Ephemeris

A tabulation of position (sometimes velocity also) of a celestial object for (usually) regular intervals of time. Frequently, such tabulations are provided in printed form, but may also be recorded in machine-readable form to facilitate computer usage (such as magnetic tape or disk). When a 24-hour time-interval is used, values are given for midnight (0h) or noon (12h) in ET (Ephemeris Time) or UT (Universal Time). Often, the Moon's position is given each 12 hours. The AENA (American Ephemeris and Nautical Almanac) gives a lunar ephemeris (Right Ascension and Declination) for every hour of ET. For the outer planets (Jupiter through Pluto), tabulations at 40-day, and even 400-day intervals are not uncommon. And, to facilitate computer operations, the ephemeris may be represented in a compressed form, e.g., by some numerical approximation (such as Chebychev Polynomials). The plural of ephemeris is ephemerides (pronounced eph-a-mehr'-a-deez), and is a collection of such tabulations for more than one body, or covering a period longer than one year. A Geocentric ephemeris is computed to represent the position of a body as it would be "seen" from the center of the Earth; a Heliocentric ephemeris represents the position of a body as viewed from the center of the Sun. The ephemeris is the astrologer's primary tool.