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.

External Links

• Introduction to the JPL ephemerides
• The effect of asteroidal perturbations on the long term accuracy of ephemerides.
• Source code for computing ephemerides - by Steve Moshier
• A Free 3200 Year Ephemeris Provided by Astro.com -- Based out of Zürich, Switzerland (available in 8 languages)
• The Original 3,000 Year High-Precision Daily Astrological Online Ephemeris from Khaldea.com -- 600BC to 2400AD -- Calculated for Midnight GMT; also with an Aspectarian included for years 1900 to 2005
• Interactive orrery and ephemeris provided by Fourmilab in Switzerland.
• Online Ephemeris from 1891 to 2100 -- A simple ephemeris that is very easy to use and extremely accurate. All times are for Midnight (00:00 Hours) Greenwich Mean Time (UTC).
• Rosicrucian Fellowship Ephemeris Software -- (1900 - 2100)