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.

by

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

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

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

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.

3. _The Horizon._--Only half of the celestial sphere is visible at a time. The plane that separates the visible from the invisible portion is called the _horizon_. This plane is tangent to the earth at the point of observation, and extends indefinitely into space in every direction. In Fig. 4, _E_ represents the earth, _O_ the point of observation, and _SN_ the horizon. The points on the celestial sphere directly above and below the observer are the poles of the horizon. They are called respectively the _zenith_ and the _nadir_. No two observers in different parts of the earth have the same horizon; and as a person moves over the earth he carries his horizon with him.

The dome of the heavens appears to rest on the earth, as shown in Fig. 5. This is because distant objects on the earth appear projected against the heavens in the direction of the horizon.

The _sensible_ horizon is a plane tangent to the earth at the point of observation. The _rational_ horizon is a plane parallel with the sensible horizon, and passing through the centre of the earth. As it cuts the celestial sphere through the centre, it forms a great circle. _SN_ (Fig. 6) represents the sensible horizon, and _S'N'_ the rational horizon. Although these two horizons are really four thousand miles apart, they appear to meet at the distance of the celestial sphere; a line four thousand miles long at the distance of the celestial sphere becoming a mere point, far too small to be detected with the most powerful telescope.

4. _Rotation of the Celestial Sphere._--It is well known that the sun and the majority of the stars rise in the east, and set in the west. In our latitude there are certain stars in the north which never disappear below the horizon. These stars are called the _circumpolar_ stars. A close watch, however, reveals the fact that these all appear to revolve around one of their number called the _pole star_, in the direction indicated by the arrows in Fig. 7. In a word, the whole heavens appear to rotate once a day, from east to west, about an axis, which is the prolongation of the axis of the earth. The ends of this axis are called the _poles_ of the heavens; and the great circle of the heavens, midway between these poles, is called the _celestial equator_, or the _equinoctial_. This rotation of the heavens is apparent only, being due to the rotation of the earth from west to east.

5. _Diurnal Circles._--In this rotation of the heavens, the stars appear to describe circles which are perpendicular to the celestial axis, and parallel with the celestial equator. These circles are called _diurnal circles_. The position of the poles in the heavens and the direction of the diurnal circles with reference to the horizon, change with the position of the observer on the earth. This is owing to the fact that the horizon changes with the position of the observer.

When the observer is north of the equator, the north pole of the heavens is _elevated_ above the horizon, and the south pole is _depressed_ below it, and the diurnal circles are _oblique_ to the horizon, leaning to the south. This case is represented in Fig. 8, in which _PP'_ represents the celestial axis, _EQ_ the celestial equator, _SN_ the horizon, and _ab_, _cN_, _de_, _fg_, _Sh_, _kl_, diurnal circles. _O_ is the point of observation, _Z_ the zenith, and _Z'_ the nadir.

When the observer is south of the equator, as at _O_ in Fig. 9, the south pole is _elevated_, the north pole _depressed_, and the diurnal circles are _oblique_ to the horizon, leaning to the north. When the diurnal circles are oblique to the horizon, as in Figs. 8 and 9, the celestial sphere is called an _oblique sphere_.

When the observer is at the equator, as in Fig. 10, the poles of the heavens are on the horizon, and the diurnal circles are _perpendicular_ to the horizon.

When the observer is at one of the poles, as in Fig. 11, the poles of the heavens are in the zenith and the nadir, and the diurnal circles are _parallel_ with the horizon.

6. _Elevation of the Pole and of the Equinoctial._--At the equator the poles of the heavens lie on the horizon, and the celestial equator passes through the zenith. As a person moves north from the equator, his zenith moves north from the celestial equator, and his horizon moves down from the north pole, and up from the south pole. The distance of the zenith from the equinoctial, and of the horizon from the celestial poles, will always be equal to the distance of the observer from the equator. In other words, the elevation of the pole is equal to the latitude of the place. In Fig. 12, _O_ is the point of observation, _Z_ the zenith, and _SN_ the horizon. _NP_, the elevation of the pole, is equal to _ZE_, the distance of the zenith from the equinoctial, and to the distance of _O_ from the equator, or the latitude of the place.

Two angles, or two arcs, which together equal 90°, are said to be _complements_ of each other. _ZE_ and _ES_ in Fig. 12 are together equal to 90°: hence they are complements of each other. _ZE_ is equal to the latitude of the place, and _ES_ is the _elevation_ of the equinoctial above the horizon: hence the elevation of the equinoctial is equal to the complement of the latitude of the place.

Were the observer south of the equator, the zenith would be south of the equinoctial, and the south pole of the heavens would be the elevated pole.

_7. Four Sets of Stars._--At most points of observation there are four sets of stars. These four sets are shown in Fig. 13.

(1) The stars in the neighborhood of the elevated pole _never set_. It will be seen from Fig. 13, that if the distance of a star from the elevated pole does not exceed the elevation of the pole, or the latitude of the place, its diurnal circle will be wholly above the horizon. As the observer approaches the equator, the elevation of the pole becomes less and less, and the belt of circumpolar stars becomes narrower and narrower: at the equator it disappears entirely. As the observer approaches the pole, the elevation of the pole increases, and the belt of circumpolar stars becomes broader and broader, until at the pole it includes half of the heavens. At the poles, no stars rise or set, and only half of the stars are ever seen at all.

(2) The stars in the neighborhood of the depressed pole _never rise_. The breadth of this belt also increases as the observer approaches the pole, and decreases as he approaches the equator, to vanish entirely when he reaches the equator. The distance from the depressed pole to the margin of this belt is always equal to the latitude of the place.

(3) The stars in the neighborhood of the equinoctial, on the side of the elevated pole, _set, but are above the horizon longer than they are below it_. This belt of stars extends from the equinoctial to a point whose distance from the elevated pole is equal to the latitude of the place: in other words, the breadth of this third belt of stars is equal to the complement of the latitude of the place. Hence this belt of stars becomes broader and broader as the observer approaches the equator, and narrower and narrower as he approaches the pole. However, as the observer approaches the equator, the horizon comes nearer and nearer the celestial axis, and the time a star is below the horizon becomes more nearly equal to the time it is above it. As the observer approaches the pole, the horizon moves farther and farther from the axis, and the time any star of this belt is below the horizon becomes more and more unequal to the time it is above it. The farther any star of this belt is from the equinoctial, the longer the time it is above the horizon, and the shorter the time it is below it.

(4) The stars which are in the neighborhood of the equinoctial, on the side of the depressed pole, _rise, but are below the horizon longer than they are above it_. The width of this belt is also equal to the complement of the latitude of the place. The farther any star of this belt is from the equinoctial, the longer time it is below the horizon, and the shorter time it is above it; and, the farther the place from the equator, the longer every star of this belt is below the horizon, and the shorter the time it is above it.

At the equator every star is above the horizon just half of the time; and any star on the equinoctial is above the horizon just half of the time in every part of the earth, since the equinoctial and horizon, being great circles, bisect each other.

8. _Vertical Circles._--Great circles perpendicular to the horizon are called _vertical circles_. All vertical circles pass through the zenith and nadir. A number of these circles are shown in Fig. 14, in which _SENW_ represents the horizon, and _Z_ the zenith.

The vertical circle which passes through the north and south points of the horizon is called the _meridian_; and the one which passes through the east and west points, the _prime vertical_. These two circles are shown in Fig. 15; _SZN_ being the meridian, and _EZW_ the prime vertical. These two circles are at right angles to each other, or 90° apart; and consequently they divide the horizon into four quadrants.

9. _Altitude and Zenith Distance._--The _altitude_ of a heavenly body is its distance above the horizon, and its _zenith distance_ is its distance from the zenith. Both the altitude and the zenith distance of a body are measured on the vertical circle which passes through the body. The altitude and zenith distance of a heavenly body are complements of each other.

10. _Azimuth and Amplitude.--Azimuth_ is distance measured east or west from the meridian. When a heavenly body lies north of the prime vertical, its azimuth is measured from the meridian on the north; and, when it lies south of the prime vertical, its azimuth is measured from the meridian on the south. The azimuth of a body can, therefore, never exceed 90°. The azimuth of a body is the angle which the plane of the vertical circle passing through it makes with that of the meridian.

The _amplitude_ of a body is its distance measured north or south from the prime vertical. The amplitude and azimuth of a body are complements of each other.

11. _Alt-azimuth Instrument._--An instrument for measuring the altitude and azimuth of a heavenly body is called an _alt-azimuth_ instrument. One form of this instrument is shown in Fig. 16. It consists essentially of a telescope mounted on a vertical circle, and capable of turning on a horizontal axis, which, in turn, is mounted on the vertical axis of a horizontal circle. Both the horizontal and the vertical circles are graduated, and the horizontal circle is placed exactly parallel with the plane of the horizon.

When the instrument is properly adjusted, the axis of the telescope will describe a vertical circle when the telescope is turned on the horizontal axis, no matter to what part of the heavens it has been pointed.

The horizontal and vertical axes carry each a pointer. These pointers move over the graduated circles, and mark how far each axis turns.

To find the _azimuth_ of a star, the instrument is turned on its vertical axis till its vertical circle is brought into the plane of the meridian, and the reading of the horizontal circle noted. The telescope is then directed to the star by turning it on both its vertical and horizontal axes. The reading of the horizontal circle is again noted. The difference between these two readings of the horizontal circle will be the azimuth of the star.

To find the _altitude_ of a star, the reading of the vertical circle is first ascertained when the telescope is pointed horizontally, and again when the telescope is pointed at the star. The difference between these two readings of the vertical circle will be the altitude of the star.

12. _The Vernier._--To enable the observer to read the fractions of the divisions on the circles, a device called a _vernier_ is often employed. It consists of a short, graduated arc, attached to the end of an arm _c_ (Fig. 17), which is carried by the axis, and turns with the telescope. This arc is of the length of _nine_ divisions on the circle, and it is divided into _ten_ equal parts. If 0 of the vernier coincides with any division, say 6, of the circle, 1 of the vernier will be 1/10 of a division to the left of 7, 2 will be 2/10 of a division to the left of 8, 3 will be 3/10, of a division to the left of 9, etc. Hence, when 1 coincides with 7, 0 will be at 6-1/10; when 2 coincides with 8, 0 will be at 6-2/10; when 3 coincides with 9, 0 will be at 6-3/10, etc.

To ascertain the reading of the circle by means of the vernier, we first notice the zero line. If it exactly coincides with any division of the circle, the number of that division will be the reading of the circle. If there is not an exact coincidence of the zero line with any division of the circle, we run the eye along the vernier, and note which of its divisions does coincide with a division of the circle. The reading of the circle will then be the number of the first division on the circle behind the 0 of the vernier, and a number of tenths equal to the number of the division of the vernier, which coincides with a division of the circle. For instance, suppose 0 of the vernier beyond 6 of the circle, and 7 of the vernier to coincide with 13 of the circle. The reading of the circle will then be 6-7/10.

13. _Hour Circles._--Great circles perpendicular to the celestial equator are called _hour circles_. These circles all pass through the poles of the heavens, as shown in Fig. 18. _EQ_ is the celestial equator, and _P_ and _P'_ are the poles of the heavens.

The point _A_ on the equinoctial (Fig. 19) is called the _vernal equinox_, or the _first point of Aries_. The hour circle, _APP'_, which passes through it, is called the _equinoctial colure_.