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We now in imagination pass into space, which stretches out in every direction without bounds or measures. We look up to the heavens and try to locate some object among the mazes of the stars. We are bewildered, and immediately feel the necessity of some system of measurement. Let us try to understand the one adopted by astronomers.
The Celestial Sphere.—The blue arch of the sky, as it appears to be spread above t us, is termed the Celestial Sphere. There are two points to be noticed here. First, that so far distant is this imaginary arch from us, that if any two parallel lines from different parts of the earth are drawn to this sphere, they will apparently intersect. Of course this cannot be the fact; but the distance is so immense, that we are unable to distinguish the little difference of four or even eight thousand miles, and the two lines will seem to unite: so we must consider this great earth as a mere speck or point at the centre of the Celestial Sphere. Second, that we must even neglect the entire diameter of the earth's orbit, so that if we should draw two parallel lines, one from each end of the earth's orbit, to the sphere, although these lines would be 183,000,000 miles apart, yet they would be extended so far that we could not separate them, and they would appear to pierce the sphere at the same point; which is to say, that at that enormous distance, 183,000,000 miles shrink to a point. Consequently, in all parts of the earth, and in every part of the earth's orbit, we see the fixed stars in the same place. This sphere of stars surrounds the earth on every side. In the daytime we cannot see the stars because of the superior light of the sun; but with a telescope they can be traced, and an astronomer will find certain stars as well at noon as at midnight. Indeed, when looking at the sky from the bottom of a deep well or lofty chimney, if a bright star happens to be directly overhead, it can be seen with the naked eye even at midday. In this way it is said a celebrated optician was first led to think of there being stars by day as well as by night. One half of the sphere is constantly visible to us; and so far distant are the stars, that we see just as much of the sphere as we would if the upper part of the earth were removed, and we were to stand four thousand miles further away, or at the very centre of ^the earth, where our view Would be bounded by a great circle of the earth. On the concave surface of the celestial sphere there are imagined to be drawn three systems of circles: the HoriZon, the Equinoctial, and the Ecliptic Systems. Each of these has (1) its Principal Circle, (2) its Subordinate Circles, (3) its Points, and (4) its Measurements. /
I. The Horizon System.
(a) The Principal Circle is the Rational Horizon. This is the great circle that, passing through the centre of the earth, separates the visible from the invisible heavens. The Sensible Horizon is the small circle where the earth and sky seem to meet; it is parallel to the rational horizon, but distant from it the semi-diameter of the earth. No two places have the same sensible horizon: any two on opposite sides of the earth have the same rational horizon.
(b) The Subordinate Circles.—These are the Prime Vertical circle and the Meridian. A vertical circle is one passing through the poles of the horizon (the zenith and nadir). The Prime Vertical is a vertical circle passing through the East and West points. The Meridian is a vertical circle passing through the North and South points.
(c) Points.—These are the Zenith, the Nadir, the N., S., E., and W. points. The Zenith is the point directly overhead, and the Nadir the one directly underfoot. They are also the poles of the horizon —i. e., the points where the axis of the horizon pierces the celestial sphere. The N, S., E., and W. points are familiar to all.
(d) Measurements.—These are Azimuth, Amplitude, Altitude, and Zenith distance.
Azimuth is the distance from the meridian, measnred East or West, on the horizon (to a vertical circle passing through the object).
Amplitude (the complement of Azimuth) is the distance from the Prime Vertical, measured on the horizon, North or South.
Altitude is the distance from the horizon, measured on a vertical circle toward the zenith.
Zenith distance (the complement of Altitude) is the distance from the zenith, measured on a vertical circle, toward the horizon.
The Horizon System is the one commonly used in observations with Mural Circles and Transit Instruments..
II. The Equinoctial System.
(a) The Principal Circle is the Equinoctial. This is the Celestial Equator, or the earth's equator, extended to the Celestial Sphere.
(b) Subordinate Circles.—These are the Hour Circles (Bight Ascension Meridians) and the Declination Parallels. The Hour Circles are thus located. The Equinoctial is divided into 360°, equal to twenty-four hours of motion—thus making 15° equal to one hour of motion. Through these divisions run twenty-four meridians, each constituting an hour of motion (time) or 15° of space. The Hour Circles may be conceived as meridians of terrestrial longitude (15° apart) extended to the Celestial Sphere. (See Colures, p. 40.)
The Declination Parallels are small circles parallel to the Equinoctial; or they may be conceived as the parallels of terrestrial latitude extended to the Celestial Sphere.
(c) The Points are the Celestial Poles and the 'Equinoxes. The Celestial Poles are the points where the axis of the earth extended pierces the Celestial Sphere, and are the extremities of the celestial axis, just as the poles of the earth are the extremities of the earth's axis. The North Point is marked very nearly by the North Star, and every direction from that is reckoned South, and every direction toward that is reckoned North, however it may conflict with our ideas of the points of the compass.
The Equinoxes are the points where the Equi-' noctial and the Ecliptic (the sun's apparent path through the heavens) intersect￼
(d) The Measurements are Right Ascension (E. A.), Declination, and Polar Distance.
Bight Ascension is distance from the Vernal Equinox, measured on the equinoctial eastward. E. A. corresponds to terrestrial longitude, and may extend to 360° East, instead of 180° as on the earth. E. A. is never measured westward. The starting point is the meridian passing through the vernal equinox; as the meridian passing through Greenwich is the point from which terrestrial longitude is measured.
Declination is distance from the equinoctial, measured on any vertical circle or meridian North or South. It corresponds to terrestrial latitude.
Polar distance (the complement of Declination) is .