Named for the Athenian astronomer Meton, it is based on the moon, counting each cycle of the phases of the Moon as one month. Days are kept approximately in step with the seasons by including 7 leap years of 13 months in each cycle of 19 years.
A period of 19 years, after which the various phases of the moon fall on approximately the same days of the year as in the previous cycle.
The Metonic cycle is the basis for the golden numbers used to determine the data of Easter. Four such cycles form a Callippic cycle.