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The great circle through the celestial poles and the object (such as a star) is the object's Hour Circle and the great circle which passes through the celestial poles and the zenith is the Meridian Circle. The coordinates in this system are given by Declination (angle between the celestial equator and the object) and the Right Ascension (angle measured from an arbitrary reference direction -- the Vernal Equinox -- to the object's hour circle). In Figure C, the darker lines express the position of a star (S) in both ecliptic and equatorial coordinates.
Equatorial Sphere -- Also called the Celestial Sphere, this is the sphere resulting from projecting the Earth infinitely into space and it is defined by the celestial equator and the north and south celestial poles.
Celestial Poles -- These are directly overhead the Earth's geographic poles and are the poles of rotation of the celestial sphere of right ascension and declination.
NCP -- North Celestial Pole
SCP -- South Celestial Pole
Celestial Equator -- A great circle projected from the Earth's equator unto the heavens, an infinite projection. The celestial equator has as its poles the celestial poles and all points on the celestial equator are equidistant from the two poles. As the Earth's equator rotates each day, it exposes each city on the equator to every degree of the celestial equator.
Equinoctial -- Another term for the celestial equator (which see).
Celestial Sphere -- An infinite extension of the sphere of the Earth in space.
Hour Circles -- Hour circles are great circles passing through any celestial object and through the celestial poles. All hour circles are at right angles to the celestial equator.
R.A. -- (See Right Ascension)
Right Ascension (R.A.) -- The angle between an hour circle passing through an object and the meridian plane, in the case of equatorial coordinates, zero degrees of Aries. R.A. is measured eastward on the celestial equator from what is called the True Equinox to the body in question. R.A. is expressed in either degrees (0° to 360°) or in Hours-Minutes-Seconds (0h to 24h).
Declination -- The angular distance of any object measured north or south of the plane of the celestial equator, from 0° to 90°.
Declination Circle -- Parallel circles of declination either north or south of the plane of the celestial equator.
Parallel of Declination -- Parallel circles of declination either north or south of the plane of the celestial equator.
Equatorial Plane -- The infinite extension of the equator of the Earth in all directions.
Meridian -- A great circle on the celestial sphere through the celestial poles, the zenith of any place or locale, and the north and south points on the horizon.
Prime Meridian -- The point of zero longitude measurement for any celestial body. In the case of the Earth, the prime meridian is considered to be Greenwich, England.
Transit -- The passage of a celestial object across a particular meridian.
Copyright (c) 1997 Michael Erlewine
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