Because of the variation in time caused by various time changes as one moves east or west across Earth, astrologers have found it convenient to construct tables such as ephemerides (tables of planetary positions) with the time at Greenwich, England, as a benchmark. Greenwich lies exactly on the 0° longitude line, which makes it relatively easy to determine the number of hours to add or subtract to local time in order to obtain Greenwich time (15° = 1 hour).
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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