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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 BabylonianIn the Babylonian calendar a year consisted of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset. This system came into use sometime before 2000 BC. The names of the months were: Nisanu Ayaru S, Hebrew, and AtticThe Attic calendar is the name of the calendar used in Ancient Athens. It is a lunisolar calendar, combining elements of solar calendars and lunar calendars. Each year began near the autumnal equinox, and was divided into 12 months, each beginning with th 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 OctaeterisIn astronomy, an octaeteris is the period of eight solar years after which the moon phase occurs on the same day of the year plus one or two days. This period is also in a very good sychronicity with five Venusian visibility cycles (the Venusian synodic y), 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 cycleEclipses may occur repeatedly, separated by some specific interval of time: this interval is called an eclipse cycle . The series of eclipses is called an eclipse series. General explanation Eclipse conditions Eclipses may occur when the Earth and Moon ar, which lasts only for about 4 or 5 recurrences of eclipses.
See also: Antikythera Mechanism
