Epoch

Epoch, in Astronomy, is an abbreviation for 'longitude at the epoch;' it means the mean heliocentric longitude of a planet in its orbit at any given time—the beginning of a century, for instance. The epoch of a planet for a particular year is its mean longitude at mean noon, on January 1, when it is leap-year, and on December 31 of the preceding year, when it is a common year. The epoch is one of the elements of a planet's orbit. For epoch in chronology, see CHRONOLOGY.