In
astronomy and
celestial navigation, an
ephemeris (plural:
ephemerides; from
Latin ephemeris, "diary", from
Greek ἐφημερίς,
ephēmeris, "diary, calendar") gives the positions of naturally occurring
astronomical objects as well as
artificial satellites in the
sky at a given
time or times. Historically, positions were given as printed
tables of
values, given at regular intervals of date and time. Modern ephemerides are often computed electronically from mathematical models of the motion of astronomical objects and the Earth. Even though the calculation of these tables was one of the
first applications of mechanical computers, printed ephemerides are still produced, as they are useful when computational devices are not available.