Ephemeris — Astrology glossary

An ephemeris is a table or dataset giving the calculated positions of the Sun, Moon, and planets for regular intervals of time, usually each day at noon or midnight. Astrologers use it to find where every body sits in the zodiac for a given birth date, which is the raw material for casting a chart. Before software, practitioners read printed ephemerides such as the long-running Swiss or Raphael's editions and interpolated for the exact birth time; today the same data is computed instantly, often from the Swiss Ephemeris engine. It matters because the accuracy of an entire natal chart depends on these positions: an error in the underlying ephemeris propagates into every placement and aspect. For example, to locate a 14 March 1990 Sun, an astrologer reads that day's solar longitude from the ephemeris.