The Metonic cycle or enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates the construction of a lunisolar calendar.

A tropical year is longer than 12 lunar months and shorter than 13 of them. The arithmetic identity 12×12 + 7×13 = 235 shows that a combination of 12 "short" years (12 months) and 7 "long" years (13 months) will be almost exactly equal to 19 solar years.