mm^ .^BBLB'S lliltl Xs :Ji .ir^^tevar CIBNGBS ^ **•! »»' NEW YORK 's ^/Ki CJ.C.L.Tf JUL 1 3 '33 ^ UNIVERSITY OF CALIFORNIA AT LOS ANGELES L-. i I " JUL 1 3 'W 7/f^ STORY OF THE STARS. NEW DESCRIPTIVE ASTRONOMY, JOEL DORM AN STEELE, Ph.D., AUTHOR OF THE FOURTEEN-VVEEKS SERIES IN NATURAL SCIENCE. ' ' The heavens cteolare the glory of God; and the fi'nmament showezn is handiwork. " — PsAlM XIX, 1. Copyright, 1869, 1884, by A. S. BARNES & COMPANY, NEW YORK AND CHICAGO. A POPULAR SERIES IN NATURAL SCIENCE, J. DoRNiAN Steele, PH.n., K.G.S., A uihor o/ the Fourteen Weeks Series in Natural Science, etc., etc. New Popular Chemistry. New Descriptive Astronomy. New Popular Physics. New Hygienic Physiology. New Popular Zoology. Popular Geology. An Introduction to Botany. The Publishers can supply (to Teachers only) a Key containing Answers to the Questions and Problems in Steele's entire Series. BARNES' HISTORICAL SERIES, ON THE FLAN OK STEELE'S FOURTEEN-WEEKS IN THE SCIENCES. A Brief History of the United States. A Brief History of France. A Brief History of Ancient Peoples. A Brief History of Mediaeval and Modern Peoples^ A Brief General History. A Brief History or Greece. A Brief History of Rome. A Popular History of the United States. Library S '^Z S PREFACE TO THE FIRST EDITION. ~r~\URING the past few years great advances -■-^ have been effected in astronomical science. Physics has come to the help of Mathematics, and, not content with the old question, where the heav- enly bodies are, has sought to find out what they are. Valuable discoveries have been made concerning Meteors, Shooting Stars, the Constitution of the Sun, the Motion of the Heavenly Bodies, &c. The investi- gations connected with Spectrum Analysis have been especially suggestive. On every hand the facts of the New Astronomy have been accumulating. Until recently, however, they were scattered through many expensive books, and were consequently beyond the reach of the most of our schools. It has been the aim to collect in this little volume the most interest- ing features of the larger works. Believing that Natural Science is full of fascina- tion, the author has sought to weave the story of those far-distant worlds into a form that may attract the attention and kindle the enthusiasm of the pupil. This work is not written for the information of scientific men, but for the inspiration of youth. 20482'? VI PREFACE TO THE FIRST EDITION. Therefore the pages are not burdened with a multi- tude of figures which no memory could retain. Mathematical tables and data, Questions for Re- view, a very valuable Guide to the Constellations, and an Apparatus for Illustrating Precession, are given in the Appendix, where they may be useful for reference. Those persons having a small telescope will find valuable assistance in the '' List of interesting Ob- jects for a common Telescope." The Index contains the pronunciation of many difficult names. Particular attention is called to the method of classifying the measurements of Space, and the practical treatment of the subjects of Parallax, Har- vest Moon, Eclipses, the Seasons, Phases of the Moon, Time, Nebular Hypothesis, Spectrum Analysis, and Precession. To teachers hitherto compelled to use a cumber- some set of charts, it is hoped that the star maps here offered will present a welcome substitute. The geometrical figures, showing the actual appearance of the constellations, will relieve the mind confused with the idea of numberless rivers, serpents, and classical heroes. Only the brightest stars are given, since in practice it is found that pupils remember the general outlines alone, while the details are soon forgotten. Many of the cuts are copied from the French edi- tion of Guillemin's '^ Heavens." Acknowledgment ASTRONOMY. vil for much valuable material is hereby made to this excellent work, and also to "Chambers's Astron- omy," "Newcomb's Astronomy," and Young's "The Sun." Finally, the author commits this little work to the hands of the young, to whose instruction he has con- secrated the energies of his life, in the earnest hope that, loving Nature in all her varied phases, they may come to understand somewhat of the wisdom, power, beneficence, and grandeur displayed in the Divine harmony of the Universe. "One God, one law, one element, And one far-off Divine event To which the whole creation moves. " READING REFERENCES. Chambers's Astronomy. — Young's The Sun.— Ball's Elements of Astronomy.— Newcomb's Popular Astronomy. — Lockyer's Spectrum Analysis. — Proctor's Other Wbrlds than Ours, Saturn, The Moon, Poetry of Astronomy, &c. — Delaunay's Cours D'Astronomie. — Haughton's Manual of Astronomy. — Newcomb and Holden's As- tronomy. — Lockyer's Elements of Astronomy. — Norton's Spherical and Physical Astronomy. — Herschel's Outlines of Astronomy. — Robinson's Astronomy, — Mitch- ell's Popular Astronomy. — Arago's Popular Astronomy. — Airy's Lectures on Astronomy. — Hind's Solar System, and Introduction to Astronomy. — Lockyer's Ele- mentary Lessons in Astronomy.— Proctor's Star Atlas. — Heis's Star Atlas. — Peck's Popular Astronom}-. — Gillet and Rolfe's Astronomy.- Sharpless and Phillips's As- tronomy. — Peabody's Elements of Astronomy. — Schellen's Spectrum Analysis. — Winchell's World-Life (excellent reading in connection with the Nebular Hypothe- sis).— Flammarion's Wonders of the Heavens.— Guillemin's The Heavens, revised by Proctor. — Loomis's Elements of Astronomy. — Proctor's Easy Star Lessons. — Olm- stead's Letters on Astronomy. — Routledge's Historj' of Science. — Buckley's History of Natural Science. — Williamson's Problems on the Globes. — The Popular Science Monthly (1872-1884;. — Rambosson's Histoire Des .Astres. SUGGESTIONS TO TEACHERS. ^l"^HIS work is designed to be recited in the topical method. On hear- ing the title of a paragraph, the pupil should be able to draw upon the blackboard the diagram, and to state the substance of what is con- tained in the book. It will be noticed that the order of topics, in treat- ing of the planets and also of the constellations, is uniform. If, each day, a portion of the class write their topics in full upon the black- board, it will be found a valuable exercise in spelling, punctuation, and composition. Every point which can be illustrated in the heavens should be shown to the class. No description or apparatus can equal the reality in the sky. After a constellation has been traced, the pupil should be practised in star-map drawing. The article on "Celestial Measurements," near the close of the work, should be constantly referred to during the term. In the figures, and especially in the star-maps, it should be remembered that the right-hand side represents the west ; and the left-hand, the east. To obtain this idea correctly, the book should, in general, be held up toward the southern sky. For the purpose of more easily finding the heavenly bodies at any time, Whitall's Movable Planisphere is of great service. It may be procured of the publishers of this work. A tellurian is invaluable in explaining Precession of the Equinoxes, Eclipses, Phases of the Moon, etc. Messrs. A. S. Barnes & Co. , New York City, furnish a good in- strument at a low price. A small telescope, or even ah opera-glass, will be useful. A good star-map, and as many advanced works upon Astronomy as can be secured, should be included in the teacher's outfit. X SUGGESTIONS TO TEACHERS. The pupil should, at the outset, get a distinct idea of the circles and planes of the celestial sphere. The subject of angular measure- ments can easily be made clear in this relation. A circle contains 360° ; 90" reach from horizon to zenith ; 180" produce opposition ; while smaller distances can be shown in the sky (see pp. 216, 228). Never let a pupU recite a lesson, nor answer a question, except it be a mere definition, in the language of the book. The text is designed to interest and instruct the pupil ; the recitation should afford him an opportunity of expressing what he has learned, in his own style and words. Teachers desiring additional information are advised to read " New- comb's Astronomy," Young's "The Sun," Proctor's Works, "Chambers's Astronomy," and Ball's " Elements of Astronomy." TABLE OF CONTENTS PAGE INTRODUCTORY REMARKS 1 I. INTRODUCTION. HISTORY OF ASTRONOMY 5 SPACE 24 The Thkee Systems op Circles 36 The Zodiac 31 II. THE SOLAR SYSTEM 35 THE SUN 36 THE PLANETS 55 Vulcan 71 Mercury 71 Venus 77 The Earth 82 The Seasons 95 Precession and Nutation 104 Refraction, Aberration, and Parai,lax 112 The Moon 122 Eclipses 138 The Tides 147 Mars 150 The Minor Planets 154 Jupiter 157 Saturn 164 Uranus 170 Neptune 172 Xll TABLE OF CONTENTS. PAGE METEORS AND SHOOTING STARS 175 COMETS 185 ZODIACAL LIGHT 196 III. THE SIDEREAL SYSTEM 201 THE STARS 203 THE CONSTELLATIONS 214 Northern Circumpolar Constellations 214 Equatorial Constellations 220 Southern Constellations 238 DOUBLE STARS, COLORED STARS, VARIABLE STARS, CLUSTERS, MAGELLANIC CLOUDS, &C 239 Nebula 246 The INIiLKY Way 253 The Nebular Hypothesis 255 CELESTIAL CHEMISTRY.— Spectrum Analysis 258 TIME 263 CELESTIAL MEASUREMENTS 271 IV. APPENDIX 289 • Tables 291 Questions 293 Guide to the Constellations 313 Apparatus 317 List of Interesting Objects Visible with an Ordinary Telescope 319 Index 323 INTRODUCTORY REMARKS* A STROif OM Y (astron, a star ; nomos, a law) treats of the /-\ Heavenly Bodies — the sun, moon, planets, stars, etc., and, as our globe is a planet, of the earth also. It is, above all others, a science that cultivates the imagination. Yet its theories and distances are based upon rigorous matliemat- ical demonstrations. Thus the study has at once the beauty of poetry and the exactness of Geometry. The great dome of the sky, lilled with glittering stars, is one of the most sublime spectacles in nature. To enjoy this fully, a night must be chosen when the air is clear, and the moon is ab- sent. We then gaze upon a deep blue, an immense expanse studded with stars of varied color and brilliancy. Some shine with a vivid light, perpetually changing and twinkling ; others, more constant, beam tranquilly and softly upon us ; while many just tremble into our sight, like a wave that, struggling to reach some far-off land, dies as it touches the shore. In the presence of such weird and wondrous beauty, the ten- derest sentiments of the heart are aroused. A feeling of awe and reverence, of softened melancholy mingled with a thought of God, comes over us, and awakens the better nature within us. Those far-off lights seem full of meaning to us, could Ave but read their message ; they become real and sentient, and, like the soft Cj es in pictures, look lovingly and inquiringlj'^ upon us. "We come into communion with another life, and the soul asserts its immortality more strongly than ever before. We are humbled as we gaze upon the infinity of suns, and strive to comprehend * This Introduction is designed merely to furnish su{;gestive material for conversa- tion at the first lesson, preparatory to beginning the study. It is not intended for com- mittal. Other topics may he found in the Questions given in the Appendix. 2 INTRODUCTORY REMARKS. their enormous distances, and their magnificent retinue of worlds. The powers of the mind are aroused, and eager questionings crowd upon us. What are those glittering fires 1 WTiat is their distance 1 Are they worlds like our own ? Do living, thinking beings dwell upon them ? Are they promiscuously scattered tlirough space, or is there a system in the universe ? Can we ever hope to fathom those mysterious depths, or are they closed to us forever 1 Some of these problems have been solved ; others yet await the astronomer whose eye shall be keen enough to read the mys- terious scroll of the heavens. Two hundred generations of study have revealed to us such startling facts, that we Avonder how man in liis feebleness can grasp so much, see so far, and penetrate so deeply into the mysteries of the universe. Astronomy has meas- ured the distance of a few stars, and of all the planets ; com- puted the mass, size, days, years, seasons, and many physical features of the planets ; made a map of the moon ; tracked many of the comets in their immense sidereal journeys ; and, at last, analyzed the structure of the sun and stars, and announced the very elements of wliich they are composed. Observing for several evenings those stars which shine with a clear, steady light, we notice that they change their position with respect to the others. They are therefore called planets (literally wanderers). Others remain immovable, and shine with a shift- ing, twinkling light. They are termed the fixed stars, although it is now knoAvn that they also are in motion — the most rapid of any known even to Astronomy — but through such immense orbits that they seem to us to be stationary. Then, too, diag- onally girdling the heavens, is a whitish, vapory belt — the Milhy Wmj. This is composed of multitudes of millions of suns — of which our OAvn sun itself is one — so far removed from us that their light mingles, and makes only a fleecy whiteness. Tliis magnificent panorama of the heavens is before us, inviting our study, and waiting to make known to us the grandest revela- tions of science. I. INTRODUCTION ( 1. Among the Chinese. 2. Among the Chaldeans. (\. Thales. 1 2. Anaximander. 3. Among the Grecians. ■{ 3. Pythagoras. I 4. Auaxagoras y Sizzi is a fair instance. " There are seven windows in the head, through which the air is admitted to the body, to enlighten, to warm, and to nourish it,— two nostrils, two eyes, two ears, and one mouth. So in the heavens there are two favorable stars, Jupiter and Venus ; two unpropitious. Mars and Saturn ; two luminaries, the Sun and Moon ; and Mercury alone, undecided and indifferent. From which, and from many other phe- nomena in Nature, such as the seven metals, etc., we gather that the number of planets is necessarily seven. Moreover, the satellites are invisible to the naked eye, can exercise no influence over the earth, and would be useless, and therefore do not exist, liesides, the week is divided into seven days, which are named from the seven planets. Now, if we increase the number of planets, this whole system falls to the ground." 22 THE HISTORY. Physics. "When a body is set in motion, it will con- tinue to move forever in a straight line, unless another force is applied. As there is no friction in space, the planets do not lose any of their original velocity, but move now with the same speed which they received at the beginning. But this would make them all pass along straight lines, and not cir- cular orbits. What causes the curve ? Obviously, another force. For example : I throw a stone into the air. It does not move in a straight line, but in a curve, because the earth constantly bends it down- ward. Application. — Just so the moon is moving around the earth, not in a straight line, but in a curve. Can it not be that the earth bends it downward, just as it does the stone ? Newton knew that a stone falls toward the earth sixteen feet the first second. He conceived, after a careful study of Kepler's laws, that the attraction of the earth diminishes according to the square of the distance. He supposed (accord- ing to the measurement then received) that a body on the surface of the earth is exactly four thousand miles from the center. He now applied this imag- inary law. Suppose the body is removed four thou- sand miles from the surface of the earth, or eight thousand miles from the center. Then, as it is twice as far from the center, its weight will be diminished 2^, or 4 times. If it were placed 3, 4, 5, 10 times fur- ther away, its weight would then decrease 9, 16, 25, 100 times. If, then, the stone at the surface of the earth (four thousand miles from the center) falls sixteen feet the first second, at eight thousand miles NEWTON. 23 it would fall only four feet ; at 240,000 miles, or the distance of the moon, it would fall only about one.- twentieth of an inch (exactly .053). Next the question arose, "How far does the moon fall toward the earth, i. e., bend from a straight line, every second ? " For sixteen years, with a patience rivaling Kepler's, this philosopher sought to solve the problem. He toiled over interminable columns of figures, to find how much the moon's path curves each second. At last, he reached a result, which was nearly, but not quite, exact. Disappointed, he laid aside his calculations. Repeatedly he reviewed them, but could not find a mistake. At length, while in London, he learned of a new and more accurate measurement of the distance from the cir- cumference to the center of the earth. He hastened home, inserted this new value in his calculations, and soon found that the result would be correct. Overpowered by the thought of the grand truth just before him, his hand faltered, and he called upon a friend to complete the computation. From the moon, Newton passed on to the other heavenly bodies, calculating and testing their orbits. Finally, he turned his attention to the sun, and, by reasoning equally conclusive, proved that the attrac- tion of that great central orb compels all the planets to revolve about it in elliptical orbits, and holds them with an irresistible power in their appointed paths.* * " Do not understand me at all as saying there is no mystery about the planets' mo- tion. There is just one single mystery,— gravitation ; and it is a very profound one. How it is that an atom of matter can attract another atom, no matter how great the distance, no matter what intervening substance there may be ; how it will act upon it, or at least behave as if it acted upon it,— I do not know, I cannot tell. Whether they 24 THE HISTORY. At last, he announced this grand Law of Gravita- tion : Every particle of matter in the universe at- tracts every other particle of matter with a force directly proportional to its quantity of matter, and decreasing as the square of the distance increases. _^ II. SPACE. We now in imagination pass into space, which- stretches out in every direction, without bounds or measures. We look up to the heavens, and try to locate some object among the mazes of the stars. Bewildered, we feel the necessity of some system of measurement. Let us try to understand the one adopted by astronomers. The Celestial Sphere. — The blue arch of the sky, as it appears to be spread over us, is termed the Celestial Sphere. There are two points to be noticed here. First, that so far distant is this imaginary arch from us, that if any two parallel lines from different parts of the earth were drawn to this Sphere, they would apparently intersect. Of course, this could not be the fact ; but the distance is so immense, that we are unable to distinguish the little difference of are puslied together l)y means of an intervening ether, or what is the action, I cannot und(T.stand. It stands with me along with the fact, that, when I will my arm to rise, it rises. It is inscrutable. All the explanations that have been given of it seem to me merely to darken counsel with words and no understanding. They do not remove the difficulty at all. If I were to say what I really believe, it would be, that the motion of the spheres of the material universe stand in some such relation to Him in whom all things exist, the ever-present and omnipotent God, as the motions of my body do to my will : I do not know bow, and never expect to know."— Pro/. Young. SPACE. 25 four or even eight thousand miles, and the two lines would seem to unite : so we must consider this great earth as a mere speck or point at the center of the Celestial Sphere. Second, that we must neglect the entire diameter of the earth's orbit, so that if we should draw two parallel lines, one from each end of the earth's orbit, to the Celestial Sphere, although these lines would be nearly 186,000,000 miles apart, yet they would appear to pierce the Sphere at the same point ; which is to say, that, at that enormous distance, 186,000,000 miles shrink to a point. Consequently, in all parts of the earth, and in every part of the earth's orbit, we see the fixed stars in the same place. This sphere of stars surrounds the earth on every side. In the daytime, we cannot see the stars be- cause of the superior light of the sun ; but, with a telescope, they can be traced, and an astronomer will find certain stars as well at noon as at midnight. One half of the sphere is constantly visible to us ; and so far distant are the stars, that we see just as much of the sphere as we should if the upper part of the earth were removed, and we were to stand four thousand miles further away, or at the center of the earth, where our view would be bounded by a great circle of the earth. On the concave surface of the Celestial Sphere, there are imagined to be drawn three systems of circles : the Horizon, the Equinoctial, and the Ecliptic System. Each of these has (1) its Prin- cipal Circle, (2) its Suhordinate Circles, (3) its Points, and (4) its Measurements. 26 SPACE. Fig. 7. z 1. THE HORIZON SYSTEM. (a) The Principal Circle is the Rational Horizon. This is the great circle whose plane, passing through the center of the earth, separates the visible from the invisible heavens. The Sensible Horizon is the small circle where the earth and the sky seem to meet : it is parallel to the rational horizon, but distant from it the semi-diameter of the earth. No two places have the same sensible horizon : any two, on opposite sides of the earth, have the same rational horizon. (6) The Subordinate Cir- cles are the Prime Verti- cal circle, and the Merid- ian. A vertical circle is one passing through the poles of. the horizon (ze- nith, and nadir). The Prime Vertical is a verti- cal circle passing througl the East and West points The Meridian is a vertical circle passing through the E, center of earth ; Z, zenith ; Z', nadir ; , . . PP', axis of earth; HAH', horizon; S, a Nortll aud SOUtll pOlUtS. star; ZSZ', rertical circle passing through a ■ AS, altitude of star ; ZS, zenith distance (q) The PolUtS are the of star ; B.' A., azimuth of star. ^ Zenith, the Nadir, and the K, S., E., and W. points. The Zenith is the point directly overhead, and the Nadir, the one directly underfoot. They are also the poles of the horizon — i. e., the points where the axis of the horizon pierces the Celestial Sphere. The N., S., E., and W. points are familiar. z' THE HORIZON SYSTEM. 2? (d) The Measurements are Azimuth, Amplitude, Altitude, and Zenith distance. Azimuth is the distance from the meridian, meas- ured east or west, on the horizon, to a vertical circle passing through the object. Amplitude (the complement of Azimuth) is the distance from the Prime Vertical, measured on the horizon, north or south. Altitude is the distance from the horizon, meas- ured on a vertical circle, toward the zenith. Zenith Distance (the complement of Altitude) is the distance from the zenith, measured on a vertical circle, toward the horizon. The Horizon system is one commonly used in observations with Mural Circles, and Transit Instru- ments. 2. THE EQUINOCTIAL SYSTEM. (a) The Principal Circle is the Equinoctial. This is the Celestial Equator^ or the earth's equator ex- tended to the Celestial Sphere. At all places between the equator and the pole, the celestial equator is in- clined to the horizon at an angle equal to the dis- tance of the zenith of the place from the pole. * {h) The Subordinate Circles are the Hour Circles (Right Ascension Meridians), the Colures, and the Declination Parallels. * Tlie latitude of a place is its distance from the equator, and this equals the distance of the zenith of the place from the equinoctial. Hence, having given the latitude of a place, to find the height of the celestial equator above its horizon, subtract the latitude from 90°, and the remainder is the required angular distance. In like manner, the lati- tude subtracted from 90° gives the co-latitude of the place— the complement of the latitude. 28 SPACE. The Hour Circles are thus located. The Equi- noctial is divided into 360°, equal to twenty-four hours of motion — thus making 15 equal to one hour of motion. Through these divisions run twenty-four meridians, each constituting an hour of motion (time) or 15^ of space. The Hour Circles may be conceived as meridians of terrestrial longitude (15" apart) extended to the Celestial Sphere. The Colures are two principal meridians ; the Equinoctial Cohire is the meridian passing through the equinoxes ; the Solstitial Colure is the meridian passing through the solstitial points. The Declination Parallels are small circles parallel to the Equinoctial ; or they may be conceived as the parallels of terrestrial latitude extended to the Celestial Sphere. (c) The Points are the Celestial Poles, and the Equinoxes. The Celestial Poles are the points where the axis of the earth extended pierces the Celestial Sphere, and are the extremities of the celestial axis, as the poles of the earth are the extremities of the earth's axis. The North Pole is marked very nearly by the Xorth Star, and every direction from that is reckoned south, and every direction toward that is reckoned north, however it may conflict with our ideas of the points of the compass. The Equinoxes are the points where the Equinoc- tial and the Ecliptic (the sun's apparent path through the heavens) intersect. (c7) The Measurements are Right Ascension (R. A.), Declination, and Polar Distance, THE EQUINOCTIAL SYSTEM. S9 Right Ascension is distance from the Vernal Equinox, measured on the equinoctial eastward to the meridian which passes through the body. R. A. corresponds to terrestrial longitude, and may extend to 360° East, instead of 180'' as on the earth. R. A. is never measured westward. The starting point is the meridian passing through the vernal equinox, as the meridian passing through Green- wich is the point from which terrestrial longitude is measured. Declination is distance from the equinoctial, measured on any Hour Circle or meridian north or south. It corresponds to terrestrial latitude. Polar Distance (the complement of Declination) is the distance from either Pole, measured on an Hour Circle. The Equinoctial System is largely used by modern astronomers, and accompanies the Equatorial Tele- scope, Sidereal Clock, and Chronographs of the best Observatories. 3. THE ECLIPTIC SYSTEM. (a) The Principal Circle is the Ecliptic. This is the apparent path of the sun in the heavens. It is inclined to the equinoctial 23^° (23° 27' 15", Jan. 1, 1884), which measures the inclination of the Earth's Equator to its orbit, and is called the obliquity of the ecliptic. (See p. 58.) The inclination of the ecliptic to the horizon, unlike that of the equinoctial, varies at different times of the year. The angle that the ecliptic makes with the horizon is greatest when the vernal equinox is 30 SPACE. on the western horizon and the autumnal on the eastern ; it is least when the vernal equinox is on the eastern horizon and the autumnal on the western.* {b) The Subordinate Circles are Circles of Celestial Longitude, and Parallels of Celestial Latitude. The Circles of Celestial, Longitude are now seldom employed. They are measured on the Eclip- tic, as circles of Right Ascension (R. A.) are meas- ured on the Equinoctial. The Parallels of Celestial Latitude are little used. They are small circles drawn parallel to the ecliptic, as parallels of declination are drawn parallel to the equinoctial. (c) The Points are the Foles of the Ecliptic, the Equinoxes, and the Solstices. The Poles of the Ecliptic are the points where the axis of the earth's orbit meets the Celestial Sphere. The Equinoxes are the points where the ecliptic intersects the equinoctial. The place where the sun crosses the equinoctial f in going north, which occurs about the 21st of March, is called the Vernal Equinox. The place where the sun crosses the equinoctial in going south, which occurs about the 21st of Septem- ber, is called the Autumnal Equinox. The Solstices are the two points of the ecliptic most distant from the Equator ; or they may be considered to mark the sun's furthest declination north and south of the equinoctial. The Summer Solstice occurs about the * In the former instance, the angle is eqnal to the co-latitude, plus 23J° (the inclina- tion of the ecliptic to the equinoctial) ; and, in the latter, the co-latitude minus 2;U°. Thus, at the latitude of New York, it varies from 90° — 41* + ■2:i\ ° = 72J " ; to 90° — 41° — 231* = 25J'. In the one case, the summer solstice ia on the meridian of the place, and, in the other, the winter. t " This is commonly called ' crossing the line.' ' THE ECLIPTIC SYSTEM. 81 31st of June ; the Winter Solstice occurs about the 21st of December. (d) The Measurements are Celestial Longitude and Latitude. Celestial Longitude is distance from the Vernal Equinox measured on the ecliptic, eastward. Celestial Latitude is distance from the ecliptic measured on a Subordinate Circle, north or south. THE ZODIAC. A belt of the Celestial Sphere, 8° on each side of the ecliptic, is styled the Zodiac. This is of very- high antiquity, having been in use among the ancient Hindoos and Egyptians. The Zodiac is divided into twelve equal parts — of 30° each — called Signs, to each of which a fanciful name is given. The following are the names of the SIGNS OF THE ZODIAC. Libra a Scorpio Ttj^ Sagittarius f Capricornus . . V5> Aquarius oji Pisoes X Aries HP Taurus y Gemini n Cancer 25 Leo ^ Virgo nj " The first, nn, indicates the horns of the Ram ; the second, « , the head and horns of the Bull ; the barb attached to a sort of letter, "1 , designates the Scor- pion ; the arrow, i , sufficiently points to Sagitta- rius ; V? is formed from the Greek letters, rp, the two first letters of rpdyog, a goat. Finally, a balance, the flowing of water, and two fishes, tied by a string, may be imagined in ^, ^, and X, the signs of Libra, Aquarius, and Pisces." (See pp. 210, 295.) 32 PRACTICAL QUESTIONS. PRACTICAL QUESTIONS. 1. How high is the Xorth Star above your horizon ? 2. What is the sun's right ascension at the autumnal equinox ? At the vernal equinox ? 3. What was the first discovery made by the telescojie ? 4. How high above the horizon of any place are the equinoctial points when they pass the meridian ? 5. Jupiter revolves around the sun in 12 of our years. Assuming the earth's distance from the sun to be 93,000,000 miles, compute Jupiter's dis- tance by applying Kepler's third law. 6. The latitude of Albany is 42° 39' X ; what is the sun's meridian altitude at that place when it is in the celestial equator ? 7. What is the co-latitude of a place ? 8. What is the declination of the zenith of the place in which you reside? 9. Why are the stars generally invisible by day ? 10. Why is the ecliptic so called ? 11. Who fii-st taught that the earth is round ? 12. What is Astrology ? 13. How can we distinguish the fixed stars from the planets ? 14. How long was the Ptolemaic System accepted ? 15. In what respect did the Copernican System differ from the one now received ? 16. For what is Astronomy indebted to Galileo ? To Newton ? 1 7. What is the amount of the obliquity of the ecliptic 1 18. Define Zenith. Xadir. Azimuth. Altitude. Equinoctial. Right Ascension. Declination. Equinox. Ecliptic. Colure. Solstice. Polar distance. Zenith distance. The Zodiac. 19. If the R. A. of the sun be 80", state in what sign he is then located ? 160° ? 280° ? 20. ^^^ly does the angle which the ecliptic makes with the horizon vary ? 21. "\Miy is the angle which the celestial equator makes with the horizon constant ? II. THE SOLAR SYSTEM. ' In them hath He set a tabernacle for the sun, " This world was once a Jluid haze of light, Till toward the center set the starry tides And eddied into sttns, that wheeling cast The planets." — Tennyson. L The Sun , 1. Distance. 2. Ljght (t Heat. 3. Apparent Size, 4. Real Dimen- sions. 5. Solar Spots.. xn CD ot to me seeming was about the bignesse of a shilling, being in 5 degrees of ititude, and still there came a great billow out of the souther board." t 1'' en the sun s surface = 430.3 miles. This spot was 2'47" acrosa {Scktoabt}. THE SUN. 43 Fig. 10. phere of the sun, what an abyss must that be at " the bottom of which our earth could lie like a boulder in the crater of a volcano ! " Spots Consist of Distinct Paets. — From the ac- companying repre- sentation, it will be seen that the spots generally consist of one or more dark portions called the umbra, and around that a grayish por- tion styled the pe- numbra {pene, al- most, and umbray black). Sometimes, however, umbrae appear without a penumbra,andvice versa. The umbra itself has generally a dense black center, called the nucleus. Besides this, the umbra is sometimes divided by luminous bridges. Spots are in Motion. — The spots change from day to day ; but all have a common movement. About fourteen days are required for a spot to pass across the disk of the sun from the eastern side, or limb, to the western ; in fourteen days, it reappears, changed in form perhaps, but generally recognizable. Spots apparently Change their Speed and Form AS THEY PASS ACROSS THE DiSK. — A Spot is SCCU OU the eastern limb ; day by day it progresses, with a gradually-increasing rapidity, until it reaches the &un-Spots. 44 THE SOLAR SYSTEM. center ; it then slowly loses its rapidity, and finally disappears on the western limb. The diagram illus- trates the apparent change which takes place in the form. Suppose at first the spot is of an oval shape ; as it approaches the center it apparently widens and becomes circular. Having passed that point, it be- comes more and more oval until it disappears. Fig. 11. Change in Spots as they Cross the Disk. This change in the Spots proves the Sun's Rota- tion ON ITS Axis. — These changes can be accounted for only on the supposition that the sun rotates on its axis : indeed, they are the precise effects which the laws of perspective demand in that case. About twenty-seven days elapse from the appearance of a spot on the eastern limb before it is seen a second time. During this period the earth has gone forward in its orbit, so that the location of the observer is changed ; allowing for this, the sun's time of rotation at the equator is about twenty-five days (25 d., 8 h., 10 m. : Langier), THE StTN. 45 Curiously enough, the equatorial regions move more rapidly, and complete a rotation in less time, than the rest of the siin. While a spot near the equator performs a rotation in twenty-seven days, one situated half- way to either pole, requires nearly twenty -eight days. Synodic and side- real REVOLUTIONS OF THE SPOTS. — We can easily under- stand why we make an allowance for the motion of the earth in its orbit. Suppose a solar spot at a, on a line pass- ing from the center of the earth to the center of the sun. For the spot to pass around the sun and come into that same Synodic and Sidereal Revolutions. position again, requires about twenty-seven days. But, during this time, the earth has passed on from T to T'. The spot has not only traveled around to a again, but also beyond that to a', or the distance from a to a' more than an entire revolution. To do this, requires about two days. A revolution from a around to a' is called a synodic, and one from a around to a again is called a sidereal, revolution. 46 the solar system. Spots do not always move in straight lines. — Sometimes their path curves toward the north, and Fig. IS. March. June. September. sometimes toward the south, as in the figure. This can be explained only on the supposition that the sun's axis is inclined to the ecliptic (7"" 15'). Spots have a motion of their own. — Besides the motion already named as assigned to the sun's rota- tion, nearly every spot seems to have an individual motion. Some spots circle about in small elliptical paths, often quite regularly for weeks and even months. Immense cyclones occasionally pass over the surface with fearful rapidity, producing rotation and sudden changes in the spots. At other times, however, the spots seem "to set sail and move across the disk of the sun like gondolas over a silver sea." Spots change their real form. — Spots break out and then disappear under the eye of the astronomer. WoUaston saw one that seemed to be shattered like a fragment of ice when it is thrown on a frozen surface, breaking into pieces, and sliding off in every direction. Sometimes one divides itself into several nuclei, while again several nuclei combine THE SUN. 47 into a single nucleus. Occasionally a spot will re- main for six or eight rotations, while often it will last scarcely half an hour. Sir W. Herschel relates Fig. lU. Solar Cyclone, May 5th, 1857. (Secchi.) that, when examining a spot through his telescope, he turned away for a moment, and on looking back it was gone. Appearance of the spots is periodical.* — It is a remarkable fact that the number of spots increases and diminishes through a regular interval of about 11.11 years. These periodic variations are closely connected with similar variations in the aurora and magnetic earth-currents which interfere with the telegraph. Are the spots influenced by the planets ? — * The regular increase and diminution in the number of the spots was discovered by Schwabe of Prussia, who watched the sun so carefully that it is said "for thirty years the sun never appeared above the horizon without being confronted by his imperturbable telescope." 48 The solar system. Many astronomers of high standing believe that the solar spots are especially sensitive to the approach of Mercury and Venus, on account of their nearness, and of Jupiter, because of its size ; that the area of the spots exposed to view from the earth is uniformly greatest when any two of the larger planets come into line with the sun ; and that when both Venus and Jupiter are on the side of the sun opposite to us, the spots are much larger than when Venus alone is in that position. Most authorities, however, doubt the accuracy of these observations, and deny this planetary influence altogether. Spots do not influence fruitfulness of the SEASON. — Herschel first advanced the idea that years of abundant spots would be years also of plentiful harvest. This is not now generally received. What two years could be more dissimilar than 1859 and 1860 ? Both abounded in solar spots, yet, in Europe, one was a fruitful year and the other one of almost famine. Whether the spots influence the weather is still a mooted question. Spots are cooler than the surrounding sur- face. — It seems that the breaking out of a spot sen- sibly diminishes the temperature of that portion of the sun's disk. The faculae, on the other hand, do not increase the temperature {Secchi). Spots are depressions. — Careful observations show that, in general, the " floor," so to speak, of the umbra is sunk from two to six thousand miles below the level of the luminous surface {Young). Comparative brightness of spots and sun. — If we represent the ordinary brightness of the 49 Photograxihic View of Spots and Faculte. sun by 1,000, then that of the penumbra would be about 800, and that of the umbra, 540 (Lang- ley). There may be much light and heat radiated by^a spot, which seems black as compared with the sun ; for we remember that even a calcium light, held between our eyes and the sun, appears as a black spot on the disk of that luminary. Appearance of the sun's surface, — Even a telescope of moderate power will show the ?5urface of the sun to have a peculiar mottled appear' 3 Fig. 16. '-"'"' '^^"" ■S:-tS3^::rj I ^ ^ 1 '^^ g P ' ''k FaculcB. 50 THE SOLAR SYSTEM. ance not unlike that of an orange skin. But, under favorable circumstances and with a telescope of high power, the solar disk is found to be covered with small, intensely bright bodies irregularly distributed. Fig. 17. WiOov-Leaf. These are now known as rice-grains.'*' They are often apparently crowded together in luminous ridges, or streaks, termed faculce {facula, a torch) ; while the rice-grains themselves, according to Prof. Langley, are composed of granules. Minute as a * Various obsen-ers describe the solar surface differently. A peculiar, elongated, leaf-shaped appearance of the rice-grains, called the willow-leaf structure, is shown in Fig. 17, as seen by Nasmyth. Newcomb compares the sun's appearance to that of a plate of rice-soup. Young says it frequently resembles bits of straw lying parallel to one another— the '• thatched-straw formation," Typical Sun-spot, uj Dec. 1S73, showing the filaments pointing to the center. THE SUN. 61 granule seems, probably the smallest has a diameter of, at least, 100 miles. Physical Constitution of the Sun.* — Of the consti- tution of the sun, and the cause of the solar spots, very little is definitely known. Wilson's Theory supposed that the sun is com- posed of a solid, dark globe, surrounded by three atmospheres. The first, nearest the black body of the sun, is a dense, cloudy covering, possessing high reflecting power. The second is called the photo- sphere. It consists of an incandescent gas, and is the seat of the light and heat of the sun, being the sun that we see. The third, or * outer one, is trans- parent — very like our atmosphere. According to this theory, the spots are to be ex- plained in the following manner. They are simply openings in these atmospheres made by powerful upward currents. At the bottom of these chasms, we see the dark sun as a nucleus at the center, and around this the cloudy atmosphere — the penumhra. This explains a black spot with its penumbra. Some- times the opening in the photosphere may be smaller than that in the inner or cloudy atmosphere ; in that case there will be a black spot without a penumbra. It will be natural to suppose that when the heated gas of the photosphere, or second atmosphere, is violently rent asunder by an eruption or current from below, luminous ridges will be formed by the heaped-up gas on every side of the opening. This would account for the faculce surrounding the sun- * For the views of various authorities on the constitution of the sun, solar spots, etc., see Newcorab's Astronomy, third edition, p. 271, 52 THE SOLAR SYSTEM. spots. It will be natural, also, to suppose that some- times the cloudy atmosphere below will close up first over the dark surface of the sun, leaving only an opening through the photosphere, disclosing at the bottom a grayish surface of penumbra. We can Fig. 19. WiUon's Theory. readily see, also, how, as the sun revolving on its axis brings a spot nearer and nearer to the center, thus giving us a more direct view of the opening, we can see more and more of the dark body. Then as it passes by the center the nucleus will disappear, THE SUN. 53 until finally we can see only the side of the fissure, the penumbra, which, in its turn, will vanish. The Present Theory* is deduced from the re- sults of Spectrum Analysis, of which we shall here- after speak. It is constantly being modified by new discoveries. But we may, in general, believe the sun to be a vast, fiery body, surrounded by an atmosphere of substances volatilized by the intense heat. Among these, we recognize familiar elements, as iron, copper, &c. The different portions of the sun are thought to be arranged thus : (1). The nucleus, probably gaseous ; \ (2). The photosphere, an envelope several thousand miles thick, which constitutes the visible part of the sun ; (3). The chromosphere, composed of luminous gas, mostly hydrogen, and the seat of enormous pro- tuberances, tongues of fire, which dart forth, some- times at the rate of 150 miles per second, and to a distance of over 100,000 miles ; (^). The corona,X an outer appendage of faint, pearly light, consisting of streamers reaching out often several hundred thou- sand miles. Of these solar constituents, the eye and the telescope ordinarily reveal only the photosphere ; the rest are seen during a total eclipse or by means of the spectroscope. The outer portion of the sun radiates its heat and * As Kirchhoff, by his discoveries in Spectrum Analysis, laid the foundation of this theory, it is often called after him. t The interior of the sun, if gaseous, must be powerfully condensed, because of the tremendous pressure of the atmosphere. The high temperature, however, prevents the gas from liquefying. Tlie rain-.storms on the sun, if such ever occur, consist of drops of molten iron, copper, zinc, &c., vaporized by the enormous heat ; and often a tempest would drive before it this white-hot, metallic blast, with a speed of 100 miles per second. t This is so called because, during a total eclipse, it forms around the moon a corona, or glory, that is the most wonderful feature of this rare event. (See p. 141.) 54 THE SOLAR SYSTEM. light, and, becoming cooler, sinks ; the hotter matter in the interior then risQS to take its place, and thus convection currents are established (Physics, p. 193). The cooler, descending currents are darker, and the hotter, ascending ones are lighter ; this gives rise to the mottled look of the sun. At times, this occurs on a grand scale, and the heated, up-rushing masses form the faculse, and the cooler, down-rushing ones produce the solar spots. The Heat of the Sun is generally considered to be produced by condensation, whereby the size of the sun is constantly decreasing, and its potential energy thus converted into kinetic. The dynamic theory accounts for the heat and the solar spots by assum- ing that there are vast numbers of meteors revolving around the sun, and that these constantly rain down upon the surface of that luminary. * Their motion, thus stopped, is changed to heat, and feeds this great central fire. Were Mercury to strike the sun in this way, it would generate sufficient heat to compensate the loss by radiation for seven years. Doubtless, the solar heat is gradually diminishing, and will ultimately be exhausted. In time, the sun will cease to shine, as the earth did long since. New- comb says that in 5,000,000 years, at the present rate, the sun will have shrunk to half its present size, and that it cannot sustain life on the earth more than 10,000,000 years longer. Of this we may be assured, there is enough to support life on our globe for millions of years yet to come. * Tlic heat of the sun could be maintained by an annual contraction of 220 feet in its diameter, a decrease so insignificant as to be imperceptible with the best instru- ments ; or by the annual impact of meteors equal in amount to y the mass of Mercury. THE PLANETS. 55 II.— THE PLANETS. INTRODUCTION. The Planets will be described in regular order, passing outward from the sun. In this journey, we shall examine each planet in turn, noticing its dis- tance, size, length of year, duration of day and night, temperature, climate, number of moons, and other interesting facts, showing how much we can know of its world-life in spite of its wonderful dis- tance. We shall encounter the earth in our imag- inary wanderings through space, and shall explain many celestial phenomena already partially familiar to us. In all these worlds, we shall find traces of the same Divine hand, molding and directing in con- formity to one universal plan. We shall discover that the laws of light and heat are invariable, and that the force of gravity, which causes a stone to fall to the ground, acts similarly upon the most distant planet. Even the elements of which the planets are composed will be familiar to us, so that a book of natural science published here might, in its general features, answer for use in a school on Mars or Jupiter. Common Characteristics (Hind). — 1. The planets move in the same direction around the sun ; their 56 THE SOLAR SYSTEM. course, as viewed from the north side of the ecliptic, being contrary to the motion of the hands of a watch. 2. The}^ describe elliptical paths around the sun, — not differing much from circles. 3. Their orbits are more or less inclined to the ecliptic, and intersect it in two points — the nodes, — one-half of the orbit lying north, and the other south of the earth's path. ■i. They are opaque bodies, and shine by reflecting the light they receive from the sun. 5. They rotate upon their axes in the same way as the earth. Tliis we know by telescopic observa- tion to be the case with many planets, and by anal- ogy the rule may be extended to all. Hence, they have the alternation of day and night. 6. Agreeably to the principles of gravitation, their velocity is greatest at that part of their orbit nearest the sun, and least at that part most distant from it ; in other words, they move quickest in perihelion, and slowest in aphelion. Comparison cf the two G-roups of the Major Planets. [Chambers.) — Separating the major planets into two groups, if we take Mercury, Venus, the Earth, and Mars as belonging to the interior, and Jupiter, Saturn, Uranus, and Neptune to the exterior group, we shall find that they differ in the following respects : 1. The interior planets, with the exception of the Earth and Mars, are not attended by any satellite, while all the exterior planets have satellites. 2. The average density of the first group consider- i'HE PLANETS. 57 ably exceeds that of the second, the approximate ratio being 5:1. 3. The mean duration of the axial rotations, or the mean length of the day of the interior planets, is much longer than that of the exterior ; the average in the former case being about twenty-four hours, but in the latter only about ten hours. Properties of the Ellipse. — In Fig. 20, S and S' are the foci of the ellipse ; A C is the major axis ; B D, the minor or conjugate axis; O, the center: or, astronomically, O A is the semi-axis-major or mean An Ellipse. distance, O B the semi-axis-minor : the ratio of O S to O A is the eccentricity ; the least distance, S A, is the perilielion distance ; the greatest distance, S C, the aphelion distance. Characteristics of a Planetary Orbit. — It will not be difficult to follow in the mind the additional characteristics of a planet's orbit. Take two hoops, and bind them into an oval shape. Incline one 58 THE SOLAR SYSTEM. slightly to the other, as shown in Fig. 21. Let the horizontal hoop represent the ecliptic. Imagine a planet following the inclined hoop, or ellipse ; at a certain point it rises above the level of the ecliptic : * this point is called the ascending node, and the op- Fig. SI. Planetary Orbits. posite point of intersection is termed the descending node. A line connecting the two nodes is the line of the nodes. The longitude of the node is its distance from the first point of Aries, measured on the eclip- tic, eastward. Comparative Size of Planets (Chambers). — The following scheme will assist in obtaining some notion of the magnitude of the planetary system. Choose a level field or common ; on it place a globe two feet in diameter for the Snn : Vulcan will then be represented by a small pin's head, at a distance of about twenty-seven feet from the center of the ideal sun ; Mercury by a mustard-seed, at a distance of eighty-two feet ; Venus by a pea, at a distance of 142 feet ; the Earth, also, by a pea, at a distance of 215 feet ; Mars by a small pepper-corn, at a distance of 327 feet ; the minor planets by grains of sand, at distances varying from 500 to 600 feet. If space wiU permit, we may place a moderate-sized * Lockyer beautifully says : " We may imagine the earth floating aroiind the sun on a boundless ocean, both sun and earth being half immersed in it. This level, this plane, the plane of the ecliptic (because all eclipses occur in it), is used by astronomers as we use the sea-level. We say a mountain is so far above the level of the sea. The astrono- mer says the star is so high above the level of the ecliptic THE PLANETS. 59 orange nearly one-quarter of a mile distant from the starting point to rep- resent Jupiter ; a small orange two-fifths of a mUe for Saturn ; a full-sized cherry three-quarters of a mile distant for Uranus ; and lastly, a plum 1| miles off for Neptune, the most distant planet yet known. Extending Fig, ss. Comparative Size of the rUinets. this scheme, we should find that the aphelion distance of Encke's comet would be at 880 feet ; the ajjhelion distance of Donati's comet of 1858 at six miles ; and the nearest fixed star at 7,500 miles. 60 THE SOLAR SYSTEM. According to this scale, the daily motion of Vulcan in its orbit would be 4| feet; of Mercury, 3 feet ; of Venus, 2 feet ; of the Earth, 1| feet ; of Mars, 1^ feet ; of Jupiter, 10| inches ; of Saturn, 7^ inches ; of Uranus, 5 inches ; and of Neptune, 4 inches. This illustrates the fact that the orbital velocity of a planet decreases as its distance from the sun increases. * Conjunction of Planets. — The grouping together of two or more planets within a limited area of the heavens is a rare event. The earliest record we have is the one of Chinese origin (p. G), stating that a conjunction of Mars, Jupiter, Saturn, and Mercury Fig. 23. ■r in Conjunction, January SO, ISOi. occurred in the reign of the Emperor Chuenhio. Astronomers tell us that this took place Feb. 28, 244G B. c, between 10° and 18^ of Pisces. There is a very general impression, however, that this conjunction was afterward calculated and chronicled in their records. In 1725, Venus, Mercury, Jupiter, and ♦ If we accept the Xelmlar Hj-pothesis (p. 255), we can easily understand the reason of this; the exterior planets, being made earlier, had the motion of the nebula during its earlier stage. The rotation-velocity of the nebula kept increasing, and so, of course, each planet Dossessed a higher rate of orbital speed than the preceding one- tHE PLANETS. 61 Mars appeared in the same field of the telescope. In 1859, Venus and Jupiter came so near each other that they appeared to the naked eye as one object. Are the Planets Inhabited? — This question is one which very naturally arises, when we think of the planets as worlds in so many respects similar to our own. We can give no satisfactory answer. Many think that the only object God can have in making a world is to form an abode for man. Our own earth was evidently fitted up, although perhaps not cre- ated, for this express purpose. Everywhere about us we find proofs of special forethought and adaptation. Coal and oil in the earth for fuel and light, forests for timber, metals in the mountains for machinery, rivers for navigation, and level plains for corn. The human body, the air, light, and heat are all fitted to one another with exquisite nicety. When we turn to the planets, we do not know but God has other races of intelligent beings who inhabit them, or even entirely different ends to attain. Of this, however, we are assured, that, if inhabited, the conditions on which life is supported vary much from those familiar to us. When we come to speak of the different planets, we shall see (1) how they differ in light and heat, from seven times our usual temperature to less than -^^^^ ; (2) in the intensity of the force of gravity, from 2^ times that of the earth to less than | ; (3) in the constitution of the planet itself, from a density ^ heavier than that of the earth to one nearly that of cork. The temperature may often sweep downward 6^ THE SOLAR SYSTEM. through a scale of 2,000° in passing from Mercury to Neptune. No human being could reside on the former, while we cannot conceive of any polar inhab- Fig. %. Size of tht' .S(t/t ay seen frtrni tke V in nets itaat who could endure the intense cold of the latter. At the sun, one of our pounds would weigh over 27 pounds ; on our moon, the pound weight would be- fHE PLANETS. 63 Come only about two ounces ; while on Vesta, one of the planetoids, a man could easily spring sixty feet in the air and sustain no shock in falling. Yet, while we speak of these peculiarities, we do not know what modification of the atmosphere or physical features may exist on Mercury to temper the heat, or on Neptune to reduce the cold. With all these diversities, we must, however, admit the power of an all-wise Creator to form beings adapted to the life and the land, however different from our own. The Power that prepared a world for us, could as easily and perfectly prepare one for other races. May it not be that the same love of diversity, that will not make two leaves after the same pattern nor two pebbles of the same size, de- lights in worlds peopled by races as diverse ? * While, then, we cannot affirm that the planets are inhabited, analogy would lead us to think that they are, and that the most distant star that shines in the arch of heaven may give light and heat to living beings under the care and government of Him who enlivens the densest forest with the hum of insects, and populates even a drop of water with its teeming- millions of animalcules. Divisions of the Planets. — The planets are divided into two classes : (1). Inferior, or those whose orbits are within that of the earth — viz., Mercurj^, Venus ; (2). Superior, or those whose orbits are beyond that * Astronomers conceive the universe to contain worlds in every possible stage of development, from the primary, gaseous nebula, to a worn-out, dead globe, like the moon. At a certain period in its existence, each world may be fitted to support life. Millions may now be in that condition ; others may be approaching, while others liave passed it. 64 THE SOLAR SYSTEM. of the earth — viz., Mars, Jupiter, Saturn, Uranus, Neptune. Motions of a Planet as seen from the Sun. — Could we stand at the sun and watch the movements of the planets, they would all be seen revolving with dif- ferent velocities in the order of the zodiacal signs. But to us, standing on one of the planets, itself in motion, the effect is changed. To an observer at the sun all the motions would be real, while to us many are only apparent. The position of a planet, as seen from the center of the sun, is called its heliocentric place ; as seen from the center of the earth, its geo- centric place. When Venus is at inferior conjunc- tion, an observer at the sun would see it in the oppo- site part of the heavens from that in which it would appear to him if viewed from the earth. Motions of an Inferior Planet — An inferior planet is never seen by us in any part of the sky opposite to the sun at the time of observation. It cannot recede from him as much as 90°, or \ the circum- ference, since it moves in an orbit entirely enclosed by the orbit of the earth. Twice in every revolution it is in conjunction ( 6 ) with the sun, — an inferior conjunction (A) wlien it comes between the earth and the sun, and a superior conjunction (B) when the sun lies between it and the earth. When the planet attains its greatest distance east or west (as we see it) from the sun, it is said to be at its greatest elongation. When passing from B to A it is east of the sun, and from A to B it is west of the sun. When east of the sun, it sets later than the sun, and hence is THE PLANETS. 65 evening star : when west of the sun, it rises earlier than the sun, and hence is morning star. An inferior ■planet is never visible when in superior conjunction, as its light is then lost in the greater brilliancy of the sun. When in inferior conjunction, it some- Conjunctions of Inferior Planet. times passes in front of the sun, and appears to us as a round, black spot swiftly moving across his disk. This is called a transit. Retrograde Motion of an Inferior Planet. — Suppose the earth at A (Fig. 26), and the planet at B. 66 THE SOLAR SYSTEM. Now, while the earth is passing to F, the planet will pass to D, — the arc AF being shorter than BD, be- cause the nearer a planet is to the sun the greater its velocity. While the planet is at B, we locate it at C on the ecliptic, in Gemini ; but at D, it appears to us to be at G, in Taurus. So that the planet has Betroffrade Motion. retrograded through an entire sign on the ecliptic, while its course all the while has been directly for- ward in the order of the signs ; and to an observer at the sun, such would have been its motion. Phases of an Inferior Planet. — An inferior planet presents all the phases of the moon. At supe- rior conjunction, the whole illumined disk is turned toward us ; but the planet is lost in the sun's rays : THE PLANETS. 67 therefore neither Mercury nor Venus ever presents a complete circular appearance, like the full moon. A little before or after superior conjunction, an inferior planet may be seen with a telescope ; but the whole of the light side is not turned, toward us, and so the planet appears gihhous, like the moon between the first quarter and full. At its greatest elongation, the planet shows us only one-half its illumined disk ; this decreases, becoming more and more crescent toward inferior conjunction, at which time the un- illumined side is toward us. Fig. 27. Phases of an Inferior Planet. Motions of a Superior Planet.— The superior planet moves in an orbit which entirely surrounds that of the earth. When the earth is at E (Fig. 28), the planet at L is said to be in opposition to the sun ( S ). It is then at its greatest distance from him— 180°. The planet is on the meridian at midnight, while the sun is on the corresponding meridian on the opposite side of the earth ; or the planet may be rising, when the sun is just setting. When the planet is at N, it is in 68 THE SOLAR SYSTEM. conjunction, and being lost in the sun's rays is invis- ible to us. When 90^ east or west of the sun, the planet is said to be in quadrature (□). Retrograde Motion of a Superior Planet. — Sup- pose the earth to be at E and the planet at L, and that we move on to G while the planet passes on to Retrogrnde Motion ofaSujKrior Planet. O— the distance EG being longer than LO, the re- verse of what takes place in the movements of the inferior planets ; at E, we should locate the planet at P on the ecliptic, in the sign Cancer ; but at G, it would appear to us at Q, in the sign Gemini, having THE PLANETS. 69 apparently retrograded on the ecliptic the distance PQ, while it was all the time moving on in the direct order of the signs. Now, suppose the earth passes on to I and the planet to U, we should then see it at the point W, further on in the ecliptic than Q, which indicates direct motion again, and at some point near Q the planet must have appeared without motion. After this, it will continue direct until the earth has completed a large portion of her orbit, as we can easily see by imagining various positions of the earth and planet, and then drawing lines as we have just done, noticing whether they indicate direct or retro- grade motion. The greater the distance of a planet the less it will retrograde, as we can perceive by drawing another orbit outside the one represented in the cut, and making the same suppositions concern- ing it as those we have already explained. Sidereal and Synodic Revolution, — The interval of time required by a planet to perform a revolution from one fixed star back to it again, is termed a sidereal revolution {sidus, a star). 1, The interval of time between two similar con- junctions of an inferior planet with the earth and the sun is termed a synodic revolution. Were the earth at rest, there would be no difference between a sidereal and a synodic revolution, and the planet would come into conjunction twice in each revolu- tion. Since, however, the earth is in motion, it fol- lows that, after the planet has completed its sidereal revolution, it must overtake the earth before they can both come again into the same position with 70 THE SOLAR SYSTEM. regard to the sun. The faster a planet moves, the sooner it can do this. Mercury, traveling at a greater speed and on an inner orbit, accomplishes it much more quickly than Venus. The synodic period always exceeds the sidereal, 2. The interval between two successive conjunc- tions or oppositions of a superior planet is also termed a synodic j^e volution. Since the earth moves so much faster than anj' superior planet, it fol- lows that, after it has completed a sidereal revo- lution, it must overtake the planet before they can again come into the same position with regard to the sun. The slower the planet, the sooner this can be done. Uranus, making a sidereal revolution in eighty-four years, can be overtaken more quickly than Mars, which makes one in less than two years. It consequently requires over a second revolution for the earth to catch up with Mars, only ^ of a second one to overtake Jupiter, and but little over ^hi of a second one to come up with Uranus. Planets as Evening and Morning Stars. — The in- ferior planets are evening stars from superior to inferior conjunction : and the superior planets, from opposition to conjunction. During the other part of their revolutions, they are morning stars. Mercury is evening st Venus Mars Jupiter Saturn FraoQUS iir. , about 2 months. H 13 6 THE PLANETS. 71 I. VULCAN (hypothetical). Supposed. Discovery. — Le Vcrrier, having detected an error in tho assumed motion of Mercury, suggested, in the autumn of 1859, that there might be an interior planet, which was the cause of this disturbance. On this being made public, M. Lescarbault, a French physician and au amateur astronomer, stated that on Mai-ch 26 of that year lie had seen a dark body pass across the sun's disk, which might have been the unknown planet. Le Verrier visited him, and found his instruments rough and home-made, but singularly accurate. His clock was a .simple pendulum, consisting of an ivory ball hanging from a nail by a silk thread. His observations were on prescription paper, covered with grease and laudanum. His calculations were chalked on a board, which he planed olf to make room for fresh ones. Le Verrier became satisfied that a new planet had been discovered by this enthusiastic observer, and congratulated liim upon his deserved success. On March 20, 1862, Mr. Lummis, of Manchester, England, noticed a rapidly-moving, dark spot, apparently the transit of an inner planet. During the total eclipse of July 29, 1878, Professor Watson, of Ann Arlior Observatory, and Dr. Lewis Swift, of Rochester, claimed to liave seen two Intra-Mercurial planets. As yet, however, the existence of the planet is not generally conceded. The name Vulcan and the sign of a hammer have been given to it. Its distance from the .sun has been estimated at 13,000,000 miles, and its periodic time (its year) at twenty days. II. MERCURY. The fleetest of the gods. Sign, s , his wand. Description. — Mercury is nearest to the sun of any of the definitely-known planets. When the sky is very clear, we maj sometimes see it, just after sun- set, as a bright, sparkling star, near the western jiorizpn. Its elevation increase's evening hj evening, 72 THE SOLAR SYSTEM. but never exceeds 28°.* If we watch it closely, we shall find that the planet again approaches the sun and becomes lost in his rays. Some days after- ward, just before sunrise, we can see the same planet in the east, rising higher each morning, until its greatest elevation equals that which it before at- tained in the west. Thus the planet appears slowly but steadily to oscillate like a pendulum, to and fro, from one side to the other of the sun. The ancients, deceived by this puzzling movement, failed to dis- cover the identity of the two stars, and called the morning star Apollo, the god of day, and the evening star Mercury, the god of thieves, who walk to and fro in the night-time seeking plunder, f On account of the nearness of Mercury to the sun, it is difficult to be detected. | It is said that Coper- nicus, an old man of seventy, lamented in his last moments that, much as he had tried, he had never been able to see it. In our latitude and climate, we. can generally easily find it if we watch for it at the time of its greatest elongation, as commonly given in the almanac. Motion in Space. — Mercury revolves around the sun at a mean distance of about 30,000,000 miles. Its * This distance varies much, owing to the eccentricity of Mercury's orbit. t The Greeks gave to Mercury the additional name of "The Sparkling One." The astrologists looked upon it as the malignant jilanet. The chemists, because of its extreme swiftness, applied the name to quicksilver. The most ancient account that we have of this planet is given by Ptolemy, in his Almagest ; he states its location on the 15th of November, 205 b. c. The Chinese also state that on June 9, llS a. d., it was near the Beehive, a cluster of stars in Cancer. Astronomers tell us tliat, according to the best calculations, it was at that date witliin less than 1° of that group. t An old English writer by the name of Goad, in 1680, humorously termed this planet, " A squinting lacquey of the sun, who seldom shows his head in these parts, as if he were in debt" Mercury. 73 orbit is the most eccentric (flattened) of any among the eight principal planets, so that, although when in perihelion it approaches to within about 28,000,000 miles, in aphelion it speeds away 15,000,000 miles further, or to the distance of over 43,000,000 miles. Being so near the sun, its motion in its orbit is cor- respondingly rapid, — viz., thirty miles per second.* The Mercurial year comprises only about eighty- eight days, or nearly three of our months. Mercury is thought to rotate upon its axis in about the same time as the earth, so that the length of the Mercurial day is nearly the same as that of the terrestrial one. Though Mercury thus completes a sidereal revolu- tion around the sun in eighty-eight days, yet to pass from one inferior or superior conjunction to the next (a synodic revolution) requires IIG days. The reason of this is, that when Mercury comes around again to the point of its last conjunction, the earth has gone forward, and it requires twenty-eight days for the planet to overtake us. The Distance from the Earth varies still more than the distance from the sun. At inferior con- junction, Mercury is between the earth and the sun, and its distance from us is the difference between the distance of the earth and of the planet from the sun : at superior conjunction, it is the stun of these distances. Its apparent diameter in these different positions varies in the same proportion as the dis- tance, or nearly three to one. The greatest and least ♦ At this rate of speed, we could cross the Atlautic Ocean in two minutes. 74 THE SOLAR SYSTEM. distances vary as either planet happens to be in aphelion or perihelion.* Dimensions. — Mercury is about 3,000 miles in di- ameter. Its volume is about ^V that of the earth — 1. e., it would require twenty globes as large as Mer- cury to make one the size of the earth, or 25,000,000 to equal the sun. It is \ denser than the earth, its mass is nearly ^V that of the earth, and a stone let drop upon its surface would fall 7^ feet the first second. Its specific gravity is not far from that of tin. A pound weight removed to Mercury would weigh only about seven ounces. Seasons. — As Mercury's axis is much inclined from a perpendicular (perhaps 70°), its seasons are peculiar. There are no distinct frigid zones ; but large regions near the poles have six weeks of continuous day and torrid heat, alternating with a night of equal length and arctic cold. The sun shines perpendicularly upon the torrid zone only at the equinoxes, while he sinks far toward the southern horizon at one solstice, and as far toward the northern horizon at the other, t The equatorial regions, therefore, during each revolution, are modified in their temperature from torrid to temperate, and the tropical heat is experienced alternately toward the north and the south of what we call the temperate zones. There is no marked distinction of zones as with us, but each zone changes its character twice during the * If at inferior conjunction Mercury is in aphelion and the earth in perihelion, its distance from us is only 91,500,000 — 43,000,000 = 48,500,000 miles. If at superior con- junction Mercury is in ai>helion and the earth in aphelion also, its distance from us is 94,500,000 + 4:i,000,000= 137,500,000 miles. t Read a chapter entitled " The Fiery World," in Proctor's Poetry of Astronomy. MERCURY. 75 Mercurial year, or eight times during the terrestrial one. An inhabitant of Mercury must be accustomed to sudden and violent vicissitudes of temperature. At one time, the sun not only thus pours down its vertical rays, and in a few weeks after sinks far toward the horizon, but, on account of Mercury's Fig. 29. Orhit and Seasons of Mermiry. elliptical orbit, when in perihelion the planet ap- proaches so near the sun that the heat and light are ten times as great as ours, while in aphelion it re- cedes so as to reduce the amount to four and a half times. The average heat is about seven times that of the earth, — a temperature sufficient to turn water into steam, and even to melt zinc. 76 THE SOLAR SYSTEM. The relative length of the days and nights is much more variable than with us. The sun, apparently seven times as large as it seems to us, must be a magnificent spectacle, and illumine every object with insufferable brilliancy. The evening sky is, however, lighted by no moon. Telescopic Features. — Through the telescope, Mer- cury presents all the phases of the moon, from a slen- der crescent to gibbous, after which its light is lost in that of the sun. These phases prove that Mercury is spherical, and shines by the light reflected from the sun. Being an inferior planet, we never see it when full, and hence the brightest, nor when nearest the earth, as then its dark side is turned toward us. t)wing to the dazzling light, and the vapors almost always hanging around our horizon, this planet has not of late received much attention ; the data here given are mainly based upon the observations of the older astronomers, and are, therefore, not universally accepted. Mercury is thought by some to have a dense, cloudy atmosphere, that materially dimin- ishes the intensity of its heat and, perhaps, makes it habitable, though others assert that the atmos- phere is too insignificant to be detected. Some dark bands about the planet's equator indicate, perhaps, an equatorial zone. There are, also, lofty heights which intercept the light of the sun. and deep valleys plunged in shade. One mountain is claimed to be over eleven miles high, or about ^ the diameter of the planet.* ♦ Tlie height of the loftiest peak of the Himalayas is only 29,000 feet, or about ^Vos part of the earth's diameter. VENUS. ?7 III. VENUS. The Queen of Beauty. Sign ? , a looking-glass. Description. — Venus, the next in order to Mercury, is the most brilliant of the planets.* She presents the same appearances as Mercury. Owing, however, to the larger size of her orbit, her greatest apparent oscillations are nearly 48° east and west of the sun,t or about 20^ more than those of Mercury. She is therefore seen much earlier in the morning and much later at night. She is morning star from in- ferior to superior conjunction, and evening star from superior to inferior conjunction. Venus is the most brilliant about five weeks before and after inferior conjunction, at which time the planet is bright enough to cast a shadow at night. If, in addition, at this time of greatest brilliancy, Venus is at or near her highest north latitude, she may be seen with the naked eye in full daylight. ;}: This occurs once in eight years — the interval required for the earth and planet to return to the same situa- tion in their orbits ; eight complete revolutions of the * When visible before sunrise, she was called by the aiicieiits Phosphorus, Lueifer, or tl;e Morning Star, and when slie shone in the evening after sunset, Hesperus, Vesjier, or the Evening Star. t This distance varies only about 3", owing to the slight eccentricity of Venus's orbit. J Arago relates that Buonaparte, upon repairing to the Luxembourg, when the Directory was about to give him a fete, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were observing with astonisliment a star which they supposed to be that of the Conqueror of Italy. The emperor himself was not inditfiirent when his piercing eye caught the clear lustre of Venus smiling upon him at midday. t8 THE SOLAli SVSTEif. earth about the sun occupying nearly the same time as thirteen of Venus. Motion in Space. — Venus has an orbit the most nearly circular of any of the principal planets. Her mean distance from the sun is about 67,000,000 miles, which varies at aphelion and perihelion 1,000,000 miles, — a contrast to Mercury, which varies 15,000,000 miles. Venus makes a complete revolution around the sun in about 225 days, at the mean rate of twenty- two miles per second ; hence her year is equal to about seven and one-half of our months. This is a sidereal revolution, as it would appear to an ob- server at the sun ; a sijuodic re »'olution requires 584 days. I\iercury, we remember, catches up with the earth in twenty-eignt days after it reaches the point where it left the earth at the last inferjr>r conjunction. But it takes Venus nearly two and a half revolutions to overtake the earth anu eome into the same conjunc- tion again. This grows out of the fact that she has a longer orbit than Mercury, and moves only about one-sixth faster than the earth, while Mer- cury travels nearly twice as fast as our planet. Venus rotates upon her axis in about twenty-four hours ; so the length of her day does not differ essen- tially from ours. Distance from the Earth. — Like that of Mercury, the distance of Venus from the earth, when in in- ferior conjunction, is the difference between the dis- tances of the two planets from the sun ; when in superior conjunction, the sum of these distances. When nearest to us, Yenus is only about 25,000,000 miles away. Figure 30 represents her apparent dimensions at the extreme, mean, and least distances from us. The variation is nearly as the numbers 10, 18, and 65. It would be natural to think that the planet is the brightest when the nearest, and thus the largest, but Extreine, Mean, and Least Apparent Size of Venus ; and Iter Piloses. we should remember that then the bright side is toward the sun, and the unillumined side toward us. Indeed, at the period of greatest brilliancy, of which we have spoken, only about one-fourth of her light is visible. At this time, however, observers have noticed the entire contour of the- planet to be of a dull gray hue, as seen in the cut. Dimensions. — Venus is about 7, GOO miles in diame- ter. The volume and density of the planet are each about nine-tenths that of the earth. A stone let fall upon her surface would fall fourteen feet in the first go THE SOLAR SYSTEM. second : a pound weight removed to her equator would weigh about fourteen ounces. From this we see that the force of gravity does not decrease exactly in proportion to the size of the planet, any more than it increases with the size of the sun. The reason is, that the body is brought nearer the mass of the small planet, and so feels its attraction more fully than when far out upon the circumfer- ence of a large body, — the attraction increasing as the square of the distance from the particles de- creases. Seasons. — Since the axis of Venus is very much inclined from a perpendicular, her seasons are similar Fig. 31. V: iia.< lit ill I- SUstic to those of Mercury. The torrid and temperate zones overlap each other, and the polar regions have, alter- nately, at one solstice a torrid temperature, and at the other a prolonged arctic cold. The inequality of the nights is very marked. The heat and light are VENUS. 81 double that of the earth, while the circular form of her orbit gives nearly an equal length to her four seasons. If the inclination of her axis is 75°, as some as- tronomers hold, her tropics must be 75° from the equator, and her polar circles 75° from the poles. The torrid zone is, therefore, 150° in width. The torrid and frigid zones interlap through a space of 60", mid- way between the equator and the poles. Telescopic Features. — Venus, being an interior planet, presents, like Mercury, all the phases of the moon. * She is thought to have a dense, cloudy atmosphere. This was suggested by the fact that at the transit of Fig. SS. i|lil!l!„,|i|,|i| i' ,1 |i||[f'i;i; i ll'l. Illlll .'''Il'l Crescent and S/iotj ./ I'l Venus over the sun in 1761, 1769, and 1883, a faint ring of light surrounded the black disk of the planet. * This was discovered by Galileo, and was among the first achievements of his tele- scopic observations. It had been argued against the Copeniican system that, if true, Venus should wax and wane like the moon. Indeed, Copernicus himself boldly declared that, if means of seeing the jdanets more distinctly were ever invented, Venus would be found to present such phases. Galileo, with his telescope, proved this feet, ^pd thus vindicated the Copemican theory. 82 THE SOLAK SYSTEM. The evidence of an atmosphere, as well as of moun- tains, however, rests upon the peculiar appearance attending her crescent shape. 1. The luminous part does not end abruptly ; on the contrary, the light diminishes gradually. This diminution can be explained by a twilight caused by an atmosphere which diffuses the rays of light into regions of the planet where the sun is already set. Thus, on Venus, as on the earth, the evenings are lighted by twilight, and the mornings by dawn. 2. The edge of the enlightened portion of the planet is uneven and irregular. This appearance is doubtless the effect of shadows cast by mountains. Spots have been noticed on her disk which are con- sidered to be traceable to clouds. Herschel thinks that we never see the body of the planet, but only her atmosphere loaded with vapors, which may mitigate the glare of the intense sunshine. Satellites. — Venus is not known to have any moon. IV. THE EARTH. Sign, ^, a circle with Equator and Meridian. The Earth is the next planet we meet in passing outward from the sun. To the beginner, it seems strange enough to class our world among the heav- enly bodies. They are brilliant, while it is dark and opaque ; they appear light and airy, while it is solid 9,nd firm ,: we see in it no motion, while they ar^ THE EARTH. 83 constantly changing their position ; they seem mere points in the sky, while it is vast and extended. Yet, at the very beginning, we are to consider the Fig. S3. The Earth in Space. earth as a planet shining brightly in the heavens, ^nd appe,g,ring to other worlds as a planet (ioes to us. 84 THE SOLAK SYSTEM. We are to learn that it is in motion, flying througli its orbit with inconceivable velocity ; that it is not fixed, but hangs in space, held by an invisible power of gravitation which it cannot evade ; * that it is small and insignificant beside the mighty globes that so gently shine upon us in the far-off sky ; that, in fact, it is only one atom in a universe of worlds, all firm and solid, and all, perhaps, equally fitted to be the abode of life. Dimensions. — The earth is not "round like a ball," but flattened at the poles. Its form is that of an oblate spheroid. Its polar diameter is about 7,899 miles, and its equatorial about 7,925|. The com- pression is, therefore, 26| miles. (See table in Appendix.) If we represent the earth by a globe one yard in diameter, the polar diameter would be one-tenth of an inch too long. The circumfer- ence of the earth is nearly 25,000 miles. Its density is about 5 1 times that of water. Its weight is 6,069,000,000,000,000,000,000 tons. The inequalities of the earth's surface, arising from valleys, mountains, etc., have been likened to the roughness on the rind of an orange. On a globe sixteen inches in diameter, the land, to be in pro- portion, should be represented by the thinnest writing paper, the hills by very fine grains of sand, and ele- vated ranges by thick drawing-paper. To represent the deepest wells or mines, a scratch should be made tha,t would be invisible except with a glass. * Were the sun's attractive force upon the earth replaced by the largest steel tele- graph wire, it would require nine wires for each square inch of the sunward side gf our plobe, to hold the earth in her orbit. THE EARTH. 85 The Rotundity of the Earth is proved in various ways : (1) By the fact that vessels have sailed around the earth ; * (2) when a ship is coming into port, we see the masts first ; (3) the shadow of the earth on the moon is circular ; (4) the polar star seems higher in the heavens as we pass north ; and (5) th^ horizon expands as we ascend an eminence, f If we climb to the top of a hill, we can see further than when on the plain at its foot. Our eyesight is not improved ; it is only because ordinarily the cur- vature of the earth shuts off the view of distant objects, but when we ascend to a higher point, we can see further over the side of the earth. The cur- vature is eight inches per mile, 2^ x 8'"- = 32 inches for two miles, 3^ x 8'"- for three miles, etc. An object of these respective heights would be just hidden at these distances. Apparent and Real Motion. — In endeavoring to understand the various appearances of the heavenly bodies, it is well to remember how in daily life we * It is curious, in connection with this well-known fact, to recall the arguments urged by the Spanisli pliilosophers against the reasoning of Columbus, wlien he assured them that he could arrive at Asia just as certainly by sailing west as east. " How," they asl