 them are telescopic, that is, are- only visible with 
 the aid of a telescope; but instances are not un- 
 common of spots sufficiently large to be visible 
 to the naked eye. The ancients knew nothing 
 about the physical constitution of the Sun, and 
 their few allusions to the subject were mere 
 guesses of the wildest character. They were, 
 
THE SUN. 25 
 
 however, able to notice now and then that when 
 the Sun was near the horizon certain black spots 
 could sometimes be distinguished with the naked 
 eye, but they took these for planets in conjunction 
 with the Sun, or phenomena of unknown origin. 
 Earliest in point of date of those who have left 
 on record accounts of naked eye sun-spots are un- 
 doubtedly the Chinese. In a species of Cyclo- 
 paedia ascribed to a certain Ma-touan-lin (whose 
 records of comets have been of the greatest pos- 
 sible use to astronomers), we find an account of 
 45 sun-spots seen during a period of 904 years, 
 from 301 A. D. to 1205 A. D. In order to convey 
 an idea of the relative size of the spots, the ob- 
 servers compared them to eggs, dates, plums, etc., 
 as the case might be. The observations often 
 extended over several days; some indeed to as 
 many as ten consecutive days, and there seem no 
 grounds for doubting the authenticity of the ob- 
 servations thus handed down to us. A few stray 
 observations of sun-spots were recorded in Eu- 
 rope before the invention of the telescope. Adel- 
 mus, a Benedictine monk, makes mention of a 
 black spot on the Sun on March 17, 807. It is 
 also stated that such a spot was seen by Aver- 
 roes in 1161. Kepler himself seems to have un- 
 consciously once seen a spot on the Sun with the 
 naked eye, though he supposed he was looking at 
 a transit of the planet Mercury. None of these 
 early observers have told us the way in which 
 they made their observations, but the smallest of 
 boys who has any claim to scientific knowledge is 
 aware of the fact, that by the use of so simple an 
 expedient as a piece of glass blackened with 
 smoke, spots which are of sufficient size can be 
 seen with the naked eye. Before telescopes came 
 
26 THE STORY OF THE SOLAR SYSTEM. 
 
 into use it was customary to receive the solar 
 rays in a dark chamber through a little circular 
 hole cut in a shutter. It was thus that J. Fabri- 
 cius succeeded in December 1610 in seeing a con- 
 siderable spot and following its movement suffi- 
 ciently well to enable him to determine roughly 
 the period of the Sun's rotation. 
 
 The spots may often be easily observed with 
 telescopes of small dimensions, taking care, how- 
 ever, to place in front of the eye-piece a piece of 
 strongly-coloured glass. For this purpose glasses 
 of various colours are used, but none so good as 
 dark green or dark neutral tint. It is not alto- 
 gether easy to say positively how large a spot 
 must be for it to be visible with the naked eye, 
 or an opera glass, but probably it may be taken 
 generally that no spot of lesser diameter than i' 
 of arc can be so seen. This measurement must 
 be deemed to apply to that central portion of a 
 normal spot already mentioned as being what is 
 called the nucleus, because penumbrse may be 
 more than i' in diameter without being visible to 
 the naked eye, for the reason that their shading 
 is so much less pronounced than the shading of 
 umbrae. Very large and conspicuous spots are 
 comparatively rare, though during the years 1893 
 and 1894 there were an unusual number of such 
 spots. It often happens that a conspicuous group 
 is the result of the merging or joining up of sev- 
 eral smaller groups. In such cases a group may 
 extend over an area on the Sun 3' or 4' of arc in 
 length by 2' or 3' in breadth. The largest spot 
 on record seems to have been one seen on Sep- 
 tember 30, 1858, the length of which in one direc- 
 tion amounted to more than 140,000 miles. 
 
 The observation of spots on the Sun by pro- 
 
THE SUN. 27 
 
 jecting them on to a white paper screen with the 
 aid of a telescope is a method so convenient and 
 so exact as to deserve a detailed description, the 
 more so as it is so little used. Let there be made 
 in the shutter of a darkened room a hole so much 
 larger than the diameter of the telescope to be 
 used as will allow a certain amount of play to the 
 telescope tube, backwards and forwards, up and 
 down, and from right to left. Direct the tele- 
 scope to the Sun and draw out the eye-piece to 
 such a distance from the object-glass as that the 
 image projected on a white screen held behind 
 may be sharply defined at its edges. If there are 
 any spots on the Sun at the time they will then 
 be seen clearly exhibited on the screen. An 
 image obtained in this way is reversed as com- 
 pared with the image seen by looking at the Sun 
 through a telescope directly. If therefore the 
 telescope is armed with the ordinary astronomical 
 eye-piece, which inverts, then the projection will 
 be direct, that is to say, on the screen the N. S. 
 E. and W. points will correspond with the same 
 terrestrial points. Under such circumstances the 
 spots will be seen to enter the Sun's disc on the 
 E. side and to go off on the W. side. The con- 
 trary condition of things would arise if a Galilean 
 telescope or a terrestrial telescope of any kind 
 were made use of. These instruments erect the 
 image, and therefore will give by projection a re- 
 versed image, in which we shall see the spots 
 moving apparently in a direction contrary to their 
 true direction. 
 
 If the reader has grasped the broad general 
 outlines now given respecting the Sun and its 
 spots he will perhaps be interested to learn a few 
 further details, but these must be presented in a 
 
28 THE STORY OF THE SOLAR SYSTEM. 
 
 somewhat disjointed fashion, because the multi- 
 tude of facts on record concerning sun-spots are 
 so great as to render a methodical treatment of 
 them extremely difficult within the limits here 
 imposed on me. These matters have been gone 
 into in a very exhaustive way by Secchi in his 
 great treatise on the Sun, and in what follows I 
 have made much use of his observations. 
 
 Let us look a little further into the laws reg- 
 ulating the movement of the spots. If it is not 
 a question of seeing a spot spring into view, but 
 of watching one already in existence, v;e shall, in 
 general, see such a spot appear on the Eastern 
 limb of the Sun just after having turned the cor- 
 ner, so to speak. The spots traverse the Sun's 
 disc in lines which are apparently oblique with 
 reference to the diurnal movement and the plane 
 of the ecliptic, and after about 13 days they will 
 disappear at the Western limb if they have not 
 done so before by reason of physical changes in 
 their condition. It is not uncommon for a spot 
 after remaining invisible for 13 days on the other 
 side of the Sun, so to speak, to reappear on the 
 Eastern limb and make a second passage across 
 the Sun ; sometimes a third, and indeed some- 
 times even a fourth, passage may be observed, but 
 more generally they change their form and vanish 
 altogether either before passing off the visible 
 disc, or whilst they are on the opposite side as 
 viewed from the Earth. 
 
 When several spots appear simultaneously, 
 they describe in the same period of time similar 
 paths which are sensibly parallel to one another 
 although they may be in very different latitudes. 
 The conclusion from this is inevitable, that spots 
 are not bodies independent of the Sun, as satel- 
 
THE SUN. 
 
 lites would be, but that they are connected with 
 the Sun's surface, and are affected by its move- 
 ment of rotation. If we make every day for a 
 few days in succession a drawing of the Sun's 
 disc with any spots that are visible duly marked 
 thereon, we shall see that their apparent progress 
 
 FIG. 6. Change of Form in Sun-spots owing to the Sun's rotation. 
 
 is rapid near the centre of the Sun, but slow near 
 either limb. These differences, however, are ap- 
 parent and not real, for their movement appears 
 to us to take place along a plane surface, whilst 
 in reality it takes place along a circle parallel to 
 the solar equator. The spots in approaching the 
 Sun's W. limb, if they happen to seem somewhat 
 circular in form when near the centre, first be- 
 
30 THE STORY OF THE SOLAR SYSTEM. 
 
 come oval, and then seem to contract almost into 
 mere lines. These changes are simple effects of 
 perspective, and are to be explained in the same 
 manner as the apparent decrease in the size of 
 many of the spots is often explicable. But this 
 condition of things proves, however, that the spots 
 belong to the actual surface of the Sun, for, on a 
 contrary supposition, we should have to regard 
 them as circular bodies greatly flattened like 
 lozenges, and this would be contrary to all we 
 know of the forms affected by the heavenly bodies. 
 Of course besides the apparent changes of form 
 just alluded to as the effect of perspective, it is 
 abundantly certain that solar spots often undergo 
 very real changes of form, not only from day to 
 day, but in the course of a few hours. Several 
 spots will often become amalgamated into one, and 
 it was ephemeral changes of this character which 
 hindered generally the early observers from deter- 
 mining with precision the duration of the Sun's 
 rotation. 
 
 The apparent movements of the spots vary 
 also from month to month during the year ac- 
 cording to the season. In March their paths are 
 very elongated ellipses with the convexity towards 
 the N., the longer axis of the ellipse being almost 
 parallel to the ecliptic. After that epoch the cur- 
 vature of the ecllipse diminishes gradually, at the 
 same time that the major axis becomes inclined 
 to the ecliptic, so that by June the flattening of 
 the ellipse has proceeded so far that the path has 
 become a straight line. Between June and Sep- 
 tember the elliptical form reappears but in a re- 
 versed position ; then, following these reversed 
 phases, the ellipticity decreases, and for the sec- 
 ond time there is an epoch of straight lines. This 
 
THE SUN. 31 
 
 happens in December, but the straight lines are 
 inclined in a converse direction to that which was 
 the case in June. It must again be impressed on 
 the reader that all these seemingly different forms 
 of path pursued by the spots are merely effects of 
 perspective, for in reality, the spots in crossing 
 the Sun's disc describe lines which are virtually 
 parallel to the solar equator. These projections 
 really depend of course on the position of the 
 observer on the Earth, and vary as his position 
 varies during the Earth's annual circuit round 
 the Sun. The number of the spots varies through 
 wide limits. Sometimes they are so numerous that 
 a single observation will enable us to recognise the 
 position of the zones of maximum frequency. 
 Sometimes, on the other hand, they are so scarce, 
 that many weeks may pass away without hardly 
 one being seen. A remarkable regularity is now 
 recognised in the succession of these periods of 
 abundance and scarcity, as we shall see later on. 
 
 It is both useful and interesting in studying 
 the spots to record methodically their number 
 and their size, but it is not easy to teach observ- 
 ers how to do this so systematically that observa- 
 tions by one person can be brought into compari- 
 son with those of another. Photography and 
 hand-drawing on a screen alone furnish a trust- 
 worthy basis of operations. Spots in general may 
 naturally be classified into (i) isolated spots or 
 points, and (2) groups of spots; but often one 
 observer will describe as a small spot an object 
 which another observer would regard as a mere 
 point; and one observer will record several 
 groups where another observer will see but one. 
 A very few days' experience with a telescope will 
 bring home to the observer's mind the difficulty 
 3 
 
32 THE STORY OF THE SOLAR SYSTEM. 
 
 of dealing with the spots where it is a question of 
 systematic methodical observation of them. 
 
 Let us now take a brief survey of some of the 
 theories which have been put forth regarding the 
 nature of the spots on the Sun. In the early days 
 of the telescope, that is to say, during the i;th 
 century, two general ideas were current. Some 
 thought the spots to be shapeless satellites re- 
 volving round the Sun; others that they were 
 clouds, or aggregations of smoke, floating about 
 in a solar atmosphere. Scheiner, the author of 
 the first theory, abandoned it towards the close 
 of his life, having arrived at the conclusion that 
 the spots were situated below the general level 
 of the Sun's surface. Another idea, but of later 
 date, was that the Sun is a liquid and incan- 
 descent mass of matter, and the spots immense 
 fragments of Scorice, or clinkers, floating upon an 
 ocean of fire. 
 
 Somewhat more than a century after the spots 
 had been generally studied with. the aid of a tele- 
 scope a Scotchman named Wilson made a memo- 
 rable discovery. He showed by the clearest evi- 
 dence that they are cavities, and he propounded 
 the first intelligible idea of the true physical con- 
 stitution of the Sun, when he compared to a 
 strongly illuminated cloud the luminous layer of 
 solar material which we now term the " photo- 
 sphere." On November 22, 1769, he observed on 
 the Sun's disc a fine round spot encompassed by 
 a penumbra, also circular, and concentric with 
 the nucleus. He watched that spot up to the 
 time that it disappeared, and he soon remarked 
 that the penumbra ceased to be symmetrical: the 
 part turned towards the centre of the Sun became 
 smaller and smaller, and eventually disappeared 
 
THE SUN. 33 
 
 altogether ; whilst the part on the opposite side 
 preserved its fulness and dimensions almost un- 
 changed. Let us suppose we chanced to turn a 
 telescope on to the Sun on a given day, and were 
 fortunate enough to discover a spot in the centre 
 of the disc, with a penumbra concentric with the 
 nucleus. When such a spot arrives about midway 
 towards the limb, it will exhibit a penumbra nar- 
 rower on the left side than on the right ; later on 
 the penumbra will disappear almost or quite com- 
 pletely on the left side : then the nucleus itself 
 will seem to be encroached upon. Finally, very 
 near the limb, there will remain only a slender 
 thread of penumbra, and the nucleus will have 
 ceased to be directly visible. Such were the 
 phases of transformation observed by Wilson and 
 often studied since. Wilson s-uspected that he 
 had come upon some great law that was ripe for 
 disclosure, and in order not to be misled he 
 waited for the return of the same spot, which in- 
 deed reappeared on the Sun's W. limb after 
 about 14 days. Then he found himself face to 
 face with the same phases reproduced, but in the 
 reverse order : the penumbra contracted on one 
 side and full on the other, widening out on the 
 contracted side as the spot came up to the Sun's 
 centre. Henceforth doubt was no longer possi- 
 ble ; the spot had sensibly preserved the same 
 shape during its passage, and the alterations 
 noticed were only apparent, and resulted from an 
 effect of perspective which was easy to be under- 
 stood. The different phases presented by such a 
 spot as that just spoken of will be so much the 
 more sensible according as the depth of the cav- 
 ity is greater ; but if the depth is inconsiderable 
 the bottom of the cavity will only disappear when 
 
34 THE STORY OF THE SOLAR SYSTEM. 
 
 a very oblique angle is attained, and this cannot 
 happen except when the spot is very near to the 
 limb. By observations carefully made under such 
 circumstances it will be possible to determine the 
 depth of the cavity, and Wilson found that the 
 depth of a spot often amounted to about one- 
 third of the Earth's radius. Wilson's theory was 
 not accepted without dispute; it was contested 
 by several astronomers, and in particular by 
 Lalande. It was however taken up by Sir W. 
 Herschel, and as modified by him has met with 
 general acceptance down to the present time ; 
 though now and again challenged, perhaps most 
 recently and most vehemently by Hewlett, a sun 
 spot observer of great experience. Wilson's dis- 
 covery was the point of departure for the grand 
 labours of Sir W. Herschel in the field of Solar 
 Physics. Man of genius that Herschel was, he 
 was above all things an observer who took his 
 own line in what he did. He saw so many phe- 
 nomena with the powerful instruments con- 
 structed by himself, he described so minutely 
 the marvels which were revealed to him, that he 
 left comparatively little for his successors to do 
 so far as regards mere telescopic observation. 
 Herschel's main idea as to the Sun was based on 
 Wilson's discovery. He remarked with reason, 
 as that astronomer had done, that if the spots 
 are cavities the luminous matter could neither be 
 properly called liquid nor gaseous; for then it 
 would precipitate itself with frightful rapidity to 
 fill up the void, and that would render it impos- 
 sible that the spots should endure as we often see 
 they do during several revolutions of the Sun. 
 Moreover, the proper movements of the spots 
 prove that the photosphere is not solid. We can 
 
THE SUN. 35 
 
 therefore only liken it to fogs or clouds, and it 
 must be suspended in an atmosphere similar to 
 ours. Such is, according to Herschel, the only 
 hypothesis which can explain the rapid changes 
 which we witness. We shall see a little later on 
 that these phenomena do admit of another expla- 
 nation. 
 
 In a second memoir Herschel followed up this 
 inquiry with an acuteness worthy of his genius. 
 Unfortunately he allowed himself to be carried 
 away with the idea that the Sun was inhabited in 
 order to sustain this theory. He needed a solid 
 kernel upon which his imaginary inhabitants could 
 dwell ; and also a means whereby he could protect 
 them from the radiations of the photosphere. 
 With this idea in view he conjectured the exist- 
 ence above the Sun's solid body of a layer of 
 clouds always contiguous to the photosphere 
 which enveloped it, and which always being rent 
 when the photosphere was rent, thus enabled us 
 to see the solid body of the Sun lying behind. 
 These notions can only be described as very ar- 
 bitrary, as unsupported by observation, and as 
 involving explanations quite out of harmony with 
 the principles of modern physics. However, the 
 labours of Herschel resulted in so many positive 
 discoveries of visible facts, and in so many just 
 conclusions, that they contributed greatly to the 
 growth of our present knowledge of the true con- 
 stitution of the Sun. 
 
 Since Wilson's time, as Secchi pointedly re- 
 marks, astronomers generally have verified his 
 observations with good instruments, and by an 
 investigation of a great number of spots. De La 
 Rue, discussing the Kew observations, found that 
 of 89 regular spots 72 gave results which con- 
 
36 THE STORY OF THE SOLAR SYSTEM. 
 
 formed to Wilson's ideas, whilst the remaining 17 
 were opposed thereto. There is nothing sur- 
 prising in the existence of a contrarient minority 
 when we consider the great changes which in 
 reality often occur in the forms of the spots. De 
 La Rue suggested a very simple expedient for 
 showing that the spots are cavities. Take two 
 photographs of the Sun made at an interval of 
 one day : during that time every point on the 
 Sun's surface will have been displaced, so far as 
 the telescope is concerned, by about 15. Place 
 these photographs in a stereoscope, and we shall 
 readily see the interior cavity, the edges of which 
 will appear raised above the photosphere. It is 
 impossible therefore to entertain the least doubt 
 as to the truth of the theory that the spots are 
 excavations in the luminous stratum which en- 
 velopes the whole of the solar globe. 
 
 If it be true that a spot is a cavity, it follows 
 that when it reaches the margin of the solar disc 
 we ought to detect a hollow place ; and this will 
 be so much the more easy to observe according as 
 the cavity is larger and deeper. As a matter of 
 fact, numerous observations of this sort have been 
 recorded from the time of Cassini down to the 
 present time under the designation of " notches " 
 on the Sun's limb. On July 8, 1873, Secchi ob- 
 served such a notch 8", or 3600 miles deep. 
 
 Faye and some other astronomers are disposed 
 to support a theory according to which the spots 
 are nothing else than aerial cyclones, but this 
 does not seem admissible. If the fundamental 
 principle of a spot is that it arises from a whirl- 
 ing movement, the rays (so to speak) which com- 
 pose the penumbrae must always be crooked, or 
 the theory falls to the ground. It is quite true 
 
THE SUN. 
 
 37 
 
 that indications of cyclonic action do sometimes 
 appear, but they are at any rate very rare, for 
 only a small percentage exhibit in a distinct man- 
 ner a spiral structure. Moreover, when such a 
 structure is seen it does not endure for the whole 
 lifetime of the spot but only for a day or two : the 
 spot may last a long time after it has lost its spiral 
 features, if it ever had any. Sometimes even the 
 
 FIG. 7. Sun-spot seen as a Notch. 
 
 whirling movement, after having slackened, begins 
 again, but in the contrary direction. Under these 
 circumstances, though this occasional spiral struc- 
 ture is very curious and interesting, we are not 
 justified in taking it as the basis of a theory which 
 has any pretensions to explain the general nature 
 of sun-spots. 
 
38 THE STORY OF THE SOLAR SYSTEM. 
 
 When we examine the Sun with instruments of 
 large aperture and high magnifying power, we no- 
 tice that its surface is far from being as smooth 
 and uniform as it appears in a small telescope. 
 On the contrary, it presents an irregular undulat- 
 ing appearance like a pond or other sheet of water 
 agitated by the wind. Careful scrutiny with a 
 powerful eye-piece reveals the fact that the Sun's 
 surface is marked by a multitude of wrinkles and 
 irregularities which it is well-nigh impossible to 
 describe in words. More or less everywhere there 
 is a general mottling visible ; it is more distinct 
 in some places than others, and especially so 
 towards the centre of the disc. This peculiar 
 appearance varies very much from time to time, 
 and its distinctness seems to depend a great deal 
 on the state of the Earth's atmosphere, for it be- 
 comes invisible when the air is disturbed ; but 
 these variations depend also on real variations of 
 the photosphere a fact which observations made 
 in very calm weather are thought clearly to indi- 
 cate. 
 
 It is often said that the Sun exhibits a granu- 
 lated structure. If we wish to realise in the most 
 precise manner what is meant by the word "gran- 
 ulation " as applied to the structure of the Sun, 
 we must abandon the method of projection and 
 examine the Sun directly with a powerful eye- 
 piece, taking advantage of a moment when the 
 atmosphere is perfectly calm, and before the eye- 
 piece has had time to get hot. It may then be 
 seen that the Sun's surface is covered with a mul- 
 titude of little grains, nearly all of about the same 
 size, but of different shape, though for the most 
 part more or less oval. The small interstices 
 which separate these grains form a net-work 
 
THE SUN. 39 
 
 which is dark without being positively black. 
 Secchi considered it difficult to name any known 
 object which exactly answers in appearance to 
 this structure, but he thought that we can find 
 something resembling it in examining with a mi- 
 croscope milk which has been a little dried up, and 
 the globules of which have lost their regular form. 
 Exceptionally good atmospheric conditions are un- 
 der all circumstances indispensable for the study 
 of these details. 
 
 In point of fact, there is a mysterious uncer- 
 tainty about the normal condition of the Sun's 
 surface, in a visual sense, which a few years ago 
 engendered a very vehement controversy, and led 
 to the use of such expressions as " willow leaves," 
 " rice grains," " sea beach," and " straw thatching," 
 to indicate what was seen. All these words are 
 too precise to be quite suitable to be taken lit- 
 erally, but perhaps, on the whole, " rice grains " 
 is not altogether a bad expression to recall what 
 certainly seems to be the granular surface of the 
 Sun as we see it. 
 
 By making use of moderate magnifying powers, 
 what we see will often convey the impression of 
 a multitude of white points on a black net-work. 
 This is very apparent during the first few mo- 
 ments that the telescope is brought to bear on 
 the Sun, but its clearness quickly passes away 
 because the eye gets fatigued, and the lenses 
 becoming warm the air in the telescope tube gets 
 disturbed because also warmed. Sometimes the 
 appearance is a little different from that just 
 described, and along with the white and brilliant 
 points little black holes are intermixed. Often- 
 times the grains appear as if suspended in a black 
 net-work and heaped together in knots more or 
 
40 THE STORY OF THE SOLAR SYSTEM. 
 
 less shaded and more or less broad. Sometimes 
 the grains exhibit a very elongated form, especially 
 in the neighbourhood of the spots. It is these 
 elongated forms to which Nasmyth applied the 
 term " willow leaf," whilst Huggins thought 
 " rice grains " a very suitable expression. 
 
 This granular or leaf- like structure call it 
 what we will cannot be made out except with 
 considerable optical assistance, for the grains 
 being intrinsically very small, diffraction in en- 
 larging them and causing them to encroach on 
 one another necessarily produces a general con- 
 fusion of image. The real dimensions of these 
 grains cannot therefore readily be determined 
 by direct measurement, but by comparing them 
 with the wires used in micrometer eye-pieces it 
 has been thought that their diameters may usu- 
 ally be regarded as equal to % or -J of a second 
 say from 120 to 150 miles. The granules seem to 
 be possessed of sensible movement, but presumably 
 it is not always or even generally a movement of 
 translation from place to place ; only an undulatory 
 movement like that of still water when a stone is 
 cast into it. Nevertheless, probably in certain 
 cases the granules actually are affected by a 
 motion of translation, for in the vicinity of spots 
 they may sometimes be seen flowing over the 
 edges of the penumbrae. In order to explain the 
 existence of the granules the strangest theories 
 have been broached. Sir William Herschel hav- 
 ing observed the granulations, applied to them 
 the term ** corrugations " or " furrows " words 
 somewhat inexact, perhaps, but by which, as his 
 descriptions clearly show, he meant to designate 
 the features which I am now treating of. He 
 even noticed the dark network which separates 
 
THESUN - 41 
 
 the grains, and he applied to it the word " inden- 
 tations." 
 
 These granulations are without doubt promi- 
 nences, probably of hydrogen gas, which rise 
 above the general surface, for this structure is 
 much more sharp and distinct at the centre of 
 the sun's disc than at the limbs ; that is to say, 
 near the limbs of the Sun they partially overlap 
 one another, as indeed Herschel remarked. The 
 idea of flames would satisfy these appearances : 
 and as the spectroscope suggests to us that the 
 Sun is habitually covered over with a multitude 
 of little jets of flame, the observations which have 
 been made compel the opinion that the grains are 
 the summits of those prominences which exist all 
 over the Sun's surface. 
 
 The surface is sometimes so thickly covered 
 over with these granulations the network is so 
 conspicuous that we can readily imagine that 
 we see everywhere pores and the beginnings of 
 spots, but this aspect is not permanent, and seems 
 to depend to some extent on atmospheric causes 
 combined also with actual changes in the Sun's 
 surface itself. There seems however no doubt 
 that the joints, so to speak, of the dark network 
 already referred to do sometimes burst asunder 
 and develope into spots. 
 
 The circumstances which accompany the for- 
 mation of a spot cannot readily be specified with 
 certainty. It is impossible to say that there 
 exists any law as to this matter. Whilst some 
 spots develope very slowly by the expansion of 
 certain pores, others spring into existence quite 
 suddenly. Yet it cannot be said that the forma- 
 tion of a spot is ever completely instantaneous 
 however rapid it may be. The phenomenon is 
 
42 THE STORY OF THE SOLAR SYSTEM. 
 
 often announced some days in advance : we may 
 perceive in the photosphere a great agitation 
 which often manifests itself by some very brilliant 
 faculce. (to be described presently) giving birth to 
 one or more pores. Very often we next notice 
 some groups of little black spots, as if the lu- 
 minous stratum was becoming thinner in such a 
 way as to disappear little by little and leave a 
 large black nucleus uncovered. At the com- 
 mencement of the business there is usually no 
 clearly denned penumbra. This developes itself 
 gradually and acquires a regular outline, just as 
 the spot itself often takes a somewhat circular 
 form. This tranquil and peaceable formation of 
 a spot only happens at a time when calm seems 
 to reign in the solar atmosphere : in general the 
 development is more tumultuous and the stages 
 more complicated. 
 
 As a rule a spot passes through three stages of 
 existence : (i) the Period of birth ; (2) a Period 
 of calm ; arid (3) the Period of dissolution. When 
 a spot is on the point of closing up, the flow of 
 the luminous matter which it, as it were, attracts, 
 is not directed uniformly towards the centre; it 
 seems that the photospheric masses, no longer 
 meeting with resistance, are precipitated promis- 
 cuously anywhere so as to fill up the hole. It is 
 impossible to describe in detail the phases which 
 irregular spots go through, but two things may 
 always be remarked : that their structure is char- 
 acterized by the existence of luminous filaments, 
 and that these filaments converge towards one or 
 several centres. 
 
 Secchi thus sums up certain conclusions which 
 he arrived at relating to spots generally: (i) It 
 is not on the surface of any solid body that the 
 
THE SUN. 43 
 
 solar spots are manifested ; they are produced in 
 a fluid mass, the fluidity of which is represented 
 by a gas, so that the constitution of this medium 
 may be likened to that of flames or clouds; (2) 
 the known details respecting the constitution of 
 the penumbra and the phenomena exhibited prove 
 that the penumbra is not a mass of obscure mat- 
 ter which floats across luminous matter, but that 
 it is on the contrary a case of luminous matter 
 invading and floating about -over darker materials 
 and so producing a half tint. 
 
 All the available evidence which we possess 
 may be said to show that the spots are not merely 
 superficial appearances, but that they have their 
 origin deep in the interior of the Sun, and are 
 produced by the operation of causes still unknown 
 to us which affect and disturb the Sun's mass to 
 an extent which is sometimes very considerable. 
 The spots then are only the results of a great 
 agitation in the materials of which the Sun is 
 composed, and this agitation extends far down 
 below the limits of the visible dark nucleus what- 
 ever that may consist of. 
 
 Besides the spots, streaks of light may fre- 
 quently be remarked upon the surface of the Sun 
 towards the margin of the disc. These are termed 
 facultz (torches), and they are often found near 
 the spots, or where spots have previously existed 
 or have afterwards appeared. When quite near 
 the Sun's limb these faculae are usually more or 
 less parallel to the limb. 'They are of irregular 
 form and may be likened to certain kinds of coral. 
 They generally appear to be more luminous than 
 the solar surface immediately adjacent to them, 
 but it is not improbable that this is an optical illu- 
 sion depending upon the fact that the edges of 
 
44 THE STORY OF THE SOLAR SYSTEM. 
 
 the Sun always appear much more luminous than 
 the centre. This last-named fact may be readily 
 recognised by the employment of a high magnify- 
 ing power, and moving the telescope rapidly from 
 the limb to the centre of the disc. If the Sun be 
 projected on a screen, as already mentioned, this 
 degradation of the Sun's light from centre, to cir- 
 cumference becomes particularly manifest. 
 
 After having studied the structure and the 
 movement of the spots, one is naturally led to ask 
 if their apparitions at different periods are sub- 
 ject to any general law. This question is one 
 which has much engaged the attention of modern 
 astronomers. The older observers noticed that 
 the number of the spots visible differed in differ- 
 ent years. There were said to have been periods 
 when months and even years passed away without 
 any spots being observed. Even allowing that 
 this statement, so far as "years" are concerned, 
 might be exaggerated, and that the absence of 
 spots was due to the want of sufficient care in 
 making the observations, and especially to the 
 want of efficient instruments, it is none the less 
 true that the number of the spots is extremely 
 variable, and that there have been epochs when 
 they were very scarce. 
 
 Sir W. Herschel was the first who devoted 
 himself to the question of seeking to establish a 
 relation between the variation of the spots and 
 terrestrial meteorology. - For the want of any 
 better object, he compared the annual number of 
 the spots with the price of wheat ; but it is easy 
 to see that nothing could result from such a 
 comparison. Without doubt the meteorological 
 phenomena of the globe must depend to some ex- 
 tent on solar changes: but the term of compari- 
 
THE SUN. 45 
 
 son selected by Herschel had no direct bearing 
 on the state of the Sun. 
 
 In our time this question has been investi- 
 gated to its very foundation by Wolf, Director for 
 many years at the Observatory of Zurich. It is 
 to his zeal that we owe a very interesting assem- 
 blage of old observations which were buried in 
 archives and chronicles. It was he who endeav- 
 oured to reduce them into a systematic form, so 
 as to supply as far as possible the numerous gaps 
 which exist in the different series. 
 
 The two most attentive observers at the period 
 when the spots were discovered were Marriott at 
 Oxford and Scheiner at Ingoldstadt, but Scheiner 
 himself has informed us that he did not note down 
 all the spots which he saw; he only recorded 
 those which were likely to assist him in his spe- 
 cial task of determining the period of the Sun's 
 rotation. Several observers after him made iso- 
 lated series of observations; but some of these 
 have been lost and the others show important 
 gaps. J. G. Staudacher, at Nuremburg, observed 
 the Sun with great perseverance during fifty years 
 from 1749 to 1799. Before him the Cassinis, Mar- 
 aldi, and others were engaged in the same sort of 
 work, but only in an indirect way : that is to say, 
 they contented themselves, whilst making merid- 
 ional observations of the Sun, with noting any- 
 thing in the way of spots which they deemed 
 important. Zucconi and Flaugergues also left 
 behind them a good collection of observations 
 which Wolf utilised, rendering them comparable 
 one with another by applying suitable corrections. 
 The great difficulty herein arises from the fact 
 that the observers were not provided with instru- 
 ments of equal power ; one man, armed with a 
 
46 THE STORY OF THE SOLAR SYSTEM. 
 
 better telescope than his contemporaries, natu- 
 rally observed and recorded spots which would 
 escape the others. The numbers entered in their 
 registers are therefore not comparable inter se. 
 Wolf endeavoured to replace these numbers by 
 others which would represent the spots which 
 might have been seen if the observers had all 
 employed telescopes of a given kind and power. 
 The result of his efforts in this direction is an al- 
 most continuous series of Sun-spot records from 
 an epoch sufficiently remote, up to the time when 
 this branch of science was taken up with the 
 vigour of modern scientific methods. 
 
 The observer who most assiduously devoted 
 himself to this subject in modern times was 
 Schwabe of Dessau. From 1826 to 1868 he never 
 failed to make daily observations when the 
 weather permitted him. His series of records is 
 specially valuable, for Carrington's fits in with it, 
 and with that in turn Sporer's is comparable, and 
 the chain is complete by the later photographic 
 and other observations. All these Sun-spot rec- 
 ords, though differing in their details, may easily 
 be used together when it is a question of working 
 out relative annual fluctuations. 
 
 At the present time there are many Astrono- 
 mers who are engaged in observing the spots 
 with care ; but just as formerly there are few 
 who possess sufficient perseverance. The photo- 
 graphic method is excellent, but it takes much 
 time and is costly. Some have decried, in a very 
 unreasonable manner, a drawing made by hand: 
 such a drawing, of sufficient size, and executed 
 by projection by a skilful draughtsman with a 
 telescope driven by clockwork, may stand com- 
 parison with a photograph, and this method has 
 
THE SUN. 47 
 
 a better chance of being persevered in. The 
 Rev. F. Hewlett's name must be mentioned in 
 this connection as a draughtsman who has ac- 
 complished much by hand drawing. Though the 
 once famous Kew observations have been discon- 
 tinued, they have been replaced by a new series 
 at Greenwich with similar appliances ; whilst 
 Janssen at Meudon has also been carrying on for 
 a number of years a splendid course of photo- 
 graphic records. 
 
 Schwabe, when he had collected a consider- 
 able number of observations, recognised clear in- 
 dications of periodicity. Very definite epochs of 
 maxima and minima succeeded one another at 
 intervals of 10 or n years. It is true that in fol- 
 lowing out such a study the observations are cer- 
 tain to be in a sense a little defective. At first it 
 was not possible to observe the Sun every day, 
 and the gaps which resulted from bad weather 
 necessarily added to the number of days which 
 had to be set down as being without spots. More- 
 over, every method of numbering the spots must 
 be a little arbitrary : there are often groups 
 which, in consequence of their sub-divisions, may 
 be counted in different ways : but in a mass of 
 observations so considerable as those of Schwabe's, 
 such uncertainties will compensate for one another 
 and will disappear in the final result. In fact the 
 law is so striking that it suffices to cast one's eye 
 over his table* to see that. 
 
 That table is both interesting and instructive 
 at the same time. The numbers exhibited in it 
 speak for themselves, and it is sufficient to exam- 
 
 * Given in full in my Handbook of Astronomy, 4th ed., vol. 
 i., p. 26. 
 
 4 
 
48 THE STORY OF THE SOLAR SYSTEM. 
 
 ine them with even a small amount of attention 
 to realise the certainty of the conclusions which 
 have been drawn. 
 
 It is therefore now to be deemed an ascer- 
 tained fact that there are periodical maxima and 
 minima in the display of spots, and that the ex- 
 tent of the period is between 10 and 12 years. In 
 order to determine this value with the utmost 
 exactness, some astronomers have had recourse 
 to early observations. Wolf of Zurich made this 
 the subject of some very interesting inquiries. He 
 was able to establish the chronology of the phases 
 which the Sun has passed through from the time 
 of the first discovery of the spots to the present 
 day more than 2^ centuries. His calculations 
 led him to a period of n^ years. Lamont fixed 
 upon 10.43 years, but this number does not repre- 
 sent the more recent observations with sufficient 
 precision. 
 
 In order to exhibit this law in the plainest 
 possible manner the dates of maxima and minima 
 should be laid down on ruled paper in proper 
 mathematical form, the abscissa of the curve rep- 
 resenting the years, and the ordinates the number 
 of spots observed. 
 
 An examination of a curve thus plotted shows 
 two things: (i) That the period is clearly an 
 eleven-year one, as has been already stated; (2) 
 that it is not however quite as simple in its form 
 as it was at first thought to be; for in reality 
 there are two periods superposed, the one rather 
 more than half a century long, and the other ex- 
 tending over the n years already spoken of. 
 We do not possess early observations sufficiently 
 numerous and sufficiently good to enable us to 
 draw any unimpeachable conclusions as to the 
 
THE SUN. 49 
 
 nature of the long period ; we can only be certain 
 that it exists. The later labours of Wolf, how- 
 ever, fixed that period at 55^ years. It is a re- 
 sult of this that, according to Loomis, a period of 
 comparative calm on the Sun existed between 1810 
 and 1825. 
 
 Each maximum lies nearer to the minimum 
 which precedes it than to the minimum which fol- 
 lows it, for the spots increase during 3.7 years, 
 and then diminish during 7.4 years. According 
 to De La Rue the increase occupies 3.52 years, 
 and diminution 7.55 years. This concurrence be- 
 tween De La Rue and Wolf is surprising consid- 
 ering the diversity of the methods. which led to 
 results almost identical, the one set being based 
 on the number of the spots, and the other on the 
 superficial extent of the spots. The different 
 periods in succession are not absolutely identical: 
 but it has been remarked that if during any one 
 period the decrease is retarded or accelerated, then 
 the increase next following will be lengthened or 
 contracted to a corresponding extent. In conse- 
 quence of this we are sometimes able to predict 
 with fair accuracy when the next ensuing maxi- 
 mum or minimum will take place. 
 
 The most striking feature of such a curve as 
 that just alluded to is the very sensible secondary 
 augmentation which happens very soon after the 
 principal maximum. 
 
 A very curious circumstance has come to light 
 in connection with the epochs of maxima and 
 minima. In arranging the spots according to 
 their latitude and longitude on a diagram suffi- 
 ciently contracted, Carrington found that their 
 latitude decreases gradually as the period of 
 minimum draws near ; then when their number 
 
50 THE STORY OF THE SOLAR SYSTEM. 
 
 begins to increase they begin to appear again at 
 a higher latitude. This seems to be a definite 
 law. At any rate Carrington's conclusion has 
 been found to hold good by the observations of 
 Sporer and Secchi. 
 
 The variations of the spots which we now rec- 
 ognise naturally recall those obscurations of the 
 Sun which are recorded in history ; but it is ne- 
 cessary to accept many of these with caution. A 
 great number of these phenomena which attracted 
 the attention of people in early times are only 
 eclipses badly observed and still more badly de- 
 scribed. In other instances the obscuration has 
 been produced by very protracted dry fogs. It is 
 probably to this last-named cause that we must 
 ascribe the obscuration which, according to Kep- 
 ler and Gemma Frisius, took place in 1547. 
 
 It was in some such way as this that, according 
 to Virgil (Georg. i, 630), who has echoed a tradition 
 which he found in history, the Sun was obscured 
 at the death of Caesar : 
 
 Hie etiam extincto miseratus Csesare Romam 
 Quum caput obscura nitidum ferrugine texit, 
 Impiaque seternam timuerunt saecula noctem. 
 
 In the year 553 A. D., and again in the year 
 626 A. D. the Sun remained obscured for several 
 months; but these facts (if facts they are) besides 
 being ill-observed, and clothed, no doubt, in ex- 
 tremely exaggerated language, are brought to our 
 notice as having occurred at epoch? which are 
 quite independent of one another, whilst the 
 variations in the markings on the Sun, which we 
 have just been talking about, present an almost 
 mathematical regularity of sequence. 
 
 We must now institute some inquiries as to 
 
THE SUN. 51 
 
 the causes of the periodicity of the spots. A 
 periodicity so well established would naturally 
 invite astronomers to seek the causes which pro- 
 duced it. The presence of spots only in the Zo- 
 diacal regions led Galileo to suspect the existence 
 of some relation between the spots and the posi- 
 tion of the planets; but there is in this a mere 
 surmise, which, when it was made, had nothing to 
 justify it, and it is still impossible for us to say 
 anything for certain on the point. The deter- 
 mining cause of the periodicity may exist in the 
 interior of the Sun, and may depend on circum- 
 stances which will for ever remain unknown to 
 us. Or it may be something external : it may be 
 due after all to the influence of the planets. It 
 remains for us, therefore, to search and see if any 
 such influence can be traced. 
 
 According to Wolf, the attraction of the planets, 
 or of some of them, is the real cause of the pe- 
 riodicity which we are dealing with ; that attrac- 
 tion producing on the surface of the solar globe 
 true tides, which give birth to the spots, these 
 tides themselves experiencing periodic variations 
 owing to the periodic changes of position of the 
 celestial bodies which cause them. It has even 
 been thought safe to assert that the fact of the 
 principal period coinciding with the revolution of 
 Jupiter is of momentous significance; but this 
 coincidence seems purely accidental, and no cer- 
 tain conclusion can be drawn as to this matter. 
 The influence of Mercury and Venus would per- 
 haps be much more potent, for their distance from 
 the Sun is not very great, and this should render 
 their influence more sensible. On the other hand, 
 their masses appear to be too small to be capable 
 of producing any sufficient effect, 
 
52 THE STORY OF THE SOLAR SYSTEM. 
 
 De La Rue, Balfour Stewart, and Lowy most 
 perseveringly studied this point of solar physics. 
 They seem to have arrived at the conclusion 
 that the conjunctions of Venus and Jupiter do 
 exercise a certain amount of influence on the 
 number of the spots and on their latitude ; and 
 that this influence is less considerable when Venus 
 is situated in the plane of the solar equator. 
 At any rate it is a fact, that a great number of the 
 visible inequalities in a duly plotted curve of the 
 spots do really correspond to special positions of 
 these two planets. 
 
 In order to determine with more precision 
 these coincidences and the importance which at- 
 taches to them, De La Rue extended his inquiries. 
 He separately analysed many different groups of 
 spots, selecting for his purpose more particularly 
 those of which the observations happened to 
 have been specially continuous and complete, 
 giving a preference moreover to those which had 
 been observed in the central portions of the Sun's 
 disc. From an investigation of 794 groups De 
 La Rue arrived at the following conclusions : 
 (i) If we take a meridian passing through the 
 middle of the disc and represented by a diameter 
 perpendicular to the equator, we find that the 
 mean size of the spots is not the same with re- 
 gard *to that meridian. It appears certain that 
 the correction required for perspective does not 
 suffice to explain this difference ; and that another 
 element must be introduced in order to secure 
 that the apparent dimensions of the spots may 
 be the same on both sides. We do not yet pos- 
 sess a very clear explanation of this fact; but 
 the most probable is this : the spots are sur- 
 rounded by a projecting bank, which seems to 
 
THE SUN. 53 
 
 disappear in part during their transit across the 
 Sun. This bank is more elevated on the pre- 
 ceding than on the following side ; accordingly, 
 the spots ought to seem smaller when they are 
 in the eastern half of the disc; larger when they 
 are in the western half ; for in the first position 
 the observer's eye meets an elevated obstacle, 
 which hides a portion of the spot itself. (2) De 
 La Rue specially studied the spots observed at 
 the times when the planets Venus and Mars were 
 at a heliocentric distance from the Earth equal'to 
 o, 90, 180, and 270 degrees, and arrived at this 
 result ; the spots are larger in the part of the Sun 
 which is away from Venus and Mars, and they 
 are smaller on the side on which these planets 
 happen to be. The same result was obtained, 
 whether Carrington's figures or the Kew photo* 
 graphs were employed. (3) Meanwhile it does 
 not appear that Jupiter emits any similar influ- 
 ence. This influence should be easily perceived, 
 for if we calculate the action of the planets in the 
 way that we calculate the tides, treating it as di- 
 rectly proportional to the masses and inversely 
 proportional to the cubes of the distances, the 
 influence of Jupiter should greatly outweigh that 
 of Venus. 
 
 Wolf thought that he had noticed traces of 
 some influence being exerted by Saturn; but this 
 remains- altogether without confirmation. 
 
 De La Rue noticed that large spots are gener- 
 ally situated at extremities of the same diameter. 
 This law also often applies to the development 
 of large prominences. The coincidence agrees 
 well with the theory that there exists on the Sun 
 some action resembling that of our tides. 
 
 may be the amount of probability 
 
54 THE STORY OF THE SOLAR SYSTEM. 
 
 which attaches to these explanations we ought 
 not to forget that we are still far off from pos- 
 sessing the power of giving a vigorous demon- 
 stration of them. If we consider with attention 
 the periodical variations of the spots we shall not 
 be long in coming to the conclusion that it is im- 
 possible to connect them directly with any one 
 astronomical function in particular, for the spots 
 appear in a sudden and irregular manner which 
 contrasts in a striking degree with the continuous 
 and. progressive action of the ordinary perturba- 
 tions which we meet with in the study of Celestial 
 Mechanics. There is but one reply possible to 
 this objection. The spots and their changes must 
 be visible manifestations of the periodical activity 
 of the Sun an activity which itself depends (as 
 assumed) on the action of the planets and on 
 their relative positions. The cause, thus defined, 
 of the Sun's activity may be very regular ; the 
 activity itself may vary in a continuous manner 
 without the resulting phenomena possessing the 
 same continuity and the same regularity. We see 
 this in the periodical succession of the Seasons on 
 the Earth. The position of the Sun, and conse- 
 quently its manner of acting upon our globe, 
 varies with a remarkable uniformity, but never- 
 theless the meteorological phenomena which re- 
 sult are irregular and capricious. Thus it comes 
 about that physicists are more and more inclined 
 to believe that the spots are only secondary 
 effects produced by causes more important and 
 more fundamental. 
 
 Whatever may be our ignorance as to the 
 causes which produce variations in the Sun's activ- 
 ity we may at least draw one conclusion from 
 the preceding remarks : it is, that the Sun is a 
 
THE SUN. 55 
 
 very long way from having arrived at a state of 
 tranquillity and freedom from internal commotion. 
 On the contrary, it is the seat of great movements. 
 Its activity is subject to numberless periodical 
 changes which ought in their turn to influence the 
 intensity of the heat and light given out by the 
 Sun ; and so re-act on the planets which receive 
 their heat, light, and life from the Sun. 
 
 No account of the periodicity of the spots on 
 the Sun can be deemed complete which does not 
 include information respecting certain other peri- 
 odical phenomena which have been found to ex- 
 hibit features of alternation closely resembling 
 in their sequence and character the periodical 
 changes which take place in regard to the spots 
 on the Sun. There is evidently a deep mystery 
 lying hid under the curious fact (which is clearly 
 established) that the n-year period of the spots 
 coincides in a manner as unexpected as it is cer- 
 tain with the period of the variation of terrestrial 
 magnetism. The magnetic needle is subject to 
 a diurnal variation which reaches its extreme 
 amount every n years, and not only so, but the 
 epoch of maximum variation corresponds with the 
 epoch of the maximum prevalence of Sun spots. 
 And similarly years in which the needle is least 
 disturbed are also years in which the Sun spots 
 are fewest. Two other very curious discoveries 
 have also been made which are in evident close 
 connection with the foregoing. The manifesta- 
 tion of the Aurora Borealis and of those strange 
 currents of electricity known as magnetic earth 
 currents (which travel below the Earth's surface 
 and frequently interfere with telegraphic opera- 
 tions), likewise exhibit periodical changes which 
 take 1 1 years to go through all their stages. This 
 
56 THE STORY OF THE SOLAR SYSTEM. 
 
 fact alone would be sufficiently curious, but when 
 we come to find that the curve which exhibits the 
 changes these two manifestations of force go 
 through, also shows that their maxima and min- 
 ima are contemporaneous with the maxima and 
 
 FIG. 8. The Sun totally eclipsed, July 18, 1860 (Feilitzsch). 
 
 minima of the Sun spots and magnetic needle 
 variations, we cannot doubt that (to use Balfour 
 Stewart's words) " a bond of union exists between 
 these four phenomena. The question next arises, 
 what is the nature of this bond ? Now, with re- 
 spect to that which connects Sun spots with mag- 
 netic disturbances we can as yet form no conjec- 
 ture." To cut a long story short, it may be said 
 generally that whilst without doubt electricity is 
 the common basis of the three last-named of the 
 four phenomena just mentioned, it seems scarcely 
 
MERCURY. 57 
 
 too great a stretch of the imagination to go one 
 step further and suggest that electricity has in 
 some or other occult manner something to do 
 with all these things and therefore with the spots 
 on the Sun. 
 
 The reader who has followed me thus far will 
 by this time be in a position to appreciate a re- 
 mark made in an earlier part of this chapter, that 
 the multitude of facts known to us in connection 
 with the Sun and its spots is so great, as to render 
 it impossible to exhibit in a single chapter any- 
 thing more than the barest outline of them. The 
 numerous observations of recent eclipses of the 
 Sun, especially since that of 1860, and the exten- 
 sive application of the spectroscope to the Sun 
 both in connection with these eclipses, and gen- 
 erally, may be said to have completely revolu- 
 tionised our knowledge of solar phenomena dur- 
 ing the present generation ; or perhaps it might 
 be more correct to say have enormously increased 
 our knowledge of the facts of the case and have 
 revolutionised in no small degree the conclusions 
 deduced from the facts. 
 
 CHAPTER III. 
 
 MERCURY. 
 
 So far as we know at present, Mercury is the 
 nearest planet to the Sun. The circumstances 
 under Vhich it presents itself to us and a brief 
 general account of its movements have already 
 been stated. In the present chapter, therefore 
 (and this remark applies in substance to each of 
 
58 THE STORY OF THE SOLAR SYSTEM. 
 
 the succeeding chapters appropriated to particular 
 planets), I shall limit myself to such topics as 
 seem to be of interest to an observer armed with 
 a telescope. Mercury, as already mentioned, ex- 
 hibits from time to time phases which may be 
 said to be the same as those of the moon; but as 
 the only chance of seeing it is when it is at its 
 greatest distance east or west of the Sun, practi- 
 cally it can only be studied when in, or rather 
 near to, what may be called the half-moon phase ; 
 and even then observations on its physical ap- 
 pearance can only be obtained with difficulty. 
 Perhaps its most definite feature is its colour. 
 This, undoubtedly, is more or less pink. Strange 
 to say, in spite of the multiplication of telescopes 
 and observers, comparatively little attention has 
 been paid to this planet, and we really know very 
 little more about it than Schroter told us nearly 
 a hundred years ago. He obtained what he con- 
 ceived to be satisfactory evidence of the existence 
 of at any rate one mountain, having a height of 
 about ii English miles a height which it will be 
 noted, far exceeds, not only relatively but abso- 
 lutely, any mountain on the earth. What Schro- 
 ter based this conclusion upon was the fact that 
 when the planet was near inferior conjunction, 
 the southern horn presented a truncated appear- 
 ance, which might be the result of a lofty projec- 
 tion arresting the Sun's light. Schroter also an- 
 nounced that Mercury rotated on its axis in 24 
 hours 5 minutes. Sir W. Herschel failed to satisfy 
 himself that Schroter's conclusions were well- 
 founded, but it must certainly be admitted that 
 some support for them is furnished by certain ob- 
 servations made within the last few years. It is 
 matter for regret, however, that most of these 
 
MERCURY. 59 
 
 were made with instruments of sizes which, for 
 the most part, cannot be said to have been equal 
 to the task to which they were applied. The 
 truncature of the southern horn first spoken of by 
 Schroter, was thought by Denning, in 1882, to be 
 obvious ; and in the same year, by watching the 
 displacement of certain bright and dusky spaces 
 on the disc, the same observer concluded that a 
 rotation period of about 25 hours was indicated. 
 
 In 1882 Schiaparelli at Milan commenced a 
 prolonged study of Mercury. Believing that it 
 was essential to observe through a good condition 
 of atmosphere, and that this was impossible if the 
 planet were only looked at in twilight, when it 
 was necessarily at a low altitude, Schiaparelli 
 made all his observations with the Sun and planet 
 high up in the heavens. He considered, in effect, 
 that the blaze of the Sun's light was a lesser evil 
 than the tremors inseparable from observations of 
 the planet, clear it might be in some degree of 
 inconvenient Sun-light, but viewed through the 
 vapours and atmospheric disturbances, which al- 
 ways spoil all observations near the horizon. 
 Schiaparelli's observations yielded various results, 
 most of them novel, and one of them very star- 
 tling. He considers Mercury to be a much spotted 
 globe and to be enveloped in a tolerably dense 
 atmosphere. He thought he noticed brownish 
 stripes and streaks (which might be regarded as 
 permanent markings), more clearly visible on 
 some occasions than on others; and that these 
 systematically disappeared near the limb, owing 
 to the increased depth there of the atmosphere 
 through which they had to be looked at. 
 
 The foregoing observations may be regarded 
 as not unreasonable ; they may even be accepted 
 
60 THE STORY OF THE SOLAR SYSTEM. 
 
 without further question. But what are we to say 
 to Schiaparelli's conclusions that these markings 
 are so nearly permanent, taking one day with an- 
 other, that Mercury's rotation cannot be meas- 
 ured in hours at all, but is a matter of days, in 
 point of fact, of 88 days; and that in reality Mer- 
 cury occupies in its rotation on its axis the whole 
 of the 88 days which constitute its sidereal year, 
 or period of revolution round the Sun. The coun- 
 terpart of this for us would be that, instead of the 
 inhabitants of the earth having a day of 24 hours, 
 they would have only one day and night every 
 365 days. Astronomers are not at present satis- 
 fied to accept this conclusion in regard to Mercury. 
 Some observers have thought that Mercury is 
 more easy to observe than Venus, and that, speak- 
 ing generally, its surface, if we could only get to 
 see it constantly under favourable circumstances, 
 might be considered to resemble in most respects 
 that of Mars. Mercury revolves round the Sun at 
 a mean distance of 36 millions of miles. Owing, 
 however, to the fact that the eccentricity of its 
 orbit (or its departure from the circular form) 
 is greater than that of any of the other major 
 planets, it may approach to within 28-^ millions of 
 miles or recede to more than 43 millions of miles. 
 Its apparent diameter varies between 4^" in su- 
 perior conjunction to 13" in inferior conjunction. 
 The real diameter may be taken at about 3000 
 miles. 
 
VENUS. 6 1 
 
 CHAPTER IV. 
 
 VENUS. 
 
 THE planet Venus has two things in common 
 with Mercury. One is, that being an inferior 
 planet, that is to say, a planet revolving round 
 the Sun in an orbit within that of the Earth, it is 
 never very far distant from the Sun, and there- 
 fore can never be seen on a distinctly dark sky. 
 The second point alluded to arises out of the 
 first; Venus exhibits from time to time a series 
 of phases which are identical in character with 
 those of Mercury, and therefore with those of the 
 Moon. Venus differs, however, from Mercury in 
 the very important point of size. Inasmuch as 
 its diameter is considerably more than double the 
 diameter of Mercury it has a surface more than 
 six times as great, and therefore exhibits a far 
 larger area of illumination than Mercury does. 
 The result of this (coupled with another fact 
 which will be stated presently) is that the planet 
 may often be easily seen in broad daylight, and 
 sometimes casts a sensible shadow at night. Un- 
 der special circumstances, which recur every 8 
 years, this planet shines with very peculiar bril- 
 liancy. True, that only about Jth of the whole 
 disc is then illuminated, but that fraction trans- 
 mits to us more light than phases of greater ex- 
 tent do, because these latter coincide with epochs 
 when the planet is more remote from the Earth. 
 
 Spots and shadings have on various occasions 
 been noticed on Venus, and though it is not easy 
 to harmonise the various accounts, there seems 
 no doubt of the reality of the facts, or that they 
 
62 THE STORY OF THE SOLAR SYSTEM. 
 
 must be ascribed to the existence of mountains. 
 Schroter found very much the same state of 
 things to exist on Venus that he found on Mer- 
 cury, and putting together what he saw he ar- 
 rived at the conclusion that Venus possesses 
 mountains of considerable height, and that his 
 observations must be taken to imply that the 
 planet revolved on its axis in rather more than 
 23 hours. This conclusion as regards the planet's 
 axial rotation was not first arrived at by Schroter, 
 for the two Cassinis, one about 1666, and the 
 other about 1740, both ascribed to Venus a rota- 
 tion period of about 23 hours, an evaluation which 
 was fully confirmed by Di Vico at Rome between 
 1839 and 1841, and by Flammarion in 1894. 
 
 What has been already said with respect to 
 Mercury is true also of Venus, namely that it has 
 been much neglected by modern observers; and 
 accordingly an announcement made by Schia- 
 parelli in 1890, that the rotation period of Venus 
 is to be measured not by hours but by months, 
 came upon the astronomical world as a startling 
 revelation ; but it is a revelation which has been 
 keenly contested, and certainly awaits legal proof. 
 Schiaparelli has not ventured to assert as he has 
 done in the case of Mercury, that Venus's rotation 
 period is identical with the period of 7^ months 
 in which it revolves round the Sun ; he only 
 claims this as a strong probability arising out of 
 what he says he is certain of, namely that its 
 period of rotation cannot be less than six months 
 and may be as much as nine months. His as- 
 sumption is that previous observers in endeav- 
 ouring to ascertain Venus's rotation period have 
 used and relied upon evanescent shadings which 
 probably were of atmospheric origin and scarcely 
 
VENUS. 63 
 
 recognisable from day to day, whereas he fixed 
 his attention upon round defined white spots, 
 which, whatever their origin, are so far permanent 
 that their existence has been spoken of for two 
 centuries. Miss Clarke thus puts the matter: 
 " His steady watch over them showed the in- 
 variability of their position with regard to the 
 terminator; and this is as much as to say that the 
 regions of day and night do not shift on the sur- 
 face of the planet. In other words she keeps the 
 same face always turned towards the Sun." 
 
 Various recent observations, some of them 
 made with the express object of throwing light 
 upon Schiaparelli's conclusions, are strangely con- 
 tradictory. Perrotin at Nice in 1890 thought his 
 observations confirmed Schiaparelli's ; on the other 
 hand Niesten at Brussels considered that numer- 
 ous drawings of Venus made by himself and 
 Stuyvaert between 1881 and 1890 harmonised 
 well with DiVico's rotation period of 23?!. 2im. 
 22S. ; which Trouvelot in 1892 only wished to in- 
 crease to about 24 hours. 
 
 There is a general consensus of opinion that 
 great irregularities exist on the surface of Venus. 
 These are made specially manifest to us in con- 
 nection with the terminator or visible edge of the 
 planet seen as an illuminated crescent. If the 
 planet had a smooth surface this line would at all 
 times be a perfect and continuous curve, instead 
 of which it is frequently to be noticed as a jagged 
 or broken line. Observations to this effect go 
 back as far as 1643, when Fontana at Naples ob- 
 served this to be the condition of the terminator. 
 La Hire, Schroter, Madler, Di Vico and many 
 others down to the present epoch have noted the 
 same thing. The fact that the southern horn of 
 5 
 
6 4 
 
 THE STORY OF THE SOLAR SYSTEM. 
 
 Venus is^constantly to be seen blunted is so well 
 established as to admit of no doubt, and this 
 
 blunting is 
 commonly as- 
 cribed to the 
 existence of a 
 lofty moun- 
 tain, to which 
 Schroter as- 
 cribed a height 
 of 27 miles. 
 Whatever we 
 may think as 
 to the precise 
 accuracy of 
 this figure, it 
 seems impos- 
 sible to doubt 
 the main fact 
 on which it de- 
 pends ; whilst 
 a Belgian observer, Van Ertborn, in 1876 repeat- 
 edly saw a point of light in this locality which he 
 regarded as due to Sun-light impinging on a de- 
 tached peak, adjacent valleys remaining in shadow. 
 This effect is common enough in the case of the 
 Moon, and is familiar to all who are in the habit 
 of studying the Moon. 
 
 The existence on Venus of an atmosphere of 
 considerable density and extent is well estab- 
 lished. Proof of this is to be found in the 
 marked diminution of the planet's brilliancy 
 towards the terminator; and in the faint curved 
 line of light which occasionally may be seen when 
 the planet is near inferior conjunction. When so 
 situated, so much of the planet itself as can be 
 
 FIG. 9. Venus, Dec. 23, 1885. 
 
VENUS. 
 
 seen illuminated shows as a narrow radiant cres- 
 cent of light, ending off in two points called in- 
 differently cusps or horns. It sometimes happens, 
 however, that from the point of each cusp there 
 runs round to the other cusp a faint continuation 
 of the crescent, resulting in the general appear- 
 ance of the planet being that of a nearly uniform 
 ring of light. 
 There is no 
 known way in 
 which the Sun 
 can illuminate 
 so much more 
 than the half 
 of Venus so as 
 to permit of a 
 perfect circle 
 being visible 
 except by sup- 
 posing that 
 an atmosphere 
 exists on the 
 planet and 
 refracts (or 
 transmits by bending, as it were, round the cor- 
 ner) a sufficient amount of Sun-light to give rise 
 to the appearance in question. Further proof of 
 the existence of an atmosphere on Venus is ob- 
 tainable on those very rare occasions when the 
 planet is seen passing across the disc of the Sun 
 a phenomenon known as a " Transit of Venus." 
 It then nearly always happens that a hazy nebu- 
 lous ring of feeble light may be detected encom- 
 passing the planet's disc indicative of course of 
 the fact that the Sun's rays are there slightly 
 obstructed in reaching the eye of an observer on 
 
 FIG. 10. Venus near conjunction as a thin 
 crescent, Sept. 21, 1887 (Flammarion). 
 
66 THE STORY OF THE SOLAR SYSTEM. 
 
 the Earth. Some observers scrutinising Venus 
 when in transit have thought that they were able 
 to obtain, by means of the spectroscope, traces of 
 aqueous vapour on the planet, but the evidence 
 of this does not appear to be altogether clear or 
 conclusive. 
 
 Everybody may be presumed to be acquainted 
 with the spectacle popularly known as " The Old 
 Moon in the New Moon's Arms " whereby when 
 the Moon is only about two or three days old and 
 exhibits but a narrow crescent of bright light,, yet 
 the whole outline of the disc is traceable on the 
 sky. A phenomenon analogous to this may often 
 be seen in the case of Venus when near its infe- 
 rior conjunction. With the Moon the cause is 
 due to the reflection of Earth-light (so to speak) 
 to the Moon, but that explanation seems inade- 
 quate in respect of Venus, because it is conceived 
 that the amount of Earth-light available is alto- 
 gether insufficient for the purpose. Many other 
 explanations have been put forward including 
 phosphorescence on the surface of Venus, elec- 
 trical displays in the nature of terrestrial aurorae, 
 and what not, but it must be frankly confessed 
 that astronomers are all at sea on the subject. 
 
 The existence of snow at the poles of Venus 
 has been suspected by observers of tried skill 
 and experience such as Phillips and Webb, though 
 the idea was first broached by Gruithuisen in 
 1813. Flammarion's observations during 1892 
 and the two following years are distinctly con- 
 firmatory of this idea. He adds that as both 
 polar caps are visible at the same time the plan- 
 et's axis cannot be much inclined to the plane of 
 its orbit. 
 
 Compared with all the other planets the ab- 
 
VENUS. 67 
 
 solute brightness of Venus stands very high. Of 
 course it must be understood that by this phrase 
 "absolute brightness" no more is meant than its 
 reflective power. Venus is what it is by virtue of 
 its power of reflecting Sun-light; presumably it 
 has no inherent brightness of its own. What iU 
 reflective power is was probably never more 
 effectively brought under the notice of a human 
 eye than on September 26, 1878, when Nasmyth 
 enjoyed an opportunity of seeing Venus ana 
 Mercury side by side for several hours in the 
 same field of view. He speaks of Venus as re- 
 sembling clean silver and Mercury as nothing 
 better than lead or zinc. Seeing that owing to 
 its greater proximity to the Sun the light incident 
 on Mercury must be some 3^ times as strong as 
 the light incident on Venus, it follows that the 
 reflective power of Venus must be very great. 
 As a matter of fact it has been calculated to be 
 nearly equal to newly fallen snow; in other words 
 to reflect fully 70 per cent, of the light which 
 impinges on it. 
 
 Venus has no satellite; this fact seems certain. 
 Yet half a dozen or more observers between 1645 
 and 1768 discovered such a satellite; observed a; 
 followed it! This startling mystery, as it realiy 
 was, attracted some years ago the attention of a 
 very careful Belgian observer, Stroobant, wno 
 examined in a most painstaking manner all me 
 recorded observations. His conclusions were 
 that in almost all cases particular stars (which he 
 identified) were mistaken for a satellite. Where 
 the object seen was not capable of identification, 
 possibly it was a minor planet ; whilst in one in- 
 stance it was probable that it was Uranus which 
 had been seen and regarded as a satellite of Venus. 
 
68 THE STORY OF THE SOLAR SYSTEM. 
 
 Venus is perhaps the planet which has most 
 impressed the popular mipd. For the earliest 
 illustration of this statement we must go as far 
 back as Homer who makes two references to it 
 in the Iliad. These, in Pope's version, run as 
 follows : 
 
 " As radiant Hesper shines with keener light, 
 Far beaming o'er the silver host of night." 
 
 xxii. 399 [318]. 
 
 " The morning planet told th' approach of light ; 
 And fast behind, Aurora's warmer ray 
 O'er the broad ocean pour'd the golden day." 
 
 xxiii. 281 [226]. 
 
 The phases of Venus were first discovered by 
 Galileo and were made known to the world, or 
 rather to Kepler, in a mystic sentence which has 
 often been quoted : 
 
 " Hac immatura, a me jam frustra leguntur oy" 
 
 " These things not ripe ; at present [read] in vain [by others] 
 are read by me." 
 
 The former sentence transposed becomes 
 
 Cynthia figuras cemulatur mater amorum. 
 
 The mother of loves [Venus] imitates the phases of Cynthia 
 [the Moon]. 
 
 Venus revolves round the Sun in 224^ days at 
 a mean distance of about 67 millions of miles. 
 Its apparent diameter varies between p-J-" in su- 
 perior conjunction, and 62" in inferior conjunc- 
 tion. The real diameter is about 7500 miles; ib 
 other words Venus is nearly as large as the 
 Earth. 
 
THE EARTH. 69 
 
 CHAPTER V. 
 
 THE EARTH. 
 
 To us, as its inhabitants, the Earth appeals in 
 two characters, and in writing a book on astron- 
 omy it is necessary, yet difficult, to keep these 
 two characters separate. The Earth is an ordi- 
 nary planet member of the solar system, amenable 
 to the same laws, impelled by the same forces, 
 and going through the same movements as the 
 other members of the Sun's entourage. Yet, by 
 reason of the fact that we are ourselves on the 
 Earth and are not spectators of it looking at it 
 from at a distance, there are many phenomena 
 coming under our notice which require special 
 treatment, and it is often very difficult to say 
 where the province of the astronomer ends and 
 that of the geographer begins. This volume 
 being specially designed to deal with astronomical 
 matters, I shall pass over many subjects which 
 may be said to be on the border line, and which 
 some of my readers may therefore be disappointed 
 not to find discussed. Besides the geographer, 
 the geologist and his scientific brother the miner- 
 alogist are concerned with the Earth regarded 
 as a planet moving through space as the other 
 planets do. The geologist studies the actual 
 structure of the Earth, its circumstances and his- 
 tory so far as they have been revealed to us, 
 whilst the mineralogist investigates and names 
 the materials of which it is composed, and classifies 
 such materials with the assistance of the geologist 
 on the one hand and of the chemist on the other. 
 All these subordinate sciences subordinate I 
 
70 THE STORY OF THE SOLAR SYSTEM. 
 
 mean from an astronomer's point of view open 
 up very varied, instructive, and interesting fields 
 of study, but they are of course foreign to the 
 purpose of the present volume. 
 
 Though the Earth is commonly regarded as a 
 sphere it is not that in reality, because it is not 
 of identical dimensions from east to west and 
 from north to south. It is somewhat flattened 
 at the poles ; its polar diameter is less than its 
 equatorial diameter, in the ratio of about 298 to 
 299, or, expressed in miles, its polar diameter is 
 about 26 miles less than its equatorial diameter. 
 If a globe 3 feet in diameter be taken to represent 
 the Earth, then the polar diameter will, on this 
 scale, be % inch too long. This flattening of the 
 poles of the Earth finds its counterpart, so far as we 
 know, in most, and probably in all of the planets. 
 It is most considerable and therefore most con- 
 spicuous in the case of Jupiter. It ought here to 
 be added that a suspicion exists that the equato- 
 rial section of the Earth is not a perfect circle, 
 but that the diameter of the Earth, taken through 
 the points on the equator marked by the merid- 
 ians 13 58' and 193 58' east of Greenwich, is one 
 mile longer than the diameter at right angles to 
 these two points. 
 
 The science which inquires into matters of this 
 kind, including besides the figure of the Earth, 
 the length of the degree at different latitudes, 
 and the distances of places from one another, 
 alike in angular measure and in time, is called 
 Geodesy ; it is, in point of fact, land-surveying 
 on a very large scale, in which instruments and 
 processes of astronomical origin are brought into 
 operation, and in which astronomers are more or 
 less required to take the lead. 
 
THE EARTH. 71 
 
 Although we all of us now perfectly under- 
 stand that the Earth is a planet moving round 
 the Sun as a centre, it is, comparatively speaking, 
 but recently that this fact has become generally 
 recognised and understood. It is true that we 
 can discover here and there in ancient writings 
 some trace of the idea, yet it is doubtful whether 
 2000 years ago more than a few " advanced " 
 thinkers thoroughly and clearly accepted it as 
 a distinct truth. It was much more in consonance 
 with popular thought and the actual appearance 
 of things that the Earth should be the centre 
 round which the Sun revolved and on which the 
 planets depended ; and accordingly, sometimes in 
 one shape and sometimes in another, the notion 
 of the Earth being the centre of the universe was 
 generally accepted. The contrary opinion had, 
 however, a few sympathisers. For instance, Aris- 
 tarchus of Samos, who lived in the third century 
 before the Christian era, supposed, if we may trust 
 the testimony of Archimedes and Plutarch, that 
 the Earth revolved round the Sun ; this, however, 
 was regarded as a " heresy," in respect of which 
 he was accused of "impiety." Some few years 
 elapsed and a certain Cleanthes of Assos is said 
 by Plutarch to have suggested that the great 
 phenomena of the universe might be explained 
 by assuming that the Earth was endued with a 
 motion of translation round the Sun together with 
 one of rotation on its own axis. The historian 
 states that this idea was so contrary to the re- 
 ceived opinions that it was proposed to put Clean- 
 thes on his trial for impiety. 
 
 In former times the philosophers who studied 
 the solar system ranged themselves in several 
 " schools of thought," to use a modern hackneyed 
 
72 THE STORY OF THE SOLAR SYSTEM. 
 
 phrase. Some upheld the Ptolemaic system, 
 which took its name from a great Egyptian as- 
 tronomer, Claudius Ptolemy, though it does not 
 appear that he was actually the first to suggest 
 it. The Ptolemaic system regarded the Earth as 
 the centre, with the following bodies, all called 
 planets, revolving round it in the order stated : 
 the Moon, Mercury, Venus, the Sun, Mars, Jupiter, 
 and Saturn. It will be observed that there are 
 seven bodies here named, and as seven was re- 
 garded as the " number of perfection," it was in 
 later times considered that only these seven 
 bodies (neither more nor less) could really be the 
 Earth's celestial attendants. Though Ptolemy 
 was in one sense an Egyptian, there yet prevailed 
 amongst the Egyptians at large another theory 
 slightly different from Ptolemy's. According to 
 the " Egyptian theory," Mercury and Venus were 
 regarded as satellites of the Sun, and not as pri- 
 mary planets appurtenant to the Earth. 
 
 After Ptolemy's era many centuries elapsed, 
 during which the whole subject of the solar sys- 
 tem lay practically dormant, and it continued 
 so until the revival of learning brought new 
 theorists upon the scene. The most important 
 of these was Copernicus, who, in the sixteenth 
 century, propounded a theory which eventually 
 superseded all others, and, with slight modifica- 
 tions, is the one now accepted. Copernicus 
 placed the Sun in the centre of the system, and 
 treated it as the point around which all the pri- 
 mary planets revolved. So far, so good ; but 
 Copernicus went astray on the question of the 
 orbits of the planets. He failed to realise the 
 true character of the curves which they follow 
 and treated these curves as " epicycles," which 
 
THE EARTH. 73 
 
 word may be described as representing a compli- 
 cated combination of little circles which taken 
 together form a big one. It was left to Kepler 
 and Newton to settle all such details on a true 
 and firm basis. But before this stage was 
 reached a man of the highest astronomical attain- 
 ments and practical experience, Tycho Brahe, 
 made shipwreck of his reputation as an astrono- 
 mer by solemnly reviving the idea of the Earth 
 being the immovable centre of everything. He 
 treated the Moon as revolving round the Earth 
 at no great distance and the Sun as doing the 
 same thing a little farther off; the five planets 
 revolving round the sun as solar satellites. The 
 " Tychonic system," as it is called, has something 
 in common with the Ptolemaic system without 
 being by any means as logical as the latter. 
 That such far fetched ideas as Tycho's should 
 have been palmed off on the world of science so 
 recently as 300 years ago is passing strange; but 
 the explanation appears to be that his action 
 arose out of a misconception of certain passages 
 of Holy Scripture, which seemed irreconcilable 
 with the Copernican theory. It must not be 
 forgotten that Copernicus's famous book, pub- 
 lished in 1543, in which he had announced his 
 views, had been condemned by the Papal " Con- 
 gregation of the Index ; " and therefore Tycho 
 might have had as a further motive a desire to 
 curry favour with the authorities of the Church 
 of Rome, and to gratify his own vanity at the 
 same time. 
 
 With these explanations it will no longer be 
 misleading if, for convenience sake, I speak of a 
 certain great circle of the heavens as apparently 
 traversed by the Sun every year, owing to the 
 
74 THE STORY OF THE SOLAR SYSTEM. 
 
 revolution of our Earth round that body. This 
 circle is called the " Ecliptic," and its plane is 
 usually employed by astronomers as a fixed plane 
 of reference. It must be distinguished from 'that 
 other great circle called the " celestial equator," 
 which is the plane of the Earth's equator ex- 
 tended towards the stars. The plane of the 
 equator is inclined to the ecliptic at an angle of 
 about 23^-, which angle is known as the " ob- 
 liquity of the ecliptic." It is this inclination 
 which gives rise to the seasons which follow one 
 another in succession during our annual journey 
 round the Sun. The two points where the ce- 
 lestial equator and the ecliptic intersect are called 
 the " equinoxes," of spring or autumn as the case 
 may be ; the points midway between these being 
 the " solstices," of summer or winter as the case 
 may be. These words need but little explana- 
 tion, at any rate, as regards those persons who 
 are able to trace the Latin origin of the words. 
 <; Equinox " is simply the place occupied by the 
 Sun twice every year (namely about March 20 
 and September 22),, when day and night are theo- 
 retically equal throughout the world, when also 
 the sun rises exactly in the east and sets exactly 
 in the west. The " solstices " represent the stand- 
 ing still of the sun at the given times and places, 
 and are the neutral points where the Sun attains its 
 greatest northern or southern declination. This 
 usually occurs about June 21 and December 21. 
 It must not be forgotten by the way, that the 
 above application of the words " summer " and 
 " winter " to the solstices is only correct so far as 
 concerns places in northern terrestrial latitudes 
 Europe and the United States, for instance. In 
 southern terrestrial latitudes for instance, when 
 
THE T^ARTH. 75 
 
 speaking of what happens at the Cape of Good 
 Hope and in Australia the words must be re- 
 versed. 
 
 We have seen in a previous chapter that 
 whilst the orbits of the planets are nearly true 
 circles, none of them are quite such : and the 
 departure from the truly circular form results in 
 some important consequences. Whilst some of 
 these are too technical to be explained in detail 
 here, one at least must be referred to because 
 of what it involves. Not only is the Earth's or- 
 bit eccentric in form, but its eccentricity varies 
 within narrow limits; and besides this the orbit 
 itself, as a whole, is subject to a periodical shift 
 of place, from the joint effect of all which changes 
 it comes about that our seasons are now of un- 
 equal length, the spring and summer quarters of 
 the year unitedly extending to 186 days, whilst 
 the autumn and winter quarters comprise only 
 178 days. The sun therefore has the chance of 
 shining for a longer absolute period of time over 
 the northern hemisphere than over the southern 
 hemisphere ; hence the northern is the warmer 
 of the two hemispheres, because it has a better, 
 because a longer, chance of storing up an accu- 
 mulation of solar radiant heat. Probably it is one 
 result of this that the north polar regions of the 
 Earth are easier of access than the south polar 
 regions. In the northern hemisphere navigators 
 have reached to 81 of latitude, whereas 71 is 
 the highest limit yet attained in the southern 
 hemisphere. Readers who have studied the his- 
 tory of explorations in the Arctic regions will not 
 need to be reminded of the controversy which has 
 so often arisen respecting the existence or non- 
 existence of an " Open Polar Sea." 
 
7 6 THE STORY OF THE SOLAR SYSTEM. 
 
 It has already been hinted that it is not an 
 easy matter to determine, when dealing with the 
 Earth, where astronomy and its allied sciences, 
 geography, geodesy and geology respectively, 
 begin and end. But as certain topics connected 
 with these sciences, such as the rotundity of the 
 Earth and its rotation on its axis, will come more 
 conveniently under consideration in other vol- 
 umes of this series, I shall pass them over and 
 only treat of a few things which more directly 
 concern the student of nature observing either 
 with or without the assistance of a telescope. 
 
 The fact that the Earth is surrounded by a 
 considerable atmosphere largely composed of 
 aqueous vapour has a material bearing on the 
 success or failure of observations made on the 
 Earth of bodies situated at a distance. It may 
 be taken as a general rule that the nearer an 
 observer is to the surface of the sea, or otherwise 
 to the surface of the land at the sea-level, the 
 greater will be the difficulty which will confront 
 him in carrying on astronomical observations. 
 Hence such observations are generally made with 
 unsatisfactory results on the sea coast or on the 
 banks of rivers. An interesting but rather an- 
 cient illustration of this last-named fact is to be 
 found in the circumstance that Copernicus, who 
 died at the age of 70, complained in his last mo- 
 ments that much as he had tried he had never 
 succeeded in detecting the planet Mercury, a fail- 
 ure due, as Gassendi supposed, to the vapours 
 prevailing near the horizon at the town of Thorn 
 on the banks of the Vistula where the illustrious 
 philosopher lived. 
 
 The phenomena depending on the presence of 
 aqueous vapour in the atmosphere which espe- 
 
THE EARTH. 77 
 
 cially come under the notice of the astronomer 
 are Refraction, Twilight, and the Twinkling of 
 the Stars. 
 
 Refraction is what it professes to be, a bend- 
 ing, and what is bent is the ray of light coming 
 from a celestial object to a terrestrial station. 
 Olmsted has put the matter in this way : "We 
 must consider that any such object always appears 
 in the direction in which the last ray of light 
 comes to the eye. If the light which comes from 
 a star were bent into fifty directions before it 
 reached the eye, the star would nevertheless ap- 
 pear in a line described by the ray nearest the 
 eye. The operation of this- principle is seen when 
 an oar, or any stick, is thrust into the water. As 
 the rays of light by which the oar is seen have 
 their direction changed as they pass out of water 
 into air, the apparent direction in which the body 
 is seen is changed in the same degree, giving it a 
 bent appearance the part below the water hav- 
 ing apparently a different direction from the part 
 above." The direction of this refraction is deter- 
 mined by the general law of optics that when a 
 ray of light passes out of a rarer into a denser 
 medium (for instance out of air into water, or out 
 of space into the Earth's atmosphere) it is bent 
 towards a perpendicular to the surface of the 
 medium ; but when it passes out of a denser into 
 a rarer medium it is bent / ~rom the perpendicular. 
 The effect of refraction is to make a heavenly 
 body appear to have an apparent altitude greater 
 than its true altitude, so that, for example, an 
 object situated actually in the horizon will appear 
 above it. Indeed it sometimes happens that ob- 
 jects which are actually below the horizon and 
 which otherwise would be invisible were it not 
 
78 THE STORY OF THE SOLAR SYSTEM. 
 
 for refraction are thus brought into sight. It 
 was in consequence of this that on April 20, 1837, 
 the Moon rose eclipsed before the Sun had set. 
 
 Sir Henry Holland thus alludes to the phenom- 
 enon : " I am tempted to notice a spectacle, hav- 
 ing a certain association with this science, which 
 I do not remember to have seen recorded either in 
 prose or poetry, though well meriting description 
 in either way. This spectacle requires, however, 
 a combination of circumstances rarely occurring 
 a perfectly clear Eastern and Western horizon, 
 and an entirely level intervening surface, such as 
 that of the sea or the African desert the former 
 rendering the illusion, if such it may be called, 
 most complete to the eye. The view I seek to 
 describe embraces the orb of the setting Sun, 
 and that of the full Moon rising in the East 
 both above the horizon at the same time. The spec- 
 tator on the sea between, if he can discard from 
 mental vision the vessel on which he stands, and 
 regard only these two great globes of Heaven 
 and the sea-horizon circling unbroken around 
 him, gains a conception through this spectacle 
 clearer than any other conjunction can give, of 
 those wonderful relations which it is the triumph 
 of astronomy to disclose. All objects are ex- 
 cluded save the Sun, the Moon, and our own 
 Globe between, but these objects are such in 
 themselves that their very simplicity and paucity 
 of number enhances the sense of the sublime. 
 Only twice or thrice, however, have I witnessed 
 the sight in its completeness once on a Medi- 
 terranean voyage between Minorca and Sardinia 
 once in crossing the desert from Suez to Cairo, 
 when the same full Moon showed me, a few hours 
 later, the very different but picturesque sight of 
 
THE EARTH. 7$ 
 
 one of the annual caravans of Mecca pilgrims, 
 with a long train of camels making their night 
 march towards the Red Sea."* 
 
 It is due to the same cause that the Sun and 
 the Moon when very near the horizon may often 
 be noticed to exhibit a distorted oval outline. 
 The fact simply is, that the upper and the lower 
 limbs undergo a different degree of refraction. 
 The lower limb being nearer the horizon is more 
 affected and- is consequently raised to a greater 
 extent than the upper limb, the resulting effect 
 being that the two limbs are seemingly squeezed 
 closer together by the difference of the two re- 
 fractions. The vertical diameter is compressed 
 and the circular outline becomes thereby an oval 
 outline with the lesser axis vertical and the 
 greater axis horizontal. 
 
 Though the foregoing information merely em- 
 braces a few general principles and facts, the 
 reader will have no difficulty in understanding 
 that refraction exercises a very inconvenient dis- 
 turbing influence on observations which relate to 
 the exact places of celestial objects. No such 
 observations are available for mutual comparison, 
 however great the skill of the observer, or the 
 perfection of his instrument, unless, and until 
 certain corrections are applied to the observed 
 positions in order to neutralise the disturbing 
 effects of refraction. In practice this is usually 
 done by means of tables of corrections, those in 
 most general use being Bessel's. Inasmuch as re- 
 fraction depends upon the aqueous vapour in the 
 atmosphere, its amount at any given moment is 
 affected by the height of the barometer and the 
 
 *" Recollections of Past Life" 2nd ed., p. 305. 
 6 
 
So THE STORY OF THE SOLAR SYSTEM. 
 
 temperature of the air. Accordingly when, for 
 any purpose, the utmost precision is required, it 
 is necessary to take into account the height of 
 the barometer and the position of the mercury in 
 the thermometer at the moment in question. At 
 the zenith there is no refraction whatever, objects 
 appearing projected on the background of the 
 sky exactly in the position they would occupy 
 were the earth altogether destitute of an atmos- 
 phere at all. The amount of the refraction 
 increases gradually, but in accordance with a 
 very complex law, from the zenith to the horizon. 
 Thus the displacement due to refraction which at 
 the zenith is nothing and at an altitude of 45 
 is only 57" becomes at the horizon more than . 
 One very curious consequence is involved in the 
 fact that the displacement due to refraction is at 
 the horizon what it is ; the diameter both of the 
 Sun and Moon may be said to be -J- , more or 
 less, so that when we see the lower edge of either 
 of these luminaries just touching the horizon in 
 reality the whole disc is completely below it, and 
 would be altogether hidden by the convexity of 
 the earth were it not for the existence of the 
 earth's atmosphere and the consequent refraction 
 of the rays of light passing through it from the 
 Sun (or Moon) to the observer. 
 
 Twilight is another phenomenon associated 
 with astronomical principles and effects which de- 
 pends in some degree on the Earth's atmosphere 
 and on the laws which regulate the reflection 
 and refraction of light. After the Sun has set it 
 continues to illuminate the clouds and upper 
 strata of the air just as it may often be seen 
 shining on the tops of hills long after it has dis- 
 appeared from the view of the inhabitants of the 
 
THE EARTH. 8 1 
 
 plains below, and indeed may illuminate the 
 chimneys of a house when it is no longer visible 
 to a person standing in the garden below. The 
 air and clouds thus illuminated reflect some of 
 the Sun's light to the surface of the earth lying 
 immediately underneath, and thus produce after 
 sun-set and before sun-rise, in a degree more or less 
 considerable according as the Sun is only a little 
 or is much depressed below the horizon, that lumi- 
 nous glow which we call " twilight.'- This word is 
 of Saxon origin and implies the presence of a twin, 
 or double, light. As soon as the Sun has disap- 
 peared below the horizon all the clouds overhead 
 continue for a few minutes so highly illuminated 
 as to reflect scarcely less light than the direct 
 light of the Sun. As, however, the Sun gradu- 
 ally sinks lower and lower, less and less of the 
 visible atmosphere receives any portion of its 
 light, and consequently less and less is reflected 
 minute by minute to the Earth at the observer's 
 station until at length the time comes when there 
 is no sunlight to be reflected and it is night. 
 The converse of all this happens before and up 
 to sun-rise; night ceases, twilight ensues, gradu- 
 ally becoming more definite ; the dawn appears, 
 and finally the full Sun bursts forth. It may here 
 be stated as a note by the way that the circum- 
 stances under which the Sun first shows itself 
 after it has risen above the horizon has some 
 bearing on the probable character of the weather 
 which is at hand. When the first indications of 
 day-light are seen above a bank of clouds it is 
 thought to be a sign of wind; but if the first 
 streaks of light are discovered low down, that is 
 in, or very near the horizon, fair weather may be 
 expected. 
 
82 THE STORY OF THE SOLAR SYSTEM. 
 
 Twilight is usually reckoned to last until the 
 Sun has sunk 18 below the horizon, but the 
 question of its duration depends on where the 
 observer is stationed,^on the season of the year, 
 and (in a slight degree) on the condition of the 
 atmosphere. The general rule is that the twilight 
 is least in the tropics and increases as the ob- 
 server moves away from the equator towards 
 either pole. Whilst in the tropics a depression 
 of 16 or 17 is sufficient to put an end to the 
 phenomenon, in the latitude of England a de- 
 pression of from 17 to 20 is required. As im- 
 plied above, it varies with the latitude ; and as 
 regards the different seasons of the year, it is 
 least on March i and October 12, being three 
 weeks before the vernal equinox and three weeks 
 after the autumnal equinox. The duration at 
 the equator may be about i hour 12 minutes; 
 it amounts to nearly 2 hours at the latitude of 
 Greenwich, and so on towards the pole. At 
 each pole in turn the Sun is below the horizon 
 for 6 months, but as it is less than 18 below 
 the horizon for about 3^- of those 6 months it 
 may be said that there is a continual twilight for 
 those 3-J- months. Something of the same sort 
 of thing as this occurs in the latitude of Green- 
 wich, for there is no true night at Greenwich 
 from May 22 to July 21, but constant twilight 
 from sunset to sunrise, or 2 months of twilight 
 in all. Though twilight at the equator is com- 
 monly set down as lasting about an hour, this 
 period is there, as elsewhere, affected by the 
 elevation of the observer above the sea-level. 
 Where the air is very rarified, as at places situ- 
 ated as Quito and Lima are, the twilight is said 
 to last no more than 20 minutes, and this would 
 
THE EARTH. 83 
 
 accord with the theory that where there is no air 
 at all (e. g., on the Moon) there is no twilight at 
 all. The greater purity and clearness of moun- 
 tain air, rarified as it is, is another cause which 
 contributes to vary by reducing the duration of 
 twilight. 
 
 It is sometimes stated that a secondary twilight 
 may be noticed, and Sir John Herschel has spoken 
 of it as "consequent on a re-reflection of the rays 
 dispersed through the atmosphere in the primary 
 one. The phenomenon seen in the clear atmos- 
 phere of the Nubian Desert, described by travel- 
 lers under the name of the * afterglow/ would 
 seem to arise from this cause." I am not ac- 
 quainted with any records which throw light on 
 these remarks of Sir John Herschel. 
 
 The phenomenon of twinkling is a subject 
 which has been much neglected, possibly on ac- 
 count of its apparent, but only apparent, sim- 
 plicity. The familiar verse of our days of child- 
 hood 
 
 *' Twinkle, twinkle little star, 
 
 How I wonder what you are, 
 
 Up above the earth so high, 
 
 Like a diamond in the sky," 
 
 contains even in this simple form a good deal of 
 food for reflection ; whilst the new version 
 
 " Twinkle, twinkle little star, 
 Now we've found out what you are, 
 When unto the midnight sky 
 We the spectroscope apply," 
 
 does so yet more. 
 
 As an optical phenomenon the twinkling, or to 
 use the more scientific phrase, the scintillation, 
 of the stars is a matter which ha.s been strangely 
 
84 THE STORY OF THE SOLAR SYSTEM. 
 
 ignored by physicists. Indeed, the only investi- 
 gators who seem to have dealt with it in any sort 
 of detail are two Italians, Secchi and Respighi, 
 Dufour, a Frenchman, Montigny, a Belgian, and 
 the Rev. E. Ledger, an Englishman. Secchi has 
 truly remarked that the twinkling of the stars is 
 one of the most beautiful of the minor phenomena 
 of the heavens. Light, sometimes bright, some- 
 times feeble, sometimes white, sometimes red, 
 darts about in intermittent gleams, like the spark- 
 ling flashes of a well-cut diamond, and works upon 
 the feelings of even the most stolid spectator. 
 The theory of twinkling is still surrounded by 
 many difficulties. One thing, however, is certain 
 it has nothing to do with recurrent changes in 
 the intrinsic light or physical condition of the 
 star itself, but arises during the passage of its 
 rays through our atmosphere ; it depends, there- 
 fore, in some way or other on the varying con- 
 ditions of the atmosphere. On the summit of 
 high mountains, according to the observations 
 of all careful observers (notably Tacchini, who 
 studied the subject on Mount Etna), the light of 
 the stars is steady, like that of the planets ; and 
 it is so likewise during the hours of calm which 
 often precede terrestrial storms. The vibrations 
 are usually more frequent near the horizon, and 
 diminish with the elevation of the star above the 
 horizon ; in other words, with the lessening of 
 the thickness of the atmospheric strata which 
 the rays of light have to traverse. Nevertheless, 
 during windy weather, and specially with north- 
 erly wind, it may be noticed that the stars 
 twinkle high up above the horizon, and even as 
 far as the zenith. From these and other similar 
 considerations we are justified in drawing the 
 
THE EARTH. 85 
 
 conclusion that twinkling largely depends on the 
 condition and movements of the atmosphere. 
 
 Secchi further points out that it is impossible 
 to study carefully with the naked eye all the 
 features of twinkling, and that telescopic assist- 
 ance is imperatively necessary. When, with the 
 aid of a telescope, we scrutinise a star during a 
 disturbed evening marked by much twinkling we 
 see an image diffused and undefined and sur- 
 rounded by rays, as if several images were super- 
 posed, and were jumping about rapidly. On such 
 occasions we do not see that little defined disc 
 surrounded by motionless diffraction rings, ordi- 
 narily indicative of a tranquil atmosphere. With 
 a telescope armed with a medium power, the field 
 of view of which is more extensive than that of a 
 high power, we find that if a light tap is given 
 to the telescope, the ordinary simple image is 
 changed into a luminous curve, the perimeter of 
 which is formed entirely of a succession of arcs 
 exhibiting the colours of the rainbow. This col- 
 oured curve does not, in principle, differ from 
 what one sees on swinging round and round in 
 the air such a thing as a stick, the end of which 
 is alight, having been freshly taken from a fire. 
 The glowing tip produces in appearance a contin- 
 uous arc, the result of the persistence of the im- 
 age of the tip on the retina. In such a case the 
 colour is constant, because the illumination re- 
 sulting from the blazing wood does not vary ; but 
 in the case of a star the arcs are differently col- 
 oured during the very brief space of time in which 
 the vibrating telescope transports the image from 
 one side to another of the visible field. This ex- 
 periment is from its nature very crude, but the 
 idea was improved upon and reduced to a syste- 
 
86 THE STORY OF THE SOLAR SYSTEM. 
 
 matic shape by Montigny, who introduced into 
 his telescope, at a certain distance from the eye- 
 piece, a concave lens eccentrically placed with 
 respect to the axis of the instrument, and endued 
 with a rapid movement of rotation imparted by 
 suitable mechanism. He thus obtained images 
 which revolved with regularity, and so was able 
 to submit certain features of the phenomenon to 
 a definite system of measurement. To cut a long 
 story short, Montigny started with the assumption 
 (made good by the sequel) that possibly stars 
 were affected in their twinkling by intrinsic con- 
 stitutional differences; and that possibly Secchi's 
 classification of stars into four types (a classifica- 
 tion which depends on the spectra which they 
 yield) might put him on the track of some intelli- 
 gible conclusions with respect to the theory of 
 twinkling.* 
 
 The results he ultimately arrived at were, that 
 the yellow and red stars of the Ilnd and Illrd 
 types twinkle less rapidly than the white stars 
 of the 1st type. Whilst the average number of 
 scintillations per second of the stars of type III. 
 were 56, those of type II. were 69, and those of 
 type I. 86. These differences may be confidently 
 said to depend upon too many observations of 
 too many different stars to be fortuitous. Mon- 
 tigny also arrived at a number of incidental con- 
 clusions of considerable interest. The one main 
 thread running through them, is that there is a 
 connection between the twinkling of a star and 
 its spectrum, which had never before been thought 
 of. We are justified, indeed, in going so far as to 
 
 * For some information respecting these Secchi " Types " 
 of Stars, see my " Story of the Stars" 2nd ed., p. 140. 
 
THE EARTH. 87 
 
 say, that Montigny's observations point distinctly 
 to a law on this subject, the law being that the 
 more the spectrum of a star is interrupted by dark 
 lines, the less frequent are its scintillations. The 
 individual character of the light, therefore, emitted 
 by any given star appears to affect its twinkling, 
 both as regards the frequency thereof and the col- 
 ours displayed. 
 
 Montigny collected some other interesting 
 facts with reference to twinkling, which may here 
 be stated in a concise form. There is a greater 
 display of twinkling in showery weather, than when 
 the atmosphere is in a normal condition ; and in 
 winter than in summer, whatever may be the 
 weather. In dry weather in Spring and Autumn 
 the twinkling is about the same, but wet has 
 more effect in Autumn than in Spring in develop- 
 ing the phenomenon. Variations in the baro- 
 metric pressure and in the humidity of the air 
 also affect the amount of twinkling ; there is 
 more before a rainy period, likely to last 2 or 3 
 days, than before a single, or, so to speak, casual 
 rainy day. Twinkling also varies with the aggre- 
 gate total rain-fall of any group of days, being 
 more pronounced as the rain-fall is greater, but 
 decreasing suddenly and considerably as soon as 
 the rainy condition of the atmosphere has passed 
 away. The number of scintillations found to be 
 observable with the aid of Montigny's instrument 
 (which he called a " scintillometre "), varied from a 
 minimum of 50 during June and July, to 97 in 
 January, and 101 in February, increasing and 
 decreasing in regular sequence from month to 
 month. When an Aurora Borealis is visible, there 
 is a marked increase in the amount of twinkling. 
 It would be interesting to follow up this last 
 
88 THE STORY OF THE SOLAR SYSTEM. 
 
 named discovery by an endeavour to ascertain 
 whether the fluctuations which are coincident in 
 point of time with an Auroral display depend 
 upon optical considerations connected with the 
 Aurora, or on physical considerations having any 
 relation to the increased development of terres- 
 trial magnetism. 
 
 I have been thus particular in unfolding some- 
 what fully the present state of our knowledge 
 concerning the twinkling of the stars, because it 
 is evident that there are many interesting, points 
 connected with it, which may be studied by any 
 patient and attentive star-gazer, and which do not 
 need the instrumental appliances and technical re- 
 finements which are only to be found in fully- 
 equipped public and private observatories. 
 
 It should be mentioned in conclusion that the 
 planets twinkle very little, or, more often, not at 
 all. This is mainly due to the fact that they ex- 
 hibit discs of sensible diameter and therefore that 
 there is, as Young puts it, u a general unchanging 
 average of brightness for the sum total of all the 
 luminous points of which the disc is composed. 
 When, for instance, point A of the disc becomes 
 dark for a moment, point B, very near to it, is 
 just as likely to become bright; the interference 
 conditions being different for the 2 points. The 
 different points of the disc do not keep step, so to 
 speak, in their twinkling/' The non-twinkling of 
 planets because they possess sensible discs is 
 often available as a means for determining when 
 a planet is looked for, which, of several objects 
 looked at, is the planet wanted and which are 
 merely stars, 
 
THE MOON. 89 
 
 CHAPTER VI. 
 
 THE MOON. 
 
 THE Moon being merely the satellite of a 
 planet, to wit, the Earth, it should, according to 
 the plan of this book, be included in the chapter 
 which deals with its primary; but for us inhabit- 
 ants of the Earth the Moon has so many special 
 features of interest that it will be better to give 
 it a special chapter to itself. 
 
 We may regard the Moon in a twofold aspect, 
 and consider what it is as a mere object to look 
 at, and what it does for us ; probably my present 
 readers will prefer that most prominence shall be 
 given to the former aspect. The Moon as seen 
 with the naked eye exhibits a silvery mass of light, 
 which at the epoch of what is called u full Moon " 
 has a seemingly even circular outline. Full or not 
 full, its surface appears to be irregularly shaded 
 or mottled. The immediate cause of this shading 
 is the fact that the surface of the Moon, not being 
 really smooth, reflects irregularly the Sun's light 
 which falls upon it. The causa causa/is of this is 
 the existence of numerous mountains and valleys 
 on its surface, and which were first discovered to 
 be such by Galileo. That there are mountains is 
 prov-ed by the shadows cast by their peaks on the 
 surrounding plains, when the Sun illuminates the 
 Moon obliquely that is, when the Moon is shin- 
 ing either as a crescent or gibbous. Such shadows, 
 however, disappear at the phase of " Full-Moon," 
 because the Sun's rays then fall perpendicularly 
 on the Moon's surface. When the Moon presents 
 either a crescent or a gibbous form (in point of 
 
9 
 
 THE STORY OF THE SOLAR SYSTEM. 
 
 fact when it presents any form except that of 
 " Full-Moon "), the boundary line which separates 
 the illuminated from the unilluminated portion 
 
 (and which boun- 
 dary line is gen- 
 erally spoken of 
 as the u termina- 
 tor") has a rough, 
 jagged appear- 
 ance ; this is due 
 to the fact that 
 the Sun's light 
 falls first on the 
 summits of the 
 peaks, and that 
 the adjacent val- 
 leys and declivi- 
 FIG. n.-Mare Crisium. ties are in shade. 
 
 (Lick Observatory photographs.) These remain SO 
 
 till by reason of 
 
 the Moon's progress in its orbit a sufficient time 
 has elapsed for the Sun to penetrate to the bot- 
 tom of the valleys. With this explanation the 
 reader will have no difficulty in realising why the 
 terminator always exhibits an irregular or jagged 
 edge. 
 
 Various mountains on the Moon to the num- 
 ber of more than a thousand have been mapped, 
 and their elevations calculated. Of these fully 
 half have received names, being those of men of 
 various dates and nationalities, who have figured 
 conspicuously in the annals of science, including 
 some, however, who have not done so. Whilst 
 many of these mountains are isolated elevations, 
 not a few form definite chains of mountains, and 
 to certain of these chains definite names, bpr- 
 
THE MOON. 91 
 
 rowed from the Earth, have been given. Thus 
 we find on maps of the Moon the " Apennines," 
 the "Alps," the "Altai Mountains," the " Dorfel 
 Mountains," the " Caucasus Mountains," and 
 so on. 
 
 Besides the mountains there exist on the Moon 
 a number of plains analogous in some sense to 
 the " steppes " of Asia and the " prairies " of 
 North America. These were termed " seas " in 
 the early days of the telescope, because it was 
 assumed that as they were so large and so smooth 
 they were vast tracts of water. This supposition 
 has long ago been overthrown, but the names 
 have been retained as a matter of convenience. 
 Hence it comes about that in descriptions of the 
 Moon one meets with such names as Mare Imbrium, 
 the " Sea of Showers " ; Mare Serenitatis, the " Sea 
 of Serenity " ; Mare Tranquillitatis, the " Sea of 
 Tranquillity " ; and so on. It seems probable 
 that the so-called seas represent in nearly its 
 original form what was once the original surface 
 of the Moon before the mountains were formed. 
 A confirmation of this idea is to be found in the 
 fact that though these plains are fairly level sur- 
 faces compared with the masses of mountains 
 which hedge them in on all sides, yet the plains 
 themselves are dotted over with inequalities (small 
 elevations and pits), which seem to suggest that 
 some of them might eventually have developed 
 into mountains if the further formation of moun- 
 tains had not been arrested by the fiat of the 
 Creator. 
 
 Though hitherto we have been speaking of 
 the mountains of the Moon under that generic 
 title, it is necessary for the reader to understand 
 that the Moon's surface exhibits everywhere re- 
 
92 THE STORY OF THE SOLAR SYSTEM. 
 
 markable illustrations of those geological pro- 
 cesses which we on the earth associate with the 
 word " volcano." There cannot be the least doubt 
 that the existing~surface of the Moon, as we see 
 it, owes all its striking features to volcanic ac- 
 tion, differing little from the volcanic action to 
 which we are accustomed on the earth. That 
 this theory is well founded may be very easily 
 inferred by comparing the structural details of 
 certain terrestrial volcanoes and their surround- 
 ings with a typical lunar mountain, or indeed, I 
 might say, with any lunar mountain. This point 
 was very well worked out some 40 years ago by 
 Professor Piazzi Smyth, who placed on pictorial 
 record his results of an examination and survey 
 of the Peak of Teneriffe. Any person seeing 
 side by side one of Smyth's pictures of Teneriffe 
 and a picture of any average lunar crater would 
 find great difficulty if the pictures were not label- 
 led in determining which was which. 
 
 The one special feature of the Moon, which 
 never fails to attract the attention of everybody 
 who looks at our satellite for the first time 
 through a telescope, are the crater mountains, 
 which indeed constitute an immense majority of 
 all the lunar mountains. Their outline almost 
 always conforms, more or less, to that of the 
 circle, but when seen near either limb of the 
 Moon they often appear considerably oval simply 
 because they are then seen considerably fore- 
 shortened. In their normal form they exhibit a 
 basin bounded by a ridge, with a conical eleva- 
 tion in the centre of the basin, the basin and the 
 cone together being evidently the result of an 
 uprush of gases breaking through the outer crust 
 of the Moon and carrying with them masses of 
 
THE MOON. 93 
 
 molten lava. This lava, with perhaps the ma- 
 terials in fragments, projected in the first instance 
 up into the air, fell back on to the Moon forming- 
 first of all the outer edge of the basin, and sub- 
 sequently, as the eruptive force became weakened, 
 the small central accumulation, which took, as it 
 naturally would do, a conical shape. An experi- 
 mental imitation of the process thus inferred was 
 carried out some years ago by a French physicist, 
 Bergeron, who acted upon a very fusible mixture 
 of metals known as Wood's alloy by forcing 
 through it a current of hot air. The success of 
 this experiment was complete, and Bergeron con- 
 sidered that his experiments, taken as a whole, 
 were calculated to throw much light on the past 
 history of the Moon. 
 
 Several observers at various times have fan- 
 cied they have seen signs that the lunar mountain 
 Aristarchus was an active volcano even up to the 
 present century ; but it admits of no doubt that 
 this idea is altogether a misconception, and that 
 what they saw as a faint illumination of the sum- 
 mit of Aristarchus was no more than an effect of 
 earth-shine. On the general question of volcanic 
 action on the Moon, Sir John Herschel summed 
 up as follows: " Decisive marks of volcanic 
 stratification arising from successive deposits of 
 ejected matter, and evident indications of lava 
 currents, streaming outwards in all directions, 
 may be clearly traced with powerful telescopes. 
 In Lord Rosse's magnificent Reflector the flat 
 bottom of the crater called Albategnius is seen to 
 be strewed with blocks not visible in inferior 
 telescopes, while the exterior ridge of another 
 (Aristillus) is all hatched over with deep gulleys 
 radiating towards its centre/' 
 
94 THE STORY OF THE SOLAR SYSTEM. 
 
 The valleys and clefts or rills visible on the 
 Moon's surface constitute another remarkable 
 feature in the topography of our satellite. The 
 valleys, properly so-called, require no particular 
 comment, because they are just what their name 
 implies hollows often many miles long and 
 several miles wide. The clefts or rills, however, 
 are more mysterious, by reason of their great 
 length and remarkable narrowness. One is al- 
 most led to infer that they are naught else but 
 cracks in the lunar crust, the result of, sudden 
 cooling, how caused is of course not known. 
 
 There is another lunar feature to be mentioned 
 somewhat akin to the foregoing in appearance 
 but apparently, however, owing its origin to a 
 different cause. I refer to the systems of bright 
 streaks which, especially at or near the time of 
 full Moon, are seen to radiate from several of 
 the largest craters, and in particular from Tycho, 
 Copernicus, Kepler and Aristarchus. These bright 
 streaks extend in many cases far beyond what 
 may fairly be considered as the neighbourhood 
 of the craters from which they start, traversing 
 distant mountains, valleys and other craters in a 
 way which renders it very difficult to assign an 
 explanation of their origin. 
 
 There are 13 areas on the Moon, which used 
 to be regarded as " seas," one of them, however, 
 bearing the name of " Oceanus Procellarum" the 
 " Ocean of Storms " ; but besides these there are 
 several bays, termed in Latin Sinus, of which the 
 most important is the Sinus Iridum or the " Bay 
 of Rainbows," a beautiful spot on the northern 
 border of the Mare Imbrium, and best seen when 
 the Moon is between 9 and 10 days old. The 
 summits of the semi-circular range of rocks 
 
THE MOON. 95 
 
 which enclose the bay are then strongly illumi- 
 nated and a greenish shadow marks the valley at 
 its base. By the way, it is worth mentioning 
 that not a few of the lunar seas, so-called, seem 
 to be pervaded by a greenish hue, though no 
 particular explanation of this fact is forthcoming. 
 
 Much controversy has ranged round the ques- 
 tion whether or not the Moon has an atmosphere. 
 Without doubt the preponderance of opinion is 
 on the negative side though it must be admitted 
 that some observers of eminence have suggested 
 that there are indeed traces of an atmosphere to 
 be had, but that it is extremely attenuated and 
 of no great extent, otherwise it must render 
 its presence discoverable by optical phenomena 
 which it is certain cannot be detected. 
 
 A brief reference may here be made to a curi- 
 ous phenomenon sometimes seen in connection 
 with occultations of stars by the Moon. Premis- 
 ing that an " occultation " is the disappearance of 
 a star behind the solid body of the Moon by rea- 
 son of the forward movement of the Moon in her 
 orbit, it must be stated that though generally the 
 Moon extinguishes the star's light instantane- 
 ously, yet this does not invariably happen, for 
 sometimes the star seems to hang upon the 
 Moon's limb as if reluctant to disappear. No 
 very clear or satisfactory explanation of this 
 phenomenon has yet been given ; the existence 
 of a lunar atmosphere would be an explanation, 
 and accordingly this anomalous appearance, seen 
 on occasions, has been advanced in support of 
 the theory that a lunar atmosphere does exist ; 
 but, nevertheless, astronomers do not accept that 
 idea. 
 
 Any one desirous of carrying out a careful 
 7 
 
96 THE STORY OF THE SOLAR SYSTEM. 
 
 study of the Moon's surface must be provided 
 with a good map, and for general purposes none 
 is so convenient or accessible as Webb's, reduced 
 from Beer and Madler's Mappa Selenographica 
 published in 1837, of which another reproduction 
 is given in Lardner's Astronomy. Those, however, 
 who would desire to study the Moon with the ut- 
 most attention to detail must provide themselves 
 with Schmidt's map published in 1878 at the 
 expense of the German Government. When it is 
 stated that this map represents the Moon on a 
 circle y-J- feet in diameter, the size and amount of 
 detail in it will be readily understood. Special 
 books on the Moon furnishing numerous engrav- 
 ings and detailed descriptions have been written 
 by Carpenter and Nasmyth (jointly) and by 
 Neison. 
 
 Various attempts have been made to deter- 
 mine the amount of light reflected by the Moon, 
 and also the question whether it yields any meas- 
 urable amount of heat. As regards the light of 
 the full Moon compared with that of the Sun, the 
 estimates range from Tnn A nnr to -gTnjW* a discrep- 
 ancy not perhaps greater than might be expected 
 under the circumstances of the case. 
 
 With respect to the heat possessed by, or 
 radiated from the Moon's surface, the conclu- 
 sions of those who have attempted to deal with 
 the matter are less consistent. As regards the 
 surface of the Moon itself Sir John Herschel was 
 of opinion that it is heated at least to the tem- 
 perature of boiling water, but that owing to the 
 radiant heat having to pass through our atmos- 
 phere, which acts as an obstacle, it is no wonder 
 that it should be difficult for us to become con- 
 scious of its existence. In 1846 Melloni, by con- 
 
THE MOON. 97 
 
 centrating the rays of the Moon with a lens 3 
 feet in diameter, thought he detected a sensible 
 elevation of temperature; and in 1856 C. P. 
 Smyth at Teneriffe, but with inferior instru- 
 mental appliances, arrived at the same conclu- 
 sion. Though Professor Tyndall in 1861 obtained 
 a contrary result, yet the most recent experi- 
 ments by the younger Earl of Rosse, Professor 
 Langley, and others, all tend to show that the Moon 
 does really radiate a certain infinitesimally small 
 amount of heat. Perhaps, however, it will be 
 best to give Langley's ideas as to this in his own 
 words : " While we have found abundant evi- 
 dence of heat from the Moon, every method we 
 have tried, or that has been tried by others, for 
 determining the character of this heat appears to 
 us inconclusive ; and without questioning that 
 'the Moon radiates heat earthward from its soil, 
 we have not yet found any experimental means 
 of discriminating with such certainty between 
 this and reflected heat that it is not open to mis- 
 interpretation." It is obvious from the foregoing 
 that we on the Earth need not concern ourselves 
 very much about lunar heat ; and I will only add 
 that F. W. Very, by an ingenious endeavour to 
 localise the Moon's radiant heat, has been able, 
 he thinks, to establish the fact that on the part 
 of the Moon to which the Sun is setting, what he 
 calls the heat-gradient (using a phrase suggested 
 by terrestrial meteorology) appears to be steeper 
 than on that part to which the Sun is rising. Gen- 
 erally, Very's observations accord fairly with Lord 
 Rosse's. 
 
 The Moon revolves round the Earth in 27 d. 
 7 h. 43 m. ii s. at a mean distance of 237,300 
 miles, in an orbit which is somewhat, but not 
 
98 THE STORY OF THE SOLAR SYSTEM. 
 
 very, eccentric. Its angular diameter at mean 
 distance is 31' 5", or, say, just over -J- . The real 
 diameter may be called 2160 miles. 
 
 A few words will probably be expected by the 
 reader on the subject of lunar influences on the 
 weather, and generally ; this being a matter highly 
 attractive to the popular mind. The truth ap- 
 pears to lie, as usual, between two extremes of 
 thought. The Moon, of course, is the main cause 
 of the tides of the Ocean, and it is not entirely 
 inconceivable that tidal changes imparted to vast 
 masses of water may be either synchronous with, 
 or may in some way engender, analogous move- 
 ments in the Earth's atmosphere; though no dis- 
 tinct proofs of this, as a determinate fact, can be 
 brought forward. 
 
 There is no doubt whatever that at or near 
 the time of full Moon, evening clouds tend to dis- 
 perse as the Moon comes up to the meridian, 
 and that by the time the Moon has reached the 
 meridian a sky previously overcast will have be- 
 come almost or quite clear. Sir John Herschel 
 has alluded to this by speaking of a " tendency to 
 disappearance of clouds under a full Moon"; 
 and he considers this " fully entitled to rank as a 
 meteorological fact." He goes on, not unnatu- 
 rally, to suggest the obvious thought that such 
 dissipation of terrestrial clouds is due to the cir- 
 cumstance that, assuming heat really comes by 
 radiation from the Moon (and we have seen on a 
 previous page the probability of this) such radi- 
 ant heat will be more potential if it falls on the 
 Earth perpendicularly, as from a Meridian Moon, 
 than if it comes to us at any one locality from a 
 Moon low down in the observer's horizon, and 
 therefore has to pass through the denser strata 
 
THE MOON. 99 
 
 of the Earth's atmosphere and surfer material 
 enfeeblement accordingly. I am aware that Mr. 
 Ellis, late of the Royal Observatory, Greenwich, 
 has sought to show by a seemingly powerful 
 array of statistics that the idea now under con- 
 sideration is unfounded, -but I consider that we 
 have here only one more illustration of the fa- 
 miliar statement that you can prove anything 
 you like by statistics. I am firmly convinced, as 
 the result of more than 30 years' observation, 
 that terrestrial clouds do disperse under the cir- 
 cumstances stated. Sir J. Herschel added that 
 his statement proceeded from his own observa- 
 tion " made quite independently of any knowledge 
 of such a tendency having been observed by 
 others. Humboldt, however, in his Personal Nar- 
 rative, speaks of it as well known to the pilots 
 and seamen of Spanish America." Sir John Her- 
 schel further remarked : " Arago has shown from 
 a comparison of rain, registered as having fallen 
 during a long period, that a slight preponderance 
 in respect of quantity falls near the * new ' Moon 
 over that which falls near the ' full.' This would 
 be a natural and necessary consequence of a pre- 
 ponderance of a cloudless sky about the ' full,' 
 and forms, therefore, part and parcel of the same 
 meteorological fact." 
 
 Bernadin has asserted it to be a fact that many 
 thunderstorms occur about the period of " new " 
 or " full " Moon. But what I want most to warn 
 the reader against is that popular idea (wonder- 
 fully wide-spread it must be admitted) that at the 
 epochs of what are called, most illogically, the 
 Moon's " changes," changes of weather may cer- 
 tainly be expected. There is absolutely no 
 foundation whatever for this, and still more void 
 
100 THE STORY OF THE SOLAR . SYSTEM. 
 
 of authority (if such a phrase is admissible) is a 
 table of imaginary weather to be expected at 
 changes of the Moon, often met with in books 
 published half a century ago, and still occasion- 
 ally reprinted in third-rate almanacs, and desig- 
 nated "Dr. Herschel's Weather Table." This 
 precious production is not only devoid of authen- 
 ticity as regards its name, but may easily be seen 
 to be fraudulent in its reputed facts any month in 
 the year. 
 
 It would be beyond both my present available 
 space and the legitimate objects of this work to 
 attempt even an outline of the influences over 
 things terrestrial ascribed to, or associated, rightly 
 or wrongly, with the Moon, and of which the 
 word " lunatic " perhaps affords the most familiar 
 exponent. 
 
 CHAPTER VII. 
 
 MARS. 
 
 MARS, though considerably smaller than the 
 Earth, is commonly regarded as the planet which, 
 taken all in all, bears most resemblance to the 
 Earth, though only one-fourth its size. Under 
 circumstances which have already been briefly 
 alluded to in Chapter L, Mars exhibits from time 
 to time a slight phase, but nothing approaching 
 in amount the phases presented by the two in- 
 ferior planets, Mercury and Venus. When in op- 
 position to the Sun, that is to say when on the 
 meridian at midnight, it has a truly circular disc; 
 but between opposition and its two positions of 
 
MARS. 10 1 
 
 quadrature it is gibbous. At the minimum phase, 
 which is at each quadrature, E. or W. as the case 
 may be, the planet resembles the Moon 3 days 
 from its "full." These phases are an indication 
 that Mars shines by the reflected light of the Sun. 
 
 FIG. 12. Four views of Mars differing 90 in longitude (Barnard). 
 
 It is a remarkable tribute to Galileo's powers of 
 observation that with his trumpery telescope, 
 only a few inches long, he should have been able 
 to suspect the existence of a Martial phase. 
 Writing to a friend in 1610 he says: " I dare not 
 affirm that I can observe the phases of Mars ; 
 however, if I mistake not, I think I already per- 
 ceive that he is not perfectly round." 
 
 The period in which Mars performs its journey 
 round the Sun (called the sidereal period) is about 
 687 days ; but owing to the Earth's motion we 
 are more concerned with what is called the 
 
102 THE STORY OF THE SOLAR SYSTEM. 
 
 planet's synodical period of 780 days than with 
 its sidereal period of 687 days. The synodical 
 period is the interval between two successive con- 
 junctions or oppositions of the planet as regards - 
 the Earth, and 780 days being twice 365 and 50 
 days over, it follows that we have an opportunity 
 of seeing the planet at its best about every 2 
 years; and this is one of the reasons why Mars 
 has been so much and so thoroughly studied as 
 regards its physical appearance. Of course Mars 
 is not equally well seen every 2 years, because it 
 may so happen at a given opposition that it may 
 be at its nearest to the Sun (perihelion), and the 
 Earth at its farthest from the Sun (aphelion), in 
 which case the actual distance between the two 
 bodies will be the greatest possible. What is 
 therefore wanted is for the planet to be nearest 
 to the Sun and nearest to the Earth at the same 
 time, under which circumstances it shines with a 
 brilliancy rivalling Jupiter. This favourable com- 
 bination occurs once in 7 synodical revolutions, 
 or about every 15 years. The most favourable 
 oppositions occur at the end of August, and the 
 least favourable at the end of February. The next 
 very favourable opposition will not occur until 
 1909. Mars may approach to within about 35 
 millions of miles from the Earth at a favourable 
 opposition, whilst under extreme circumstances 
 the other way it may be no nearer than 61 millions 
 of miles at opposition. 
 
 Mars in opposition is a very conspicuous ob- 
 ject in the Heavens, shining with a fiery red light 
 which has always been regarded as a peculiar 
 attribute of the planet, so much so that its name, 
 or epithet, in many languages conveys the idea 
 of " fiery " or " blazing." It is recorded that in 
 
MARS. 103 
 
 August 1719 its brilliancy was such as to cause 
 a panic amongst the public. 
 
 Telescopically examined, Mars is always found 
 to exhibit patches of shade of various sizes and 
 shapes, and, on the whole, fairly permanent from 
 year to year. During the last few years in par- 
 ticular these markings have been subjected to 
 very careful scrutiny and measurement at the 
 hands of numerous observers ot skill and experi- 
 ence, and armed in many cases with very power- 
 ful telescopes. The conjoint effect of the ob- 
 servations obtained has been largely to augment 
 our knowledge of the planet's geography, or (to 
 use the proper term) k< areography." Before de- 
 scribing the minutest details recorded and pen- 
 cilled by the best observers, it will be best to 
 speak of the leading general features which are 
 within the grasp of comparatively small telescopes 
 say, refractors of 6 inches and reflectors of 12 
 inches in aperture. The first thing which pre- 
 sents itself as very obvious on the disc of Mars, 
 is the fact that certain portions are ruddy, whilst 
 others are greenish in hue. It is generally as- 
 sumed that the red areas represent land and the 
 green areas water. On this subject Sir John 
 Herschel's remarks, penned about half a century 
 ago, may be said still to stand good He ascribes 
 the ruddy colour to " an ochrey tinge in the gen- 
 eral soil, like what the red sandstone districts 
 on the Earth may possibly offer to the inhab- 
 itants of Mars, only more decided." The pro- 
 priety of this thought will be best appreciated 
 by a reader who has travelled through parts of 
 North Gloucestershire, and seen a succession of 
 ploughed fields in that locality. The deep red 
 colour of the soil is in many places very con- 
 
104 THE STORY OF THE SOLAR SYSTEM. 
 
 spicuous. It has often been remarked that the 
 redness of Mars is much more noticeable with the 
 naked eye than with a telescope ; and Arago car- 
 ried this idea one step further in suggesting that 
 the higher the optical power the less the colour. 
 This, however, might naturally be expected. 
 
 The most prominent surface marking on Mars 
 is that known as the " Kaiser Sea," sometimes 
 called the "V-mark" from its resemblance to 
 that letter, though a leg of mutton would be 
 quite as good a simile. East of the Kaiser Sea 
 and a little north of the planet's equator is a 
 well-defined dark streak known as " Herschel 
 II. Strait"; whilst on the west side is another 
 shaded area which has been called " Flammarion 
 Sea." These three features are so very conspic- 
 uous, that, provided the hemisphere in which they 
 are situated is fairly in front of the observer, 
 his telescope, if it will show anything on Mars, 
 will show these. The white patches seen on 
 certain occasions at Mars's N. pole and close to its 
 S. pole form another special feature of interest 
 connected with this planet. It admits of no 
 doubt whatever that these are immense masses 
 of snow and ice which undergo at stated intervals 
 changes analogous to the changes which we know 
 happen in the great fields of ice situated in the 
 regions of the Earth surrounding the Earth's two 
 poles. Not only do these white patches look 
 like snow, but if attention is paid to the changes 
 they undergo and the epochs at which the changes 
 take place there will be found abundant confir- 
 mation of this theory, for these patches decrease 
 in size when brought under the Sun's influence 
 on the approach of summer and increase again in 
 size when the summer is over and winter draws 
 
MARS. 105 
 
 near. In the second half of 1892 the Southern 
 Pole was in full view, and during especially July 
 and August the diminution of the snow area from 
 week to week was very evident. Schiaparelli, 
 who observed it with great attention during that 
 season, noted at the commencement of the season 
 that the snow reached at the first as far as lati- 
 tude 70 and formed a polar cap some 1200 miles 
 in diameter. Its subsequent decrease, however, 
 was so marked that two or three months later the 
 diameter of the snow patch had dwindled to no 
 more than 180 miles, and became indeed still 
 smaller at a later period. The summer solstice 
 on Mars occurred on October 13, 1892, which was 
 therefore the epoch of midsummer for Mars's 
 southern hemisphere. Whilst these changes were 
 taking place in the southern hemisphere, no doubt 
 changes of the reverse character were going on in 
 the northern hemisphere, but they were not visi- 
 ble from the Earth because the North Pole was 
 situated in that hemisphere of Mars which was 
 turned away from the Earth. In previous years, 
 however, the North Pole being turned towards the 
 Earth its snow was also seen to undergo the same 
 sort of change ; in other words, was seen to melt. 
 This happened, and was seen in 1882, 1884, and 
 1886. These observations of the alternate in- 
 crease and decrease of the polar snow on Mars 
 may be viewed with telescopes of moderate power, 
 but of course it is more interesting and profitable 
 to watch them with a large telescope. The fact 
 (for it is an undoubted fact) that the north polar 
 snow is concentric with the planet's axis whilst 
 the southern polar patch is eccentric to the extent 
 of about 180 miles from the southern pole is one 
 which has not yet received a satisfactory explana- 
 
106 THE STORY OF THE SOLAR SYSTEM. 
 
 tion. If both patches were eccentric so as to be 
 exactly opposite to one another an explanation 
 wpuld be much more easy for we might say that 
 the poles of rotation lay in one direction and the 
 poles of cold in another. 
 
 I have spoken on a previous page of three 
 specially conspicuous shadings of Mars, and other 
 similar shadings to the number perhaps of a 
 couple of dozen were generally recognised by 
 astronomers (having been mapped and named) 
 down to about the year 1877. In that year the 
 astronomical world was startled by the announce- 
 ment that Schiaparelli of Milan, an able and 
 competent observer, had discovered that those 
 shaded areas which all previous astronomers had 
 regarded as continents or vast tracts of land, 
 were in reality islands, that is to say, so far, that 
 the continents in question were cut up by in- 
 numerable channels intersecting one another at 
 various angles. When this discovery was an- 
 nounced, and older observations and drawings 
 came to be examined, it was found, or at any- 
 rate thought, that these so-called canals might be 
 traced in drawings of earlier dates by Dawes, 
 Secchi, and Holden. So much for 1877. In De- 
 cember, 1881, the planet was again in opposition, 
 but farther off in distance, and therefore smaller 
 in size than in 1877. It was, however, higher up 
 in the Heavens as seen at Milan and the weather 
 appears to have been more favourable.. In these 
 altered circumstances Schiaparelli again saw his 
 canals, but this time they were in at least as many 
 as twenty instances seen in duplicate ; that is to 
 say, a twin canal was seen to run parallel to the 
 original one at a distance of from 200 to 400 
 miles, as the case mig^t be. The existence of 
 
MARS. 107 
 
 not only single canals but of twin canals seems 
 an established fact, for Schiaparelli's drawings 
 and descriptions have been confirmed by compe- 
 
 FIG. 13. Mars, August 27, 1892 (Guiot). 
 
 tent testimony ; but explanation is nowhere; es- 
 pecially in view of Schiaparelli's own idea that 
 the duplication of his canals is perhaps not a per- 
 manent feature but a periodical phenomenon de- 
 pending on, or connected in some way with, Mars's 
 seasons. 
 
 Several points stand out clearly established 
 by the observations of Mars during the opposition 
 of 1894, especially the correctness of Schiaparelli's 
 discoveries and maps. Most of the canals origi- 
 nally seen by him were again seen, and thus their 
 existence was confirmed, whilst new ones were 
 also noticed. Many of these canals were double. 
 
Io8 THE STORY OF THE SOLAR SYSTEM. 
 
 The great extent of the S. Polar cap and its rapid 
 disappearance as Mars's summer approached was 
 also a special feature of the observations of 1894. 
 It dwindled until it became almost invisible, or at 
 best showed as a tiny speck. It is thought by 
 some observers that as the Polar cap melts, the 
 water collects round the Pole, and thence flows 
 over the planet's surface, giving rise to the phe- 
 nomenon of canals, and that this is the way the 
 planet's surface is irrigated. It may here be re- 
 marked that the word " canal," which has been 
 given to these dark streaks crossing and cutting 
 up the large areas of land in Mars, is an unfor- 
 tunate one, suggesting as it does artificial agency. 
 But these Martial canals are probably, especially 
 the largest, a great many miles in w r idth and 
 hundreds of miles in length, though some are 
 smaller; and they are probably nature's method 
 of distributing over the continents and lands of 
 Mars the water which collects round the Pole 
 during the rapid melting of the Polar snows. 
 
 The idea of the presence of cloud or mist on 
 Mars also received strong confirmation in 1894. 
 Large portions of the planet's disc were found to 
 be hidden from view. "Herschel I. Continent" 
 and the " Maraldi Sea " (both well-known mark- 
 ings on Mars, readily visible with small telescopes) 
 were at times quite obscured by cloud. Indeed, 
 the Maraldi Sea was occasionally quite blotted 
 out : other well-known markings were also either 
 blotted out or only faintly seen. These facts 
 seem almost to prove conclusively the existence 
 of cloud and vapour in Mars, especially as some 
 of these markings subsequently again assumed 
 their ordinary form and colour. Bright projec- 
 tions too were seen at times on the terminator of 
 
MARS. 109 
 
 Mars, giving rise to the belief that there are high 
 mountains on the planet, though some observers 
 regarded these projections as high clouds power- 
 fully reflecting the Sun's light. 
 
 Mars rotates on its axis in 24.^1. 37m. 225., a 
 period so nearly coincident with the period of the 
 Earth's rotation as greatly to facilitate the map- 
 ping of Mars's features by work continued from 
 day to day by observers who have the necessary 
 instrumental means and artistic skill in handling 
 the pencil. 
 
 Mars has an atmosphere which may be said to 
 be no more than moderately dense ; that is to say 
 much less dense than the Earth's atmosphere. Of 
 course the existence of snow, which has been 
 taken for granted on a previous page, carries with 
 it the existence of water and aqueous vapour a 
 fact capable of independent spectroscopic proof. 
 
 The inclination of Mars's axis to the ecliptic 
 has not been ascertained with all desirable cer- 
 tainty, but if Sir W. Herschel's estimate that the 
 obliquity on Mars is 28f (the Earth's obliquity 
 being 23-^-) is correct, it is evident that there must 
 be a very close similarity between the seasons of 
 the Earth and the seasons of Mars, thereby furnish- 
 ing another link of proof to support the statement 
 made at the commencement of this chapter that, 
 taken all in all, Mars is the planet which bears 
 most resemblance to the Earth. 
 
 The apparent absence of satellites in the case 
 of Mars was long a matter of regret to astrono- 
 mers ; they seemed to think that such a planet 
 ought to have at least one companion. At last, 
 in 1887, two were found by Hall at Washington, 
 U. S., using a very fine refractor of 26 inches 
 aperture. These satellites, which have been 
 
110 THE STORY OF THE SOLAR SYSTEM. 
 
 named Phobos and Deimos, are, however, very 
 small, for Phobos at its best only resembles a star 
 of mag. ii-j-, whilst Deimos is no brighter than 
 a star of mag. 13**; from this it will be under- 
 stood that only very large telescopes will show 
 either of them. Phobos revolves round Mars in 
 7^- hours at a distance of about 6000 miles, w'hilst 
 Deimos revolves in 30 hours at a distance of 
 about 15,000 miles. It has been thought that 
 neither of them can be more than about 6 or 7 
 miles in diameter, and therefore that they can 
 not afford much light to their primary. 
 
 Mars revolves round the Sun in 686d. 230. 
 3om., at a mean distance of 141 million of miles, 
 which the eccentricity of its orbit may increase 
 to 154 millions or diminish to 128 millions. The 
 planet's apparent diameter varies between 4" in 
 conjunction and 30'' in opposition. Owing to the 
 great eccentricity of the orbit the planet's appar- 
 ent diameter as seen from the Earth varies very 
 much at different oppositions. The real diameter 
 is rather more than 4000 miles. 
 
 CHAPTER VIII. 
 
 THE MINOR PLANETS. 
 
 IN 1772 a German astronomer named Bode, 
 of Berlin, drew attention to certain curious nu- 
 merical relations subsisting between the distances 
 of the various planets. This " law," as it has 
 been sometimes called, usually bears Bode's name, 
 though it was not he but J. D. Titius of Wittem- 
 berg who really first discovered it. 
 
THE MINOR PLANETS. 
 
 Ill 
 
 Take the numbers 
 
 o, 3, 6, 12, 24, 48, 06, 192, 384; 
 each of which (the second excepted) is double the 
 preceding ; adding to each of these numbers 4 we 
 obtain 
 
 4, 7, 10, 16, 28, 52, 100, 196, 388; 
 which numbers approximately represent the dis- 
 tances of the planets from the sun expressed in 
 radii of the Earth's orbit. A little table will 
 make the matter more clear. 
 
 Planets. 
 
 Distance : 
 Bode's Law. 
 
 True distance 
 from Sun. 
 
 Mercury 
 
 4 
 
 3-9 
 
 Venus 
 
 7 
 
 7-2 
 
 Earth 
 
 10 
 
 10. 
 
 Mars 
 
 16 
 
 15.2 
 
 [Ceres] 
 
 [28] 
 
 [27.7] 
 
 Jupiter 
 Saturn 
 
 52 
 
 100 
 
 52.0 
 95-4 
 
 [Uranus] 
 [Neptune] 
 
 [196] 
 [388] 
 
 [191.8] 
 [300.0] 
 
 Bode having examined these relations and 
 noticing the void between 16 and 52 (Ceres and 
 the other minor planets, and Uranus and Neptune 
 also, being then unknown) ventured to predict 
 the discovery of new planets, and this idea stimu- 
 lated him to organise a little company of astron- 
 omers to hunt for new planets. Before, however, 
 this scheme was got into working order, Piazzi, 
 director of the Observatory at Palermo, on Jan- 
 uary i, 1801, noted an 8th magnitude star in 
 Taurus, which on the next and succeeding nights 
 he saw again, and found had moved. He ob- 
 served the strange object for 6 weeks, when ill- 
 8 
 
112 THE STORY OF THE SOLAR SYSTEM. 
 
 ness interrupted him. However he wrote letters 
 announcing what he had seen, one of them to 
 Bode himself; but this letter, though dated Jan. 
 24, did not reach Bode at Berlin, till March 20 
 a striking illustration of the state of the Postal 
 service on the Continent less than 100 years ago. 
 The new body, at first assumed to be a tailless 
 comet, was eventually recognised to be a new 
 planet; and the name of Ceres, the tutelary 
 goddess of Sicily, was at Piazzi's instance be- 
 stowed upon it. 
 
 Looking for Ceres in March, 1802, Olbers at 
 Bremen, came upon another new planet, which 
 was afterwards named Pallas. At first he thought 
 he had got hold of a new variable star, but two 
 hours sufficed to show that the object under 
 notice was in motion. The two new bodies were 
 found to be so much alike in size and appearance, 
 and in their orbits, that Olbers suggested both 
 were but fragments of some larger body w r hich 
 had been shattered by some great convulsion of 
 nature. The idea was a daring one, and it was 
 an attractive one, though now regarded as un- 
 tenable. However it served the purpose of stim- 
 ulating research, and the discovery of Pallas was 
 followed by that of Juno, by Harding, at Lilien- 
 thal 1804; and of Vesta, by Olbers, at Bremen in 
 1807. 
 
 The organised search for minor planets was 
 relinquished in 1816, presumably because no 
 more planets seemed to be forthcoming, and it 
 does not appear that any further attempts were 
 made by anybody till about 1830, when a Prussian 
 amateur, named Hencke of Driessen, profiting by 
 the publication of some new star maps put forth 
 by the Berlin Academy, commenced a methodical 
 
THE MINOR PLANETS. 113 
 
 search for small planets. These Berlin maps, one 
 for each hour of R. A., were only completed in 
 1859, and, therefore, Hencke had only a small 
 number of them at his command during the early 
 years of his labours. Still it is strange that 15 
 years elapsed before his zeal and perseverance 
 were rewarded, his first discovery, the planet 
 Astraea, not taking place till December 1845. 
 Once however the ice was broken new planets 
 followed with considerable rapidity, and begin- 
 ning with 1847, no year has elapsed without sev- 
 eral or many having been found. During the 
 last decade the number detected annually has 
 been very great sometimes as many as 20 in a 
 year, but this has been the result of photography 
 being brought to bear on the work. It is obvious 
 that if a photograph of a given field taken on 
 any one day is compared with a photograph 
 taken a few days earlier or later, and any of the 
 objects photographed have moved, their change 
 of place will soon be noticed and will be a dis- 
 tinct proof of their planetary nature. 
 
 It seems quite certain that all the larger of 
 these planets have now been found, for the aver- 
 age brilliancy (and this no doubt means the 
 average size) of those recently discovered has 
 been steadily diminishing year by year, and it 
 looks as if the limit of visibility will soon be 
 reached, if it has not been reached already. 
 
 The three largest of these bodies, in order of 
 size, have generally been thought to be Vesta, 
 Ceres, and Pallas; but Barnard, from observa- 
 tions made in 1894, concluded that Ceres is 520 
 miles in diameter ; Pallas, 304 miles ; and Vesta, 
 241 miles. As to all the rest of the minor planets, 
 excepting Juno, Hornstein is of opinion that 
 
(t 
 
 114 THE STORY OF THE SOLAR SYSTEM. 
 
 those having a greater diameter than 25 geo- 
 graphical miles are few in number, and that the 
 majority of them are no larger than from 5 to 15 
 miles in diameter. 
 
 From what has gone before the reader will 
 readily infer that these minor planets are of no 
 sort of interest to the casual amateur who dabbles 
 in Astronomy ; and indeed that they are of very 
 little interest to anybody. With a few general 
 statistics, therefore, this chapter may be con- 
 cluded. The total number of minor planets now 
 known nearly reaches 500, and every year in- 
 creases the list; but not, however, at as rapid a 
 rate as was once the case, because the German 
 mathematicians, who alone latterly have been 
 willing to trouble themselves with the computa- 
 tion of the orbits, are understood to have an- 
 nounced that they are no longer able to keep 
 pace with the discoveries made. Those who care 
 to investigate in detail the circumstances of these 
 planets will find great extremes in the nature of 
 the orbits. Whilst the planet nearest to the Sun 
 has a period of only 3 years, the most distant oc- 
 cupies nearly 9 years in performing its journey 
 round the Sun. So, also, there are great differ- 
 ences in the eccentricities of the orbits and in 
 their inclinations to the ecliptic. Whilst one 
 planet revolves almost in the plane of the ecliptic, 
 another (Pallas) has an orbit which is inclined no 
 less than 34 to the ecliptic. One word, in con- 
 clusion, as to the names applied to these bodies. 
 At the outset the names given were, without ex- 
 ception, chosen from the mythologies of ancient 
 Greece and Rome, but, latterly, the most fantastic 
 and ridiculous names have in many cases been 
 selected, names which in too many instances have 
 
JUPITER. 115 
 
 served no other purpose than that of displaying 
 the national or personal vanity of the astronomers 
 who applied them to the several planets. The 
 French are great offenders in this matter. 
 
 CHAPTER IX. 
 
 JUPITER. 
 
 THE planet Jupiter occupies, in one sense, the 
 first position in the planetary world, it being the 
 largest of all the planets Moreover, with the ex- 
 ception of Venus, it is the brightest of the planets. 
 As with Mars, and for the like reason, Jupiter, 
 when in the positions known as the Quadratures 
 (or near thereto), exhibits a slight phase, but 
 owing to the far greater distance from the Sun of 
 Jupiter, compared with Mars, the deviation of the 
 illuminated surface from that of a complete circle 
 is very small ; it is, however, perceptible at or 
 near the time of quadrature, a slight shading off 
 of the limb farthest from the Sun being trace- 
 able. 
 
 Jupiter is noteworthy on account of two fea- 
 tures, both of them more or less familiar, at least 
 by name, to most people its belts and its satel- 
 lites, both of which will be described in due 
 course. 
 
 The belts are dusky streaks, which vary from 
 time to time both in breadth and number: most 
 commonly two broad belts will be seen with two 
 or three narrower ones on either side; but some- 
 times all are rather narrow, and their narrowness 
 is made up for by an increa.se in their number. 
 
Il6 THE STORY OF THE SOLAR SYSTEM. 
 
 Under all circumstances they lie practically par- 
 allel, or nearly so, to the planet's equator. It is 
 generally thought that the planet, whatever may 
 be its actual structure or constitution, is sur- 
 rounded by a dense cloudy envelope, and that the 
 
 FIG. 14. Jupiter, November 27, 1857 (Dawes). 
 
 shaded streaks which we call belts are rifts in this 
 atmosphere, which expose to view the solid body 
 of the planet underneath. Whether, however, the 
 term " solid body " is an accurate one to be used 
 in this connection is thought by some to be open 
 to doubt. The laws which regulate the existence 
 of these belts are quite unknown ; indeed it seems 
 doubtful whether any laws exist at all, for the 
 belts at one time .appear to undergo constant 
 change, whilst at another time they remain almost 
 unchanged for several months. It has been sug- 
 gested that when the changes are rapid it must 
 t>e presumed that great atmospheric storms a.re. 
 
JUPITER. 117 
 
 to be considered as in progress, and possibly this 
 may be the true explanation. Belts are commonly 
 non-existent immediately under the equator ; 
 whilst north and south of this void space it most 
 usually happens that there is one broad belt and 
 several narrower ones in each hemisphere. At 
 each pole the planet's brightness is less than the 
 average brightness, but it cannot exactly be said 
 that this is due to the existence there of belts 
 properly so called. 
 
 It was formerly considered that no tinges of 
 colour could be traced on Jupiter except a silvery 
 gray of different degrees of intensity ; but during 
 the last thirty years there can be no doubt that 
 shades of brown, red, and orange, of no great 
 depth, but yet quite definite have been traceable. 
 Many observers concur in this opinion. Whether 
 this detection of colour is due to an absolute de- 
 velopment of colour during the period in ques- 
 tion ; or whether its detection is merely the result 
 of more careful scrutiny with better instruments 
 is a matter as to which the evidence is not clear. 
 Though the general position of the belts is such 
 that they are parallel to the planet's equator, yet 
 there are sometimes exceptions to this rule, for 
 in a few very rare instances a streak in the 
 nature of a narrow belt has been seen, inclined to 
 the equator at a decided angle, perhaps 20 or 
 even more. 
 
 It occasionally happens that spots are seen on 
 Jupiter's belts. Sometimes these remain visible 
 for a considerable period. They are either dark 
 or luminous, and their origin is unknown. Besides 
 these casual spots, which are always small in size, 
 there was visible during many years following 
 1878 a very remarkable and conspicuous large 
 
Il8 THE STORY OF THE SOLAR SYSTEM. 
 
 spot, strongly red in colour for several years, 
 though it afterwards became much fainter. This 
 spot exhibited an oval outline and was about 
 27,000 miles long and 8000 miles broad. For 
 about 4 years it maintained its intense red colour 
 and its shape almost unaltered; but after 1882, 
 the shape remaining, the colour sensibly faded. 
 The observations which were made on this spot 
 during 1886 by Professor Hough at Chicago, 
 U. S., with an i8-inch refractor, led him to the 
 opinion that the persistence of the red spot for 
 so many years rendered untenable the generally 
 accepted theory that the phenomena seen on the 
 surface of the planet are due to atmospheric 
 causes. 
 
 Some astronomers have thought that a rela- 
 tionship subsists between the spots on the Sun 
 and the spots on Jupiter. There certainly seems 
 an apparent identity in point of time between the 
 two classes of spots, and on the assumption that 
 the spots on Jupiter are indicative of disturb- 
 ances on the planet, Ranyard broached the idea 
 that both classes of phenomena are dependent on 
 some extraneous cosmical change ; and are not 
 related as cause and effect. Browning suggested 
 many years ago that the red colour of the belts 
 is a periodical phenomenon coinciding with the 
 epoch of the greatest display of sunspots, but this 
 thought does not appear to have been followed up 
 by any one. Spots on Jupiter seem to have been 
 first recorded by Robert Hooke in 1664. In the 
 following year Cassini saw a spot which he found 
 to be in motion, and by following it attentively 
 he inferred that the planet rotated on its axis in 
 9h. 56m. It is a remarkable illustration of the 
 great care bestowed by Cassini on his astronom- 
 
JUPITER. 119 
 
 ical work that the best modern determinations of 
 Jupiter's rotation-period differ from Cassini's esti- 
 mate by only half a minute. 
 
 Bearing in mind the enormous size of Jupiter 
 compared with the Earth, whilst" its period of ro- 
 tation is considerably less than half the Earth's, 
 it will be at once seen that the velocity of matter 
 at the planet's equator is immensely great 466 
 miles per minute against the Earth's 17 miles per 
 minute. One result of this is the great intensity 
 of the centrifugal force at the equator, and like- 
 wise the greatness of the compression of the 
 planet's body at the poles. Hind has suggested 
 that the great velocity which thus evidently exists 
 may have the effect, by reason of the develop- 
 ment of the heat which it gives rise to, of com- 
 pensating the planet for the small amount of 
 heat which owing to its distance it receives from 
 the Sun. 
 
 On favourable occasions the brilliancy of Ju- 
 piter is very considerable ; so much so that it 
 rivals Venus and Mars. And besides this, there 
 appears to be something special in the nature of 
 Jupiter's surface, for not only does it seem to 
 radiate a much larger proportion of the solar 
 light which falls on it than do the planets gener- 
 ally, but some observers have expressed the opin- 
 ion that it possesses inherent light of its own. 
 Speculations, however, such as this must always 
 be received with reserve, because of the evident 
 difficulty of making sure of the facts on which 
 they must be based. One thing, however, seems 
 less open to doubt. Bearing in mind the small 
 amount of heat which reaches Jupiter from the 
 Sun, there is reason to infer that the clouds which 
 certainly exist on Jupiter must owe tneir origin to 
 
120 THE STORY OF THE SOLAR SYSTEM. 
 
 the influence of some other heat than solar heat; 
 in other words that Jupiter possesses sources of 
 heat within itself. 
 
 Jupiter has satellites, 5 in number. The dis- 
 covery of four of these, was one of the first fruits 
 of the invention of the telescope, for they were 
 found by Galileo in January, 1610. The 5th sat- 
 ellite is so small that it escaped notice until as 
 recently as 1892, having been discovered on Sep- * 
 tember 9 of that year by Professor Barnard, with 
 the great Lick telescope in California. It is, how- 
 ever, so minute that one can count on one's fin- 
 gers the telescopes capable of showing it. 
 
 The four old satellites of Jupiter shine as 
 stars of about the yth magnitude; in other words, 
 they are sufficiently bright to be visible with tele- 
 scopes however small : indeed several instances 
 are on record of persons gifted with very good 
 sight, having been able to see them with the naked 
 eye. For the study of their physical appearance 
 very powerful optical assistance is necessary, but 
 their movements are so rapid, and the phenomena 
 which result from those movements are so inter- 
 esting, that these bodies may be considered to 
 occupy the first place in the stock-in-trade of 
 every amateur astronomer, who lays himself out 
 for planet-gazing, with the object of profiting 
 himself or his friends. The phenomena here 
 alluded to are known as eclipses, transits, t and 
 occupations. 
 
 The four old satellites do not bear any names, 
 but are numbered from the innermost outwards, 
 and are always alluded to by their numbers as I, 
 II, III and IV. 
 
 An eclipse of a Jovian satellite is identical in 
 principle with an eclipse of the Moon ; that is to 
 
JUPITER. 121 
 
 say, just as an eclipse of the Moon happens when 
 the Moon passes into and is lost in the Earth's 
 shadow, so an eclipse of a Jovian satellite hap- 
 pens when such satellite becomes lost in the 
 shadow cast by the planet into space. The 1st 
 Ilnd and Illrd satellites in consequence of the 
 smallness of the inclination of their orbits, un- 
 dergo eclipse once in every revolution round their 
 primary, but the IVth is less often eclipsed, owing 
 to the joint effect of its considerable orbital incli- 
 nation, and of the distance to which it recedes from 
 its primary. 
 
 An occultation of a Jovian satellite is akin in 
 principle to an occultation of a star by the Moon. 
 As the Moon moving forwards suddenly covers a 
 star, so the planet, on occasions, suddenly covers 
 one of its satellites. If the satellite in question 
 is the IVth, its disappearance behind the planet 
 and its reappearance from behind the planet will 
 both be visible in due succession. This is often 
 true also of the Illrd satellite, but for reasons 
 connected with the proximity to their primary of 
 the 1st and Ilnd satellites, only their disappear- 
 ance or reappearance (not both) can, as a rule, be 
 observed on the same occasion. The most inter- 
 esting, by far, however, of the phenomena con- 
 nected with Jupiter's satellites are their transits 
 in front of, that is across, the visible disc of the 
 planet. Though these transits are of frequent 
 occurrence, yet they are always interesting be- 
 cause of the diverse appearances which the satel- 
 lites exhibit at different times, and which cannot 
 be said to be in accordance with any recognised 
 laws. Moreover, in observing the transit of a 
 satellite, we may often see the black shadow cast 
 by the satellite on the planet's disc ; and this 
 
122 THE STORY OF THE SOLAR SYSTEM. 
 
 shadow will sometimes precede and sometimes 
 follow the satellite itself. From the fact that the 
 satellite generally appears as a bright spot on a 
 bright background whilst the shadow is black, or 
 blackish, an inexperienced observer is apt to 
 look at the shadow and think he is seeing the 
 satellite. 
 
 Jupiter revolves round the Sun in not quite 12 
 years at a mean distance of 483 millions of miles. 
 Its apparent diameter varies between 50" and 30" 
 according to its position with respect to the Earth. 
 Its true diameter is about 88,000 miles. Owing 
 to its large size and rapid rotation, as has already 
 been mentioned, Jupiter is very much flattened at 
 the poles. The amount of this (the polar " com- 
 pression '' as it is called) is about T *g-. 
 
 CHAPTER X. 
 
 SATURN. 
 
 NEXT beyond Jupiter, proceeding outwards 
 from the Sun, we reach the planet Saturn, which 
 beyond any doubt is the most beautiful and most 
 interesting of all the planets. Nobody who has 
 ever had a fairly good chance of seeing it can 
 have the least doubt that this is the case. Briefly 
 stated the three main features which constitute its 
 claims are : (i) Its belts, (2) its rings, (3) its sat' 
 ellites. 
 
 The belts of Saturn resemble generally those 
 of Jupiter, but they are more faint and less 
 changeable. Their physical cause, however, may 
 be assumed to t>e the same. Taking the planet 
 
SATURN. 
 
 123 
 
 as a whole, it may be said that its ordinary colour 
 is yellowish white, the belts inclining to grayish 
 white; though the dark belts have often been 
 thought to exhibit a greenish hue. Lassell con- 
 sidered that the south pole is generally darker 
 than the north pole and more blue in tinge. 
 
 There is one important particular in which 
 the belts of Saturn differ from those of Jupiter. 
 Jupiter's belts are straight, whereas Saturn's are 
 sensibly curved. Supposing, as is probable,' that 
 Saturn's belts are parallel to the planet's equator, 
 then we must assume that the plane of this equa- 
 tor makes a rather considerable angle with the 
 ecliptic. Spots on Saturn are very rare. Whether 
 Saturn has an atmosphere seems uncertain, or 
 perhaps it may be said that one has not been 
 proved to exist but may exist. The question of 
 polar snow is also uncertain, but Sir W. Herschel 
 
 FIG. 15. Saturn, Jan. 26, 1889 (Antoniadi). 
 
 thought he could trace changes of hue at the poles 
 which might be due to the melting of snow. 
 
 It is usual to speak of the planet itself under 
 
124 THE STORY OF THE SOLAR SYSTEM. 
 
 the name of the " Ball " when it is not a question 
 of referring to the whole Saturnian system collect- 
 ively. In consequence of its distance from the 
 Sun, Saturn undergoes no equivalent to a phase; 
 or to be more exact, no phase can be detected, 
 though theoretically when the planet is in quad- 
 rature the disc must undergo an infinitesimally 
 small loss of light. 
 
 Though the point has now-a-days no scientific 
 importance, it may perhaps be desirable just to 
 make a brief allusion to Sir W. Herschel's- curious 
 theory that Saturn was seen by him to be com- 
 pressed not only at the poles but at the equator, 
 so that it resembled a parallelogram with the cor- 
 ners rounded off. It is difficult to imagine what 
 could have given rise to this strange idea, though, 
 of course, Herschel's good faith in advancing it 
 cannot be called in question. I refer to it because 
 it will be found mentioned in so many books on 
 astronomy, often under the name of the " square- 
 shouldered " figure of Saturn. As a theory it may 
 be regarded as quite exploded in consequence of 
 accurate measures by Bessel, Main and others hav- 
 ing conclusively shown that the form of the ball 
 does not depart from that of a regular spheroid. 
 
 In referring to Saturn generally, we speak of 
 its ring in the singular number, but, in point of 
 fact, there are several rings three in particular. 
 The principal bright ring is really double, and 
 within the innermost bright ring there is a dusky 
 one, perfect as a ring, but not luminous as the 
 outer rings are. By way of distinguishing one 
 ring from another, it is usual to adopt Struve's 
 nomenclature, whereby the outermost bright ring 
 is called A, the inner bright ring B, and the dusky 
 ring C. 
 
SATURN. 125 
 
 A good engraving will convey more fully and 
 more clearly an idea of what the Saturnian sys- 
 tem consists of than the fullest verbal description 
 will do. (See Frontispiece^] 
 
 To the -earliest astronomers who possessed tel- 
 escopes, Saturn proved a great puzzle, because it 
 seemed to undergo changes of shape which were 
 quite inexplicable on any principles then known. 
 Galileo, when first he saw it, thought it presented 
 an oval outline which might be due to a central 
 planet having a smaller planet on each side of it, 
 and accordingly he announced to his friend, Kep- 
 ler, that the most distant planet was tergeminum or 
 tri-form. But greater magnifying power led him 
 to arrive at the conclusion that the planet was not 
 a triple combination of spheres, but one body, 
 either oblong or oval in outline. This conclusion, 
 however, was soon found to be untenable, because 
 the two (supposed) tributary bodies gradually de- 
 creased in size until they entirely disappeared. 
 Galileo writing to his friend, Welser, in December 
 1612, thus expressed himself: 
 
 " What is to be said concerning so strange a 
 metamorphosis ? Are the two lesser stars con- 
 sumed after the manner of the solar spots ? 
 Have they vanished or suddenly fled ? Has Sat- 
 urn, perhaps, devoured his own children ? Or 
 were the appearances indeed illusion or fraud, 
 with which the glasses have so long deceived me, 
 as well as many others to whom I have shewn 
 them ? Now, perhaps, is the time. come to revive 
 the well-nigh withered hopes of those who, guided 
 by more profound contemplations, have discovered 
 the fallacy of the new observations, and demon- 
 strated the utter impossibility of their existence. 
 I do not know what to say in a case so surprising, 
 
126 THE STORY OF THE SOLAR SYSTEM. 
 
SATURN. 127 
 
 so unlocked for, and so novel. The shortness of 
 the time, the unexpected nature of the event, the 
 weakness of my understanding, and the fear of 
 being mistaken have greatly confounded me." 
 
 Galileo seems to have become so out of heart 
 in consequence of the difficulty of determining 
 what these changes really meant, that he gave up 
 altogether observing Saturn. In the course of 
 time, but by very gradual steps, astronomers 
 came to realise what the facts were. The next 
 idea that was broached, was that the planet con- 
 sisted of simply one central ball, and that the 
 excrescences which Galileo had been puzzled by 
 were merely handles as they were called, (ansce) 
 projecting like the handles, say of a soup tureen, 
 though why they should vary in size at stated 
 intervals remained as great a mystery as ever. 
 It was not until about 1656 that the true ex- 
 planation was arrived at by a Dutchman, named 
 Christopher Huygens. It was the fashion in 
 those days for scientific men to intimate to the 
 world discoveries which they had made by resort 
 to mysterious anagrams, which served in some 
 degree the purpose which in the present day is 
 served by the law regulating copyright or patent 
 rights. Accordingly Huygens published the fol- 
 lowing singular memorandum : 
 
 aaaaaaa cccc d eeeee g h i 
 iiiiii 1111 mm nnnnnnnnn oo 
 oo pp q rr s ttttt uuuuu. 
 
 These letters arranged in their proper order 
 furnish the following Latin sentence : 
 
 Annulo cingitur, tenui, piano, nusquam cohaerente, 
 ad eclipticam inclinato ; which Latin sentence be- 
 comes in the English tongue : 
 
 " [The planet] is surrounded by a slender flat 
 
 9 
 
128 THE STORY OF THE SOLAR SYSTEM. 
 
 ring inclined to the ecliptic, but which nowhere 
 touches [the body of the planet.] " 
 
 Huygen's discovery was not a mere piece of 
 guesswork, for he spent several years carefully 
 observing the alterations of form which Saturn 
 underwent, before he came to the conclusion that 
 it was only the existence of a ring surrounding 
 the planet which would explain the various ob- 
 served changes. 
 
 It was by way of guarding himself from being 
 robbed of the fruits of his discovery whilst he 
 was accumulating the necessary proof of its truth, 
 that he buried his thoughts in the logogriph or 
 anagram just quoted. Having arrived at the 
 conclusion which he did, he thought himself suf- 
 ficiently sure of his facts to predict that in July 
 or August 1671, the planet would again appear 
 round, the ring becoming invisible. This surmise 
 proved practically correct, in so far, that in May 
 1671, or within 2 months of the time predicted 
 by Huygens, Cassini saw the planet as a simple 
 ball unaccompanied by any ring. ^ 
 
 This is a convenient place at which to offer a 
 brief explanation of the changes of appearance as 
 regards the ball and rings which Saturn under- 
 goes. These changes depend jointly on Saturn's 
 motion in its orbit round the Sun, and on the 
 corresponding motion of the Earth in its orbit. 
 Neither Saturn nor the Earth revolve round the 
 Sun exactly in the ecliptic, and this want of coin- 
 cidence results in the fact, that twice in the 29^ 
 years occupied by Saturn in journeying round the 
 Sun, the plane of its ring is seen edgeways by us 
 on the Earth ; whilst at two other periods inter- 
 mediate but equi-distant the ring is seen opened 
 out to the widest possible extent ; that is, so far 
 
SATURN. 
 
 129 
 
130 THE STORY OF THE SOLAR SYSTEM. 
 
 as we on the Earth can by any possibility have a 
 chance of seeing it. 
 
 The appearances presented by the rings when 
 undergoing the transformations to which they 
 are subject, will be readily understood by an in- 
 spection of the annexed engravings. Fig. 17, 
 indicates the actual appearances in the years 
 specified, and these years may be considered as 
 carried forward and brought up to date by sub- 
 stituting 1877 for 1848, 1885 for 1855, 1891 for 
 1862, and 1898 for 1869. 
 
 Adverting to fig. 16, it will suffice to remark 
 that the two central phases of the rings, opened 
 wide, are to be deemed co-related, or indeed 
 identical in a geometrical sense (so to speak) 
 the difference being that one of them is to be 
 deemed to show the northern side of the ring 
 (which is now in view and will continue in view 
 till 1907) whilst the other represents the southern 
 side, which was in view from 1877 till 1891. The 
 foregoing is a brief statement of the general prin- 
 ciple involved in the changes which take place, 
 but the motions of the two planets introduce cer- 
 tain technical complications into the details which 
 would be seen by an observer using a large tele- 
 scope ; with these, however, the ordinary reader 
 will not care to concern himself, and need not 
 do so. 
 
 A great deal might be said with respect to the 
 rings treated descriptively. I will now mention 
 a few matters of general interest. Huygens re- 
 garded the appendage to Saturn, whose existence 
 he established, to be a single ring, but as far back 
 as 1675, Cassini determined that Huygen's single 
 ring was really made up of two, one lying inside 
 the other. Cassini in this conclusion outstepped 
 
SATURN. 131 
 
 not only all the observers of his own century, but 
 those of the succeeding century, for Sir W. Her- 
 schel even 100 years after Cassini, was for a long 
 time unable to satisfy himself, even with his 
 superior telescopes, that the black streaks seen 
 in the ring by Cassini, and regarded by him as 
 indicative of a severance of the ring into two 
 parts, really implied a severance. It is now, how- 
 ever, accepted as a fact that not only are the 
 rings which are known as A and B absolutely 
 distinct, but that A also is itself certainly duplex, 
 that is, that it certainly consists of two independ- 
 ent rings. In addition to this many competent 
 observers armed with powerful telescopes have 
 obtained traces of other sub-divisions, both in A 
 and B; and though there is some want of har- 
 mony in the details, as stated by the different 
 observers, yet undoubtedly we must speak of 
 Saturn's rings collectively as forming a multiple 
 system. 
 
 What the rings are is a highly debatable 
 point, but the preponderating idea is that they 
 are not what they appear to be, namely solid 
 masses of matter, but are swarms of independent 
 fragments of matter. Yet " fragment " is n9t the 
 best word to use, because it implies that some- 
 thing has been broken up to make the fragments. 
 Rather, perhaps, we should say with Professor 
 Young, that the rings are 4< composed of a swarm 
 of separate particles, each a little independent 
 moon pursuing its own path around the planet. 
 The idea was suggested long ago, by J. Cassini in 
 1715, and by Wright in 1750, but was lost sight of 
 until Bond revived it in connection with his dis- 
 covery of the dusky ring. Professor Benjamin 
 Pierce soon afterwards demonstrated that the 
 
132 THE STORY OF THE SOLAR SYSTEM. 
 
 rings could not be continuous solids; and Clerk 
 Maxwell finally showed that they can be neither 
 solid nor liquid sheets, but that all the known 
 conditions would be answered by supposing them 
 to consist of a flock of separate and independent 
 bodies, moving in orbits nearly circular, and in 
 one plane in fact, a swarm of meteors." 
 
 The thickness of the rings seen edgeways has 
 been variously estimated. Sir J. Herschel sug- 
 gested 250 miles as an outside limit, which G. P. 
 Bond reduced to 40 miles. It is generally con- 
 sidered, however, that 100 miles is probably not 
 far from the truth. Young has pointed out that 
 if a model of them were constructed on the scale 
 of i inch to represent 10,000 miles, so that the 
 outer ring of such a model would be nearly 17 
 inches in diameter, then the thickness of the ring 
 would be represented by that of an ordinary sheet 
 of writing paper. 
 
 Considered as a system, the rings are distinctly 
 more luminous than the planet, and of the two 
 bright rings, the inner one is brighter than the 
 outer one; and the inner one is less bright at its 
 inner edge than elsewhere. It is also to be 
 noticed that when seen edgeways just about the 
 time of the Saturnian equinoxes, when the Sun is 
 shifting over from one side of the ring to the 
 other, and the ring is dwindling down to a narrow 
 streak, its edges (forming the ans<z as they are 
 termed) do not disappear and reappear at the 
 same time, and are not always of the same ap- 
 parent extent. One ansa, indeed, is sometimes 
 visible without the other, and most commonly it 
 is the Eastern one that is missing. To what 
 causes these various peculiarities are due is un- 
 known. 
 
SATURN. 133 
 
 Many physical peculiarities have been either 
 noticed or suspected with reference to the bright 
 rings. For instance, on comparing one with 
 another, some persons have thought that their 
 surfaces are convex, and that they do not lie in 
 the same plane. The existence of mountains on 
 their surface has more than once been suspected. 
 Again, it has been fancied that they are sur- 
 rounded by an extensive atmosphere. It seems 
 hardly likely that the rings would have an atmos- 
 phere and not the ball (or vice versa), and, there- 
 fore, no wonder that we have no observations 
 which countenance the idea that the ball does 
 really possess an atmosphere. This, indeed, 
 seems to flow from Trouvelot's observation, that 
 the ball is less luminous at its circumference than 
 at its centre. 
 
 The circumstances of ring C, otherwise called 
 the " Dusky " or " Crape " ring are as curious 
 historically, as they are mysterious physically. 
 In 1838, Galle of Breslau, noticed what he 
 thought to be a gradual shading off of the in- 
 terior bright ring towards the ball. Though he 
 published a statement of what he saw, the matter 
 seems to have attracted little or no notice. In 
 1850, G. P. Bond in America perceived something 
 luminous between the ring and the ball, and after 
 repeated observations in concert with his father, 
 came to the conclusion that the luminous appear- 
 ance which he saw, was neither less nor more 
 than an independent and imperfectly illuminated 
 ring lying within the old rings and concentric 
 with them. Before, however, tidings of Bond's 
 discovery reached England, but a few days after 
 the discovery in point of actual date, Dawes sud- 
 denly noticed one evening as Bond had done, a 
 
134 THE STORY OF THE SOLAR SYSTEM. 
 
 luminous shading within the bright rings, which 
 he was not long in finding out to be in reality a 
 complete ring, except so far that a portion of it 
 was of course hidden from view behind the ball. 
 He, and O. Struve likewise, noticed that this new 
 Dusky Ring was occasionally to be seen divided 
 into two or more rings. The Dusky Ring is 
 transparent, though this fact was not ascertained 
 until 1852, or two years after Bond's discovery of 
 the ring. 
 
 The Dusky Ring is now recognised as a per- 
 manent feature of Saturn, but how far it used to 
 be permanent, or how long it has been so, is a 
 matter wrapped in doubt. Recorded observations 
 by Picard in 1673, and by Hadley in 1723, made 
 of course with telescopes infinitely less powerful 
 than those of the present day, seem to suggest 
 that both the observers named saw the Dusky 
 Ring, without, however, being able to form a 
 clear conception that it was a ring. It is strange 
 that during the long period from 1723 to 1838, 
 no one not even Sir W. Herschel, with his vari- 
 ous telescopes should have obtained or at least 
 have recorded any suspicion of its existence. 
 There is, however, direct evidence that the Dusky 
 Ring is wider and less faint than formerly. This 
 was directly confirmed by Carpenter in 1863, who 
 says he saw it "nearly as bright* as the illumi- 
 nated ring, so much so, that it might easily have 
 been mistaken for a part of it." In 1883, David- 
 son found a marked difference in the brilliancy of 
 the two ends (ansce) of the ring. 
 
 In 1889 Barnard was fortunate enough to ob- 
 serve an eclipse of one of Saturn's satellites by 
 the ring, but the eclipse, that is the concealment 
 of the satellite, was only effected when it passed 
 
SATURN. 135 
 
 behind the bright rings; the dusky ring did not 
 obliterate it, and hence there was obtained a con- 
 clusive proof of the transparency of the dusky 
 ring. Barnard further concluded from his obser- 
 vations that there was no separating space or 
 division between the inner bright ring and the 
 dusky ring, as has frequently been represented in 
 drawings. This transparency of the Dusky Ring, 
 as a matter of fact, is therefore undoubted ; yet 
 what are we to consider to be the meaning of an 
 observation by Wray in 1861, that whilst looking 
 at the dusky ring edgeways the impression was 
 conveyed to his eye that that ring was very much 
 thicker than the bright rings ? 
 
 A very interesting question which has been 
 much discussed has reference to the stability of 
 the rings. It is generally agreed that the con- 
 stituent particles of the rings must be in motion 
 round the primary or their equilibrium could not 
 be maintained : almost equally certain is it, and 
 for the like reason, that the rings cannot be solid. 
 Of actual change in the rings as regards their 
 dimensions, we have no satisfactory proof, though 
 authorities differ on the point, some thinking that 
 the rings are expanding inwards, so that ulti- 
 mately they will come into contact with the ball, 
 whilst others consider no proof whatever of such 
 change can be obtained from any of the observa- 
 tions yet made in the way of measurements. 
 
 We must now proceed to consider the satel- 
 lites of Saturn. These are 8 in number, 7 of 
 which move in orbits whose planes coincide 
 nearly with the planet's equator, whilst the re- 
 maining one is inclined about 12 thereto. One 
 consequence of this coincidence in the planes of 
 these satellites, which, it should be stated, are the 
 
136 THE STORY OF THE SOLAR SYSTEM. 
 
 7 innermost, is that they are always visible to the 
 inhabitants of both hemispheres when they are 
 not actually undergoing eclipse in the shadow of 
 Saturn. The satellites are of various sizes, and 
 succeed one another in the following order, reck- 
 oning from the nearest, outwards : Mimas, En- 
 celadus, Tethys, Dione, Rhea, Titan, Hyperion 
 and lapetus. Any good 2-inch telescope will 
 show Titan ; a 3-inch will sometimes show lape- 
 tus ; a 4-inch will show lapetus well, together 
 with Rhea and Dione, but hardly Tethys,; all the 
 others require large telescopes. If Saturn has 
 any inhabitants at all constituted like ourselves, 
 which is highly improbable, they will have a 
 chance of seeing celestial phenomena of the 
 greatest interest. What with the rings surround- 
 ing the planet and 8 moons in constant motion, 
 there will be an endless succession of astronom- 
 ical sights for them to study. The amount of 
 light received from the Sun cannot be much 
 barely y^o-th of what the earth receives. The 
 ring and satellites will therefore be useful as sup- 
 plementary sources of light ; yet the satellites 
 will not furnish much, for it has been calculated 
 that the surface of the sky occupied by all the 
 satellites put together would to a dweller on 
 Saturn only amount to 6 times the area of the 
 sky covered by our Moon ; whilst the intrinsic 
 brightness of all put together would be no more 
 than -^g-th part of the light which we receive from 
 our Moon. 
 
 The only physical fact worth noting here in 
 connection with the satellites concerns lapetus. 
 Cassini two centuries ago with his indifferent tel- 
 .escopes thought he had ascertained that this satel- 
 lite was subject to considerable variations of bril- 
 
SATURN. 137 
 
 liancy. Sir W. Herschel confirmed Cassini as to 
 this. He found that it was much less brilliant 
 when traversing the eastern half of its orbit than 
 at other times. Two conclusions have been drawn 
 from this fact. One is that the satellite rotates 
 
 FIG. 18. Saturn with the shadow of Titan on it, March u, 
 1892 (Terby). 
 
 once on its axis in the same time that it performs 
 one revolution round its primary ; and that there 
 are portions of its surface which are almost en- 
 tirely incapable of reflecting the rays of the Sun. 
 This last named supposition may perhaps be well 
 founded, but the former needs more proof than is 
 as yet forthcoming. lapetus on the whole may 
 be said to shine as a star of the 9th magnitude. 
 To this it may be added that Titan is of the 8th 
 magnitude, but all the others much smaller. 
 
 Saturn revolves round the Sun in a little under 
 29^ years at a mean distance of 886 millions of 
 miles. Its apparent diameter varies between 15" 
 and 2o ;/ ; its true diameter may be put at 75,000 
 miles. The flattening of the poles, or " polar 
 compression " as it is called, is greater than that 
 
138 THE STORY OF THE SOLAR SYSTEM. 
 
 of any other planet, but is usually less noticeable 
 than in the case of Jupiter, because the ring is 
 apt to distract the eye, except when near the edge- 
 ways phase. The compression may be taken at . 
 
 CHAPTER XI. 
 
 URANUS. 
 
 To the Ancients Saturn was the outermost 
 planet of the System, nothing beyond it being 
 known. Nor indeed was it to be assumed that 
 any more could possibly exist, because Mercury, 
 Venus, the Earth, Mars, Jupiter, and Saturn, with 
 the Sun, made 7 celestial bodies of prime impor- 
 tance; and 7 was the number of perfection; and 
 there was thus provided one celestial body to give 
 a name to each of the days of the week. 
 
 But Science is not sentimental ; and when men 
 of Science come upon what looks like a discovery 
 they do their best to bring their discovery to a 
 successful issue, however much people's prejudices 
 may seem to stand in the way at the moment. 
 
 On a certain evening in March, 1781, Sir Wil- 
 liam Herschel, then gradually coming into notice 
 as a practical astronomer, was engaged in looking 
 at different fields of stars in the constellation 
 Gemini when he lighted on one which at once at- 
 tracted his special attention. Altering his eye- 
 piece, and substituting a higher magnifying power 
 he found the apparent size of the mysterious ob- 
 ject enlarged, which conclusively proved that it 
 was not a star; for it is a well-known optical 
 property of all stars that whatever be the size of 
 
URANUS. 139 
 
 telescope employed on them, and however high 
 the magnifying power no definite disc of light can 
 be obtained when in focus. Herschel's new find, 
 therefore, was plainly not a star, and no idea hav- 
 ing in those days come into men's minds of there 
 being any new planets awaiting discovery, he an- 
 nounced as a matter of course that he had found 
 a new comet, so soon as he ascertained that the 
 new body was in motion. The announcement 
 was not made to the Royal Society till April 26, 
 more than six weeks after the date of the actual 
 discovery, an indication, by the way, of the dila- 
 tory circulation of news a hundred years ago. 
 The supposed comet was observed by Maskelyne, 
 the Astronomer Royal, four days after Herschel had 
 first seen it, and Maskelyne seems to have at once 
 got the idea into his head that he was looking at 
 a planet and not at a comet. As soon as possible 
 after the discovery of a new comet the practice 
 of astronomers is to endeavour to determine what 
 is the shape of the orbit which it is pursuing. All 
 attempts to carry out this in the case of Herschel's 
 supposed new comet proved abortive, because it 
 was found impossible to harmonise, except for a 
 short period of time, the movements of the new 
 body with the form of curve usually affected by 
 most comets, namely, the parabola. It is true, as 
 we shall see later on in speaking of comets, that 
 a certain number of those bodies do revolve in 
 the closed curve known as the ellipse, but it is 
 usual to calculate the parabolic form first of all, 
 because it is the easier to calculate ; and to perse- 
 vere with it until it plainly appears that the parab- 
 ola will not fit in with the observed movements 
 of the new object. This practice was carried out 
 in the case of Herschel's new body, and it was 
 
140 THE STORY OF THE SOLAR SYSTEM. 
 
 eventually found that not only was its orbit not 
 parabolic; that not only was its orbit not an elon- 
 gated ellipse of the kind affected by comets; but 
 that it was nearly a circle, and as the body itself 
 showed a defined disc the conclusion was inevita- 
 ble : it was in real truth a new planet. It has not 
 taken long to write this statement, and it will 
 take still less time for the reader to read what 
 has been written, but the result just mentioned 
 occupied the attention of astronomers many 
 months in working out, step by step, in such a 
 way as to make sure that no* mistake had been 
 made. 
 
 When it was once clearly determined that 
 Herschel had added a new planet to the list of 
 known planets it became an interesting matter 
 of inquiry to find out whether it had ever been 
 seen before; and to settle the name it should bear. 
 A little research soon showed that the new planet 
 had been seen and recorded as a fixed star by 
 various observers on 20 previous occasions, be- 
 ginning as far back as Dec. 13, 1690, when Flam- 
 stead registered at Greenwich as a star. These 
 various observations, spread over a period of 91 
 years, and all recorded by observers of skill and 
 eminence materially helped astronomers in their 
 efforts to calculate accurately the shape and na- 
 ture of the new planet's orbit. One observer, a 
 Frenchman named Le Monnier, saw the planet no 
 less than 12 times between 1750 and 1771, and if 
 he had had (which it is known he had not) an or- 
 derly and methodical mind, the glory of this dis- 
 covery would have been lost to England and 
 obtained by France. Arago has left it on record 
 that he was once shown one of these chance ob- 
 servations of Uranus, which had been recorded 
 
URANUS. 141 
 
 by Le Monnier on an old paper bag in which hair 
 powder had been sold by a perfumer. 
 
 A long d scussion took place on the question 
 of a name for the new planet. Bode's suggestion 
 of " Uranus " is now in universal use, but it is 
 within the recollection of many persons living 
 that this planet bore sometimes the name of the 
 " Georgium Sidus " and sometimes the name of 
 " Herschel." The former designation was pro- 
 posed by Herschel himself in compliment to his 
 sovereign and patron George III. of England; 
 whilst a French astronomer suggested the latter 
 name. However, neither of these appellations 
 was acceptable to the astronomers of the Con- 
 tinent, who declared in favour of a mythological 
 name, though it was a long time before they 
 agreed to accept Bode's " Uranus." The symbol 
 commonly used to represent the planet is formed 
 of Herschel's initial with a little circle added 
 below, though the Germans employ something 
 else, " made in Germany," to quote a too familiar 
 phrase. 
 
 The visible disc of Uranus is so small that 
 none but telescopes of the very largest size can 
 make anything of it. A few sentences therefore 
 will dispose of this part of the subject. The disc 
 is usually bluish in tinge, and most observers who 
 look at it consider it uniformly bright, but there 
 is satisfactory testimony to the effect that under 
 the most favourable circumstances of instrument 
 and atmosphere two or more belts, not unlike the 
 belts of Jupiter, may be traced. From the posi- 
 tion in which these belts have been seen it is in- 
 ferred that the satellites of Uranus (presently to 
 be mentioned) are unusually much inclined to the 
 planet's equator, and revolve in a retrograde direc- 
 
142 THE STORY OF THE SOLAR SYSTEM. 
 
 tion, contrary to what is the ordinary rule of the 
 planets and satellites. It is assumed as the basis 
 of these ideas, (and by analogy it is reasonable 
 to do this) that the belts are practically parallel 
 to the planet's equator, and at right angles to the 
 planet's axis of rotation. To speak of the planet's 
 axis of rotation is, in one sense, another assump- 
 tion, because available observations can scarcely 
 be said to enable us to demonstrate that the planet 
 does rotate on its axis, yet we can have no moral 
 doubt about it. Taylor has suggested, grounds 
 for the opinion that " there can be very little 
 doubt that Uranus is to a very large extent self- 
 luminous, and that we do not see it wholly by re- 
 flected light." To this Gore adds the idea that 
 there is " strong evidence in favour of the exist- 
 ence of intrinsic heat in the planet." 
 
 Uranus is attended by several satellites. It 
 was once thought that there were eight, of which 
 six were due to Sir W. Herschel, the other two 
 being of modern discovery. Astronomers are, 
 however, now agreed that no more than four 
 satellites can justly be recognised as known to 
 exist, and they are so minute in size that only the 
 very largest telescopes will show them; and there- 
 fore our knowledge of them is extremely limited. 
 Sir W. Herschel's idea that he had seen six satel- 
 lites appears to have resulted from his having on 
 some occasions mistaken some very small stars 
 for satellites. Two only of his six are thought 
 to have been real satellites. The other two rec- 
 ognised satellites were found both in 1847, one by 
 Lassell, and the other by O. Struve. 
 
 Uranus revolves round the Sun in rather more 
 than 84 years, at a mean distance of 1781 millions 
 of miles. Its apparent diameter, seen from the 
 
NEPTUNE. 143 
 
 Earth, does not vary much from 3^, which corre- 
 sponds to about 31,000 miles. It has been calcu- 
 lated that the light received from the Sun by 
 Uranus would be about the amount furnished by 
 300 full Moons seen by us on the Earth, though 
 another authority increases this to 1670 full 
 Moons. From Uranus Saturn can be seen, and 
 perhaps Jupiter, both as inferior planets, just as 
 we see Venus and Mercury ; but all the other 
 inner planets, including Mars and the Earth, 
 would be hopelessly lost to view, because per- 
 petually too close to the Sun. Possibly, how- 
 ever, they might, on rare occasions, be seen in 
 transit across the Sun's disc. Neptune, of course, 
 would be visible and be the only superior planet. 
 The Sun itself would appear to an observer on 
 Uranus as a very bright star, with a disc of i J' of 
 arc in diameter. 
 
 CHAPTER XII. 
 
 NEPTUNE. 
 
 WE now come to the best known planet of 
 the solar system, reckoning outwards from the 
 Sun, and though this planet itself, as an object to 
 look at, has no particular interest for the general 
 public, yet the history of its discovery is a matter 
 of extreme interest. Moreover, it is very closely 
 mixed up with the history of the planet Uranus, 
 which has just been described. After Uranus 
 had become fully recognised as a regular member 
 of the solar system, a French astronomer named 
 Alexis Bouvard set himself the task of exhaust- 
 
144 THE STORY OF THE SOLAR SYSTEM. 
 
 ively considering the movements of Uranus with 
 a view of determining its orbit with the utmost 
 possible exactness. His available materials 
 ranged themselves in two groups: the modern 
 observations between 1781 and 1820, and the 
 early observations of Flamsteed, Bradley, Mayer, 
 and Le Monnier, extending from 1690 to 1771. 
 Bouvard found in substance that he could frame 
 an orbit which would fit in with each group of 
 observations, but that he could not obtain an 
 orbit which would reconcile both sets of observa- 
 tions during the 130 years over which they jointly 
 extended. He therefore rashly came to the con- 
 clusion that the earlier observations, having been 
 made when methods and instruments were alike 
 relatively imperfect, were probably inaccurate or 
 otherwise untrustworthy, and had better be re- 
 jected. This seemed for awhile to solve the diffi- 
 culty, and results which he published in 1821 rep- 
 resented with all reasonable accuracy the then 
 movements of the planet. A very few years, how- 
 ever, sufficed to reveal discordances between ob- 
 servation and theory, so marked and regular as 
 to make it perfectly clear that it was not Bou- 
 vard's work which was faulty but that Uranus 
 itself had gone astray through the operation of 
 definite but as yet unknown causes. What these 
 causes were could only be a matter of surmise 
 based upon the evident fact that there was some 
 source of disturbance which was evidently throw- 
 ing Uranus out of its proper place and regular 
 course. First one and then another astronomer 
 gave attention and thought to the matter, and 
 eventually the conclusion was arrived at that 
 there existed, more remote from the Sun than 
 Uranus, an undiscovered planet which was able 
 
NEPTUNE. 145 
 
 to make its influence felt by deranging the move- 
 ments of Uranus in its ordinary journey round 
 the Sun every 84 years. This conclusion on the 
 part of astronomers becoming known, a young 
 Cambridge student, then at St. John's College, 
 John Couch Adams by name, resolved, in July 
 1841, to take up the subject, though it was not 
 until 1843 tnat ne actually did so. The problem 
 to be solved was to suggest the precise place in 
 the sky at a given time of an imaginary planet 
 massive enough to push, or pull, out of its nor- 
 mal place the planet Uranus, which was evidently 
 being pushed at one time and pulled at another. 
 It would also be part of the problem to predict 
 the distance from the Sun of the planet thus im- 
 agined to exist. Adams worked patiently and 
 silently at this very profound and difficult prob- 
 lem for if years when he found himself able to 
 forward to Airy, who had become Astronomer 
 Royal after being a Cambridge Professor, some 
 provisional elements of an imaginary planet of a 
 size, at a distance, and in a position to meet the 
 circumstances. It is greatly to be regretted, on 
 more grounds than one, that Airy did nothing but 
 pigeon-hole Adams's papers. Had he done what 
 might have been, and probably was, expected, 
 that is, had he made them public, or better still 
 had he made telescopic use of them, a long and 
 unpleasant international controversy would have 
 been avoided, and Adams would not have been 
 robbed in part of the well-deserved fruits of his 
 protracted labours. 
 
 We must now turn to consider something that 
 was happening in France. In the summer of 
 1845, J ust before Adams had finished his work, 
 and one and a half years after he commenced it 
 
146 THE STORY OF THE SOLAR SYSTEM. 
 
 a young Frenchman, who afterwards rose to 
 great eminence, U. J. J. Le Verrier, turned his 
 attention to the movements of Uranus with a 
 view of ascertaining the cause of their recognised 
 irregularity. In November 1845 he made public 
 the conclusion that those irregularities did not 
 exclusively depend upon Jupiter or Saturn. He 
 followed this up in June 1846 by a second memoir 
 to prove that an unknown exterior planet was the 
 cause of all the trouble, and he assigned evidence 
 as to its position very much as Adams -had do*ne 
 8 months previously. Airy on receiving a copy 
 of Le Verrier's memoir seems so far, at last, to 
 have been roused that he took the trouble to 
 compare Le Verrier's conclusions with those of 
 Adams so long in his possession neglected. Find- 
 ing that a remarkably close accord existed be- 
 tween the conclusions of the two men, he came to 
 realise that both must be of value, and he wrote 
 a fortnight later to suggest to Professor Challis 
 the desirability of his instituting a search for the 
 suspected planet. Challis began within two days, 
 but was handicapped by not having in his posses- 
 sion any map of the stars in the neighbourhood 
 suggested as the locale of the planet. He lost no 
 time however in making such a map, but, of 
 course, the doing so caused an appreciable delay, 
 and it was not until September 29, 1846, that he 
 found an object which excited his suspicions and 
 eventually proved to be the planet sought for. It 
 was subsequently ascertained that the planet had 
 been recorded as a star on August 4 and 12, and 
 that the star of August 12 was missing from the 
 zone observed on July 30. The discovery of the 
 planet was therefore just missed on August 12 
 because the results of each evening's work were 
 
NEPTUNE. 147 
 
 not adequately compared with what had gone 
 before. 
 
 Meanwhile things had not been standing still 
 in France. In August 1846, Le Verrier published 
 a third memoir intended to develope information 
 respecting the probable position of the planet in 
 the heavens. In September 23 a summary of this 
 third memoir was received by Encke at Berlin, 
 accompanied by the request that he would co- 
 operate instrumentally in the search for it. Encke 
 at once directed two of his assistants named 
 D'Arrest and Galle to do this, a^nd they were for- 
 tunately well circumstanced for the task. Unlike 
 Challis, who, as we have seen, could do nothing 
 until he had made a map for himself, the Berlin 
 observers had one ready to hand, which by good 
 chance had just been published by the Berlin 
 Academy for the part of the heavens which both 
 Adams and Le Verrier assigned as the probable 
 locality in which the anxiously desired planet 
 would be found. Galle called out the visible 
 stars one by one whilst D'Arrest checked them 
 by the map, and suddenly he came upon an un- 
 marked object which at the moment looked like 
 an 8th magnitude star. The following night 
 showed that the suspicious object was in motion, 
 and it was soon ascertained to be the trans-Ura- 
 nian planet which was being searched for. The 
 discovery when announced excited the liveliest 
 interest all over the world. It did more ; it cre- 
 ated a bitter feeling of resentment on the part of 
 French astronomers that the laurels claimed by 
 them should have been also claimed in an equal 
 share by a young and unknown Englishman, and 
 accordingly the old cry of " perfide Albion " arose 
 on a.ll sides. I have been particular in stating 
 
148 THE STORY OF THE SOLAR SYSTEM. 
 
 the various dates which belong to this narrative, 
 in order to make as clear as possible the facts of 
 the case. This is even now necessary, because 
 though the astronomers of England and Ger- 
 many are willing to give Adams and Le Verrier 
 each their fair share of this great discovery, the 
 same impartial spirit is not to be found in France, 
 for nothing is more common, even in the present 
 day, in looking at French books of astronomy, 
 than to find Adams's name either glossed over or 
 absolutely suppressed altogether when the planet 
 Neptune is under discussion. 
 
 How remarkable a discovery this was, will 
 perhaps be realized, when it is stated that Adams 
 was only 2j out in assigning the position of the 
 new planet, whilst Le Verrier was even nearer, 
 being barely i out. 
 
 We know practically nothing respecting the 
 physical appearance of Neptune, owing to its im- 
 mense distance from us, and for the like reason 
 the Neptunian astronomers, if there are any, will 
 know absolutely nothing about the Earth ; in- 
 deed, their knowledge of the Solar System will 
 be restricted to Uranus, Saturn, and the Sun. 
 Even the Sun will only have an apparent diameter 
 of about i' of arc, and, therefore, will only seem 
 to be a very bright star, yielding light equal in 
 amount, according to Zollner, to about 700 full 
 moons. There is one satellite belonging to Nep- 
 tune, and as this has been calculated to exhibit a 
 disc 10 in diameter, a certain amount of light 
 will no doubt be afforded by it especially if, as is 
 not unlikely, Neptune is itself possessed, of some 
 inherent luminosity independently of the Sun. 
 
 The fact that Neptune seems destitute of vis- 
 ible spots or belts, results in our being igno- 
 
NEPTUNE. 149 
 
 rant of the period of its axial rotation, though it 
 should be stated that in 1883, Maxwell Hall in 
 Jamaica, observed periodical fluctuations in its 
 light, which he thought implied that the planet 
 rotated on its axis in rather less than 8 hours. 
 Several observers thought 20 or 30 years ago, 
 that they had noticed indications of Neptune 
 being surrounded by a ring like Saturn's ring, 
 but the evidence as to this is very inconclusive. 
 It is quite certain that none but the very largest 
 telescopes in the world would show any such 
 appendage, and this planet seems to have been 
 neglected of late years, by the possessors of such 
 telescopes. Moreover, if a ring existed it would 
 only open out to its full extent once in every 82 
 years, being the half of the period of the planet's 
 revolution round the Sun (just as Saturn's ring 
 only opens out to the fullest extent every 14^ 
 years), so that, obviously, supposing suspicions 
 of a ring dating back 30 or 40 years were well 
 founded, it might well be that another 30 or 40 
 years might need to elapse, before astronomers 
 would be in a position to see their suspicions 
 revive. 
 
 Neptune revolves round the Sun in 164^ 
 years, at a mean distance of 2791 millions of 
 miles. Its apparent diameter scarcely varies 
 from 2f". Its true diameter is about 37,000 
 miles. No compression of the Poles is percepti- 
 ble. Its one satellite revolves round Neptune in 
 5f days, and in a retrograde direction, at a mean 
 distance of 223,000 miles, and shines as a star of 
 the i4th magnitude. This is a peculiarity which 
 it only shares with the satellites of Uranus, so far 
 as it regards the planetary members of the Solar 
 System, though there are^many retrograde Comets, 
 
150 THE STORY OF .THE SOLAR SYSTEM. 
 
 The question has often been mooted, whether 
 there exists, and belonging to the Solar System, 
 a planet farther. off than Neptune. There does 
 seem some evidence of this, as we shall better 
 understand, when we come to the subject of long- 
 period Comets, though it cannot be said that much 
 progress has yet been made in arriving at a solu- 
 tion of the problem. 
 
 Unless there does exist a trans-Neptunian 
 planet, a Neptunian astronomer will know very 
 little about planets, for Uranus and Saturn will 
 alone be visible to him. Both will of course be 
 what we call " inferior planets," and under the 
 best of circumstances will cut a poor figure in the 
 Neptunian sky. 
 
 CHAPTER XIII. 
 
 COMETS. 
 
 I SUPPOSE that it is the experience of all those 
 who happen to be in any sense, however humble, 
 specialists in a certain branch of science, that 
 from time to time, they are beset with questions 
 on the part of their friends respecting those par- 
 ticular matters which it is known that they have 
 specially studied. There is no fault to be found 
 with this thirst for information, always supposing 
 that it is kept within due bounds; but my motive 
 for alluding to it here, is to see whether any well- 
 marked conclusion can be drawn from it, within 
 my own knowledge as regards astronomical facts 
 or events. Now in the case of the science of 
 astronomy (for which in this connection I, for the 
 
COMETS. 151 
 
 moment, will venture to speak), there is certainly 
 no one department which so unfailingly, at all 
 times and in all places, seems to evoke such pop- 
 ular sympathy and interest as the department 
 which deals with Comets. 
 
 Sun-spots may come and go ; bright planets 
 may shine more brightly; the Sun or Moon may 
 be obscured by eclipses ; temporary stars may 
 burst forth, all these things are within the ken 
 of the general public by means of newspapers or 
 almanacs, but it is a comet which evokes more 
 questionings and conversations than all the other 
 matters just referred to put together. When a 
 new and bright comet appears, or even when any 
 comet not very bright gets talked about, the old 
 question is still fresh and verdant " Is there any 
 danger to the Earth to be apprehended from col- 
 lision with a Comet?" followed by " What is a 
 Comet ? " " What is it made of ? " " Has it ever 
 appeared before ? " " Will it come back again ? " 
 and so on. Questions in this strain have more 
 often than I can tell of been put to me. They 
 seem the stock questions of all who will con- 
 descend to replace for five minutes in the day 
 the newest novel or the pending parliamentary 
 election. 
 
 It may be taken as a fact (though in no proper 
 sense a rule) that a bright and conspicuous comet 
 - comes about once in 10 years, and a very remark- 
 able comet every 30 years. Thus we have had 
 during the present century bright comets in 1811, 
 1825, 1835, 1843, 1858, 1861, 1874 and 1882, whereof 
 those of 1811, 1843, and 1858 were specially cele- 
 brated. Tested .then by either standard of words 
 "bright and conspicuous," or "specially cele- 
 brated," it may be affirmed that a good comet is 
 
152 THE STORY OF THE SOLAR SYSTEM. 
 
 now due, so let us prepare for it by getting up the 
 subject in advance. 
 
 I will not attempt to answer in regular order 
 or in any set form the questions which I have 
 just mentioned as being stock questions, but they 
 will be answered in substance as we go along. 
 There is one matter in connection with comets 
 which has deeply impressed itself upon the public 
 mind, and that is the presence or absence of a 
 " tail." It is not too much to say that the gener- 
 ality of people regard the tail of a comet as the 
 comet; and that though an object may be a true 
 comet from an astronomer's point of view, yet if 
 it has no tail its claims go for nought with the 
 mass of mankind. We have here probably a 
 remnant of ancient thought, especially of that 
 line of thought which in bygone times associated 
 Comets universally with the idea that they were 
 especially sent to be portents of national disasters 
 of one kind or another. This is brought out by 
 numberless ancient authors, and by none more 
 forcibly than Shakespeare. Hence we have such 
 passages as the following in Julius Ccesar (Act ii., 
 sc. 2) : 
 
 " When beggars die there are no comets seen, 
 The Heavens themselves blaze forth the death of princes." 
 
 In Henry VI. (Part I., act i., sc. i) we find the 
 well-known passage : 
 
 " Comets importing change of times and states 
 Brandish your crystal tresses in the sky, 
 And with them scourge the bad revolting stars 
 That have consented unto Henry's death." 
 
 There are in point of fact two distinct ideas 
 evolved here : (i) that comets are prophetic of 
 evil, and (2) stars potential for evil, 
 
COMETS. 153 
 
 There is another passage in Henry VI. (Part 
 I., act iii., sc. 3) even more pronounced: 
 
 "Now shine it like a Comet of revenge, 
 A prophet to the fall of all our foes." 
 
 Again ; in Hamlet (Act i., sc. i) we find : 
 
 " As stars with trains of fire, and dews of blood, 
 Disasters in the Sun." 
 
 Once more ; in the Taming of the Shrew (Act 
 iii., sc. 2) we have the more general, but still em- 
 phatic enough, idea expressed by the simple words 
 of reference to 
 
 " Some Comet or unusual prodigy." 
 
 Shakespeare may be said to have lived at the 
 epoch when astrology was in high favour, and it 
 may be that he only gave utterance to the current 
 opinion^ prevalent among all classes in those still 
 somewhat " Dark Ages " (so called). This, how- 
 ever, can hardly be said of the author of my next 
 quotation John Milton {Paradise Lost, bk. II.) : 
 
 " Satan stood 
 
 Unterrified, and like a Comet burned, 
 That fires the length of Ophiuchus huge 
 In th' Arctic sky, and from its horrid hair 
 Shakes pestilence and war." 
 
 Jumping over a century we find the ancient 
 theory still in vogue, or Thomson (Seasons ^ Sum- 
 mer) would never have written : 
 
 " Amid the radiant orbs 
 That more than deck, that animate the sky, 
 The life-infusing suns of other worlds ; 
 Lo ! from the dread immensity of space, 
 Returning with accelerated course, 
 The rushing comet to the sun descends ; 
 And, as he sinks below the shading earth, 
 With awful train projected o'er the heavens. 
 The guilty nations tremble." 
 
154 , THE STORY OF THE SOLAR SYSTEM. 
 
 Even Napoleon I. had servile flatterers who, as 
 late as 1808, tried to extract astrological influence 
 out of 'a comet by way of bolstering up " Old 
 Bony." But enough of poetry and fiction, let us 
 hasten back to prosaic fact. 
 
 Comets as objects to look at may be classed 
 under three forms, though the same comet may 
 undergo such changes as will at different epochs 
 in its career cause it to put on each variety of 
 form in succession. Thus the comet of 1825 seen 
 during that year as a brilliant naked-eye object, 
 after being lost in the sun's 
 rays, was again found on 
 April 2, 1826 by Pons. 
 Lamentable were his cries 
 at the miserable plight it 
 was in. He described it 
 as totally destroyed : 
 without tail, beard, coma 
 or nucleus, a mere spectre. 
 The simplest form of 
 comet is a mere nebulous 
 mass, almost always cir- 
 cular, or perhaps a little 
 oval, in outline. It may maintain this appearance 
 throughout its visibility ; or, growing brighter 
 may become a comet of the second class, with a 
 central condensation, which developing becomes 
 a "nucleus " or head. It may retain this feature 
 for the rest of its career, or may pass into the 
 third class and throw out a "coma" or beard, 
 which will perhaps develop into a tail or tails. 
 Doing this it will not unfrequently grow bright 
 enough and large enough to become visible to 
 the naked eye. In exceptional cases the nucleus 
 will become as bright as a 2nd or even ist mag- 
 
 FIG. 19. Telescope Comet 
 with a nucleus. 
 
COMETS. 155 
 
 nitude star, and the tail may acquire a length of 
 several or many degrees. In the last named case 
 of all the comet becomes, /#r excellence according 
 to the popular sentiment, " a comet." It will now 
 be readily inferred that the astronomer in his ob- 
 servatory has to do with many comets which the 
 public at large never hear of, or if they do hear 
 of, treat with contempt, because they are destitute 
 of tails. 
 
 The tails of comets exhibit very great varieties 
 not only of size but of form ; some are long and 
 
 FIG. 20. Wells's Comet of 1882, seen in full daylight near the 
 Sun on Sept. 18. 
 
 slender; some are long and much spread out 
 towards their ends, like quill pens, for instance; 
 some are short and stumpy, mere tufts or excres- 
 cences rather than tails. Not unfrequently a tail 
 seems to consist of two parallel rays with no 
 cometary matter, or it may be only a very slight 
 amount of cometary matter traceable in the in- 
 
156 THE STORY OF THE SOLAR SYSTEM. 
 
 FIG. 21. Quenisset's Comet, July 9, 1893 (Quenisset). 
 
COMETS. 157 
 
 terspace; some have one main tail consisting of 
 a pair of rays such as just described, together 
 with one or more subsidiary or off-shoot tails. 
 The comet of 1825 had five tails and the comet 
 of 1744 had six tails. It might be inferred from 
 all this that the tails of comets are so exceedingly 
 irregular, uncertain and casual as to be amenable 
 to no laws. This was long considered to be the 
 case; but a Russian observer named Bredichin, 
 as the result of much study and research, has 
 arrived at the conclusion that all comet tails may 
 be brought under one or other of three types; 
 and that each type is indicative of certain dis- 
 tinct differences of origin and condition which he 
 considers himself able to define. The first type 
 comprises tails which are long and straight; 
 "they are formed " (to quote Young's statement 
 of Bredichin's views) " of matter upon which the 
 Sun's repulsive action is from twelve to fifteen 
 times as great as the gravitational attraction, so 
 that the particles leave the comet with a relative 
 velocity of at least four or five miles a second ; 
 and this velocity is continually increased as they 
 recede, until at last it becomes enormous, the 
 particles travelling several millions of miles in a 
 day. The straight rays which are seen in the 
 figure of the tail of Donati's Comet, tangential to 
 the tail, are streamers of this first type; as also 
 was the enormous tail of the comet of 1861. 
 The second type is the curved plume-like train, 
 like the principal tail of Donati's Comet. In this 
 type the repulsive force varies from 2.2 times 
 gravity (for the particles on the convex, edge of 
 the tail) to half that amount for those which form 
 the inner edge. This is by far the most common 
 type of cometary train. A few comets show tails 
 
158 THE STORY OF THE SOLAR SYSTEM. 
 
 of the third type short, stubby, brushes violently 
 curved, and due to matter of which the repulsive 
 force is only a fraction of gravity from T Vto ." 
 
 Bredichin wishes it to be inferred that the 
 tails of the ist type are probably composed of 
 hydrogen ; those of the 2nd type of some hydro- 
 carbon gas; and those of the 3rd of the vapour 
 of iron, probably with some admixture of sodium 
 and other substances. Bredichin, as a reason for 
 these conclusions, supposes that the force which 
 generates the tails of comets is a repulsive force, 
 with a surface action the same for equal surfaces 
 of any kind of matter ; the effective accelerating 
 force therefore measured by the velocity which 
 it would produce would depend upon the ratio 
 of surface to mass in the particles acted upon, 
 and so, in his view, should be inversely propor- 
 tional to their molecular weights. Now it hap- 
 pens that the molecular weights of hydrogen, of 
 hydro-carbon gases, and of the vapour of iron 
 bear to each other just about the required pro- 
 portion. 
 
 I am here stating the views and opinions of 
 others without definitely professing to be satis- 
 fied with them, but as they have met with some 
 acceptance,' it is proper to chronicle them, though 
 we know nothing of the nature of the repulsive 
 force here talked about. It might be electric, it 
 might be anything. The spectroscope certainly 
 lends some countenance to Bredichin's views, but 
 we need far more knowledge and study of comets 
 before we shall be justly entitled to dogmatise on 
 the subject. 
 
 This has been rather a digression. I go back 
 now to prosaic matters of fact, of which a vast 
 and interesting array present themselves for con- 
 
COMETS. 
 
 159 
 
 sideration in connection with comets. Let us 
 consider a little in detail what they are, to look 
 
 FIG. 22. Holmes's Comet, Nov. 9, 1892 (Denning). 
 
 at. We have seen that a well-developed comet 
 of the normal type usually comprises a nucleus, 
 a head or coma, and a tail. Comets which have 
 
 FIG. 23. Holmes's Comet, Nov. 16, 1892 (Denning). 
 
 no tails generally exhibit heads of very simple 
 structure; and if there is a nucleus, the nucleus 
 is little else than a stellar point of light. But in 
 ii 
 
160 THE STORY OF THE SOLAR SYSTEM. 
 
 the case of the larger comets, which are almosi 
 or quite visible to the naked eye, the head ofter 
 exhibits a very complex structure, which in not 
 few cases seems to convey very definite indica 
 tions of the operations going on at the time 
 Figs. 22 and 23 may be taken as samples of c. 
 complex cometary head, though no two comets- 
 resemble one another exactly in details. Fig. 24 
 forcibly conveys the idea that we are looking at 
 a process of development analogous to an uprush 
 of water from a fountain, or perhaps I might bet- 
 ter say, from a burst waterpipe. There is a dis- 
 tinct idea of a jet. This self-same idea, in another 
 form, presents itself in the case of those comets 
 which exhibit what astronomers are in the habit 
 of calling " luminous envelopes." The jet in this 
 case is not strictly a jet 
 because it is not a con- 
 tinuous outflow, or over- 
 flow, of matter ; the idea 
 rather suggests itself of an 
 intermittent overflow re- 
 sulting in accumulated lay- 
 ers, or strata, of matter 
 becoming visible. But 
 with this we come to a 
 standstill; we cannot tell 
 FIG. 24. Comet in. of 1862, where the matter comes 
 
 on Aug 22, showing jet of from and still less where 
 luminous matter (Challis). 
 
 it goes to; we can only 
 
 record what our eyes, assisted by telescopes, tell 
 us. There can, however, I think, be no doubt 
 that the matter of a comet becomes displayed to 
 our senses as the result of a process of expulsion, 
 or repulsion, from the nucleus ; and then, having 
 become launched into space, it comes under the 
 
COMETS. l6l 
 
 influence, also repulsive, of the Sun. All these 
 things are visible facts. As to causes, we suggest 
 little, because we know so little. Anyone who 
 has seen a comet and has watched the displays of 
 jets and luminous envelopes, such as I have en- 
 deavoured to set forth, will realise at once how 
 impossible it is to describe these things in words. 
 They must be seen either in actual being or in 
 picture. Some further allusions to this branch of 
 the subject may perhaps be more advantageously 
 made after we have considered the movements 
 and orbits of comets. 
 
 There is often a slight general resemblance 
 between a planet and a comet, as regards the 
 path which each class of body pursues. Probably 
 the least reflective person likely to be following 
 me here understands the bare fact, that all the 
 planets revolve round the Sun, and are held to 
 defined orbits by the Sun's influence, or attrac- 
 tion, as it is called. Perhaps, it is not equally 
 
 i realised, that in a somewhat similar, but not quite 
 the same way, comets are influenced and con- 
 trolled by the Sun. 
 
 Comets must be considered as regards their 
 motions to be divisible into two classes : (i) 
 Those which belong to the Solar System ; and (2) 
 
 i those which do not. Each of these two classes 
 must again be sub-divided, if we would really ob- 
 tain a just conception of how things stand. 
 
 By the Comets which belong to the Sun, I 
 mean those which revolve round the Sun in 
 
 i closed orbits ; * and are, or may be, seen again 
 and again at recurring intervals. There are 2 
 
 - 
 
 * The circle and the ellipse are what are called " closed " 
 ves. 
 
162 THE STORY OF THE SOLAR SYSTEM. 
 
 or 3 dozen comets which present themselves to 
 our gaze at stated intervals, varying from about 
 3 to 70 years. There are again other comets 
 which without any doubt (mathematically) are 
 revolving round the Sun in closed orbits, but in 
 orbits so large and with periods of revolution so 
 long (often many centuries), that though they 
 will return again to the sight of the inhabitants 
 of the earth some day, yet no second return hav- 
 ing been actually recorded, the astronomer's pre- 
 diction that they will return, remains at present 
 a prediction based on mathematics but nothing 
 more. 
 
 There is another class of Comet of which we 
 see examples from time to time, and having seen 
 them once shall never see again. This is be- 
 cause these Comets move in orbits which are not 
 closed, and which are known as parabolic or hy- 
 perbolic orbits respectively, because derived from 
 those sections of a cone which are called the 
 Parabola and the Hyperbola. It must be under- 
 stood that what I am now referring to is purely 
 a matter of orbit, and that no relationship sub- 
 sists between the size and physical features of a 
 Comet and the path it pursues in space. The 
 only sort of reservation, perhaps, to be made to 
 this statement is, that the comets celebrated for 
 their size and brilliancy, are often found to be 
 revolving in elliptic orbits of great eccentricity, 
 which means that their periods may amount to 
 many centuries. 
 
 It may be well to say something now as to 
 what is the ordinary career of a comet, so far as 
 visibility to us, the inhabitants of the Earth, is 
 concerned. Though this might be illustrated by 
 reference to the history of many comets, perhaps 
 
COMETS. 163 
 
 there is no one more suitable for the purpose 
 than Donati's Comet of 1858. In former times, 
 when telescopes were few or non-existent, bril- 
 liant comets often appeared very suddenly, just as 
 a carriage or a man does, as you turn the corner 
 of a street. Such things even happen still : for 
 instance, the great comet of 1861 burst upon us 
 all at once at a day's notice. Usually, however, 
 now in consequence of the large size of the tele- 
 scopes in use, and the great number of observ- 
 ers who are incessantly on the watch, comets are 
 discovered when they are very small, because re- 
 mote both from the Earth and Sun, and many 
 weeks, or even months, it may be, before they 
 shine forth in their ultimate splendour. Now, let 
 us see how these statements are supported by the 
 history of Donati's comet in 1858. On June 2 in 
 that year, it was first seen by Donati at Florence, 
 as a faint nebulosity, slowly journeying north- 
 wards. June passed away, and July, and August, 
 the comet all the while remaining invisible to the 
 naked eye; that is to say, it first became percep- 
 tible to the naked eye on August 29, having put 
 forth a faint tail about August 20. After the 
 beginning of September its brilliancy rapidly 
 increased. On September 17, the head equalled 
 in brightness a 2nd magnitude star, the tail being 
 4 long. Passing its point of nearest approach to 
 the Sun on September 29, it came nearest to the 
 Earth on October 10 ; though, perhaps, its ap- 
 pearance a few days previously, namely on Octo- 
 ber 5, is the thing best remembered by those who 
 saw it, because it was on that night that the comet 
 passed over the ist magnitude star Arcturus. 
 For several days about this time, the comet was 
 an object of striking beauty in the Western 
 
1 64 THE STORY OF THE SOLAR SYSTEM. 
 
 Heavens, during the hours immediately after 
 sun-set. After October 10, it rapidly passed 
 away to the Southern hemisphere, diminishing 
 in brightness, as it did so, because receding from 
 the Earth and the Sun. It continued its career 
 through the winter ; became invisible to the 
 naked eye ; and finally invisible altogether in 
 March 1859. It remained in view, therefore, for 
 more than nine months, not to return again till 
 about the year 3158 A. D., for its period of revo- 
 lution was found to be about 2000 years. 
 
 I have been particular in sketching somewhat 
 fully the history of this comet so far as we are 
 concerned, because, as I have already said, it is 
 typical of the visible career of many comets. 
 Halley's comet in 1835 and 1836, went through 
 a somewhat similar series of changes. This 
 comet a well-known periodical one of great 
 historic interest and brilliancy may be com- 
 mended to the younger members of the rising 
 generation, because it is due to return again 
 to these parts of space a few years hence, or in 
 1910. 
 
 What is a comet made of ? Men of Science 
 equally with the general public would like to be 
 able to answer this question, but they cannot do 
 so with satisfactory certainty. A great many 
 years ago Sir John Herschel wrote thus : " It 
 seems impossible to avoid the following conclu- 
 sion, that the matter of the nucleus of a comet is 
 powerfully excited and dilated into a vaporous 
 state by the action of the Sun's rays escaping in 
 streams and jets at those points of its surface 
 which oppose the least resistance, and in all prob- 
 ability throwing that surface or the nucleus it- 
 self into irregular motions by its reaction in the 
 
COMETS. 
 
 165 
 
 a 
 
 o 
 U 
 
 I 
 
1 66 THE STORY OF THE SOLAR SYSTEM. 
 
 act of so escaping, and thus altering its direc- 
 tion." This passage was written of course before 
 the spectroscope had been brought to bear on the 
 observations of comets, but so far as Sir John 
 Herschel's remark implies the presence of va- 
 pour, that is gas, in a comet, the surmise has 
 been amply borne out by later discoveries. The 
 fact that as a comet approaches the Sun some 
 forces, no doubt of solar origin, come into opera- 
 tion to vaporise and therefore expand the matter 
 composing the comet is sufficiently shown by the 
 great developement which takes place as we have 
 seen in the tails of comets, but in regard to the 
 heads of comets we are face to face with a strange 
 enigma. Though the tails expand the heads con- 
 tract as the comet approaches its position of 
 greatest proximity to the Sun. Having passed 
 this point the head expands again. This curious 
 circumstance, first pointed out by Kepler in 1618, 
 has often been noticed since, and noticed indeed 
 not as the result of mere eye impressions, but 
 after careful micrometrical measurement with suit- 
 able instruments. I think the confession must be 
 made that we are hopelessly ignorant of the nature 
 of comet's except that gases are largely concerned 
 in their constitution. 
 
 It seems impossible to doubt that some tails 
 of comets are hollow cylinders or hollow cones. 
 Such a theory would account for the fact, so 
 often noticed, that single tails are usually much 
 brighter at their two edges than at the centre. 
 This is the natural effect of looking transversely 
 at any translucent cylinder of measureable thick- 
 ness. 
 
 It was long a moot point whether comets are 
 self-luminous, or whether they shine by reflected 
 
COMETS. 167 
 
 light; but it is now generally admitted that whilst 
 a part of the light of a comet may be derived by 
 reflection from the Sun yet as a rule they must 
 be regarded as shining by their own intrinsic light. 
 
 It should be stated here by way of caution 
 that the observations on this subject are not so 
 consistent as one could wish, and it seems neces- 
 sary to assume that all comets are not constituted 
 alike, and that therefore what is true of one does 
 not necessarily apply to another. 
 
 To those who possess telescopes (not neces- 
 sarily large ones) opportunities for the study of 
 comets have much multiplied during the last few 
 years, for we are now acquainted with a group of 
 small comets which are constantly coming into 
 view at short intervals of time. The comets have 
 now become so numerous that seldom a year 
 passes without one or more of them coming into 
 view. Whilst that known as Encke's revolves 
 round the Sun in 3^ years, Tuttle's doing the 
 same in 13^ years, there are four others whose 
 periods average about 5^ years, 5 which average 
 6J- years, together with one of y years and one 
 of 8 years. It is thus evident that there is a con- 
 stant succession of these objects available for 
 study, and that very few months can ever elapse 
 that some one or more of them are not on view. 
 They bear the names of the astronomers who either 
 discovered them originally, or who, by studying 
 their orbits, discovered their periodicity. The 
 names run as follows, beginning with the shortest 
 in period and ending with the longest : 
 
 Encke's. 
 
 Temple's Second (1873, 
 n.) 
 
 Winnecke's. 
 
 Brorsen's. 
 
 Temple's First (1867, n.) 
 
1 68 THE STORY OF THE SOLAR SYSTEM. 
 
 Swift's (1880, v.) 
 Barnard's (1884, 11.) 
 D'Arrest's. 
 Finlay's. 
 
 Wolf's (1884, m.) 
 Faye's. 
 Denning's. 
 Tuttle's. 
 
 I cannot stay to dwell upon either the history 
 or description of these comets separately, but 
 must content myself by saying generally t'hat 
 whilst as a rule they are not visible to the naked 
 eye, yet several of them may occasionally become 
 so visible when they return to perihelion- under 
 circumstances which bring them more near than 
 usual to the earth. 
 
 Several other comets are on record which it 
 W 7 as supposed at one time would certainly have 
 been entitled to a place in the above list, but 
 three of them in particular have, under very 
 mysterious circumstances, entirely disappeared 
 from the Heavens. 
 
 Chief amongst the mysterious comets must be 
 ranked that which goes by the name of Biela. 
 This comet, first seen in 1772, was 'afterwards 
 found to have a period of about 6f years, and on 
 numerous occasions it reappeared at intervals of 
 that length down to 1845, when the mysterious 
 part of its career seems to have commenced. In 
 December of that year this comet threw off a 
 fragment of nearly the same shape as itself, and 
 the two portions travelled together side by side 
 for four months, the distance between the frag- 
 ments slowly increasing. At the end of the four 
 months in question the comet passed out of sight 
 owing to the distance from the earth to which it 
 had attained. The comet returned again to peri- 
 helion in 1852, remaining visible for three weeks. 
 The two portions of the comet noticed in 1846 
 
COMETS. 169 
 
 retained their individuality in 1852, but the dis- 
 tance between them had increased to about eight 
 times the greatest distance noticed in 1846. As 
 a comet Biela's Comet has never been seen since 
 1852, and it must now be regarded as having per- 
 manently disappeared. But what seems to have 
 happened is this, that Biela's Comet has become 
 
 FIG. 26. Biela's Comet, February 19, 1846. 
 
 broken up into a mass of meteors. On November 
 27, 1872, and again in November 1885, when the 
 earth in travelling along its own orbit reached 
 a certain point where its orbit intersected the 
 former orbit of Biela's Comet the Earth encoun- 
 tered, instead of the comet which ought to have 
 been there, a wonderful mass of meteors ; and it 
 is now generally accepted that these meteors, 
 
1 70 THE STORY OF THE SOLAR SYSTEM. 
 
 which apparently are keeping more or less to- 
 gether as a fairly compact swarm, are nought else 
 than the disintegrated materials of what once was 
 Biela's Comet. 
 
 It is extremely probable that as time goes on 
 we shall be able to say that an intimate connec- 
 tion subsists between particular comets which 
 have been and particular meteoric swarms. We 
 already possess proof that other comets which 
 once came within our view were at that time re- 
 volving round the Sun in orbits so comparatively 
 small that they should have reappeared at inter- 
 vals of half-a-dozen or so years, yet they have 
 not reappeared. The question therefore suggests 
 itself, Have they been subject to some great in- 
 ternal disaster which has led to their disintegra- 
 tion ? It may be said without doubt that this is 
 in the highest degree probable ; but short of this, 
 that is short of total disintegration into small 
 fragments, we have several cases on record of 
 what I may, for the moment, call ordinary comets 
 breaking up into two or three fragments. For a 
 long while astronomers were naturally loath to 
 believe that this was possible, and therefore they 
 discredited the statements to that effect which 
 had been made. Though it would occupy too 
 much space to give the particulars of these 
 comets in full it may yet be worth while just to 
 mention the names of some of them, presumed to 
 be of short period, which seemed nevertheless to 
 have eluded our grasp. I would here specially 
 mention Liais's Comet of 1860 and the second 
 comet of 1881 as seemingly having undergone 
 some sort of disruption akin to what happened in 
 the case of Biela's Comet. 
 
 There is another group of periodical comets 
 
COMETS. 171 
 
 to be mentioned. These are six in number and 
 seem to have periods of 70 years or a little more. 
 Of these three have not yet given us the chance 
 of seeing them again ; two have paid us a second 
 visit, and therefore their periods are not open 
 to doubt; whilst the most famous of this group, 
 " Halley's," has been recorded to have shown 
 itself to the Earth no less than 25 times, begin- 
 ning with the year n B. c. It was Halley's 
 comet which shone over Europe in April 1066, 
 and was considered the forerunner of the con- 
 quest of England by William of Normandy. It 
 figures in the famous Bayeux tapestry as a hairy 
 star of strange shape. 
 
 It would seem that there exists in some in- 
 scrutable manner a connection between each of 
 the three great exterior planets and certain 
 groups of comets. In the case of Jupiter the 
 association is so very pronounced as long ago to 
 have attracted notice ; but the French astron- 
 omer, Flammarion, has brought forward some 
 suggestions that Saturn has one comet (and per- 
 haps two) with which it is associated ; Uranus, 
 two (and perhaps three) ; and Neptune, six ; 
 whilst farther off than Neptune the fact that 
 there are two comets, supposed periodical, with- 
 out a known planet to run with them has inspired 
 Flammarion to look with a friendly eye on the 
 idea (often mooted) that outside of Neptune 
 there exists another undiscovered planet revolv- 
 ing round the sun in a period of about 300 years. 
 
 The Jupiter group of comets deserves a few 
 additional words. There are certainly nine, and 
 perhaps twelve comets revolving round the Sun 
 in orbits of such dimensions that they either 
 reach up to or slightly overreach the orbit of 
 
172 THE STORY OF THE SOLAR SYSTEM. 
 
 Jupiter. The effect of this condition of things is 
 that on occasions Jupiter and each of the comets 
 may come into such proximity that the superior 
 mass of Jupiter may exercise a very seriously 
 disturbing influence over a flimsy and ethereal 
 body like a comet. There is reason to suppose 
 that some of the comets now belonging to the 
 Jupiter group have not done so for any great 
 length of time, but having been wandering about, 
 either in elliptic orbits of great extent, or even in 
 parabolic orbits, have accidentally come within 
 reach of Jupiter, and so have been, as it were, 
 captured by him. Hence the origin of the term, 
 the "capture theory," as applied to these family 
 groups of comets which I have just stated to 
 exist, each presided over, as it were, by a great 
 planet. It may be that at some future time this 
 theory will help us to a clue to the fact that be- 
 sides the comets of Lexell of 1770, Blainpain of 
 1819, and Di Vico of 1844, short period comets 
 unaccountably missing, there are several others 
 presumed to have been revolving in short period 
 orbits when discovered, and as to which it is very 
 strange that they should not have been seen before 
 their only recorded visit to us, and equally strange 
 that they should never have been seen since. 
 
 Is there any reason to fear the results of a 
 collision between a comet and the Earth ? None 
 whatever. However vague may be, and in a cer- 
 tain sense must be, our answer to the question, 
 "What is a comet?" certain is it that every 
 comet is a very imponderable body a sort of 
 airy nothing, a mass of gas or vapour.* At the 
 
 * It is not a little singular that the Chinese in bygone 
 centuries have often alluded to comets under the name of 
 
COMETS. 173 
 
 same time it always has been and perhaps still is 
 difficult to persuade the public that whatever 
 might be the effect on a comet if it were to strike 
 the Earth, the effect on the Earth, were it to be 
 struck by a comet, would be nil. This is not 
 altogether a matter of speculation, for according 
 to a calculation by Hind, on June 30, 1861, the 
 Earth passed into and through the tail of the 
 great comet of that year at about two-thirds of 
 its distance from the nucleus. Assuredly there 
 was no dynamical result ; but it seems, however, 
 not unlikely that there was an optical result ; at 
 any rate, traces of something of this sort were 
 noted. Hind himself, in Middlesex, observed a 
 peculiar phosphorescence or illumination of the 
 sky which he attributed at the time to an au- 
 roral glare. Lowe, in Nottinghamshire confirmed 
 Hind's statement of the appearance of the heav- 
 ens on the same day. The sky had a yellow 
 auroral glare-like look, and the Sun, though shin- 
 ing, gave but feeble light. The comet was plainly 
 visible at 7.45 p. m. (during sunshine), and had a 
 much more hazy appearance than on any subse- 
 quent evening. Lowe adds that his Vicar had 
 the pulpit candles lighted in the Parish Church 
 at 7 o'clock (it was a Sunday), though only five 
 days had elapsed since Midsummer day, which 
 itself proves that some sensation of darkness was 
 felt even while the Sun was shining. 
 
 So far as I remember there has been no such 
 thing as a comet panic during the present genera- 
 tion, at any rate in civilised countries, but it is 
 on record that there was a verv considerable 
 
 vapours; e.g., the comet of 1618 is recorded as having been 
 " a white vapour 20 cubits long." 
 
174 THE STORY OF THE SOLAR SYSTEM. 
 
 panic in 1832 in connection with the return of 
 Biela's Comet in the winter of that year. Olbers 
 as the result of a careful study in advance of the 
 comet's movements found that the comet's centre 
 would pass only 20,000 miles within the Earth's 
 orbit, and that as the nebulosity of the comet 
 had in 1805 been more than 20,000 miles in diam- 
 eter, it was certain, unless its dimensions had 
 diminished in the 27 years, that some of the 
 comet's matter would overlap the Earth's orbit ; 
 in other words would envelop the Earth itself, if 
 the Earth happened to be there. This conclusion 
 when it became public was quite enough to create 
 a panic and make people talk about the forth- 
 coming destruction of our globe. It was nothing 
 to the point (in the public mind) that astronomers 
 were able to predict that the Earth would not 
 reach the place where the comet would cross the 
 Earth's orbit until four weeks after the comet 
 had come and gone. However, we now know 
 that nothing happened, and I am justified in 
 adding that even if there had been contact, Earth 
 meeting comet face to face, nothing (serious) 
 would have occurred so far as the Earth was 
 concerned. 
 
 This seems a convenient place for referring to 
 a matter which when it was first broached excited 
 a great deal of interest, but about which one does 
 not hear much now-a-days. The period of the 
 small comet known as Encke's (which, revolving 
 as it does round the Sun in a little more than 
 three years, has the shortest period of any of the 
 periodical comets) was found many years ago to 
 be diminishing at each successive return. That 
 is to say, it always attained its nearest distance 
 from the Sun at each apparition 2^ hours sooner 
 
COMETS. 175 
 
 than it ought to have done. In order to account 
 for this gradual diminution in the comet's period 
 Encke conjectured the existence of a thin ethereal 
 medium sufficiently dense to affect a light flimsy 
 body like a comet, but incapable of obstructing a 
 planet. It has been remarked by Hind that " this 
 contraction of the orbit must be continually pro- 
 gressing, if we suppose the existence of such a 
 medium; and we are naturally led to inquire, 
 What will be the final consequence of this resist- 
 ance ? Though the catastrophe may be averted 
 for many ages by the powerful attraction of the 
 larger planets, especially Jupiter, will not the 
 comet be at last precipitated on the Sun ? The 
 question is full of interest, though altogether 
 open to conjecture." 
 
 Astronomers are not altogether agreed as to 
 the propriety of this explanation. One argument 
 against it is that w\\h perhaps one exception none 
 of the other short-period comets (all of them 
 small and presumably deficient in density) seem 
 affected as Encke's is. On the other hand Sir 
 John Herschel favoured the explanation just 
 given, as also does Hind who is the highest living 
 authority on comets. A German mathematician, 
 Von Asten, who devoted immense labour to the 
 study of the orbit of Encke's Comet, thought 
 there should be no hesitation in accepting the 
 idea of a resisting medium, subject to the limita- 
 tion that it does not extend beyond the orbit 
 of Mercury. Von Asten's allusion to Mercury 
 touches a subject which belongs more directly to 
 the question of Mercury's orbit and to that other 
 very interesting question, " Are there any planets, 
 not at present known, revolving round the Sun 
 within the orbit of Mercury." 
 12 
 
176 THE STORY OF THE SOLAR SYSTEM. 
 
 Which is the largest and most magnificent 
 comet recorded in history ? It is virtually impos- 
 sible to answer this question, because of the ex- 
 travagant and inflated language made use of by 
 ancient and medieval (I had almost added, and 
 modern) writers. There is no doubt that the 
 comet of 1680, studied by Sir I. Newton, the tail 
 of which was curved, and from 70 to 90 long, 
 must have been one of the finest on record, as it 
 was also the one which came nearest to the Sun, 
 for it almost grazed the Sun's surface. 
 
 The comet of 1744, visible as it was in broad 
 daylight, was, no doubt, the finest comet of the 
 i8th century, though in size it has been surpassed ; 
 yet its six tails must have made it a most remark- 
 able object. So far as the ipth century is con- 
 cerned, our choice lies between the comets of 
 1811, 1843, 1858, and 1861. The comet of 1811 
 is spoken of by Hind as " perhaps the most famous 
 of modern times. Independently of its great 
 magnitude, the position of the orbit and epoch of 
 perihelion passage, were such as to render it a 
 very splendid circumpolar object for some 
 months." The tail as regards its length was not 
 so very remarkable, for at its best, in October 
 1811, it was only about 25 long, its breadth, how- 
 ever, was very considerable ; at one time 6, the 
 real length of the tail, about the middle of Octo- 
 ber, was more than 100,000,000 of miles, and its 
 breadth about 15,000,000 of miles. The visibility 
 of this comet was coincident with those events 
 which proved to be the turning-point in the career 
 of Napoleon I., and there were not wanting those 
 who regarded the comet as a presage of his disas- 
 trous failure in Russia. Owing to the long period 
 (17 months), during which this comet was visible, 
 
COMETS, 
 
 177 
 
 F IG . 27. The Great Comet of 1811, 
 
178 THE STORY OF THE SOLAR SYSTEM. 
 
 it was possible to determine its orbit with unusual 
 precision. Argelander found its period to be 3065 
 years, with no greater uncertainty than 43 years. 
 The great dimensions of its orbit will be realised 
 when it is stated that this comet recedes from the 
 Sun to a distance of 14 times that of the planet 
 Neptune. 
 
 Donati's comet of 1858, has already received 
 a good deal of notice at my hands, but the ques- 
 tion remains, what are its claims, to be regarded 
 as the comet of the century, compared with that 
 of 1843? It is not a little strange that though 
 there must have been many persons who saw both, 
 yet it was only quite recently that I came across, 
 for the first time, a description of both these 
 comets from the same pen. It ought, however, 
 to be mentioned by way of explanation, that the 
 inhabitants of Europe only saw the comet of 1843, 
 when its brilliancy and the extent of its tail had 
 materially diminished, about a fortnight after it 
 was at its best. 
 
 The description of these two comets to which 
 I have alluded, will be found in General J. A. 
 Ewart's " Story of a Soldier s Life" published in 
 1881. Writing first of all of the comet of 1843, 
 General Ewart says : 
 
 " It was during our passage from the Cape of 
 Good Hope to the Equator, and when not far from 
 St. Helena, that we first came in sight of the great 
 comet of 1843. 1 tne fi rst instance a small portion 
 of the tail only was visible, at right angles to the 
 horizon ; but night after night as we sailed along, 
 it gradually became larger and larger, till at last 
 up came the head, or nucleus, as I ought properly 
 to call it. It was a grand and wonderful sight, 
 for the comet now extended the extraordinary 
 
COMETS. 179 
 
 FIG. 28. The Great Comet of 1882, on October 19 (Artus). 
 
l8o THE STORY OF THE SOLAR SYSTEM. 
 
 distance of one-third of the heavens, the nucleus 
 being, perhaps, about the size of the planet 
 Venus." (Vol. i., p. 75.) 
 
 As regards Donati's comet of 1858, what the 
 General says is : 
 
 " A very large comet made its appearance 
 about this time, and continued for several weeks 
 to be a magnificent object at night; it was, how- 
 ever, nothing to the one I had seen in the year 1843, 
 when on the other side of the equator." (Vol. ii., 
 P- 205.) 
 
 Passing over the great comet of 1861, on which 
 I have already said a good deal, I must quit the 
 subject of famous comets by a few words about 
 that of 1882, which, though by no means one of 
 the largest, was, in some respects, one of the most 
 remarkable of modern times. It was visible for 
 the long period of nine months, and was con- 
 spicuously prominent to the naked eye during 
 September, but these facts, though note- worthy, 
 would not have called for particular remark, had 
 not the comet exhibited some special peculiarities 
 which distinguished it from all others. In the 
 first place, it seems to have undergone certain 
 disruptive changes, in virtue of which the nucleus 
 became split up into four independent nuclei. 
 Then the tail may have been tubular, its cxtrem- 
 tiy being not only bifid, but totally unsymmetrical 
 with respect to the main part. The tubular 
 character of the tail was suggested by Tempel. 
 To other observers, this feature gave the idea of 
 the comet, properly so-called, being enclosed in a 
 cylindrical envelope, which completely surrounded 
 the comet, and overlapped it for a considerable 
 distance at both ends. Finally (and in this re- 
 sembling Biela's comet) the comet of 1882 seems 
 
COMETS. l8l 
 
 to have thrown off a fragment which became an 
 independent body. 
 
 YVhat has gone before, will, I think, have 
 served abundantly to establish the position with 
 which I started, namely, that comets occupy (and 
 deservedly so) the front rank amongst those 
 astronomical objects in" which, on occasions, the 
 general public takes an interest. 
 
 I have thus completed, so far as the space at 
 my disposal has permitted, a popular descriptive 
 Survey of the Solar System. Those who have 
 perused the preceding pages, however slight may 
 have been their previous acquaintance with the 
 Science of Astronomy taken as a whole, will have 
 no difficulty in realising that what I have said 
 bears but a small proportion to what I have left 
 unsaid. They will equally, I hope, be able to see, 
 without indeed the necessity of a suggestion, that 
 all those wondrous orbs which we call the planets 
 could neither have come into existence nor have 
 been maintained in their allotted places for so 
 many thousands of years, except as the result of 
 Design emanating from an All-powerful Creator. 
 
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 184 
 
GENERAL INDEX. 
 
 A. 
 
 Adams, J. C., 145. 
 
 Airy, Sir G. B., 145. 
 
 Anagram on Venus, 68 ; on Saturn, 
 
 127. 
 
 Aphelion, u, 102. 
 Apparent movements of the Planets, 
 
 12, 14. 
 
 Arago, D. J. F., 99, 104, 140. 
 Arctic regions of the Earth, 75. 
 Argelander, F. G. A., 178. 
 Aristarchus of Samos, 71. 
 Aristarchus (Lunar Mountain), 93, 
 
 94- 
 
 Asteroids, no. 
 Aurora Borealis and spots on the 
 
 Sun, 55 ; and twinkling, 87. 
 Autumnal Equinox, 74. 
 Axial rotation of the Planets, 17, 183. 
 
 B. 
 
 Barnard. E. E., 120, 134. 
 
 Barnard's Comet, 168. 
 
 Bayeux Tapestry, 171. 
 
 Beer and Madler ; their map of the 
 Moon, 96. 
 
 Belts on Jupiter, 115, 123 ; on Sat- 
 urn, 122. 
 
 Bergeron's Experiment, 93. 
 
 Bessel, W., 124. 
 
 Biela's Comet, 168. 
 
 Bode's so-called " Law," no. 
 
 Bond, G. P., 132, 133. 
 
 Bouvard, A., ii3. 
 
 Bradley, Rev. J., 144. 
 
 Bredechin's theory of Tails of Com- 
 ets, 157. 
 
 Brorsen's Comet, 167. 
 
 C. 
 
 44 Canals " on Mars, 106. 
 Carpenter, J., 134. 
 
 Carrington, R. C., 46, 49. 
 Cassini, J. D., 36, n8, 128, 130, 136. 
 Ceres (Minor Planet), 112. 
 Challis, Rev. J., 146. 
 Charts, Celestial, 112, 147. 
 Chinese observations, 25, 172. 
 Clouds influenced by the Moon, 98. 
 Coggia's Comet, 1874, 151. 
 Comets, 150; periodic, 162, 170; 
 
 remarkable, 176. 
 
 Comparative sizes of the Planets, n. 
 Comparative size of the Sun from 
 
 the Planets, 17. 
 Compression of the Planets, 70, 122, 
 
 137. 
 Conjunction of the Planets, 14, 15, 
 
 66. e 
 
 Copernican System, 72. 
 Copernicus, 72, 73, 76. 
 
 D. 
 
 D' Arrest, 147. 
 
 D' Arrest s Comet, 168. 
 
 Dawes, W. R., 106, 133. 
 
 Day and night, 12. 
 
 Day, length of, 74. 
 
 De La Rue, W., 35, 36, 49, 52. 
 
 Denning's Comet, 168. 
 
 Density of the Planets, 17, 183. 
 
 Diameter of the Sun and Planets, 
 
 183. 
 
 Di Vico. 62, 63. 
 Di Vico s Comet, 172. 
 Distance of the Planets from the 
 
 Sun, 20, 183. 
 
 Donati's Comet of 1858, 157, 163, 178. 
 Dufour on Twinkling, 84. 
 
 E. 
 
 Earth, annual motion of, 75 ; figure 
 
 of, 70. 
 Earthshine, 66, 93. 
 
 185 
 
1 86 THE STORY OF THE SOLAR SYSTEM. 
 
 Eccentricity of Planetary orbits, 10, 
 
 ^60, 75. 
 Eclipses of the Sun, 56, 57 ; of the 
 
 Moon, 120. 
 
 Ecliptic, plane of, g, 10, 74. 
 Egyptian System, 72. 
 Elliptic Orbits, 10. 
 Elongation of Planets, 14. 
 Encke, J. F., 147. 
 Encke's Comet, 167, 174, 175. 
 Equinoxes, 74. 
 Ewart, Gen. J. A., 178. 
 
 F. 
 
 Faculae on the Sun, 42, 43. 
 Faye's Comet, 168. 
 Finlay's Comet, 168. 
 Flammarion, C., 62, 66, 171. 
 Flamsteed, Rev. J., 140, 144. 
 
 G. 
 
 Galileo, 89, 101, 120, 125. 
 Galle, J. G., 147. 
 Gassendi, 76. 
 Geodesy, 70. 
 Georgium Sidus, 141 
 Goldschmidt, H., 4. 
 Granules, Solar, 38. 
 Gravity on the Sun and Planets, 
 183. 
 
 H. 
 
 Hall, A., 109. 
 
 Halley's Comet, 164, 171. 
 
 Heat rays of the Sun, 20 ; of the 
 
 Moon, 96. 
 Herschel, Sir W., 34, 40, 44, 58, 109, 
 
 103, 134, 137, 138, 142. 
 Herschel, Sir J. F. W., 24, 96, 99, 
 
 103, 132, 175. 
 
 Hind, J. R., 119, 173, 175. 
 Holden, E. S., 106. 
 Holland, Sir H., 78. 
 Holmes's Comet, 1892, 159. 
 Homer's Iliad, cited, 68. 
 Hewlett, Rev. F., 47. 
 Huggins, W., 40. 
 Huygens, C., 127, 130. 
 Hyperbola, 162. 
 Hyperbolic Comets, 162. 
 
 I. 
 
 Inclination of planetary orbits, 9. 
 Inferior Planets, 9, 12. 
 
 J. 
 
 anssen J., 47. 
 
 unp (Minor Planet), 112. 
 
 upiter : its influence on Sun-spots 
 
 doubtful, 51, 53 ; on Comets, 
 
 171 ; its light, 7. 
 
 K. 
 
 Kepler 55, 125, 166. 
 Kew Observatory, 47. 
 
 La Hire, 63. 
 Lalande 34. 
 
 Lassell, W., 123, 142, 185. 
 Ledger, Rev. E., 84. 
 Le Monnier, 140, 144. 
 LeVerner, U.J. J., 147. 
 Lexell's Comet, 172. 
 Liais's Comet, 170. 
 Logogriphs as to Venus, 68 ; as to 
 Saturn, 127. 
 
 M. 
 
 Madler, J. H., 63, 96. 
 
 Magnetism, Terrestrial, 55. 
 
 Maps, Astronomical, 112, 147. 
 
 Mars, loo. 
 
 Maskelyne, Rev. N., 139. 
 
 Mass of the Sun, and of the Planets, 
 20, 183. 
 
 Medium, Resisting, 175. 
 
 Mercury, 57 ; phases of, 15 ; its in- 
 fluence on Sun-spots, 51 ; its 
 luminosity, 67. 
 
 Milton, J., Paradise Lost, cited, 
 
 Minor Planets, no. 
 Montigny on Twinkling, 84. 
 Moon, 80. 
 
 Moonlight, Brightness of, 96. 
 Motions of the Planets, 10. 
 Mountains of the Moon, 90. 
 
 N. 
 
 Napoleon I., 154, 176. 
 Nasmyth, J., 40. 
 Needle, Magnetic, 55. 
 Neptune, 143 ; its influence on Ura- 
 nus, 143. 
 Newton, Sir J., 176. 
 
GENERAL INDEX. 
 
 I8 7 
 
 Nubian after-glow, 83. 
 Nucleus of a Sun-spot, 22. 
 
 O. 
 
 Obliquity of the ecliptic, 74. 
 Occultations of Jupiter's Satellites, 
 
 Olbers/W., 112, 174. 
 Opposition of Planets, 13. 
 Orbits of the Planets, 9, 10 ; of 
 Comets, 161. 
 
 P. 
 
 Pallas (Minor Planet), 112, 113, 114. 
 
 Parabola, 162. 
 
 Penumbra of a Sun-spot, 22. 
 
 Perihelion, IT, 102. 
 
 Periodic Comets, 162, 170. 
 
 Perturbations of Uranus by Nep- 
 tune, 145. 
 
 Phases of an inferior Planet, 15 ; of 
 a superior Planet, 16 ; of Mars, 
 loo- of Jupiter, 115. 
 
 Piazzi, G., in. 
 
 Planets, classification of, 7 ; com- 
 parative sizes of, ii ; move- 
 ments of, 10. 
 
 Plutarch, 71. 
 
 Poles of Mars, snow at, 104. 
 
 Primary Planets, 7. 
 
 Ptolemaic System, 72. 
 
 Q. 
 
 8uadrature of a Planet, 16. 
 uenisset's Comet, 1893, 156. 
 
 R. 
 
 Rays of the Sun, 20. 
 Refraction, 77 ; effect of, 77. 
 Red soot on Jupiter, 118. 
 Resisting Medium, 175. 
 Retrograde motion of a Planet, 15. 
 " Rice grains " on the Sun, 39, 40. 
 Rings of Saturn, 128. 
 Rosse, Earl of, 93. 
 Rotation of the Sun, 24, 28 ; of the 
 
 planets, 17, 183. 
 Rotundity of the Earth, 76. 
 
 S. 
 
 Satellites of Mars, 16, 184 ; of Jupi- 
 ter, 16, 184 ; of Saturn, 16, 185 ; 
 
 of Uranus, 16, 185 ; of Neptune, 
 16, 185. 
 
 Saturn, 122. 
 
 Sawerthai's Comet, 165. 
 
 Schiaparelli, J. V., 59, 62, 63, 106. 
 
 Schmidt, J. F. F., 96. 
 
 Schroter, J. J., 58, 62, 63. 
 
 Schwabe's observations of Sun- 
 spots, 46. 
 
 Seas, Lunar, 94. 
 
 Seasons on the Earth, 54, 74. 
 
 Secchi, A., 28, 35, 42, 50, 84, 86. 
 106. 
 
 Secondary Planets, 8. 
 
 Shakespeare, citations from, 152. 
 
 Smyth, C. P., 92. 
 
 Snow on Venus, 66 ; on Mars, 104 ; 
 doubtful on Saturn, 123. 
 
 Solstices, 74. 
 
 Spherical form of the Earth, 70. 
 
 Sporer, 46, 50. 
 
 Spots on the Sun, 21, n8 ; on Ve- 
 nus, 61 ; on Jupiter, 117 ; on 
 Saturn, 123. 
 
 Stewart, B., 52, 56. 
 
 Struve, O., 134, 142. 
 
 Sun, 18, 183. 
 
 Superior Planets, 8 ; movements of, 
 J 5- 
 
 Surfaces of the Sun and Planets, 
 .183. 
 
 Swift's Comet, 168. 
 
 Systems of the Universe, 72. 
 
 T. 
 
 Tables of the Major Planets, 183. 
 
 Tails of Comets, 154. 
 
 Temple's Periodical Comets, 167. 
 
 Terminator of the Moon, 90. 
 
 Thomson's Seasons cited, 153. 
 
 Tides, 98. 
 
 Total Eclipses of the Sun, 57. 
 
 Transits of inferior Planets, 13, 25, 
 
 65- 
 
 Trouvelot, 133. 
 
 Tuttle's Periodical Comet, 167, 168. 
 Twilight, 80. 
 Twinkling, 83. 
 
 Tycho (Lunar Mountains), 94. 
 Tycho Brahe, 73. 
 Tychonic System, 73. 
 
 U. 
 
 Umbra of a Sun-spot, 22. 
 Uranus, 138 ; the influence of Nep- 
 tune on, 143. 
 
THE STORY OF THE SOLAR SYSTEM. 
 
 V. 
 
 Velocity of the planets, 183. 
 
 Venus, 61 ; phases of, 15 ; its influ- 
 ence on Sun-spots, 51, 52, 53. 
 
 Vernal Equinox, 74. 
 
 Vesta (minor planet), 112, 113. 
 
 Virgil, citation from, 50. 
 
 Volume of the Sun, 20 ; of the 
 Planets, 183. 
 
 W. 
 
 Weather influences ascribed to the 
 Sun, 44 ; to the Moon, 98. 
 
 " Willow leaves " on the Sun, 39, 40. 
 Wilson's theory of Sun-spots, 32. 
 Winnecke's Periodical Comet, 167. 
 Wolf, R., observer of Sun-spots, 45, 
 
 48. 
 Wolf's Periodical Comet, 168. 
 
 Y. 
 
 Young, C. A., 131. 
 
 Z. 
 
 Zodiac, movement of Planets in, 15. 
 
 (7) 
 
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