Chapter I: 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.

The illustrations contained in the present work are worthy of as careful study as is the text, and many of them are intended as an aid to experimental work and accurate measurement, e. g., the star maps, the diagrams of the planetary orbits, pictures of the moon, sun, etc. If the school possesses a projection lantern, a set of astronomical slides to be used in connection with it may be made of great advantage, if the pictures are studied as an auxiliary to Nature. Mere display and scenic effect are of little value.

A brief bibliography of popular literature upon astronomy may be found at the end of this book, and it will be well if at least a part of these works can be placed in the school library and systematically used for supplementary reading. An added interest may be given to the study if one or more of the popular periodicals which deal with astronomy are taken regularly by the school and kept within easy reach of the students. From time to time the teacher may well assign topics treated in these periodicals to be read by individual students and presented to the class in the form of an essay.

The author is under obligations to many of his professional friends who have contributed illustrative matter for his text, and his thanks are in an especial manner due to the editors of the Astrophysical Journal, Astronomy and Astrophysics, and Popular Astronomy for permission to reproduce here plates which have appeared in those periodicals, and to Dr. Charles Boynton, who has kindly read and criticised the proofs.

GEORGE C. COMSTOCK.

UNIVERSITY OF WISCONSIN, _February, 1901_.

CONTENTS

CHAPTER PAGE
I.--DIFFERENT KINDS OF MEASUREMENT 1
The measurement of angles and time.

II.--THE STARS AND THEIR DIURNAL MOTION 10
Finding the stars--Their apparent motion--
Latitude--Direction of the meridian--Sidereal
time--Definitions.

III.--FIXED AND WANDERING STARS 29
Apparent motion of the sun, moon, and
planets--Orbits of the planets--How to find
the planets.

IV.--CELESTIAL MECHANICS 46
Kepler's laws--Newton's laws of motion--The law
of gravitation--Orbital motion--Perturbations--
Masses of the planets--Discovery of Neptune--
The tides.

V.--THE EARTH AS A PLANET 70
Size--Mass--Precession--The warming of the
earth--The atmosphere--Twilight.

VI.--THE MEASUREMENT OF TIME 86
Solar and sidereal time--Longitude--The
calendar--Chronology.

VII.--ECLIPSES 101
Their cause and nature--Eclipse limits--Eclipse
maps--Recurrence and prediction of eclipses.

VIII.--INSTRUMENTS AND THE PRINCIPLES INVOLVED IN THEIR USE 121
The clock--Radiant energy--Mirrors and lenses--
The telescope--Camera--Spectroscope--Principles
of spectrum analysis.

IX.--THE MOON 150
Numerical data--Phases--Motion--Librations--Lunar
topography--Physical condition.

X.--THE SUN 178
Numerical data--Chemical nature--Temperature--
Visible and invisible parts--Photosphere--Spots--
Faculæ--Chromosphere--Prominences--Corona--The
sun-spot period--The sun's rotation--Mechanical
theory of the sun.

XI.--THE PLANETS 212
Arrangement of the solar system--Bode's law--
Physical condition of the planets--Jupiter--
Saturn--Uranus and Neptune--Venus--Mercury--
Mars--The asteroids.

XII.--COMETS AND METEORS 251
Motion, size, and mass of comets--Meteors--Their
number and distribution--Meteor showers--Relation
of comets and meteors--Periodic comets--Comet
families and groups--Comet tails--Physical nature
of comets--Collisions.

XIII.--THE FIXED STARS 291
Number of the stars--Brightness--Distance--Proper
motion--Motion in line of sight--Double stars--
Variable stars--New stars.

XIV.--STARS AND NEBULÆ 330
Stellar colors and spectra--Classes of stars--
Clusters--Nebulæ--Their spectra and physical
condition--The Milky Way--Construction of the
heavens--Extent of the stellar system.

XV.--GROWTH AND DECAY 358
Logical bases and limitations--Development of the
sun--The nebular hypothesis--Tidal friction--Roche's
limit--Development of the moon--Development of stars
and nebulæ--The future.