Project Gutenberg's A Textbook of General Astronomy, by Charles A. Young 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.net Title: A Textbook of General Astronomy For Colleges and Scientific Schools Author: Charles A. Young Release Date: August 30, 2011 [EBook #37275] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK A TEXTBOOK OF GENERAL ASTRONOMY *** Produced by Susan Skinner, Nigel Blower, Brenda Lewis and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images generously made available by the Biblioth`equenationale de France (BnF/Gallica) at http://gallica.bnf.fr, illustrations generously made available by The Internet Archive/Canadian Libraries) Transcriber's Notes A small number of minor typographical errors and inconsistencies have been corrected. See the DPtypo command in the LATEX source for more information. FRONTISPIECE. The Great Telescope of the Lick Observatory, Mt. Hamilton, Cal. Object-Glass made by A. Clark & Sons: Aperture, 36 in.; Focal Length, 56 ft. 2 in. Mounting by Warner & Swasey. ATEXT-BOOK OF GENERAL ASTRONOMY FOR COLLEGES AND SCIENTIFIC SCHOOLS BY CHARLES A. YOUNG, Ph.D., LL.D., Professor of Astronomy in the College of New Jersey (Princeton). Boston, U.S.A., and London: GINN & COMPANY, PUBLISHERS. 1889. Entered at Stationers' Hall. Copyright, 1888, by CHARLES A. YOUNG, All Rights Reserved. Typography by J. S. Cushing & Co., Boston, U.S.A. Presswork by Ginn & Co., Boston, U.S.A. PREFACE. The present work is designed as a text-book of Astronomy suited to the general course in our colleges and schools of science, and is meant to supply that amount of information upon the subject which may fairly be expected of every \liberally educated" person. While it assumes the previous discipline and mental maturity usu- ally corresponding to the latter years of the college course, it does not demand the peculiar mathematical training and aptitude nec- essary as the basis of a special course in the science|only the most elementary knowledge of Algebra, Geometry, and Trigonometry is required for its reading. Its aim is to give a clear, accurate, and justly proportioned presentation of astronomical facts, principles, and methods in such a form that they can be easily apprehended by the average college student with a reasonable amount of effort. The limitations of time are such in our college course that proba- bly it will not be possible in most cases for a class to take thoroughly everything in the book. The fine print is to be regarded rather as collateral reading, important to anything like a complete view of the subject, but not essential to the course. Some of the chapters can even be omitted in cases where it is found necessary to abridge the course as much as possible; e.g., the chapters on Instruments and on Perturbations. While the work is no mere compilation, it makes no claims to special originality: information and help have been drawn from all available sources. The author is under great obligations to the astronomical histories of Grant and Wolf, and especially to Miss Clerke's admirable \History of Astronomy in the Nineteenth Cen- tury." Many data also have been drawn from Houzeau's valuable \Vade Mecum de l'Astronomie." It has been intended to bring the book well down to date, and to indicate to the student the sources of information on subjects which are necessarily here treated inadequately on account of the PREFACE. v limitations of time and space. Special acknowledgments are due to Professor Langley and to his publishers, Messrs. Ticknor & Co., for the use of a number of illustrations from his beautiful book, \The New Astronomy"; and also to D. Appleton & Co. for the use of several cuts from the au- thor's little book on the Sun. Professor Trowbridge of Cambridge kindly provided the original negative from which was made the cut illustrating the comparison of the spectrum of iron with that of the sun. Warner & Swasey of Cleveland and Fauth & Co. of Washing- ton have also furnished the engravings of a number of astronomical instruments. Professors Todd, Emerson, Upton, and McNeill have given most valuable assistance and suggestions in the revision of the proof; as indeed, in hardly a less degree, have several others. The author will consider it a great favor if those who may use the book will kindly communicate to him, either directly or through the publishers, any errata, in order that they may be promptly corrected. Princeton, N. J., August, 1888. Note. In this issue of the book a number of errors which appeared in the first impression have been corrected. April, 1889. TABLE OF CONTENTS. PAGES INTRODUCTION . 1{4 CHAPTER I.|The Doctrine of the Sphere; Definitions and General Considerations . 5{20 CHAPTER II.|Astronomical Instruments: the Telescope; Time-Keepers and Chronograph; the Transit Instrument and Accessories; the Meridian Circle and Reading Mi- croscope; the Altitude and Azimuth Instrument; the Equatorial Instrument and Micrometer; the Sextant . 21{58 CHAPTER III.|Corrections to Astronomical Observa- tions: the Dip of the Horizon; Parallax; Semi-Diameter; Refraction; and Twilight . 59{71 CHAPTER IV.|Problems of Practical Astronomy: the Determination of Latitude, of Time, of Longitude, of a Ship's Place at Sea, of Azimuth, and of the Apparent Right Ascension and Declination of a Heavenly Body; the Time of Sunrise or Sunset . 72{93 CHAPTER V.|The Earth: the Approximate Determination of its Dimensions and Form; Proofs of its Rotation; Accurate Determination of its Dimensions by Geodetic Surveys and Pendulum Observations; Determination of its Mass and Density . 94{120 CHAPTER VI.|The Earth's Orbital Motion: the Motion of the Sun among the Stars; the Equation of Time; Precession; Nutation; Aberration; the Calendar . 121{148 CHAPTER VII.|The Moon: her Orbital Motion; Distance and Dimensions; Mass, Density, and Superficial Gravity; Rotation and Librations; Phases; Light and Heat; Physical Condition; Influence exerted on the Earth; Surface Structure; and Possible Changes . 149{176 TABLE OF CONTENTS. vii PAGES CHAPTER VIII.|The Sun: Distance and Dimensions; Mass and Density; Rotation; Solar Eye-Pieces, and Study of the Sun's Surface; General Views as to Constitution; Sun Spots: their Appearance, Nature, Distribution, and Periodicity; the Spectroscope and the Solar Spectrum; Chemical Composition of the Sun; the Chromosphere and Prominences; the Corona . 177{216 CHAPTER IX.|The Sun's Light and Heat: Comparison of Sunlight with Artificial Lights; the Measurement of the Sun's Heat, and Determination of the Solar Constant; the Pyrheliometer, Actinometer, and Bolometer; the Sun's Temperature; Maintenance of the Sun's Radiation; and Conclusions as to its Age and Future Endurance . 217{232 CHAPTER X.|Eclipses; Form and Dimensions of Shadows; Lunar Eclipses; Solar Eclipses, Total, Annular, and Partial; Ecliptic Limits, and Number of Eclipses in a Year; the Saros; Occultations . 233{251 CHAPTER XI.|Central Forces; Equable Description of Areas; Areal, Linear, and Angular Velocities; Kepler's Laws and Inferences from them; Gravitation demonstrated by the Moon's Motion; Conic Sections as Orbits; the Problem of Two Bodies; the \Velocity from Infinity" and its Relation to the Species of Orbit described by a Body moving under Gravitation; Intensity of Gravitation . 252{273 CHAPTER XII.|The Problem of Three Bodies; Disturb- ing Forces; Lunar Perturbations and the Tides . 274{299 CHAPTER XIII.|The Planets: their Motions, Appar- ent and Real; the Ptolemaic, Tychonic, and Copernican Systems; the Orbits and their Elements; Planetary Perturbations . 300{322 CHAPTER XIV.|The Planets themselves; Methods of determining their Diameters, Masses, Densities, Times of Rotation, etc.; the \Terrestrial Planets,"|Mercury, Venus, and Mars; the Asteroids; Intra-Mercurial Planets and the Zodiacal Light . 323{355 CHAPTER XV.|The Major Planets,|Jupiter, Saturn, Uranus, and Neptune . 356{383 TABLE OF CONTENTS. viii PAGES CHAPTER XVI.|The Determination of the Sun's Hor- izontal Parallax and Distance; Oppositions of Mars and Transits of Venus; Gravitational Methods; Determi- nation by Means of the Velocity of Light . 384{402 CHAPTER XVII.|Comets: their Number, Motions, and Orbits; their Constituent Parts and Appearance; their Spectra, Physical Constitution, and Probable Origin . 403{438 CHAPTER XVIII.|Meteors: Aerolites, their Fall and Physical Characteristics; Shooting Stars and Meteoric Showers; Connection between Meteors and Comets . 439{455 CHAPTER XIX.|The Stars: their Nature and Number; the Constellations; Star-Catalogues; Designation of Stars; their Proper Motions, and the Motion of the Sun in Space; Stellar Parallax . 456{476 CHAPTER XX.|The Light of the Stars; Star Magni- tudes and Photometry; Variable Stars; Stellar Spectra; Scintillation of the Stars . 477{503 CHAPTER XXI.|Aggregations of Stars: Double and Multiple Stars; Clusters; Nebulæ; the Milky Way, and Distribution of Stars in Space; Constitution of the Stellar Universe; Cosmogony and the Nebular Hypothesis . 504{539 APPENDIX.|Tables of Astronomical Data . 540{547 INDEX . 548{567 INTRODUCTION. 1. Astronomy (ἄστροn nomoc´ ) is the science which treats of the heavenly bodies. As such bodies we reckon the sun and moon, the planets (of which the earth is one) and their satellites, comets and meteors, and finally the stars and nebulæ. We have to consider in Astronomy:| (a) The motions of these bodies, both real and apparent, and the laws which govern these motions. (b) Their forms, dimensions, and masses. (c) Their nature and constitution. (d) The effects they produce upon each other by their attrac- tions, radiations, or by any other ascertainable influence. It was an early, and has been a most persistent, belief that the heavenly bodies have a powerful influence upon human affairs, so that from a knowledge of their positions and \aspects" at critical moments (as for instance at the time of a person's birth) one could draw up a \horoscope" which would indicate the probable future.