An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at a given moment in time; the astrological positions are usually given for either noon or midnight depending on the particular ephemeris that is used. For scientific uses, a modern planetary ephemeris comprises software that generates positions of the planets and often of their satellites, or of asteroids or comets at virtually any time desired by the user. Often there is an option to find the velocities of the bodies of interest, as well. Typically, such ephemerides cover several, or even many centuries, past and future; the future ones can be covered because celestial mechanics is an accurate theory. The biggest uncertainties, nowadays, are due to the perturbations on the planets of numerous asteroids, most of whose masses are poorly known, rendering their effect a bit uncertain.
Solar system ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars and galaxies.
An ephemeris also sometimes provides data on astronomical phenomena of interest to astrologers and astronomers such as solar and lunar eclipses, apparent retrogradation/planetary stations, planetary ingresses, sidereal time, positions for the Mean and True nodes of the moon, the phases of the Moon, and sometimes even the position(s) of Chiron, Lilith, and other minor celestial bodies. Some ephemerides also contain a monthly aspectarian, while others often include the declination of the planets as well as their longitudes, right ascensions or Cartesian coordinates.
Though astrology is and always has been geocentric, Heliocentric Astrology is an emerging field; for this purpose a standard ephemeris cannot be utilized, and because of this specialized heliocentric ephemerides must be calculated and used instead of the default geocentric ephemerides that are used in standard Western astrology to construct the astrological chart/natal chart.