Ephemeris

A table or computational dataset that gives the precise position of celestial bodies (planets, the Sun, the Moon) at specified moments in time, calculated from gravitational mechanics and observation.

An ephemeris is the foundation of every accurate birth chart. It is a dataset that records, for any given date and time, where each planet sits in the sky as seen from Earth. Without an ephemeris there is no astrology, because there is no way to know which sign Mercury was in when you were born or where Saturn is right now.

The most widely used modern ephemeris in astrology software is the Swiss Ephemeris, derived from NASA JPL's DE431 planetary integration. This is the same gravitational model used to plan spacecraft trajectories. The positional accuracy for the inner planets is within fractions of an arcsecond across thousands of years.

ZODIA uses Swiss Ephemeris-grade calculations for every chart. When the system says your Venus is at 14 degrees Libra, that figure is derived from the same astronomical data NASA uses to track interplanetary missions. The astronomy is precise. The interpretation of what 14 degrees Libra means is the astrological layer on top.

Historically, ephemerides were published as printed books — the printed table of planetary positions for each day of a year. Astrologers calculated charts by hand using these tables and a set of trigonometric formulas. Modern software has automated the lookup, but the underlying data structure is the same.