REESE LIBRARY . UNIVERSITY OF CALIFORNIA. Deceived 7^f. Accession No. {) / Cla&s No. SPECTRA OF DIFFERENT TYPES. (i) Continuous spectrum. (2) Solar spectrum. (3) Of Sirius. (4) Of Aldebaran. (5) Of Alpha in Hercules. (6) Of a red star. (7) Of a temporary star. (8) Of Comet of 1874. (9) f the nebula of Orion. (10) Of sodium. (11) Of hydrogen. (la) Of nitrogen. MR. HUMPHREY B. CHAMBERLIN, TO WHOSE MUNIFICENCE THE AUTHOR IS DEEPLY INDEBTED, IS GRATEFULLY DEDICATED. INTRODUCTION. THIS book is intended for the use of students who have a fair knowledge of elementary algebra and plane geometry, and is the outcome of several years of experience with classes of this sort. The author hopes that the volume will also be acceptable to more advanced scholars. The teacher may here be reminded of the fact that among the most urgent needs in the study of astronomy is the exercise of the geometric imagination ; that is, the faculty which forms mental pictures of the relative positions of planes and circles, and of the motions, real and apparent, of the celestial bodies. The initial step in astronomical instruction is to teach pupils the use of their eyes, to insist that they observe the heavens and watch the celestial motions. Though they may be bewildered by such work at first, they will soon learn to delight in it, and will derive much profit from it. The earliest work in the line of observation is the study of the constellations, Acquaintance with the principal stars of the chief constellations visible in his latitude will prove a source of lifelong enjoyment to the pupil. Each student should keep a small blank book in which to make sketches, and to record the results of obser- vations. The Star Maps, at the end of the volume, are on a generous scale, and include all stars not fainter than the fifth magnitude, from the north pole to 40 of south declination. By the use of these maps and star groups drawn on the blackboard, the teacher may greatly aid his pupils, who should copy the pictures and then find the corre- sponding objects in the sky. The use of a telescope adds much of interest to this study, espe- cially if the students are taught to manipulate it themselves, and to iv INTRODUCTION. V find by its aid the telescopic objects mentioned in Chapter XIV. Even a good opera-glass is very serviceable. The observation exer- cises given at times will be found very helpful, and will assist in the cultivation of the geometric imagination. A globe, with blackboard surface, may be useful in various ways, but as soon as the pupils have derived a geometric conception by its aid they should be led at once to transfer the mental picture to the heavens. It is also recommended that constant recourse be had to such periodicals as " Popular Astronomy," " Knowledge," etc., in order to follow the progress of astronomical research, month by month, and thus to supplement the text-book. The list of works, given in Appendix VII., is intended as a guide in the selection of an astro- nomical library. The optical principles of the telescope and spectroscope have been explained very simply, for students not familiar with descrip- tions of them in elementary text-books on physics. Especial attention has been paid to the Meridian Circle and to the Equatorial, because accurate knowledge of the positions and motions of the heavenly bodies depends chiefly on observations made with these instruments. The purely descriptive matter about the sun, moon, planets, etc., has been kept quite free from such statistics as the values of the masses of the planets, and the intensity of the pull of gravitation at the surface of each. The student should, however, learn the distance, diameter, time of revolution, and time of rotation of each planet. More extended data for purposes of reference are to be found in the Appendices. In this edition the results of the latest important investigations and discoveries have been stated. The work of the Lick Observa- tory, as set forth in the publications of the Astronomical Society of the Pacific, merits and has received much attention. The columns of astronomical periodicals have furnished a large amount of reliable information. 1 The author will welcome for a second edition any suggestions or corrections. The Exercises, which are a special feature of this book, and are placed at the end of each chapter, will be of great help to the pupils 1 A good Star Atlas is a desideratum. Almost every school-book publishing house can furnish one. It will supplement the Star Maps at the end of this volume. VI INTRODUCTION. in reviewing the lessons, and also to the teacher in the work of the class-room. The Appendices contain, along with other useful material, questions for examination, topics for essays, and short reviews of a number of valuable works on Astronomy suitable for reference and general reading. A large number of the illustrations have never before appeared in any text-book. For many of the finer ones the author is in- debted to Prof. E. S. Holden, Prof. Wm. W. Payne, Mr. A. Cowper Ranyard, and Dr. E. E. Barnard. Prof. E. C. Pickering kindly furnished a fine set of lithographs, made from observations at Har- vard College Observatory, many of which have been reproduced. Prof. G. E. Hale has contributed photographs of the solar disturb- ance of July 15, 1892. Messrs. G. W. Saegmuller, of Washington, D. C., and J. A. Brashear, of Allegheny, Pa., supplied the pictures of some of the instruments. The author desires to state that he is specially indebted to his wife, Fannie Shattuck Howe, for her assist- ance in the preparation of the manuscript. UNIVERSITY PARK, COLORADO, 1896. CONTENTS. CHAPTER I. GENERAL SURVEY OF THE HEAVENS. PAGE Celestial Objects classified ; the Star Maps explained ; Names of the Constellations ; how to find the Constellations ; Hints on Constella- tion Study i CHAPTER II. APPARENT DAILY MOTION OF THE STARS. The Daily Motion ; the Celestial Sphere ; the Celestial Equator and Horizon; Exercises 6 CHAPTER III. THE TELESCOPE. Reflection and Refraction of Light; Lenses; Formation of an Image; Object-glass and Eyepieces ; Dispersion of Light ; Achromatism ; Refractors and Reflectors ; Equatorial Mountings ; Exercises .... 19 CHAPTER IV. THE SUN. Its Distance and Diameter ; how to Observe it with a Small Telescope ; Photosphere; Faculae; Spots; Solar Disturbances; Magnetic Storms; the Spectroscope ; Laws of Spectrum Analysis ; the Chromosphere ; Prominences ; the Corona ; Light and Heat; Constitution; Exercises . 36 vii Vlll CONTENTS. CHAPTER V. THE EARTH. PAGE Dimensions; Latitude and Longitude; its Orbit; the Ecliptic; the Equi- nox and Solstices ; the Zodiac ; the Day ; the Seasons ; Precession and the Calendar; Aberration; Atmospheric Refraction; Twilight; Exercises 64 CHAPTER VI. CELESTIAL MEASUREMENTS. Circles of Reference ; Parallax; Time; Solar and Sidereal Days ; Civil and Astronomical Days; Mean Solar and Sidereal Time; Standard Time, and its Determination by Means of a Meridian Circle and a Chrono- graph ; Determination of Latitude and Longitude ; Exercises .... 87 CHAPTER VII. THE MOON AND ECLIPSES. Distance; Diameter; Orbit; Period; Rotation; Librations ; Phases; Plains; Craters ; Mountains ; Water and Air ; Light and Heat ; Effect on the Weather; Eclipses, Solar and Lunar ; Exercises 108 CHAPTER VIII. MOTIONS OF THE PLANETS. Their Orbits; Newton's and Kepler's Laws ; Aspects; Periods; Exercises. 133 CHAPTER IX. MERCURY, VENUS, MARS, THE ASTEROIDS. Their Distance; Diameter; Revolution; Rotation; Phases; Satellites; Atmosphere ; Telescopic Appearance ; Physical Condition ; Exercises. 143 CHAPTER X. JUPITER, SATURN, URANUS, NEPTUNE. Their Distance ; Diameter ; Revolution ; Rotation ; Discovery ; Satellites ; Atmosphere ; Telescopic Appearance ; Physical Condition ; Exercises. 164 CONTENTS. IX CHAPTER XI. COMETS AND METEORS. PAGE Comets, their Discovery ; Designation ; Parts ; Orbits ; Appearances; Tails; Mass ; Light ; Spectra; Fate. Meteors, their Classes; Paths; Light and Heat; Constituents; Showers; Orbits, Exercises 186 CHAPTER XII. THE FIXED STARS. Number; Milky Way; Constellations; Names; Magnitudes; Dimen- sions; Distances; Clusters; Parallax; Spectra; Motions; Double and Multiple ; Variable ; Exercises 225 CHAPTER XIII. THE NEBUUE. Various Forms ; Spectra ; Notable Ones ; the Nebular Hypothesis ; the Future of the Visible Universe 257 CHAPTER XIV. THE CONSTELLATIONS IN DETAIL. The Greek Alphabet ; Detailed Descriptions of the Constellations visible in the United States, with Tabular Lists of Prominent Double Stars, Clusters, Nebulae, Colored and Variable Stars 272 APPENDICES. I. Names of Stars . . , . 303 II. Astronomical Constants 304 III. Planetary Data t 305 IV. Landmarks in the History of Astronomy . . . 307 V. Topics for Essays 313 VI. Queries for Use in Reviews and Examinations 315 VII. List of Reference Books 320 INDEX 327 STAR MAPS ..,.,'. 342 LIST OF ILLUSTRATIONS. PAGE FRONTISPIECE. Spectra of Different Types Fig. i. Solar Prominences . . opposite i " 2. Revolution of the Sphere . . 7 " 3. The Great Dipper and Polaris . 8 Figs. 4-6. Diagrams illustrating Definitions 9 " 7, 8. Apparent Daily Motion of the Stars 16 Fig. 9. Apparent Daily Motion of the Stars 17 " 10. Reflection by a Plane Mirror . . 19 " ii. Reflection by a Concave Mirror . 20 " 12. Refraction 20 " 13. Refraction by Prisms .... 20 " 14. Lenses 21 " 15. Rays made Divergent .... 21 " 16. Visual Angle 21 " 17. Object and Image 22 18. Lens as Eyepiece 23 19. Object-Glass and Eyepiece . . 23 20. Dispersion 25 21. Dispersion Corrected .... 25 22. Achromatic Object-Glass ... 26 23. Portrait of Alvan Clark ... 26 24. Eyepieces 27 25. Path of Rays of Light in a Re- flector 27 26. A Newtonian Reflector made by Brashear 28 27. Lord Rosse's Six-foot Reflector . 29 28. Scheme of an Equatorial Mount- ing 30 29. A German Equatorial . . . . 31 30. The Lick Telescope .... 32 31. The Sun's Image on a Screen . 37 32. Absorption of Light by the Sun's Atmosphere 37 33. Faculae observed Visually ... 38 34. Sun Spot 40 35. Large Sun Spot 41 36. The Sun at the Time of a Spot Maximum 42 37. Photographs of the Disturbance of July 15, 1892 43 PAGE Fig. 38. Cyclonic Motion 44 " 39. A Spectroscope made by Bra- shear 47 " 40. Plan of a Spectroscope .... 47 " 41. Slit of a Spectroscope .... 48 " 42. Production of Spectra .... 49 " 43. Spectra 49 " 44. Protrait of Kirchhoff . . . . 50 " 45. A Geissler's Tube 50 " 46. A Portion of the Solar Spectrum 51 " 47. Correspondence of Bright and Dark Lines in Two Spectra . 51 " 48. Solar Prominences . . opposite 53 " 49. A Spectroscope attached to a Telescope 54 " 50. The Corona on July 29, 1878 . . 55 " 51. The Corona, photographed on December 21, 1889 .... " 52. Illustrations of Schaeberle's The- ory of the Corona .... " 53 194- ii 17,8. Proper Motions of the Pleiades 240 " 195- PAGE Double Stars 242 A Spectroscopic Binary . . . 244 Multiple Stars ...... 245 Portrait of Tycho Brahe . . 247 Tycho's Star in Cassiopeia . 248 How to find Algol .... 249 Y Cygni 250 Real Velocity of a Star . . . 254 The Pleiades: Photographed by Roberts 258 The Trifid Nebula .... 260 The Nebula in Andromeda : Photographed by Roberts . 26 1 The Nebula in Orion : Drawn by Bond 262 The Ring Nebula in Lyra : Drawn by Bond 264 The Spiral Nebula in Canes Venatici : Photographed by Roberts 265 Portrait of La Place .... 266 The Star Finder 295 The Declination Circle . . . 295 THE SUN. SOLAR PROMINENCES. nr c / ' OF THE 1 o (UNIVERSITY ) DESCRIPTIVE ASTRONOMY. CHAPTER I. GENERAL SURVEY OF THE HEAVENS. " The sky Spreads like an ocean hung on high, Bespangled with those isles of light So wildly, spiritually bright. Who ever gazed upon them shining, And turned to earth without repining, Nor wished for wings to flee away, And mix with their eternal ray ? " BYRON. 1. The Fixed Stars. -The fixed stars are points of light, of vari- ous degrees of brightness, which bestrew the sky. They are called fixed, because, as seen with the naked eye, they do not change their relative positions from year to year. In the earliest ages men divided them into various groups, which we call constellations ; the appearance of each of these constella- tions is almost the same to-day as when it was named by the ancients. A star just visible to an average eye is said to be of the sixth magnitude ; one a little brighter is said to be of the fifth magnitude, and so on, a few of the brightest being called first magnitude stars. All fixed stars are at inconceivably great distances from us. 2. The Planets. The word planet is derived from a Greek word meaning "a wanderer." The designation is applied to certain star- like objects which appear to move among the fixed stars. The 2 DESCRIPTIVE ASTRONOMY. brightest ones have received the names of ancient divinities, as Jupiter, Saturn, and Venus. They revolve about the sun in paths nearly circular ; the earth is considered one of them. 3. The Moon. This familiar object revolves about the earth in 27 J days. It is the nearest of the celestial bodies, being a little less than a quarter of a million of miles away. It belongs to the class of objects known as satellites, which revolve about the planets, held fast by their attractive force. 4. The Sun. The sun, like the moon, is so familiar that no particular description of it is needed here. Though its distance from the earth is nearly 93,000000 miles, it is very much nearer to us than any one of the fixed stars. To its abounding light and heat we owe the preservation of our lives, the maintenance of our vigor, the physical comforts which we enjoy, and the marvellous beauties of nature. 5. Comets. The word " comet " is derived from a Greek word meaning " long-haired." Comets are usually invisible to the naked eye, but sometimes attain great splendor and beauty. In ancient and mediaeval times their appearance was usually regarded as a dire omen. Even in 1861 it was rumored in Italy that the great comet of that year presaged the death of Pope Pius IX. 6. Meteors. These are' evanescent objects which flash across the sky, and usually fade from sight in a few seconds. Occasionally they are so brilliant as to be seen in broad daylight, and are accompanied by terrific detonations. 7. Nebulae. Nebulae, as their name implies, are cloudlike in appearance. A few of them are conspicuous enough to be faintly seen without telescopic aid, and appear as feeble patches of light on the dark background of the sky. They are large and diffuse masses of matter, at vast distances from us. 8. The Star Maps. In the maps at the end of this book, the magnitudes are indicated very simply ; especial care has been taken in drawing the dotted lines connecting the stars in the constellations, so that figures easily remembered may be obtained. Careful direc- tions are given for learning the constellations, together with lists of telescopic objects, most of which are within the power of a three- inch telescope. Stars of the first magnitude are indicated by heavy black dots. Those of the fifth magnitude are represented by small GENERAL SURVEY OF THE HEAVENS. dots. A star of the second magnitude has two short arms project- ing from a central dot. Stars of the third and fourth magnitude have, respectively, three and four projecting arms. The figures at the top and bottom of each map, except the first, denote the right ascensions of the stars ; those at the sides indicate the declinations. On Map I. right ascensions are given around the circumference, declinations along a diameter. These terms are explained in 122. They are not needed in learning the constellations. Each con- stellation is bounded by a heavy dotted line, and its name is printed in large letters. The proper names of the brightest stars, such as Sirius, Vega, etc., are given. Most of the stars are marked by letters or numbers. The name of such a star is formed by adding to its letter the Latin genitive of the name of the constellation in which it lies. Thus the star m in Orion is called m Orionis. 9. Names of the Constellations. The names of the constellations shown on the Star Maps are given in the table below. The Greek alphabet is found in 405. LATIN (Nominative) An-drom'-e-da A-qua'-ri-us Aquila (Ak'-wi-la) Ar-go Na-vis Aries (A'-ri-ez) Au-ri'-ga Bootes (Bo-6'-tez) Cam-el-o-par'-dus Can-cer Canes Venatici (Ka'-nez Ve-nat'-i-si) Ca-nis Ma-jor Ca-nis MT-nor Cap-ri-cor'-nus Cassiopeia (Kas-si-6-pe'-ya) Centaurus (Sen-taw'-rus) Ce'-phe-us Ce-tus Co-lum'ba Coma Berenices Co-ro'-na Bo-re-a'-lis Corvus Cra-ter Cyg'-nus Del-phi'-nus Dra-co LATIN (Genitive) Andromedae Aquarii Aquilae Argus Arietis Aurigae Bootis Camelopardi Cancri Can. Ven. Canis Majo'ris Canis Mino'ris Capricorni Cassiope'iae Centau'ri Cephei Ceti Columbae Comae Berenl'ces Coronae Borea'lis Corvi Cra'teris Cygni Delphihi Draco'nis ENGLISH Andromeda The Water Carrier The Eagle The Ship The Ram The Charioteer The Bear Keeper The Camelopard The Crab The Hunting Dogs The Great Dog The Little Dog The Goat The Lady in the Chair The Centaur Cepheus The Whale The Dove The Hair of Berenice The Northern Crown The Crow The Cup The Swan The Dolphin The Dragon DESCRIPTIVE ASTRONOMY. LATIN (Nominative) Equuleus (E-kwu'-le-us) E-rid'-a-nus Gem'-i-m Her'-cu-les Hy'-dra La-cer'-ta Le-o Le-o Mi-nor Le-pus Li-bra Lu-pus Lynx Ly'-ra Mohoceros (Mo-nos'-e-ros) Oph-i-u'-chus O-rT'-on Peg'-a-sus Per'-se-us Pisces (Pis'-sez) Pis'-cis Aus-tra'-lis Sagitta (Sa-jit'-ta) Sagittarius (Saj-i-ta'-ri-us) Scor'-pi-6 Sculp-tor Scu-tum Serpens (Ser'-penz) Sextans (Seks'-tanz) Taurus (Taw'-rus) Triangulum (Tri-ang'-gu-lum) Ur'-sa Ma-jor Ur'-sa Mi-nor Vir'-go Vul-pec'-u-la LATIN (Genitive) Equu'lei Eridani Gemino'rum Herculis Hydrae Lacertae Leo'nis Leo'nis Mino'ris Lep'-o-ris Libras Lupi Lyncis Lyras Monocero'tis Ophiuchi Orionis Pegasi Persei Piscium Piscis Austr. Sagittae Sagittarii Scorpii Sculpto'ris Scuti Serpentis Sextantis Tauri Trianguli Ursae Majo'ris Ursae Mino'ris Vir'-gTnis Vulpeculae ENGLISH The Little Horse The River The Twins Hercules The Snake The Lizard The Lion The Little Lion The Hare The Scales The Wolf The Lynx The Harp The Unicorn The Serpent Bearer Orion The Winged Horse Perseus The Fishes The Southern Fish The Arrow The Archer The Scorpion The Sculptor The Shield The Serpent The Sextant The Bull The Triangle The Great Bear The Little Bear The Virgin The Fox 10. How to Find the Northern Constellations. The constellations visible in the northern sky are to be found on Map I. The appear- ance of these constellations at 8 o'clock on any evening may be found by using the dates around the circumference of the map. If the aspect of the northern sky on April ist, for instance, is desired, hold the map so that the date April ist shall be uppermost. Face the north and hold the map up toward the sky. The Great Dipper, which is a portion of the constellation of the Great Bear (Ursa Major), can then be found readily. After the Great Dipper has been fixed in mind, the Pole Star can be located by the help of Fig. 3. Cassiopeia is on the opposite GENERAL SURVEY OF THE HEAVENS. 5 side of the Pole Star from the Great Dipper, and at about the same distance from it. The five brightest stars in this constellation form a straggling W and are quickly discovered. Beginning at the Pole Star, one can then trace out the Little Bear (Ursa Minor) with the assistance of the map. When the outlines of these constellations have been learned thor- oughly, there will be very little difficulty in becoming acquainted with the adjacent ones. 11. How to Find the Constellations in the South. These constella- tions are pictured on Maps II. -V. The stars underneath any particular date at the top of the map are to be seen in the south at 8 P. M. on that date. For example, Map II. shows that on February i6th Orion is in the south at 8 P. M. In winter, Orion is the best southern constellation to learn first, because of its conspicuousness. In spring, Leo (see Map III.) is recommended as a starting point; in summer, Scorpio (see Map IV.) will answer the same purpose; it will be seen low down in the south. For autumn, Pegasus (see Map V.) is available. Four of the principal stars of this constellation form a large square. 12. Hints on Constellation Study. When trying to learn any particular constellation, the student should find from the maps about half a dozen or a dozen of its principal stars. The configuration of these should be impressed upon the mind so thoroughly that a drawing showing their relative positions can be made without looking at the maps. With this picture well in mind, the student may confront the sky. It is well to note the brightness and color of any first magnitude star in the constellation, and to learn its name. In many cases one can trace in a constellation a resemblance to the object after which it is named. Orion, for instance, resembles the figure of a man. The best way to secure a thorough acquaint- ance with the constellations which have been learned is to draw them from memory frequently, and to look at the heavens whenever occasion offers. The descriptive matter in Chapter XIV. will be found useful in this work. DESCRIPTIVE ASTRONOMY. CHAPTER II. APPARENT DAILY MOTION OF THE STARS. " The sad and solemn night Hath yet her multitude of cheerful fires ; The glorious host of light Walk the dark hemisphere till she retires ; All through her silent watches, gliding slow, Her constellations come, and climb the heavens, and go." BRYANT. 13. The Daily Motion. The most casual observer cannot fail to notice that the majority of the visible stars daily rise, travel across the sky, and set. The Greek philosopher, Pythagoras, is said to have taught that the thousands of fixed stars which stud the sky were set in a crystal sphere, which, by its daily revolution, carried them around the earth as a centre. By this theory the apparent daily motion of the stars was ex- plained very simply and accurately, so far as the unassisted eye could judge. 14. Cause of the Motion. When a passenger train, which has been standing near a motionless freight train, starts without per- ceptible jar, a passenger looking at the freight train has the impres- sion at first that the latter train is moving, and his own standing still. A person on the deck of a steamer, which is slowly turning about in a harbor, sees the objects on shore apparently revolving about him. If a visitor to an astronomical observatory looks upward at the dome while it is being turned, the floor on which he stands seems to be revolving. Similarly, the apparent daily revolution of the heavens is an illusion. The earth turns upon its axis once a day, but makes the rotation without noise or jar, so that the observer is unconscious of it, and is led to think that the earth is at rest, and that the sky moves. 15. The Celestial Sphere. As one looks by night at the heavenly bodies, they all seem to lie on the surface of an immense dome. APPARENT DAILY MOTION OF THE STARS. 7 Were the earth to vanish suddenly, the observer would seem to be in the centre of a hollow star-spangled sphere. The distance from his eye to any celestial object he could not tell. But his reason would quickly declare that the stars might really be at widely different distances from him, while they appeared to be upon a spherical surface which lay beyond them all. Each star would seem to be situated where a straight line drawn from his eye through the star, met the spherical surface. This imaginary spherical surface is called the celestial sphere. 16. Radius of the Celestial Sphere. Astronomers find it conven- ient to assume that the radius of the celestial sphere is infinite, that is, too great for human comprehension. Were it possible for a man to take his stand upon the celestial sphere and look back at the vast assemblage of worlds which we call the physical universe, their combined mass would appear to him to be a mere point of light, which would be situated at the centre of the sphere. Hence it is evident that the observer's eye, or any point on the earth's surface, or the earth's centre, or the sun's centre, may be considered without palpable error as the centre of the celestial sphere. 17. Revolution of the Sphere. The Poles. Imagine that the earth's axis is prolonged until it strikes the celestial sphere at two opposite points, called the north and south celestial poles, respect- ively. The heavens appear to revolve about an axis drawn from one celestial pole to the other. In Fig. 2, P is the north pole of the earth and F the south pole ; O is the position of an observer upon its surface ; AB is drawn through O parallel to PP r . Now there is no fixed star which is 8 DESCRIPTIVE ASTRONOMY. known to be nearer to us than 20,000000,000000 miles. To give greater definiteness to our ideas, conceive the radius of the celestial sphere to be 20,000000,000000 miles, and its centre to be at the earth's centre. Prolong AB and PP', until they strike the surface of the sphere : suppose the places where they strike to be marked by brilliant points of light. So enormous is the radius of the sphere when compared with the distance between AB and PP', that to an observer on the earth an extremity of AB would seem to coincide with the corresponding extremity of PP', even if the observer's eye were assisted by the most powerful telescope of modern times. We may therefore consider the sky as revolving on AB as an axis, and may state the following principle. The celestial sphere appears to revolve on an axis drawn from the eye of the observer to either pole of the celestial sphere. 18. Location of the North Celestial Pole. Nearly every one is familiar with the configuration of seven bright stars which is called 2 N . X Fig. 3. THE GREAT DIPPER AND POLARIS. " The Great Dipper." It is represented in Fig. 3. The two stars in the bowl of the dipper which point nearly to the Pole Star (Polaris) are called "The Pointers." The distance from Polaris to the nearer one of the Pointers is about five times the distance be- tween the latter. The position of the pole is shown in the figure ; APPARENT DAILY MOTION OF THE STARS. 9 it lies very near a line drawn from Polaris to Mizar ; its distance from Polaris is one fourth of the distance between the Pointers. 19. Definitions. If a straight line does not lie in a plane, but meets it at some point, the point is called the foot of the line. A straight line is perpendicular to a plane when it is perpendicular to every straight line that can be drawn in the plane through its foot. The corner of a room, where two walls meet, is a line perpendic- ular to the plane of the ceiling, or of the floor. Fig. 4. Fig- 5- A straight line is parallel to a plane when they cannot meet, however far they may be extended. When two planes meet, their line of intersection is called the edge of the angle which they make with each other. The planes AC and CF meet in the edge BC. At any point, H, in the edge, two perpendiculars to the edge are drawn, one lying in each plane. The angle GHK made by these perpendiculars measures the angle between the planes. To find the angle which a line, prolonged if necessary, makes with a plane which it meets, a per- pendicular to the plane is dropped from some point A in the line : the foot of the perpendicular is then joined with the foot of the original line. The angle which the last line drawn makes with the original line is j the angle sought. In the figure, AB is the original line, AC the per- pendicular, BC the joining line, and ABC the angle. 20. The Celestial Equator. The equator of the earth is an imagi- nary line encircling it, midway between the poles. The plane of the Fig. 6. 10 DESCRIPTIVE ASTRONOMY. equator extended indefinitely cuts the celestial sphere in a circle, which is called the celestial equator. Since the plane of the ter- restrial equator is perpendicular to the earth's axis, the plane of the celestial equator is perpendicular to the axis of the celestial sphere. 21. The Horizon. The word " horizon " is commonly used to designate the line where the earth and sky appear to meet. But astronomers use the word in a different sense. If the observer holds a plumb-line so that it hangs vertically, any plane surface, like a book-cover, when held so that it is perpendicular to the plumb-line, will represent a portion of the plane of the horizon of the place of observation. If the flat surface of the book-cover be extended hori- zontally in all directions until it reaches the sky, the plane thus formed is called the plane of the horizon. This plane cuts the celestial sphere in a circle called the horizon. 22. The Zenith, for any place on the earth, is the point where a plumb-line prolonged upward strikes the celestial sphere. 23. The Nadir is the point where the plumb-line prolonged downward strikes the celestial sphere. The zenith and nadir are those points on the celestial sphere which are most remote from the horizon. In the language of geometry, they are called the poles of the horizon. EXERCISES. 24. I. Narrate some personal experience of illusory motion similar to those mentioned in 14. 2. The diameter of the earth is about eight thousand miles. What is the greatest possible distance between AB and PP' in Fig. 2 ? 3. The observer is on the terrestrial equator. (a) What is the distance in miles between AB and PP' in Fig. 2? (b) If he walked toward either pole, and the earth were a perfect sphere, would this distance increase or decrease? 4. If another line were drawn through O in Fig. 2, making an angle of 10 (one ninth of a right angle) with AB, would AB and the new line, when prolonged, meet the celestial sphere at the same points apparently? APPARENT DAILY MOTION OF THE STARS. II [If there be any doubt in the student's mind concerning this, he should take two straight sticks, and put an end of one in contact with an end of the other, so that the sticks make an angle of 10 with each other. Then, placing his eye near the vertex of the angle, he can look along each stick at the sky.] 5. If the nearest fixed star were 20,000000,000000 miles from us, how many years would it take its light, travelling 186,330 miles per second, to reach us? Ans. 3.4+ years. 6. Look at the Great Dipper and Polaris in the sky, and draw a map of them ; first make a dot for Polaris and draw a line below it to represent the horizon. Draw another line through Polaris per- pendicular to the horizon line. Draw the Dipper, showing its position with reference to these lines, and state the hour at which your observation was made. 7. The observer, facing north, notices that the Dipper is below the pole. {a) Will the Dipper appear, on account of the earth's rotation, to move towards his right hand, or towards his left? (#) What would be the direction of motion of the Dipper if it were above the pole, near the zenith? (c) What direction (up or down), if at the right of the pole? (d) What direction, if at the left? 8. If a line be drawn from your eye to each of the two " Pointers " in the Dipper, the two lines make an angle of about 5, Astronomers commonly say that the distance between the Point- ers is 5. Estimate the distance from the Pole Star to the Pointer nearest to it. 9. Suppose that the earth's centre is fixed in the centre of the celestial sphere, and that the earth rotates on its axis. Imagine all the stars to be fixed on the surface of the celestial sphere. If the earth's axis were then tipped a few degrees, would the north celestial pole remain at the same point among the stars as before? Would the position of the celestial equator be changed ? 10. If, instead of tipping the axis of the earth, as in the pre- ceding exercise, the earth were moved toward some point on the celestial equator and were placed 1,000000 miles away from its former position, the new direction of its axis being parallel to its former direction, would the new celestial poles appear coincident 12 DESCRIPTIVE ASTRONOMY. with the former celestial poles? Would the old and new celestial equators coincide? 11. Conceive that the earth, in exercise 9, is rotating about a straight wire stretched from the north celestial pole to the south celestial pole. If it were slid along this wire to a place 10,000000 miles from its first position, would the new celestial equator appear to coincide with the old? 12. The earth makes its annual journey around the sun in a path nearly circular. (a) If the earth's axis at every instant during the year were parallel to its position at every other instant, would the celestial poles change their position during the year? (b) Would the celestial equator change its position? [The earth's axis remains almost parallel to itself during a year. Its deviations will be explained later. So far as naked-eye obser- vations are concerned, it may be considered as remaining parallel to itself.] 13. Take a ball or an orange to represent the earth. Mark on it the north and south poles and the equator. Find from a geography or other source the latitude of your place of observation to the nearest degree, and locate the place on the ball. (If the latitude were 45, the place would be halfway between the pole and the equator.) Take a flat stiff card, and lay one of its surfaces against the ball, at the point representing the place of observation. Fasten the card by a pin thrust through it into the ball at the point of contact. The pin should point toward the centre of the ball. The surface of the card will then represent the plane of the horizon of the place, and will be tangent to the spherical surface of the ball. (#) Is your horizon parallel or perpendicular to the earth's axis? (&) Is it parallel to the earth's equator? (c) If you were at some point on the earth's equator, would your horizon plane be perpendicular to the earth's axis, or parallel to it? (d) If you were at the north pole, would your horizon plane be parallel to the earth's axis, or perpendicular to it? (e) As the earth tuVns on its axis, does the inclination of the axis to the plane of your horizon change? APPARENT DAILY MOTION OF THE STARS. 13 14. If the polar axis of the ball in the last exercise be prolonged both ways, which prolongation (north or south) would pierce the plane of the card representing your horizon plane? If the ball be held so that the card is horizontal, what points on the celestial sphere would the pin strike if prolonged indefinitely each way? REMARK. In obtaining the answers to the following exercises, the apparatus described in exercise 1 3 can be used ; but it would be much better to imagine the earth itself, with its poles and equator, and with horizon planes touching it at different points. The student should make every endeavor to picture to himself the realities of nature, rather than the apparatus or the geometrical diagrams used to explain principles. Wherever possible, he should observe the celes- tial motions about which he studies. 15. If you were at the north pole, would your horizon plane be parallel to the plane of the earth's equator, or perpendicular to it? If you lived at the equator, would your horizon plane be parallel to the earth's equator, or perpendicular to it? 1 6. (#) If one man were at the north pole and another at the south, would their horizon planes be parallel? () If one man were at the north pole and another at any point of the equator, would their horizon planes be perpendicular to each other? (c) Could the horizon planes at two points on the equator be parallel ? ( as the area of the hole is to the area of the sphere, so is the amount of light the earth receives in one second to the total light given out by the sun in one second. The area of the surface of a sphere = 4 X 3.1416 X the square of its radius. The area of a circle = 3.1416 X the square of its radius. Compute the fractional part of the sun's radiation which strikes the earth. 4. Why cannot the prominences and corona be- seen with a good telescope on any bright day? 5. What change does the vapor which is shot off from the sun ( 77) undergo, in passing through space?" \ 64 DESCRIPTIVE ASTRONOMY. CHAPTER V. THE EARTH. " The earth, Though in comparison of heaven so small, Nor glistering, may of solid good contain More plenty than the sun that barren shines, Whose virtue in itself works no effect, But in the fruitful earth ; there first received His beams, inactive else, their vigor find." e . MILTON. 90. Dimensions and Shape. The earth is a globular body, nearly 8,000 miles in diameter. The surface of the ocean is not truly spher- ical, but bulges at the equator. If a soft rubber ball be spun rapidly on an uncarpeted floor, it will assume a form like that of the earth : its shortest diameter will be that on which, as an axis, it rotates. This is due to the tendency of every particle of matter which is whirling around a centre to fly away from it. Mathematicians call the earth an oblate spheroid. Such a solid is formed by revolving an ellipse ( 96) about its shortest diameter as an axis. 91. Direction of the Plumb-line. A string, at the lower end of which a plumb-bob hangs, is a plumb-line. If the earth were truly spherical and homogeneous, and did not rotate, its attraction for the bob would cause the line to point directly to its centre. In Fig. 56, PP' is the earth's axis, and EQ its equator. When the earth whirls, the plumb-bob at Q tends to fly directly away from C, in the direction of the line CQ prolonged. But gravity pulls the bob directly toward C, and overcomes its tendency to fly away. At D, on account of the earth's rotation, the bob tends to fly away from the axis PP', in the direction indicated by the arrow A. This ten- dency causes the plumb-line, instead of pointing toward C, to swing a trifle, taking the position shown. At P, since the plumb-line is in the prolongation of the earth's axis, the rotation causes no side- wise swing. Hence, at the poles and at any point on the equator the THE EARTH. plumb-line points towards the earth's centre (if the earth be homoge- neous); at other places it does not point quite towards the centre. The plumb-line is perpendicular to the surface of still water. Fig. 56. DIRECTION OF THE PLUMB-LINE, 92. How the Earth's Diameter is Found. While the details of this process are too difficult for us to understand at this stage of our progress, we may get a notion of the principles involved by assum- ing that the earth is a perfect sphere, and that a plumb-line points toward its centre. In Fig. 57, C is the earth's centre, and AB an arc of a meridian. At A, AP is drawn toward the north celestial pole, and AZ in the direction of the plumb-line. At B, BZ' is drawn " plumb," and BD is made parallel to AP. BE is parallel to AZ. Now the angle PAZ DBE, because their sides are parallel. For the same reason, EBZ' = ACB. Then ACB = EBZ', = DBZ' - DBE, = DBZ' - PAZ. 5 66 DESCRIPTIVE ASTRONOMY. An astronomer at B measures the angle DBZ'; then, using his utmost skill and refined apparatus, he determines the length of AB in feet or meters. On arriving at A, he measures the angle PAZ. Subtracting one of these angles from the other, he gets the value of ACB, as explained above. If ACB were one degree, the circum- ference of the sphere would be 360 times the length of AB in feet or meters. Geometry teaches that the circumference divided by 3.1416 gives the diameter. The real spheroidal shape and the lengths of the polar and equa- torial diameters are deduced from measurements made in various parts of the world. 1 Fig. 57- Fig. 58. LATITUDE AND LONGITUDE. 93. Latitude and Longitude, if the Earth were a Perfect Sphere. Assuming for a moment that the earth is a perfect sphere, we get the following definitions. The terrestrial meridian of any point on the earth's surface is the circle drawn through the point and the poles. In Fig. 58, NGG'SR is the terrestrial meridian of Greenwich, if G represent that place. The latitude of any point on the earth is the arc of its meridian, between the equator and the point. The arc is measured in de- grees. A city in 30 north latitude is one third of the way from 1 The lengths of the diameters are respectively : Polar 7,901.476 miles. Equatorial ... 7,926.592 miles. THE EARTH. south latitude is half the equator to the north pole. A place in 45' way between the equator and the south pole. The longitude (from Greenwich) of any point on the earth is the arc of the equator embraced between the meridian of the point and the meridian of Greenwich. It is reckoned either east or west of the Greenwich meridian. Sometimes it is expressed in degrees, but more often in hours, by putting 360 equal to 24 h. Thus 15 equals I h. ; a place in longitude 105 west of Greenwich is said to be 7 h. west of Greenwich. 94. Latitude and longitude accurately Defined. Astronomers are accustomed to regard the earth as a perfect spheroid ( 90), making allowances, when ne- cessary, for the irregularities caused by the unevenness of its surface. The astronomical latitude of any point on the earth's surface is the angle made by the plumb- line suspended at that point with the plane of the equator. In Fig. 59, the astronomical latitude of D is the angle DAE. The plane of the meridian of any point on the earth's surface is the plane passing through the point and the poles. Any two meridian planes intersect, their line of intersection being the earth's axis. In Fig. 59, PF is the line of intersection of the planes PGP' and PEF. The angle between these planes is ( 19) equal to ECB, because EC and BC are both perpendicular to PF. The angle ECB is measured by the arc EB. The longitude (from Greenwich) of any point on the earth's sur- face is the angle made by its meridian with the Greenwich meridian ; or it is the arc of the equator lying between the two meridians. Thus, in Fig. 59, the longitude of D, reckoned from G, is the angle between the planes PGP' and PEP', which, as we have found, is measured by the arc EB. Fig. 59. THE ASTRONOMICAL LATITUDE. 68 DESCRIPTIVE ASTRONOMY. Latitude is measured in degrees ; longitude in degrees or hours, as stated in the last section. 95. Variation of Latitude. During the past few years it has been shown that the latitudes of various observatories are not con- stant, but change by slight amounts, according to a law which has been pretty thoroughly determined by Dr. S. C. Chandler. 1 The change is due to a "wobbling" of the earth upon its axis, so that the north and south poles do not remain at the same points on the earth's surface. This movement of the poles amounts to a few yards in the course of a year, and takes place in a very sinuous path. It is believed to be due to slight changes in the earth's form, due to the varying forces which act upon it. A rude illus- tration is given by a base ball, which is usually rotating about some axis just before it is struck by a bat ; immediately after the stroke, it commonly rotates about an entirely different axis. B THE ECLIPTIC AND THE ZODIAC. 96. The Orbit. The earth completes its circuit about the sun in a year, moving in an ellipse. This curve may be drawn as shown in Fig. 60. A string runs from F around the pencil at P to F'. The pencil, being moved in such a way that the string slips around it and is continually kept taut, describes the curve. F and F' are the foci. A A' is the major axis ; BB' is the minor axis ; C is the centre The sun is not at the centre of the earth's orbit, but at one of the foci. When the earth is nearest the sun, it is said to be in peri- helion ; when farthest away, in aphelion. Half the major axis is called the mean distance (CA) ; the mean distance of the earth is 93,000000 miles. The earth travels most swiftly when in perihelion, and with the least velocity at aphelion. 97. The Ecliptic. If the plane of the earth's orbit be extended till it cuts the celestial sphere, their intersection will be a circle, known as the ecliptic. In Fig. 61 is a pond of water in which a 1 Of Cambridge, Mass. Fig. 60. AN ELLIPSE. THE EARTH. 6 9 croquet ball is floating, half submerged. The surface of the water represents the plane of the ecliptic, and the croquet ball the earth, the centre of which is just in the plane. The point S, near the centre of the pond, represents the sun's centre. If the earth's axis were perpendicular to the plane of its orbit, the planes of the equator and ecliptic would coincide. But the axis is tipped so as to make an angle of 23 27' with the perpen- - Fig. 61. ILLUSTRATION OF THE ECLIPTIC. dicular. (See Fig. 62.) The plane of the equator, being per- pendicular to the axis, is also tipped, and makes the same angle with the plane of the ecliptic. This angle has been named the obliquity of the ecliptic. 98. The Equinoxes. The Sun's Yearly Path. The planes of the ecliptic and equator, when extended, cut the celestial sphere in two circles, which cross each other at two opposite points (A and B) in Fig. 63. The points are the vernal and autumnal equinoxes respect- ively. It is evident, from Fig. 61, that a straight line drawn from the earth's centre through that of the sun, and prolonged to meet the celestial sphere, will strike at some point of the ecliptic. As the earth moves in its orbit around the sun, the other end of this line moves along the ecliptic. To an observer on the earth, the sun appears to be on the sur- face of the celestial sphere, at the end of the line just mentioned. Hence the sun seems to us to creep along the ecliptic, taking a year to make one complete circuit. On or near the 2Oth of March, the DESCRIPTIVE ASTRONOMY. sun is at A ; this point is called an equinox, because at that time the days and nights are equal, as will be demonstrated later. Six months afterwards, about September 22d, the sun is at B, the autumnal equinox. Fig. 62. THE OBLIQUITY OF THE ECLIPTIC. Fig. 63. THE ECLIPTIC AND THE EQUATOR. 99. The Solstices : The Sun's Eastward Motion. The point C in Fig. 63, midway between A and B, is the summer solstice. The sun is at this point about June 2Oth. In travelling from A, the vernal equinox, to C, the summer solstice, the sun keeps getting farther away from the celestial equator every day, and nearer to the north pole. While travelling from C to B, the sun continually lessens its distance from the celestial equator. After passing through the autumnal equinox, it is between the south celestial pole and the equator, and gets farther south of the equator every day until it reaches D, the winter solstice, about December 2ist. When going from D to A, it is approaching the equator every day. It must be diligently remembered that during the entire year, the sun, whether north or south of the celestial equator, is contin- ually moving eastward along the sphere. If one could look right past the sun and see the stars beyond, the amount of the sun's motion among the stars in a day would be very plain to the naked eye. Fig. 64 shows such a view of the sun's daily motion. 100. The Zodiac. The zodiac is a belt 16 broad, encircling the sky, like the colored band on a croquet ball. The ecliptic is its central line. Twelve constellations lie in this belt or zone. They are : Aries, Taurus, Gemini ; Cancer, Leo, Virgo ; Libra, Scorpio, Sagittarius ; and Capricornus, Aquarius, Pisces. The moon and the THE EARTH. 71 planets are always to be found in the zodiac. The signs of the zodiac are twelve arbitrary divisions of it, each of which is 30 long. The first three of them stretch from the vernal equinox to the summer solstice. The signs have the same names as the constella- ECLIPTIC Fig. 64. THE SUN'S DAILY MOTION AMONG THE STARS. tions just mentioned, but do not coincide with them. The expres- sion, " The sun enters Aries," found in almanacs, means that the sun passes the vernal equinox. 101. Is the North Celestial Pole Fixed? When a heavy top spins rapidly on a smooth surface, its axis keeps the same direction, no matter how much the top wanders around. The successive posi- tions taken by the axis of the top are parallel lines. As the earth goes spinning around the sun once a year, all the posi- tions of its axis from moment to moment are parallel lines. ._. . . *-. Fi - 65. A SPINNING TOP. We here neglect certain minute tippings caused by the attraction of other bodies for the earth. These parallel lines when extended appear to strike the celestial sphere at the same point, because of the infinite distance of the sphere from us. This explains why the north celestial pole can be found at the same place among the stars ( 18) during the entire year. It does not move enough during his lifetime to attract the attention of one observing with the naked eye. 102. Are the Celestial Equator and Ecliptic Fixed ? Since the plane of the earth's equator is perpendicular to its axis, all the positions DESCRIPTIVE ASTRONOMY. which it takes during a year will constitute a series of parallel planes. These planes, being extended to the celestial sphere, cut it in great circles, which appear to us to coincide on account of their infinite distance from us. Since the earth's axis is tipped a little this way and that, as mentioned in the last article, the equator suffers slight shiftings. But for purposes of naked eye obser- vation, we regard the celestial Its place among them is shown Fig. 66. SUCCESSIVE POSITIONS OF THE EARTH'S EQUATOH. equator as fixed among the stars, on the maps. Similarly the ecliptic is to be regarded as a circle practically fixed among the stars. DAY AND NIGHT: THE SEASONS. 103. The Length of The Day. Everybody knows that in summer the days are long and the nights short, and that the case is reversed in winter. We refer now to middle latitudes, leaving the polar regions and the equator out of consideration for the moment. One may get a clear grasp of this matter by a simple experi- ment. Take an orange, with a knitting needle thrust through it to serve as an axis of rotation, and mark upon it with a penknife the equator, together with three or four circles on each side of the equator and parallel to it. Submerge half the orange in a basin of water in the position shown in Fig. 67. The surface of the water represents our horizon, and the upper part of the or- ange the half of the celestial sphere visible to us at any mo- ment, PP' being its axis and EQ its equator. It is plain that just Fig. 67. AN ORANGE HALF SUBMERGED. one half of the equator is below the horizon. Hence, if any ce- lestial object is on the equator, as the sphere turns, the object will be above the horizon for twelve hours and below for the same length THE EARTH. 73 of time. More than half of every circle between P and EQ is above the horizon. Therefore, if any celestial object be north of the equator, it will be above the horizon more than twelve hours out of the twenty-four. If it be near the north celestial pole, it will be above the horizon all the time. Less than half of every circle between P' and EQ is above the horizon. Consequently any celestial object south of the equator is above the horizon less than twelve hours out of the twenty-four. If it be near the south celestial pole, it will never rise above the horizon. On March 2Oth the sun is near the vernal equinox all day ; and since this point is on the equator, it rises nearly at the east point of the horizon and sets nearly at the west point, so that both day and night are almost exactly twelve hours long. After March 2Oth the sun, creeping along the ecliptic, gets farther and farther north every day until June 2Oth, so that the days continue to grow longer and the nights shorter until that date, which is the longest day of the year. After June 2Oth the sun approaches the equator and the days shorten and the nights lengthen until September 22d, when the sun reaches the autumnal equinox, and the days and nights are again equal. The sun then passes south of the equator, and the days grow shorter till December 2ist, when the sun is farthest south. The sun approaches the equator during the next three months, so that the days continually lengthen until the days and nights again become equal. 104. Day and Night at the Equator. In Chapter II. we learned that, if a man lived at the equator, the celestial poles would be on his horizon at the north and south points. The orange used in the last section is now to be placed in the water, with the knitting needle lying horizontal in the liquid surface. Then every circle marked on the orange, as previously described, will be half submerged, and the sun, wherever it may be on the sphere on any given day, will be above the horizon twelve hours, and below for the same number of hours. 105. Day and Night at the Poles. The horizon of a man at the north pole would be parallel to the terrestrial equator. Both planes, when extended to the celestial sphere, would seem to cut it in the same circle, the celestial equator. The north celestial pole would be in the zenith. Therefore, if the sun were north of the equator 74 DESCRIPTIVE ASTRONOMY. it would be above the horizon, and if south of the equator it would be below the horizon. For the possible polar bears or seals in that locality there is continuous day from March to September, and night for the remaining months of the year. During much of the night there is a strong twilight, because the sun is not far below the horizon. 106. The Midnight Sun. In Fig. 68, PP' is the earth's axis, and EQ is the equator. O is the situation of an observer who is in a high northern latitude. OA, drawn from the observer's position at O, parallel to the earth's axis, is directed toward the north celestial pole. OZ points toward the zenith, and NS represents the horizon. Suppose that the observer is in 70 north latitude, then the angle OCQ will be 70, and OCP will be 20. But OCP= AOZ. Since OA points to the pole and OZ to the zenith, and the angle AOZ is small, the pole is near the observer's zenith. N MIDNIGHT E .< -> . \^^ Fig. 69. THE MIDNIGHT SUN. The appearance of the celestial sphere, as seen by an observer at O is shown in Fig. 69. NESW is the horizon ; EQW is half of the celestial equator ; P is the north pole, and OP is the apparent rota- tion axis of the sphere. In June the sun is so far north of the equator that its daily path through the sky, ABCD, lies entirely above the horizon, so that it is visible even at midnight ; this may be made very plain by using the orange of 103. 107. The Seasons in Middle Latitudes. We consider first a place at a north latitude of about 45. The change of seasons is due principally to two causes, i. The sun is above the horizon during THE EARTH. 75 more hours on a day in summer than on a day in winter. 2. The sun heats the earth's surface at any place the more powerfully the higher up it is in the sky. This is shown in Fig. 70, where a bundle of rays from the sun, coming nearly vertically downward, heats up a square foot, while an equal bundle, striking obliquely, spreads its heat out over a greater surface ; consequently it heats each square inch Fi s- ?- ~ EFFECT OF THE SLANT - , r OF THE SUN'S RAYS. of the surface less. The oblique rays also traverse a longer path in the atmosphere, and are more absorbed by it, before reaching the ground. 108. The Seasons at the Equator. The change of seasons is much less marked than in middle latitudes : for the sun is above the horizon the same time (twelve hours) every day of the year, and its rays come down nearly vertically at noon, throughout the year. THE PRECESSION OF THE EQUINOXES, AND THE CALENDAR, 109. Attraction of the Earth's Equatorial Ring. Since the earth bulges at the equator, we may consider it as a true sphere, around the equatorial regions of which an extra ring of matter has been placed. This conception is rudely represented in Fig. 71. The attraction of the earth and moon upon this ring-like protuberance cause the precession, which we proceed to explain. Sup- pose the moon to lie in the direction of. the arrows, and by its attractive force to be tu gg in g aw *y at the ring. Its pull on the half of the ring at the left (in the figure) of C tends to tip the earth, so that P will move 7 6 DESCRIPTIVE ASTRONOMY. to the left. Its pull on the half of the ring at the right of C tends to tip the earth in the other direction, so that P will move to the right. But since the left half of the ring is nearer the moon than the right half, it is pulled the more strongly, and therefore P will move slowly toward the left. In consequence of this attraction, the earth will tend to turn slowly about an axis going through C, but perpendicular to the plane of the page on which Figure 71 is drawn. But meanwhile the earth is spinning with prodigious energy about the axis P P', and the moon's attraction on the ring produces only a slight disturbance of this energetic rotation. 110. Illustration with a Top. If a top, which is not spinning, be placed in a slanting position on a floor, the force of gravity pulling in the direction of the arrow in Fig. 72, will cause it to fall over; in falling it will move just as if it were turning about a line AB, drawn Fig. 72. A LEANING TOP. Fig. 73. POSITIONS OF THE Axis OF THE TOP. on the floor, as an axis. When the top is spinning swiftly in an inclined position, it rotates about the axis PS, and gravity is at the same time trying to make it rotate about AB. Any one who has watched a top knows that, under these circumstances, the axis PS moves slowly around, taking the successive positions PS, PS', PS'', etc., Fig. 73. PZ is perpendicular to the floor. The more swiftly the top spins, the slower is this motion of PS around PZ. 111. The Earth compared with the Top. Both spin rapidly. The force of gravity attempts to tip the axis PS of the top. The pulls of the sun and moon on the equatorial protuberant ring of the earth tend to tip its axis, PP'. The axis of he top swings around PZ, a THE EARTH. 77 perpendicular to the plane of the floor. The earth's axis swings around a perpendicular to the plane of the ecliptic, as shown in Fig. 74. CA is perpendicular to the ecliptic ; PC, the earth's axis, takes successively the positions PC, P'C, P"C, etc., 25,800 years being required for a complete journey around the circle PP'P"P'". Fig. 74. THE PRECESSIONAL MOTION OF THE EARTH'S EQUATOR. The point where the equator cuts the plane of the ecliptic likewise moves around, taking the positions V and V" successively. On extending the planes of the equator and the ecliptic to the sky, the point V, where the celestial equator and ecliptic intersect, becomes the vernal equinox. DESCRIPTIVE ASTRONOMY. DENEB POLARIS Hence, the vernal equinox moves slowly along the ecliptic, west- ward, taking 25,800 years for a complete revolution. The autumnal equinox does likewise. This is the precession of the equinoxes. This motion of the equinoxes may be made plain by using the orange described in 103, and representing the ecliptic by a sheet of paper in which a circle a little larger than the orange has been cut. Then, by moving the orange in a way suggested by Fig. 74, the motion of the equinoxes becomes easily visible. 112. Some Effects of Precession. On account of precession the north celestial pole, moving as described in the preceding section, comes near different stars as the centuries pass away. About twelve thousand years hence, the north pole will be so near the brilliant star Vega, in the constellation of the Lyre, that it will be called the pole star. The path of the pole among the stars is shown in Fig. 75. Polaris will be only half a degree from the pole in the year 2000. The non-coinci- dence of the sign Aries with the constellation Aries is due to precession. For the sign Aries begins at the vernal equinox, which, as we have seen, shifts slowly westward among the stars. It is now in the constellation Pisces. If Greenwich and its merid- ian moved perceptibly on the earth's surface, the longitudes of all other cities would be chajiged. If the earth's equator kept shift- ing its position on the surface, the latitudes of cities would keep changing. In the same way the shifting of the celestial equator and vernal equinox, by precession, causes changes in the right ascensions and declinations (which correspond to longitudes and latitudes of cities) of the stars. Fig. 75. THE PATH OF THE NORTH CELESTIAL POLE AMONG THE STARS. THE EARTH. 79 Since the vernal equinox moves a little westward every year, the sun in his apparent annual march eastward reaches the vernal equinox sooner than if it were fixed. This makes the year twenty minutes shorter than it would be otherwise. 113. Different Kinds of Years. The sidereal year is the time required for the earth to make a complete revolution about the sun. If the sun, as seen by us, is now in line with some fixed star, a sidereal year must elapse before it will get into lin with the same star again. The length of the year is 365 d. 6 h. 9 m. 9 sec. STAR Fig. 76. DIFFERENT KINDS OF YEARS. The tropical year is the time which elapses between two suc- cessive passages of the sun through the vernal equinox. Suppose that the vernal equinox, when the sun appears to be in it, in March, 1900, is at V in Fig. 76. As the earth moves from E' to E", E'", etc., the sun appears to move from V through A to B, and so on. But meanwhile the vernal equinox, because of precession, has been moving westward a slight distance, so that the sun will meet it in March, 1901, at V. 8O DESCRIPTIVE ASTRONOMY. A sidereal year will be completed when the sun reaches V again. A tropical year is therefore shorter than a sidereal year. Its length is 365 d. 5 h. 48 m. 46 sec. 114. The Julian Calendar. Julius Caesar found the Roman calen- dar in great confusion. It was decidedly complex, and the priests, whose duty it was to regulate the religious festivals in accordance with it, sometimes introduced alterations capriciously, to subserve their own interests. Matters had come to a pretty pass, in Roman eyes, when a festival of Bacchus must be celebrated while grapes were green. Acting upon the advice of Sosigenes, a noted Alexan- drian astronomer, Csesar ordained that three years out of every four consist of 365 days, the fourth being 366 days long. He did this because the year was known to be about 365^ days in length. If the number of a year be divisible by 4 it is a leap year, and contains 366 days according to the Julian calendar. He also directed that the year begin on Jan. I, instead of in March, as before. This Julian calendar went into effect in 45 B. c., and is still employed in Russia. Dates reckoned according to it are now called " Old Style." 115. The Gregorian Calendar The true tropical year being 365 d. 5 h. 48 m. 46 sec. in length, the Julian year of 365 d. 6 h. is too long by ii m. 14 sec. This discrepancy amounts to over 3 days in 400 years. In 1582 Pope Gregory XIII. introduced a reform which dropped 3 days in every four centuries. This was accomplished by ordering that the year which rounds out each century ( 1 300, 1 800, etc.) should be a leap year only when its date number is divisible by 400. Thus 1600 was a leap year according to this calendar, while 1,700 was not. Accordingly three out of every four century years are not reckoned as leap years, and 3 days are thus dropped from the Gregorian calendar which are retained in the Julian. At the time of the famous Council of Nice (A. D. 325) the sun was in the vernal equinox on March 2ist; but in 1582 the same event happened on March nth, because of the imperfection of the Julian calendar. The Pope therefore ordered that the 10 days lost should be made up by calling the day following October 4th October I5th. The new calendar was at once adopted by all nations which recognized the Pope's authority. In England, the Old Style was used until 1752, when by act of Parliament the New Style was intro- THE EARTH. 8l duced, in the face of opposition and noting on the part of some of the people, who acted as though they believed that by the change of date from September 3d to September I4th, eleven days were to be stolen from them. It is now quite common in Russia to write a date according to both styles : thus, Mar. 3 /i 5 . ABERRATION AND REFRACTION. 116. Aberration of Light. One who walks briskly along the street, when the rain is descending vertically, does not hold his umbrella straight up, but slants it forward. Were he to stand, holding a tube vertical, raindrops would pass through it without touching its sides. But if he walked briskly, still holding the tube upright, a drop of rain entering at the top would strike against the side. However, if he slanted the tube forward at the proper angle, drops would go through freely. While the drop was failing the dis- tance AB in Fig. 77, the man would walk a distance equal to CB. In the same way, if the earth were still, and a man pointed a telescope directly at a fixed star, the rays from the star would come down through the telescope tube and emerge at the eyepiece. But since the earth moves, it is neces- sary to slant the telescope by a minute angle, so that rays from a star, after passing through the object-glass, may come out at the centre of the eyepiece. Now the star seems to us to lie in the direction in which the telescope points, and a ray which has the direction AB appears to have the direction AC. This apparent change in the direction of the ray is called its aberration. It is a quantity altogether too small to be detected with the naked eye. 117. Astronomical Refraction. : In 28 we learned that a ray of light passing from one medium into another of different density is bent out of its course unless it strikes the surface of the new medium perpendicularly. 77- ILLUSTRATION OF ABERRATION. 82 DESCRIPTIVE ASTRONOMY. Fig. 78. REFRACTION. In Fig. 78 a ray coming from S through the earth's atmosphere is bent from its course. The air may be considered as made up of a large number of strata of different densities, the stratum nearest the surface of the earth having the greatest density, because it is com- pressed by the weight of all the air above it. Therefore the ray is being continually bent, coming through denser and denser strata, till, when it reaches the eye at E, it is coming in the direction S'E. The star appears to lie at S' instead of S. Since S' lies nearer the zenith than S, the effect of refraction is to make celestial objects appear nearer the zenith, or farther above the horizon, than they really are. When a star is in the zenith, its rays strike perpendicularly on the atmosphere and suffer no deviation. The nearer the horizon a star is, the more its rays are refracted. The sun and the moon, when rising or setting, are ele- vated by refraction a little over half a degree, so that we see them when they are really just below the horizon. 118. Twilight. In the air there are not only clouds, but also minute particles of dust and globules of water, which reflect the sunlight. For some time after the sun has set, he still shines on the clouds over our heads, and on the particles in the upper strata of the air. The light is reflected from these down to the ground, and thus produces twilight. When the sun gets about 18 below the horizon, his rays are shot above the clouds and other reflecting objects, so that they are no longer reflected to us, and twi- light ceases. In England and other countries of like northern Fig. 79. (See Exercise i.) latitude, twilight lasts all night in midsummer, because the sun is less than 18 below the horizon, even at midnight. THE EARTH. 83 EXERCISES. 119, i. Suppose the earth to be a perfect and smooth sphere and a body to be placed on it, as shown in Fig. 79 ; gravity would pull it toward the centre, while it would tend to fly off in the direc- tion of the arrow F, because of the earth's rotation. (a) As a result of this state of affairs, would the body slide toward the equator? (d) If the earth were a perfect sphere, would its rotation cause the water of the ocean to move toward the equator? (c) Because of the bulge of the earth, is the surface of the water at the mouth of the Mississippi River nearer the earth's centre than at St. Paul, or farther from it? (y) If the earth did not rotate, which way would gravity make the river flow? (e) Why does the river run south? (y) Why does not the water of the ocean leave the poles and rush to the equator? (g) If the earth were composed entirely of water, and did not rotate, what would be its form? 2. If the earth were a sphere the diameter of which was 7,920 miles, w r hat would its circumference be according to 92 ? Ans. 24,881.472 miles. 3. In Fig. 60, if the sun be at F, what points are perihelion and aphelion? 4. In Fig. 60, the sun being at F, prove that the semi-major axis (AC) is half the sum of the greatest and least distances (A'F and AF) of the earth from the sun. 5. Why does the earth move most rapidly in its orbit when at perihelion? 6. Cut two equal circles out of stiff paper ; in each cut a narrow slit from a point on the circumference to the centre. Then fit the circles together in such a way that they will represent the relative positions of the planes of the equator and ecliptic. 7. Let the surface of a small round body, like an apple or orange, represent the celestial sphere. On it mark in their proper positions the north and south celestial poles, the equator, the ecliptic, the 84 DESCRIPTIVE ASTRONOMY. equinoxes, and the solstices. Also mark the circle in which the north celestial pole makes its precessional journey of 25,800 years. Does the south celestial pole describe a similar circle on the sphere? 8. The sun, when at C or D in Fig. 63, ceases to recede from the equator and begins to approach it. Show that this fact harmonizes with the derivation of the word " solstice." 9. Why is that equinox in which the sun is on March 2Oth called the vernal equinox? 10. If the earth's axis lay in the plane of the ecliptic, and always pointed directly at the sun, the north pole being toward the sun, in what portion of the earth would the sun never be visible? 11. If the earth's axis lay in the plane of the ecliptic, but always pointed toward one particular fixed star as the earth performed its yearly journey around the sun, in what part of the earth would the sun never be seen? 12. If the earth's axis were perpendicular to the plane of the ecliptic, and the earth were rotating on its axis and revolving about the sun as fast as at present, would all of that part of the earth which is in darkness at any instant be in the light twelve hours thereafter ? 13. If the earth's axis made an angle of 70 with the plane of the ecliptic, and the earth rotated as at present, but did not revolve around the sun, is there any part of the earth in which there would be no night? 14. At the moment when the sun lies in the plane of the equator, about March 2Oth or September 22d, does its pull on the equatorial protuberance of the earth tend to tip the earth's axis? 15. When a common almanac makes the statement, " The sun enters Aries," does it refer to the sign or to the constellation? 1 6. From an almanac which gives the times of rising and setting of the sun, find out the number of hours of daylight on the longest day of the year, and also on the shortest, at the place for which the almanac is computed. 17. When it is summer in the northern hemisphere, what is the season in the southern hemisphere? 1 8. At the place where you live does the sun rise very near the east point of the horizon in midsummer (June 2Oth) ? Answer the same question for Christmas time. THE EARTH. 85 19. In each of the four figures here given, the earth is repre- sented as illuminated by the sun at a certain time of the year. Determine the time of year which corresponds to each figure. Fig. So. Fig. 81. Fig. 82. Fig. 83. 20. If a man were perched on the top of a balloon, which was rising straight up through a shower of rain falling vertically, in what position would he have to hold the glass tube of 116 so that the raindrops would go straight through it, without touching the sides? If the earth be moving directly toward a certain star, would aberra- 86 DESCRIPTIVE ASTRONOMY. tion cause a telescope pointed at a star to deviate a trifle from the real direction in which the star lay? 21. (a) Does refraction make sunrise come earlier than it other- wise would, or later? {b} What is its effect on the time of sunset? (c) Does it lengthen the number of hours of daylight, or shorten them? 22. (a) When the moon is seen near the horizon, which edge (or limb, as it is called) is lifted more by refraction, the upper or lower? (#) What effect does this have on the apparent shape of the moon? (c) Does refraction cause the moon to look larger, when near the horizon, than when high in the heavens? 23. At your home does twilight last longer in midsummer, or in midwinter? 24. Denver is in longitude 105 W. of Greenwich, and in north latitude 39 40'. Considering the earth as a perfect sphere, what are the latitude and longitude of the point on the earth's surface which is diametrically opposite Denver? 25. Two places in England have the same latitude, and differ i in longitude. Two places in Ohio differ i in latitude, but have the same longitude. Are the places in England as many miles apart as those in Ohio? CELESTIAL MEASUREMENTS. CHAPTER VI. CELESTIAL MEASUREMENTS. " Snatch me to heaven ; thy rolling wonders there, World beyond world, in infinite extent, Profusely scattered o'er the blue immense, Show me : their motions, periods, and their laws, Give me to scan." THOMSON. 120. Position of Points on a Sphere. In describing the posi- tions of points on the surface of a sphere, a fundamental circle is first assumed, bisecting the sphere. In Fig. 84, ABCD is the funda- mental circle. Great circles are those whose planes pass through the centre of the sphere. All others drawn on the spherical surface are small circles, GPH is a small circle. Secondary circles are great circles which are perpendicular to the funda- mental circle. All secondaries pass through two points called the poles of the fundamental circle. E and F are the poles of ABCD ; AECF and EBFD are secondaries. The fundamental point is a point of reference, which lies on the fundamental circle. In the figure, A is chosen as the fundamental point. In finding the position of a point on a sphere, we first draw a secondary through the point. If P is the point, EPBFD is the secondary drawn. We then find out two things: first, how many degrees (measured on the secondary) the point is above or below the fundamental circle, that is, the length of PB ; second, the number of degrees in that H Fig. 84. CIRCLES OF REFERENCE. 88 DESCRIPTIVE ASTRONOMY. arc (AB) of the fundamental circle which lies between the funda- mental point (A) and the secondary (EPBFD). This system has already been employed in defining the latitude and longitude of a place on the earth ( 93). 121. The Horizon System. In this system the horizon is the fundamental circle. As shown in 21, the horizon of any place is the circle in which the Celestial sphere is cut by^i plane perpen- dicular to a plumb-line suspended at the place. The secondaries; being perpendicular to the horizon, are called vertical circles. All vertical circles pass through the zenith and nadir ( 22, 23). The south point of the horizon is usually taken as the fundamental point. The altitude of a celestial object is the portion of its vertical circle lying between it and the horizon. Its azimuth is the arc of the horizon, measured from the south point around towards the west, to its vertical circle. The zenith distance of a celestial object is the portion of its vertical circle lying between it and the zenith. In Fig. 84 the altitude of P is PB, its zenith distance is EP, and its azimuth (if A be. the south point of the horizon) is ADCB. The celestial meridian of any place is that vertical circle which passes through the north and south points of the horizon. It is divided by the poles into two equal branches ; the upper branch is that in which the zenith lies. The prime vertical is that vertical circle which passles through the east and west points of the horizon. 122. The Equator System. Here the celestial equator is the / fundamental circle. The secondaries are called hour circles ; all hqur circles pass through the celestial poles. The fundamental point is the vernal equinox. The declination of a celestial object is the portion of its hour circle between it and the celestial equator. When a star is north of the equator, it is in north declination ; when south, it is in south declination. North declinations are accounted positive ; south, negative. The right ascension of a celestial object is the arc of the celestial equator, measured eastward from the vernal equinox to the hour circle on which the star lies. CELESTIAL MEASUREMENTS. 8 9 R Right ascension may be measured in degrees, but is usually reckoned in hours, like longitude, 15 being equivalent to one hour. The right ascensions and declinations of \h.z fixed stars change very little from year to year. The north polar distance of a celestial object is the portion of its hour circle lying between it and the north celestial pole. Instead of the vernal equinox, another fundamental point is fre- quently taken, viz. the point where the celestial meridian of the place of observation cuts the celestial equator. Then the hour angle of a celestial object is the arc of^the equator em- braced between the meridian and the hour circle of the pbject. Hour angles are reck- oned either east or west of the meridian, east hour angles being accounted negative : they are usually reckoned in hours. In Fig. 85, NESW is the horizon, EQWR the celestial equator, PABP' half of the hour circle of the star A, and V the vernal equinox. AB is the declination of A, VWQB its right ascension, and OB its east hour angle. The hour angle of W, the west point of the horizon, is Q W, which equals +6 h. : that of the east point E is QE, which equals 6h. 123. Parallax. The word parallax is a broad term. In general, it means the difference in direction of an object when viewed from two different standpoints : it is the angle formed by two lines drawn from the object to the two standpoints respectively. Thus, the parallax of the moon, as seen from Boston and the Cape of Good Hope, is the angle between lines drawn from the moon's centre to these places. Usually the earth's centre is regarded as one of the standpoints : then the parallax of Venus, for example, as seen from Denver at any instant, is the angle made at Venus by two lines drawn from its Fig. 85. THE EQUATOR, HORIZON, MERIDIAN, ETC. 9 o DESCRIPTIVE ASTRONOMY. centre to Denver and the earth's centre respectively. It is the angle DVC in Fig. 86. If the object is in the plane of the hori- zon of the place of observation, its parallax is called the horizon- tal parallax. DV'C is the horizontal parallax of an object at V'. If we consider the earth as a perfect sphere, the angle V'DC is a right angle. We know 'V the length of DC, the earth's radius. When the value of the angle V is known, it is easy to compute the distance CV by elementary trig- onometry. If D be a point on the earth's equator, CV'D is the equatorial hori- zontal parallax of an object situated at V. The parallaxes of the sun, moon, and planets are large enough to be measurable ; but the fixed stars are so distant that the angle CV'D is too small to be measured. In getting the parallax of a fixed star, the two lines are drawn from the star to the earth and sun respectively. Stellar parallax is discussed in 350. Fig. 86. PARALLAX. TIME. 124. The Year. The two principal kinds of years have been explained already in 113. The principles of the Julian and Gre- gorian calendars have been set forth in 114 and 115. 125. Solar Days. There are two kinds of solar days, apparent solar and mean solar. An apparent solar day is the interval of time which elapses between two successive passages of the sun across the upper branch of the celestial meridian of any place. The earth rotates on its axis at a uniform rate, so that, if the sun were fixed on the celestial sphere, all solar days would be of ' equal length. But the sun appears to creep slowly eastward on the sphere, on account of the yearly revolution of the earth, and at an irregular rate, creeping more on some days than others, as will be explained CELESTIAL MEASUREMENTS. in the next section. Hence apparent solar days vary in length, the greatest variation being nearly a minute. The sun being so irregular a timekeeper, astronomers have devised a fictitious sun, called the mean sun, which moves in the equator at a uniform rate^ completing its apparent journey around the celestial sphere in a year. The mean sun crosses the meridian sometimes a few minutes in advance of the true sun, at other times a few minutes behind it. The greatest difference is sixteen minutes. A mean solar day is the interval between two successive passages of the mean sun over the upper branch of the celestial meridian of any place. All ordinary clocks and watches are regulated by mean solar time. 126. Causes of the Unequal Lengths of Apparent Solar Days. ^- There are two principal causes : I. The earth in travelling around the sun does not move at a uniform speed. When at perihelion it moves most swiftly; at aphelion, with the least velocity. Since the apparent motion of the sun eastward is due to the earth's revolution about it, the distance over which the sun creeps each day on the sphere varies, being greatest when the earth's motion is most rapid. II. But even if the sun moved at a uniform rate in the ecliptic, apparent solar days would still vary in length. Suppose that the mean and true suns are to- gether at the vernal equinox, V, in Fig. 87. After one fourth of a year has elapsed, each sun, if moving uniformly, will have described an arc of 90 ; the true sun will then be at S, the summer solstice, and the mean sun at Q, both lying on the same hour circle, PSQ. Let M be the middle point of VQ, and draw the hour circle PM cutting the ecliptic at R. One easily sees that the arc VR is longer than VM, so that, when the mean sun arrived at M. the true sun had M Fig. 87. UNEQUAL LENGTHS OF APPARENT SOLAR DAYS. DESCRIPTIVE ASTRONOMY. not reached R. Hence the mean sun and the true sun, though together on the same hour circle at the beginning and at the end of their three months' race, did not keep on the same hour circle during the race. Now any celestial objects which are on the same hour circle cross the meridian at the same time. Hence, while the mean sun by its successive passages across any given meridian was mark- ing off days of uniform length, those marked off by the true sun were not of uniform length. 127. Another Explanation. On a sphere, (the larger the better,) draw the ecliptic and equator. Mark the summer solstice, and put another mark, a quarter of an inch away, on the ecliptic. Draw an hour circle through each. Place a mark on the ecliptic a quarter of an inch from the vernal equinox and draw hour circles through these two points. One sees at once that the first pair of hour circles make a larger angle with each other than the second pair. The quarter of an inch represents the amount that the sun creeps on the ecliptic in a day. At the time of the vernal equi- nox an apparent solar day would be the time of a rotation of the earth plus the time required for it to turn through the angle between the first pair of hour circles. At the time of the summer sol- stice an apparent solar day would be the time of a rotation of the POLE POLE ECLIPTIC EQUATOR Fig. !. VARIABLE MOTION IN HOUR ANGLE. earth plus the time required for it to turn through the angle between the second pair of hour circles. Since the second angle is larger than the first, the two days would be of unequal length. Fig. 88 exhibits the arcs drawn as directed. 128. A Sidereal Day. A sidereal day is the interval between two successive passages of the vernal equinox over the upper branch of the celestial meridian of any place. Since the apparent daily rotation of the celestial sphere is caused by the real rotation of the earth, sidereal days are of, uniform length. The length of this day might be changed by various causes. A gradual shrinkage of the CELESTIAL MEASUREMENTS. 93 earth in cooling would make it rotate faster : the friction caused by the tidal movement of the water of the ocean tends to impede the rotation. However, no change in the length of a day has ever been detected by observations. It is considered certain that it has not changed a hundredth of a second in the past thirty centuries. 129. Civil Day and Astronomical Day. Each of these days con- sists of 24 hours of mean solar time. The civil day, employed for the ordinary purposes of life, begins at midnight. The astronom- ical day begins 12 hours later than the civil day, at noon. Thus, Sept. 3, 9 P. M. of civil time is equivalent to Sept. 3, 9 h. by astronomical time; but Sept. 3, 9 A. M. of civil time is Sept. 2, 21 h. by astronomical time. An astronomer when observing at night is saved the trouble of changing the date in his record-book when midnight comes. A movement is on foot to discontinue the use of the astronomical day at the end of the nineteenth century. 130. Mean Solar and Sidereal Time. An astronomical clock keeping mean solar time at Denver, for instance, reads oh. o m. o sec., when the mean sun is on the upper branch of the meridian of Denver, at noon. Its face is graduated from o h. to 23 h., and the hour hand sweeps around the dial once in a mean solar day. Such a clock is said to keep " local " mean time. A sidereal clock, on the other hand, reads oh. om. osec., when the vernal equinox is on the upper branch of the meridian. When it reads roh. 30 m. osec., it shows that the vernal equinox crossed the meridian loj sidereal hours ago. A reading 23 h. om. osec. indicates that the vernal equinox crossed the upper branch of the meridian 23 sidereal hours ago, and will cross again in an hour. If the mean sun and the vernal equinox were together on the celestial meridian of a given place, both the sidereal and the mean time clock at that place should indicate o h. o m. o sec. After a lapse of 24 sidereal hours, the vernal equinox would be on the meridian again ; but the mean sun, on account of its eastward motion on the sphere, would be east of the vernal equinox, and would cross the meridian a little while after it. A mean solar day is therefore longer than a sidereal day. As the mean sun travels entirely around the sphere in a year, in a day it moves about o-J^ of the circumference of the sphere, making the mean solar day ^.^ h., or nearly four minutes, longer than the sidereal day. 94 DESCRIPTIVE ASTRONOMY. 131. Right Ascension vs. Sidereal Time. By the definition of right ascension ( 122), we see that a star whose right ascension is 4 h. will cross the meridian of any place 4 h. after the vernal equinox has crossed the same meridian. But the sidereal clock will read 4 h. We therefore have the following principle. The right ascension of any star is equal to tJie sidereal time at any place at the instant when the star is on the meridian of that place. Thus when a star whose right ascension is exactly 16 h. is on the meridian of the U. S. Naval Observatory at Washington, the Washington sidereal clock should read 16 h. o m. o sec. When the sidereal time is 13 h., the star mentioned has an east hour angle of 3 h., because 3 h. must elapse before it will reach the meridian. If the sidereal time be 18 h., the star crossed the meridian two hours before, and hence has a west hour angle of 2 h. 132. Relation between Longitude and Time. If the longitude of a place is one hour west of Greenwich, the mean sun arrives at its meridian one hour after it has crossed the Greenwich meridian ; hence at noontime at the place in question it is I P. M. at Green- wich. When a city is two hours east of Greenwich, the sun crosses its meridian two hours before it reaches the meridian of Greenwich ; when it is 10 A. M. at Greenwich, it is noon at the city. 133. Where the Date Changes. A place whose longitude is 1 80, or 1 2 h. west of Greenwich, is also I2h. east of Greenwich. Reckoning it as 12 h. west of Greenwich, its time will be 12 h. less than the Greenwich time ; so that when it is 1 1 A. M. on July 4 at Greenwich, it will be 1 1 P. M. on July 3 at this place. But if we say that the place is 12 h. east of Greenwich, its time will be I2h. more than the Greenwich time at the same instant, mak- ing ii P. M. of July 4. Thus there is a discrepancy of one day, according as we count east or west from Greenwich. Mariners when crossing the iSoth meridian change the date; in going west, an entry in the log-book just before crossing the line might be dated Wednesday, Sept. 12, 3 P.M. An entry made an hour after- ward, if the ship had crossed the line meanwhile, would be dated Thursday, Sept. 13, 4P.M. Similarly, in crossing from the west, Tuesday, October 9, would be changed to Monday, October 8. 134. Standard Time. There is now in use throughout North America a system of standard time, which is a great boon to the CELESTIAL MEASUREMENTS. 95 business world. Five standard meridians have been adopted, west of Greenwich 4, 5, 6, 7, and 8 hours respectively. A city generally adopts the time of that standard meridian to which it is nearest. The standard times are called respectively Colonial, Eastern, Central, Mountain, and Pacific. In a few large cities there is still confusion. At Pittsburg both Eastern and Central times are in use. It would be well if all towns in the same State kept the same time. Several European countries have adopted standard times based upon the meridian of Greenwich. 135. Clocks and Watches. Accurate time obtained from various astronomical obser- vatories (chiefly from Washington) is tele- graphed daily all over the United States. The clocks in the observatories are regu- lated with the greatest care, so that they are rarely over a second in error. The pendulum jar of the clock shown in Fig. 89 is nearly filled with mercury. In warm weather the pendulum rod lengthens be- cause of the heat; this lengthening would make the clock go slow, but the mercury expanding rises in the jar, and thus coun- teracts the elongation of the rod. In cold weather there is a similar compensation : such a clock is said to be compensated for temperature. Good watches should be wound regularly and kept in the same position by day and by night. When the second hand is at sixty, the minute hand should be over some minute mark. A sudden change of rate is not a sure sign that the regulator needs to be moved. The best watches exhibit anomalous variations at times, and frequently right themselves without being regulated anew. 136. The Meridian Circle. -- The telescope of this instrument is put in the middle of, and at right angles to, an axis which turns in A STANDARD CLOCK. 9 6 DESCRIPTIVE ASTRONOMY. Fig. 90. A PORTABLE MERIDIAN CIRCLE. CELESTIAL MEASUREMENTS. 97 a bearing at each end as shown in Fig. 90. The axis is horizontal, and points due east and west. Upon the axis are mounted a couple of graduated circles, which are used for finding the altitude of stars when they cross the meridian. At the focus of the object-glass near the eye-end of the telescope is the " reticle," which, as seen through the eyepiece, presents the appearance shown in Fig. 91. It is usually made of spider-webs, or of fine lines ruled on a thin piece of glass. The " wire " AB is parallel to the axis of the instrument. Fig. 91. A RETICLE. 137. A Star passes the Me- ridian. If the telescope be pointed in the direction of a star just before it passes the meridian, it will be seen to move across the field of view in a path parallel to AB, cross- ing each of the five parallel wires. The reason for this is shown in Fig. 92. A line drawn from the star at S through the centre of the object-glass meets the star's image at I. As the its Fig. 92. MOTION OF A STAR'S IMAGE. star moves to S' and S" image moves to V and V . Therefore, the observer sees the star's image crossing the wires of the reticle. If ST is perpendicular to the rotation axis, it lies in the plane 7 98 DESCRIPTIVE ASTRONOMY. of the meridian, for the meridian plane is perpendicular to a hori- zontal east and west line. Therefore both the star S' and its image I' lie in the plane of the meridian. The reticle is so placed that the star's image when at I' lies on the middle wire in Fig. 91. 138. Determination of Clock Error. In the Nautical Almanac, 1 which is issued yearly, is to be found an extensive list of bright stars, the right ascensions of which are given. The right ascension of any star, as we have learned, equals the sidereal time when that star crosses the observer's meridian. The* astronomer who wishes to find the error of his sidereal clock selects from the list a star, Sirius, for example, and points the telescope of the meridian circle in such a direction that Sirius when it crosses the meridian will pass through the field of view. As it crosses each wire of the reticle he notes the reading of the clock as below: h. 6 m. 39 sec. 46.3 39 5 6.2 40 II.4 40 26.6 40 36.6 6 40 11.42 Average, The average of these readings is a pretty accurate value of the clock reading when the star crossed the middle wire, which repre- sents the meridian. Turning to the almanac, he finds that on the date of observation the right ascension of Sirius was 6 h. 40 m. 26.94 sec. Hence, if the instrument was in perfect adjustment, the error of the clock was 15.52 sec. Was the clock fast or slow ? To attain greater accuracy, the astronomer observes a number of stars. Four stars, after allowance had been made for errors in the position of the meridian circle, might give him for the clock error respectively 15.52 sec., 15.46 sec., 15.53 sec., and 15.45 sec. The average of these is 15. 49 sec. By comparing observations made on two dates, the amount that a clock gains or loses in a day can be found. This amount is the 1 A copy of this work, if not obtained through a Senator or Representative, may be had by sending one dollar to the Nautical Almanac office, Washington, D. C. The British, German, French, and some other nations publish such almanacs. CELESTIAL MEASUREMENTS. 99 daily rate. Sidereal time is easily reduced to mean time by tables given in the Nautical Almanac. 139. The Chronograph. A chronograph is employed to facilitate noting the time when any phenomenon occurs. Its most common use is to record the reading of a timepiece at the instant when an astronomer sees a star cross a wire in the reticle of his meridian circle. - Fig. 93. A CHRONOGRAPH. A sheet of paper is wrapped around the barrel, which is rotated once a minute by the mechanism, driven by a weight not shown in the figure. If this were the only motion, the pen which rests on the barrel would draw a circle around the cylinder in a minute, and repeat the same circle the next minute, and so on, as long as the mechanism ran. But the carriage which holds the pen is mounted on a long screw, shown in front of the barrel : the screw rotates once a minute, and continually moves the pen carriage, which is set near one end of the barrel at starting, toward the other end of the barrel. Consequently the pen, instead of making a single circle over and over, makes a long spiral line, like a screw thread, running from one end of the barrel to the other. IOO DESCRIPTIVE ASTRONOMY. 140. Records of a Clock and Key. By suitable electrical connec- tions, which cannot well be explained here, a clock causes the pen to give a quick vibration each second, so that a series of notches are made in the line, as shown in Fig. 94. No notch is made at the /6 /5 /** I/ 10 9 8 7 6 S 4* 3 2. Fig. 94. A CHRONOGRAPHIC RECORD. fifty-ninth second of each minute : the first notch thereafter marks the beginning of a new minute. The observer notes the time which the clock read when some particular notch was made, and marks that notch. In Fig. 94 the notch for 9 h. 6 m. o sec. was thus marked : from this record he can tell the time when any other notch was made by the clock. The astronomer, when observing, has at his side a telegraph key, connected with the chronograph : when he wishes to note the time, he presses the key quickly, and the pen makes a notch at that instant. Fig. 94 shows some records made between 9 h. I m. and 9 h. 6 m., on a small portion of a sheet. In the place where the fifty-ninth second of each minute is omitted, the observer has written on the lines the numbers of the successive minutes. The numbers of the seconds of each min- ute are written along one of the lines. There are three extra marks, made when the observer pressed his key. One of the marks was made at 9h. I m. 4.5 sec., another at 9h. 3 m. 9.4 sec., and the third at Qh. 4m. 13. 8 sec. 141. Determination of Latitude. Let D in Fig. 95 be a point on the earth's surface, NS its horizon, P'P" the earth's axis, and EQ Fig. 95- CELESTIAL MEASUREMENTS. 101 the equator. ZDO is perpendicular to NS. PD is parallel to FP", and therefore points to the north pole of the celestial sphere. DT is parallel to EO. PDT is a right angle. By definition ( 94) the latitude of D is DOE. This equals ZDT = 90 - ZDP = PDN, which by definition ( 12 1) equals the altitude of the celestial pole. Hence the latitude of the place of observation equals the alti- tude of the pole. Though an astronomer cannot see the north celes- tial pole, he can find its altitude by observing the Pole Star when it is on the meridian. In Fig. 96, D, is the place of observation, NS the horizon, NZS the meridian, P trie north celestial pole, P' and P" the two positions of the Pole Star when it crosses the meridian. By M Fig. 96. LATITUDE FOUND BY OBSERVATION OF POLARIS. means of the divided circle on the axis of the meridian circle, the astronomer measures the angle P'DN, the altitude of the Pole Star when it is on the meridian above the pole. Twelve hours later he measures P"DN. P'DN = PDN + PDF F'DN = PDN - PDP" FDN + P"DN = 2 PDN + PDF PDF'. But PDP' == PDP". Therefore FDN + P"DN = 2 PDN PDN = $ (FDN + P"DN). PDN is the latitude required. The method by which the angles P'DN and P"DN are measured will be understood readily by examining an engineer's transit, such as is shown in Fig. 97. 102 DESCRIPTIVE ASTRONOMY, . 97. AN ENGINEER'S TRANSIT. CELESTIAL MEASUREMENTS. 103 142. Determination of Longitude. We have seen ( 132) that the difference in longitude between two places, such as Washington and Chicago, is equal to the difference at any instant between the readings of two clocks, one of which keeps correct Washington time, while the other keeps Chicago time. If a chronometer keeping Washington time be carried to Chicago and compared with the Chicago clock, the difference between their readings, if both are Fig. 98. A CHRONOMETER. correct, is the difference of longitude sought. In practice several chronometers are used, the errors of which for no chronome- ter or clock runs exactly right are very carefully determined by observations of the stars. .The electric telegraph, however, furnishes a much more accurate method. By quite simple mech- anism the Washington clock, as it ticks, makes a telegraphic sounder at Chicago click, so that an astronomer at Chicago can 104 DESCRIPTIVE ASTRONOMY. compare the telegraphic beats of the Washington clock with his own. The times at which the signals are to be sent are agreed upon beforehand. 143. The Position of a Ship. A mariner usually finds the lati- tude and longitude of his ship by observations of the sun made with a sextant, a little instrument easily held in the hand. A chronometer keeping some standard time, as that of Greenwich, is also necessary. In Fig. 96, let M represent the sun when on the meridian. The mariner measures its altitude, MS, with the Fig. 99. A SEXTANT. sextant. From the almanac he finds PM, the distance of the sun from the pole. The sum of these gives PMS, which, subtracted from 1 80, leaves PM, the latitude desired. To find the longitude, the altitude of the sun is measured about the middle of the forenoon or afternoon, and the reading of the chronometer, keeping Greenwich time, is noted at the same time. Suppose that the measured altitude was 62 15' 20." By means of astronomical tables the mariner computes the time at which the sun had that altitude; it might have been 3 h. i6m. 27 sec. If CELESTIAL MEASUREMENTS. 1 05 the Greenwich time, found from the chronometer, 1 was 4h. 29m. 48 sec., the ship was evidently in longitude I h. 1301. 21 sec. west of Greenwich. EXERCISES. 144. i. Is the arctic circle on the earth a great circle or a small circle? 2. Consider the earth as a perfect sphere, and the equator as the fundamental circle ; what is the name applied in geography to the secondaries? 3. Consider the earth as a perfect sphere. In estimating the longitudes of points from Greenwich, what point is taken as the fundamental point? -^ 4. (a) When the sun is just rising, what is its altitude? -(^) When a star is in the zenith, what is its altitude? 5/'(tf) What is the azimuth of a point on the prime vertical, the p\>int being west of the zenith? ~(b) What is the azimuth of the north celestial pole? -() What is the azimuth of the east point of the horizon? 6. (a) Does the celestial meridian of any place pass through the nadir? _(#) Do the celestial poles lie on this meridian? (<;) Is there any point on the earth where the meridian coincides with the celestial equator? *-(d) Does your celestial meridian cut the celestial equator? (V) If so, can you point your finger toward a point of inter- section? ^(/) Where does the prime vertical cut the meridian? ~{g) What position has the plane of the prime vertical with refer- ence to that of the meridian ? 7. (a) The declination of a star is +20 ; what is its north polar distance? 1 The reading of the chronometer face is not the true Greenwich time. Before leaving port the error of the chronometer (usually a few seconds) and the daily rate ( 138) were found by astronomical observations. From these the error of the chro- nometer at any time during the voyage can be computed, and allowance made for it to get the true Greenwich time. While chronometers do not keep exactly the same rate from week to week, they run closely enough for the practical purposes of navigation. IO6 DESCRIPTIVE ASTRONOMY. ~~(^) What is the north polar distance of a star whose declination is -30 43'? -(c) What is the right ascension of the autumnal equinox? ^d) W T hat is the sun's right ascension, when it is in the summer solstice? j(^) What is its right ascension when in the winter solstice? (f) If the vernal equinox be on the meridian of Chicago, what is the right ascension of a star which is rising at that instant at the east point of the Chicago horizon? "(") At the same instant as above a star is setting at the west point of the horizon; what is its right ascension? 8. (a) If a star having a right ascension of .18 hours is now on the meridian, what is the right ascension of a star which now has an east hour angle of 3 hours? ^. (b) When a star the right ascension of which is 8 hours is on the meridian, what is the right ascension of a star which has a west hour angle of 5 hours? (c) May a number of different stars have the same right ascension? (d) May a number of stars have the same declination? (e) Is it possible for two stars to lie on the same hour circle, and have different right ascensions? r (/) If the right ascension of a fixed star now is 1 1 h. 28 m., what is it 3 hours hence? ^- 9. (a) When a correct sidereal clock reads 17 h., the hour angle of a certain star is 5 h. west. What is the star's right ascension ? * () The sidereal time being 17 h. 26m., a star is found to have an east hour angle of 4h. ; what is the star's right ascension? 10. Reduce March 9, 7 A. M., civil time, to astronomical time. ^-u. The sun and a star are in the vernal equinox, and both are setting below the horizon of some place. - (#) A month afterwards, which will set the earlier? \b) Which will rise the earlier? ' \c) On July 2Oth, a certain star sets below the horizon of Boston at 8 P. M. A month afterwards, will it set at about the same time? 12. By use of the telegraph it was found that when a Washing- ton clock read 9h. o m. o sec., a St. Louis clock read 8 h. 7m. 22.93 sec. The Washington clock was 26.37 sec. f ast > and the St. CELESTIAL MEASUREMENTS. IO7 Louis clock was 15. 92 sec. slow. What is the difference of longi- tude between the two places? 13. An astronomer noted the following readings of his sidereal clock when a certain star crossed the wires in the reticle of his meridian circle. h. 111. sec. 13 14 iy.2 14 30.6 14 44.0 14 57-4 15 10.7 The right ascension of the star was I3h. 1401. 3 1.68 sec. If the meridian circle was in perfect adjustment, what was the clock error? 14. When Polaris is on the meridian above the pole, an observer measures its altitude, finding it to be 39 46'. Twelve hours later its altitude is 37 18'. What is the latitude of the place of observation ? 15. (a) If the altitude of a star is 16, what is its zenith distance? rr () If the declination of a star is +34 5', what is its north polar distance? ^.Ajc) The north polar distance of a star is 116 35'. What is its declination? 1 6. A mariner measures the altitude of the sun at noon, getting 69 47' 25" as his result. From the almanac he finds that the sun's declination at the time when the altitude was measured was 6 37' 49". Find the latitude of the ship's position. 17. At midnight on July 25th, a chronometer was 38.92 sec. slow. Its daily rate was 0.84 sec. gaining. Assuming that it kept this rate, what was its error at noon of July 3 1st? 1 8. The captain of a vessel measures the altitude of the sun on the afternoon of May i6th; his chronometer (keeping Greenwich time) reads 4 h. i6m. 29 sec. when the altitude is measured. By means of data given in the almanac, he computes that the local time when the sun had the measured altitude was 3 h. 27m. 58 sec. What was the longitude of the ship at the time of the observation ? 108 DESCRIPTIVE ASTRONOMY. CHAPTER VII. THE MOON AND ECLIPSES. " In full-orbed glory yonder moon divine Rolls through the dark blue depths." SOUTHEY. 145. Distance, Diameter, Orbit, Nodes. The moon, though the most conspicuous of all the heavenly bodies except the sun, is really quite small, being only 2,163 miles in diameter. Its average distance from the earth's centre is 238,840 miles. As the earth journeys around the sun in an ellipse, so the moon travels around the earth in a similar orbit, the earth being at one of the foci of the ellipse. The orbit is nearly a circle, and is inclined to the plane of the ecliptic only 5. The moon appears to us to de- scribe a circle on the face of the sky every month, the circle being roughly coincident with the ecliptic. The moon's path intersects the ecliptic at two opposite points, called the moon's nodes. 146. Periods, Sidereal and Fi?. 100. ORBITS OF THE EARTH AND MOON. ,. -r^, 7 / , Synodic. The sidereal period of the moon is the time required for making one revolution about the earth. In Fig. 101 the sun, the earth, and the moon are in line, in the positions S, E, and M. The earth and the moon pursue their appointed paths until they arrive at E' and M' respectively, the line E'M' being parallel to EM. The moon has now accomplished a complete revolution ; the time required is nearly 2/| days, which is therefore the sidereal period. But the moon is not yet opposite the sun, as it was at the start. When it reaches M", the earth being at E", it is opposite the sun again, and a synodic revolution has been accomplished. The synodic period 'is over 29! days. THE MOON AND ECLIPSES. IO9 147. Time of Crossing the Meridian. Since the moon moves rap- idly eastward among the stars, it does not cross the meridian of the observer at the same time every day, but crosses about 5 1 min- utes later on the average each day than the preceding. The amount of daily retardation varies considerably from causes analogous to those which cause the sun to be an irregular timekeeper. ( 126.) A; Fig. 101. SIDEREAL AND SYNODIC PERIODS. Fig. 102. ILLUSTRATION OF THE MOON'S ROTATION. 148. Rotation. The moon always presents the same face to us : an unthinking person might conclude from this that it did not rotate at all. Let a boy trace a circle on the ground around a tree, and station himself south of the tree and facing it ; he then faces north. Let him walk around the circle, continually facing the tree. At S in Fig. 1 02, he faces north, at E west, at N south, and at W east. When he arrives at S again, he has turned completely around once. 149. Librations. The moon rotates on its axis at a uniform rate, but since it does not move with uniform rapidity in its orbit, we sometimes see a short distance around one edge or the other. In Fig. 103, when the moon is at M, we see the portion ACB. When moving more swiftly than its average, it will describe 90 of its orbit and arrive at M' in a little less than one fourth of its sidereal period. So it will not have rotated one fourth of a complete turn, and an observer at the earth, though not able to see the point A, will look past B, as shown by the figure. no DESCRIPTIVE ASTRONOMY. Furthermore, the moon does not stand quite upright on its orbit, that is, its axis is not perpendicular to the plane of its orbit. Hence, as shown in Fig. 104, we sometimes see past the north pole and sometimes past the south pole. These apparent oscillations are called librarians. There is a minute libration due to the fact that we are not at the earth's centre. N EARTH Fig. 103. LIBRATION. Fig. 104. LIBRATION. When the moon is rising, we can see farther over its upper edge than a man whose eye is at the earth's centre. Fifty-nine per cent of the moon's surface has been seen by astronomers. 150. Phases of the Moon. The moon shines by reflecting the sunlight which strikes it. When the moon is between us and the sun, its illuminated side being toward the sun, we see the dark side. The moon is then said to be new. A week later, when it has moved from A to B (Fig. 105), half of the illuminated hemisphere is visible to us, and the moon is said to be at its first quarter. After the lapse of another week it is at C, opposite the sun, and the whole of the illuminated hemisphere can be seen by us ; this phase is full moon. A week thereafter, when the moon has reached D, it is in its last quarter, half the bright hemisphere being visible. For a week before and after new moon, when but a small part of the moon looks bright to us, it is crescent. During the week preceding full moon, and also during the following week, when more than one half and less than the whole of the bright part of the moon is turned THE MOON AND ECLIPSES. I II toward us, the moon is gibbons. The appearances of the moon are shown in Fig. 106. 151. Earth Shine. When the moon is crescent, one easily sees the dark portion of it, as well as the brilliant crescent of light. The dark part is bounded on the side next to the sky by a narrow rim of silvery light, so that the whole looks not unlike a cake basket hung in the sky : it is popularly called " the old moon in the new moon's arms." Why is the dark part seen so easily? Some of the sunlight which falls upon the earth is reflected away, and, striking upon the side of the moon turned toward us, illumi- nates it sufficiently to render the dark part visible to us. 152. Occupations. The moon in its monthly round passes be- tween us and many of the stars, hiding them from view so that they are occulted (hidden) for a time. The occultation of bright stars 105. THE MOON ILLUMINATED. NEW CRESCENT WAXING FIRST QUARTER FULL GIBBOU5 WANING LA5T QUARTER GIBBOUS WAXING CRESCENT WANING Fig. 106. THE MOON'S PHASES. can be observed with the naked eye. The fainter ones are blotted 112 DESCRIPTIVE ASTRONOMY. out by the moon's brilliancy as it approaches them. At the instant when the limb of the moon gets into line between the star and the observer's eye, the star vanishes as if annihilated. After an hour or less, the star reappears on the opposite side of the moon. Both the disappearance and the reappearance are instantaneous. Observa- tions of occultations are sometimes made by mariners to find the Fig. 107. THE MOON: PHOTOGRAPHED AT THE LICK OBSERVATORY. errors of their chronometers : from the data given in the Nautical Almanac the Greenwich time of the occurrence of an occultation of a given star, as seen at a given place, is computed. The chronom- eter reading is noted at the time of the star's disappearance ; by comparing this reading with the computed time the error of the timepiece is found. UNIVERSITY THE MOON AND ECLIP^So f 0=0^^ I 13 153. Appearance to the Naked Eye. To the naked eye, the face of the full moon appears to be diversified with irregularly shaped dark spots. Most people see a strong resemblance to a human face. Many perceive a complete human figure, said among the French to be Judas Iscariot, transported thither as a punishment. Humboldt states that it is a popular belief among the people of Asia Minor that the moon is a mirror, which reflects back the image of the earth. When one examines the moon with an opera- glass, the dark spots are seen to be the smoother portions of the moon's surface. They are simply vast plains : on the maps they are designated as seas, being thought by the early lunar cartogra- phers to be such. 154. Use of the Telescope. It is well at first to put on a low magnifying power, so that the whole of the moon may be in the field of view at once. At the time of full moon the view is very much less satisfactory than at the first quarter (Fig. 107), and for three days thereafter. For at the time of full moon the shadows of the mountains on the moon are invisible to us, because they are cast directly behind them, and we lose the effect of contrast between the objects and their shadows. At the time of the first quarter, those mountains which are near the terminator (boundary between the illuminated and unilluminated portions of the moon) cast magnificent shadows ; the rugged details of these mountainous forms are then very conspicuous in the telescope (Fig. 113). After viewing the entire lunar disk with a low power, higher powers may be tried with advantage on the more conspicuous objects. They bring out a wealth of detail, which wellnigh baffles delineation. No telescope ever yet constructed can bear with advantage, even on the finest night, a power exceeding 3,000 diameters : such a power would bring the moon within 80 miles. Objects as large as the largest buildings on the earth might be perceived, if they differed considerably in color from the back- ground upon which they were seen. 155. Lunar Topography. The features of the landscape maybe divided into the following classes : plains, craters, mountain peaks, mountain ranges, rills, clefts, and rays. Some hundreds of these objects have received names : a few of the most prominent ones DESCRIPTIVE ASTRONOMY. are shown on the accompanying skeleton map (Fig. 108), in which the moon is represented as seen in an inverting telescope. The numbered craters have the following names : 1. Clavius. 2. Schiller. 3. Schickard. 4. Tycho. 5. Catharina. 6. Cyrillus. 7. Theophilus. 8. Arzachael. 9. Alphonsus. 10. Ptolemy. ii. 12. 13- 14. 1 S- Gassendi. Maskelyne. Copernicus. Kepler. Eratosthenes. /^'o & q X -N D 7 ^ ,--. fV C 1 6. Archimedes. 17. Burg. 1 8. Aristotle. 19. Plato. Fig. 108. SKELETON MAP OF THE MOON. 156. The Plains. These are, as before remarked, darker than the rest of the surface, and smoother. With low powers they look much as if they were dry beds of ancient seas. Under a high power many minute pits are discovered besprinkling the plains ; THE MOON AND ECLIPSES. 115 the surface is found to be really quite rough and wrinkled. The boundaries of these " seas," as they are denominated on the map, are not always sharply defined ; in some cases, the bounding " sea-wall " is nearly complete, and composed in part of pre- cipitous cliffs, which exceed in grandeur any similar terrestrial formations. Fig. 109. CONSPICUOUS CRATERS. (Nasmyth and Carpenter.) 157. Craters. Even with an opera-glass one may see that the brighter portions of the moon's surface are thickly bestrewn with irregular ring-shaped mountains. Kepler 1 conjectured that these 1 The great astronomer, 1571-1630, who discovered that the orbits of the. planets are ellipses, and formulated certain famous laws concerning their motion. n6 DESCRIPTIVE ASTRONOMY. were pits dug by the inhabitants of the moon to shelter themselves from the sun during the long lunar day. Had he known that a large number of these pits are over fifty miles in diameter, and that the walls of some of them are three or four miles high, he would hardly have broached this theory. The craters present a wonderful diversity of size and aspect. Thou- ''.-; *&' %fe |il|^il lilll _ Fig. no. THE CRATER COPERNICUS. (Nasmyth and Carpenter.) sands are so minute that in the most powerful telescope they look like mere pin-pricks, being half a mile or less in diameter. The largest ones are over one hundred miles across, and are perhaps more properly called walled plains, especially if the floor of the crater is quite smooth. In the centre of a crater a mountain or a small group of peaks is usually found. THE MOON AND ECLIPSES. 117 The interiors of most of these craters are lower than the general surrounding level, but in some cases the interiors are elevated above the general surface. Some are isolated : others are crowded so thickly together that they overlap. Some walls are very pre- cipitous ; others are a series of magnificent terraces. The im- mensity of some of these formations is realized by looking at the cut of Copernicus (Fig. no). The larger of the craterlets around it are as big as Vesuvius. In the figure, these, as well as the other features to be described, are easily distinguished. Fig. in. COPERNICUS. 158. Nature and Cause of the Craters. The craters are frequently referred to as the moon's volcanoes. It would be a mistake to infer from this that there are any signs of present volcanic activity on the moon. The entire appearance points to the theory that the moon was once a molten mass, and that by its cooling and solidification its various topographical features were formed. Similar appearances are to be found upon the earth; Fig. 112 shows the marked similarity of the neighborhood of Vesuvius to a portion of the moon. On the cooling tap cinder from the fur- naces for the production of iron there is formed a thin crust, which is soon broken open in spots by the pressure of the con- n8 DESCRIPTIVE ASTRONOMY. fined gases ; the molten matter exudes through the holes and cracks, and frequently forms miniature volcanic cones. The cen- tral mountains in lunar craters were probably formed by the last sluggish oozings from the heated interior. 159. Mountains and Mountain Ranges. Isolated mountain peaks are comparatively rare, though occasionally one can be found rising to a height of a mile or more out of a comparatively smooth landscape. There are a few mountain chains, the most prominent Fig. 112. THE TERRESTRIAL CRATER VESUVIUS. (Nasmyth and Carpenter.) of which is named the Apennines ; it is 450 miles long, and bristles with peaks which rival the Andes in altitude, and cast magnificent shadows nearly 100 miles long athwart the neighboring plains. By measurement of these shadows the heights of many peaks have been determined. On one side, as shown in Fig. 113, they rise gradually from the plain, but on the other they are terminated by dizzy precipices, some of which are over three miles high. 160. Rills, Clefts, and Rays. Rills are narrow, deep, and tor- tuous valleys, which look like the beds of dried up streams. THE MOON AND ECLIPSES. 119 Clefts are narrow rifts of great depth. Two fine ones are shown in Fig. 113, starting from opposite sides of the largest crater: each is over 100 miles in length ; near the centre it is a mile in width. They are thought to be not less than ten miles in depth, and must be appalling in grandeur to a lunar traveller. Fig. 113. THE LUNAR APENNINES. (Nasmyth and Carpenter.) Rays are streaks which diverge in all directions from some of the craters. They are best seen at the time of full moon. The finest system radiates from Tycho, which, with an opera-glass, looks as if it were a pole of the moon, the rays being meridians diverging from it. Copernicus has a smaller system, shown in Fig. 109. The rays are on the general surface, being neither elevated 120 DESCRIPTIVE ASTRONOMY. above nor depressed below it; they go over crater walls and through valleys, just as if some one had painted them there with a gigantic brush after the landscape had assumed its present form. No satisfactory explanation of these has been given. Possibly Fig. 114. THE CRATER VENDELINUS : DRAWN FROM A LICK PHOTOGRAPH. they are discolorations of the surface by vapors rising from cracks too narrow to be visible to us. 161. Changes. There has been considerable discussion among astronomers as to whether any changes have ever been noted in the THE MOON AND ECLIPSES. 121 lunar topography. A given crater may change its aspect in an hour, because of the shifting of its shadow ; such changes are most noticeable when the sun is just rising or setting at the crater (Fig. 114). For this reason, a careful examination, extending over several nights, is necessary to enable one to gain a correct idea of the details of the form of any crater or mountain. When we add to this the usual blurring caused by the unsteadiness of our atmos- phere, and the minute errors which the most skilful draughtsmen are liable to make in their delineations, we can see how easy it is to imagine slight changes where none have really taken place. There is not the slightest evidence that any eruptive forces are at work. Possibly a very few land-slips have occurred. Lunar pho- tography may after some years give us decisive results. 162. Atmosphere. It has been demonstrated that the atmosphere, if it exists, is extremely rare, the pressure not exceeding a thou- sandth of that at the earth's surface. When a star is occulted, it ought, if there were a lunar atmosphere one tenth as dense as that of the earth, to suffer a change of brightness and color, when close to the moon's limb ; further, as one sees the sun after it has really set, on account of refraction, so the time of the star's disappearance would be much retarded by the refraction of the lunar atmosphere, and its reappearance would be accelerated. Twilight causes an illumination of the terrestrial landscape for some time after the sun has set. No marked illumination of this sort has been seen at any point on the moon. The lunar spectrum is identical with the solar ; this shows that the sun's rays when reflected from the moon suffer no noticeable absorption by its atmosphere. 163. Water. What may be on the side of the moon which we never see, we cannot affirm, but there is no reason to think it different from the face presented to us. Any lake covering as much as a square mile, if not hidden from view by some obstruction, would have been discovered ere this. On account of the coldness of the lunar days, as well as nights, it is not unlikely that water, if present, would exist only in a frozen state. There are no indications of either ice or snow. 164. The Water and Air Formerly. If the moon was once, as is generally supposed, a portion of the earth, it must have carried 122 DESCRIPTIVE ASTRONOMY. some water and air with it when they separated, though the earth kept the lion's share, on account of its stronger power of attraction. What has become of the water we can only conjecture : great cav- erns may have been formed in the process of cooling, into which both the air and water have sunk. The water may have become chemically united with the molten rock in the process of crystal- lization. A rock when heated expels gases formerly absorbed ; in cooling slowly it can absorb them again ; perchance the lunar atmosphere was absorbed in this way. Still another theory is based on the exceedingly swift motion of the molecules of gases. The force of gravity at the moon's surface is but one sixth of that at the earth's, so that a rifle bullet there would " carry" 100 miles. Some have thought that the molecules of the lunar atmosphere may have escaped from this feeble attraction and gone off into space, never to be recovered ; but this theory will not stand a critical examination. 165. Light and Heat reflected to the Earth. Five sixths of the sunlight which falls upon the moon is absorbed, the rest being reflected away. The sun gives 600,000 times as much light as the full moon ; yet the full moon in mid-heaven gives sufficient light to enable one to read this page. The measurement of the heat sent us by the moon is difficult, because its amount is " vanishingly small." If the full moon could shine upon us steadily for a year, it would give us as much heat as the sun does in three minutes. 166. Temperature at the Moon. During the long lunar day the sun blazes upon the moon's plains with a fury unmitigated by a protecting atmosphere, and untempered by the presence of clouds ; yet the plains are cold. The air of our own planet acts as a blanket to keep us warm. The solar rays pierce the atmosphere readily and find lodgment in the earth ; but when the earth strives to radi- ate its heat back into space, the air checks the radiation. On lofty mountain tops, over which there is a much thinner air blanket than at their base, the rigors of eternal winter reign. The lunar atmosphere is entirely inadequate to check radiation, so that under direct sun- shine the temperature of the moon's surface probably never rises above the freezing point of. water. During the lunar night the tem- perature is believed to be no higher than 200 below zero on the Fahrenheit scale. THE MOON AND ECLIPSES. 123 167. Life on the Moon. Enough has been said to show that there is no such animal and vegetable life on the moon as on the earth. It is a land of death. The sky is a pall of black, studded with stars by day as well as by night. The rising sun, unheralded by the beautiful sky tints which accompany the dawn on earth, darts his garish beams athwart the desolate landscape, causing the lofty peaks to cast long shadows which vie with the sky in blackness. No bird song greets him; there is no rustle of a breeze, or plash of a brook, or murmur of an ocean. Should " lips quiver and tongues essay to speak," no sound from them would break the eternal silence. Dark pits innumerable yawn on every hand. The silvery rims of mighty craters encircle abysses of darkness. As the sun slowly rises in the sky, the fierce chill of the departing night is slowly mitigated ; but no manlike being welcomes returning warmth. The earth hangs continually in mid-heaven, waxing from crescent to full and waning again, swiftly spinning on its axis and bringing into view an ever shifting panorama of cloud and continent and ocean. No star forgets to shine ; the weird glory of the solar corona and the fantastic forms of the protuberances can be seen in all their beauty by screening off the direct light of the sun. The Milky Way girdles the sky, bejewelled with thousands of glittering orbs. The eye is enchanted by the glories above, though the mind shrinks from contemplation of the desolation all about. After four- teen terrestrial days have elapsed, the long shadows stretch them- selves eastward, the sun slowly sinks beneath the western horizon, and night with its terrible rigors of cold comes on apace. Such is a lunar day. 168. The Moon and the Weather. Various fanciful notions con- cerning the moon's influence upon the weather are rife among igno- rant persons. One hears of wet and dry moons ; when the cusps of the new moon have a decided upward slant, fair weather is said to be presaged; when they do not slant upwards, foul weather is to be expected. Such ideas are arrant nonsense. The positions of the moon's cusps can be foretold for thousands of years ; the weather, not for a single week. The connection of changes of the weather with changes of the moon's phases is likewise unfounded. Since the moon changes its phase every week, all weather changes must occur within four days of some change of lunar phase. We 124 DESCRIPTIVE ASTRONOMY. know of no ways in which the moon would affect the weather except by its heat, or by raising aerial tides, or by disturbances of the magnetic conditions. Its heat is almost immeasurably small ; the effect of aerial tides on the readings of the barometer is insignificant. Certain minute magnetic disturbances have been detected, which seem to be de- pendent upon the varying distance of the moon. The idea that the full moon clears away clouds probably has its foundation in the fact that the moon renders the rifts in a lightly clouded sky conspicuous, while they would otherwise escape notice. 169. The Moon's Worth to Man. The most stupendous work done by the moon for man is the rise of the tides, of which it is the chief cause. The flood tide lifts ponderous ships over dangerous bars at the entrances of harbors. Merchantmen are carried from the mouth of the Thames up the river to the busy wharves of London on the bosom of the tide. The tides scour the mouths of rivers, carrying away the pestilence breeding matter which tends to accumulate there. The enormous power of the tides may some day be utilized in driving dynamos to charge storage batteries, from which electricity can be taken when desired. The moon also helps the navigator to guide his ship, as explained in 152. In this capacity the moon has frequently been likened to the hand of a stupendous clock, whose dial is the starry vault. In historical researches dates are frequently fixed by reference to eclipses, which inspired awe in the beholders, and were carefully recorded. The date of the beginning of the Christian era is deter- mined by means of a lunar eclipse, which took place on the night of Herod's death. The moon's light is of use in various ways, which readily suggest themselves. ECLIPSES. 170. How Caused : Shape of the Shadow. An eclipse of the moon occurs when it is in the shadow of the earth ; one of the sun is caused by the interposition of the moon between it and us. In order to understand them, we must study the shape of the shadow cast by the earth or moon. THE MOON AND ECLIPSES. 125 In Fig. 115, S, E, and M represent the sun, earth, and moon, respectively. The heavily shaded portion CDV is called the umbra of the earth's shadow; it is of a conical shape. The lightly shaded portions, FCV and GDV, represent the penumbra of the shadow. An eye situated at X, between CF and CV, would see (neglecting the effect of refrac- tion of the sun's rays, where they graze the earth at C) only a portion of the sun's disk. An object between CF and CV would not be as brilliantly illuminated as one at the left of CF, where light from every part of the sun's disk would strike it. In Fig. 116, CHKD is the umbra of the moon's shadow; the penumbra occupies the space represented by FCH and KDG. A cross-section of the shadow of either the earth or the moon is shown in Fig. 117, the dark portion being the umbra, the lighter the penumbra. 171. Cause of a Lunar Eclipse. Since the centres of the sun and earth lie in the plane of the ecliptic, the axis of the earth's shadow, a line drawn from E to V in Fig. 115, lies there also. If the moon moved exactly in the plane of the ecliptic, it would pass through the earth's shadow every month, and suffer eclipse. But as the moon is above or below the ecliptic, except when at its nodes ( 145) it usually passes above or below the earth's shadow and escapes eclipse. On those occasions when it encounters the shadow, the eclipse is total if the entire Fig. 115. UMBRA AND PENUMBRA OF THE EARTH'S SHADOW. Fig. 116. UMBRA AND PENUMBRA OF THE MOON'S SHADOW. Fig. 117. CROSS-SECTION OF A SHADOW. 126 DESCRIPTIVE ASTRONOMY. moon passes into the umbra, and partial if only a portion of the moon is immersed in the umbra. 172. Phenomena of a Total Lunar Eclipse. When the moon is in the penumbra of the earth's shadow, enough sunlight still strikes it to make it shine brightly ; no one would surmise from its appear- ance that it was about to suffer eclipse. But as soon as it reaches. the umbra, the portion of its limb in the dark shadow disappears from view, the moon having the appearance exhibited in Fig. 118. The dark notch grows until the entire moon is immersed in the umbra. But, strange to say^ the whole moon usually becomes visible, shin- ing with a dull copper-colored light. The explanation is not far to seek. Many of the sun's rays pass through the earth's atmos- here ^ c and D (pj -) and, being re- TOTAL LUNAR ECLIPSE. fracted, pass into the umbra and light up the moon with the sunset tinge. Should the earth's atmosphere be charged with clouds where the sun's rays attempt to struggle through it, the sunlight will be stopped by the clouds, and the moon will be entirely invisible ; this happens rarely. When the forward edge of the moon emerges from the umbra, totality is past ; the eclipse ends when the entire moon has emerged from the umbra. Any phase of a lunar eclipse is visible from the whole of that hemi- sphere of the earth which is turned toward the moon. 173. Cause of a Solar Eclipse. A solar eclipse is caused, as shown in Fig. 116, by the moon's passing between the earth and the sun, so as to obscure the sun either partially or wholly. Since the moon does not move in the plane of the ecliptic, it does not get within the conical space ABCD (Fig. 115), every month, but usually passes above or below it. But when any part of the moon enters this conical space, the sun is at least partially obscured at some point of the earth's surface. 174. The Moon's Shadow. In Fig. n 6, the moon's shadow, where it falls upon the earth, is quite narrow. On account of the variations of the earth's distance from the sun, and of the moon's distance from the earth, the moon's distance from the sun changes. The nearer it is to the sun, the shorter is its shadow (umbra) : the farther away,. THE MOON AND ECLIPSES. C/) m THE MOON AND ECLIPSES. 127 the longer the shadow. Usually the shadow is not quite long enough to reach the earth's surface. Under the most favorable circum- stances, the diameter HK (Fig. 116) of the cross-section of the shadow at the earth's surface is 168 miles. The penumbra of the moon's shadow is shown by the light shading in Fig. 117. The moon hides a portion of the sun from an eye situated anywhere in the penumbra. The moon moves eastward, but as the earth turns in the same direction, the shadow does not skim over the c^ atinents as fast as it otherwise would. A projectile from a modern rifled cannon would keep up with it for a few seconds. 175. Varieties of Solar Eclipses. A total solar eclipse occurs when the whole sun is hidden from view. This happens only when the observer is within the umbra of the moon's shadow. Since the diameter of the cross-section of the umbra at the earth Fig. 120. PATH OF THE CENTRAL LINE OF THE ECLIPSE OF MAY 27, 1900. is always less than 170 miles, the path of the shadow on the earth's surface is long and narrow ; a total eclipse is visible only to those who are in this path. Fig. 116 shows that the penumbra is much wider; the average diameter of its cross-section at the earth is 4,400 miles. 128 DESCRIPTIVE ASTRONOMY. OF THE SUN DURING AN ANNULAR ECLIPSE. For any one situated within the penumbra there will be a partial eclipse. The nearer he is to the true shadow path, the more of the sun will be hidden. The next total solar eclipse visible in the United States occurs on May 27, 1900. The path of the shadow is shown in Fig. 120. When the umbra is not long enough to reach the earth, any one at R in Fig. 121, where the axis of the umbra prolonged cuts the earth's surface, can look past Fig. 122. APPEARANCE the moon's edge and see a part of the sun, which will then have the appearance shown in Fig. 122. Such an eclipse is called annular. 176. Phenomena of Partial and Annular Eclipses. At the beginning of the eclipse, the moon appears to eat away the edge of the sun's disk, forming a notch similar to that shown in Fig. 118 for a lunar eclipse. The notch increases to its maximum size, and then diminishes. One may get a good idea of the appearance by taking two equal circles, one black, the other white, and passing the black one slowly over the face of the white one, leaving a greater or less portion of the white one exposed to view. To represent an annular eclipse, the black circle must be smaller than the white one. With a telescope the lunar mountains are easily seen, projecting from the moon's limb where it eats into the sun. 177. Phenomena of Total Eclipses. A total eclipse is perhaps the grandest of natural phe- nomena. It begins in the same way as a partial one ; just before the sun is entirely covered, the landscape assumes an unearthly hue. Awe seizes the beholder: one sometimes sees the moon's shadow advancing through the air with terrifying swiftness, as if to smite him. In a few seconds it reaches him, and the last ray of sunlight Fig. 121. CAUSE OF AN ANNULAR ECLIPSE. THE MOON AND ECLIPSES. I2Q is gone; the planets and bright stars appear. Around the black ball now hanging in the sky the pearly corona flashes out in all its weird beauty. At its base glow the prominences, like rubies set in pearl. Men's faces grow ghastly. The silence of death is upon the beholders. Soon there is a sudden flash of sunlight at the western limb of the moon : the corona and prominences fade apace. The gloom is overpast, and silence gives place to exclamations of wonder and delight. 178. Observations During Totality. Some of the more important of the observations made by astronomers are given below. 1. Photographs of the corona and prominences are taken. 2. The structure of the inner portions of the corona, which can be seen only during a total eclipse, is carefully studied with the telescope. 3. Spectroscopic observations are made on the corona, the pro- tuberances, and the low-lying regions of the chromosphere. 4. Search is prosecuted for possible small planets near the sun : it is claimed that such objects were seen during the eclipse of July 29, 1878, by two American astronomers. 1 Diligent search has been made for these during more recent eclipses, but without success. 5. Drawings are made of the outer corona to determine its extent and boundaries. 179. Duration and Number of Eclipses. An eclipse of the moon, if total, may last for four hours. During half this time, the whole of its disk will be in eclipse. A total solar eclipse, from first to last contact, occupies about two hours. Totality may, on the rarest occasions, last nearly eight minutes. Its duration is ordinarily only two or three minutes. In some years there are no lunar eclipses : three may occur in a year, as will happen in 1898. Every year there are at least two solar eclipses ; there may be five. The greatest number of eclipses that can occur in any year is seven, of which two are lunar. NOTE. The effects of a total solar eclipse on animals are interesting. Bees return to the hive. Chickens go to roost. Caged birds put their heads under their wings. Bats and owls fly out of their accustomed retreats. Dogs are terri- fied, and sometimes howl dismally. Horses have been known to lie down in the public highway and refuse to advance. Some oxen were once seen to range them- selves in a circle, back to back, with horns outward, as if to resist an attack. 1 Lewis Swift and James C. Watson. 9 I3O DESCRIPTIVE ASTRONOMY. EXERCISES. 180. i. If the moon should cease to rotate on its axis, would its entire surface become visible to us? 2. When the moon is new, does it rise and set at about the same time that the sun does? 3. (a) When the moon is full, where is it to be looked for just after sunset? (b) Where just before sunrise? (c) Where at noon? (d) Where at midnight? 4. (a) When the moon becomes a crescent, shortly after being new, does it set a little while before the sun, or after it? (b) Does it rise before the sun, or after? 5. When the moon is at its first quarter, does the terminator (the straight edge) lie on the left hand side of the illuminated portion, or on the right hand side, as we look at it? 6. When the moon is at its first quarter, does it cross the meridian a few hours before the sun, or a few hours after? 7. When the moon is at its first quarter, and the sun is setting, do we look in the south for the moon, or in the east? 8. Does the full moon shine all night, if the sky is clear? 9. (a) When the moon is in its last quarter, in what direction (north, east, south, or west) is it to be seen at sunrise? (b) In what direction at sunset? 10. On the ecliptic are four cardinal points; viz. the vernal equinox, the summer solstice, the autumnal equinox, and the winter solstice. ( 98, 99.) (a) The sun being in the vernal equinox, if the moon were full, near what point of the ecliptic would it be? (b) Where, if at first quarter? (c) Where, if at last quarter? 11. If the moon on a given night be near either equinox, near what points of the horizon will it rise and set? 12. The moon on a given night is near the summer solstice. (#) Will it, as seen from your home, rise in the northeast, or in the southeast? THE MOON AND ECLIPSES. ($) When crossing the meridian, will it be near the zenith, or low down near the southern horizon? 13. Is the moon visible an hour after sunrise, when it is at the last quarter? 14. If the moon be full about Christmas time, will it run high (that is cross the meridian near the zenith), or low? [To answer this question first find, from the time of year, where the sun is in the ecliptic ; then from the relative positions of the sun and moon determine what point of the ecliptic the moon is near.] 15. Why is not the dark part of the moon rendered plainly visible by earth shine at the first quarter, as well as when the moon is a narrow crescent? 1 6. A mariner computes that upon a certain date the moon will occult the bright star Aldebaran, the disappearance occurring at 8 h. 26 m. 47 sec., Greenwich mean time. By observing the occul- tation, he finds that his chronometer reads 8 h. 25 m. 58 sec. at the time of the star's disappearance. Is his chronometer fast or slow, and how mnch? 17. Can a lunar crater ever be filled with the shadow of its own wall, so that the bottom of the crater will be invisible to us? 1 8. Why are not the rays radiating from lunar craters overflows of lava? 19. How does the atmosphere prevent our seeing stars by day as well as we see them at night? 20. The lines AD and BV in Fig. 115 are tangent to the circle E. Are they tangent at exactly the same point? 21. If the moon were as large as the earth, would it ever suffer a total eclipse ? 22. (a) At what kind (175) of a solar eclipse does the moon look smaller than the sun? (b) At what kind does it look larger? 23. If the earth were suddenly robbed of its atmosphere when the moon was visible during a total lunar eclipse, what change would there be in the moon's appearance? 24. In a daily paper, of wide circulation, the head lines of an article on a solar eclipse read thus : " The Moon Casts its Shadow on the Sun." Change that sentence so that it will express the truth. 132 DESCRIPTIVE ASTRONOMY. 25. When an observer at Chicago sees an annular eclipse, can an observer in some other city see the eclipse as total? 26. (V) When a solar eclipse is partial for one observer, may it be total for another at the same time ? (b) When a lunar eclipse is partial for one observer, may it be total as seen by another at the same time ? 27. The total solar eclipse of January, 1889, was visible in both California and Nevada. Did an observer on the coast of California see it before or after one in Nevada saw it? 28. Does a total lunar eclipse end for an observer in Boston at the same instant as for one in Chicago ? 29. What is the derivation of the word " annular"? 30. (#) Do lunar eclipses happen when the moon is new? () Do solar eclipses happen when the moon is new? MOTIONS OF THE PLANETS. 133 CHAPTER VIII. MOTIONS OF THE PLANETS. " 'T is by the secret, strong attracting force, . As with a chain indissoluble bound, The system rolls entire." THOMSON. 181. Their Orbits. The orbit of each planet is an ellipse (96), one focus of which is in the sun. The planes of all the planetary orbits, excepting those of some of the asteroids ( 224), are but little inclined to the ecliptic. If a dot were placed in the centre of this page, to represent the sun, and all the planetary orbits were accu- rately drawn around it, the deviation of any one of them from true circularity would not be perceived. The definitions of major axis, minor axis, perihelion, aphelion, and mean distance, given in 96, apply to the orbit of any planet. The radius vector of a planet is a line drawn from the focus of its orbit to the planet's centre. 182. Motion in Orbit. If one could station himself in space between the north star and the sun, and a billion miles from the latter, on looking back at the planets he would see that they were all moving about the sun in a direction opposite to that of the hands of a watch. This is an easterly motion. When a planet is at peri- helion, it moves more swiftly than at any other point of its orbit. The planet nearest the sun moves more rapidly than any other. 183. Newton's Law of Gravitation. This law, to which all bodies in the universe are supposed to be subject, may be stated in the following way. The mutual attraction between any two particles is proportional to the product of their masses, and inversely proportional to tiie square of the distance between tJiem. This law may be expressed as an algebraic equation : let m and m' be the masses of two bodies, d the distance between them, and k a number, the value of which depends on the units of mass and dis- tance employed. . mm 1 Attraction = k . 134 DESCRIPTIVE ASTRONOMY. To make this clearer, suppose that two lead balls a mile apart attracted each other with a force of an ounce. If one ball were suddenly made five times as massive, the resulting attraction would be five times as great as before. If at the same time the other were made three times as massive, the new attraction between the bodies would not be 5 + 3, or 8 times the old attraction, but 5 X 3, or 15 times the old attraction. Again, suppose that the masses of the Fig. 123. SIR ISAAC NEWTON. balls remained the same as at first, but the distance between the balls was doubled, the new attraction would not be | of the old, but the square of |, which is J, of the old. The force of gravity binds each planet to the sun. 184. What keeps the Planets Moving? Persons ignorant of the laws of mechanics frequently think that gravity alone cannot keep the planets moving, but that some other force is pushing them. But there is no such extra pushing force. One of the laws of mechanics is that a body once set in motion will continue to move MOTIONS OF THE PLANETS. 135 in a straight line with a uniform velocity, unless ac^^^K by some external force. The tendency to keep going if once .set in motion, or to remain at rest if stopped, is known as inertia. So a planet needs no pushing force behind it : having in some way been set in motion, its tendency is to move in a straight line ; but the gravita- tional pull of the sun compels it to describe a curve instead. .;: Fig. 124. KEPLER. 185. Kepler's Laws. Before the time of Kepler, who was born In 1571, the heavenly bodies were supposed to move in circles. He discovered three laws concerning the motions of the planets : I. The orbit of each planet is an ellipse, the sun being at one of its foci. II. The radius vector of a planet describes equal areas in equal times. 136 DESCRIPTIVE ASTRONOMY. III. The squares of the times of revolution of any two planets are to each other as the cubes of their mean distances from the sun. The second law is illustrated in Fig. 125. If a planet describes the arcs AB and CD in the same length of time, the area of SAB is equal to that of SDC. Let t and f denote the times of revo- lution of two planets, while a and a 1 are their mean distances. Then, by the third law, ^:/ /2 ::fl 8 :a /8 . The discovery of this law, known as the harmonic law, after seventeen years of arduous labor, caused Kepler the great- est exultation. He wrote concerning it: " The die is cast : the book is written, to be read either now or by posterity, I care not which. It may well wait a century for a reader, as God has waited six thousand years for an observer." 186. Perturbations. Gravitation being universal, it follows that the planets attract each other. These attractions cause disturbances of their elliptic motions. The computation of these perturbations has taxed the highest skill of mathematical astronomers ; by a series of profound and elegant researches it has been proved that the stability of the planetary system is not endangered by them. Fig. 125. EQUAL AREAS IN EQUAL TIMES. APPARENT MOTIONS. 187. Two Classes of Planets. For convenience in discussing their apparent motions the planets are divided into two classes. The inferior planets are those the orbits of which lie within that of the earth : these are Mercury and Venus. The superior planets are those whose orbits are exterior to that of the earth : they are Mars, the asteroids, Jupiter, Saturn, Uranus, and Neptune. 188. Aspects. One is aided in remembering the following ex- planations of the aspects of the planets by the thought that they all refer to the position of a planet with relation to the sun, as we, looking out from the earth, see the two bodies. MOTIONS OF THE PLANETS. 137 Conjunction 1 occurs when a planet appears to be close to the sun. A superior planet is then beyond the sun. An inferior planet may be beyond the sun, in which case it is in superior conjunction, or it may be between the earth and the sun ; in that case it is in inferior conjunction. Conjunction is indicated by the sign & . Opposition occurs when the planet is in the opposite direction (from us) to that in which the sun lies. If the sun were neaa* the OPPOSITION Fig. 126. ASPECTS OF THE PLANETS. east point of the horizon, a planet in opposition would be near the west point. Opposition is denoted by the sign 8 . A planet's elongation from the sun is the angle formed at the earth by two lines drawn from it to the planet and the sun respect- 1 There are different kinds of conjunction, opposition, etc., because the planets' orbits do not coincide with the ecliptic. Planets are at conjunction in longitude when their longitudes are the same: they are in conjunction in right ascension when their right ascensions are the same. 138 DESCRIPTIVE ASTRONOMY. ively. The greatest elongation of an inferior planet is illustrated in Fig. 126. A superior planet is in quadrature when its elongation is 90. Quadrature is denoted by the sign n . 189. Apparent Movement of an Inferior Planet. In Fig. 1 26 the orbit of Venus is drawn to illustrate that of an inferior planet. After being at inferior conjunction, Venus, moving in a direction opposite to the hands of a watch, goes to its greatest western elongation. If the earth stood still, Venus would arrive at its elongation in four weeks ; but as the earth chases after it, the interval between inferior conjunction and greatest western elongation is lengthened to 2^ months. The planet then passes on through superior conjunction and greatest eastern elongation to inferior conjunction again. While it is travelling from inferior to superior conjunction, a man facing the sun will see the planet at the right (or west) of the sun. During the other half of its course it will be east of the sun. 190. Apparent Movement of a Superior Planet. In Fig. 127 are represented the orbits of the earth and of Mars, P, P', and P" being on the celestial sphere. E and M are the positions of the earth and Mars when the latter is at opposition. Mars then appears to be at the point P on the celestial sphere. Two weeks thereafter the earth has moved to E', and Mars, moving more slowly on account of its greater distance from the sun, has traversed the arc M M'. Mars then appears to be at P', which is west of P. So a superior planet, though really moving eastward in its orbit, appears when near oppo- sition to move toward the west among the stars, because its motion is slower than that of the earth, and the two bodies are moving in nearly the same direction. This westward motion is said to be the retrograde. Again', let Mars at M be in conjunction, the earth being at E", and Mars appearing to us to be at P. In a few days the earth ar- rives at E"' and Mars at M', so that it appears to be at P", having moved east from P. Had the earth remained at E", Mars in moving from M to M' would have appeared to go east, but would not have reached P". Summing the matter up, we reach three conclusions : I. If the earth were stationary, the planet's easterly motion in its orbit would cause it to appear to move eastward among the stars. MOTIONS OF THE PLANETS. 139 II. When the planet is near opposition, the more rapid motion of the earth causes its eastward motion to be apparently reversed, so that it retrogrades or moves westward among the stars. III. When the planet is near conjunction, its apparent eastward motion among the stars is swifter than it would be, were the earth at rest. Fig. 127. APPARENT MOVEMENT OF A SUPERIOR PLANET. When changing its apparent eastward motion to a westward, and vice versa, the planet is said to be at a stationary point. 191. Evening and Morning Stars. When a planet rises between midnight and the ensuing sunrise, it is called a morning star. When 140 DESCRIPTIVE ASTRONOMY. it is above the horizon at some instant between sunset and the fol- lowing midnight, it is an evening star. Hence an evening star can be seen before midnight (if not too near the sun), and a morning star cannot. 192. Periods, Sidereal and Synodic. The sidereal period of a planet is the time of a complete revolution around the sun. The synodic period is the time which elapses between two successive conjunctions of the planet with the sun. If the planet be an inferior one, the two conjunctions must be both inferior or both superior. EXERCISES. 193. i. (a) At what point of its orbit is a planet, when its radius vector has its greatest length ? (b) At what point when it is the shortest? 2. (a) Why does a planet move most swiftly when at its peri- helion? ($) Why does Mercury travel more miles in an hour than any other planet? 3. Two balls, a rod apart, attract each other with a force of one grain. If the mass of one ball be made ten times as great, while that of the other is halved, what will be the attraction between them, the distance remaining the same? 4. In exercise 3 what would have been the mutual attraction had the original balls been placed ten rods apart? 5. What would have been the mutual attraction had the original balls been placed one fourth of a rod apart? 6. What would have been the attraction, if each ball had been halved, and the distance had been halved also? 7. What would have been the attraction if the mass of one ball had been made three times as great, while that of the other was made ten times as great, and the distance between them shortened to one fifth of a rod ? 8. If the gravitational pull between the earth and the sun were suddenly to cease, how would the former move? 9. How does Kepler's second law show that a planet when at aphelion must describe a shorter arc of its orbit in a day than when at perihelion? MOTIONS OF THE PLANETS. 141 10. The mean distance of the earth from the sun being 93 millions of miles, while that of Jupiter is 483 millions, show by Kepler's third law that the period of Jupiter's revolution about the sun is nearly twelve years. 11. The mean distance of Neptune being thirty times that of the earth, show that its period is over 164 years. 12. The period of Uranus being eighty-four years, show that its mean distance from the sun is over nineteen times that of the earth. 13. Does a superior planet which is in conjunction set about sunrise? 14. Does a planet when in opposition rise about sunset? 15. (a) At about what time of day does an inferior planet, when in' inferior conjunction, cross the meridian? (b} At what time, when in superior conjunction? 1 6. What is the aspect of a superior planet which is on the meridian at midnight? 17. Why does a superior planet look brightest when at oppo- sition ? 1 8. Why are planets not easily observed when they are in con- junction ? 19. When Mercury is at its greatest eastern elongation, being 28 from the sun, is it visible in the evening twilight? 20. If Mercury was seen going across the face of the sun, would it move from the sun's eastern limb towards its western, or vice versa ? 21. If Venus is at its greatest eastern elongation, being 47 from the sun, does it rise before the sun? 22. When at its greatest western elongation, is Venus a morning star or an evening star, or both? 23. Just after an inferior planet passes its superior conjunction, is it a morning star or is it an evening star? 24. If Venus is at its eastern elongation, does it cross the upper branch of the meridian in the forenoon, or in the afternoon? 25. Could Jupiter and Venus ever appear to be close together? 26. At one of its oppositions. Mars was near perihelion, while the earth was near aphelion. At another opposition Mars was near aphelion, while the earth was near perihelion. At which of the two oppositions could Mars be best seen by us? 142 DESCRIPTIVE ASTRONOMY. 27. When does a superior planet appear to have the smaller diameter, at opposition or at conjunction? 28. When does an inferior planet appear to have the smaller diameter, at inferior conjunction or at elongation? 29. Should an inferior planet, shining by reflecting the sun's light, show phases similar to those of the moon? 30. Draw a picture containing two concentric circles, one repre- senting the orbit of the earth, the other that of a superior planet. Mark the positions of the sun, earth, and planet, when the planet is in conjunction with the sun. Determine whether the synodic period of the planet is longer than the sidereal. 31. Find, by making a drawing similar to that described in the preceding exercise, whether the sidereal period of an inferior planet is longer than the synodic. MERCURY, VENUS, MARS, THE ASTEROIDS. 143 CHAPTER IX. MERCURY, VENUS, MARS, THE ASTEROIDS. " Now glowed the firmament With living sapphires : Hesperus, that led The starry host, rode brightest." MILTON. 194. Two Groups of Planets. When the planets themselves are considered, instead of their orbits, they fall naturally into two divis- ions. Mercury, Venus, the earth, and Mars are all comparatively small bodies, are doubtless solid, and are quite dense. Each is sup- Fig. 128. RELATIVE SIZES OF THE PLANETS. posed to have an atmosphere of small mass compared with the mass of the body enveloped by it. Their equipment of moons is meagre. The relative sizes of the planets are shown in Figure 128. Jupiter, Saturn, Uranus, and Neptune, in comparison with the other planets, are giants in size. But their densities, are small, so 144 DESCRIPTIVE ASTRONOMY. that they have perhaps only little kernels of solid matter at their centres. Their atmospheres are very extensive and dense. They are liberally provided with satellites, except Neptune. Were he nearer, we might discover that he had a goodly retinue of them. The planets just mentioned, eight in all, are called the major planets. The minor planets, or asteroids, are quite insignificant in point of size. Very little is known of their physical constitution. MERCURY, . 195. Distance and Diameter. 1 The mean distance of Mercury from the sun is 36,ooopoo miles. The actual distance varies 7,500000 miles each side of this value, the orbit being much more eccentric than that of any other of the large planets. The diameter of Mercury is 3,000 miles. 196. Revolution and Rotation. The sidereal period is 88 days, so that it performs a revolution in less than one fourth of the time required by the earth. The time of its rotation upon its axis cannot be said to be cer- tainly known. One astronomer, a century ago, thought he saw certain appearances on the planet due to the presence of high mountains ; by observing them, he obtained a rotation period of about 24 hours. But his observations have not been confirmed by more powerful telescopes. Schiaparelli, a distinguished Italian astronomer, who has made a special study of some very faint markings on Mercury since 1881, has concluded that Mercury rotates on its axis in the same time that it revolves about the sun. Thus, as our moon continually pre- sents the same face to the earth, Mercury turns the same side to the sun. Schiaparelli announced his discovery to a friend in 1882 in the following lines : " Cynthiae ad exemplum versus Cyllenius axe Aeternum noctem sustinet, atque diem : Altera perpetuo facies comburitur aestu, Abdita pars tenebris altera Sole caret." 1 The distance, diameter, sidereal period, and rotation time of each planet, should be thoroughly committed to memory. MERCURY, VENUS. 145 197. Transits. When Mercury is near inferior conjunction, it sometimes gets into line between the earth and the sun, so that it is seen by us as a small black circle crossing the solar disk. Thirteen transits occurred during the nineteenth century, the last one being on Nov. 10, 1894. 198. Appearance to the Naked Eye. The planet keeps so close to the sun, that it is not readily seen with the naked eye. The times of its elongation are the best times to look for it: it can be well seen about a week before elongation, as well as a week after. The dates of elongation are given in the Nautical Almanac. The best conditions for seeing it in the evening occur at those eastern elonga- tions which happen in March or April. It then appears like a star of exceptional brilliancy, near the western horizon, distinguishable in strong twilight, and conspicuous as soon as night sets in. Copernicus is said never to have . seen it. 199. Telescopic Appearance. The telescope shows that the planet has phases like the moon : it therefore shines by reflecting the light of the sun. When near inferior conjunction it is a narrow cres- cent, as is the moon when new. Its phase at superior conjunction is like that of the full moon. Favorable views of the planet are rare, since it must be observed either during the daytime, or at night when it is near the horizon. Faint dark markings are some- times seen on its disk, but they are so indistinct that their nature can only be guessed at. They may be dark plains, like those on the moon, or possible lakes or seas. 200. Physical Condition. There is spectroscopic evidence of the presence of water vapor ; from this we conclude^ that both air and water are to be found on the planet. But it is probable that the atmosphere is not as dense as ours. The sun shines seven times as hotly as on the earth. VENUS, 9. 201. Morning and Evening Star. Venus is the most brilliant of the planets, and when at its maximum brightness casts distinct shadows of objects at night, in the absence of the moon or bright artificial lights near at hand. It is then visible to the naked eye in full daylight. Because of its brightness it has received the distinct- ive appellations of the Evening Star and the Morning Star. The 146 DESCRIPTIVE ASTRONOMY. Greeks called it Hesperus when it was an evening star, and Phos- phorus when it was a morning star. Of late years many ignorant people, seeing it by day, have supposed it to be a reappearance of the Star of Bethlehem. 202. Distance and Diameter. The mean distance of Venus from the sun is 67,000000 miles ; its distance, when in different parts of its orbit, varies little, because its orbit is more nearly circular than that of any other planet. The planet's diameter is 7,700 miles: it is therefore nearly as large as the earth. When at inferior conjunc- tion it is nearer to us than any other planet ever is. 203. Revolution and Rotation. Venus accomplishes a revolution about the sun in 225 days. The time of its rotation is not now in much doubt. Until lately the only evidence (and that very insuffi- cient) was that it revolved in 23 h. 21 m. But the recent researches of Schiaparelli, the keenness of whose vision is remarkable, render it probable that the time of rotation is 225 days, agreeing with the sidereal period. This result has been corroborated by two other Italian astronomers. 204. Transits. Transits of Venus across the sun's face are much rarer than those of Mercury. The last one occurred on Dec. 6th, 1882, and the next one will not come until 2004. Another is due in 2012. These transits have a high degree of interest, because they have been used in finding the distance of the earth from the sun. Expensive scientific expeditions have been sent to various parts of the world, by the governments of the most progressive nations, to observe these transits. Halley's ' method of observation consists of observing the times of external and internal contact as seen from two stations widely different in latitude. As shown in Fig. 129, the planet as seen from these stations has different paths across the sun's disk. The dis- tance and direction of one station from the other being known, and the lengths of the two paths being measured, it is possible by trig- onometric methods, too difficult to be explained here, to find the sun's distance. The accuracy of the final result depends upon the precision with which the times of contact are noted. Unfortunately, when the black circle of the planet's disk is inter- nally tangent to the sun's limb at the beginning of the transit, it has 1 Halley was an English Astronomer Royal in Newton's time. VENUS. the appearance shown in Fig. 130. As it moves away from the limb, it is, for a number of seconds, apparently attached to it by a black ligament, called the " black drop." The ligament stretches, contracts, and finally breaks. Thus it is very difficult to note the VENU5 BLACK DROP Fig. 129. A TRANSIT OF VENUS. Fig. 130. THE BLACK DROP. time of real internal contact. The action of the planet's atmos- phere also vitiates the accuracy of the observation. A phenomenon similar to the black drop may be seen by placing the thumb and forefinger close together, and holding them six inches or less from the eye. On account of these troubles astronomers now place more re- liance upon other methods. 205. Phases and Maximum Brightness. The phases of Venus are similar to those of Mercury and the moon. They are almost visible to the naked eye. A good spyglass brings out the crescent phase well. The time of greatest brightness does not occur when the planet looks like a full moon, for then it is farthest from us. It comes during the crescent phase, five weeks before, and the same time after, inferior conjunction. The discovery of the phases of Venus was one of the first fruits of the invention of the telescope. Galileo l made the discovery and announced it in the following anagram: " Haec immatura a me jam frustra leguntur, o. y." This he afterwards transposed so that it read, " Cynthiae figuras aemulat mater amorum." 1 Galileo Galilei (1564-1642), the famous Italian philosopher. 148 DESCRIPTIVE ASTRONOMY. 206. Telescopic Appearance. The planet is of dazzling splendor, even in a small telescope ; it looks almost as if made of quicksilver, Fig. 131. GALILEO. and is surrounded by a marked purplish aureole caused by the lack of achromatism ( 39) of the telescope. On rare occasions ill defined spots of a leaden hue are seen on its surface. They may be continents or seas dimly descried. Cer- tain very bright spots, said by some to be occasionally visible near the limb, have been thought to be due possibly to polar ice and snow. 207. Atmosphere. When Venus is near inferior conjunction, being a very slender crescent, the horns or cusps of the crescent appear to be much prolonged, so that they really surround the dark disk of the planet. When about to enter upon a transit, a ring of light is seen surrounding the entire disk (Fig. 132). This is VENUS, MARS. 149 the sunlight shining through the planet's atmosphere, and being refracted by it to our eyes. The atmosphere has been shown to be Fig. 132. THE RING OF LIGHT. denser than ours, and probably less than twice as dense. The vapor of water has been detected in it by spectroscopic observations. 208. Physical Condition. The density of Venus being nearly equal to that of the earth, we conclude that it is a solid body. It probably owes its brilliancy to the fact that its sky is almost totally cloudy at all times. Any one looking at bright white masses of cumulus cloud in a summer sky will be convinced that such clouds reflect light much better than the general landscape does. The excessive cloudiness in turn, combined with the spectroscopic evi- dence of water vapor, indicates that water is abundant on the planet's face. There may not be a square foot of dry land to vary the monotony of a universal ocean. MARS, $. 209. Distance and Diameter. The mean distance of Mars from the sun is 141,500000 miles. Its orbit is, excepting Mercury's, the most eccentric of all the orbits of the major planets, so that the planet's distance varies 13,000000 miles each side of the average distance. Its diameter is 4,200 miles, which is not much more than half the earth's diameter. 210. Revolution and Rotation. The sidereal period is 687 days, only 43 days short of two years. By comparison of drawings of the 150 DESCRIPTIVE ASTRONOMY. planet made soon after the invention of the telescope with those made during this century, the rotation time has been determined with great precision. It is 24 h. 37 m. 22. 67 sec. 211. Appearance to the Naked Eye. Mars, being a superior planet, is best seen at the time of opposition, when it is near the earth. At some oppositions it comes within 36,000000 miles of us, at others it is as much as 61,000000 miles away. This variation of distance is due to the eccentricity of the orbit. The favorable oppo- sitions, which come when the planet is near its perihelion, occur about every fifteen years. The last was in August, 1892. At such times Mars is a brilliant object, shining with a fiery red light, and fairly rivalling Jupiter in splendor. It is then more than fifty times as bright as when faintest, at conjunction. When not near opposi- tion, it might frequently be mistaken by an unpractised eye for one of the brightest of the fixed stars. Its motion among them would lead to its speedy identification. No other planet looks red, except when near the horizon. 212. Phases: Appearance in a Small Telescope. When at oppo- sition the planet's disk looks round, as seen with a small telescope, but at quadrature it is plainly gibbous. For at that time we are not in line between Mars and the sun, and so do not see all of its illuminated hemisphere. (See exercise 21 at the end of this chapter.) Besides the phase, an eye armed with a small telescope (three or four inches in aperture) sees at opposition that the surface is not all red, but bears certain darker markings, generally thought to be of an olive-green hue. At times a small white spot may be seen at one of the poles. The dark spots are supposed to be due to the presence of water ; the white polar spot suggests snow. 213. The Polar Caps. The caps are generally believed to be composed of snow and ice, not only because of their white appear- ance and their situation, but also because the northern one dimin- ishes in size during summer time in Mars's northern hemisphere, and increases during the winter. The southern cap behaves in a similar fashion. During the opposition of 1892, the southern polar cap seemed to diminish with great rapidity. Its area was estimated to lose 1,500000 square miles in a month. First a dark spot was seen in the snow : this spot gradually enlarged, splitting the cap into two parts, each of which melted away at an astonishing rate. On several MARS. occasions white spots, apparently detached snowfields, were seen lying close to the main cap. In 1894 a similar melting took place: in May hundreds of square miles of the polar cap disappeared daily. During the melting a dark band surrounded the cap, keeping at its edge continually, as would be expected if the snow and ice were turning into water. In October the cap had become so small that it was seen with the Fig. 133. MARS: DRAWN BY BARNARD. Lick telescope only ; the remnant of it seemed to be almost hidden by some overhanging veil. Fig. 133 shows the planet as it appeared to Dr. E. E. Barnard 1 with the Lick 36-inch telescope on August 19, 1892. The polar cap was then only one third as large in area as in June of the same year. 1 Now of the Yerkes Observatory. 152 DESCRIPTIVE ASTRONOMY. 214. Seas. The dark areas, if really seas, as generally supposed, are not as permanent in form as the oceans on the earth. The permanent water area has been estimated at about 500,000 square miles, which is only half as great as that of the Mediterranean Sea. When the polar cap melted in the summer of 1892, a portion of the region between it and one of the seas became dark, and the sea apparently increased in size. While we cannot be confident about the cause of such changes, it has been suggested that the water produced by the quick melting of the polar cap flowed across the land to a sea, increasing its size temporarily. Though some of the dark portions of the planet's surface are probably bodies of water, there is reason to believe that much of the dusky area is not permanently covered with water. For canals (to be described later) have been seen in these " seas," and many different shades of color exist there : the same regions hav,e different tints at different times. Sometimes a vast amount of detail is perceived, which would scarcely be found upon a water surface. 1 215. Continents and Islands. The reddish portion of the planet's disk is supposed to be dry land. But these hypothetical continents are not secure in their boundary lines. Disappearances of portions of them have been noted ; they seem to be inundated by the waters of neighboring seas. Dr. S. P. Langley, in his work entitled " The New Astronomy," states that Lockyer Land is sometimes seen white, as if covered with ice ; further, that Hall Island has this white appearance so frequently as to suggest the idea that some mountain or table-land on it rises into the region of perpetual snow. The changes on the surface of Mars during the opposition of 1892 were so noteworthy that Dr. E. E. Barnard was led to write as follows : 1 Dr. Barnard describes some of his observations with the Lick telescope in 1894 as follows : " Under the best conditions these dark regions, which are always shown with smaller telescopes of nearly uniform shade, broke up into a vast amount of very fine details. I hardly know how to describe the appearance of these ' seas ' under these conditions. To those, however, who have looked down upon a mountainous country from a considerable elevation, perhaps some conception of the appearance presented by these dark regions may be had. From what I know of the appearance of the coun- try about Mount Hamilton, as seen from the observatory, I can imagine that, as viewed from a very great elevation, this region, broken by canyon and slope and ridge, would look just like the surface of these Martian 'seas.' " MARS. 153 " These striking changes are enough to make us pause and question whether what we see before us in the heavens is really another world like our own, with relatively fixed oceans and continents, or whether it is not a world like our own in its younger days, when continents were shifting and oceans changing, before the surface of the earth became firm and fixed by the process of cooling." Some of the apparent changes in the forms of the continents have been ascribed to the spread of vegetation along their borders. 216. Clouds. - - Transient spots, having the general aspect of cloud masses, have been observed. Sometimes they are small and of tolerably definite outline, but usually they are diffuse and of large extent. They have also appeared as long streaks projecting a trifle beyond the planet's limb. At times portions of the land- scape have been so obscured as to give rise to the theory that the obscuration was caused by a passing cloud. But many details of the Martian landscape are usually seen so plainly that clouds must be considered as rarities. Their comparative absence would naturally follow from the small proportion of water surface. 217. Atmosphere. Were the atmosphere dense, like that of Venus, we should never have discovered the mass of topographical detail now known. The rarity of Mars's atmosphere has been ac- counted for by the moderate size of the planet, and the weakness of the force of gravity at its surface. Could a rifle ball be shot up- ward with a velocity of 3.5 miles a second, it would, unless checked by atmospheric resistance, leave the planet never to return. The speed of molecules of hydrogen, in their incessant vibration, may considerably exceed this, so that free hydrogen is not to be looked for as a constituent of Mars's atmosphere. The best spectroscopic observations indicate that the atmosphere of Mars exerts no meas- urable absorptive effect upon the sunlight which strikes through it, and is reflected back to us. We are therefore ignorant of its com- position, and can only say that, if it be similar to that of our air, its average density can scarcely be one fourth as great. 218. Description of the Canals. -^- In 1877 Schiaparelli discovered several of the markings which are commonly called " Schiaparelli's canals." A few had been seen previously. Many more have since been found by him and by others. The map of Mars made at the 154 DESCRIPTIVE ASTRONOMY. Lowell observatory 1 in 1894, exhibits a bewildering network of canals, connecting small dark spots scattered over the surface. Not infrequently half a dozen canals radiate from a single spot, going straight to other spots. Most of the canals choose the shortest path from one spot to another: a few are curved. Some do not run from one small dark spot to another, but connect large dark areas, or go from a small spot to a large area, or occasionally con- nect two other canals. The small spots are less than 1 50 miles in diameter. The length of the canals ranges from a few hundred to 3,500 miles. Their average breadth is 30 miles. The most myste- rious fact about them is that they become double at times, the two new canals being about 200 miles apart, and veritable twins. Schiaparelli thinks that the doubling may be periodical, and con- nected in some way with the planet's seasons. 219. Explanations of the Canals. The canals have naturally been supposed to be water-ways. When a polar cap melts, the canals in the neighborhood become darker and wider, and remain dark until the snow stops melting. Then the width of the canals diminishes. These appearances have led Schiaparelli to the con- clusion that the canals are natural furrows, through which the water is carried from the poles equatorward. Mr. Percival Lowell advocates the theory that the canals are strips of vegetation, which are watered by canals too small to be visible to us. A small spot at the junction of several canals is an oasis, according to this view. No satisfactory explanation of the doubling of the canals has been given. The majority of astronomers, while freely admitting the existence of the markings called canals, are inclined to be conservative with reference to any explanation of their nature. It has been aptly said that it is better not to know so much, than to know so many things that are not so. 220. Colors. Orange and grayish green are the prevailing col- ors, outside of the polar caps. But various colors have been seen in different spots. The same spot has been of different hues at different times, though the utmost care was taken to avoid optical illusions. Light greens and bright greens have been seen often. At times places supposed to be bodies of water have exchanged 1 A temporary observatory set up at Flagstaff, Arizona, by Mr. Percival Lowell, of Boston. MARS. ^-..^ V- /--MH; 5j ^ *\" >*H\> 5^a - *' f ^3k ^ '\ HK^ ii r#i r . c ^>4 iVtf^^ff/ls'^x 1 ^^r - rr s o f : >^ t-Ti^ J^T** gr r< v 7 li^^to^l^ - :: i o, ii ^CTM> ?iN x ax --^ \ 1 3= v '< 156 DESCRIPTIVE ASTRONOMY. their ordinary color for dark blue. Gray and yellow tints are of common occurrence. Even so extraordinary a color as violet-lake was once perceived. Some of these colors may be explained as due to haziness or partial cloudiness. Perhaps some are due to the presence of vegetation. 221. Satellites. Mars is attended by two moons, discovered by Prof. Hall 1 in August, 1877. Their names are Deimos and Phobos. 2 The distance of Deimos from the planet's centre is 14,600 miles; it completes a revolution about Mars in 30 h. 18 m. Phobos is at a distance of but 5,800 miles and takes only 7 h. 39 m. for one revolution. The time of rotation of Mars being over 24 hours, Phobos by reason of its rapid motion rises in the west and sets in the east, as our moon would if its orbital motion were swift enough. These moons are so minute that it is not possible to measure their diameters directly ; but by measures of the amount of light that they give, Prof. Pickering 3 has concluded that the inner one is 7 miles in diameter, the outer one 5 or 6. Estimates made at the Lowell observatory give a diameter of 36 miles for Phobos and 10 miles for Deimos. Their discovery was as great a feat of telescopic vision as for a man in Boston to see a tennis ball at Philadelphia. 222. Habitability. If we have simply to answer the question, " Would a man, as constituted at present, if transported to Mars, find it possible to exist there?" the most probable answer is, " No." While one must not be dogmatic, it may be said, with some assurance, that the man would gasp a few times and die. However, it is conceivable that manlike beings might find a home there. Plans for communication with the supposititious inhab- itants of Mars by means of huge signals displayed in deserts or on table-lands, or by gigantic combinations of electric lights, are little better than phantasies of a disordered imagination. If some enter- 1 Prof. Asaph Hall, formerly of the U. S. Naval Observatory, Washington, D. C. 2 These names were given by Homer to the steeds which drew the chariot of the god of war. In one passage in the fifteenth book of the Iliad they are personified, and refer to the attendants of Mars. Bryant's translation is : " He spake, and summoned Fear and Flight to yoke His steeds and put his glorious armor on." 3 Edward C. Pickering, Director of the Harvard College Observatory. MARS. 157 prising and athletic individual on Mars should wave a flag as large as the State of New York, terrestrial astronomers might notice his greeting. There have been curious anticipations of the discovery of the moons of Mars. 1 Kepler, in a letter written after the discovery of four satellites of Jupiter by Galileo, said : " I am so far from disbelieving the existence of the four circumjovial planets, that I long for a tele- scope to anticipate you, if possible, in discovering two around Mars, as the proportion seems to require, six or eight around Saturn, and perhaps one each around Mercury and Venus." Swift, in his satire, " The Travels of Mr. Lemuel Gulliver," puts the following language into the mouth of Gulliver, who had arrived among the inhabitants of Laputa, a flying island : "The knowledge I had in mathematics gave me great assistance in acquiring their phraseology, which depended much upon that science, and music : and in the latter I was not unskilled. Their ideas were perpetu- ally conversant in lines and figures. If they would, for example, praise the beauty of a woman, or any other animal, they describe it by rhombs, circles, parallelograms, ellipses, and other geometrical terms, or by words of art drawn from music, needless here to repeat. . . . They spend the greatest part of their lives in observing the celestial bodies, which they do by the assistance of glasses far excelling ours in goodness. For although their largest telescopes do not exceed three feet, they magnify much more than those of a hundred with us, and show the stars with greater clearness. This advantage has enabled them to extend their discoveries much farther than our astronomers in Europe ; for they have made a catalogue of ten thousand fixed stars, whereas the largest of ours do not contain above one third of that number. They have likewise discovered two lesser stars or satellites which revolve about Mars : whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost five ; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of the periodical times are very nearly in the same proportion with the cubes of their distances from the centre of Mars ; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies." 1 The following interesting information is derived from Professor Hall's monograph on the satellites. 158 DESCRIPTIVE ASTRONOMY. Voltaire represents, in one of his works, that Micromegas, an inhabitant of Sirius who visited our system, discovered that Mars had two moons, which made perpetual compensation for the lack of the brilliant sunlight which we enjoy. THE ASTEROIDS, OR MINOR PLANETS. 223. Bode's Law. In 1772 the astronomer Bode brought into prominence a relation between the distances from the sun of the then known planets. This relation had been discovered some years previously by Titius, but it is commonly called Bode's Law, and is found as follows. The series o, 3, 6, 12, etc., in which each number except the first is double the preceding one, is written. To each term 4 is added. 03 6 12 24 48 96 192 4 7 10 16 28 52 100 196 The resulting numbers represent fairly the relative distances. Taking the earth's distance as ten, the real distances are given below. Mercury . . . 3.9 Jupiter . . . 52.0 Venus ' V . . 7.2 Saturn . . . 95.4 Earth .... 10.0 Uranus . . . 191.8 Mars . . . . 15.2 The number 28 in the scheme of Titius corresponded to no known planet. Astronomers generally became imbued with the notion that there was a planet to be discovered, which would fill the gap. Neptune was then unknown : its distance does not conform to the law, being only 300.5. 224. Discovery of the First Minor Planet. After Uranus was dis- covered in 1781, and its distance had been found to conform to the law of Titius, an association of astronomers was formed, to hunt for the missing planet between Mars and Jupiter. But the honor of the discovery was reserved for Piazzi, of Palermo, who was not a member of the association, but was engaged upon a star catalogue. It was his habit to observe the right ascension and declination of each star on several different nights, that he might determine its place accurately. THE ASTEROIDS, OR MINOR PLANETS. 159 Upon the evening of the first day of the nineteenth century, he observed a list of stars, one of which was destined to bring him renown. On January 2d, 3d, and 4th he reobserved the same list, and upon comparing his observations, discovered that the thirteenth star on the list changed its position from night to night. Here at last was a planet, but was it the one sought? For six weeks he observed it upon every opportunity, and then he fell ill : when he recovered, the planet was so near to the sun that it could no longer be found. 225. Gauss computes the Orbit. Here was a dilemma. The news reached Germany in the early spring, and Gauss, of Gottingen, then a rising young mathematician, set himself at the task of finding some method of computing its orbit. Finally he discovered the now classic method of computing a planet's orbit, when its right ascension and declination are known at three different dates. He applied the new method to Piazzi's observations, and predicted the future place of the planet; at last it was bound by the chains of mathematical analysis, more ethereal than a spider's web, but stronger than bronze. The association was already hard on the track of the wanderer, and when Gauss's results reached them, one of them speedily rediscov- ered it, on the last day of the year. Its distance from the sun was in fair agreement with the law of Titius, being 25.7 instead of 28. It received the name Ceres. 226. Further Discoveries. In 1802, 1804, and 1807, Pallas, Juno, and Vesta were found. Astronomers were not again successful in the quest until 1845, when Astraea was brought to light. Soon after- ward the pace of discovery quickened, and now the asteroid hunters find them with almost embarrassing rapidity. The family of these little strangers is so large, and increasing so rapidly, that the problem of taking care of them is becoming a very serious one. The orbit of each new one has to be computed, and its place in the sky (right ascension and declination) calculated from year to year, so that fresh observations can be taken, and a more accurate orbit figured out. 227. Methods of Search. Almost all of these bodies are so small that they look like fixed stars, and can be detected only by their motion. An observer makes a chart of a certain region of the sky, containing all the stars visible there through his telescope. Then night after night he compares the heavens with his chart. If a i6o DESCRIPTIVE ASTRONOMY. SUN star-like object not on the chart is seen, its position is carefully noted. In a few hours its motion will betray it, if it be a minor planet. This process is quite laborious, and is now little used because photography has entered the field. A picture of a certain region of the sky is taken, the plate being exposed for a couple of hours. While the images of stars on the plate are circular, the image of the planetoid is a short streak, caused by its motion. 228. Orbits: Distances: Periods. The orbits of the planetoids are more eccentric than those of the major planets. The orbit planes have greater inclinations to the ecliptic, the orbit of Pallas being inclined 35. The mean distances vary from 198,000000 to 400,000000 miles, the pe- riods from three years to nine. The computations of the orbits, as well as of the annual ephemerides> giving the right ascen- sion and declination of the minor planets, are made chiefly by German computers. The Berliner Jahr- buch, issued from the Fig. 135.- THE ZONE OF ASTEROIDS. Imperial Observatory at Kiel, Prussia, contains the results of their work. The late Prof. Watson J left a fund to pay for the twenty-two which he discovered, intrusting it to the American National Academy of Sciences. 229. Designations. Each asteroid has a number, which is usually printed thus, , the numbers being given in the order of discov- ery. Names are also given, chosen chiefly from those of female divinities in classical mythology. The number of these is about exhausted. Some of the unfortunate planetoids have been afflicted with such names as Xantippe, Vindobona, and Sophia, to say nothing of Walpurga and Chicago. The asteroid last mentioned 1 James C. Watson, Director of the Observatory at Ann Arbor, Mich., and later of the Washburn Observatory at Madison, Wis., author of Watson's Theoretical Astronomy. THE ASTEROIDS, OR MINOR PLANETS. l6l was named by the Astronomical Congress which met at Chicago during the World's Fair. 230. Number and Size. The number is now (1896) over 400. Most of them are of quite insignificant dimensions. Vesta, the brightest one, is 250 miles in diameter. At opposition it is barely visible to a good eye. Ceres, the largest, is nearly 500 miles in diameter. The majority are less than fifty miles in diameter. Most of the faint ones now being discovered have diameters of only about ten miles. Compared with the earth they are as flour-dust to a foot- ball. Half a billion of them compacted together might equal the earth in bulk. 231. Atmosphere : Gravity. The bodies are so small that they probably have very rare atmospheres. There are no reliable obser- vations bearing on this point. On the assumption that they are as dense as the earth, the force of gravity at the surface of one of the small ones is about one thousandth part 1 of that which we daily contend with. A sharply batted base-ball would leave the planet; in a jumping match the spectators could eat lunch while waiting for the contestants to come back to terra firma. 232. Origin. One view is that they have been formed from a larger body by a series of explosions. A single explosion would not account for them, for the fragments, after coursing about the sun would all return to the point where the explosion occurred. After the lapse of ages their orbits would be considerably modified by the attractions of the major planets, especially by that of Jupiter. But yet the theory of a single explosion is considered untenable by those mathematicians who have given particular attention to the matter. The present tendency of scientific opinion is to discard catastrophes, and to believe in an orderly evolution manifested throughout the universe. According to the nebular hypothesis, the material composing the minor planets was once collected in a ring surrounding the sun. The ring in condensing formed many planets instead of one: the cause of the disruption of the ring may have been the powerful attraction of Jupiter. 1 The principles of mechanics show that, for spheres of equal densities the force of gravity at their surfaces varies as their diameters. Thus, if two leaden spheres were respectively one foot and ten feet in diameter, a grain of sand lying on the surface of the latter would be attracted by it ten times as strongly as an equal grain on the surface of the former. II 1 62 DESCRIPTIVE ASTRONOMY, EXERCISES. 233. i. If the axis of Mercury is perpendicular to the plane of its orbit, and the planet rotates on its axis at a uniform rate from west to east once in every revolution, are there alternations of day and night at a point on the surface directly between the centres of the sun and the planet? 2. In the case above, would the sun shine on more than half of Mercury's surface in 88 days, considering the eccentricity of its orbit? 3. On account of the relative sizes of the sun and Mercury, does the former, at each instant, illuminate more than a hemisphere of the latter's surface? 4. Why do Mercury and Venus look black during transit? 5. Mercury at times attains an elongation of 28 from the sun. If it is at eastern elongation about the last of March, its orbit being roughly coincident with the ecliptic, will it be above the celestial equator, or below? [In answering this, first fix in mind the position of the sun in the ecliptic.] 6. Should a planet, in order to be seen most advantageously from your home, be above or below the equator? 7. If Mercury be near its western elongation on April I, will it be in a favorable position to be seen from your home? 8. If Mercury be near its eastern elongation when school opens in the fall, will it be in a favorable position for observation by you ? 9. If you were searching for Mercury with the naked eye, would it be more convenient to have it at eastern or western elongation, considering only the time of night at which you would look for it? 10. When Venus is morning star, is it east of the sun, or west? 11. Draw two concentric circles of the proper relative sizes to represent the orbits of Venus and the earth. Locate the two planets upon them, so that Venus shall be at its greatest elongation. Then measure the angle of elongation with a protractor. 12. What phase has Venus when at elongation? 13. Is Venus gibbous between its superior conjunction and its greatest eastern elongation ? 14. Is Venus gibbous between its greatest western elongation and superior conjunction? MERCURY, MARS, VENUS, THE ASTEROIDS. 163 15. Just before inferior conjunction do the cusps of the crescent Venus point toward the sun? 1 6. Find from the Nautical Almanac when Mercury and Venus next reach elongation. 17. If you have a telescope, make the following test of it and your eye. Make on white paper a drawing like Fig. 136. Look at it with the telescope, and estimate the width of the black drop as compared with the diameter of the black circle representing the planet. It is well to perform the experi- ment out of doors, with your back to the sun, so that the paper may be well illuminated. l 6 1 8. Why is not a ring of light seen around Venus, when it is in transit? 19. The volumes of two spheres are to each other as the cubes of their diameters. The earth is how many times as large as Mars? 20. Assume that the diameter of the earth is 8,000 miles, and that of some asteroid is 10 miles. The earth is how many times as large as the asteroid? 21. Draw two concentric circles of the proper relative diameters to represent the orbits of the earth and Mars. (The representation will not be very accurate, because Mars's orbit is quite eccentric.) Mark a point on the inner circle for the earth, and another on the outer to represent the position of the centre of Mars, when in quad- rature. Around this centre draw a little circle, to represent the planet itself, on an exaggerated scale. Shade one half of this little circle, so that it will represent the unilluminated hemisphere of Mars. How does the picture show that Mars, as seen from the earth, would be gibbous? 22. Does any planet, as seen from the sun, exhibit phases? 23. Deimos and Phobos move in the plane of Mars's equator. Could an observer at a pole of the planet see them? 24. Compute the area of the surface of Deimos, assuming it to be a sphere six miles in diameter. 25. Is Bode's Law anything more than a chance coincidence? 26. The first day of this century was January 1st in what year? 27. Why does the small size of an asteroid militate against its having a dense atmosphere? 164 DESCRIPTIVE ASTRONOMY. CHAPTER X. JUPITER, SATURN, URANUS, NEPTUNE. " Some displaying Enormous liquid plains, and some begirt With luminous belts, and floating moons, which took, Like them, the features of fair earth." BYRON. 234. The Outer Group. We come now to the consideration of the outer group of planets, in comparison with which the planets before considered are but pygmies. JUPITER, y.. 235. Distance and Diameter. The mean distance of Jupiter from the sun is 483,000000 miles; this is five and one fifth times the distance of the earth. Its mean diameter is 88,000 miles. The planets heretofore considered are nearly spherical, but Jupiter's disk as seen in a telescope is a marked oval, showing that the polar diameter of the planet is shorter than the equatorial. 1 Jupiter is larger than all the other planets put together, being 1,300 times as large as the earth. 236. Revolution and Rotation. The sidereal period of this planet is nearly twelve ( n.86) years. Despite its huge bulk, it rotates with amazing swiftness, in about 9 h. 55m. The rotation period cannot be obtained with exactness, because, like the sun, different parts of the surface rotate in different times, those near the equator moving most rapidly. Even a particular feature of his surface does not have a uniform rotation time. The time for the great red spot ( 240), for example, slowly increased from year to year during the period 1878-86, the total increase being seven seconds. Since 1886 there has been no change. 1 The polar diameter is 84,300 miles and the equatorial 89,790 miles. The mean diameter is found by adding the polar diameter to twice the equatorial, and dividing the sum by 3. JUPITER. 165 237. Appearance to the Naked Eye. To the naked eye, Jupiter, when near opposition, attains a greater brilliancy than any other planet, except Venus. At all times, except when too near con- junction, it is much brighter than any of the fixed stars. Its light is white, with the merest tinge of yellow. Except when near one of its stationary points, its planetary nature can be detected in less than a week by its motion among the stars. Men of extremely acute vision, under the fairest of skies, have occasionally seen one of its satellites with the naked eye. One who suspects that he sees a satellite should move his head from side to side, and see if the suspected satellite moves also ; if it does not, it may be a satellite or a fixed star. 238. Belts. These are readily discerned with a small telescope. The surface seems banded by parallel belts, the most conspicuous ones being near the planet's equator. The belts are dark colored. In small instruments the dark belts appear of a grayish or brownish cast ; but in larger ones a reddish hue is observed. At times they even appear pink. The belts are supposed to be rifts in the clouds, through which we look down deeper into his atmosphere than else- where. The light portions of the disk, as shown in Fig. 137, re- semble masses of cumulus cloud. The appearance of the belts is sometimes reproduced in the terrestrial clouds lying below the sum- mit of Mt. Hamilton, Cal., the home of the Lick observatory. The belts change their outlines somewhat from month to month. 239. Pickering's Theory of the Belts. Prof. W. H. Pickering, 1 observing from a high Peruvian table-land, where the atmospheric conditions are very fine, says that the appearance of Jupiter is that of a uniform white mass of cloud, overlaid by a thin gauze veil of a brown material, not unlike our cirrus clouds in structure. This veil is more dense in some places than in others ; the dense portions, by obscuring the white cloud beneath, cause the dark belts. The well known white spots are thought to be due to round or elliptical holes in the cirrus layer, through which we see the white surface below. He says : " In short, it appears that, were it not for this insignificant light gauzy veil of brown cloud, we should find the surface of Jupiter, like that of most of the other planets in the solar system, almost a perfect blank. This gauzy structure must float in 1 Of the Harvard College Observatory. i66 DESCRIPTIVE ASTRONOMY. /, Fig. 137. JUPITER, AS SEEN WITH THE LICK TELESCOPE: DRAWN BY KEELER JUPITER. 167 a nearly transparent atmosphere, surrounding and rising above it ; it is this atmosphere which causes the absorption, and which almost completely obscures the belts at the limb of the planet." 240. The Great Red Spot. This was discovered in 1878; it is 30,000 miles long and 7,000 broad. It has had various degrees of brightness in different years, being almost invisible in 1883 and 1884, and again in 1892. It is no longer a conspicuous object, even in large telescopes. The spot is thought to be a rift in the clouds, similar in color to the belts, though usually more vivid. Its cause is a mere matter of conjecture. Perhaps it is due to some terrific ebullition in the depths of the planet, which causes heated vapors to arise and clear away the clouds which would otherwise be above it. Its remarkable persistency of form, and its movement ( 236) are against this supposition. 241. Smaller Spots. These have been seen frequently of late years, and are chiefly either white or black. They are usually round or elliptical ; their average size is about that of our moon. Perhaps they are cloud masses which are above the general level, and therefore show more plainly. Their motions are independent, as is shown by the fact that the rotation time of the planet, deter- mined by observations of one spot, does not generally agree with that found by employing some other spot. 242. Atmosphere: Spectrum. All the evidence points to the con- clusion that the atmosphere is very extensive. There is no reason to believe that any permanent markings have ever been seen on the planet, so deep and dense is the enveloping atmosphere. The spectroscope gives no certain evidence concerning the com- position of the atmosphere, the spectrum of Jupiter being almost identical with that of the sun. This shows that the light which gives the spectrum is merely reflected sunlight. If the light pene- trated to any considerable depth it would suffer marked absorption, which would be manifested by bands in the spectrum. There is one large faint band in the spectrum, the origin of which is unknown. 243. Light and Heat. If Jupiter were hot enough to give out light of its own, its moons would not suffer total eclipse when they passed into its shadow. But a body may be quite hot without being perceptibly luminous, as any disbeliever may discover by experimenting with a poker recently withdrawn from the fire. So 1 68 DESCRIPTIVE ASTRONOMY. cloudy is the planet, so great are the changes continually taking place on its surface, and so feeble is the action of the sun upon it on account of its great distance, that we are compelled to believe that the planet is hot. It may be nearly self-luminous, and has been called a semi-sun. 244. Physical Condition. The average density of the planet is only a third greater than that of water. It is therefore chiefly, if not entirely, composed of matter in a fluid state ; hot water may be one of its chief constituents. We are not to regard it as possessing a solid crust, like the earth, but rather as being a seething caldron, in which the hot fluids rise from the interior, become cooler, and sink back, in a ceaseless round of motion. 245. The Major Satellites. These are four in number, and are designated in the Nautical Almanac by the Roman numerals I, II, III, and IV, according to their distances from the planet, I being Fig. 138. THE ORBITS OF THE MAJOR SATELLITES. nearest, and IV the most remote. The smallest, II, is as large as the moon, and the largest, III, is nearly as big as Mars. They are all visible in a good opera-glass. Their orbits are nearly in the plane of the planet's equator. 246. Eclipses. Jupiter casts so long and large a shadow away from the sun that all the satellites except the fourth suffer eclipse once during every revolution. The times of these eclipses are given in the Nautical Almanac, and they can be easily observed with a small telescope. For obvious reasons the satellite does not disappear instantaneously, but fades from sight gradually. 247. Occultations. When a satellite, as seen from the earth, is behind Jupiter, it has suffered occultation. When it has been just disappearing behind the limb of Jupiter, or reappearing, many at- tempts have been made to see it shining through Jupiter's atmos- phere, with the hope of measuring its refractive power. But most of the attempts (even with the Lick telescope) are acknowledged failures. JUPITER. 169 OCCULTATIOM 248. Transits. A satellite, when passing between us and the planet, appears to cross its face and is in transit. The limb of Jupiter is darker than the cen- tre of its disk : on this account a satellite is visible at the be- ginning or end of a transit as a bright spot on a dark back- ground. When the satellite is projected upon some portion of the disk which has the same brightness and color, it be- comes invisible. One occa- sionally appears dark, or even black at some time during tran- sit: this may be because the background, on which it is pro- jected at the time, is unusually bright. The shadows of the satellites also make transits : at times, a satellite and its shadow are seen journeying across Ju- piter's face in company. 249. Markings and Rotation. Fig. 139. PHENOMENA OF THE SATELLITES. Many observers have reported dark markings on the satellites. Some of the most authoritative recent work is that of Professors Schaeberle and Campbell l with I Fig. 140. MARKINGS SEEN WITH THE LICK TELESCOPE. the Lick telescope. Their drawings of the markings on satellite III, made with very high magnifying powers, lend strong support 1 W. W. Campbell, astronomer at the Lick Observatory. OF THE UNIVERSITY I7O DESCRIPTIVE ASTRONOMY. to the theory that it continually keeps the same face toward Jupiter. Their observations also show that satellite I is perceptibly elongated, and that its long axis points toward the planet's centre. These satellites therefore resemble the moon in that their times of revolu- tion and rotation are coincident. Schaeberle and others have seen a bright polar cap ort satellite III. Barnard discovered a white belt on satellite I, which causes it to appear double when crossing a white part of Jupiter. 250. Pickering's Observations of the Satellites. In addition to studying the belts, Prof. W. H. Pickering has made careful observa- tions of the satellites. He found the first satellite to be at times very plainly elliptical, the major axis exceeding the minor by ten per cent. For the time of rotation he has deduced a value of 13 h. 3 m., from a series of observations. The rotation time of the second satellite was much more difficult to obtain, but a value of over 41 hours was settled upon. The observations of the third and fourth satellites favor the theory that their times of rotation and revolution are coincident. Prof. Pickering's results are in only partial agreement with those of other observers : the matter needs much further study. 251. The Fifth Satellite. On Sept. 9, 1892, Dr. E. E. Barnard discovered a fifth satellite, with the Lick telescope. It is a tiny point of light, which can be observed with the most powerful telescopes only. Its distance from the centre of Jupiter is 112,000 miles and its time of revolution n h. 57 m. 22. 56 sec. Its Fig. 141. JUPITER AND diameter is estimated as 100 miles. If it were THE ORBIT OF THE a few thousand miles nearer to the planet, it FIFTH SATELLITE. < 1 1111 1.1 would probably be torn in pieces by the attrac- tion of the latter, which would be much more powerful upon the side of the satellite turned toward it at any time than on the opposite side. 252. Velocity of Light. The fact that light does not travel from one point to another instantaneously was discovered in 1675 by Roemer, a Danish astronomer. The discovery was made by the discussion of observations of Jupiter's satellites. The eclipses recur at nearly equal intervals. By noting the times of the eclipses of satellite I, for example, during the period of one revolution of Jupiter about the sun, one can calculate with great accuracy the JUPITER, SATURN. I 71 average interval of time between two successive eclipses : the satellite suffers some 2,500 eclipses during that period. When Jupiter is in opposition, let the time of an eclipse be noted : by means of the known interval between two successive eclipses, com- pute the day, hour, and minute when an eclipse will occur three months after opposition, at which time the earth will be farther from Jupiter. The eclipse will happen later than the predicted time. Predict another eclipse near the time of the planet's conjunction with the sun, when the earth is 186,000000 miles farther from Jupiter than at opposition. The eclipse will again be behindhand, and by a larger amount than before. Roemer sagaciously guessed that the eclipse which took place near the time of conjunction really happened on time, but that the light which brought the message to the observer took time to cross the extra distance of 186,000000 miles. In this he was right : the time required for light to cross the earth's orbit is close to 1,000 seconds. SATURN, b. 253. Distance and Diameter. The distance of this most enchant- ing of planets from the sun is 886,000000 miles. Its mean diameter is 74,000 miles. 254. Revolution and Rotation The sidereal period is 29^ years. The rotation time is hard to determine because so few small well defined spots have ever been seen on its surface. Prof. Asaph Hall has derived a value of loh. 14 m. 23. 8 sec. from his observations of a white spot which appeared on the ball in December, 1876, and was visible for a month. 255. Appearance to the Naked Eye. Because of its greater dis- tance, Saturn is much fainter than Jupiter. It alone of the planets has a decided yellowish tint; it is generally as bright as a first magnitude star, and may be distinguished from a star by the fact that it does not twinkle. All planets have this peculiarity except when near the horizon. A person who is acquainted with the con- stellations may find it easily by looking up its right ascension and declination in the Nautical Almanac, and locating it on a star map. 256. Telescopic Appearance. The first view of Saturn with a large telescope usually calls forth an exclamation of wonder and delight. For the globe is seen to be surrounded by a marvellous 172 DESCRIPTIVE ASTRONOMY. ring system, which is unique in the solar system, and, so far as we know, in the entire universe. A goodly retinue of satellites is also seen attending this majestic orb. The ball is encircled by rather bright belts near the equator, and by fainter ones at higher latitudes. These belts are not sub- ject to much change of appearance, except that due to imperfect seeing caused by the fluctuations of our own atmosphere. Serenity is natural for the oldest of the gods. Fig. 142. SATURN, AS SEEN WITH THE LICK TELESCOPE: DRAWN BY KEELER. 257. Discovery of the Kings. In 1610 Galileo discovered that the planet appeared triform. This was due to the imperfection of his telescope. He said that Saturn had two servants who aided him on his way. Great was his perplexity and chagrin to find that the attendants disappeared after a year or two. In a letter to a friend he said : "What is to be said concerning so strange a metamorphosis? Are the two lesser stars consumed after the manner of solar spots? Have they vanished, or suddenly fled? Has Saturn, perhaps, devoured his own children? Or were the appearances indeed illusion and fraud, with which the glasses have so long deceived me, as well as many others to whom I have shown them? . . . The shortness of the time, the unex- pected nature of the event, the weakness of my understanding, have greatly confounded me." SATURN. 173. Nevertheless, in the latter part of the letter he ventures to predict that the lost bodies will reappear ; and he himself, as we learn from a later letter, saw them again as " ears," one on each side of the central ball. Forty odd years later, Huyghens, a Dutch astronomer,, advanced the theory that Saturn had a ring, announcing it in the form " aaaaaaa ccccc d eeeee g h iiiiiii 1111 mm nnnnnnnnn oooo pp q rr s ttttt uuuuu." These letters, properly arranged, form the Latin sentence, Annulo cingitur, tenui, piano, nusquam cohae- rente, ad eclipticam inclinato. The translation is : " It is encircled by a thin flat ring, nowhere touching, inclined to the ecliptic." 258. Divisions and Dimensions of the Ring System. The ring announced by Huyghens has been found with powerful telescopes, to be composed of three, two of which are bright and the third dark. They are shown clearly in Fig. 142. The division between the outer ring and the middle one has been named Cassini's di- vision, after the Italian astronomer who first noticed it, twenty years, after Huyghens's announcement; it is about 2,200 miles in width. There is a finer division in the outer ring, known as the Encke division. Many others have been suspected. One has been found by Keeler 1 at the Lick Observatory. The inner ring is much darker and fainter than the others ; it is known as the dark or dusky ring. The extreme diameter of the ring system is 173,000 miles, and its breadth is about equal to the semidiameter of the ball. The rings are not of uniform thickness,, as is shown when the edge of the system is turned toward us. The average thickness is not far from one hundred miles. 259. Disappearance of the Rings. This phenomenon, which was so sore a trial to Galileo, is explained by Fig. 143. The plane of the rings coincides with that of the planet's equator, but is in- clined 27 to the plane of the planet's orbit. Saturn keeps the successive positions of its rings parallel to each other in its journey around the sun. As the plane of the earth's equator passes through the sun in March and September, so the plane of Saturn's rings- passes through the sun twice in one of its revolutions. Since the earth, as seen from Saturn, is close to the sun, the plane of the rings will pass through the earth within a few weeks of the same time. At such a time, the rings disappear because they are thin, 1 James E. Keeler, Director of the Allegheny Observatory. 174 DESCRIPTIVE ASTRONOMY. and edgewise to us. The disappearances happen at intervals of fifteen years: at times midway between them the rings are seen most favorably. Fig. 143. DIFFERENT POSITIONS OF THE RINGS. 260. The Dark Ring. -- This ring is sometimes called the crape or gauze ring, because it is semi-transparent. Dr. Barnard, at the Lick telescope, on Nov. I, 1889, observed an eclipse of one of the satellites. After emerging from the shadow of the ball it recovered its normal brightness, and soon plunged into the shadow of the dusky ring; it then became fainter and fainter, but did not disap- pear until it had passed through the shadow of the crape ring and into the shadow of the inner one of the two bright rings ; then it dis- appeared entirely. This shows that sunlight sifts through the dark ring, and that the transparency of the latter decreases regularly from its inner edge to its outer, where it joins the inner bright ring. 261. Structure of the Rings. The researches of mathematicians have demonstrated that neither ring can be an unbroken mass, either solid or liquid. In either case the ring would have been destroyed long ago by the attraction of the ball. The hypothesis now adopted is that it is composed of myriads of minute bodies, a con- geries of closely packed moons, each of which has an orbit of its own. In the dark ring the bodies are less closely packed together than in the bright ones. At the outer edge of the dark ring they are thought to be more densely crowded than at its inner edge. SATURN. 175 This hypothesis concerning the structure of the rings has been confirmed by observation. The separate particles are much too small to be seen separately, but their existence did not escape the spectroscope in the hands of Keeler in April, 1895. If the bright ring were solid, its outer edge would travel faster than the inner, just as a point on a tooth of a circular saw moves more swiftly than one nearer the centre. On the other hand, if the ring is made up of moonlets, those near its outer edge must move with less velocity than those near the inner. Professor Keeler 's beautiful photo- graphs of the spectrum of the bright ring showed that the outer and inner edges had respectively velocities of 10. 1 and 12. 4 miles per second. These values agree well with theoretical ones computed according to Kepler's Laws. 262. Stability of the System. The Cassini division is supposed to be due to the attraction of the largest satellite, which has changed the orbits of the bodies which once occupied the division. The outer divisions were presum- ably caused in the same manner. These minute bodies must be continually colliding with each other, so that some of them lose velocity, and are drawn into smaller orbits Fi s- J 44- OLD DRAW. , , . c , , ,. rru ING OF SATURN. by the attraction of the ball. The appearance of the dark ring suggests that the ring system is being thus disintegrated. A comparison of the old drawing shown in Fig. 144 with Fig. 142 indicates that the space between the ball and the inner edge of the rings is now smaller than formerly. All these considerations have led to the hypothesis that Saturn is indeed u devouring his children." However, no evidence of such a change is given by accurate measures of the dimensions of the ring made during the past one hundred years. 263. The Satellites. These are eight in number. Japetus, the outermost, is at a distance of 2,212000 miles, and occupies seventy- nine days in a revolution. Mimas, the innermost, is only 30,000 miles beyond the outer edge of the ring system, and completes its circuit in less than a day. Titan, the largest, is nearly as big as Mercury. All move in the plane of the rings excepting Japetus, the orbit of which is inclined 10 to it. Japetus suffers remark- 176 DESCRIPTIVE ASTRONOMY. able and regular variations in brightness, which are explained by assuming that one hemisphere of it is much brighter than the other, and that it always presents the same face to the planet, as our moon does to the earth. 264. Physical Condition of the Planet. The mean density of the ball is less than that of water, or even alcohol, closely agreeing with that of ether. The cloud shell surrounding the kernel of the planet is so deep that it hides beneath its placid exterior nearly all the commotions which are taking place. The central nucleus seems to possess heat sufficient to maintain this cloud mantle, but. not sufficient to give rise to such activity as Jupiter manifests. The spectrum of the ball is that of the sunlight reflected from its surface, with the addition of some dark bands caused by the absorption of the sunlight by an unknown constituent of the planet's atmosphere. The spectrum of the rings contains no- absorption bands. URANUS, & OR #. 265. Discovery. 1 William Herschel discovered Uranus in March, 1781. He was an organist at Bath, England, a man of no mean musical attainments. In studying the mathematical theory of music, he had occasion to enlarge his knowledge of mathematics ; from this he was led to optics, and became exceed- ingly interested in telescopes and astronomy. He resolved to make a reflecting telescope : supporting himself by his profession, he devoted his leisure to grinding and polishing specula and lenses. Rushing home from a concert, he would plunge at once into work on his mirrors, without even stopping to take off his lace ruffles. Mirror after mirror was constructed, put to use, and laid aside or sold, each giving place to a new one, more perfect, or of larger size. When engaged in putting the finishing touches on one of his great mirrors, he often sat at his work for hours, food being put into his mouth by his devoted sister Caroline, who sat by his side, and beguiled the time by reading "The Arabian Nights." 1 This article is chiefly a condensation of material found in Ball's "Story of the: Heavens." URANUS. 177 Such unremitting enthusiasm and genius must find their reward. After half a dozen years of this assiduous toil, he succeeded in constructing a seven-inch reflector of exquisite optical perfection. He resolved to examine all the stars above a certain order of brightness. Now a fixed star is the merest point of light; the more perfect the telescope, the smaller is the image of the star. Star after star passed in review before him. Finally, on the night of March 13, 1781, he perceived an object which looked like Fig. 145. SIR WILLIAM HERSCHEL. a star, except that its disk was a trifle larger than that of a star of the same brightness. Many a time had this object been observed by other astronomers, but they had noticed no peculiarity in its appearance. Herschel soon found that it was in slow motion; he reasoned that it must be nearer than the fixed stars, and not dreaming that he had discovered a new major planet, the others having been known since the dawn of astronomical science, an- nounced that he had found a comet. Astronomers at once set to 12 178 DESCRIPTIVE ASTRONOMY. work observing it. Computations of its orbit followed. Within a year the mathematicians had demonstrated that the orbit was nearly a circle, twice as large as the path of Saturn. The object was therefore a planet. Herschel proposed the name Georgium Sidus in honor of his sovereign; Laplace suggested the designa- tion Herschel. The name finally adopted was proposed by Bode. This notable extension of the confines of the solar system was hailed with the greatest enthusiasm. King George knighted Herschel, and gave him ,200 a year. The further career of Her- schel, who finally constructed a reflector four feet in aperture and forty feet in focal length, stamps him as foremost among astronom- ical observers. 266. Distance and Diameter. The distance of Uranus from the sun is 1,782,000000 miles. Its diameter is 32,000 miles, four times that of the earth. 267. Revolution and Rotation. The sidereal period is eighty- four years. The time of rotation is unknown, because no suffi- ciently definite markings have ever been seen on its surface. 268. Appearance. To the naked eye it appears as a small star just on the limit of visibility. It may be found by the use of the Nautical Almanac, which gives its right ascension and declination throughout the year. In a large telescope it exhibits a greenish disk occasionally marked by faint belts. Granting, as mathematical theory de- mands, that the planes of the orbits of the satellites nearly coin- cide with that of the planet's equator, the belts are not parallel to the equator, an unexplained anomaly. 269. The Satellites. Uranus is attended by four of these bodies, no one of which can be seen by an ordinary eye with a telescope less than eight inches in aperture. The diameter of the largest is probably five hundred miles. Their orbits lie in one plane, which, strange to say, is nearly perpendicular to the ecliptic. They also revolve backwards, that is from east to west, in their orbits, unlike any other satellites before considered. 270. Physical Condition. Of this little is known. The spec- trum of the planet exhibits some conspicuous bands, thought to be due to the absorption of a dense atmosphere. Sunlight at Uranus being only 3- J-g- as intense as at the earth, the processes of cloud NEPTUNE. 179 formation must be dependent chiefly on internal heat. Its mean density is less than that of bituminous coal. NEPTUNE, ^. 271. Discovery. The discovery of Neptune is esteemed the most notable triumph of mathematical astronomy. It was no mere accident, nor was it brought about simply by a diligent search with the telescope. Forty years after the discovery of Uranus, Bou- vard, a French astronomer, published tables of its motion, by means of which its place could be predicted for the future. But the planet refused to follow the path marked out for it; farther and farther it departed from the appointed course. In twenty years the discrepancy between theory and observation had become intol- erable. To be sure, the difference could not yet be perceived by the naked eye, but the unfailing accuracy of the observations loudly proclaimed that there was some fault in the theory of the planet's motion. Was the law of gravitation partially inoperative at this enormous distance from the sun ? Had a flaw been found at last in the marvellous researches of Newton ? By no means. From many quarters came the suggestion that some unknown body was displacing Uranus by its powerful attraction. But could the position of the troublesome stranger be pointed out? John Couch Adams, a tutor in the University of Cambridge, England, grappled with the problem. In October, 1845, ne com- municated to the Astronomer Royal of England the elements of the orbit of the suspected planet, together with a prediction of its place in the sky. But the Astronomer Royal l did not regard these investigations of a young and comparatively unknown man as entitled to much confidence. He however called the attention of a few of his friends to them, and wrote Adams asking some further information : no reply reached him. He therefore pigeon- holed the manuscript. One of the friends wrote to Lassell, who possessed a fine two-foot reflector which was mounted near Liver- pool, begging him to search for the planet. But Lassell was suffering from a sprained ankle, and when he recovered, the letter was nowhere to be found, and the telescopic search was not made. 1 Sir George Biddell Airy. i8o DESCRIPTIVE ASTRONOMY. Meanwhile Leverrier, a brilliant French astronomer, likewise a young man, had employed his powers upon the same problem. On June i, 1846, he sent a communication to the French Academy of Sciences giving the direction in which the planet was to be found. Fig. 146. JOHN COUCH ADAMS. The English astronomers, finding that Leverrier's results agreed with those of Adams, awoke from their lethargy, and began to NEPTUNE. l8l bestir themselves. Professor Challis, the astronomer of the University of Cambridge, commenced a search. Doubting the accuracy of the predictions, he began to map a large area of the sky, hoping by comparison of maps of the same region made on different nights to detect the planet by its change of position if it were really there. Sir John Herschel (son of Sir William), in a public address, said concerning the unknown body: "We see it as Columbus saw America from the coast of Spain. Its movements have been felt, trembling along the far-reaching line of our analysis, with a cer- tainty hardly inferior to that of ocular demonstration." Three times Challis observed the planet, but did not look sharply enough to notice its disk, which was larger than that of the stars. While he was laboriously heaping up observations and neglect- ing to compare them, the prize of discovery slipped from his grasp. Leverrier had written to Galle, of Berlin, where some excellent star charts were being made, asking him to direct his telescope to a certain point on the ecliptic, and saying that he would find within a degree of that point a new planet, as bright as a star of the ninth magnitude ( i) and having a perceptible disk. Galle did as he was bidden, and found the planet within half an hour, on Sept. 23, 1846. Success is to the confident. 272. Distance and Diameter. The mean distance of Neptune from the sun is 2,792,000000 miles. It is therefore a billion miles farther than Uranus, and thirty times as far as the earth. The diameter is 35,000 miles. 273. Revolution and Rotation. The sidereal period is nearly 165 years. The time of rotation is unknown, because no well defined spots have ever been seen on the surface. 274. Appearance. Neptune is too faint to be visible to the naked eye. A good opera-glass will show it. It may be found by using the Nautical Almanac and a star map, as formerly explained ( 255). In a large telescope its greenish disk is readily perceived, but no marks have been seen upon it. 275. Satellite. There is one satellite, a very faint object, sup- posed to be of the size of our moon. The plane of its orbit is in- clined 35 to the ecliptic, and the satellite, like those of Uranus, moves backwards from east to west. 1 82 DESCRIPTIVE ASTRONOMY. 276. Physical Condition. The spectrum is similar to that of Uranus, showing faintly the same absorption bands, which are presumably due to a dense atmosphere. The sunlight is only 9^ as intense as at the earth ; perhaps no cheering ray of sunlight penetrates the clouds in which the planet is entirely enveloped. The density of Neptune is a little less than that of Uranus. The two planets are almost identical in size and general make up. 277. Planets beyond Neptune. Such planets have been suspected on various insufficient grounds ; they have been hunted for with large telescopes, both visually and by means of photography, which brings to light stars too faint to be seen with the most powerful telescopes. No success has yet attended these efforts. The 24-inch Bruce photographic telescope, 1 if used for long exposure photographs in the vicinity of the ecliptic, would reveal hosts of new asteroids, and might bring to notice ultra-Neptunian planets. EXERCISES. 278. i. Why is Jupiter's disk elliptical? 2. The volumes of spheres are to each other as the cubes of their radii or diameters. Verify the statement that Jupiter is i, 300 times as large as the earth. 3. What reasons are there for thinking that Jupiter has no solid crust ? 4. Though Jupiter is i, 300 times as large as the earth, its density is only o. 24 as great. Its mass is therefore how many times that of the earth ? 5. Ought Jupiter to appear gibbous, when at quadrature, like Mars ? 6. Why do Jupiter's belts, if they are due to the absence of clouds, look darker than the cloudy portions? 7. If the interior of Jupiter were so hot as to shine through his atmosphere, wouJd the spectrum be continuous, or crossed by dark lines ? 8. Can one of Jupiter's satellites be in occupation and also in eclipse at the same time? 1 By far the most powerful telescopic camera in existence, the property of Harvard College Observatory. JUPITER, SATURN, URANUS, NEPTUNE. 183 9. (a) If one of Jupiter's satellites were in transit, and were almost exactly between us and its own shadow, would Jupiter be near opposition or near quadrature ? (b} Might it be near conjunction ? 10. (a) Why is an eclipse of one of Jupiter's satellites not instan- taneous ? (b) Is an occupation instantaneous? 11. If Jupiter's atmosphere were sufficiently transparent to let the light of a satellite through when it was disappearing in occulta- tion, would the time of disappearance be delayed ? 12. Would the refractive power of Jupiter's atmosphere delay the time at which a satellite entered upon a transit over his disk? 13. Our moon, when eclipsed, is usually visible. Why are not eclipsed satellites of Jupiter similarly visible? 14. Suppose that the shadow of one of Jupiter's satellites, when moving across its disk, fell upon some portion that was decidedly brighter than the average, would the shadow look darker in conse- quence, or lighter? 15. A person on Jupiter, in the shadow of one of its satellites, would see an eclipse of what ? 16. If satellite I was once, or is now, a fluid mass, why is it elongated ? 17. If satellite I was once fluid, and rotated more swiftly than now, what force has checked its velocity of rotation ? 1 8. If satellites III and IV were fluid, being of the same size and composition, why should III be more elongated than IV? 19. What is the velocity of light, in miles per second, on the assumption that light takes just 1,000 seconds to cross the earth's orbit? 20. If Jupiter's fifth satellite has the same albedo as satellite IV, that is, reflects sunlight just as well, state two reasons why it is difficult to see. 21. The volume of Saturn is how many times that of the earth, if its diameter is nine times as great? 22. The earth's diameter being taken as unity, the diameters of the other planets are roughly as follows : Mercury -J, Venus I, Mars J, Jupiter 11, Saturn 9, Uranus 4, Neptune 4^. Show that 184 DESCRIPTIVE ASTRONOMY. the volume of Jupiter is greater than that of all the other major planets together. 23. Assuming the approximate data in the preceding exercise, find whether the surface of Jupiter is as great as the combined surfaces of the other major planets. 24. Could we ever see an occultation of Mars by Jupiter? 25. Why did Galileo's two attendants of Saturn disappear? 26. What appearance in Fig. 142 shows that the dark ring of Saturn is transparent ? 27. Is the shadow of the ball of Saturn, as cast upon the rings, visible in Fig. 142? 28. Is the shadow of the bright rings of Saturn, cast upon the ball, visible in Fig. 142? 29. Does the plane of Saturn's rings, when extended indefinitely, ever pass between the earth and the sun? 30. Does the sun ever illuminate both sides of Saturn's rings at the same time ? 31. Is one side of Saturn's rings perpetually unilluminated by the sun? 32. If the plane of the rings ever passed between the sun and the earth, could we then see the bright side of the rings ? 33. Suppose that the ball of Saturn was a perfect sphere of uniform density throughout ; also that the ring system was a solid sheet of matter, truly circular, uniform in both thickness and density, and concentric with the ball ; suppose further that one of the satellites attracting the ring pulled it to one side, so that it was no longer concentric with the ball, would the attraction of the ball pull it farther until it struck the surface of the ball ? 34. Would a great difference in brightness between the outer edge of Saturn's dusky ring and the inner edge of the bright ring next to it militate against the theory advanced in 262, that Saturn is "devouring his chijdren " ? 35. What does the absence of absorption bands from the spec- trum of the rings indicate concerning their atmosphere? 36. Why is the direction east to west called backwards in 269? 37. How can the statement in 276, that sunlight at Neptune is only -g-J-g- as intense as at the earth, be figured out from the JUPITER, SATURN, URANUS, NEPTUNE. 185 statement in 272, that Neptune's distance is thirty times that of the earth ? 38. Ought Neptune to look very gibbous when a* quadrature? 39. When Neptune is at opposition, show that the light by which we see it left the sun 8 h. 1 1 m. 40 sec. before it reached us. Assume that light takes five hundred seconds to come from the sun to the earth, and that Neptune's distance from the sun is thirty times ours. 40. Sunlight at Neptune would be how many times as intense as our moonlight ? ( 165.) 41. (a) Does Neptune disturb the motion of Mercury at all? (&) Does Mercury disturb that of Neptune ? l86 DESCRIPTIVE ASTRONOMY. CHAPTER XL COMETS AND METEORS. " Stranger of heaven, I bid thee hail ! Shred from the pall of glory riven, That flashest in celestial gale, Broad pennon of the King of Heaven ! " HOGG. 279. Comets in General. The word "comet " is derived from a Greek word, which means the long-haired one; the designation evidently came from the resemblance of the tail to dishevelled tresses. These bodies are very different in behavior from the staid and trusty planets. They usually come unheralded, change their form and brightness from night to night, display all their antics in a few weeks or months, and are off again, perchance to whisk about some other world in like gay fashion. 280. Discovery. In the early ages only those comets were discovered which were bright enough to be conspicuous to the naked eye. But of late years a comet does not often become visible to the naked eye before one of the comet-hunters l has detected it with his telescope. These observers usually employ small telescopes equipped with low powers, so that the field of view may be large. Hour after hour they scan the face of the sky, hunting for nebulous-looking 1 The following extract about Messier, a comet-hunter of the eighteenth century, is taken from Langley's New Astronomy ; it is given there as a translation from Delambre's History of Astronomy : " He has passed his life in nosing out the tracks of comets. He is a very worthy man, with the simplicity of a baby. Some years ago he lost his wife, and his attention to her prevented him from discovering a comet he was on the search for, and which Montaigne of Limoges got away from him. He was in despair. When he was condoled with on the loss 'he had met, he replied, with his head full of the comet, * Oh dear ! to think that when I had disco'vered twelve, this Montaigne should have got my thirteenth ! ' And his eyes filled with tears, till, remembering what it was he ought to be weeping for, he moaned, ' Oh my poor wife ! ' but went on crying for his comet." COMETS AND METEORS. 187 objects. Faint comets ordinarily look so much like nebulae that they cannot be distinguished from them, except by their motion. A comet-hunter, finding such an object, looks at his catalogue of nebulae to see if it is given there. If not, it may be a comet, and he watches it until he has found out whether it is in motion; if in motion, he announces it as a comet. Photography has now scored its first success in this field. Dr. Barnard was the first to discover a comet by photography. 1 Special photographic lenses are employed which enable the astronomer to photograph on one plate a large region of the sky. One drawback to this method is that the exposure times are necessarily long, since a faint object does not impress itself on the plate quickly. 281. Number : Designation. During the past three thousand years there have been recorded about seven hundred of these bod- ies. Before the invention of the telescope the rate of discovery was slow, because only a few comets are conspicuous objects to the naked eye. At present about half a dozen are found annually, the majority of them being merely telescopic, i. e. too faint to be seen without a telescope. There may be thousands of comets which never come near enough to the earth to be discovered. It has been estimated that millions of them never come nearer to the sun than Neptune does. Kepler thought comets to be as numerous in the heavens as fishes in the ocean. Comets especially noteworthy receive special names. The great comet of 1858 received the name of Donati's comet, Donati being its discoverer. Encke's comet was named for him, because he made some striking researches concerning its movements. The wonderful comet found by Finlay, at the Cape of Good Hope, in the fall of 1882, was so majestic that no man's name has been attached to it. It is known as "The Great Comet of 1882." Other designations are used for the convenience of astronomers. Comet a 1892 denotes the first comet discovered in that year: Comet f would be the sixth comet. Roman numerals are also 1 So faint was the photographic impression that, when another keen-sighted astrono- mer was asked to find the comet on the plate, he was unable to do so, though he succeeded in seeing it after it was pointed out to him. 1 88 DESCRIPTIVE ASTRONOMY. used to denote the order of arrival at perihelion. Comet 1889 V. was the fifth in that year to arrive at its perihelion. 282. Brightness and Visibility. Comets vary greatly in bright- ness, some being so faint that only a powerful telescope reveals them, others being so brilliant that they can be seen in full day- light though close to the sun. The brightness continually changes as the distances of the comet from the earth and sun change. But there are also irregular fluctuations of brightness. There is rarely a week through the year when some comet is not in sight. Some of them are seen only a few weeks before they become too faint to be observed longer. But the large telescopes scattered throughout the world now enable astronomers to follow comets for a much longer time than before. One comet was seen with the Lick telescope over two years after its discovery. 283. Parts of a Comet. Three parts of a comet are usually men- tioned, the coma, or head, the nucleus, and the tail. The coma is the cloudlike form, which is the distinguishing mark of a comet. Faint comets are frequently all coma, no tail or nucleus being visible. The nucleus is a starlike or planetary point in the coma, the most condensed portion of the comet. It is likewise the most brilliant part, and usually contains most of the comet's mass. The tail is the train of tenuous matter which, streaming from the head, is the chief glory, to the naked eye, of a large comet. 284. Forms of Orbits. If a right triangle be revolved about one of its perpendicular sides as an axis, it will generate a cone. The base of the cone will be a circle (Fig. 147). A section CD of the cone, made by a plane parallel to the base, is a circle. If the cutting plane is inclined to the base by a less angle than VAB, the section EM is an ellipse. When the cutting plane FGH is parallel to VA, the section is a parabola. A plane IKL, which is more nearly parallel to the axis VO than FGH was, cuts an hyperbola out of the conical surface. The circle and ellipse are closed curves, but the parabola and hyperbola are not. The three curves are delineated in Fig. 148. The branches of a parabola become more nearly parallel to each other the farther they extend. The branches of an hyperbola continually diverge. A comet which travels in an ellipse will return to our vision at COMETS AND METEORS. 189 regular periods, unless it undergoes some powerful disturbance, which alters its patn. A parabolic or hyperbolic comet never returns. L H Fig. 147. CONIC SECTIONS. Fig. 148. VARIETIES OF ORBITS. 285. Significance of these Forms. Suppose that a small body is at a very great distance from the sun, and both bodies are motionless. The body will begin to fall toward the sun, its path being a straight line directed toward the sun's centre. Another small body, likewise at a distance practically infinite, has a slight motion of its own, but is not moving directly toward the sun; urged on by the sun's imperious attraction, its velocity will contin- ually increase; however, as it is not going directly toward the sun, it will not strike it, but as it goes past the pull of the sun will cause its path to be violently curved ; whirling around the sun, it will return toward the infinite depths of space from which it came ; its orbit is a parabola. A body which has originally a very con- siderable velocity of its own will come down to the sun in an hyperbolic orbit, and then retreat, never again to visit us. A body moving in a parabola may have its velocity checked, as it approaches the sun, by the attraction of some planet : its orbit will thus be changed to an ellipse. Were the movement of the body accelerated by the planet's action, the orbit would become an hyperbola. DESCRIPTIVE ASTRONOMY. 286. Groups of Comets A comparison of the orbits of comets shows that there are certain groups of them, pursuing nearly the same paths. The Great Comet of 1882 belonged to one of these groups, the orbits of those of 1668, 1843, 1880, and 1887 being very similar to its orbit. Since each of these four comets approaches very close to the sun's surface, when at perihelion, a startling theory was promulgated in 1882 that these were one and the same, the periodic time being continually lessened by passing through the sun's atmosphere, and that the comet would plunge into the sun in a few months. It is needless to say that this anticipation was not verified. Tisserand l has recently shown that comet groups may be caused by the disruption of the nucleus of a single comet, in consequence of the heat or tidal action of the sun. The fragments would thereafter pursue very similar orbits, the chief difference being in the periodic times. A comet moving in an ellipse is called aperiodic comet, because it returns at regular intervals. 287. Planetary Families of Comets. Fig. 149 shows the orbits of a number of recent periodic comets. Inspection shows that the aphelion of each orbit lies near the orbit of Jupiter. This fact suggests that Jupiter is the planet which retarded these comets as they were sweeping down towards the sun from interstellar space, and transformed their orbits into ellipses. Jupiter, having a much more powerful attractive force than any other planet, is credited with a larger family of comets, about twenty individuals in all. Neptune, the outpost of the solar system, has succeeded in capturing half a dozen. It should be borne in mind that a planet's influence does not always work in favor of a comet's capture. The planet may be so placed with reference to the comet as to accelerate its motion. 288. Successive Changes in Orbits : Exact Parabolas. A comet which approaches near a planet and suffers a change of orbit may come near it again after a few years and suffer another change. An ellipse may be changed into another ellipse, in which the comet revolves in a longer or shorter period than formerly. A number of such instances are known. Comet 1889 V. (see 292) was in 1884 revolving in an ellipse having a period of about thirty years. In 1886 it came so near to Jupiter that it was 1 Late Director of the Paris Observatory. COMETS AND METEORS. for a short time almost dominated by that planet, and described an hyperbola about it ; when it finally escaped from Jupiter, and yielded to the power of the sun again, its period was only seven years. In 1921 Jupiter will modify its orbit seriously again, prob- ably enlarging it greatly. The ellipse may become a parabola or even an hyperbola. When this is said, it must be remembered Fig. 149. ORBITS OF SOME COMETS OF JUPITER'S FAMILY. that an orbit is called a parabola when its form approaches that curve so nearly that our observations detect no appreciable devia- tion from it. The chances are that no orbit is an exact parabola, for if a comet were moving at any instant in a parabolic orbit, the slightest attraction from any body (it must suffer many such pulls) would change the orbit into an ellipse of very long period, or an hyperbola. The orbits of most comets are sensibly parabolic. 289. Changes of Appearance. When a telescopic comet is first seen, it appears, as has been said, like a filmy cloud on the bosom DESCRIPTIVE ASTRONOMY. of the sky. The coma usually looks more condensed toward the centre. As it approaches the sun, it grows brighter, and the nucleus, if it has any, comes into view, like a blurred star shining through a mass of foggy light. As it is warmed by the sun signs of activity become manifest. The tail gradually forms, increasing in size and splendor as the comet comes nearer the sun. The nucleus seems to be in ebulli- tion, throwing off masses of vapor, or ejecting jets. After peri- helion passage the nucleus gradually becomes fainter, the jets feebler, the head larger, and the tail shorter, until the comet has reached its former low estate, having laid aside the gay trappings with which it was ornamented at perihelion. 290. Jets and Envelopes. The jets or fountains of matter which spurt out from the nucleus as the body nears perihelion are emitted from the sunward side of the nucleus, and are directed toward the sun. They rise higher and higher and become more diffuse, until they are lost in the head. One is exhibited in Fig. 150. A well behaved nucleus throws off en- velopes (Fig. 157). These rise sunward, one after another, becoming fainter and more diffused as they approach the outside of the head. 291. Tails : their Dimensions and Varieties. The tail is by far the bulkiest part of a comet. Tails long enough to reach from the earth to the sun are not uncommon. The extremity of such a tail is millions of miles in thickness. The tail of the comet of 1843 was estimated to be 581,000000 miles long at one time. A tail a tenth as long as this is reckoned highly respectable. Some tails are narrow and straight, like prodigious spines. The forms of others are like half-opened fans. A comet occasionally has several tails, pointing in widely different directions. 292. Companion Comets. Comet 1889 V. (Brooks) was very re- markable on account of the group of companions which attended it. There were four of these comrades, moving along a little in advance of the main comet. Two of the companions were excessively faint, and finally disap- COMETS AND METEORS. 193 peared. The two brighter ones were perfect miniatures of the main comet, having tiny nuclei and shapely tails. But their beauty was evanescent. For a while they receded from the prin- cipal comet. In three weeks the nearer companion ceased to recede; it then enlarged, became very diffuse, and disappeared completely, as if blotted out of existence. The farther corn- Fig. 151. COMPANIONS OF BROOKS'S COMET. panion continued to recede, until it had become (a month after discovery) brighter than the main comet. In a month more it began to come back and shed its tail ; its head swelled and became diffuse and faint, so that it appeared to be in a sorry plight. The companions may have been caused by a disruption of the parent mass by the attraction of Jupiter in 1886. At that time the comet was within the system of Jupiter's satellites for over two days and a half, and may have been struck by one of them ; it may even have '3 194 DESCRIPTIVE ASTRONOMY. grazed the planet's surface. Companion comets are not very common. 293. Constitution of the Head and of the Nucleus. The most plausible theory is that the head is composed of a mixture of solid and gaseous matter. The presence of gas is shown by the spec- trum. The connection known to exist between certain comets and meteors ( 332) renders it wellnigh certain tnat solid bodies are scattered throughout the head. That portions of the solid matter become liquid temporarily, when a comet like that of 1882 dashes through the sun's corona, is almost inevitable. The size of the solid bodies is largely a matter of conjecture. Some think that they are like grains of sand ; others liken them to paving-stones and brick-bats. The nucleus is supposed to be the densest portion of this swarm of meteoric bodies. The nuclei of some large comets may be small solid bodies of great density. 294. Evolution of the Tail. Comets' tails point away from the sun, except in a few rare and anomalous instances. The material carried up by the jets and envelopes seems to be repelled by the sun, and driven away from it, despite its gravitational pull. The nature of the repulsive force is un- known, but it is generally thought to be electrical. Many experiments described in works on physics show that electrical attractions and repulsions, acting upon Fig. 152. DEVELOPMENT light bodies which have a large surface in OF A TAIL. . .,1 ,1 r , -i comparison with their mass, frequently overcome the force of gravity. A body, on the other hand, which has a small surface, but considerable weight, obeys the force of gravity. So the lightest portions of a comet's head might be driven away by an electrical repulsion originating in the sun, while 'the heavier portions, being but slightly affected by this re- pulsion, obeyed the law of gravitation. There is evidence that the nucleus, as well as the sun, repels the finely divided matter. Photographs of Rordame's comet, taken in 1893^ showed that cer- tain condensations in its tail were receding from the nucleus at the rate of fifty miles a second. 1 By Prof. W. J. Hussey, at the Lick Observatory. COMETS AND METEORS. 1 95 295. Types of Tails. A Russian astronomer, Bredichin, has made an elaborate investigation of the forms of comets' tails, and has divided them into three types. Tails of Type I. are nearly straight, and point almost directly away from the sun. They are thought to be composed of hydrogen. SUNWARD LINE Fig. 153. TYPE 1. Type II. consists of the gently curving trains which are most common. These trains are probably composed of compounds of hydrogen and carbon, known to chemists as hydro-carbons : marsh gas and olefiant gas are two of these. SUNWARD LINE ^ ^ SUNWARD LINE Fig. 154. TYPE II. Type III. is not common; it includes short tails of great curva- ture. The axes of such tails are far from pointing away from the sun. Hence they are composed of heavy materials, such as iron vapor. 296. Mass and Density. The masses of comets must be very small compared with those of the major planets; otherwise the earth and other planets would have been disturbed appreciably by comets which have come near to them. No such perturbation has ever been manifest. The combined mass of 100,000 bodies, each as massive as one of the greatest comets, would not equal that of the earth. Since they are of prodigious size, their mean density must be exceedingly small. Sir John Herschel watched a comet as it passed in front of a cluster of very minute stars ; the lustre of the stars was not perceptibly dimmed. Even when a star is nearly behind the nucleus of a large comet, its light is scarcely dimmed. But careful observations of stars which were shining 196 DESCRIPTIVE ASTRONOMY. through hundreds of thousands of miles of cometary matter have shown that their light was refracted a trifle by the gases of the comet. While the mean density of a comet is low, the density of the particles of solid matter which make up most of its mass is probably comparable with that of the materials composing the earth's crust. Were the particles closely packed, the mean density would be largely increased. The tail of a comet is so tenuous that it may be appropriately likened to "such stuff as dreams are made ofc " 297. ^Xight and Spectra. The brightness of a comet generally varies according to the changes of its distance from the earth and the sun. But there are frequent anomalous variations of brilliancy which would not take place were the light merely reflected sun- light. The spectrum of a comet is ordinarily a combination of two spectra: one is a faint continuous spectrum due to reflected sun- light; the other is a spectrum of bright bands, like that produced by the flame of a Bunsen burner. It is due to glowing hydro- carbon gases. Comets which go near the sun, so as to be especially excited by its influence, exhibit spectra in which the lines due to sodium and iron are plainly visible. There is a rapidly accumulating weight of evidence in favor of the theory that a comet derives its light, except that portion which is merely reflected sunlight, almost wholly from electrical dis- charges between its particles. The electrical action is stimulated by the sun, being more intense the nearer the comet is to it. 298. Fate of Comets. The tail of a comet is not to be regarded as the same to-day that it was yesterday. When one of those comets which dash through the corona goes from one side of the sun to the opposite side in a few hours, the tail, though millions of miles in length, appears also to be swung around, like a gigantic scimetar brandished athwart the sky. No known force could cause the tail to swing around with such prodigious velocity. We there- fore conclude that the tail resembles the cloud of smoke puffed out from the smoke-stack of a locomotive. Fresh material is driven off from the head each second, to form the tail. As the smoke of a locomotive does not return to it again, so the particles driven off from the comet in such profusion at its perihelion pas- sage are lost in space. COMETS AND METEORS. 1 97 Thus at each perihelion passage a periodic comet loses a portion of its mass. It must therefore in the course of ages be much reduced in mass and brightness. Sir Isaac Newton expressed the opinion that it was the fate of comets diffundi tandem et spargi per coelos universes, "to be finally diffused and scattered through the celestial spaces." Comets which are not periodic, and which therefore do not visit the sun every few years, may in their infinite journeyings encounter other suns, swing about them in unwonted splendor, and continue their wanderings till they are captured for some sun by the aid of one of its planets, and are then gradually shorn of their beauty. 299. Superstitions. 1 Milton says, in the second book of Paradise Lost : " On the other side, Incensed with indignation, Satan stood Unterrified; and like a comet burned, That fires the length of Ophiuchus huge In the arctic sky, and from his horrid hair Shakes pestilence and war." Such superstitions have been rife from early times. Josephus mentions, among the prodigies which foretold the destruction of Jerusalem, a comet with a tail like the blade of a sword, which hung over the city a year. The Roman Emperor Vespasian, when nearing his end, heard some of his courtiers conversing in a low tone about a comet then visible. But he treated the matter lightly, saying, "This hairy star does not concern me; it menaces rather the king of the Parthians, for he is hairy and I am bald." Throughout the Middle Ages comets seem to have been regarded as especially presaging the death of kings. The comet afterward known as Halley's appeared in April, 1066, when William the Conqueror was about to invade England. "Nova Stella, novus rex," was the proverb of the time. A monk, apostrophizing the comet, said : " Here art thou, source of the tears of many mothers. Long have I seen thee; but now thou appearest to me more terrible, for thou menacest my country with complete ruin." 1 See the English edition of Guillemin's " World of Comets." 198 DESCRIPTIVE ASTRONOMY. The comet of 1528 must have struck terror to the hearts of the beholders. Ambrose Pare, one of the most learned men of that time, writes of it as follows : <' This comet was so horrible, so frightful, and it produced such great terror in the vulgar, that some died of fear and others fell sick. It appeared to be of excessive length and was of the color of blood. At the summit of Fig. 156. COMET OF 1528. it was seen the figure of a bent arm, holding in its hand a great sword, as if about to strike. At the end of the point there were three stars. On both sides of the rays of this comet were seen a great number of axes, knives, blood-colored swords, among which were a great number of hideous human faces, with beards and bristling hair." UNIVERSITY COMETS AND METEORS^^CALIFORjj^^ 199 Through the instrumentality of modern science the terror formerly inspired by great comets has largely given place to a lively delight in watching their beautiful forms and wonderful changes. 300. Collisions. Comets are still feared by many people, on the ground that they may collide with the earth and arrest its motion, so that it will begin to fall toward the sun, or that they may produce such intense heat by the impact that, in the words of Prospero, " The great globe itself, Yea, all which it inherit, shall dissolve, And, like this insubstantial pageant faded, Leave not a rack behind." From what we have already learned concerning the masses of comets, it is plain that there is no ground for apprehension that the earth's orbit would be much changed by collision with even the largest of them. Computations of the orbit of the magnificent comet of 1 86 1 (Fig. 157) showed that the earth probably passed through its tail on a certain night. The result was no more serious than if our planet had been smitten by the club of a phantom. If the earth encountered the head of an ordinary comet, the meteoric masses of which it is presumably composed might be dissipated into vapor when they struck the atmosphere. Should these particles prove to be metallic masses as large as the fist, able to plough through the atmosphere, and to make a fiery rain upon the earth's surface, the bombardment would be memorable. While there are no very definite data to reason from, it is be- lieved that an encounter with the nucleus of one of the largest comets is not to be desired. So vast are the celestial spaces in comparison with the bodies by which they are peopled, that the chance that the earth will strike a good-sized comet some time during the next 100,000 years is exceedingly slight. Most astronomers would be delighted with the prospect that the earth was going to blaze a pathway through some ordinary comet. They would also be pleased to watch some large comet dashing 200 DESCRIPTIVE ASTRONOMY. headlong into the sun. Professor Young thinks that the heat evolved by the collision would be chiefly liberated below the Fig. 157. COMET OF 1861. solar surface, and would simply add a trifle to the sun's store of energy. In November, 1892, when it was supposed that Biela's comet was about to strike the earth, there was considerable fright. The COMETS AND METEORS. 2OI following despatch from Atlanta, Georgia, was printed in a daily paper. " The fear which took possession of many citizens has not yet abated. The general expectation hereabouts was that the comet would be heard from on Saturday night. As one result, the confessionals of the two Catholic churches here were crowded yesterday evening. As the night advanced there were many who insisted that they could detect a change in the atmosphere. The air, they said, was stifling. It was wonderful to see how many persons gathered from different sections of the city around the newspaper offices with substantially the same statement. As a conse- quence, many families of the better class kept watch all night, in order that if the worst came they might be awake to meet it. The orgies around the colored churches would be laughable, were it not for the seriousness with which the worshippers take the matter. To-night (Saturday) they are all full, and sermons suited to the terrible occasion are being delivered." REMARKABLE COMETS. 301. Halley's Comet. Halley, who was a contemporary of New- ton, having learned that, according to the recently propounded theory of gravitation, comets might move in elliptic orbits, and thus be visible at several returns, computed the orbits of two dozen comets. On comparing the computations he observed that the orbits of the comets of 1531, 1607, and 1682 were strikingly simi- lar; he reasoned from this that these three were one and the same body, revolving about the sun in about seventy-five years. He predicted its return about 1758; knowing that he would be in his grave before that time, he expressed a modest hope that, if the comet should return then, "posterity would not refuse to acknowl- edge that this was discovered by an Englishman." In 1757 astronomers began to watch for it. For weary months the quest was vain. Clairaut, a French mathematician, showed by an elab- orate investigation, which challenged the admiration of the world, that the comet would be retarded 618 days by the attraction of Jupiter and Saturn, and would arrive at perihelion in the middle of April, 1759. He said that this might be a month in error. The comet was first seen on Christmas night, 1758, and arrived at perihelion on March 13, a month before the time set. Thus the Newtonian principle of gravitation received a striking verification. 2O2 DESCRIPTIVE ASTRONOMY. This comet was seen many times before Halley's day. Its earliest recorded appearance is supposed to be that of B. C. 1 1. It is expected again in 1910 or 1911. 302. Encke's Comet. This comet is insignificant in appearance, rarely exhibiting a sharply defined nucleus, and sporting the scan- tiest of tails. It was discovered in 1786, and thirty-two years after- wards was found to be revolving in a period of only three years and a quarter, the shortest known period. It is specially interest- ing because its motion suffers much at the hands of Mercury, and is also accelerated in a strange way. Encke's laborious computa- tions showed that its periodic time was shortened nearly three hours at each revolution. More recent observations and discussions show a reduction of the acceleration to one half its former value. The acceleration was at one time considered a triumphant proof of the existence in space of a resisting medium, the luminiferous ether. Such a medium, by retarding the comet's motion, would cause it to drop toward the sun, and revolve in a smaller orbit, in which it would move with greater speed. But the recent diminution of the acceleration does not bear out this theory. The comet's disturbance may be due to collisions with small bodies coming across its path, or to its own feeble activities in the line of throwing off envelopes or jets. Its perturbations by Mercury afford a means of determining the mass of that planet. 303. Biela'i, JJpmet. This was discovered by Biela, an Austrian, in 1826; its periodic time was computed to be 6^ years. It was due again in 1832, and then gave rise to the first comet scare of this century. For calculation showed that it came close to the earth's orbit, and ignorant people became much alarmed at the prospect of a collision. Though it came near the earth's orbit, the earth itself was at no time nearer to it than 15,000000 miles. In December, 1845, it was found to be elongated, and ten days afterward it split into two comets, one of them being much smaller than the other. Both became brighter and developed well defined nuclei and tails. The smaller one grew in brightness until it surpassed the other for a time. Their distance apart became 157,000 miles, and they were lost to view in April, 1846. In 1852 the twins were seen again; the distance between them COMETS AND METEORS. 203 had increased to a million and a half miles; sometimes one was the brighter, sometimes the other. They faded from sight again, and have never been seen since, though they were in a very favorable position in 1872. Comet Holmes, which appeared in November, 1892, was at first thought to be the long lost Biela, but the compu- tation of its orbit showed that this was not the case. 304. The Great Comet of 1882. This was discovered early in September by observers in the southern hemisphere. On Septem- ber 17, the observers at the Cape of Good Hope saw this aston- ishingly brilliant body move swiftly up to the limb of the sun, Fig. 158. THE GREAT COMET OF 1882. and then vanish completely as it swept over its broad disk. On the following day it was seen close to the sun by observers all over the world. It was only necessary to screen off the sun's light by holding up the hand at arm's length in order to see the comet. In less than a week the nucleus, formerly round, became oval, and by the end of a month two centres of condensation were seen. During the next three months the nucleus became divided into four or five condensations, ranged in a line and connected by a hazy mass of light. Meanwhile a magnificent train, 100,000000 miles in length, had been developed. More than half a dozen companion comets were 2O4 DESCRIPTIVE ASTRONOMY. seen, all evanescent. The fiery object seemed to be strewing its path with filmy debris, thrown off by some unknown force. Possi- bly they were fragments driven off by the intense repulsive action of the sun, as the comet dashed through the corona. When at perihelion, it was less than 300,000 miles from the sun's surface. There was a faint but prodigious sheath of cometary matter which enveloped most of the comet, and projected millions of miles Fig. 159. NUCLEUS OF THE GREAT COMET OF 1882, AT DIFFERENT TIMES. in front of the head. The orbit was not appreciably changed by any resistance due to the coronal matter. The periodic time is 772 years. Besides the hydro-carbons ordinarily found in comets, there were in this body sodium and iron; some of the numerous bright lines in the spectrum were probably due to calcium and manganese. 305. Swift's l Bright Comet of 1892. This is mentioned on ac- count of the marvellous changes observed in its tail. On April 4 it was 20 long, straight and slender; in the telescope it was seen to consist of two branches, between which scarcely any cometary matter was visible. The next morning a new tail had formed between the other two, and each tail was composed of several lying close together. At least a dozen could be counted. After the lapse of another day, one of the original three tails had van- ished, and the other two were blended. Then one of these grew bright, and the other faded away; the bright one had a sharp bend in it, as if turned aside by some 1 Lewis Swift, of Echo Mountain, California. COMETS AND METEORS^ 2O5 Fig. 160. SWIFT'S COMET: PHOTOGRAPHED BY BARNARD. 2O6 DESCRIPTIVE ASTRONOMY. obstacle. Near the point of deflection were two dark spots in the brightest part of the tail. Finally the tail split up into six branches. All these changes and some others took place in five days. 306. Comet Holmes. This was discovered on Nov. 6, 1892, by Mr. Holmes, an English amateur astronomer. Its position in the sky was near that in which Biela's comet might appear, and the latter (if still in existence) was due. Therefore the comet was supposed by some to be Biela's, and preliminary computations led to the interesting result that the comet was likely to collide with the earth. On this account a wide-spread popular interest was awak- ened ( 303). The comet changed its position in the sky so little, for several weeks, that its orbit could not be computed with much accuracy at first. It was finally found to be moving in a small ellipse, which is more nearly circular than the orbit of any other known comet; the period is less than seven years. The orbit resembles that of an asteroid. The comet, which was at first vis- ible to the naked eye, grew faint and diffuse in a few weeks. But in the middle of January, 1893, it suffered a strange transformation, changing into a starlike object surrounded by a small circular nebulosity. On January 16, Dr. Barnard saw the nucleus brighten considerably while he was observing it. During the next week the nucleus suffered marked changes of brightness, being very plain at times, and almost invisible at others. The surrounding nebulosity soon grew larger and fainter, and faded away. 307. Comet c 1893 (Brooks). This comet was discovered on the morning of Oct. 17, 1893, by William R, Brooks, of Geneva, N.Y. At first it had a short tail issuing from the northern side of the head, in addition to the main tail, which was straight and of a graceful form. But a photograph taken by Dr. E. E. Barnard on the morning of October 21 (October 20 by astronomical time) showed remarkable changes in the tail, which Dr. Barnard thus describes. 1 " It presented the comet's tail as no comet's tail was ever seen before. The graceful symmetry was destroyed ; the tail was shattered. It was bent, distorted, and deflected, while the larger part of it was broken up 1 Popular Astronomy, December, 1893. COMETS AND METEORS. 2O/ into knots and masses of nebulosity, the whole appearance giving the idea of a torch flickering and streaming irregularly in the wind. The short northern tail was swept entirely away, and the comet itself was much brighter. " The very appearance at once suggested an explanation, which is prob- ably the true one. If the comet's tail, in its flight through space, had suddenly encountered a resisting medium which had passed through the tail near the middle, we should have precisely the appearance presented by the comet. It is not necessary that the medium should be a solid body ; if it possessed only the feeblest of ethereal lightness it would de- flect, distort, and shatter the tail. What makes this explanation all the more probable is that the disturbance was produced from the side of the tail that was advancing through space." Fig. 161. BROOKS'S COMET: PHOTOGRAPHED BY BARNARD. The appearance may also be explained by variations in the amount and in the direction of motion of matter driven off from the comet's head. METEORS. 308. The Two Classes. Meteors are divided into two classes, meteorites and shooting stars. Meteorites are the bright bodies which from time to time dash through the air, like balls of fire, and fall to the ground. There are various other names for them, the most common one being aerolites. Brilliant meteoric objects which do not fall to the earth are ordinarily designated only by the general term "meteor." Shooting stars, on the other hand, are less conspicuous bodies, which can be seen on any clear dark 2O8 DESCRIPTIVE ASTRONOMY. night, darting across the sky; they usually attract no especial attention by their brightness, and never fall to the earth. 309. Past Appearances of Meteorites. Falls of meteorites are recorded before the present era. Though there are many records of the falling of stones from the sky, they were at the close of the last century regarded by most scientific men as old wives' fables. There is one at Mecca, which is adored by the faithful Mussul- man. An Emperor in the Middle Ages was said to have a sword which was fashioned from one of these celestial visitors. In April, 1889, copper earrings plated with meteoric iron were found in an Indian mound in Ohio. During the last decade of the eighteenth century, several falls of meteorites occurred, which were so reli- ably substantiated that scientific men began to inquire into the matter earnestly. In 1803 such a shower fell at L'Aigle in Normandy, that the French Academy sent one of their number to inquire into the matter. So exhaustive was his investigation, and so convincing his report, that the most sceptical were forced to admit that stones fell from the sky. The story of one of the best authenticated as well as most remarkable of meteorites is told in the next section. 310. The Ensisheim Meteorite. This meteorite fell on Nov. 7, 1492, at Ensisheim, in Alsace, and an account of it, together with the stone itself, was put in the church at that place. The account states that on the day in question, some minutes before noon, there was a loud noise, like the rumbling of thunder, and a stone weighing 280 Ibs. was seen by a child to dash into a ploughed field and bury itself about three feet in the earth. Some small pieces of it were taken for examination, but the parent mass was suspended in the choir of the church. There it remained until it was ruthlessly taken away during the French Revolution. It has since been restored to the town hall of the village. 311. A Detonating Meteor. On Dec. 21, 1876, a superb fireball appeared over the State of Kansas, and moved thence eastward south of Chicago across Indiana, over Lake Erie, to Lake Ontario, where it disappeared. When nearly 200 miles from Bloomington, Indiana, the meteor burst, and the inhabitants of that city saw a magnificent array of fireballs sweeping through the evening sky. After the ex- citement aroused by the marvellous spectacle was over, there came a COMETS AND METEORS. 2OQ tremendous crash, like the heaviest reverberations of thunder. The concussion which accompanied it led some to think that a light earthquake had shaken the town. How terrific must a detonation have been, which was so startling nearly 200 miles away, after the sound waves had been on their journey a quarter of an hour ! 312. Kiowa County (Kansas) Meteorites. In March, 1890, the attention of scientific men was called to several strange pieces of iron, which had been ploughed up from time to time in Brenham township, Kiowa County, Kansas. They had been put to various uses, such as holding down the cover of a rain barrel, and keeping the roof of a stable from blowing away, and helping to stop a fence hole through which hogs escaped from their feeding ground. One had risen to the dignity of ornamenting the sidewalk in front of a real estate office. A cowboy and a woman were the only people in the vicinity who seemed to realize the value of these articles. The cowboy, being unable to carry his off, buried them, and died shortly afterward. The woman sent for a college professor, and disposed of hers for enough to pay off the mortgage on her farm. The meteorites were found scattered over an oval area about a mile long. The largest mass, called by the farmers the " moon me- teorite," weighed 466 pounds. The total weight of the twenty odd pieces found was about a ton. 313. A Meteorite in Flight. A flying meteorite is an object of dazzling splendor, when seen by night. It is generally followed by a luminous train, which may remain for some minutes. A noise like the roar of artillery is heard, with occasional crashes like those caused by the explosion of shells, which signalize the breaking of the main body. When the meteorite is many miles away, one may see a flash of light without hearing the accompanying detonation. In a very few seconds all is over, save that there is a bright irregular streak of light on the sky, marking the meteor's path. A recent meteorite is reported as having the distinct shape of a banana, and as turning end for end as it flew, scattering sparks along its pathway: it glowed like a piece of red-hot iron, though the sun was half an hour above the horizon. The meteor's velocity is so effectually checked by the resistance 14 2IO DESCRIPTIVE ASTRONOMY. of the air, that it finally comes to earth like a spent cannon ball or a shower of grape. 314. Path and Velocity. Meteorites move in orbits about the sun : ninety per cent of those orbits which have been computed are elliptical. These bodies generally become inflamed at a height of eighty miles or more above the earth's surface, despite the rarity of the air at that elevation. The path is occasionally hundreds of miles Fig. 162. METEOR SEEN AT BASSEIN, BURMAH. (From WinchdFs " World Life," by permission.) in length. The velocity of a meteorite with reference to the earth depends upon the relative directions of their motions. When the collision occurs " head on" the relative velocity exceeds forty miles a second. When the meteor catches up with the earth, both being in motion in the same direction, the relative velocity sinks below ten miles a second. The velocity with which the meteor reaches the ground is often considerably less than that of a cannon ball, as is shown by the slight depth to which it penetrates. Notwithstanding this comparatively low velocity, a meteorite is no insignificant mis- sile. There are a few records of the destruction of buildings by them. The Chinese, not to be outdone by Western nations, have a record of one which came to earth 2,500 years ago, destroying several chariots and killing ten men. COMETS AND METEORS. 2 I I Though the meteorite which flew into fragments over Madrid on Feb. 10, 1896, was about fifteen miles above the city, the con- cussion caused strong vibrations of partitions of houses, and exten- sive damage to windows. 315. Cause of Light and Heat. When the motion of a cannon ball is arrested by striking an armor plate, the ball and the plate are heated, so that the armor plate becomes viscous at the point of striking, and flows like tar. The energy possessed by the ball be- cause of the swiftness of its motion is transformed into heat when the motion is arrested. By the same principle a nail is heated, when struck repeatedly by a hammer. The energy in the mov- ing hammer is changed into heat. As the speed of a rifle ball is checked when it is fired into water, the speed of a meteorite, which may be one hundred times as great as that of the rifle ball, suffers disastrous diminution even in the upper regions of the atmosphere, and is almost destroyed before the body reaches the ground. The energy lost by the meteor, as its speed diminishes, reappears in heat. The air is heated enormously ; the quantity of heat developed, if it were all spent on the meteorite, would liquefy it, were it of iron. The meteor shines, because its surface is intensely hot: most of the light which we see undoubtedly comes from the incandescent gases surrounding the meteor. The train left behind remains luminous for so long a time that its light cannot be accounted for by heat alone : it may possibly be due to phosphorescence. 316. Effect of the Heat. If a candle were thrown through the flames of a large bonfire, some of its surface would be melted off, but the interior of the candle might not be heated perceptibly. In like manner, a meteor when dashing through the heated air is affected as if it were passing through a sea of burning gas raging with uncontrollable fury. The outside of the meteor is fused at once, and wiped off to form the train. As the exterior is very hot, the meteorite is liable to crack, and strew its path with its own debris. Sometimes the heating causes a terrific explosion, or a series of explosions, which reduce the meteor to small Tragments. So rapid is the entire process that the heat at times does not penetrate to the interior of the mass. While most pieces of freshly fallen meteorites are too hot to handle, some are cold. A portion 212 DESCRIPTIVE ASTRONOMY. of one which fell in India was found, about half an hour afterward, embedded in moist earth, and coated with ice. The intensity of the heat to which a meteor is exposed may be illustrated by the case of a fireball which was observed in England in 1869. The luminous fiery envelope was more than four miles in diameter, and the entire meteor was vaporized in five seconds, while travelling 170 miles. There remained a cloud of glowing vapor about fifty miles long and four miles broad, which was visible for fifty minutes. 317. Meteoric Stones. Most of the meteoric masses which fall to the earth are of a stony nature. When found they are glazed over with a thin crust formed by the fusion of the exterior during the flight. When a meteorite bursts just |^fore falling, the freshly broken surfaces are not thus incrusted, and the pieces have in some cases been fitted together again. The surface is usually indented with numerous pits caused by the fusion of parts of the meteoric mass. The structure of some of these objects is like that of certain volcanic rocks, which are formed of irregular masses of various ma- terials held together by a cement. In half a dozen meteorites com- pounds of carbon have been found, which are like those resulting from the decay of vegetable life ; but no forms of vegetation such as we frequently find in terrestrial sandstones have been discovered. 318. Meteoric Iron : Intermediate Forms. A small percentage of meteorites are composed of iron, which is alloyed with nickel in all yet analyzed. Only about a dozen of these have actually been seen to fall. The others have been found lying on or near the surface, in places where there is no other iron in the vicinity. A few of these masses weighed several tons. There are forms intermedi- ate between meteoric iron and Fig. 163. A METEORITE. stone. In some of these there (From " Science," by permission.) . , is a honeycombed mass of iron, the cavities in which are filled with various minerals. In others bits of iron are found scattered through a stony mass. COMETS AND METEORS. 213 319. Elements found in Meteorites. More than a third of the known chemical elements are found in meteorites ; no new element has been discovered in them, but some new compounds have been found. Most of the elements are common ones, such as sulphur, phosphorus, copper, tin, aluminum, calcium, etc. There are, as Fig. 164. THE CANYON DIABLO METEORITE. yet, no traces of gold or silver. Small black diamonds, and also minute white ones, have been found in cavities of meteoric iron, which came from the Canyon Diablo, in Arizona, where about three hundred fragments of meteoric iron were discovered in 1891 : the largest piece, weighing 1,015 pounds, was on exhibition at the World's Fair in Chicago. Dust from the diamonds was employed in the Tiffany pavilion at the Fair, to polish other diamonds : the success of the experiment proved conclusively that the hard black grains from the meteorite were genuine diamonds. 320. Origin. The Austrian mineralogist, Tschermak, after a care- ful study of the structure of meteorites, reached" the conclusion that they had a volcanic origin. The volcanoes of the moon are now ex- tinct. No terrestrial one has sufficient power to eject a missile with 214 DESCRIPTIVE ASTRONOMY. a velocity of six miles a second, which would be necessary to carry it away from the earth's attraction, if the earth had no atmosphere, and to render it obedient to the sun. But when these bodies or other planets were young, their volcanoes may have possessed the necessary energy. The sun, as we have mentioned ( 77), has been seen to eject masses of heated vapor with such a velocity that they would not fall back again. Such masses, exposed to the cold of space, would be condensed into solid bodies, like the meteorites. It has been the general belief among astronomers that meteorites originated like the large heavenly bodies, from the condensation of nebulous matter scattered throughout the universe. But Tscher- mak's theory is considered worthy of careful examination. 321. How to Observe a Meteorite. The observations necessary for determining a meteor's path can be easily made by any intelligent person who gets a fair view of it. Two things are to be observed, the time and the direction of movement. At night one familiar with the constellations may note its path among the stars, and the time at which it disappears. By repeating " Mary had a little lamb," etc., beginning when the meteor is first seen and stopping at its disappearance, the length of time during which it is visible can be estimated ; for the number of seconds required to repeat the same snatch of rhyme is easily obtained afterwards by rehearsing it, watch in hand. In the daytime, the position of the body when first and last seen can be noted with refer- ence to surrounding objects. The altitude and azimuth ( 1 21) of the object at each of these times can be measured later with a sur- veyor's transit, placed at the point where the observer stood. 1 SHOOTING STARS. 322. Numbers. One can scarcely look at the face of the sky for fifteen minutes on a clear moonless night without seeing at least one of these objects dart harmlessly across the sky, and disappear in silence, leaving no trace behind. It has been estimated that if ob- 1 A careful report of such observations, however rude they may seem to the ob- server, would be welcomed by any astronomical periodical. Such communications may be sent to the "Astronomical Journal," Cambridge, Mass., or to " Popular Astronomy," Northfield, Minn. COMETS AND METEORS. 215 servers were uniformly distributed over the earth, in such a way as to command a view of all the shooting stars entering the atmosphere, ten or fifteen million of them would be found to strike the earth during a day. * ; They are twice as frequent at 6 A. M. as at 6 P. M. For in the morning we are in front of the earth, as it moves in its orbit, while in the evening we are in the rear, as shown in Fig. 165. At A the sun is rising : at B it is setting. The meteors are supposed to be com- ing from all directions. SUN EARTH 1 Fig. 165. RELATIVE FREQUENCY OF METEORS IN THE MORNING AND EVENING. 323. Paths and Velocity. A shooting star which is coming di- rectly toward the observer has no visible path. It is simply an evanescent bright spot on the sky. Those which shoot to one side of him usually have paths several degrees in length. The paths of some meteors exhibit abrupt changes of curvature : the meteors appear to glance, as a skipping stone does on the water. By means of observations taken by men stationed several miles from each other, shooting stars have been found to be on the aver- age seventy-five miles above us when they are ignited, and fifty miles when they disappear. While glowing they travel forty or fifty miles at rates of from ten to fifty miles per second. Like the meteorites, they have orbits about the sun. A few double meteors have been seen moving side by side, some of them being connected by a ligament of light. They were telescopic. 324. Masses and Constituents. Since these bodies perish when they have encountered the extremely rare atmosphere which exists at heights of from fifty to seventy-five miles, they must be insignifi- 2l6 DESCRIPTIVE ASTRONOMY. cant in mass. Most of those which compose a shower are believed to be less than a grain in weight. One as brilliant as Jupiter or Venus at their best, may weigh 50 or 100 grains. These rather insecure estimates are based upon measurements of their light, combined with those of their velocity. The spectroscope shows that sodium and magnesium are constituents of shooting stars. Some years since, the Swedish naturalist Nordenskiold melted a quantity of snow taken from polar regions, and discovered in the water minute particles which proved to be compounds of iron : they were hailed as the debris of shooting stars. But the great eruption of Krakatoa in 1883 taught us that fine volcanic ashes may be carried in the air for thousands of miles be- fore they finally settle to the earth. The particles found by Nor- denskiold may therefore be the products of volcanic eruptions. Fig. 166. THE RADIANT. 325. Radiant. When a shower of shooting stars comes, the ap- parent paths of the bodies when produced backward meet at a spot on the sky, which is called the radiant point, or simply the radiant. One who looks upward during a gentle fall of snow will observe the same phenomenon with reference to the paths of the snowflakes. The paths prolonged backward seem to converge toward the zenith. But the snowflakes are descending in parallel paths, and the convergence is explained as an effect of perspective. Hence COMETS AND METEORS. 217 we infer that the shooting stars are coming in parallel paths. As the hours of the night wear away, the radiant remains in the same place among the stars : from this we conclude that the meteors come from the same direction, so that there must be a stream of them, pouring a bootless fusillade in upon the earth. Meteoric showers are frequently named from the constellation in which the radiant lies. The Leonids, Perseids, and Andromedes come from the constellations Leo, Perseus, and Andromeda, re- spectively. 326. The August Shower. The August meteors are popularly known as the " Tears of St. Lawrence." They are most numerous about August 10, when an observer may see one every minute, the radiant being in the constellation Perseus. Perseids are visible in greater or less numbers during the latter half of July and the first three weeks of August (Fig. 167). This shows that the meteoric stream is very broad, for the earth moves about 60,000000 miles while passing through it. The meteors are distributed rather uniformly along the orbit, though there are occasional gaps. In August, 1892, the shower did not come. The orbit is an ellipse extending beyond the orbit of Neptune, and the period of revolution is over 100 years. 327. The November Leonids : Appearance : Velocity : Orbit. Every year about November 13 there is a shower of meteors emanating from the constellation Leo. In most years they are rather insig- nificant, but once in 33 years the magnificence of the display is appalling. When the encounter takes place, the meteors come in a direction nearly opposite to that in which the earth is moving. The velocity of the meteors is 26 miles a second, and that of the earth 18 miles a second, so that the missiles pelt the earth as furi- ously as if they were going 44 miles a second, and the earth were at rest. They have a brilliant bluish light, and leave vivid trails. This meteoric system is not diffused around its orbit, like the August meteors, but is largely condensed in a single shoal about 2,000,000000 miles long. Their orbit is a long ellipse, the peri- helion of which is near the path of Uranus. The periodic time is 33! years. 328. The November Leonids : Past Showers. The earliest re- corded appearance of this shower was in 902 A. D. On the very ai8 DESCRIPTIVE ASTRONOMY. Fig. 167. THE ORBIT OF THE AUGUST SHOWER. (From Winchell's " World Life," by permission.) night when a Moorish tyrant died, " by the judgment of God," there were seen, " as it were lances, an infinite number of stars, which scattered themselves like rain to right and left." That year was COMETS AND METEORS. 2IQ called the "Year of the Stars." On the night of Nov. 12, 1833, the display was probably the most magnificent on record. The falling stars were as thick as flakes in a snowstorm ; there were many fire- balls brighter than Venus ; one is said to have looked larger than the full moon. The utmost terror was inspired in the ignorant. A South Carolina planter wrote : " I was suddenly awakened by the most distressing cries that ever fell on my ears. Shrieks of horror and cries for mercy I could hear from most of the negroes of the three plantations, amounting in all to about six or eight hundred. While earnestly listening for the cause I heard a faint voice near the door calling my name. I arose, and, taking my sword, stood at the door. At this moment I heard the same voice still beseeching me to arise, and saying, 'The world is on fire ! ' I then opened the door, and it is difficult to say which excited me the most, the awfulness of the scene, or the distressed cries of the negroes. Upwards of one hundred lay prostrate on the ground, some speechless, and some with the bitterest cries, with their hands raised, imploring God to save the world and them. The scene was truly awful ; for never did rain fall much thicker than the meteors fell to the earth : east, west, north, and south, it was the same." In 1866 there was another wonderful display, on the night of November 13. The next great shower is expected in 1899; the meteoric shoal is so long, that there may be a fair shower in 1898. 329. History of the Leonids. Fig. 168 illustrates the supposed introduction of the Leonids into our system. Their probable his- tory is as follows. Prior to the second century of our era they were coming toward the solar system in a tolerably compact swarm mov- ing in a parabolic orbit. Neptune, the first picket, was successfully passed. But Uranus came along opportunely, and in A. D. 126 gave them so powerful a tug that their orbit was changed to the ellipse shown in the figure, and they found themselves subject to the sun. The attraction of Uranus also distorted the shoal, and caused its various components to move in slightly different orbits : the sepa- rate meteors were made to move with slightly different velocities, so that the shoal became elongated. Time and time again the earth dashed through it. Furthermore, the attractions of Jupiter and Saturn kept shifting its orbit, until it now occupies the position shown by the dotted lines in the figure. Looking ahead, we may 220 DESCRIPTIVE ASTRONOMY. prophesy that the shoal will, in the course of ages, become dis- tributed around the orbit, as are the August meteors. 330. The Bielids. This meteoric shower comes in the latter part of November each year. It overtakes the earth, striking it with a Fig- 168. CAPTURE OF THE LEONIDS. relative velocity of only 12 miles a second. The radiant point is in Andromeda. Brilliant displays were seen in 1872 and 1885. In 1892 the meteors were expected on November 27, but arrived on November 23. The early arrival was afterwards discovered to be an effect of Jupiter's attraction upon the meteoric stream. Com- parison with old records shows that the shower, though somewhat irregular in the date of its appearance, comes gradually earlier and earlier, gaining a fortnight in a century. The swarm derives great interest from its supposed connection with Biela's comet ( 303). The comet seems to be lost, but a meteoric shower more pronounced than the average comes when the comet is due. Some writers speak of these meteors as fragments of Biela's comet. In the shower of 1892 several could be seen every minute by a single observer. Another fine display is expected in 1898. COMEtS AND METEORS. 221 331. Meteoric and Cometic Orbits. As soon as the orbits of the great meteoric swarms were computed, it was perceived that they were similar to those of certain comets. The August meteor swarm moves in an orbit which is identical with that of the bright comet of 1862. The meteors which cause the great 33 year November shower follow hot on the trail of Tempel's comet (1866, I.). Several other similar coincidences are now known. 332. Relation between Comets and Meteors. The Bielids are con- sidered by many to be portions of Biela's comet. The bright comet of 1862 seems to be simply a condensed por- tion of the August meteoric swarm. Tempel's comet and the 33 year shower may both be parts of the original mass brought into our system by the attraction of Uranus. The general opinion is that shoals of meteoric matter ac- companying comets are the products of the disintegration of the cometary masses. 333. How to Observe a Meteoric Shower. 1 In order to calculate the orbit of a shower it is necessary to know the position of the radiant point. A careful map of the stars visible to the naked eye in the vicinity of the radiant is first prepared. This may be done easily by putting a piece of tracing paper or tracing cloth over a good star map. On tracing cloth the stars may be well marked by small dots of ink. The completed map should be securely fastened to a smooth board. A watch of the sky for a few minutes during the shower will enable one to locate the radiant point fairly. With eyes directed toward this spot, the observer notes the apparent path of some meteor ,' he then traces it upon the map, marking as accu- rately as possible the beginning and end of the path. A stick held in the hand, and placed parallel to the meteor's path, will be of de- cided assistance. Those meteors having short paths are best suited for fixing the position of the radiant. When the observations are finished, the paths marked on the map should be prolonged back- ward till they intersect : they will not all intersect at the same point. After studying the map, the observer should mark the spot which 1 See articles by W. F. Denning in " Popular Astronomy " for October, November, and December, 1893. 222 DESCRIPTIVE ASTRONOMY. >%*. he deems to be the true position of the radiant, and find its right ascension and declination. Another person, or several others, may make repeated counts of the number of meteors visible in ten minutes, noting the time when each set of observations was begun. Still other observers may note the paths of brilliant ones, and in- teresting phenomena concerning them, such as their brightness, color and length of train. Successful observations should be pub- lished as recommended in the note to 321. The apparent paths of the brighter meteors can now be de- termined with more accuracy by photography, than by visual observations. THE ZODIACAL LIGHT. 334. In the early spring one may see, when twilight has faded away, a faint hazy beam of light projecting up from the horizon in the west. It lies in the ecliptic, and can be traced 90 or more from the sun without much difficulty. In autumn it can be well seen in the morning before sunrise. It is said to have been observed to follow the ecliptic clear around the sky. There are many theories as to its cause : the most widely ac- cepted is that it is due to a countless host of meteoric bodies revolv- ing about the sun, and constituting a huge figure resembling in shape a double convex lens. The Gegenschein, or Zodiacal Counterglow, lies opposite the sun, and is a very faint round appearance, a trifle brighter than the adjoining portions of the zodiacal light. Observations of it are exceedingly difficult. 1 EXERCISES. 335. I. May Comet a 1907 be also Comet 1907 II.? 2. Why are jets spurted out from the nucleus of a comet on the side next to the sun rather than on the other side ? 3. Why are comets' tails when composed of a heavy material (Type III.) more sharply curved than when composed of a light material (Type I.)? 1 See Dr. Barnard's interesting and important observations, published in No. 308 of the " Astronomical Journal." COMETS AND METEORS. 223 4. If a comet were a compact sphere 80 miles in diameter, the average density of which equalled that of the earth, its mass would be what fractional part of the earth's mass, the earth's diameter be- ing called 8,000 miles ? 5. A circular shield in front of the earth, to protect it in case of collision with a comet, would have an area of about 50,000000 square miles if it were 8,000 miles in diameter. Verify this statement. Suppose that 1,000,000000 masses of the size of a man's fist uni- formly distributed throughout the comet pelted the shield during such a collision. Is it likely that a particular house located on the front of the shield would be struck? 6. If the nucleus of a comet spurts out a jet toward the sun, does that action tend to drive the nucleus away from the sun? 7. If a meteorite overtake the earth, is it less liable to be shattered in the air than if it meet the earth? 8. What evidence is there that a meteorite, before it strikes the atmosphere, is a cold body? 9. On some clear moonless night observe a meteor, estimating its time of flight, and the direction in which it moved. Note its color and the length of its train (in degrees), remembering that the dis- tance between " The Pointers " in the Great Dipper is five degrees,. 10. Do meteors gradually increase the size of the eajth? 11. Do those meteors which meet the earth, and thus resist its motion, tend to lengthen or shorten the year? ( 302.) 12. The sun must be struck by meteors, as well as the earth. Do meteors tend to increase the attraction between the sun and the earth? 13. If the attraction between the sun and the earth be thus in- creased, does the increase operate to shorten the year? 14. Would a decrease of the distance between the sun and the earth operate to shorten the year? 15. Judging by the effect of meteors on the temperature of the earth, do you think that the sun's heat can be accounted for by their impact? 224 DESCRIPTIVE ASTRONOMY. Fig. 169. STARS VISIBLE TO THE NAKED EYE. THE FIXED STARS. 225 CHAPTER XII. THE FIXED STARS. " What involution ! what extent ! what swarms Of worlds, that laugh at earth ! immensely great ! Immensely distant from each other's spheres ! " YOUNG. 336. Number Visible. The stars visible to the naked eye are by no means countless. ,. Over 2,000 of them can be seen at one time, under favorable circumstances. Were it possible to see the entire celestial sphere as well as one sees the portion of it near the zenith, more than 6,000 stars could be enumerated. There are multitudes of stars just below the limit of. naked-eye vision which a spy-glass brings out without difficulty (Fig. 169). It will show twenty stars for every one seen without its aid. The forty-inch Yerkes telescope (Fig. 170) of the University of Chicago may be capable of revealing 1,000 times as many stars as a lady's opera-glass. As faint stars are much more numerous than bright ones, only a few hundred stars can be seen without a telescope when the full moon dominates the heavens. 337. Scintillation. Comparison of stars near the zenith with those at a greater distance from it shows that stars twinkle the more, the nearer they are to the horizon. Since the light coming from those near the horizon passes through a greater thickness of air than that from those at higher altitudes, it is more violently disturbed. The strata of air through which it passes have many degrees of density, so that the light is refracted hither and thither on its way to the eye. The image of a star in a telescope is made to boil or dance. The irregular refraction of the light also causes a phenome- non described in works on physics as u interference." In conse- quence of this there are continual changes of color; the flashes of many colors which emanate from Sirius, the brightest of the fixed stars, when it is near the horizon in the early evening in midwinter, are very beautiful. An electric arc light scintillates when seen at a distance of several blocks. Scintillation is generally most marked on windy or frosty nights. When the stars twinkle vio- 226 DESCRIPTIVE ASTRONOMY. lently, telescopic observations are usually of little value. A little haze uniformly suffused through the atmosphere reduces scintillation Fig. 170. THE YERKES TELESCOPE AT THE WORLD'S FAIR, 1893. to a minimum : on a slightly hazy night the stellar images seen in a telescope are in general small and neat, and bear magnifying well. THE FIXED STARS. 22 J The planets, having much broader disks than the fixed stars, twinkle very little ; when one point of their disks is temporarily dark- ened by interference, another may become brighter, so that the total quantity of light from all points of the disk remains about the same. Fig. 171. A PORTION OF THE MILKY WAY. 338. The Milky Way. The Milky Way, or Galaxy, is the broad bright stream of stars which encircles the heavens, and exhibits a 228 DESCRIPTIVE ASTRONOMY. fine contrast with the blackness of the sky at times when the moon is not visible. The Galaxy is not of uniform brightness ; in places there are striking dark spots, many of which look like vast abysses : one in the constellation of the Centaur is called the Coal Sack : in Cygnus there is another starless space, smaller than the Coal Sack. The portion of the Milky Way seen south of the zenith in middle north latitudes in midsummer is divided into two streams, lying side by side. The sheen of the Galaxy is due to the fact that it is com- posed of millions of closely packed faint stars. In the brighter por- tions of it, hundreds of stars can be seen in the field of view of an opera-glass. Photographs taken with large lenses and long expos- ures show that there is a marvellous complexity of structure ; there are sprays of stars, and vast cloud-like appearances, 1 which are crossed by dark lanes and bestrewn with dark spots. 339. Tree-like Structures. - Many solar prominences have tree-like forms : one is astonished to find such forms in the Milky Way, but photography gives indubitable evidence of their exist- ence. Some of these are dark, and others are bright. Fig. 172 represents a dark plant-like structure near Alpha Cygni, which appears on a photograph taken by Dr. Wolf 2 with an exposure time of eleven hours. It has been conjectured that these forms are due to colossal uprushes into a resisting medium. But the dimensions of these mysterious objects are so enor- mous that this explanation seems inadequate. The matter is still further complicated by the fact that Fig. 172. PLANT- manv o f the dark structures are bordered by lines of LIKE STRUCTURE. stars. 340. The Constellations. In the early ages men grouped the brighter stars fantastically, and gave names to the groups. The Latin forms of these names are now employed. Some of the most commonly known constellations visible in the United States are Cassiopeia (the Lady in the Chair), Cygnus (the Swan), Leo (the Lion), Lyra (the Lyre), Orion, and Ursa Major (the Great Bear). Most of the constellations are named after mythological personages, or after animals. Ptolemy, who died A. D. 170, revised the scheme 1 Dr. Barnard first photographed these. 2 Dr. Max Wolf, of Heidelberg, one of the foremost of celestial photographers. THE FIXED STARS. 22Q of constellations known to the ancients and transmitted to us forty- eight of them. More modern astronomers have added a large num- ber of other constellations to fill in the spaces not covered by the old ones. Nineteen of these are now generally recognized. 341. Names of Individual Stars. Many of the brighter stars have received proper names, drawn from the Latin, Greek, and Arabic languages. Such are Sirius, Polaris, Rigel, Aldebaran, and Vega. Practical astronomers use only a few of them. Such names as Skat, Rotanev, Muphrid, and Zavijava are sinking into deserved oblivion. Naked-eye stars are most commonly designated by letters or numbers prefixed to the genitive case of the Latin name of a con- stellation. The general plan is to call the brightest star in a given constellation by the Greek letter Alpha, the next brightest being Beta, and so on through the alphabet. Thus Alpha Lyrae is the brightest star in Lyra. After the Greek alphabet ( 405) is ex- hausted, the Roman is used. Thus we have such names as Delta Herculis and/"Herculis. The system of Greek and Roman letters does not follow the order of brightness accurately. There is a sys- tem of numbering which is independent of the other two, the stars of a constellation being numbered according to the order in which they cross the meridian. For instance I Orionis crosses before 2 Orionis. A star may have both a letter and a number, the former being preferred : Eta Aurigae is the same star as 10 Aurigae. For telescopic stars the names are taken from catalogues. Lalande 19,486 is the 1 9,486th star in Lalande's catalogue of stars -( 344). 342. Orders of Brightness. Stars visible without telescopic aid are divided into six orders of brightness, called magnitudes ( i). Stars of the sixth magnitude are just perceptible by an ordinary eye : two thousand of them are visible in the United States. A few of them may be seen within the bowl of the Great Dipper. Those of the fifth magnitude are plain to persons who are not short-sighted. The uppermost of the three distinct stars in the sword-handle of Orion is of the fifth magnitude. There are twenty stars of the first magni- tude, fourteen of which lie between the north pole and 35 of south declination ( 122), and can be seen from any point in the United States. Polaris is of the second magnitude. An average star of 230 DESCRIPTIVE ASTRONOMY. the first magnitude is one hundred times as bright as one of the sixth magnitude. 343. Magnitudes of Telescopic Stars : Ratio of Magnitude. The sys- tem of magnitudes outlined in the preceding section is extended to telescopic stars. Of late years a uniform ratio of brightness between stars of successive magnitudes has been adopted by astronomers ; it is called the "light ratio." A star of a given magnitude is 'V/ioo times as bright as a star of the next lower magnitude: -v/ioo = 2.5 nearly. A star of the third magnitude is 2.5 times as bright as one of the fourth. The forty-inch telescope mentioned in 336 should reveal stars of the seventeenth magnitude. Stars even fainter than this can be photographed. The brightness of a star the magnitude of which is between two integral magnitudes is expressed by the aid of decimals. Thus a star of the 6.4 magnitude is fainter than one of the sixth, and brighter than one of the seventh magnitude. 344. Star Catalogues. An observer with a' meridian circle and a clock can determine the right ascensions and declinations of a large number of stars, as explained in Chapter VI. These, when arranged in the order of their right ascensions, constitute a star catalogue. The names of the stars and their magnitudes are also given. A small catalogue is given each year in the Nautical Almanac. Each of the star places given in this catalogue depends upon hundreds of observations. 345. Photographic Star Charts. Stars whose right ascension and declination have been found can be charted, but the work is very laborious. Charts are made much more expeditiously by the use of photographic plates. A number of observatories, scattered throughout the world, are now (1896) engaged in securing photo- graphs of the entire sky ; the plates will exhibit millions of stars which have never been catalogued. Prof. E. C. Pickering has planned a similar campaign with the Bruce photographic telescope. This instrument should do such work with much greater rapidity than others hitherto constructed. Its objective is a quadruple lens, two feet in aperture. 346. Distribution. The stars visible to the naked eye are distrib- uted over the face of the sky with tolerable uniformity. This is shown in Fig. 169. When telescopic stars are taken into consider- ation the case is very different. These are massed in great profu- THE FIXED STARS. 23! sion in and near the Milky Way. The farther one goes from the Galaxy, the fewer the stars become. This fact was established by the observations of the Herschels, father and son, who pointed a large telescope equipped with a certain magnifying power to a few thousand different places in the sky, and counted the number of stars visible in the field of view each time. In the Galaxy, 122 stars, on the average, were visible at one time in the field of view; 15 from the Galaxy, only thirty were similarly brought into view; at a distance of 30, only eighteen were seen; at 45, ten were counted ; 90 away, the average number sank to four. There is here a resemblance to the distribution of vegetation on the earth : it is most luxuriant at the equator, and diminishes as one goes toward the poles. 347. Clusters. The unassisted eye reveals several coarse clusters, of which the best known is the Pleiades, in the constellation Taurus. The Hyades in the same constellation, Praesepe in Cancer, and the cluster in the sword-handle of Perseus are plain to the naked eye, and will well reward the trouble of looking at them with an opera- glass. In some telescopic clusters, the stars crowd upon one another so closely that the telescope cannot separate them ; near the centre of the well known cluster in Hercules star crowds upon star in a blaze of glory (Fig. 173). When looking at one of the closely packed clusters with a large telescope, one is apt to get the impression that he is gazing across measureless vistas of space at a remote system of stars, which appear faint and crowded together on account of their stupendous distance. But such is not the case. The best evidence at command renders it practically certain that their distances from us are no greater than those of more scattered stars. Though the stars in a given cluster are supposed to be near enough to one another to be subject to considerable mutual attrac- tion, no motion due to such a force has yet been detected. Their motions will probably be recorded on the photographic plates in due time. ^^^ Ranyard 1 held that there is evid^fcfc|| c Ui s i ns are taking place between the stars, as a result of jKir mutual attraction. If two such bodies collided, the^rapid^TOlution of heat at their point of contact would expand t!?!*^gtiguous gases so violently 1 The late Mr. A. C. Ranyard, who was editor of "Knowledge," London, Eng. 232 DESCRIPTIVE ASTRONOMY. that the effect of an explosion would be produced, and the stars might rebound like caroming billiard balls, while the gases heated at the point of impact would diffuse themselves in the surrounding space. To this he attributed the radiated appearance of the outlying stars, which are frequently arranged in streams, as if ejected from the central mass. The stars in a stream are often connected by a band of nebulous matter. Photography has revealed the fact that nebulosity is associated with very many star clusters. Fig. 173. THE GREAT CLUSTER IN HERCULES. 348, Dimensions and Nature of the Stars. The larger the magnify- ing power employed, the larger does a near object, like the moon or a major planet, appear. The diameters of these bodies have been measured. But with the fixed stars the case is entirely different. The larger and more perfect the telescope, the smaller the disk of a star, under the best atmospheric conditions. The visible disk is a spurious one, the cause of which is explained in works on optics. THE FIXED STARS. 233 The diameter of the sun, as seen from the nearest fixed star, is equivalent to that of a small marble a thousand miles from the ob- server. Though the diameters of stars subtend so small a visual angle as to defy direct measurement, yet the spectroscope has given us a little knowledge concerning them. The star Algol ( 379) has been shown to be over a million miles in diameter : it has an Fig. 174. THE CLUSTER OMEGA CENTAURI : PHOTOGRAPHED BY DR. GILL, AT THE CAPE OF GOOD HOPE. invisible companion of nearly the same size as the sun. Arcturus, Capella, and Vega are believed to be much larger than our sun : the diameter of Arcturus may be a hundred times that of the sun. The second magnitude star in the crook of the handle of the Great Dipper (Zeta Ursae Majoris) is forty times as massive as the sun. The spectroscope has also shown that the stars are self-luminous bodies, similar to the sun. 234 DESCRIPTIVE ASTRONOMY. 349. Distances. The distances of the stars are inconceivably great, though easily expressed in figures. Alpha Centauri is nearer than any other star the distance of which has yet been measured. Its distance from us is 275,000 times the distance of the earth from the sun ; light consumes over four years in traversing this distance. Sirius, the Dog Star, is twice as far away. Most of the stars are at distances so stupendous as to defy measurement. It is considered probable that the vast majority of the stars are so far away that their light occupies over a century in coming to us. The light which reaches our eyes from many a one of them, may have been on the journey for thousands of years. Less than one hundred stars have been found to lie within measurable distances. In estimating such stupendous distances it is convenient to em- ploy as a unit the " light-year," that is, the distance over which light would travel in a year. A locomotive with driving-wheels 60 miles in diameter, which were revolving 1,000 times a second, would trav- erse the distance in a few days less than a year. EARTH STAR Fig. 175. STELLAR PARALLAX. 350. Stellar Parallax. The annual parallax of a star is the apparent semidiameter of the earth's orbit as seen from the star. This angle, formed at the star by the two lines shown in Fig. 175, is very small. In the case of Alpha Centauri, our nearest neighbor, the parallax is only as great as the apparent diameter of a sphere one foot through, located fifty miles from the observer. FAINT . STARS Fig. 176. METHOD OF OBSERVING STELLAR PARALLAX. The method most employed for determining parallax is illustrated in Fig. 176. Usually some stars much fainter than the star the THE FIXED STARS. 235 parallax of which is sought can be seen in the same telescopic field of view with it. It is reasonable to suppose that these are much further away. When the earth is at E the star A will appear to be situated at the point X among the fainter stars. Six months later, when the earth is at E', 186,000000 miles from its former position, the star A will appear to be located at Y. This apparent shift of position, when accurately measured, gives an astronomer the means of computing the star's parallax. The explanation of the methods of observation and calculation by which the parallax is found lies beyond the scope of this book. The parallax being known, it is a simple matter to find out how far away the star is. For the semi- diameter of the earth's orbit is 93,000000 miles, and the parallax is the angle at the star in Fig. 175. The problem then is to find how far from the star a line 93,000000 miles long must be, in order to subtend a visual angle equal to the star's parallax. Its solution is given in the next section. \ Fig. 177. RELATION OF PARALLAX TO DISTANCE. 351. Solution of the Problem. In Fig. 177 let the star be at A, the sun at S, and the earth at E. Then the angle SAE is the star's parallax ; this angle is so minute that the arc SE is practically of the same length as its chord, and we may use d to designate either of them. Represent the distance AS by R, and let p equal the number of seconds in SAE. The circumference of the circle of which SE is an arc is 2 TT R. There are 360, or 1,296000", in the circumference; hence the length of an arc of i" is l 2 2 9 g^ 00 > which equals 2 52 e 5 * ^ e length of an arc of p seconds equals p times this expression. Hence <*= whence R= 236 DESCRIPTIVE ASTRONOMY. According to this formula, if any star had a parallax as large as five seconds, its distance from the sun would be 2 6 5 2 6 5 , or 41, 253 times the earth's distance from the sun. 352. Colors. Most of the stars are white : there are many of a yellowish or reddish tinge. A few are of very pronounced colors : Sirius is white ; Vega has a bluish tinge ; Arcturus is reddish. A few faint stars are deep red. Those which are close to brighter ones are usually bluish or greenish. It was once thought that the color gave a clue to the temperature of a star, the white stars being much hotter than the red ones, just as white-hot iron is at a higher temperature than red-hot. But this theory is now abandoned : the colors are probably dependent on the materials which enter into the composition of the star, as well as its temperature. The references to the color of Sirius by ancient writers render it highly probable that it was red at the beginning of the Christian era. 353. Spectra. Secchi, an Italian astronomer, divided stellar spectra into four arbitrary classes, or types. 1 Type I. The dark lines due to hydrogen are very pronounced ; other lines are few and inconspicuous. This type embraces the majority of the stars ; their colors are white or bluish. They are called Sirian stars, as Sirius belongs to the group. Type II. The spectrum resembles that of the sun, being crossed by numerous dark lines, indicating the presence of various metals. These are called solar stars, and their colors are mostly yellow. Our nearest neighbors among the stars have recently been shown to belong to this type. Type III. In spectra of this type, shaded bands are seen, each of which is darkest at the edge nearest the violet end of the spectrum, and shades off toward the red end. The color of these stars is orange or red. Type IV. As in Type III. we have here a banded spectrum, but the bands are darkest at the edge nearest the red end of the spec- trum, and shade off toward the violet end. These stars are faint, red, and few in number. To these a fifth class is now added, embracing the so called " Wolf-Rayet " stars, which have bright line spectra. More than fifty of these are known. 1 Some of these spectra are shown in the frontispiece. THE FIXED STARS. 237 354. Discussion of Stars of Different Spectral Types. Two thirds of the Sirian stars are situated in the Milky Way, while the solar stars are about evenly divided between galactic and non-galactic regions. Each square mile of the surface of a Sirian star is brighter than an equal area of a solar star, but solar stars are on the average much more massive than Sirians, and give a greater quantity of light. The Wolf-Rayet or " bright-line " stars lie in or near the Milky Way : these stars are of special interest, because they apparently form a connecting link between the nebulae ( 388) and other stars. Bright lines are not uncommonly found in the spectrum of the sun itself, and are thought to be due to masses of vapor hotter than the underlying photosphere. The stars in Orion (with the notable exception of Betelgeuse) have a special variety of spectrum, scarcely found outside of that constellation. This indicates that these stars have a similar structure ; probably they are " chips off the same block." " In general, it may be stated that, with a few exceptions, all the stars may be arranged in a sequence, beginning with the planetary nebulae ( 385), passing through the bright-line stars to the Orion stars, thence to the first type stars, and by insensible changes to the second and third type stars. The evidence that the same plan governs the constitution of all parts of the visible universe is thus conclusive." l Different spectra doubtless indicate, in many cases, different stages of evolution, but many more observations must be made before any far reaching theory can be suitably fortified. 355. Light and Heat. The amount of light received from some of the stars has been compared with that given us by the sun. Though Sirius far outshines any other fixed star, being nearly six times as bright as Vega, 7,000,000000 stars like it would be required to fur- nish daylight ; 9,000000,000000 stars of the sixth magnitude would be necessary for the same purpose. Professor Young estimates that the full moon gives sixty times as much light as the entire starry sphere ; and that ninety-five per cent of the latter comes from stars invisible to the naked eye. 1 Prof. E. C. Pickering, in " Astronomy and Astrophysics" for October, 1893. 238 DESCRIPTIVE ASTRONOMY. No trustworthy measures of the heat reaching the earth from any particular star have been made. It is too small to affect the most delicate thermometric appliances. 356. Bird's-eye View of the Stellar System. The following conclu- sions have been reached by a study of the star gauges made by the Herschels, assuming that the faint stars are, as a class, more distant than the bright ones. Though subject to considerable uncertainty, they are generally given in works on descriptive astronomy. (a) Most of the stars are not arranged in the form of a sphere, but in that of a thin disk ; the shape of the disk is about that of a, silver dollar. (b) Only a small proportion of the stars lie on one side or the other of the disk, but the majority of the nebulae find their homes there ; i. e. outside of the disk. (c) Within the disk are the stars embraced in the Milky Way,, which contains most of the Sirian stars. (d) The stars are not evenly distributed throughout the disk. The fainter ones are grouped in clusters and streams, as a nation is. divided into families. Many of the brighter stars are thus grouped, but each " family" consists of fewer individuals than in the case of the faint ones. (e) The sun lies near the centre of the disk. 357. Kapteyn's Investigations. Prof. J. C. Kapteyn, a Dutch as- tronomer, has made the most exhaustive discussion of the form of the sidereal universe. The most interesting of his conclusions may be summed up under three heads. (a) The nearer stars are chiefly of the solar spectral class, and are scattered about the sun on all sides, independently of the position of the Milky Way. They form with the sun a scattered cluster. (#) Those stars the distances of which from us are immeasurably great, whether Sirian of solar, are more numerous the nearer they lie to the plane of the Milky Way. (c) Of the stars of any given brightness (say sixth magnitude),, those which lie in or near the Milky Way are, on the whole, more remote from us than those which lie in other parts of the heavens. The stellar universe thus bears a rude resemblance to the planet Saturn, consisting of a central ball of stars, surrounded at a great distance by an apparently ring-shaped collection of stars. Professor- THE FIXED STARS. 239 Kapteyn likens it to the nebula in Andromeda ( 389), the cen- tral nucleus of which corresponds to the solar cluster, while the out- lying whorls are miniatures of the Galaxy. 358. Proper Motions. Though the stars are called " fixed," they are far from being so. They are moving with various degrees of rapidity in all conceivable directions ; but on account of their pro- digious distances from the earth, their positions with reference to one another change by minute amounts only, from year to year. Proper motion is this apparent shifting of a star's position on the celestial sphere. The proper motion is not the star's real motion in space. If the earth were at rest, and a star were coming directly toward it or going directly away from it, the star would appear to be fixed on the celestial sphere, and would have no proper motion. The largest proper motion yet discovered is that of the star Groombridge 1830, which has been graphically termed the " runaway star." In 270 years it moves over a space equal to the apparent diameter of the moon. The bright stars have, on the average, larger proper motion than the faint ones. This is probably due to the fact that their average distance from us is less than that of the faint stars. Arc- turus has apparently moved over a space equal to one fifth of the distance between the Pointers in the Great Bear, during the Chris- tian era. It has been shown, by combining a mass of observations on numer- ous stars, that the average proper motion of a first magnitude star is six times that of one of the sixth magnitude. Stars having a large proper motion are at less distances from us, on the whole, than those of small proper motion. 359. Proper Motion Groups. While proper motions have all sorts of directions, there are many groups of stars the components of which move in the same direction. The stars in the Great Dipper, with the exception of the one at each end of the figure, belong to such a group. By spectroscopic observations ( 360) it has been found that these five stars are all retreating from us to greater depths of interstellar space. They are separated from one another by distances inconceivably vast. Yet there seems to be some common bond, and the laws of motion of this stupendous system may for- ever elude the keenest search of man. 240 DESCRIPTIVE ASTRONOMY. f I Fig. 178. PROPER MOTIONS OF THE PLEIADES. The Pleiades constitute a similar system. Of the four hundred stars catalogued in this cluster, only a few refuse to conform to a common proper motion pos- sessed by the others. These outre stars are probably be- tween us and the cluster proper, or beyond it. The other stars all agree in hav- ing the same spectra. 360. Motions in the Line of Sight. Every star is con- stantly sending forth lumi- nous vibrations of various wave lengths. If the star be approaching, the number of waves which reach us in any given time is increased, and their wave length is shortened. Hence, when a star is coming to- ward us, its light is rendered more refrangible, and all of the lines in its spectrum are shifted toward the violet end of the spectrum. By comparing the position of the hydrogen lines, for example, in the spectrum of the star, with the spectrum of hydrogen obtained in the laboratory ( 73), the shifting of the lines can be measured. The velocity of approach or recession of the star can then be com- puted, due allowance being made for the earth's motion. The majority of the stars yet observed in this way exhibit velocities of less than thirty miles a second. The best modern results in this line of work are not subject to errors exceeding a mile a second. 361. The Sun's Path. A man is in a boat on a small lake sur- rounded by a forest. The boat is drifting, he knows not whither. He watches the trees carefully, and finally perceives that the trees at his right appear to be spreading apart and growing taller. Those on the left seem to be crowding more closely together. He can de- tect no change in the relative situations of the trees ahead of him, or those behind him. He at once decides that his boat is drifting toward the right. In this manner astronomers have discovered the direction in which the sun with its attendant planets is drifting. While the THE FIXED STARS. 24! proper motions of the stars are in all directions, when we combine large numbers of them in a single discussion, a prevailing common drift comes out clearly. The stars in the constellations Lyra and Hercules are slowly separating from one another. Those in the opposite part of the sky are crowding together. The proper mo- tions are so small that it is not possible to fix the point toward which we are moving with much precision. Spectroscopic observa- tions of the velocities of stars make the sun's velocity only from 8 to 12 miles a second. 362. The Central Sun. There is a persistent idea that there is a central sun. One theory, which has obtained a wide currency, is that Alcyone, the brightest of the Pleiades, is the central sun. This theory arose fifty years ago from a study of the proper motions of the Pleiades. In the light of our present knowledge concerning proper motions, the theory is considered untenable. The hypothesis that the sun is sweeping around a gigantic curve is a reasonable one ; but no deviation from a straight line has yet been detected in its motion. Even if the centre of its motion be found, it by no means follows that all the other bodies in the uni- verse move about that centre. 363. The System of the Stars. Evidences of order and obedience to law are so numerous in the entire domain of physical science, that the human mind instinctively seeks for some law or set of laws, in accordance with which all the stars pursue their journeyings. The only law now known, which the motions of the heavenly bodies follow, is that of gravitation. But while there are many systems more or less similar to the solar system among the stars, each is so far from its neighbors that it experiences very little attrac- tion from them. They exist in great variety, from the largest and most complicated clusters, down to simple double stars ; their con- nection with one another is only a matter of conjecture. It seems very probable that there is no central sun, or even central point, about which the universe moves in orderly fashion. The solar system is a fairly well regulated family. The stellar system seems to be made up of families and tribes which are largely independent; while each family or tribe exercises some influence upon the neighboring ones, it apparently attends pretty strictly to its own affairs. 16 242 DESCRIPTIVE ASTRONOMY. DOUBLE AND MULTIPLE STARS. 364. Appearance to the Naked Eye. By surveying the heavens for a few minutes one may find several places where two stars lie in close proximity to each other. Theta Tauri, in the head of the Bull, and Alpha Capricorni, are good examples. But neither of these is ordinarily classed as a double star, for the components of each pair are too far apart. The stars which make up a real " double " are so close together that a telescope is required to separate them. 365. Appearance through a Telescope. Fig. 1 79 exhibits some of the double stars, when seen under a high magnifying power. When GAMMA LEONIS ALPHA HERCULIS BETA CYGNI ANTARE5 Fig. 179. DOUBLE STARS. the two stars are of the same brightness they are also of the same color. When they differ considerably in brightness the smaller star is apt to have a bluish cast. Beta Cygni, in the foot of the Cross, is one of the finest of those colored doubles which can be seen with a small telescope. The large star is reddish yellow, the small one greenish blue. The colors may be seen beautifully, by putting the stars out of focus. The two stars are often so close to each other that even a very powerful telescope shows them bunched together THE FIXED STARS. in an oblong mass of light. At times the blaze of a bright star quite overpowers the feeble light of its faint companion. 366. Numbers and Nomenclature. The number of doubles thus far catalogued is over 10,000. New ones are being discovered con- tinually, but not at a rapid rate, since there are few stars above the eighth magnitude which have not been scrutinized carefully with large instruments. Doubles, the principal stars of which are no brighter than the ninth magnitude, are rarely catalogued. Each double retains its ordinary name, such as Sirius, Gamma Virginis, 61 Cygni, etc., and acquires an additional one taken from the name of the discoverer. Thus h 1064 is one of Sir John Herschel's discoveries : ft 462 was found by Prof. S. W. Burnham of Chicago, the greatest living double-star astronomer. 367. Optical Double Stars. An optical double star is one the components of which seem to be near each other, but are not; one of the stars lies far beyond the other. Optical pairs form but a very small percentage of doubles. They are detected by the absence of such motion as would ensue were the stars so near together that their mutual attraction caused relative motion. 368. Physical Double Stars. The stars forming a physical double are subject to the sway of their own mutual attraction. Observa- tions of them reveal the fact that they move in elliptical orbits. This leads to the belief that gravity is the force which controls their motions. A force acting according to some other law might pro- duce elliptical motion, as is proved in works on mechanics. But since the spectroscope shows that the stars are composed of much the same materials as the sun, it is reasonable to suppose that their attractions for one another follow the same law which holds good in the solar system. Gravitation may therefore be considered as universal. Physical double stars are usually termed binaries. Many of the periods of revolution are hundreds of years in length ; a few are less than a year. Some of the orbits are several times as large as that of Neptune. Others are smaller than that of Mercury. 369. Spectroscopic Binaries. When the components of a binary are so close together that the most powerful telescopes fail to separate them, or to give any indication that the star is double, the spectroscope in a few instances has revealed the duplicity. The star 244 DESCRIPTIVE ASTRONOMY. Mizar at the crook of the handle of the Great Dipper is a case in point. A small telescope easily resolves this into two stars. Prof. E. C. Pickering, in 1889, found by his photographs of this double, that the spectrum of the brighter component exhibited strange anomalies. At regular intervals of a few weeks the dark lines in the spectrum were doubled. The explanation of this depends upon the principle ( 360) that when a star is approaching us the lines of its spectrum are shifted toward the violet, and when it is receding the lines are shifted toward the red. When two bright stars, close together and composed of the same substances so that they give the same spectra, are revolving about their common centre of gravity in an orbit the plane of which is nearly edgewise to us, one star will be approaching when the other is receding. Were the stars at rest, their spectra would coincide in position. But when, on account of their motion, the lines in one spectrum are shifted in one direction, and those in the other in the opposite direction, the lines which formerly coincided will appear side by side. EARTH Fig. 180. A SPECTROSCOPIC BINARY. When the stars are at A and B in Fig. 180, they are neither approaching the earth nor receding from it so far as their orbital motion is concerned. The two close stars in Mizar complete a revolution in one hun- dred and four days, 1 in an orbit of the same size as that of Mars. Spica, in Virgo, is a yet more wonderful double. The compo- nents are only about 6,000000 miles apart, and complete a revolution in four days. 370. Sirius. Certain minute movements of Sirius on the face of the sky, hither and thither, were for a long time a source of perplex- ity to astronomers. Fifty years ago the illustrious German astron- omer, Bessel, announced that the observations of Sirius indicated that it was describing a tiny ellipse. He also advanced the theory that the motion was caused by the proximity of a companion. Less 1 Possibly in just half that time. THE FIXED STARS. 245 than ten years thereafter, two other German astronomers declared, as the result of an elaborate investigation, that the period of orbital revolution of Sirius and his unseen satellite was fifty years ; they also pointed out the direction in which the companion lay from the larger star, and the direction of its motion. Eight years later, when the Clarks were testing an i8|-inch object-glass, they turned it upon Sirius. The keen eye of Alvan Clark, Jr. quickly detected a faint star in the blaze of light surrounding the large star. It was soon found to be moving in the way predicted. The mass of the system is six times that of the sun. The faint star, which gives less than 10 o 00 as much light as the main star, may contain one third of the mass of the system. 371. Planetary Systems. As the sun is the ruler of a planetary system, many of the stars may be centres about which troops of planets roll and shine. Such planets, in order to be discovered by us, must be much more brilliant in comparison with their suns than are those of the solar system. Jupiter himself, if searched for from Alpha Centauri with the most powerful telescope ever constructed by man, would elude the most searching scrutiny. Professor Young has computed that a refracting telescope ten feet in aperture would be needed. Such companions, if not too near their primaries, may in the future impress themselves on photographic plates of great sensitiveness. 372. Multiple Stars. Epsilon Lyrae is a fine specimen of a multiple star. It is one of the two fourth magnitude stars near Vega EPSILON LYRAE THETAORIONI5 ZETA CANCRI Fig. 181. MULTIPLE STARS. which form with it an equilateral triangle. To a good eye the star appears oblong; a keen eye separates it into two. An opera-glass shows them finely. A telescope three inches or more in aperture 246 DESCRIPTIVE ASTRONOMY. reveals each star as a double. We have, therefore, a quadruple star. Each pair is a binary : it is probable that the two pairs revolve about their common centre of gravity, completing a single revolution in many thousands of years. Theta Orionis is composed of six stars. It is located in the sword-handle of Orion, and is involved in the great nebula of Orion ( 39) There is good evidence that these stars have been formed by the condensation of a portion of the nebula. Zeta Cancri is visually a triple star, two being close together, the other farther away. The close pair is a binary, and the third star apparently revolves about the binary, but with singular irregularities of motion. The irregularities are thought by some astronomers to be due to a fourth star near by, but invisible. The system is in that case composed of two binaries, which revolve about their com- mon centre of gravity. VARIABLE STARS. 373. Definition : Number : Names. Variable stars are those the brightness of which has been observed to change. Those that repeat the same series of changes over and over are known as periodic, the period being the time required for the star to pass through one com- plete cycle of change. Some naked eye stars become too faint to be seen with a telescope four inches in aperture. New variables are discovered from time to time, and the number now (1896) well authenticated is 400. This number does not include those variables which were discovered in I895, 1 in certain globular star clusters; nearly 100 variables were noted in a single cluster. When such stars already have names ( 341) other than mere numbers in some star catalogue, no new name is added to denote variability. But when the stars are faint, so that they have not received such names, the first such variable discovered in the constellation An- dromeda, for instance, is named R Andromedae. The second would be S, and so on through the alphabet. After the letter Z has been reached, further discoveries receive the designations RR, RS, etc. 374. Classes. These are classified in five groups. Class I. embraces temporary stars, which suddenly experience an enormous increase in brightness, and then fade away gradually. 1 By Prof. S. I. Bailey, at Arequipa, Peru. THE FIXED STARS. 247 Class II. includes periodic stars which suffer great variations of light in not less than several months. In Class III. are found stars which exhibit slight irregular fluctua- tions of brightness. For Class IV. are taken those stars of short periods, the light of which varies smoothly and regularly. Class V. is devoted to those periodic stars which suffer a remark- able diminution of light for a few hours, every few days. They be- have as if temporarily partially eclipsed. Fig. 182. TYCHO BRAHE. 375. Temporary Stars. One evening, in November, 1572, when Tycho Brahe was taking his usual walk, he perceived in the con- stellation of Cassiopeia a new star, brighter than Sirius, and com- parable with Venus at her best. Doubting the evidence of his eyes, he called the attention of several others to the splendid object. 248 DESCRIPTIVE ASTRONOMY. For some days the star could be discerned in full daylight, and at night shone through light clouds which obscured all other stars. In December it began to wane ; at the end of that month it had become fainter than Jupiter. Finally, in March, 1574, it disappeared from view. There were no telescopes then to watch it further. Its color changed from white to yellow and red successively, and returned to _ white before it faded from vision. STAR Tycho determined its place, but his obser- vations are so rude, from lack of telescopic aid, that it is impossible to tell whether the 9 star was any one of half a dozen now visible Fig.zSj.-TYCHO-SSTAR IN CASSIOPEIA. Nova Aurigae (381) belongs to this class of stars. 376. Mira. Class II. is fitly represented by Mira, which is Omicron Ceti. The period of this star is eleven months. Most of the time it is invisible to the unassisted eye, but once .during its period it rises to its maximum brightness, which Varies from the second to the fifth magnitude, remains there about a week, and then sinks more slowly back. The rise and fall together consume about one hundred days. During the remainder of its period it is of about the ninth magnitude, and can therefore always be seen with a good field-glass. It is visible to the naked eye about six weeks, when near its maximum. 377. Class III. To this belong Alpha Orionis and Alpha Cassio- peiae. Alpha Orionis is the bright reddish star in the shoulder of Orion. The amount of fluctuation is small, and no period or regu- larity of fluctuation has been found. 378. Beta Lyrae. This star is a good example of Class IV. It is of the fourth magnitude, and varies half a magnitude on each side of this. The period is nearly thirteen days ; during this time the star first reaches a maximum of the 3.4 magnitude, then sinks to the 3.9 magnitude, next rises again to the 3.4 magnitude, and finally sinks to the 4.5 magnitude. These changes are thought to be due in some way to the action of one or more companions, revolving about the main star. 379. Algol. Beta Persei was called by the Arabians Algol, which THE FIXED STARS. 249 means the Demon Star; they had therefore undoubtedly noticed the variation of its light. Its mean period has been very accurately determined, and is given by Chandler as 2 d. 20 h. 48 m. 55 sec. During most of the time it is of the second magnitude. Its variation occupies ten hours, the magnitude falling to the fourth, remaining there for twenty minutes, and rising again to the normal amount. ALGOL GAMMA -.. ANDROMED"AL. , B T A '"* -?''' SQUARE OF \ ANDROMEDAE. -. V Fig. 184. How TO FIND ALGOL. The cause of this sudden diminution of light has long been sus- pected to- be the presence of a dark star revolving about Algol and partially eclipsing it at each revolution. The truth of this has re- cently been rendered nearly certain by spectroscopic observations ( 360) which show that Algol alternately approaches us and recedes, just as if it were one component of a binary system. The following approximate data concerning this binary have been derived : Diameter of the principal star, i,ooopoo miles. Diameter of the dark companion, 800,000 " Distance between their centres, 3,000,000 " Velocity of the companion, 55 miles per second. Mass of the principal star, f of the sun's mass. Mass of the companion, f of the sun's mass. There are certain small inequalities in. the period of variability which Chandler explains by the theory that the binary already mentioned is involved in an orbital revolution with a heavy faint star, in a period of about 130 years. The size of this new orbit is about equal to that of the orbit of Uranus. It is possible that there are other bodies in the system. Algol belongs to Class V. 380. Y Cygni. This star is one of the most interesting of vari- ables : it belongs to the Algol type. Twice in every three days it has 250 DESCRIPTIVE ASTRONOMY. a minimum, at which the light is one half of the maximum amount. This fluctuation is explained by the supposition that the star is 'a close double, the two components being equal in size and brightness, and revolving in a plane which is turned edgewise to us. Twice in each revolution about their common centre of gravity one star eclipses the other : the period of revolution is thus 72 hours. If we consider several successive minima, calling them first, second, third, etc., we find that the time from the first to the third is 72 hours, as is also the interval between the second and fourth, but the interval from the first to the second is not 36 hours as would be ex- pected. The interval between the first and second minima may be 32 hours, for instance : then 40 hours would elapse between the second and third, and the interval between the third and fourth LINE OF VISION Fig. 185. Y CYGNI. would be 32 hours again. This irregularity is accounted for by the assumption that the stars revolve in ellipses which lie " broad- side " to our line of vision, as shown in Fig. 185. When the stars are at A and B respectively, there is an eclipse or minimum. Be- tween the first and second minima the stars are describing the short parts (ACB and BDA) of their orbits. Between the second and third minima the long portions, BEA and AFB, are described. This simple explanation does not wholly account for the observed irregularities in the times of the eclipses. In 1886 each period was 36 hours, while in 1891 the successive intervals were respectively 31 THE FIXED STARS. 25! hours and 43 hours. This anomaly can be explained upon the hy- pothesis that the ellipses are shifting their position with reference to our line of sight, the disturbance being due to the attraction of some neighboring unseen body. 381. Nova Aurigse. Nova Aurigae was discovered in the latter part of January, 1892, by Dr. Thomas D. Anderson of Edinburgh, an amateur astronomer, who was in the habit of observing with a hand telescope magnifying only ten diameters. It was of the fifth mag- nitude. Soon astronomers all over the world were observing the spectrum and changes of brightness of this new star. The question at once arose whether it had previously been a telescopic star, and when it first displayed itself. Fortunately photographs of the re- gion of sky in which it lay were at hand. On Dec. 10, 1891, six weeks before its visual discovery, it had impressed itself on one of the photographic plates exposed at the Harvard College Observa- tory. A photograph taken in Europe on December 8 showed no trace of the star. The photographic evidence shows that it was somewhat brighter in the latter part of December, 1891, than a month later, when the attention of astronomers was called to it. The spectrum was found to be of bewildering complexity ; there were fine bright lines and broad dark ones ; some of the lines were shifted in one direction, and others in the opposite, as if there were two bodies moving in widely different directions. In a few weeks it began to decline in brightness rapidly. On April 24 it was only of the sixteenth magnitude, and two days later it was hardly percepti- ble with the Lick telescope. The complexity of its spectrum led to the greatest variety of theories as to the cause of the outburst. By some it was attributed to the near approach of two bodies moving with immense speed ; their proximity caused great mutual disturbances of a tidal nature, leading to the production of enormous eruptions similar to solar prominences, though on a vastly greater scale. Another hypothesis was, that some unknown heavenly body, speeding along its far distant path, came into collision with a cloud of cosmical matter, similar to the meteoric aggregations encountered by the earth, but much denser. Photography has revealed the pres- ence of such clouds ( 338) in the Milky Way, and the Nova, like most temporary stars, was situated in the Galaxy. 252 DESCRIPTIVE ASTRONOMY. But another strange chapter is to be added to the history of this remarkable object. In August it was found to have brightened up, having attained the tenth magnitude. It then appeared like a small star surrounded by a nebulous atmosphere, and its spectrum was that of a planetary nebula ( 385). It still (1896) retains, this appearance. It is not improbable that this wonderful object is at so stupendous a distance that all these changes occurred before the astronomers who have observed them were born. The amount of light given out, when the Nova was at its best, may have been many times greater than that radiated by the sun. 382. Causes of Variability. There have been many theories upon this topic. The variability of stars of the Algol type is well ex-- plained by the hypothesis of eclipses by unseen bodies revolving about the variables. The behavior of many variables can be explained by the hypoth- esis that they have spots, like the sun's, though much larger, and that these spots have their times of maximum and minimum frequency, as do the solar spots. If a star had one or more large companions revolving about it, their attractions might cause consid- erable tidal disturbances, which would give rise to variability. The sudden appearance of temporary stars may be explained by terrific outbursts of heated vapors, analogous to the solar prom- inences. Lockyer has advanced the theory, that the variables are not single masses, but are rather compact swarms of meteoric bodies, attended by satellite swarms revolving in very eccentric or- bits. The satellite swarms are supposed, when nearest the main swarm once in every revolution, to collide with its outlying meteors, thus producing a temporary increase of light. Much research must yet be made before the complex phenomena exhibited by variable stars can receive any adequate explanation. 383. How to Observe Variables. The observations of the varia- tions, of these stars in brightness often do not require the use of any telescope larger than an opera-glass. When a star is near its maximum or minimum, it is compared with adjoining stars of nearly the same brightness : it is noted as being equal in brightness to some particular one of its neighbors, or a trifle brighter or fainter than others, at a given time. The object of the observations is to determine the time of maximum or minimum brightness. THE FIXED STARS. 253 The approximate times are given in various publications, 1 for the observer's guidance. Most of the observations made in this country on these interesting objects are by amateur astronomers. EXERCISES. 384. i. With an opera-glass or spy-glass look at some portion of the sky, which appears to the naked eye to be barren of stars, and count the number in the field of view. 2. On a night when the moon is not shining, direct an opera-glass or spy-glass toward some bright spot in the Milky Way, and find out whether the light from that particular locality is due to a number of faint stars, or to a few brighter ones. 3. On a night when the moon is not shining, find a dark spot in the Milky Way, and make a drawing showing its location among the stars. 4. Observe five of the brightest stars visible at any given hour, and write down the name of each, together with its color. 5. Estimate the color of Zeta Ursae Majoris (Mizar), and of the minute star (Alcor) close by it. 6. What is the origin of the name Groombridge 1830? ( 341.) 7. Count the stars visible to the naked eye inside the bowl of the Great Dipper, when the moon is not shining, and the Dipper is not low down. 8. The light of a sixth magnitude star is equivalent to the com- bined light of how many of the eighth magnitude? 9. If a cluster were spherical in form, and the stars distributed uniformly through it, would it appear to be more condensed near the centre than at the edge ? 10. The intensity of light varies inversely as the square of the distance ; that is, if two equal lights are at distances of one mile and three miles from the eye, the farther one would not look one third as bright, but one ninth as bright. If a given star were placed at half its present distance from us, it would look how many times as bright as before? 1 The times of minima of variables of the Algol type are given in " Popular Astron- omy," every month. For methods of observation, see articles by Mr. J. A. Parkhurst in " Popular Astronomy " for December, 1893, an ^ January, 1894. 254 DESCRIPTIVE ASTRONOMY. 1 1. The semidiameter of the earth's orbit being 93,000000 miles, how far off is a star which has an annual parallax of one tenth of a second of arc ? 12. Show that the time required for light to come to us from a star having a parallax of one hundredth of a second of arc is over three hundred years. 13. Do Sirian stars have atmospheres of large absorptive power? 14. Do the spectra of solar stars indicate that they are probably more dense than Sirian stars or less dense? 15. Do the spectra of the Wolf-Rayet stars show that they are surrounded by extensive atmospheres, which absorb the rays com- ing from within? ( 73.) 1 6. Show that the sun, if removed to the distance of Sirius, would appear to be less than one fortieth as bright as the latter. 17. If the stellar system were in the form of a sphere, throughout which the stars were distributed uniformly, and we were at its centre, would the stars appear to be uniformly distributed over the face of the sky? 1 8. Does the fact that there are many more stars visible when we look toward the Galaxy than when we look in other directions indicate that the stellar universe is shaped somewhat like a thin cheese? (In answering this question, assume that the stars are distributed with some uniformity through the space which they occupy.) 19. Might the appearance of the Galaxy be accounted for on the supposition that it is an irregular ring of closely packed stars sur- rounding us? 20. Spectroscopic observations show that a star is approaching us at the rate of thirty miles a second, and visual observations show that it is apparently moving perpendicular to the line of sight with a velocity of forty miles a second; according to the principles of Fi s- l86 - mechanics its real velocity is repre- sented by the diagonal of the rectangle in Fig. 186. Prove that the real velocity is fifty miles a second. THE FIXED STARS. 255 21. If the earth and a certain star are moving, at a given time, with the same velocity in the same direction, will the lines of the star's spectrum be shifted from their normal place? 22. Give some reasons why the stars differ in brightness. 23. Examine Theta Tauri with the naked eye; if you cannot see it double, your vision is defective. 24. If the orbit of a certain binary were a perfect circle, and a line drawn from the observer's eye to either of the stars were oblique to the plane of the circle, would the orbit appear to us to be a circle or an ellipse? 25. If the plane of the orbit of a binary passed through an ob- server's eye, would one of the stars ever occult the other, if they were equal? 26. If one star of a binary is more massive than the other, to which one will their common centre of gravity lie the nearer? 27. If one component of a binary is much brighter than the other, does it follow that it is more massive? 28. If the earth were fixed, and the plane of the orbit of a binary were perpendicular to a line drawn to the star from the observer's eye, would the spectroscope enable us to determine the velocity of either star? 29. As binaries revolve, do the components sometimes appear closer together than at others ? 30. If the plane of the orbit of a binary passed through the ob- server's eye, how would the star appear in a telescope, when one body was between us and the other? 31. What does the name Y Cygni signify? 32. What is the signification of the designation Nova? 33. If the outburst of a temporary star be due to the collision of some star with a meteor-like cloud of comparatively small bodies, why does it gradually fade away? 34. Upon the collision theory how can the reappearance of Nova Aurigae in August, 1892, be explained? 35. Suppose Mira to be a dense cluster of meteoric bodies, about which another cluster is revolving, in a very elliptical orbit. Could the variability of Mira be accounted for by the hypothesis of a peri- odic collision between Mira and its companion? 36. Could the fact that Mira, when brightest, may be anywhere 256 DESCRIPTIVE ASTRONOMY. from the second to the fifth magnitude, be explained by the collision theory advanced in the preceding exercise? 37. If Y Cygni is a binary consisting of two stars of equal size and brightness, will its minima occur when one of the stars is behind the other? 38. If the ellipses in Fig. 185 lay "endwise" to the earth, so that our line of vision went through the points E and F, would each in- terval between successive minima of Y Cygni be 36 hours? 39. If the ellipses in Fig. 185 did not lie either exactly " broad- side " or " endwise " to our line of vision, would the time intervals between successive minima of Y Cygni be equal? 40. Is there any reason not mentioned in 380 why one of the stars would traverse the arc BDA of Fig. 185 more quickly than the arc AFB? THE NEBULA. 257 CHAPTER XIII. THE NEBULA. " This world was once a fluid haze of light, Till towards the centre set the starry tides, And eddied into suns, that wheeling cast The planets." TENNYSON. 385. Various Forms. Nebulae are cloud-like objects of a bewilder- ing variety of forms. They are to be carefully distinguished from clusters, which are aggregations of stars. A true nebula does not consist of separate stars. Many clusters, however, have nebulous matter associated with them, and many nebulae contain stars within their borders. A large nebula is in general of an irregular shape. In it are to be seen many spots brighter, and presumably more condensed, than the rest of the nebula : there are also found dark spots, rifts, and streams of various shapes. The cuts of the nebulae of Orion and Andromeda (Figs. 189 and 190) illustrate these pe- culiarities. Spiral nebulae, of which there are several, exhibit convolutions like those of the hair-spring of a watch ; the appearance resembles that of a Catherine wheel. Annular nebulae are ring-shaped objects, darker in the centre than at the edge. Planetary nebulae have small round disks of approximately uniform brightness throughout. They are usually brightest in the centre. A nebulous star has a strong central condensation, surrounded by a nebulous envelope. It is frequently difficult to decide whether an object should be called a planetary nebula or a nebulous star. Double and variable nebulae are known : no orbital revolution has been detected in the double nebulae : no law of variation is known for the variable ones. 386. Number, Distance, and Grouping. The number of known nebulae is about eight thousand. New ones are continually being 17 258 DESCRIPTIVE ASTRONOMY. discovered, especially by photography, but most of the discoveries are exceedingly faint and uninteresting. No nebula has yet revealed any parallax ( 350). Yet the many and intimate associations of nebulae with stars lead to the belief that they are at the same distances. Fig. 187. THE PLEIADES: PHOTOGRAPHED BY ROBERTS.! In the Pleiades, photography has revealed the presence of a mass of nebulous matter surrounding four of the bright stars, and con- nected with another by a faint ray. The brightest star, and some smaller ones near it, are involved in a similar nebula. Other faint stars in the vicinity are connected by wisps of nebulous matter emanating from the vicinity of one of the bright stars. 1 An English amateur astronomer. THE NEBULAE. 259 The multiple star Theta Orionis ( 372) lies in a dark space in the Great Nebula of Orion, the four brighter stars looking like eggs in a bird's nest. The appearance suggests that the stars are con- densations formed from the surrounding nebulous matter. Further- more, certain lines in the spectrum of Theta Orionis are matched by corresponding ones in the spectrum of the nebula. While the stars are crowded together in the vicinity of the Milky Way, the majority of the nebulae lie outside of it. Their law of distri- bution over the sky is opposite to that of the stars ( 346). They are most numerous where the stars are least numerous, and vice versa. 387. Sizes : Changes of Appearance. Nebulae vary greatly in size. Some are so small as to look like stars in a small telescope. Others are the most gigantic objects ever revealed to the eye of man. The Great Nebula in Orion, as recently photographed, covers a large part of the entire constellation. There are serious discrepancies between old drawings of some of the nebulae and recent delineations of them. Drawings of so faint objects, made with telescopes of different sizes and under widely different circumstances, would naturally fail to agree. While ma/iy of the apparent changes are due to such causes, there remains a small residuum of cases which cannot be explained reasonably, ex- cept on the hypothesis that real changes in the nebulae have taken place. The " Trifid " Nebula shown in Fig. 188 is an illustration in point. A star, which was located in one of the dark lanes at the opening of the nineteenth century, is now involved in the nebulous matter. The star has not changed its position with respect to the neighboring stars: therefore the nebula must have changed. Such is the result of an investigation made by Professor Holden, Director of the Lick Observatory. 388. Spestra. About half of the nebulae give spectra containing bright lines; thus showing ( 73) that they may be composed of glowing gas under low pressure. Four of these lines are generally seen without difficulty with the powerful spectroscopes now em- ployed. Two of them demonstrate the presence of glowing hydro- gen. The origin of the other two is unknown. Besides these characteristic nebular lines, several others have been seen ; by some of these, helium and sodium are fairly recognized. The Great Nebula in Orion is the finest specimen of this class. 26O DESCRIPTIVE ASTRONOMY. Most of those nebulae which do not exhibit bright-line spectra give continuous spectra ( 73) simply. They may be composed of gaseous matter under high pressure, or of glowing liquid matter, or Fig. 188. THE TRIFID NEBULA. of a mixture of both. The Nebula in Andromeda belongs to this class: it is plentifully besprinkled with stars. Incandescent solid matter, unenveloped by a gas, would give a continuous spectrum. THE NEBULA. 26l We have no proof, however, that matter exists in that form any- where in the universe. A few nebulse give both continuous and bright-line spectra. Fig. 189. THE NEBULA IN ANDROMEDA : PHOTOGRAPHED BY ROBERTS. 389. The Nebula in Andromeda. This nebula is plainly visible to the naked eye, and has often been mistaken for a comet. It has a tolerably regular elliptical outline, and a strong central condensation. 262 DESCRIPTIVE ASTRONOMY. Fig. 189 gives the impression that it is surrounded by rings like those of Saturn, or that it is a gigantic spiral. The appearance of this nebula is very interesting in its relation to the nebular hypothe- sis ( 394), that all stars and planets were formed by the conden- sation of nebulous matter. According to this theory, the two condensations outside of the main elliptical portion may be planets Fig. 190. THE NEBULA IN ORION: DRAWN BY BOND AT THE HARVARD COLLEGE OBSERVATORY. in the process of formation. In August, 1885, a new star appeared close to the nucleus of the nebula ; at first it was bright enough to be seen with an opera-glass, but it faded away to invisibility in a few months. Its spectrum was almost the same as that of the nebula ; hence the star was probably in the nebula. It exhibited no sensible parallax. THE NEBULA. 263 390. The Great Nebula of Orion. In the sword-handle of Orion are three stars in a line, easily seen with the naked eye. The central one of these appears hazy : it is the multiple star Theta Orionis, shown in Fig. 190, near the centre of the nebula. This star is com- monly known as the Trapezium, because the four brighter stars in it form that geometrical figure. The spectrum of one of these stars has been photographed, and exhibits bright lines corresponding to lines in the spectrum of the nebula. This indicates that the star is a sphere of nebulous matter, not yet condensed as much as stars ordinarily are. The brightest portion of the nebula is in the immediate vicinity of the multiple star. Thence it branches off in wonderful forms, which contrast beautifully, in their delicate tracery, with the blackness of the adjacent regions. Photography reveals a vast extension of the nebulosity which the most powerful telescopes fail to show. Keeler has determined spectroscopically that the nebula is retreat- ing from us at the rate of nearly eleven miles per second. If the moon be absent, the nebula, even in a small telescope, must call forth the admiration of the beholder. It is the finest object of its class in the heavens. 391. Other Notable Nebulae. The Dumb-bell Nebula in Vulpecula (between Lyra and Delphinus) appears in a small telescope to be composed of two oval masses in contact. The Ring Nebula in Lyra is situated a third of the way from Beta to Gamma Lyrse. It is the only one of its kind which can be seen with a small telescope, and is shown in Fig. 191, as seen in a 1 5-inch telescope. Of Spiral Nebulae, one of the most remarkable is the one in Canes Venatici, shown in Fig. 192. It is three degrees from the star in the end of the tail of the Great Bear. The appearance is as if a slow rotation were taking place. The Trifid Nebula is situated in Sagittarius : it is distinguished by the curious triple-pronged dark area, which gives it the appear- ance of being cracked open. This is the nebula previously men- tioned, which affords distinct evidence of change. It is shown in Fig. 188. 392. Real Form of Spiral Nebulae. While these nebulae exhibit to the eye, more or less perfectly, the appearance described in 385, 264 DESCRIPTIVE ASTRONOMY. this, may not be their real form, since we see simply their projec- tions on the sky. In 1888 Professor Holden discovered, at the Lick Observatory, that one of the planetary nebulae had a spiral filament Fig. 191. THE RING NEBULA IN LYRA: DRAWN BY BOND AT THE HARVARD COLLEGE OBSERVATORY. within it. This led him to a study of the best extant drawings of the spiral nebulae. He found that their various forms can be ex- plained on the assumption that the filaments which give the spiral appearance are really of the form of a corkscrew. He bent a wire THE NEBULA. 265 into this shape, and was able, by holding it in different positions, to represent the shapes of the various spirals shown in the drawings. 393. The Magellanic Clouds. The Magellanic Clouds, or Nube- culae, are situated in the southern celestial hemisphere, and are not visible in middle north latitudes. They are two cloudy masses of Fig. 192. THE SPIRAL NEBULA IN CANES VENATICI : PHOTOGRAPHED BY ROBERTS. light, the larger one of which has an area about equal to that of the bowl of the Great Dipper; the smaller one is only one fourth as large. Both are plainly visible to the naked eye, and resemble por- tions of the Milky Way. They exhibit a marvellous structure in a telescope : nebulae, both regular and irregular in form, and star clus- ters of all degrees of condensation, are mingled promiscuously. The larger cloud contains about three hundred of these objects. 266 DESCRIPTIVE ASTRONOMY. THE NEBULAR HYPOTHESIS. 394. General Statement. The celebrated Nebular Hypothesis is an attempt to account for the present form of the solar system by a process of orderly evolution. Its name indicates that it pre- supposes the existence of a nebulous mass, the parent of the well ordered worlds which we now behold. The chaotic mass of world stuff may be described, in Milton's words, as " A dark, Illimitable ocean, without bound, Without dimension, where length, breadth, and height, And time and place, are lost ; where eldest Night And Chaos, ancestors of Nature, hold Eternal anarchy, amidst the noise Of endless wars, and by confusion stand." Various writers suggested, and partially worked out, the nebular hypothesis, but the first to give it an extensive mathematical devel- opment was Laplace. We proceed to state his theory, and its modi- fications, setting forth afterwards the facts which give color to it. 395. Laplace's Theory. Accord- ing to this theory the original neb- ula was a mass of intensely heated gas, which had by reason of the mutual attractions of its particles assumed a globular form, and had acquired a motion of rotation. As its heat was radiated away, the nebula contracted, and rotated more swiftly ; the mass became flattened at the poles, and when the " cen- trifugal force " at the equator bal- anced the force of gravity there, a ring of equatorial matter was abandoned. The spheroid left within the ring rotated still more rapidly until another ring was left behind, etc. The matter in each ring gradually condensed into a planet, which in turn rotated and abandoned rings, which usually Fig. 193. LAPLACE. THE NEBULAE. 267 condensed into satellites : in the case of Saturn some of the rings failed to break up into satellites of goodly size. 396. Changes in the Theory. As facts and laws unknown in La- place's day have been discovered, various modifications of the original theory have been proposed. It is no longer necessary to suppose that the parent nebula was originally at a high temper- ature : it is regarded as more probable that it was a cold cloud of finely divided matter, which became heated in the process of contraction. Since some parts of the parent mass were probably denser than others, it is not likely that rings were usually abandoned, but rather that balls of matter were left behind. When the material at the equator was unusually homogeneous, a ring similar to Saturn's might be formed. Laplace supposed that the outermost planet was formed first, but it is now believed that several of the planets may have been liberated at about the same time. Faye, a French astronomer, has shown that the inner planets may have been formed before the outer ones. The retrograde motions of the satellites of Neptune and Uranus contradict Laplace's supposition that the rings from which they were formed rotated as if solid, before they broke up to form the satellites. But if it is admitted that different portions of the ring were of differ- ent degrees of density, and that the inner edge rotated more swiftly than the outer, mathematicians find no difficulty in accounting for the retrograde motions of the satellites. 1 According to Laplace's theory alone the inner satellite of Mars should not complete a revolution in less time than that planet re- quires to rotate on its axis. This anomaly has been explained in a marvellous manner as a result of the tides which the sun causes on Mars. 2 397. The Evolution of Double Stars. Laplace's theory of the aban- . donment of rings, which gradually condensed into satellites, answers very well for the solar system, but fails for the double stars. In the solar system we have a number of comparatively small planets re- 1 See Young's General Astronomy, Art. 914. 2 An elucidation of this matter in a popular way is found in Ball's Story of the Heavens, Chapter XXVII. 268 DESCRIPTIVE ASTRONOMY. volving about a central body, which is 750 times as massive as all its planetary attendants put together. But the two bodies composing a double star are more nearly equal to each other. If the original nebula were quite homogeneous, rings might be formed as supposed by Laplace. But as there were probably great differences in density in different parts of the parent nebula, the densest portions would attract to themselves the surrounding matter. Under such con- ditions it is probable that the rotating and contracting nebula would separate into two or more portions. Dr. See 1 has specially emphasized the fact that, while the ring formation is ideally possible, the nebula would be more likely to separate into two globular masses. Many double nebulae are known, which seem to substantiate this theory of " fission." Probably such a double nebula will condense into a double star after thousands or millions of years have elapsed. 398. Testimony of the Nebulae. We have seen that the nebulae are aggregations of tenuous matter, ranging from the vast filmy irregular nebulae to the neat round compact planetary nebulae. Be- tween these two extremes there seems to be every gradation of size and brightness. The great nebula of Andromeda and those which are distinctly spiral give the impression that they may be rotating. The globular bright spots found in some of the larger nebulae look as if they were condensations of the surrounding matter. Planetary nebulae usually have a brightening at the centre, and nebulous stars seem to be approaching the end of the process of transformation into stars. The nuclei of planetary nebulae and stars like those in the Trape- zium of Orion (Theta Orionis) exhibit spectra similar to those of the nebulae in which they are involved. Immediately around the stars of the Trapezium there is a dark place, as if the matter once there had been used up to form the stars. 399. Testimony from the Stars. The wonderful associations of nebulae and stars, such as are found in the Pleiades and in Orion, point to a close connection. Some of these stars have wisps of neb- ulous matter clinging to them, as photography has shown. Others, 1 Dr. T. J. J. See, Professor in the University of Chicago, who has worked out an. elaborate theory of the evolution of double stars. THE NEBULA. 269 though giving the ordinary spectra of stars ( 353), have quite an extensive nebulosity connected with them. The Wolf-Rayet stars give bright line spectra, and one class of the nebulae does. One naturally concludes that we have different stages of a process of con- densation, which will finally lead to the production of such highly finished orbs as Sirius, or the sun. 400. Testimony of the Earth and Moon. The deeper we go into the crust of the earth, the warmer we find it. Volcanoes give abun- dant evidence of the presence of intensely heated matter in the in- terior of the earth. The granite which we prize so highly owes its toughness to its having passed through primeval fires. Statuary marble is but common limestone which has been metamorphosed by heat. Mountain chains are thought to have been formed by the wrinkling of the earth's crust, as it contracted in the process of cool- ing. The earth and the sun are composed of the same substances in large part. Were the former heated to incandescence it would give essentially the same spectrum as the latter. The moon bears the marks of its igneous origin, written in large characters over its face. The following extract is taken from Nasmyth and Carpenter's book on the Moon : " We trust that we, on our part, have shown that the study of the moon may be a benefit not merely to the astronomer, but to the geologist, for we behold in it a mighty ' medal of creation,' doubtless formed of the same ma- terial and struck with the same die that moulded our earth ; but while the dust of countless ages and the action of powerful disintegrating and denud- ing elements have eroded and obliterated the earthly impression, the super- scriptions on the lunar surface have remained with their pristine clearness unsullied, every vestige sharp and bright as when it left the Almighty Maker's hands." 401. Testimony from the Planetary Systems. We note the following harmonies in the motions and densities of the planets. I. They all revolve eastward about the sun, in orbits nearly cir- cular, which lie approximately in the same plane. II. They rotate eastward on their axes (except probably Uranus and Neptune), the planes of their equators being but little inclined to those of their orbits (except probably that of Uranus). III. The satellites revolve in the same direction in which the 270 DESCRIPTIVE ASTRONOMY. planets rotate, their orbit planes being nearly coincident with the equators of their respective planets. IV. The four inner major planets are small bodies of great density; they rotate slowly, as far as is known. The four outer major planets are great bodies of small density ; they rotate swiftly, as far as is known. 402. Testimony of the Sun. The accepted theory of the source of the tremendous quantity of heat continually radiated by the sun is the contraction theory ( 86). If the sun be now contracting it must have been larger 1,000 years ago than to-day. Reasoning backward, we find it highly probable that at one time the diameter of the sun equalled that of the orbit of Mercury. But we may go yet farther back in imagination and see the sun as a tenuous nebu- lous mass, the confines of which lie beyond the orbit of the farthest planet. 403. Is the Testimony Sufficient? The human mind is irresistibly attracted toward a grand and far-reaching theory, which explains a variety of observed results by a single process of development. With a limitless duration of time and an infinite extent of space at its disposal, it leaps over the most stupendous chasms in knowledge as nimbly as a mountain goat leaps from rock to rock, scaling the precipitous heights of its native wilds. The limitations of our knowledge are so great that the Nebular Hypothesis must probably remain a mere theory, as long as man in- habits the earth ; Bacon has said that the subtlety of nature tran- scends in many ways the subtlety of the intellect and senses of man. Yet the theory explains many facts of observation so simply and so reasonably, that the speculations of men will probably be guided by its broad lines for centuries to come. So inadequate is the sum total of our present knowledge of the processes of celestial evolution that we are led to cry out with Job : " Lo, these are parts of His ways : but how little a portion is heard of Him? but the thunder of His power who can understand? " 404. The Future of the Visible Universe. As the sun is continually radiating its heat away, with boundless prodigality, it is reasonable to suppose that the stars, which are but distant suns, are doing like- wise. We know of no way in which this expenditure is to be repaid. We can look forward to the time when the sun will become a cold THE NEBULA. 271 cinder, feeling its way by the starlight through the darkness of infi- nite space. But will there be starlight then? Many of the stars are larger and hotter than the sun, and, though much diminished in radi- ance, will yet be able to shed a kindly though feeble light upon his pathway. But the time will come when even the brightest and hot- test, having radiated its heat away, will roll a cold corse among its dead compeers. Such is the gloomy teaching of our philosophy. Once there lived a race of ephemerans, whose dwelling place was upon a thermometer. The span of life of one of them was but a second. Being of a scientific turn of mind they made records of the readings of the instrument. After observations had been made for ten generations they promulgated the theory that the mercury was rising one hundredth of a degree every second. After the lapse of ten generations more the theory was confirmed, and was then called a law. When one hundred generations had passed away, the law was considered so firmly established, that no reasonable ephemeran could doubt it. It was the one grand and inexorable law of nature : one might question everything else, but never this. During the next ten generations they executed a laborious triangulation, determin- ing the distance over which the mercury must still travel before it reached the top of the thermometer, and burst the glass tube. Then it was an easy matter to calculate that the utter ruin of their beauti- ful dwelling place could not be delayed beyond the ten-thousandth generation. Great was the humiliation of their scientists, but still greater the joy of the ephemerans at large, when it was found, after the lapse of two thousand generations, that the mercury was actually going the other way. Even the scientists were constrained to admit that there were more things in heaven and earth than were dreamed of in their philosophy. 272 DESCRIPTIVE ASTRONOMY. CHAPTER XIV. THE CONSTELLATIONS IN DETAIL. " Sit, Jessica. Look how the floor of heaven Is thick inlaid with patines of bright gold : There 's not the smallest orb which thou behold'st But in his motion like an angel sings, Still quiring to the young-eyed cherubins." SHAKESPEARE. 405. The Greek Alphabet. Since very many of the stars are named by means of Greek letters, the Greek alphabet is subjoined. In pronouncing the names of the letters e should be given like ay in bay. I is pronounced like ee. a Alpha. fi Beta. Gamma. Delta. Epsilon. Zeta. T] Eta. Theta. 1 lo'ta. K Kappa. X Lambda. , Mu. v NU. $ XI (Ksee). o Omicron. 7T PI. p Rho. cr Sigma. T Tau. v Upsilon. Phi. X Chi. $ PsI. to Ome'ga. 406. Use of the Data in this Chapter. Before making use of the data in this chapter, the student should be familiar with I and 8-12. Under each constellation the directions for finding its principal stars are first given. These directions presuppose an acquaint- ance with Ursa Major, Ursa Minor, and Cassiopeia ( 10). They should be used in conjunction with the maps. When attention has been called to the configuration of the princi- pal stars of the constellation, the chief objects of interest in it are THE CONSTELLATIONS IN DETAIL. 273 mentioned. The numbers in [ ] refer to the lists at the end of the chapter. For the mythological history of the constellations, the reader is referred to a classical dictionary. At the end of the chapter the right ascensions and declinations of interesting telescopic objects are given, together with simple directions for finding them by means of a telescope equatorially mounted, and provided with graduated circles. 407. Andromeda, the Chained Lady. [Map L] Andromeda may be easily learned after Cassiopeia. A line drawn from Polaris to ft Cassiopeiae, and prolonged an equal distance, strikes a, which is the head of Andromeda : it is also one corner of the square of Pegasus. A line drawn from Polaris to e Cassiopeiae, and prolonged nearly the same distance beyond, ends very near 7, which is in one foot of the figure. The row of stars from a to 7 bounds the left side of An- dromeda, so that her body lies between this row and Cassiopeia. Her outstretched arms run from X to 77. The great nebula [83] which is plainly visible to the naked eye is in line with /3 and /a, ^ being half way between the other two. It has been described in 389. In a small telescope it is simply a bright oval mass, none of the wonderful details of its structure being perceptible. 7 [6] is a fine double, the larger star being orange and the smaller sea-green : the small star is a very close double ; 7 is there- fore really a triple star. The small star is of the sixth magnitude, and u" distant. 408. Aquarius, the Water Bearer. [Map V.] A line drawn from ft Pegasi to a of the same constellation, and prolonged as far again, terminates just east of a group of fourth magnitude stars having the form of a Y. This is the jar from which Aquarius pours a never exhausted stream of water which meanders southward into the mouth of the Southern Fish. The remainder of this dull-looking constellation lies south of the jar, and extends to quite a distance east and west of it. Thirty degrees south and a little east of the Y is the first magnitude star Fomalhaut, in the Southern Fish. Between the two lies a portion of Aquarius which has been likened to the contour of South Amer- ica. It is formed by the stars 0, X, r, , c 2 , and i. The stream from the water jar to the Southern Fish is marked by pretty groups of 18 274 DESCRIPTIVE ASTRONOMY. stars, and is indicated on the map by a dotted line. The stars a, /?, v, e, and 3, in the western portion of the constellation, form a rude short-handled dipper. f [81], the central star of the Y is a fine binary, the components being nearly equal ; they are 3" apart now. A little over a degree west of v lies a small bright planetary neb- ula [136], with a stellar nucleus: it is of a greenish blue cast. Almost directly north of fi, at a distance of 5, lies an exceedingly compact globular cluster [138] of faint stars. 409. Aquila, the Eagle. [Map V.] Altair, the brightest star, is easily found by means of /3 and 7, which lie on either side of it ; the three stars lie athwart the Milky Way, there being no other very bright stars in the immediate vicinity. The triangle formed by 7, 0, and X embraces most of the bright stars of the constellation, which bears no resemblance to an eagle. A degree and a half northeast of 7 lies the sixth magnitude star TT [69], which is a close double, a test for the power of a three- or four-inch telescope, on a fine night; the components are nearly equal, and only 1^.5 apart. Three degrees east and a trifle south of 12 Aquilae lies a fan- shaped cluster [131] of telescopic stars, rj varies between the fourth and fifth magnitudes in a period of seven days and a fraction. 410. Argo Navis, the Ship. [Map III.] Only a portion of this huge constellation is visible in the United States. The rest is too far south. The few bright stars visible to us lie east and south of Canis Major, and can be identified by the use of the map, after that constellation is known. A line from Sirius to 7 Canis Majoris, when prolonged nearly twice as far, terminates just north of a diffuse cluster [102] of stars, some of which are visible to the naked eye. A little over a degree east of the preceding and 20' south of it is a circular telescopic cluster [103] 30' in diameter. A degree west of [102] lies a red star [150] of the sixth magnitude. 411. Aries, the Ram. [Map II.] A line from Polaris through e Cassiopeiae to 7 Andromedse, when prolonged a distance equal to that between the latter stars, pierces the triangle composed of a, /3, and 7 Arietis, which is the distinguishing mark of the constellation. The triangle is in the Ram's head. His body lies THE CONSTELLATIONS IN DETAIL. 275 to the eastward, and bears no resemblance to the configuration of the stars. 7 [4] is a fine double star, the components of which are nearly equal ; the distance between them is 9". 412. Auriga, the Charioteer. [Map I.] Capella, the brightest star, is of the first magnitude, and forms a rude square with Polaris, e Cas- siopeiae, and o Ursae Majoris, the star in the nose of the Great Bear. It is one of the brightest stars in the sky, and shines with a pure white light, ft, 6, L, and ft Tauri form with it an irregular five-sided figure, which is readily discerned. 8 is in the head of the Charioteer : L and ft Tauri mark his feet. He carries in his arms a kid marked by the stars e, f, and 77. ! / 14 Aurigae [14] is a triple star having a seventh magnitude com- panion at a distance of 14" and one of the eleventh magnitude at a distance of 13". Inside of the triangle formed by X, t, and % are a number of star clusters most of which lie near the line between X and %. Halfway between X and L is a rich field of stars [90] fainter than the seventh magnitude. A line from ft to a point midway between and z>, pro- longed as far again, strikes a beautiful cluster [96] of small stars : the whole field seems strewn with gold dust. These stars are so closely associated that one must believe them to be really near to- gether, and not merely in the same line of vision. This combina- tion of stars of very different degrees of brightness is an evidence that a faint star is not necessarily at a great distance from us. A line from t to 0, prolonged half its length, ends near a deep red star [149] of the sixth magnitude. 413. Bootes, the Bear Keeper. [Maps I. and IV.] A line from 8 Ursae Majoris to 77 of the same constellation, prolonged an equal distance, strikes a very small triangle composed of the fourth mag- nitude stars 0, L, and K, which are in the uplifted hand of Bootes. They lie midway between Polaris and Arcturus, the most brilliant star in Bootes, and one of the brightest in the heavens: it has a pronounced ruddy hue. It is at the lower end of an immense kite- shaped figure formed by ft, 8, e, a, p, and 7. ft is in the head of Bootes ; 7 and 5 are in his shoulders ; p and e form his belt. In his right foot is the triangle ?, o, TT; in his left foot is another triangle, 77, T, v. Arcturus is in his sword. 276 DESCRIPTIVE ASTRONOMY. e [41] is a fine slow binary : the companion is of the sixth magni- tude, and 3" away. The large star is yellow, the small one blue. f [42] is a fine binary, having a period of about 130 years. The companion is now less than 4" distant, of the seventh magnitude, and purple. The distance is diminishing. 1 [39] i s 5 east an d 2 north of 77 Ursae Majoris, and has a companion of the eighth magnitude, 38" away. TT [40] is 6 east and 3 south of Arcturus, and has a companion of the sixth magnitude, 6" distant. 414. Camelopardus, the Camelopard. [Map I.] The stars in this constellation are faint. The head of the creature consists of four fifth magnitude stars situated one fifth of the way from Polaris to the bowl of the Great Dipper. His fore feet are almost on the head of Auriga, while his hind feet are in position to give Perseus a kick in the stomach. If he were not such a weakling, he might give trouble. There is a rich, though coarse cluster [89] two thirds of the way from a Persei to & Aurigae. It is close to the fifth magnitude star 7 Camelopardi. 415. Cancer, the Crab. [Maps III. and I.] The principal start form an inverted Y, as shown on Map III. A line drawn from Polaris to h Ursae Majoris, a fifth magnitude star in the head of the Great Bear, and prolonged i^ times its own length, strikes i, the uppermost star in the A. f [27] is found by alignment with Castor and Pollux. It is a fine multiple star, and has been described in 372. The two stars forming the bright one are in rather rapid motion, sixty years sufficing for a revolution. The visible companion of this binary is 5" distant. The large star looks oblong with a high power on a three- or four-inch telescope. Between 7 and & lies the cluster [104] Praesepe, the Beehive, which is visible to the naked eye on a moonless night. A good- sized opera-glass shows it better than a larger telescope. 416. Canes Venatici, the Hunting Dogs. [Map I.] This con- stellation is not especially noteworthy. Its brightest star, a or 12, is called Cor Caroli (the Heart of Charles II. of England), and is found by prolonging a line from Polaris to e Ursae Majoris half its length. THE CONSTELLATIONS IN DETAIL. 277 Cor Caroli [36] has a sixth magnitude companion 20" distant. 2 [33] is an orange star of the fifth magnitude, having a blue companion of the ninth magnitude, n" distant. There is a bright globular cluster [114] containing upwards of 1,000 stars, lying nearly midway between Cor Caroli and a Bob'tis (Arcturus), but a little nearer the latter: it is close to a star of the sixth magnitude. The Great Spiral Nebula [113] ( 391) lies about one fourth of the way from 77 Ursae Majoris to Cor Caroli. It is called the Whirl- pool Nebula, but in small telescopes it looks simply like a faint double nebula. The entire constellation is plentifully besprinkled with faint nebulae. 417. Canis Major, the Great Dog. [Map III.] This constella- tion is best learned after Orion. The line of the three stars in Orion's belt prolonged eastward passes near Sirius, which is a in this constellation, and by far the brightest fixed star in the heavens. The triangle S, e, 77 is in the haunches of the animal, and .Sirius is in his head : ft is the extremity of one uplifted fore paw. The animal sits upright, in the attitude of begging his master Orion for permission to put his teeth into the Hare, which is under Orion's feet. Sirius is a very interesting double star ( 370), but is much too difficult for a small telescope, the faint companion being in the blaze of light surrounding the bright star. The period of revolution is about fifty years. fji [24] has a ninth magnitude companion at a distance of less than 4". A superb cluster [100] visible to the naked eye lies about one third of the way from Sirius to e. There is a ruddy star near the centre : many of the brighter stars are arranged in curves. 418. Canis Minor, the Little Dog. [Map III.] It is well to learn Gemini before Canis Minor. A line from Polaris to 13 Geminorum (Pollux), prolonged one third as far again, reaches Procyon, a first magnitude star, which is a Canis Minoris. The only other con- spicuous star is /3, which is 4 northwest of Procyon. Procyon is of interest because of its irregular proper motion, supposed to be caused by the presence of close companions, which have often been searched for by the largest telescopes, but without 278 DESCRIPTIVE ASTRONOMY. success. In the same field with Procyon is a star of the seventh magnitude which is a close double. The components are only i".5 apart, but the star can be elongated by a good four-inch glass. 419. Capricornus, tne Goat. [Map V.] It is well to know Cyg- nus before attempting to learn Capricornus. A line from Polaris to 7 Cygni (where the arm of the cross is fixed to the upright piece), prolonged an equal distance, reaches the naked-eye double a, below which is ft at a distance of 2. 5. S and 7 form another such pair of stars, and ^ and co a third. The constellation i-s chiefly embraced in the triangle formed by these three pairs. a and ft are in the head of the animal, while i/r and co are in his knees ; the rest of the Goat may be supplied to suit the fancy. p [73] and TT [72] are pretty doubles, each having a companion of the ninth magnitude, less than 4." away. A good night is needed for their observation with a four-inch glass. 420. Cassiopeia, the Lady in the Chair. [Map I.] This brilliant constellation is quickly found by using Map I. according to the directions in 10. The stars e, 8, 7, a, ft, and K form a rude broken- backed chair. The Lady, however, refuses to sit in it, preferring to sit on empty space. The stars ft, a, 7, and K form her body; 8 is in her knee, and i in her foot; is in her head; her arms are uplifted, possibly in prayer to the gods to spare her lovely daughter Andromeda, who has been chained to a rock, as prey for a sea monster. 77 [i] is a splendid binary, having a purple companion of the eighth magnitude, 5" distant. It is less than half the way from a to 7. The period of revolution is 200 years : the combined mass of the two stars is thought to be from five to ten times that of the s-un. Near tc appeared Tycho's new star described in 375. As the Milky Way runs through Cassiopeia, there are many beautiful fields which can be best seen with a low power. Near ft, between p and a, is a large cloud of minute stars [142] discovered by Caroline Herschel, the sister of Sir William Herschel. Between TT and o in the uplifted left hand of the Lady is a magnificent region. One degree east and a little north of 8 is a beautiful field [84]. A line from B to a, prolonged i-J- times its former length, ends near THE CONSTELLATIONS IN DETAIL. 279 R, a vivid red star which varies from the fifth to the twelfth magni- tude in a period of 433 days. 421. Centaurus, the Centaur. [Maps III. and IV.] Centaurus, even when most favorably situated, is too near the southern horizon for satisfactory observation in the United States, except in Florida and Southern Texas. It is of especial interest, because it contains our nearest neighbor among the stars, a Centauri. 422. Cepheus. [Map I.] This constellation lies between Cassi- opeia and Draco. Cepheus is the husband of Cassiopeia, who, with her daughter Andromeda, nearly monopolizes the brilliancy of the family. The five brightest stars are a, ft, 7, t, and f, which form a figure composed of a rude square surmounted by a triangle which is nearly isosceles, a forms with Polaris and 7 Cassiopeiae an isos- celes triangle which is nearly equilateral. Near f are 8 and e : the three are in the head of the figure. ft [77] is a double star, the companion being blue, of the eighth magnitude, and 14" distant. [82] has a companion of the seventh magnitude, 41" distant: the primary is yellow, the companion blue : the main star is a noted variable, having a period of 5^ days. f [80] has a blue seventh magnitude companion, 6" distant. 423. Cetus, the Whale. [Maps II. and V.] A line from Polaris to 8 Cassiopeiae, prolonged so that the prolongation is 2\ times the ori- ginal length of the line, reaches the centre of this huge and ungainly constellation, which can be best learned by following the dotted lines given on the map. The monster has about the shape of a walrus. The most noticeable portion of the constellation is an irregular pen- tagon, rudely kite-shaped, formed from the third magnitude stars /3, 77, 1 , f, and r. The pentagon formed by a, 7, f 2 , /x, and X marks the head. Nearly half way from 7 to flies o (Mira) [143], the wonderful variable described in 376. It is visible to the naked eye only six weeks in the year. 7 [8] is not a very difficult double for a four-inch glass. A star of the seventh magnitude nestles close to the larger star : the dis- tance is 2". 5. a [144] is a fine orange-colored star, having a blue neighbor in the same low-power field. 280 DESCRIPTIVE ASTRONOMY. 424. Columba, the Dove. [Map II.] The full name is Columba Noachi or Noah's Dove. The asterism lies south of Lepus, and is too low down in the south to be seen well. A line drawn from /3 Orionis (Rigel) to fi Leporis, and prolonged as far again, terminates near a and {3, the two brightest stars. 425. Coma Berenices, the Hair of Berenice. [Map I.] This con- stellation consists of faint stars ; most of those visible to the naked eye are of the fifth and sixth magnitudes. They are well crowded together. A line from Polaris to 8 Ursae Majoris, when prolonged an equal distance, terminates near the most crowded part of the asterism. It is a fine sight in a small opera-glass. A little over one third of the way from ?? Bootis (near Arcturus) to Leonis is the fifth magnitude star 42 ; 50' northeast of this is a condensed mass of minute stars [112], which cannot be well seen with a telescope of less than four inches aperture. 426. Corona Borealis, the Northern Crown. [Map I.] Corona lies a little south of a line from a Bootis (Arcturus) to a Lyrse (Vega) r at about one third the distance from the former to the latter. Seven of its principal stars form a figure so similar to a crown that it is in- stantly recognized. f [44] has a bluish green companion of the sixth magnitude 6' r distant. A degree south of e is a ninth magnitude star called T Coronae [155]* It suddenly blazed up in May, 1866, and equalled a in brightness ; it then slowly declined, and after a month reached its former low estate, which it has held ever since. 427. Corvus, the Crow. [Map IV.] A line from Polaris to B Ursae Majoris, prolonged until it is 3^ times its former length, strikes a snjall but conspicuous quadrilateral, 15 west and 10 south of aVir- ginis (Spica). a is in the Crow's bill; the Crow stands upon and pecks at Hydra. 8 [34] is accompanied by a purple star of the eighth magnitude, at a distance of 24". 428. Crater, the Cup. [Map III.] Crater adjoins Corvus on the west, and stands upon Hydra. The stars 77, f, 7, S, e, and 6 form the bowl of a crooked goblet, in the base of which are a and /3. The gob- let leans as if to discharge its contents upon its neighbor, the Crow. Just east of a and in the same field of view with a very low power THE CONSTELLATIONS IN is R [152], a notable red star of the eighth magnitude. Sir William Herschel described it as " scarlet, almost blood-colored ; a most intense and curious color." 429. Cygnus, the Swan. [Map I.] Cygnus is readily discovered by following the directions for using Map I. given in 10. It lies in the Milky Way, just east of Lyra, and is quickly recognized by the cross, the upright piece of which is composed of a, 7, 77, ^, and /8, and has the same trend as the Milky Way. The cross arm consists of the stars 8, 7, and e. ft is in the Swan's head, and a in its tail. The cross piece of the cross, extended, forms the wings of the bird. ft [66] has a blue seventh magnitude companion at a distance of 34". It is the finest colored double for a small telescope in the northern sky ; the colors are beautifully seen by putting the telescope slightly out of focus. fi [78] is a much closer double, the fifth magnitude companion being only 4" distant. A third star of the seventh magnitude is over 200'' distant. 17 [68] lies in a beautiful field, and has a ninth magnitude com- panion 26" distant. 61 [76], which is in one corner of a parallelogram formed by a, 7, e, and itself, is a pretty double when seen with a low power : the components are nearly equal. This star is celebrated as the first one the distance of which from us was measured. It is about 550,000 times as far off as the sun. There are fine fields in many places, especially within a few de- grees of a (Deneb). One of the best is a little north of the middle of a line from a and 8, near o. In the northeast corner of the con- stellation, about half way between p and Tr 1 , is a large cluster [139] in a rich vicinity. 430. Delphinus, the Dolphin. [Map V.] A line from Polaris to a Cygni, when prolonged until it is two thirds longer than before, strikes a small diamond, composed of three stars of the fourth mag- nitude and one of the third. These, with a fifth of the fourth mag- nitude, which lies southwest of them, form a narrow wedge, called Job's Coffin. This is the principal portion of Delphinus. 7 [74] is a golden yellow star having a greenish blue companion of the sixth magnitude, at a distance of u". 282 DESCRIPTIVE ASTRONOMY. 431. Draco, the Dragon. [Map I.] The head of the Dragon consists of a bright quadrilateral formed of ft, 7, f , and v, which is so situated as to form an equilateral triangle with Cassiopeia and the bowl of the Great Dipper, Polaris being inside of the triangle. It also forms a much smaller right triangle with a Lyrae (Vega) and a Cygni, the right angle being at Vega. From the head the constellation winds in magnificent convolu- tions, shown by the dotted line on the map, around between the two Bears. X, the last bright star in the tail, is two thirds of the way from Polaris to the centre of the bowl of the Great Dipper. About half way between Ursse Majoris (Mizar) and 7 Ursae Minoris (one of the two brighter stars in the Little Dipper) lies a, which is distinguished as having been the pole star four or five thousand years ago. About half way between 8 and f lies the pole of the ecliptic, which is near a bright planetary nebula [125], 35" in diameter. Unlike most such objects, it can be seen very well with a four-inch glass. fji [52] is a neat double, the two stars being nearly equal in bright- ness, and less than 3" apart. A planetary nebula has been men- tioned above. 432. Equuleus, the Little Horse. [Map V.] a lies 7 west and nearly 5 south of e Pegasi, which is in the nose of the animal. It contains only five stars above the sixth magnitude. e [75] has a companion of the seventh magnitude at a distance of n" ': the main star is a close rapid binary, which now looks elon- gated in a four-inch telescope, armed with a high power. 433. Eridanus, the River. [Map II.] Three degrees north and two west of /3 Orionis lies @ Eridani, which may be considered as the source of the river. Thence it flows west, following the sinuous line on the map, till it reaches the star TT Ceti, where it laves the paws of Cetus; then it drops south about 5, thence east, southeast, and southwest in succession, till it is lost beneath our horizon. 32 [n], which has a right ascension of 3 h. 49m. and a south declination of 3 15', is a fifth magnitude star having a companion of the seventh magnitude 7" distant. The primary has been called topaz-yellow, and the companion sea-green. 434. Gemini, the Twins. [Maps I. and II.] A line drawn from the bowl of the Little Dipper to the head of the Great Bear, and THE CONSTELLATIONS IN DETAIL. 283 prolonged an equal distance, terminates near the two bright stars a and /3 (Castor and Pollux). Pollux is the brighter of the two. These two are in the heads of the twins, who stand side by side. The chief stars can be traced by the dotted lines on Maps I. and II. The entire figure is much like an end view of an upright piano. a and /3 are at the top, p, 7, and f at the bottom, while X and f are at the key-board. The summer solstice is close to the fifth magni- tude star I, which is a little west and north of rj and /JL. Castor [26] is a magnificent double, the components differing one magnitude in brightness, and being nearly 6" apart. It is a binary, the period of which is thought to be about 1,000 years. 8 [25] has an eighth magnitude companion at a distance of f. Four degrees west and two north of /x (at the base of the back of the piano) is a cluster [97], visible to the naked eye as a faint cloud on the sky. It is 20' in diameter and consists of stars from the ninth magnitude down to the faintest points of light. 435. Hercules. [Maps I. and IV.] Directly east of Corona lies the belt of Hercules, composed of the stars e and f ; (3 and 8 are in the shoulders ; ?? and TT are in the thighs ; a marks the head. The limbs and arms are traced by the dotted lines on the maps. The whole forms a fair picture of a giant, with his head toward the equator. a [54] is a fine double, having an emerald companion of the sixth magnitude 5" away : it is also variable. p [57] is a binary, having a greenish companion of the fifth mag- nitude at a distance of 4" : it is near TT in one thigh. 8 [55] in one shoulder has an eighth magnitude companion, which has, if one compares the estimates of different observers, nearly all the colors of the rainbow, and is at a distance of 19". The finest globular cluster [118] in the northern hemisphere, pictured in Fig. 173, is one third of the way from 77 to , and is just visible to the naked eye. The stars are so thickly crowded near the centre, that a small telescope shows them simply as a neb- ulous mass. About one third the way from t, in one foot, to rj, in the opposite thigh, is a very condensed cluster [121], which is fine, but inferior in interest to the preceding. 436. Hydra, the Snake. [Maps II. and III.] A line from Polaris 284 DESCRIPTIVE ASTRONOMY. through the middle of the triangle which forms the head of the Great Bear, carried on through Cancer, meets the head of Hydra, which is just beyond Cancer ; the head is a good representation of that of a hissing snake. Thence it may be traced in a south and east direction by following the dotted line on the map. A line from Polaris through h at the vertex of the obtuse angle of the triangle in the Great Bear's head, passing in front of the Sickle in Leo (through K Leonis) meets a, which is also called Cor Hydrse. The distance from a to K Leonis is one half the distance of the latter from Polaris. One is helped in tracing the eastern end of the constellation by the recognition of Corvus, which stands upon it. e [28], the northernmost star in the head, has a blue companion of the eighth magnitude at a distance of 3". 5. At a right ascension of loh. 20 m., 2 south of ^, is a bright plan- etary nebula [108], which appears as large as Jupiter when the latter is at opposition. 437. Lacerta, the Lizard. [Map I.] Lacerta lies between Cyg- nus and Andromeda. The middle point of a line connecting a Cygni with a Andromedae lies a little south of the centre of the constellation. Two and a half degrees west of 7, which i-s the brightest star in the constellation, lies a fair cluster [141]. The constellation furnishes some fine fields, when viewed with a low power. 438. Leo, the Lion. [Map III.] A line from Polaris to the middle point of a line connecting a Ursae Majoris and k of the same constellation, when prolonged to nearly three times its original length, passes through a conspicuous figure known as The Sickle, and terminates at a (Regulus), in the end of the handle of the Sickle. At the east of this figure is a conspicuous right-angled tri- angle which lies in a line drawn from Polaris through the bowl of the Great Dipper. The Sickle constitutes the head and the fore part of the body of the crouching lion. The large triangle is in his haunches. Regulus is sometimes called The Lion's Heart. 7 [30] is a golden yellow star, having a companion of the fourth magnitude, at a distance of 3". 5. It is one of the finest binaries in the northern sky : its period is about 400 years. i [32], the nearest bright star south of the west end of the right triangle, has a bluish companion of the seventh magnitude, less than 3" away. THE CONSTELLATIONS IN DETAIL. 285 A little over half the way from a to f is the crimson variable R [151], which ranges between the fifth and tenth magnitudes ; the period is 312 days. 439. Leo Minor, the little Lion. [Map I.] Adjoining the Sickle, in a line from it to the bowl of the Great Dipper, lies Leo Minor, a shapeless constellation containing a few naked-eye stars, three of which are as bright as the fourth magnitude. 440. Lepus, the Hare. [Map II.] Lepus crouches under Orion's feet, and does not particularly resemble a hare. 7 [21] is a triple star; the larger companion is of the seventh magnitude, and is 93" distant; the small companion is 45" from the other one, and is visible with a three-inch glass. 45 [ J 9] i s a seventh magnitude star I J east of a ; it has four com- panions visible with a small telescope, at distances varying from 60" to 126". There are four other companions to be seen with larger telescopes. A line from a to p, prolonged two thirds of its length, ends close to the crimson star R [147], which varies from the sixth to the ninth magnitude; the period is 438 days. 441. Libra, the Scales. [Map IV.] Libra is best learned after Virgo and Scorpio, between which it lies, a lies a little more than half way from a Virginis ( Spica ) to /3 Scorpii. The chief config- uration is a quadrilateral formed by a, fi, 7, and i. a looks elongated to a keen eye ; an opera-glass shows that it has a fifth magnitude companion. B [153] is a variable, situated 4. 5 west and i north of /3. Its period is 2\ days, and it varies from the fifth to the sixth magnitude. The change in brightness consumes 12 hours. $ [154] is a pale green star. 442. Lupus, the Wolf. [Map IV.] Lupus lies south of Libra, and even when best seen is too near the southern horizon for observ- ers in middle north latitudes. 443. Lynx, the Lynx. [Map I.] The Lynx occupies a dull region between Ursa Major on one side, and Auriga and Gemini on the other. The leading stars form an irregular line, traced on the map. 38 [29] in the southeastern corner of the constellation, has a lilac companion of the seventh magnitude, 3" distant. The pair 38 and 286 DESCRIPTIVE ASTRONOMY. 40 form an equilateral triangle with two pairs in the feet of the Great Bear. 5 [148] is a fiery red star of the sixth magnitude, in a fine group, 444. Lyra, the Harp. [Map I.] The leader of this constellatioa (Vega) is one of the brightest of the first magnitude stars. To the naked eye its color is pale sapphire. It is easily identified by means of the two fourth magnitude stars, e and f, which form with it an equilateral triangle, each side of which is nearly 2 in length. The constellation lies between Hercules and Cygnus. The equilateral triangle is perched on one corner of a rhomboid, being common to both figures. a (Vega) [60] has a blue companion of the tenth magnitude, 48" distant. /3 [63] is a multiple star, having three companions of about the eighth magnitude, at distances of 46", 66", and 86", respectively. It is also one of the noted variables. See 378. e [61] is one of the equilateral triangle, and appears elongated to the average eye : a sharp eye splits it into two stars. An opera- glass separates them widely, and a small telescope shows each star as a double. The distance between the components of one pair is 3" ; the other pair is a little closer. [62] has a fifth magnitude companion, 44" distant. 8 east of Vega are the two stars rj \6$] and 0. The former has a blue companion of the ninth magnitude, 28" distant. 8, one of the stars of the rhomboid, is double in an opera-glass, and is sit- uated in a fine field. Beautiful fields lie between e and R, which is 5 northeast of it. The only annular nebula [132] which small telescopes reveal lies one third of the way from ft to 7. It has been described in 391. 445. Monoceros, the Unicorn. [Maps III. and II.] This constel- lation contains only four stars as bright as the fourth magnitude. It lies east of Orion, and stretches itself in the Milky Way between Canis Major and Canis Minor. 8 [22] lies in the northwestern part of the constellation, at a right ascension of 6 h. 19 m. A line from X Orionis (in his head) to a Orionis, prolonged i| times its own length, stops just south of 8. It is a golden yellow star with a lilac companion of the seventh magnitude, 13" distant: it is in a splendid field. THE CONSTELLATIONS IN DETAIL. 287 II [23], which lies in the southwestern part of the constellation, has a double companion of the sixth magnitude, 7" distant. The components of the companion are 2". 3 apart. It is a star of the fourth magnitude, about three eighths of the way from Sirius to a Orionis (Betelgueuse), a little east of a direct line. 2 east of 8, and i south of the middle point of the line joining 7 Orionis (Bellatrix) with a Canis Minoris (Procyon) is a cluster [99] visible to the naked eye, and very pleasing with a low power. Some of the faintest stars are arranged in straight lines. A line from Sirius to 6 Canis Majoris, when prolonged three fourths of its length, reaches a brilliant coarse cluster [101], in a " superb " neighborhood. There is a fine field one fifth of the way from 1 1 to 8 ; the fifth magnitude star 10 is in it. 446. Ophiuchus, the Serpent Bearer. [ Map IV. ] Ophiuchus lies between Hercules and Scorpio. The two portions of Serpens lie respectively at the east and west sides of this constellation. Ophiu- chus is represented as standing on the Scorpion and grasping the Serpent with both hands. A line from Polaris to j3 Draconis (in the Dragon's head), pro- longed an equal distance, ends near a, which is in the head of Ophi- uchus and near a Herculis. (3 and 7 mark his right shoulder, t and K the left; v and T are in his right hand, 8 and e in his left. His right knee contains 77 and his left f. The right foot is at 0, the left at p. The parallelogram (nearly) formed by and X Ophiuchi with a and p Serpentis is shown by the dotted lines on the map, and is note- worthy to the eye : one diagonal of it contains five bright stars. X [51] is a binary, having a period of about 230 years. The com- panion is of the sixth magnitude, and is now (1896) i".7 distant. 36 [53]> a fifth magnitude star in the southernmost part of the constellation, 11 east of a Scorpii (Antares), has a sixth magnitude companion at a distance of 5". 70 [58], 4. 5 east of 7, is a fine binary, completing a revolution in less than a century : the seventh magnitude companion is reddistu The distance is now (1896) 2". p [49], in the left foot, has an eighth magnitude companion at a distance of 4". A cluster [120] 3' in diameter lies 9. 5 due east of a Scorpii 288 DESCRIPTIVE ASTRONOMY. (Antares), nearly in line with 36. There are a number of other clusters in the vicinity. 3 south and i west of f lies a cluster [i 17] 5' in diameter. One third of the way from e to /3 lies a cluster [119] 8' in di- ameter, in the centre of which the stars are very closely crowded. A line from 1 f'frtl t'l t 8 00 3 ^3 ~~ B * * OJ ^^ ** >2 ^ ^ o 1-4 MO ^U "* P _r^ "^ ^H U ffipQU i-3 ^ i-J = 111 CO t^ ^"} O O O O O vO O O O O O M 1-1 10 O \O CO OOOOOOOO OOOO O II s ci cT pf co ro M M -> C fl N si ^O NM .cor^.coo O O O O O CNOO O IS ^co r^r>* "coooONt^ 00000*0^-- ^k^i 2 l-c h-i CO CO Jo ^ nf ^ f -o ^O ^O If! ,0 vo^n > w^^^^^T^^ CNCNO^C^ ^ O "o 5 _; r^^ cocor5co 10 c^ MNf^co N fO N CO i * ro ON r-N* t N s. r^ ro w ci TJ- row vnwi-iTj-ro ?0^5-^5-^^ c?c7^^ ^ i & r^ r^\o - oo ro rovO NOO>-itv.r)civOr^ N fO^O >-i M i k-l 1 1 H-t h-l N N M M N w -ON & s CO i < H- i \o i 1 ^O vO M 1-1 W Tj-vO w i^ t^ ro 1000 CN^-W Or^H-ro ^ h- 1000 ^O N vo ro *^ f^ *O r^vO ^ Q * 0) 4) Satellite. cL ^.2 M 0) 2 05^ e ^ s^r3U > o w _ . S )5 ' HH illl 1 ^HHQ^HK^ 473- Rosse, Lord, his great reflector, 43. Rotation of the sun, 58. Sagitta, 452. Sagittarius, 453. Satellites, attending upon stars, 371 ; of the planets, 472. Saturn, distance and diameter, 253 ; revolution and rotation, 254 ; appearance, 255, 256 ; discovery of the rings, 257 ; divisions and dimensions of the rings, 258; disappearance of the rings, 259; the dark ring, 260; structure of the rings, 261 ; stability of the rings, 262 ; the satellites, 263 ; physical condi- tion, 264. Schaeberle, J. M., theory of the corona, 81 ; theory of the markings on Mars, 215; observes Jupiter's moons, 249. Schiaparelli, determines rotation time of Mercury, 196 ; determines rotation time of Venus, 203 ; discovers canals of Mars, 218. Schott, Doctor, 39. Schwabe, observations of the sun, 59. Scintillation of the stars, 337. Scorpio, 454. Screen, used in solar observations, 51. Sculptor, 455. Scutum, 456. Seasons, in middle latitudes, 107; at the equator, 108. Secchi classifies stellar spectra, 353. Secondary Circles defined, 120. See, T. J. J., investigates the origin of double stars, 397. Serpens, 457. Sextans, 458. Sextant, 143. Shadows, of the earth and moon, 170, 174; shadow of the moon visible, 177. Ship, the position of, determined, 143. Shooting Stars, described, 308 ; numbers, 322 ; paths and velocity, 323; masses and constituents, 324; radiant, 325 ; the August shower, 326; the November Leonids, 327-329; the Bielids, 330; orbits of, 331. Shower, Meteoric, at L'Aigle, 309 ; the August, 326 ; of the November Leonids, 327-329; of the Bielids, 330 ; orbits of, 331 ; how to observe a, 333. Sidereal Time, 130, 131, 466. Sidereal Year defined, 1 13, Signs of the zodiac, 100. Sirian stars, 353, 354. Sirius, its distance, 349; color, 352 ; spectrum, 353 ; double, 370, 417. Small Circles defined, 120. 22 INDEX. Solar stars, 353, 354. Solstices defined, 99. Spectra, classes of stellar, 353 ; of the nebulae, 388. Spectroscope, description of, 69-72. Spectrum, the solar, 74. Spectrum Analysis, the laws of, 73. Sphere, the celestial, 15-17; position~of points on, 120. Spica, 369. Spiral Nebula, 391 ; real form of, 392; location, 416. Square of Pegasus, 448. Stability, of the planetary system, 186; of Saturn's rings, 262. Standard Time, 134. Star Finder, 465. Star Light, 355. Star of Bethlehem, 201. Stars, fixed, defined, i ; morning and evening, 191, 201 ; number visible, 336; scintillation, 337 ; Milky Way, 338 ; tree-like structures, 339 ; constellations, 340 ; names, 341 ; orders of brightness, 342 ; magnitudes, 343 ; catalogues, 344 ; charts, 345 ; distribution, 346 ; clusters, 347 ; dimensions and nature, 348 ; distances, 349; parallax, 350, 351 ; colors, 352; spectra, 353 ; of differ- ent spectral types, 354 ; light and heat, 355 ; the stellar system, 356, 357; proper motions, 358 ; groups, 359; motions in the line of sight, 360; the system of, 363 ; double and multiple, 364-372 ; variable, 373-383 ; nebulous, 385 ; hour angle at any instant, 467 ; list of double, 469 ; list of variable, 469 ; list of colored, 469 ; proper names of, 470. Stationary Point of a planet's path, 190. Structure, of the Milky Way, 338 ; of the stellar universe, 356, 357. Structures, tree-like, 359^ Sun, distance and diameter, 49; how to view, 50, 51; photosphere, 52; rice- grains, 53 ; faculae, 54 ; general appearance of a spot, 55 ; changes in appear- ance of spots, 56 ; dimensions of spots, 57 ; rotation, 58 ; periodicity of the spots, 59; observation by Carrington and Hodgson, 60; disturbance on July 15, 1892, 61 ; cyclonic motion of spots, 62; nature of spots, 63; causes of weather changes, 64; magnetic storms, 65, 66 ; the storm of February, 1892, 67 ; frequency of magnetic storms, 68 ; the solar spectrum, 74 ; constituents of, 75 ; chromosphere, 76 ; prominences, 77-79 ; appearance of the corona, 80 ; Schaeberle's theory of the corona, 81 : nature of the corona, 82 ; light, 83 ; heat, 84; causes of radiation, 85 ; contraction theory, 86; past and future, 87 ; con- stitution, 88; the midnight, 106; enters Aries, 112; the mean, 125; cause of eclipses, 173; varieties of eclipses, 175; partial, annular, and total eclipses, 176-178; duration and number of eclipses, 179; its path, 361 ; testimony to the nebular hypothesis, 402. Sun Spots. (See under Sun, 55-64 ) Superstitions about comets, 299. Synodic Period, of the moon, 146; of a planet, 192. INDEX. 339 System, stability of the planetary, 186; of the stars, 363; systems of planets attending upon stars, 371 ; planetary, 401. Swift, Lewis, claims discovery of intra- Mercurial planets, 178 ; his bright comet of 1892, 305. Swift, the satirist, writes of the moons of Mars, 222, note. Tails of comets, 291 ; evolution of, 294 ; types of, 295 ; of Swift's comet, 305 ; of Brooks's comet shattered, 307. Taurus, 459. Telegraph, used in determining longitude, 142- Telescope, management of, 47 ; reflecting, 25, 41, 42, 43 ; refracting, 25, 34, 42 ; equatorial, 44-46 ; Lick, 48 ; invention of, 473. Telescopic Objects, 469. Telluric Lines, in a spectrum, 75. Tempel's comet, 332. Temperature at the moon, 166. Temporary Stars, denned, 374; Tycho's, 375; Nova Aurigae, 381. Terminator of the moon, 1 54. Theta Orionis, 372 ; condensed from a nebula, 386 ; spectrum, 398 ; position of, 447. Thomson, Sir William, on the future of the sun, 87. Tides, 169. Time, the years, 113; the calendars, 114, 115; mean and apparent solar days, 125; inequalities of apparent solar days, 126, 127 ; sidereal day, 128 ; civil and astronomical day, 129; mean solar and sidereal compared, 130; relation between sidereal time and right ascension, 131; longitude and time, 132; where the date changes, 133; standard, 134; determination of, by a meridian circle, 138; sidereal at any instant, 466. Tisserand, investigates comet groups, 286. Titan, 263. Topics for Essays, 474. Total Eclipses, lunar, 172; solar, 177, 178. Train of a meteor, 313, 315. Transits, of Mercury, 197 ; of Venus, 204; of Jupiter's satellites, 248. Triangulum, 460. Trifid Nebula, 387, 391. Tropical Year defined, 113. Tschermak, theory of the origin of meteorites, 320. Twilight, 1 1 8. Twinkling of the stars, 337. Tycho Brahe, his star, 375. Ultra-Neptunian planets, 277. Umbra, of a sun spot, 55 ; of a shadow, 170, 174. Universe, future of the visible, 404. 34 INDEX. Uranus, discovery, 265 ; distance and diameter, 266 ; revolution and rotation, 267 ; appearance, 268; satellites, 269; physical condition, 270; captures the Leo- nids, 329 ; captures Tempel's comet, 332. Ursa Major, 10, 406, 461. Ursa Minor, 10, 406, 462. Variable Stars, definition, number, names, 373 ; classes, 374 ; temporary stars, 375; Mira, 376; Class III., 377; Beta Lyrae, 378; Algol, 379; Y Cygni, 380; Nova Aurigae, 381 ; causes of variability, 382; how to observe, 383; list of, 469. Velocity, of light, 83 ; of meteors, 314 ; of shooting stars, 323 ; of the stars, 360. Venus, morning and evening star, 201 ; distance and diameter, 202 ; revolution and rotation, 203 ; transits, 204 ; phases and maximum brightness, 205 ; tele- scopic appearance, 206; atmosphere, 207; physical condition, 208; transit first observed in 1639, 473. Vernal Equinox defined, 98. Vertical Circles defined, 121. Vespasian jokes about a comet, 299. Vesta, discovered, 226; diameter, 230. Virgo, 463. Visual Angle defined, 31. Volcanoes, lunar, 157, 158; terrestrial, 400. Voltaire writes of the moons of Mars, 222, note. Vulpecula, 464. Watches, 135. Water on the moon, 163, 164. Watson, J. C., claims discovery of intra-Mercurial planets, 178; leaves a fund for the computation of the orbits of asteroids, 228. Weather, changes due to sun spots, 64 ; effect of the moon on, 168. Wolf, Max, photographs tree-like structures in the Milky Way, 339. Wolf-Rayet Stars, 353, 354, 399. Y Cygni, 380. Years, different kinds of, 113. Young, C. A., theory of sun spots, 63; observation of prominences, 79 ; opinion about the collision of a comet with the sun, 300; estimate of the light of the stars, 355. Zenith defined, 22. Zenith Distance defined, 121. Zeta Cancri, 372. Zodiac defined, 100. Zodiacal Light, 334. *