The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the tropical year and the synodic month. 19 tropical years differ from 235 synodic months by about 2 hours.

19 tropical years  = 6939.602 days
235 synodic months = 6939.688 days

This approximation is used by the Hebrew calendar. It was known to the Greek astronomer Meton , who introduced it about 432 BC, and the Chaldean astronomer Kidinnu (4th cent. BC). It is also used in the computation of the date of Easter.

In a typical lunisolar calendar, most years are lunar years of 12 months, but some years have an extra month, known as an intercalary or embolismic month. There are 7 of these intercalary months in the 19 years of a Metonic cycle. Traditionally (in the ancient Babylonian, Hebrew, and Attic calendars), the years: 3, 6, 8, 11, 14, 17, and 19, are the long (13-month) years of the Metonic cycle.

The Cycle incorporates two less accurate subcycles, for which 8 years = 99 lunations to within 1.5 days (an Octaeteris), and 11 years = 136 lunations.

Meton approximated the cycle to a whole number (6940) of days, obtained by 125 long months of 30 days and 110 short months of 29 days.

The 19-year cycle is also close (to somewhat more than half a day) to 255 draconic months, so it also is an eclipse cycle, which lasts only for about 4 or 5 recurrences of eclipses.

See also Antikythera mechanism.