Chapter 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_.

14. _Declination and Right Ascension._--The _declination_ of a heavenly body is its distance north or south of the celestial equator. The _polar distance_ of a heavenly body is its distance from the nearer pole. Declination and polar distance are measured on hour circles, and for the same heavenly body they are complements of each other.

The _right ascension_ of a heavenly body is its distance eastward from the first point of Aries, measured from the equinoctial colure. It is equal to the arc of the celestial equator included between the first point of Aries and the hour circle which passes through the heavenly body. As right ascension is measured eastward entirely around the celestial sphere, it may have any value from 0° up to 360°. Right ascension corresponds to longitude on the earth, and declination to latitude.

15. _The Meridian Circle._--The right ascension and declination of a heavenly body are ascertained by means of an instrument called the _meridian circle_, or _transit instrument_. A side-view of this instrument is shown in Fig. 20.

It consists essentially of a telescope mounted between two piers, so as to turn in the plane of the meridian, and carrying a graduated circle. The readings of this circle are ascertained by means of fixed microscopes, under which it turns. A heavenly body can be observed with this instrument, only when it is crossing the meridian. For this reason it is often called the _transit circle_.

To find the declination of a star with this instrument, we first ascertain the reading of the circle when the telescope is pointed to the pole, and then the reading of the circle when pointed to the star on its passage across the meridian. The difference between these two readings will be the polar distance of the star, and the complement of them the declination of the star.

To ascertain the reading of the circle when the telescope is pointed to the pole, we must select one of the circumpolar stars near the pole, and then point the telescope to it when it crosses the meridian, both above and below the pole, and note the reading of the circle in each case. The mean of these two readings will be the reading of the circle when the telescope is pointed to the pole.

16. _Astronomical Clock._--An _astronomical clock_, or _sidereal clock_ as it is often called, is a clock arranged so as to mark hours from 1 to 24, instead of from 1 to 12, as in the case of an ordinary clock, and so adjusted as to mark 0 when the vernal equinox, or first point of Aries, is on the meridian.

As the first point of Aries makes a complete circuit of the heavens in twenty-four hours, it must move at the rate of 15° an hour, or of 1° in four minutes: hence, when the astronomical clock marks 1, the first point of Aries must be 15° west of the meridian, and when it marks 2, 30° west of the meridian, etc. That is to say, by observing an accurate astronomical clock, one can always tell how far the meridian at any time is from the first point of Aries.

17. _How to find Right Ascension with the Meridian Circle._--To find the right ascension of a heavenly body, we have merely to ascertain the exact time, by the astronomical clock, at which the body crosses the meridian. If a star crosses the meridian at 1 hour 20 minutes by the astronomical clock, its right ascension must be 19°; if at 20 hours, its right ascension must be 300°.

To enable the observer to ascertain with great exactness the time at which a star crosses the meridian, a number of equidistant and parallel spider-lines are stretched across the focus of the telescope, as shown in Fig. 21. The observer notes the time when the star crosses each spider-line; and the mean of all of these times will be the time when the star crosses the meridian. The mean of several observations is likely to be more nearly exact than any single observation.

18. _The Equatorial Telescope._--The _equatorial_ telescope is mounted on two axes,--one parallel with the axis of the earth, and the other at right angles to this, and therefore parallel with the plane of the earth's equator. The former is called the _polar axis_, and the latter the _declination axis_. Each axis carries a graduated circle. These circles are called respectively the _hour circle_ and the _declination circle_. The telescope is attached directly to the declination axis. When the telescope is fixed in any declination, and then turned on its polar axis, the line of sight will describe a diurnal circle; so that, when the tube is once directed to a star, it can be made to follow the star by simply turning the telescope on its polar axis.

In the case of large instruments of this class, the polar axis is usually turned by clock-work at the rate at which the heavens rotate; so that, when the telescope has once been pointed to a planet or other heavenly body, it will continue to follow the body and keep it steadily in the field of view without further trouble on the part of the observer.

The great Washington Equatorial is shown in Fig. 22. Its object-glass is 26 inches in diameter, and its focal length is 32-1/2 feet. It was constructed by Alvan Clark & Sons of Cambridge, Mass. It is one of the three largest refracting telescopes at present in use. The Newall refractor at Gateshead, Eng., has an objective 25 inches in diameter, and a focal length of 29 feet. The great refractor at Vienna has an objective 27 inches in diameter. There are several large refractors now in process of construction.

19. _The Wire Micrometer._--Large arcs in the heavens are measured by means of the graduated circles attached to the axes of the telescopes; but small arcs within the field of view of the telescope are measured by means of instruments called _micrometers_, mounted in the focus of the telescope. One of the most convenient of these micrometers is that known as the _wire micrometer_, and shown in Fig. 23.

The frame _AA_ covers two slides, _C_ and _D_. These slides are moved by the screws _F_ and _G_. The wires _E_ and _B_ are stretched across the ends of the slides so as to be parallel to each other. On turning the screws _F_ and _G_ one way, these wires are carried apart; and on turning them the other way they are brought together again. Sometimes two parallel wires, _x_ and _y_, shown in the diagram at the right, are stretched across the frame at right angles to the wires _E_, _B_. We may call the wires _x_ and _y_ the _longitudinal_ wires of the micrometer, and _E_ and _B_ the _transverse_ wires. Many instruments have only one longitudinal wire, which is stretched across the middle of the focus. The longitudinal wires are just in front of the transverse wires, but do not touch them.

To find the distance between any two points in the field of view with a micrometer, with a single longitudinal wire, turn the frame till the longitudinal wire passes through the two points; then set the wires _E_ and _B_ one on each point, turn one of the screws, known as the _micrometer screw_, till the two wires are brought together, and note the number of times the screw is turned. Having previously ascertained over what arc one turn of the screw will move the wire, the number of turns will enable us to find the length of the arc between the two points.

The threads of the micrometer screw are cut with great accuracy; and the screw is provided with a large head, which is divided into a hundred or more equal parts.

These divisions, by means of a fixed pointer, enable us to ascertain what fraction of a turn the screw has made over and above its complete revolutions.

20. _Reflecting Telescopes._--It is possible to construct mirrors of much larger size than lenses: hence reflecting telescopes have an advantage over refracting telescopes as regards size of aperture and of light-gathering power. They are, however, inferior as regards definition; and, in order to prevent flexure, it is necessary to give the speculum, or mirror, a massiveness which makes the telescope unwieldy. It is also necessary frequently to repolish the speculum; and this is an operation of great delicacy, as the slightest change in the form of the surface impairs the definition of the image. These defects have been remedied, to a certain extent, by the introduction of silver-on-glass mirrors; that is, glass mirrors covered in front with a thin coating of silver. Glass is only one-third as heavy as speculum-metal, and silver is much superior to that metal in reflecting power; and when the silver becomes tarnished, it can be removed and renewed without danger of changing the form of the glass.

_The Herschelian Reflector._--In this form of telescope the mirror is slightly tipped, so that the image, instead of being formed in the centre of the tube, is formed near one side of it, as in Fig. 24. The observer can then view it without putting his head inside the tube, and therefore without cutting off any material portion of the light. In observation, he must stand at the upper or outer end of the tube, and look into it, his back being turned towards the object. From his looking directly into the mirror, it is also sometimes called the _front-view_ telescope. The great disadvantage of this arrangement is, that the rays cannot be brought to an exact focus when they are thrown so far to one side of the axis, and the injury to the definition is so great that the front-view plan is now entirely abandoned.

_The Newtonian Reflector._--The plan proposed by Sir Isaac Newton was to place a small plane mirror just inside the focus, inclined to the telescope at an angle of 45°, so as to throw the rays to the side of the tube, where they come to a focus, and form the image. An opening is made in the side of the tube, just below where the image is formed; and in this opening the eye-piece is inserted. The small mirror cuts off some of the light, but not enough to be a serious defect. An improvement which lessens this defect has been made by Professor Henry Draper. The inclined mirror is replaced by a small rectangular prism (Fig. 25), by reflection from which the image is formed very near the prism. A pair of lenses are then inserted in the course of the rays, by which a second image is formed at the opening in the side of the tube; and this second image is viewed by an ordinary eye-piece.

_The Gregorian Reflector._--This is a form proposed by James Gregory, who probably preceded Newton as an inventor of the reflecting telescope. Behind the focus, _F_ (Fig. 26), a small concave mirror, _R_, is placed, by which the light is reflected back again down the tube. The larger mirror, _M_, has an opening through its centre; and the small mirror, _R_, is so adjusted as to form a second image of the object in this opening. This image is then viewed by an eye-piece which is screwed into the opening.

_The Cassegrainian Reflector._--In principle this is the same with the Gregorian; but the small mirror, _R_, is convex, and is placed inside the focus, _F_, so that the rays are reflected from it before reaching the focus, and no image is formed until they reach the opening in the large mirror. This form has an advantage over the Gregorian, in that the telescope may be made shorter, and the small mirror can be more easily shaped to the required figure. It has, therefore, entirely superseded the original Gregorian form.

Optically these forms of telescope are inferior to the Newtonian; but the latter is subject to the inconvenience, that the observer must be stationed at the upper end of the telescope, where he looks into an eye-piece screwed into the side of the tube.

On the other hand, the Cassegrainian Telescope is pointed directly at the object to be viewed, like a refractor; and the observer stands at the lower end, and looks in at the opening through the large mirror. This is, therefore, the most convenient form of all in management.

The largest reflecting telescope yet constructed is that of Lord Rosse, at Parsonstown, Ireland. Its speculum is 6 feet in diameter, and its focal length 55 feet. It is commonly used as a Newtonian. This telescope is shown in Fig. 27.

The great telescope of the Melbourne Observatory, Australia, is a Cassegranian reflector. Its speculum is 4 feet in diameter, and its focal length is 32 feet. It is shown in Fig. 28.

The great reflector of the Paris Observatory is a Newtonian reflector. Its mirror of silvered glass is 4 feet in diameter, and its focal length is 23 feet. This telescope is shown in Fig. 29.

21. _The Sun's Motion among the Stars._--If we notice the stars at the same hour night after night, we shall find that the constellations are steadily advancing towards the west. New constellations are continually appearing in the east, and old ones disappearing in the west. This continual advancing of the heavens towards the west is due to the fact that the sun's place among the stars is _continually moving towards the east_. The sun completes the circuit of the heavens in a year, and is therefore moving eastward at the rate of about a degree a day.

This motion of the sun's place among the stars is due to the revolution of the earth around the sun, and not to any real motion of the sun. In Fig. 30 suppose the inner circle to represent the orbit of the earth around the sun, and the outer circle to represent the celestial sphere. When the earth is at _E_, the sun's place on the celestial sphere is at _S'_. As the earth moves in the direction _EF_, the sun's place on the celestial sphere must move in the direction _S'T_: hence the revolution of the earth around the sun would cause the sun's place among the stars to move around the heavens in the same direction that the earth is moving around the sun.

22. _The Ecliptic._--The circle described by the sun in its apparent motion around the heavens is called the _ecliptic_. The plane of this circle passes through the centre of the earth, and therefore through the centre of the celestial sphere; the earth being so small, compared with the celestial sphere, that it practically makes no difference whether we consider a point on its surface, or one at its centre, as the centre of the celestial sphere. The ecliptic is, therefore, a great circle.

The earth's orbit lies in the plane of the ecliptic; but it extends only an inappreciable distance from the sun towards the celestial sphere.

23. _The Obliquity of the Ecliptic._--The ecliptic is inclined to the celestial equator by an angle of about 23-1/2°. This inclination is called the _obliquity of the ecliptic_. The obliquity of the ecliptic is due to the deviation of the earth's axis from a perpendicular to the plane of its orbit. The axis of a rotating body tends to maintain the same direction; and, as the earth revolves around the sun, its axis points all the time in nearly the same direction. The earth's axis deviates about 23-1/2° from the perpendicular to its orbit; and, as the earth's equator is at right angles to its axis, it will deviate about 23-1/2° from the plane of the ecliptic. The celestial equator has the same direction as the terrestrial equator, since the axis of the heavens has the same direction as the axis of the earth.

Suppose the globe at the centre of the tub (Fig. 31) to represent the sun, and the smaller globes to represent the earth in various positions in its orbit. The surface of the water will then represent the plane of the ecliptic, and the rod projecting from the top of the earth will represent the earth's axis, which is seen to point all the time in the same direction, or to lean the same way. The leaning of the axis from the perpendicular to the surface of the water would cause the earth's equator to be inclined the same amount to the surface of the water, half of the equator being above, and half of it below, the surface. Were the axis of the earth perpendicular to the surface of the water, the earth's equator would coincide with the surface, as is evident from Fig. 32.

24. _The Equinoxes and Solstices._--The ecliptic and celestial equator, being great circles, bisect each other. Half of the ecliptic is north, and half of it is south, of the equator. The points at which the two circles cross are called the _equinoxes_. The one at which the sun crosses the equator from south to north is called the _vernal_ equinox, and the one at which it crosses from north to south the _autumnal_ equinox. The points on the ecliptic midway between the equinoxes are called the _solstices_. The one north of the equator is called the _summer_ solstice, and the one south of the equator the _winter_ solstice. In Fig. 33, _EQ_ is the celestial equator, _EcE'c'_ the ecliptic, _V_ the vernal equinox, A the autumnal equinox, Ec the winter solstice, and _E'c'_ the summer solstice.

25. _The Inclination of the Ecliptic to the Horizon._--Since the celestial equator is perpendicular to the axis of the heavens, it makes the same angle with it on every side: hence, at any place, the equator makes always the same angle with the horizon, whatever part of it is above the horizon. But, as the ecliptic is oblique to the equator, it makes different angles with the celestial axis on different sides; and hence, at any place, the angle which the ecliptic makes with the horizon varies according to the part which is above the horizon. The two extreme angles for a place more than 23-1/2° north of the equator are shown in Figs. 34 and 35.

The least angle is formed when the vernal equinox is on the eastern horizon, the autumnal on the western horizon, and the winter solstice on the meridian, as in Fig. 34. The angle which the ecliptic then makes with the horizon is equal to the elevation of the equinoctial _minus_ 23-1/2°. In the latitude of New York this angle = 49° - 23-1/2° = 25-1/2°.

The greatest angle is formed when the autumnal equinox is on the eastern horizon, the vernal on the western horizon, and the summer solstice is on the meridian (Fig. 35). The angle between the ecliptic and the horizon is then equal to the elevation of the equinoctial _plus_ 23-1/2°. In the latitude of New York this angle = 49° + 23-1/2° = 72-1/2°.

Of course the equinoxes, the solstices, and all other points on the ecliptic, describe diurnal circles, like every other point in the heavens: hence, in our latitude, these points rise and set every day.

26. _Celestial Latitude and Longitude._--_Celestial latitude_ is distance measured north or south from the ecliptic; and _celestial longitude_ is distance measured on the ecliptic eastward from the vernal equinox, or the first point of Aries. Great circles perpendicular to the ecliptic are called _celestial meridians_. These circles all pass through the poles of the ecliptic, which are some 23-1/2° from the poles of the equinoctial. The latitude of a heavenly body is measured by the arc of a celestial meridian included between the body and the ecliptic. The longitude of a heavenly body is measured by the arc of the ecliptic included between the first point of Aries and the meridian which passes through the body. There are, of course, always two arcs included between the first point of Aries and the meridian,--one on the east, and the other on the west, of the first point of Aries. The one on the _east_ is always taken as the measure of the longitude.

27. _The Precession of the Equinoxes._--The equinoctial points have a slow westward motion along the ecliptic. This motion is at the rate of about 50'' a year, and would cause the equinoxes to make a complete circuit of the heavens in a period of about twenty-six thousand years. It is called the _precession of the equinoxes_. This westward motion of the equinoxes is due to the fact that the axis of the earth has a slow gyratory motion, like the handle of a spinning-top which has begun to wabble a little. This gyratory motion causes the axis of the heavens to describe a cone in about twenty-six thousand years, and the pole of the heavens to describe a circle about the pole of the ecliptic in the same time. The radius of this circle is 23-1/2°.

28. _Illustration of Precession._--The precession of the equinoxes
may be illustrated by means of the apparatus shown in Fig. 36. The
horizontal and stationary ring _EC_ represents the ecliptic; the
oblique ring _E'Q_ represents the equator; _V_ and _A_ represent the
equinoctial point, and _E_ and _C_ the solstitial points; _B_
represents the pole of the ecliptic, _P_ the pole of the equator,
and _PO_ the celestial axis. The ring _E'Q_ is supported on a pivot
at _O_; and the rod _BP_, which connects _B_ and _P_, is jointed at
each end so as to admit of the movement of _P_ and _B_.

On carrying _P_ around _B_, we shall see that _E'Q_ will always
preserve the same obliquity to _EC_, and that the points _V_ and _A_
will move around the circle _EC_. The same will also be true of the
points _E_ and _C_.

29. _Effects of Precession._--One effect of precession, as has already been stated, is the revolution of the pole of the heavens around the pole of the ecliptic in a period of about twenty-six thousand years. The circle described by the pole of the heavens, and the position of the pole at various dates, are shown in Fig. 37, where o indicates the position of the pole at the birth of Christ. The numbers round the circle to the left of o are dates A.D., and those to the right of o are dates B.C. It will be seen that the star at the end of the Little Bear's tail, which is now near the north pole, will be exactly at the pole about the year 2000. It will then recede farther and farther from the pole till the year 15000 A.D., when it will be about forty-seven degrees away from the pole. It will be noticed that one of the stars of the Dragon was the pole star about 2800 years B.C. There are reasons to suppose that this was about the time of the building of the Great Pyramid.

A second effect of precession is the shifting of the signs along the zodiac. The _zodiac_ is a belt of the heavens along the ecliptic, extending eight degrees from it on each side. This belt is occupied by twelve constellations, known as the _zodiacal constellations_. They are _Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_, _Libra_, _Scorpio_, _Sagittarius_, _Capricornus_, _Aquarius_, and _Pisces_. The zodiac is also divided into twelve equal parts of thirty degrees each, called _signs_. These signs have the same names as the twelve zodiacal constellations, and when they were first named, each sign occupied the same part of the zodiac as the corresponding constellation; that is to say, the sign Aries was in the constellation Aries, and the sign Taurus in the constellation Taurus, etc. Now the signs are always reckoned as beginning at the vernal equinox, which is continually shifting along the ecliptic; so that the signs are continually moving along the zodiac, while the constellations remain stationary: hence it has come about that the _first point of Aries_ (the _sign_) is no longer in the _constellation_ Aries, but in Pisces.

Fig. 38 shows the position of the vernal equinox 2170 B.C. It was then in Taurus, just south of the Pleiades. It has since moved from Taurus, through Aries, and into Pisces, as shown in Fig. 39.

Since celestial longitude and right ascension are both measured from the first point of Aries, the longitude and right ascension of the stars are slowly changing from year to year. It will be seen, from Figs. 38 and 39, that the declination is also slowly changing.

30. _Nutation._--The gyratory motion of the earth's axis is not perfectly regular and uniform. The earth's axis has a slight tremulous motion, oscillating to and fro through a short distance once in about nineteen years. This tremulous motion of the axis causes the pole of the heavens to describe an undulating curve, as shown in Fig. 40, and gives a slight unevenness to the motion of the equinoxes along the ecliptic. This nodding motion of the axis is called _nutation_.

31. _Refraction._--When a ray of light from one of the heavenly bodies enters the earth's atmosphere obliquely, it will be bent towards a perpendicular to the surface of the atmosphere, since it will be entering a denser medium. As the ray traverses the atmosphere, it will be continually passing into denser and denser layers, and will therefore be bent more and more towards the perpendicular. This bending of the ray is shown in Fig. 41. A ray which started from _A_ would enter the eye at _C_, as if it came from _I_: hence a star at _A_ would appear to be at _I_.

Atmospheric refraction displaces all the heavenly bodies from the horizon towards the zenith. This is evident from Fig. 42. _OD_ is the horizon, and _Z_ the zenith, of an observer at _O_. Refraction would make a star at _Q_ appear at _P_: in other words, it would displace it towards the zenith. A star in the zenith is not displaced by refraction, since the rays which reach the eye from it traverse the atmosphere vertically. The farther a star is from the zenith, the more it is displaced by refraction, since the greater is the obliquity with which the rays from it enter the atmosphere.

At the horizon the displacement by refraction is about half a degree; but it varies considerably with the state of the atmosphere. Refraction causes a heavenly body to appear above the horizon longer than it really is above it, since it makes it appear to be on the horizon when it is really half a degree below it.

The increase of refraction towards the horizon often makes the sun, when near the horizon, appear distorted, the lower limb of the sun being raised more than the upper limb. This distortion is shown in Fig. 43. The vertical diameter of the sun appears to be considerably less than the horizontal diameter.

32. _Parallax._--_Parallax_ is the displacement of an object caused by a change in the point of view from which it is seen. Thus in Fig. 44, the top of the tower _S_ would be seen projected against the sky at _a_ by an observer at _A_, and at _b_ by an observer at _B_. In passing from _A_ to _B_, the top of the tower is displaced from _a_ to _b_, or by the angle _aSb_. This angle is called the parallax of _S_, as seen from _B_ instead of _A_.

The _geocentric parallax_ of a heavenly body is its displacement caused by its being seen from the surface of the earth, instead of from the centre of the earth. In Fig. 45, _R_ is the centre of the earth, and _O_ the point of observation on the surface of the earth. Stars at _S_, _S'_, and _S''_, would, from the centre of the earth, appear at _Q_, _Q'_, and _Q''_; while from the point _O_ on the surface of the earth, these same stars would appear at _P_, _P'_ and _P''_, being displaced from their position, as seen from the centre of the earth, by the angles _QSP_, _Q'S'P'_, and _Q''S''P''_. It will be seen that parallax displaces a body from the zenith towards the horizon, and that the parallax of a body is greatest when it is on the horizon. The parallax of a heavenly body when on the horizon is called its _horizontal parallax_. A body in the zenith is not displaced by parallax, since it would be seen in the same direction from both the centre and the surface of the earth.

The parallax of a body at _S'''_ is _Q'''S'''P_, which is seen to be greater than _QSP_; that is to say, the parallax of a heavenly body increases with its nearness to the earth. The distance and parallax of a body are so related, that, either being known, the other may be computed.

33. _Aberration._--_Aberration_ is a slight displacement of a star,
owing to an apparent change in the direction of the rays of light
which proceed from it, caused by the motion of the earth in its
orbit. If we walk rapidly in any direction in the rain, when the
drops are falling vertically, they will appear to come into our
faces from the direction in which we are walking. Our own motion has
apparently changed the direction in which the drops are falling.

In Fig. 46 let _A_ be a gun of a battery, from which a shot is fired
at a ship, _DE_, that is passing. Let _ABC_ be the course of the
shot. The shot enters the ship's side at _B_, and passes out at the
other side at _C_; but in the mean time the ship has moved from _E_
to _e_, and the part _B_, where the shot entered, has been carried
to _b_. If a person on board the ship could see the ball as it
crossed the ship, he would see it cross in the diagonal line _bC_;
and he would at once say that the cannon was in the direction of
_Cb_. If the ship were moving in the opposite direction, he would
say that the cannon was just as far the other side of its true
position.

Now, we see a star in the direction in which the light coming from
the star appears to be moving. When we examine a star with a
telescope, we are in the same condition as the person who on
shipboard saw the cannon-ball cross the ship. The telescope is
carried along by the earth at the rate of eighteen miles a second:
hence the light will appear to pass through the tube in a slightly
different direction from that in which it is really moving: just as
the cannon-ball appears to pass through the ship in a different
direction from that in which it is really moving. Thus in Fig. 47, a
ray of light coming in the direction _SOT_ would appear to traverse
the tube _OT_ of a telescope, moving in the direction of the arrow,
as if it were coming in the direction _S'O_.

As light moves with enormous velocity, it passes through the tube so
quickly, that it is apparently changed from its true direction only
by a very slight angle: but it is sufficient to displace the star.
This apparent change in the direction of light caused by the motion
of the earth is called _aberration of light_.

34. _The Planets._--On watching the stars attentively night after night, it will be found, that while the majority of them appear _fixed_ on the surface of the celestial sphere, so as to maintain their relative positions, there are a few that _wander_ about among the stars, alternately advancing towards the east, halting, and retrograding towards the west. These wandering stars are called _planets_.

Their motions appear quite irregular; but, on the whole, their eastward motion is in excess of their westward, and in a longer or shorter time they all complete the circuit of the heavens. In almost every instance, their paths are found to lie wholly in the belt of the zodiac.

Fig. 48 shows a portion of the apparent path of one of the planets.

II.
THE SOLAR SYSTEM.