Metonic cycle 

The Metonic cycle in astronomy and calendar studies is an approximate common multiple[?] of the orbital periods of the Earth and the Moon. 19 tropical years differ from 235 synodic months by just about 2 hours.

This approximation is used by the Hebrew calendar. It was known to the Greek astronomer Meton[?], who introduced about 432 BC, and the Chaldean astronomer Kidinnu (4th cy. BC). It is also used in the calculation of the date for Easter.

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