The Heavens Above: a Popular Handbook of Astronomy

Part 1

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Transcriber's note:

Text enclosed by underscores is in italics (italics).

Superscripts, such as P to the second power, are shown by the caret character "^" before the superscript, such as P^2.

Subscripts are similarly shown by an underscore before the subscript which is wrapped in curly braces, such as M_{2}.

The Heavens Above: A Popular Handbook of Astronomy

THE HEAVENS ABOVE:

A Popular Handbook of Astronomy.

J. A. GILLET,

Professor of Physics in the Normal College of the City of New York,

and

W. J. ROLFE,

Formerly Head Master of the High School, Cambridge, Mass.

With Six Lithographic Plates and Four Hundred and Sixty Wood Engravings.

Potter, Ainsworth, & Co., New York and Chicago. 1882.

Copyright by J. A. Gillet and W. J. Rolfe, 1882.

Franklin Press: Rand, Avery, and Company, Boston.

PREFACE.

It has been the aim of the authors to give in this little book a brief, simple, and accurate account of the heavens as they are known to astronomers of the present day. It is believed that there is nothing in the book beyond the comprehension of readers of ordinary intelligence, and that it contains all the information on the subject of astronomy that is needful to a person of ordinary culture. The authors have carefully avoided dry and abstruse mathematical calculations, yet they have sought to make clear the methods by which astronomers have gained their knowledge of the heavens. The various kinds of telescopes and spectroscopes have been described, and their use in the study of the heavens has been fully explained.

The cuts with which the book is illustrated have been drawn from all available sources; and it is believed that they excel in number, freshness, beauty, and accuracy those to be found in any similar work. The lithographic plates are, with a single exception, reductions of the plates prepared at the Observatory at Cambridge, Mass. The remaining lithographic plate is a reduced copy of Professor Langley's celebrated sun-spot engraving. Many of the views of the moon are from drawings made from the photographs in Carpenter and Nasmyth's work on the moon. The majority of the cuts illustrating the solar system are copied from the French edition of Guillemin's "Heavens." Most of the remainder are from Lockyer's "Solar Physics," Young's "Sun," and other recent authorities. The cuts illustrating comets, meteors, and nebulæ, are nearly all taken from the French editions of Guillemin's "Comets" and Guillemin's "Heavens."

CONTENTS.

I. THE CELESTIAL SPHERE 3

II. THE SOLAR SYSTEM 41

I. THEORY OF THE SOLAR SYSTEM 41

The Ptolemaic System 41

The Copernican System 44

Tycho Brahe's System 44

Part 2

Kepler's System 44

The Newtonian System 48

II. THE SUN AND PLANETS 53

I. The Earth 53

Form and Size 53

Day and Night 57

The Seasons 64

Tides 68

The Day and Time 74

The Year 78

Weight of the Earth and Precession 83

II. The Moon 86

Distance, Size, and Motions 86

The Atmosphere of the Moon 109

The Surface of the Moon 114

III. Inferior and Superior Planets 130

Inferior Planets 130

Superior Planets 134

IV. The Sun 140

I. Magnitude and Distance of the Sun 140

II. Physical and Chemical Condition of the Sun 149

Physical Condition of the Sun 149

The Spectroscope 152

Spectra 158

Chemical Constitution of the Sun 164

Motion at the Surface of the Sun 168

III. The Photosphere and Sun-Spots 175

The Photosphere 175

Part 3

Sun-Spots 179

IV. The Chromosphere and Prominences 196

V. The Corona 204

V. Eclipses 210

VI. The Three Groups of Planets 221

I. General Characteristics of the Groups 221

II. The Inner Group of Planets 225

Mercury 225

Venus 230

Mars 235

III. The Asteroids 241

IV. Outer Group of Planets 244

Jupiter 244

The Satellites of Jupiter 250

Saturn 255

The Planet and his Moons 255

The Rings of Saturn 261

Uranus 269

Neptune 271

VII. Comets and Meteors 274

I. Comets 274

General Phenomena of Comets 274

Motion and Origin of Comets 281

Remarkable Comets 290

Connection between Meteors and Comets, 300

Physical and Chemical Constitution of Comets 314

II. The Zodiacal Light 318

III. THE STELLAR UNIVERSE 322

Part 4

I. General Aspect of the Heavens 322

II. The Stars 330

The Constellations 330

Clusters 350

Double and Multiple Stars 355

New and Variable Stars 358

Distance of the Stars 364

Proper Motion of the Stars 365

Chemical and Physical Constitution of the Stars 371

III. Nebulæ 373

Classification of Nebulæ 373

Irregular Nebulæ 376

Spiral Nebulæ 384

The Nebular Hypothesis 391

IV. The Structure of the Stellar Universe 396

I. THE CELESTIAL SPHERE.

I. The Sphere.--A sphere is a solid figure bounded by a surface which curves equally in all directions at every point. The rate at which the surface curves is called the curvature of the sphere. The smaller the sphere, the greater is its curvature. Every point on the surface of a sphere is equally distant from a point within, called the centre of the sphere. The circumference of a sphere is the distance around its centre. The diameter of a sphere is the distance through its centre. The radius of a sphere is the distance from the surface to the centre. The surfaces of two spheres are to each other as the squares of their radii or diameters; and the volumes of two spheres are to each other as the cubes of their radii or diameters.

Distances on the surface of a sphere are usually denoted in degrees. A degree is 1/360 of the circumference of the sphere. The larger a sphere, the longer are the degrees on it.

A curve described about any point on the surface of a sphere, with a radius of uniform length, will be a circle. As the radius of a circle described on a sphere is a curved line, its length is usually denoted in degrees. The circle described on the surface of a sphere increases with the length of the radius, until the radius becomes 90°, in which case the circle is the largest that can possibly be described on the sphere. The largest circles that can be described on the surface of a sphere are called great circles, and all other circles small circles.

Any number of great circles may be described on the surface of a sphere, since any point on the sphere may be used for the centre of the circle. The plane of every great circle passes through the centre of the sphere, while the planes of all the small circles pass through the sphere away from the centre. All great circles on the same sphere are of the same size, while the small circles differ in size according to the distance of their planes from the centre of the sphere. The farther the plane of a circle is from the centre of the sphere, the smaller is the circle.

By a section of a sphere we usually mean the figure of the surface formed by the cutting; by a plane section we mean one whose surface is plane. Every plane section of a sphere is a circle. When the section passes through the centre of the sphere, it is a great circle; in every other case the section is a small circle. Thus, AN and SB (Fig. 1) are small circles, and MM' and SN are large circles.

In a diagram representing a sphere in section, all the circles whose planes cut the section are represented by straight lines. Thus, in Fig. 2, we have a diagram representing in section the sphere of Fig. 1. The straight lines AN, SB, MM', and SN, represent the corresponding circles of Fig. 1.

The axis of a sphere is the diameter on which it rotates. The poles of a sphere are the ends of its axis. Thus, supposing the spheres of Figs. 1 and 2 to rotate on the diameter PP', this line would be called the axis of the sphere, and the points P and P' the poles of the sphere. A great circle, MM', situated half way between the poles of a sphere, is called the equator of the sphere.

Every great circle of a sphere has two poles. These are the two points on the surface of the sphere which lie 90° away from the circle. The poles of a sphere are the poles of its equator.

2. The Celestial Sphere.--The heavens appear to have the form of a sphere, whose centre is at the eye of the observer; and all the stars seem to lie on the surface of this sphere. This form of the heavens is a mere matter of perspective. The stars are really at very unequal distances from us; but they are all seen projected upon the celestial sphere in the direction in which they happen to lie. Thus, suppose an observer situated at C (Fig. 3), stars situated at a, b, d, e, f, and g, would be projected upon the sphere at A, B, D, E, F, and G, and would appear to lie on the surface of the heavens.

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