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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.
(6) 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, measured East or West, on the horizon (to a vertical cirole 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 Prinoipal 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), the Coheres, 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.
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 Declination Parallels are 6mall 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 Right Ascension (R. A.), Declination, and Polar Distance.
Right Ascension is distance from the Vernal Equinox, measured on the equinoctial eastward. R. A. corresponds to terrestrial longitude, and may extend to 360° East, instead of 180° as on the earth. R. 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 Hour Circle or meridian North or South. It corresponds to terrestrial latitude.
Polar distance (the complement of Declination) is the distance from the Pole, measured on an hour circle.
The Equinoctial System is largely used by modern astronomers, and accompanies the Equatorial Telescope, Sidereal Clock, and Chronographs of the best Observatories.
III. The Ecliptic System.
(a) 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 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 (P. A.) are now measured on the Equinoctial.
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.
(c) The Points are the Poles of the Ecliptic, the Equinoxes, and the Solstices.
The Poles of the Ecliptic are the points where the axis of the earth's orbit meets the Celestial Sphere. (Little used.)
The Equinoxes are the points where the ecliptic intersects the equinoctial. The place where the sun crosses the equinoctial* in going North, which occurs about the 21st of March, is called the Vernal Equinox. The place where the sun crosses the equinoctial in going South, which occurs about the 21st of September, is called the Autumnal Equinox. The Solstices are the two points of the ecliptic most distant from the Equator; or they may be considered to mark the sun's furthest declination, North and South of the equinoctial. The Summer Solstice occurs about the 22d of June; the Winter Solstice occurs about the 22d of December.
(d) The Measurements are celestial longitude and latitude.
Celestial longitude is distance from the Vernal Equinox measured on the ecliptic, eastward.
Celestial latitude is distance from the ecliptic measured on a Subordinate circle, north or south.
A belt of the Celestial Sphere, 8° on each side of the ecliptic, is styled the Zodiac. This is of very
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