Ephemeris | Celestial navigation, Planetary motion, Timekeeping | Britannica (original) (raw)

ephemeris, table giving the positions of one or more celestial bodies, often published with supplementary information. Ephemerides were constructed as early as the 4th century bc and are still essential today to the astronomer and navigator.

Modern ephemerides are calculated when a theory (mathematical description) of the motion of a heavenly body has been evolved, based on observations. Heavy computing and careful checking are involved. Until the 20th century, tables of logarithms were the chief aid to computation. The gradual introduction of mechanical calculators increased the speed and accuracy of the work. Of greater effect was the development of electronic calculators and computers. These have made feasible the solution of problems formerly considered impossible because of the tremendous labour involved. The simultaneous integration of the equations of motion of the five outer planets, for every 40th day, from the year 1653 to 2060 is typical.

A number of national ephemerides are published regularly. The oldest is the Connaissance des temps, founded in Paris in 1679 as the direct successor to a series of ephemerides originally begun by the German astronomer Johannes Kepler in 1617. The British Nautical Almanac and Astronomical Ephemeris commenced through the initiative of Nevil Maskelyne in 1766. The American Ephemeris and Nautical Almanac was first published in Washington, D.C., in 1852 for the year 1855. From 1877, under the direction of the astronomer Simon Newcomb, it became the best of the national ephemerides. To avoid duplication of costs, it has since 1960 been unified with the British national publication, which at the same time was renamed The Astronomical Ephemeris. The two are of identical content, reproduced separately in each country; the work of computing is shared. Beginning in 1981, both national ephemerides were renamed The Astronomical Almanac. Ephemerides of Minor Planets, compiled and published annually by the Institute of Theoretical Astronomy, St. Petersburg, represents further international cooperation.

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