Project Gutenberg’s An Introduction to Astronomy, by Forest Ray Moulton This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: An Introduction to Astronomy Author: Forest Ray Moulton Release Date: April 24, 2010 [EBook #32000] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK AN INTRODUCTION TO ASTRONOMY *** Produced by Brenda Lewis, Andrew D. Hwang, Bup, and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.) transcriber’s note Figures may have been moved with respect to the surrounding text. Minor presentational changes have been made without comment. Typographical corrections are documented in the LATEX source file. The spelling “Leverrier” has been retained from the original. This PDF file is formatted for screen viewing, but may be easily formatted for printing. Please consult the preamble of the LATEX source file for instructions. AN INTRODUCTION TO ASTRONOMY 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 Fig. 1. — The Lick Observatory, Mount Hamilton, California. AN INTRODUCTION TO ASTRONOMY BY FOREST RAY MOULTON, Ph.D. PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF CHICAGO RESEARCH ASSOCIATE OF THE CARNEGIE INSTITUTION OF WASHINGTON NEW AND REVISED EDITION New York THE MACMILLAN COMPANY 1916 All rights reserved Copyright, 1906 and 1916, By THE MACMILLAN COMPANY. Set up and electrotyped. Published April, 1906. Reprinted November, 1907; July, 1908; April, 1910; April, 1911; September, 1912; September, 1913: October, 1914. New and revised edition November, 1916. Norwood Pre& J. S. Cushing 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 rewriting it. As in the first edition, the aim has been to present the great subject of astronomy so that it can be easily comprehended even by a person who has not had extensive scientific training. 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 intellectual development that he demands the rea- sons for whatever conclusions he is asked to accept. The first 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 under- gone many, and in some cases important, modifications. For example, the work on reference points and lines has been deferred to Chapter iv. If one is to know the sky, and not simply know about it, a knowledge of the coördinate 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 difficult. The chapter on telescopes has been regretfully omit- ted because it was not necessary for understanding the remainder of the work, and because the space it occupied was needed for treating more vital parts of the subject. The numerous discoveries in the si- dereal universe during the last ten years have made it necessary greatly to enlarge the last chapter. As now arranged, the first chapters are devoted to a discussion of the earth and its motions. They present splendid examples of the char- acteristics 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 elasticity of the earth. They show how closely various sciences are in- terlocked, 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 re- vi PREFACE vii garded 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 energy, the disintegration of radioactive elements, and the principles of spectrum analysis. Then follows the evolution of the solar system. In this chapter new and more serious demands 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 transcendental and absorbingly interesting problems respecting the organization and evolution of the sidereal universe. Lists of problems have been given at the ends of the principal di- visions 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 illustrated, with the exception of Figs. 23 and 30, which were kindly furnished by Mr. George Otis. He is indebted to Professor David Eugene Smith for photographs of Newton, 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. R. MOULTON. The University of Chicago, September 25, 1916. CONTENTS CHAPTER I Preliminary Considerations ARTS. PAGE 1. Science . .1 2. The value of science . .2 3. The origin of science . .3 4. The methods of science . .5 5. The imperfections of science . .8 6. Great contributions of astronomy to science . 11 7. The present value of astronomy . 12 8. The scope of astronomy . 14 CHAPTER II THE EARTH I. The Shape of the Earth 9. Astronomical problems respecting the earth . 20 10, 11. Proofs of the earth’s sphericity . 21, 24 12, 14, 15. Proofs of the earth’s oblateness . 24–28 13. Size and shape of the earth . 26 16. The theoretical shape of the earth . 29 17. Different kinds of latitude . 31 18. Historical sketch on the shape of the earth . 31 II. The Mass of the Earth and the Condition of its Interior 19. The principle by which mass is determined . 34 20. The mass and density of the earth . 35 21–23. Methods of determining the density of the earth . 36–39 24. Temperature and pressure in the earth’s interior . 40 25, 26. Proofs of the earth’s rigidity and elasticity . 41, 47 27. Historical sketch on the mass and rigidity of the earth . 49 viii CONTENTS ix III. The Earth’s Atmosphere ARTS. PAGE 28. Composition and mass of the earth’s atmosphere . 50 29–31. Methods of determining height of the atmosphere . 51–53 32. The kinetic theory of gases . 54 33. The escape of atmospheres . 55 34. Effects of the atmosphere on climate . 56 35. Importance of the constitution of the atmosphere . 57 36. Rôle of the atmosphere in life processes . 58 37. Refraction of light by the atmosphere . 59 38. The twinkling of the stars . 59 CHAPTER III THE MOTIONS OF THE EARTH I. The Rotation of the Earth 39. The relative rotation of the earth . 62 40. The laws of motion . 63 41–43. Proofs of the earth’s rotation . 66–67 44. Consequences of the earth’s rotation . 68 45. Uniformity of the earth’s rotation . 70 46. The variation of latitude . 72 47. The precession of the equinoxes and nutation . 74 II. The Revolution of the Earth 48. Relative motion of the earth with respect to the sun . 77 49–52. Proofs of the revolution of the earth . 79–82 53. Shape of the earth’s orbit . 83 54. Motion of the earth in its orbit . 84 55. Inclination of the earth’s orbit . 85 56. The cause of the seasons . 86 57. Relation of altitude of pole to latitude of observer . 87 58. The sun’s diurnal circles . 88 59. Hours of sunlight in different latitudes . 90 60. The lag of the seasons . 91 CONTENTS x ARTS. PAGE 61. Effect of eccentricity of earth’s orbit on seasons . 91 62. Historical sketch of the motions of the earth . 93 CHAPTER IV Reference Points and Lines 63. Object and character of reference points and lines . 98 64. The geographical system . 99 65. The horizon system . 100 66. The equator system . 101 67. The ecliptic system . 102 68. Comparison of systems of coördinates . 103 69, 70. Finding the altitude and azimuth . 105, 107 71, 72.