Why “Ephemeris” Becomes “Ephemerides”
An ephemeris is a table or reference book listing where celestial bodies are expected to appear at particular times. Its plural, ephemerides, comes through Latin from a Greek word related to a daily record. The ending changes rather than adding the familiar English -s, much as index can become indices.
A Table for a Particular Moment
An ephemeris is useful because the planets and Moon do not stay fixed against the background of the sky. Their listed positions vary with the date and time, so readers look up the relevant entry rather than rely on a single diagram. Astronomers have used such tables to track celestial positions; astrologers use them to find the planetary positions needed to interpret a chart.
That practical role explains why the word often appears in astrology: an ephemeris is a lookup tool, not the interpretation itself. One book is an ephemeris; several reference books, or tables covering different periods, are ephemerides.
