Produced by Chris Curnow, Iris Schimandle, Lindy Walsh and the Online Distributed Proofreading Team at http://www.pgdp.net TWENTIETH CENTURY TEXT-BOOKS EDITED BY A. F. NIGHTINGALE, PH.D., LL.D. FORMERLY SUPERINTENDENT OF HIGH SCHOOLS, CHICAGO [Illustration: A TOTAL SOLAR ECLIPSE. After Burckhalter's photographs of the eclipse of May 28, 1900.] TWENTIETH CENTURY TEXT-BOOKS A TEXT-BOOK OF ASTRONOMY BY GEORGE C. COMSTOCK DIRECTOR OF THE WASHBURN OBSERVATORY AND PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF WISCONSIN [Illustration] NEW YORK D. APPLETON AND COMPANY 1903 COPYRIGHT, 1901 BY D. APPLETON AND COMPANY PREFACE The present work is not a compendium of astronomy or an outline course of popular reading in that science. It has been prepared as a text-book, and the author has purposely omitted from it much matter interesting as well as important to a complete view of the science, and has endeavored to concentrate attention upon those parts of the subject that possess special educational value. From this point of view matter which permits of experimental treatment with simple apparatus is of peculiar value and is given a prominence in the text beyond its just due in a well-balanced exposition of the elements of astronomy, while topics, such as the results of spectrum analysis, which depend upon elaborate apparatus, are in the experimental part of the work accorded much less space than their intrinsic importance would justify. Teacher and student are alike urged to magnify the observational side of the subject and to strive to obtain in their work the maximum degree of precision of which their apparatus is capable. The instruments required are few and easily obtained. With exception of a watch and a protractor, all of the apparatus needed may be built by any one of fair mechanical talent who will follow the illustrations and descriptions of the text. In order that proper opportunity for observations may be had, the study should be pursued during the milder portion of the year, between April and November in northern latitudes, using clear weather for a direct study of the sky and cloudy days for book work.