Ephemerides

Ephemerides are tables or data sets that provide the predicted positions of astronomical objects at specified times. The term comes from Greek roots meaning a daily record; in English, ephemeris is singular and ephemerides is the common plural. Modern ephemerides are generated from models of celestial mechanics and numerical integration, and they are essential for both observational astronomy and space navigation.

Most ephemerides concern solar-system bodies such as planets, moons, asteroids, comets, and artificial satellites. They may deliver geocentric or topocentric coordinates (right ascension and declination, or Cartesian vectors), distances, and sometimes velocities or orbital elements. Time scales and reference frames are important details, with common choices including Terrestrial Time (TT), Barycentric Dynamical Time (TDB), and Julian dates.

Several well-known series and formats exist. Planetary ephemerides are produced by organizations such as NASA’s Jet Propulsion Laboratory (the Development Ephemeris or DE series) and the INPOP project, among others. For space missions, ephemerides are often distributed in SPICE kernels (NAIF), which provide spacecraft-centric positions and orientations. Ephemerides for Earth-orbiting satellites are frequently published as Two-Line Elements (TLEs), used to propagate satellite orbits.

Applications include navigation of spacecraft and spacecraft mission planning, prediction of occultations and eclipses, telescope pointing and scheduling, and historical reconstruction of celestial positions. Ephemerides are publicly published in astronomical almanacs and mission data sets, serving as a foundational tool for precise celestial positioning.