# Metonic cycle

The Metonic cycle is a period of 19 years, after which the phases
of the Moon recur on the same calendar date and within two hours of
the same time. Discovered by Meton of Athens in 432 B.C.,
it arises from the fact that 235 lunations equal 19 tropical years almost exactly – about 6939.5 days. The Metonic cycle formed the basis
for the Greek calendar until 46 B.C. when the Julian
calendar was introduced. The **Callipic cycle** is four
19-year Metonic cycles, or 76 years, and was introduced by the Greek astronomer
and mathematician Callipus of Cyzicus (c.370–c.300 B.C.)
in order to reconcile more closely the lunar month with the solar year.