TRANSCRIBER'S NOTE Italic text is denoted by _underscores_. Obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources. More detail can be found at the end of the book. PRACTICAL TALKS BY AN ASTRONOMER [Illustration: The Moon. First Quarter. Photographed by Loewy and Puiseux, February 13, 1894.] PRACTICAL TALKS BY AN ASTRONOMER BY HAROLD JACOBY ADJUNCT PROFESSOR OF ASTRONOMY IN COLUMBIA UNIVERSITY ILLUSTRATED NEW YORK CHARLES SCRIBNER'S SONS 1902 COPYRIGHT, 1902, BY CHARLES SCRIBNER'S SONS Published, April, 1902 TROW DIRECTORY PRINTING AND BOOKBINDING COMPANY NEW YORK PREFACE The present volume has not been designed as a systematic treatise on astronomy. There are many excellent books of that kind, suitable for serious students as well as the general reader; but they are necessarily somewhat dry and unattractive, because they must aim at completeness. Completeness means detail, and detail means dryness. But the science of astronomy contains subjects that admit of detached treatment; and as many of these are precisely the ones of greatest general interest, it has seemed well to select several, and describe them in language free from technicalities. It is hoped that the book will thus prove useful to persons who do not wish to give the time required for a study of astronomy as a whole, but who may take pleasure in devoting a half-hour now and then to a detached essay on some special topic. Preparation of the book in this form has made it suitable for prior publication in periodicals; and the several essays have in fact all been printed before. But the intention of collecting them into a book was kept in mind from the first; and while no attempt has been made at consecutiveness, it is hoped that nothing of merely ephemeral value has been included. CONTENTS PAGE NAVIGATION AT SEA 1 THE PLEIADES 10 THE POLE-STAR 18 NEBULÆ 27 TEMPORARY STARS 37 GALILEO 47 THE PLANET OF 1898 58 HOW TO MAKE A SUN-DIAL 69 PHOTOGRAPHY IN ASTRONOMY 81 TIME STANDARDS OF THE WORLD 111 MOTIONS OF THE EARTH'S POLE 131 SATURN'S RINGS 140 THE HELIOMETER 152 OCCULTATIONS 161 MOUNTING GREAT TELESCOPES 170 THE ASTRONOMER'S POLE 184 THE MOON HOAX 199 THE SUN'S DESTINATION 210 ILLUSTRATIONS THE MOON. FIRST QUARTER _Frontispiece_ _Photographed by Loewy and Puiseux, February 13, 1894._ FACING PAGE