This is a reproduction of a library book that was digitized by Google as part of an ongoing effort to preserve the information in books and make it universally accessible. https://books.google.com I ;r-:-iii \'5.i tfA'K THE PULL MOOH, FBOM A PHOTO&RAPH. ECLECTIC EDUCATIONAL SERIES. THE ELEMENTS ASTRONOMY: COLLEGES, SCHOOLS, AND PRIVATE STUDENTS. WEITTEN FOB THE MATHEMATICAL COURSE OF JOSEPH RAY, M.D., BY SELIM H. PEABODY, M.A., TEACHER OF NATURAL SCIENCES IN TH*! CH'CAGQ BIQK SuHOuL CINCINNATI. WILSON, HINKLE &, CO. PHIL'A: CLAXTON, REMSE*& HAFFELFINGER. NEW YORK : CLARK & MAYNARD. f--3LIC LIBRARY 4787.25 »«T"«, LFMOX ANV F, ICO t, Entered according to Act of Congress, in the year 1869, by WILSON, HINKLE & CO., In the Clerk's Office of the District Court of the United States, for the Southern District of Ohio. ■ • • • • ♦ 1 • • • • « • ELECTROTYPED AT THE FRANKLIN TYPE FOUNDRY. CINCINNATI > g PREFACE. 92 CO C"i In this work I have endeavored to describe and explain the q Mechanism of the Solar System and of the Stellar Universe. ^5 Though written for pupils in the higher grades of public schools, it may be found useful in institutions of still higher rank, and as a foundation for more extended research by the private student. Although well aware that the rigid principles of mathematics and mechanics form the sole foundation of high astronomical attainment, I have carefully avoided abstruse mathematical demonstrations. Most who study Astronomy desire an accurate tiowledge of facts and principles, but need neither for mental - culture nor for practical use such a mastery of methods' as should fit them to become even amateur astronomers. For such I have aimed to furnish needed information and instruction. I have assumed that my readers know only the simplest principles of geometry and algebra, and the plainest facts of mechanics and physics. The rest I have endeavored to supply as needed. The liberality of Publishers has enabled me to insert an a @" unusual number of illustrations. Of the telescopic views, selected from the best authorities, some have lately found their eS Jti way into American text-books; others appear now for the first time. The beautiful experiments of Foucault on the Eotation O of the Earth, of Fizeau on Light, and of Plateau on Eotation, «S= (iii) cz IV ELEMENTS OF ASTRONOMY. have not been described hitherto in works of this grade. The same is true of the elegant apparatus of Bache for measuring base lines, reference having usually been made to the clumsier machinery of the English or French. Many of the diagrams are new, the fruits of hard work in the class-room. If the omission of the figures of men, animals, and serpents from the star-maps seems to any a questionable innovation, I have to say that my own experience as a teacher long since con vinced me that those monstrosities hinder rather than help ; and that my practice is sanctioned by Arago, Herschel, Lockyer, Proctor, Guillemin, and others, foremost astronomical writers of the present day. The Circumpolar Map is drawn on the equidistant projection, the increase in polar distance being always equal to the increase in circular arc. The Equatorial Maps have for base lines the meridian and the equinoctial — circles easily found, and always in the same position relative to the observer. The projection is the Polyconic, adopted by the U. S. Coast Survey for terrestrial maps, and now first used, so far as I am informed, for astronomical maps. Each tenth declination-parallel is assumed to be the base of a cone, tangent to the sphere in that circle; the spherical surface between that and the next higher parallel is projected into the conical surface, which is then developed upon a plane. As the maps extend but 30° on either side of the meridian, it is believed that the distortion, caused when a spherical surface is represented* on a plane, is reduced to a minimum. The stars have been carefully platted from Proctor's Tables. The Astronomical Tables have been compiled chiefly from Chambers. Following the example of Herschel in his " Outlines," and the advice of eminent American astronomers, I have retained PREFACE. V in the text the values of dimensions and distances in the solar system as based upon Encke's determination of the sun's parallax. The coming transits of Venus in 1874 and 1882 will furnish " a full and precise settlement of this important question." Both new and old values are inserted in the tables. I am indebted to the courtesy of the Director of the U. S. Naval Observatory, and to the Assistant in charge of the Coast Survey Office, at Washington, for valuable information ; to Prof. Henry Morton, Ph. D., of the Franklin Institute, Philadelphia, through whose unsolicited kindness I am able to present the superior engraving of the moon, reduced from a 24-inch photo graph in his possession, taken by Mr. Louis M. Eutherford, of New York ; and to Mr. H. H. Vail, whose careful scrutiny of the proof-sheets has been invaluable. S. H. P. High School, Chicago,'} Jane 15, 1869. f TABLE OF CONTENTS. CHAPTER I. PAQK. Astronomical Ideas referred to the Position of the Observer, ... 9 CHAPTER II. Form and Rotation of the Earth, 15 CHAPTER III. Astronomical Ideas derived from the Motion of the Earth, .... 23 CHAPTER IV. The Terrestrial Meridian, 28 CHAPTER V. Astronomical Instruments, 34 CHAPTER VI. Time. Longitude. Right Ascension, 52 CHAPTER VII. Atmospheric Refraction. Day and Night. Twilight, 63 CHAPTER VIII. Shape of the Earth. Gravitation, 73 CHAPTER IX. Distance of the Heavenly Bodies. The Ellipse, 88 CHAPTER X. The Earth's Orbit. Change of Seasons. Equation of Time. The Calendar, 96 (vi) TABLE OF CONTENTS. VU CHAPTER XI. PAQX. Planetary Motion, 124 CHAPTER XII. The Sun, 149 . CHAPTER XIII. The Moon. Eclipses. Tides, 170 CHAPTER XIV. The Planets, 209 CHAPTER XV. The Comets, 241 CHAPTER XVI. Star-showers. The Zodiacal Light, 258 CHAPTER XVII. The Progressive Motion of Light, 266 CHAPTER XVIII. The Fixed Stars, 272 CHAPTER XIX. The Nebular Hypothesis, 303 CHAPTER XX. The Constellations, 309 APPENDIX, 321 TABLES. Equation of Time, 328 Elements of the Planets, 329 Elements of the Minor Planets, 330 Elements of the Satellites, 332 STAR MAPS. ELEMENTS ASTRONOMY. CHAPTER I. ASTRONOMICAL IDEAS REFERRED TO THE POSITION OF THE OBSERVER. 1. The Horizon. — Any person in the open air upon level ground, or on the water, finds himself at the center of a large circle bounded by the sky. The sky seems to be the half of a hollow sphere, or a dome, which rests upon the outer edge of the plane, on which he stands. The line at which the earth and sky appear to meet is called the visible, or sensible, horizon. The plane which contains this line is the plane of the horizon; any plane which is parallel to the plane of the horizon is a hori zontal plane. 2. The horizon is often hidden by houses, forests, or hills, while high Fig. x. objects, as masts or sails of ships, towers, or mount ain-tops, some times appear beyond. If the observer ascends some high place, -HoSii°H 10 ELEMENTS OF ASTRONOMY. as a spire or hill, the horizon seems to remove as he goes up, and is lower than himself. The amount of its depression below a horizontal line passing through the observer is the dip of the horizon. The angle BAC, Fig. 1, measures the dip of the horizon at the top of the hill. 3. Each observer has his own Horizon. — Strictly speak ing, the horizon of one person diners from that of any other, and each person's horizon accompanies him as he goes from place to place. Practically, persons in the same vicinity have the same horizon. 4. The Sky. — The observer is at the center, not only of the visible part of the earth's surface, but also of the sky which covers it. The sky seems to be a surface at a uniform distance, which is immeasurable, or infinite. On this surface the sun, moon, and stars seem to move. 5. Astronomy. — The science which treats of the nature and motions of the heavenly bodies is called Astronomy,* The Law of the Stars. Because these bodies were sup posed to influence the fortunes of men, a pretended sci ence, which assumed to read the stars, and to foretell by them future events, was called Astrology,f The Language of the Stars. 6. The real Horizon. — We shall assume that the earth is a globe, surrounded by the sky. The center of the sky is, therefore, readily understood to be at the center of the earth. If a plane pass through the center of the earth parallel to the plane of the sensible horizon, and be extended every way, the line in which it meets the sky * From aoTTjp, cuter, a star ; and vopoq, nomas, a law. Aster is allied in derivation to straw and ttrew ; the stars are strewn oyer the sky. t From aaryp, and toyoc:, logos, a word. DEFINITIONS. 11 is called the real, or astronomical, horizon. The visible and astronomical horizons seem to meet the sky in the same line, because the distance between them is too NADIR small to be perceived on a surface so far away as the apparent surface of the sky. DEFINITIONS. "7. A Sphere is a space bounded by a surface which is at every point equally distant from a point within, called its center. The line in which a plane cuts the surface of a sphere is the circumference of a circle (Geom.