The celestial sphere can be understood by imagining that all the objects in the sky are stuck against the inside of a gigantic hollow sphere, with Earth located at the exact center. The basic notion of the celestial sphere is assumed in various systems for locating celestial bodies in terms of celestial coordinates. Very much like terrestrial coordinates, which involve specifying a location in terms of longitude and latitude, celestial coordinates require two measurements of distance, expressed in terms of degrees of a circle. The altitude-azimuth system begins by situating itself at a specific location on the Earth's surface, and uses the horizon, the zenith, and the north-south axis as points of reference. The equatorial system uses the celestial equator (Earth's equator extended out into space and projected against the backdrop of the celestial sphere) and the vernal point (where the Sun is located at the point of the spring equinox) as its points of reference. Astronomers most often use the equatorial system. The ecliptic system uses the ecliptic (the orbit of Earth around the Sun, projected outward against the celestial sphere) and the vernal point. Astrologers use the ecliptic system.
Brau, Jean-Louis, Helen Weaver, and Allan Edmands.
Larousse Encyclopedia of Astrology. New York: New American Library, 1980. Filbey, John, and Peter Filbey. The Astrologer's Companion. Wellingborough, Northamptonshire, UK: Aquarian Press, 1986.
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