Ephemeriden

An ephemeride (plural Ephemeriden) is a dataset that lists the positions of astronomical objects in the sky at specified times. The term, from Greek ephemeros meaning temporary or daily, is used in astronomy to refer to tables or files that predict where bodies will be. Ephemerides are essential for planning observations, navigating, and guiding spacecraft; modern ephemerides are produced digitally and cover planets, the Moon, the Sun, minor planets, comets, and artificial satellites.

A typical ephemeris provides coordinates such as right ascension and declination or ecliptic longitude and latitude, sometimes with distances, velocities, and uncertainties, for each object at a series of epochs. The time scale used is usually Terrestrial Time or Barycentric Dynamical Time, with dates given as Julian days or calendar dates. The reference frame may be the International Celestial Reference Frame or a heliocentric or geocentric frame.

Ephemerides are generated by numerical integration of gravitational models and perturbation theories, often constrained by observations from radar, spacecraft tracking, astrometry, and VLBI. Leading modern ephemeris series include NASA's JPL Development Ephemerides (for planets and the Moon), such as DE430/DE431 and newer versions, the European INPOP series, and the Russian EPM/ETP ephemerides. Ephemerides are distributed as data tables and as SPICE kernels for software tools used in astronomy and mission design.

In practice, ephemerides enable predicting planetary conjunctions, lunar phases, solar eclipses, and the positions of minor bodies, and they underpin spacecraft navigation, astronomical planning, and educational use. They continue to be updated as new observations and refined models become available.