Astrology
Ephemeris
Definition
A table or dataset giving the precise positions of the Sun, Moon, and planets for each date and time — the astronomical data from which accurate astrological charts are computed.
An ephemeris is a table — historically printed, now digital — giving the precise positions of the Sun, Moon, and planets for each date and time. It’s the astronomical backbone of astrology: every natal chart and transit is calculated from ephemeris data.
The accuracy of a chart depends entirely on the quality of this data. The modern standard is the Swiss Ephemeris, derived from NASA’s JPL planetary data, which is why a well-built tool’s chart positions match a professional astronomer’s to the arc-minute. It’s also the layer that lets an AI mystic companion compute your chart precisely rather than approximate it.