What Is an Ephemeris?

An ephemeris is a table that provides information on the positions of celestial objects such as stars and planets on given dates in the past or future. It can also show the positions of man-made satellites. The term, whose plural is ephemerides, comes from the Greek word for a diary or journal. Since ancient times, man has plotted the positions of stars and planets in the sky, for a variety of reasons relating to religion, navigation and, more recently, space travel. A cave painting found at Lascaux, France dating from around 15,000 BCE appears to depict the phases of the moon and could be regarded as the earliest known moon ephemeris, but the earliest known planetary ephemerides appear to have been produced by the Babylonians around 523 BCE.

In early times, ephemerides seem to have been used to predict solar eclipses and, in some cultures, to attempt to predict the future. It was often believed that the positions of the planets in the sky, relative to constellations and to one another, had significance for affairs on Earth: certain configurations were regarded as favorable and others not. The configuration of the planets at the time of a person’s birth were also thought to influence that person’s character and destiny, and ephemerides were used to attempt to predict what lay in store for people and for society in general. They are still used in this way today by astrologers; however, there are differences between an astrological ephemeris and an astronomical ephemeris. Astrological ephemerides usually give planetary positions on the ecliptic relative to the vernal equinox and show the zodiacal constellation in which the planet resides on a given date.

The main items of astronomical ephemeris data, for a given planet and date, are the right ascension and the declination. These can be regarded as essentially the horizontal and vertical positions of the object. Some may, however, provide additional information. For example, ephemerides published in the U.S. by the National Aeronautics and Space Administration (NASA) give, among other things, the planet’s distance from the earth in astronomical units and its velocity, in kilometers per second, relative to Earth.

Modern ephemerides are used by professional and amateur astronomers to locate objects of interest, which may include planetoids and large asteroids, as well as planets. The object can be looked up in the appropriate table for the required date so that the user will know where to point the telescope. Ephemerides are also employed to plot the courses of space probes used to explore the planets of the solar system. Global Positioning System (GPS) satellites broadcast ephemerides, giving their own precise positions at regular intervals.

Since the movements of the planets in orbit around the Sun are very predictable, it is possible to compile ephemerides far into the future. The planets, however, are influenced slightly by the gravitational pulls of numerous asteroids whose precise orbits are not known, so over very long periods the predicted positions may drift slightly out of step with reality. For this reason, astronomical ephemeris tables are regularly revised.