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 BC, 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 BC when the Julian calendar was introduced. The Callipic cycle is four 19-year Metonic cycles, or 76 yr, and was introduced by the Greek astronomer and mathematician Callipus of Cyzicus (c. 370–c. 300 BC) in order to reconcile more closely the lunar month with the solar year.