ephemeris

An ephemeris is a table or data set that gives the predicted positions of astronomical objects as a function of time. Etymology: from Greek ephemeris, from ephēmeros meaning "lasting only a day" or "daily record."

Ephemerides specify coordinates (often right ascension and declination or ecliptic longitude/latitude), distances, and sometimes velocities, for objects such as planets, moons, asteroids, comets, or spacecraft, for given times. They can be geocentric, heliocentric, or topocentric, and may include apparent positions as seen from a particular location or the center of Earth.

Generation: derived from observations and dynamical models. They combine orbital mechanics with perturbations from gravitation of other bodies, relativistic corrections, and sometimes non-gravitational forces. Timescales such as International Atomic Time (TAI) or Terrestrial Time and ephemeris time are used.

Uses and history: Important for celestial navigation, astronomy, and space mission planning. Modern ephemerides are produced by space agencies and research groups; widely used examples include NASA's Jet Propulsion Laboratory (JPL) planetary ephemerides (the DE-series), the VSOP87 theory for planetary positions, and the lunar ephemerides ELP. Ephemerides can be published as tables or provided as digital files for interpolation.

Notes: The term may also apply to star catalogs predicting proper motions and parallax over time, or to the ephemerides of artificial satellites. The accuracy depends on data quality and the dynamical model.