CORNELL;. UNIVERSITY LIBRARY V BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 189I BY HENRY WILLIAMS SAGE Cornell University Library QB 43.M927I6 1916 An introduction to astronomy, 3 1924 012 499 756 AN INTRODUCTION TO ASTEONOMY THE MACMILLAN COMPANY NEW YORK BOSTON • CHICAGO - DALLAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., Limited LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, Ltd. TORONTO Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924012499756 AN INTRODUCTION TO ASTRONOMY BY FOREST RAY MOULTON, Ph.D. PKOFESSOR OP ASTRONOMY IN THE UNIVERSITY OF CHICAGO RESEARCH ASSOCIATE OF THE CAKNBGIE INSTITUTION OF WASHINGTON NEW AND REVISED EDITION Weto goris THE MACMILLAN COMPANY 1916 All rights reserved OOPYEIGHT, 1906 AND 1916, By TBE MACMILLAN COMPANY. Set up and electrotyped. Published April, 1906. Reprinted November, 1907; July, igo8; April, 1910; April, 1911; September, 1912; September, 1913; Optober, 1914. New and revised edition November, igi6. J. S. Gushing Co. — Berwick & Smith Co. Norwood, Mass., U.S.A. PREFACE The necessity for a new edition of " An Introduction to Astronomy" has furnished an opportunity for entirely re- writing it. As in the first edition, the aim has been to pre- sent the great subject of astronomy so that it can be easily comprehended even by a person who has not had extensive scientific trainiag. It has been assumed that the reader has no intention of becoming an astronomer, but that he has an interest in the wonderful universe which surrounds him, and that he has arrived at such a stage of iatellectual development that he demands the reasons for whatever conclusions he is first asked to accept. The two of these , assumptions have largely determined the subject matter which is presented ; the third has strongly influenced the method of presenting it. While the aims have not changed materially since the first edition was written, the details of the attempt to accomplish them have undergone many, and in some cases important, modifications. For example, the work on reference points and liues has been deferred to Chapter IV. If one is to know the sky, and not simply know about it, a knowledge of the coordi- nate systems is indispensable, but they always present some difficulties when they are encountered at the beginning of the subject. It is believed that the present treatment prepares so thoroughly for their study and leads so naturally to them that their mastery will not be found dif&cult. The chapter on telescopes has been regretfully omitted because it was not necessary for understanding the remainder of the work, and because the space it occupied was needed for treatrag more vital parts of the subject. The numerous discoveries in the sidereal universe during the last ten years have made it neces- sary greatly to enlarge the last chapter. VI PREFACE discus- As now arranged, the first , chapters are devoted to a sion of the earth and its motions. They present splendid examples of the characteristics and methods of science, and amply illustrate the care with which scientific theories are established. The conclusions which are set forth are bound up with the development of science from the dawn of recorded history to the recent experiments on the rigidity and the elas- ticity, of the earth. They show how closely various sciences are interlocked, and how much an understanding of the earth depends upon its relations to the sky. They lead naturally to a more formal treatment of the celestial sphere and a study of the constellations. A familiarity with the brighter stars and the more conspicuous constellations is regarded as important. One who has become thoroughly acquainted with them will always experience a thrill when he looks up at night into a cloudless sky. The chapter on the sun has been postponed until after the treatment of the moon, planets, and comets. The reason is that the discussion of the sun necessitates the introduction of many new and difficult topics, such as the conservation of en- ergy, the disintegration of radioactive elements, and the prin- ciples of spectrum analysis. Then follows the evolution of the solar system. In this chapter new and more serious de- mands are made on the reasoning powers and the imagination. Its study in a measure develops a point of view and prepares the way for the consideration, in the last chapter, of the tran- scendental and absorbingly interesting problems respecting the organization and evolution of the sidereal universe. Lists of problems have been given at the ends of the prin- cipal divisions of the chapters. They cannot be correctly answered without a real comprehension of the principles which they involve, and in very many cases, especially in the later chapters, they lead to important supplementary results. It is strongly recommended that they be given careful consideration. The author is indebted to Mr. Albert Barnett for the new star maps and the many drawings with which the book is illus- trated, with the exception of Tigs. 23 and 30, which were PREFACE VU kindly furnished by Mr. George Otis. He is indebted to Professor David Eugene Smith for photographs of Newtcta, Kepler, Herschel, Adams, and Leverrier. He is indebted to the Lick, Lowell, Solar, and Yerkes observatories for a large amount of illustrative material which was very generously furnished. He is under deeper obligations to his colleague, Professor W. D. MacMillan, than this brief acknowledgment can express for assistance on the manuscript, on the proofs, and in preparing the many problems which appear in the book. F. E. MOULTON. The Uniteksitt of Chicago, September 25, 1916. CONTENTS CHAPTER I Preliminary Considerations PAGE 1. Science 1 2. The value of science 2 3. The origin of science ... 4 4. The methods of science 6 5. The imperfections of science 10 6. Great contributions of astronomy to science 14 7. The present value of astronomy . 16 8. The scope of astronomy 19 CHAPTER U THE EARTH I. The Shape op the Earth 9. Astronomical problems respecting the earth 26 10, 11. Proofs of the earth's sphericity 27 12, 14, 15. Proofs of the earth's oblateness 31 1-3. Size and shape of the earth 33 16. The theoretical shape of the earth 38 17. Different kinds of latitude . 39 18. Historical sketch on the shape of the earth 40 II. The Mass of the Earth and the Conditioi« of its Interior 19. The principle by which mass is determined .... 43 20. The mass and density of the earth . .45 21-23. Method? pf determining the density of the earth ... 46 CONTENTS PAGE 24. Temperature and pressure in the earth's interior 51 25, 26. Proofs of the earth's rigidity and elasticity . 52 27. Historical sketch on the mass and rigidity of the earth 62 III. The Earth's Atmosphere 28. Composition and mass of the earth's atmosphere 64 29-31. Methods of determining height of the atmosphere 65 32. The kinetic theory, of gases .... 33. The escape of atmospheres 34. Effects of the atmosphere on climate . 71 35. Importance of the constitution of the atmosphere 72 36. R61e of the atmosphere in life processes 74 37. Kefraction of light by the atmosphere 74 38. The . twinkling of the stars . 76 CHAPTER III THE MOTIONS OF THE EARTH I. The Rotation of the Earth 39. The relative rotation of the earth 77 40. The laws of motion 79 41-43. Proofs of the earth's rotation 82 44. Consequences of the earth's rotation . 85 45. Uniformity of the earth's rotation 87 46. The variation of latitude 89 47. The precession of the equinoxes and nutation 92 II. The Revolution of the Earth 48. Relative motion of the earth with respect to the sun 96 49-52. Proofs of the revolution of the earth 98 53. Shape of the earth's orbit 102 54. Motion of the earth in its orbit . 103 55. Inclination of the earth's orbit . 105 56. The cause of the seasons .... 107 .57. Relation of altitude of pole to latitude of observer 108 58. The sun's diurnal circles 109 59. Hours of sunlight in different latitudes HI 60. The lag of the seasons 112 61. Effect of eccentricity of earth's orbit on seasons u;j 62. Historical sketch of the motions of the earth 115 CONTENTS XI CHAPTER IV EErBRENCE Points and Lines ARTS. PAGE 63. Object and character of reference points and lines 121 64. The geographical system . 122 65. The horizon system . 123 66. The equator system . 125 67. The ecliptic system . 127 68. Comparison of systems of coSrdinates 127 69, 70. Finding the altitude and azimuth 130 71, 72. Finding the right ascension and declination 133 73. Other problems of position .... 135 CHAPTER V The Constellations 74. Origin of the constellations 138 75. Naming the stars 138 76. Star catalogues 141 77. The magnitudes of the stars 142 78. The first-magnitude stars 143 79. Number of stars in first six magnitudes 145 80. Motions of the stars 145 81. The Milky Way, or Galaxy 146 82. The constellations and their positions (Maps) .... 148 83. Finding the pole star 149 84. Units for estimating angular distances 150 85-101. Ursa Major, Cassiopeia, Locating the equinoxes, Lyra, Hercules, Scorpius, Corona Borealis, Bootes, I^eo, An- dromeda, Perseus, Auriga, Taurus, Orion, Canis Major, Canis Minor, Gemini 150 102. On becoming familiar with the stars 167 CHAPTER VI Time 103. Definitions of equal intervals of time 169 104.