Word of the day: Ephemeris

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.


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