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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 bo 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 -r—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, measured 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 Sour Circles (Right 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 Hoar Circles may be conceived as meridians of terrestrial longitude (15° apart) extended to the Celestial Sphere. (See Colures, p. 40.)
T/ie 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 Equinoctial and the Ecliptic (the sun's apparent path through the heavens) intersect.
(d) The Measurements are Bight 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) ia the distance from the Pole, measured on a vertical circle.
The Equinoctial System is largely used by modern astronomers, and accompanies the Equatorial Telescope, Sidereal Clock, and Chronographs of the best Observatories.//^
in. The Ecliptic System.
(o) The Principal Circle is the Ecliptic. This is the earth's orbit about the sun, or the apparent path of the sun in the heavens. It is inclined to the equinoctial 23° 28', which measures the inclination of the Earth's Equator to its orbit, and is called the obliquity of the ecliptic.
(b) The Subordinate Circles are Circles of Celestial Longitude, the Colures, and Parallels of Celestial Latitude.
The Circles of Celestial Longitude are now less employed. They are measured on the Ecliptic, as circles of Right Ascension (R. A.) are now measured on the Equinoctial.
The Colures are two principal meridians; the Equinoctial Colure is the meridian passing through the equinoxes; the Solstitial Colure is the meridian passing through the solstitial points.
The Parallels of Celestial Latitude are now little used, but are small circles drawn parallel to the ecliptic, as parallels of declination are now drawn parallel to the equinoctial.