Metonic cycle

The Metonic cycleastronomycalendar studiesan approximate common multiple oforbital periods ofEarth andMoon. 19 tropical years differ from 235 synodic months by just about 2 hours.

This approximationused byHebrew calendar. It was known toGreek astronomer Meton, who introducedabout 432 BC, andChaldean astronomer Kidinnu (4th cy. BC). Italso used incomputation ofdateEaster.

Intypical lunisolar calendar, most yearslunar years12 months, but7 of19 years have an extra month, known as an intercalary or embolismic month. Traditionally (inancient BabylonianHebrew calendars),years: 3, 6, 8, 11, 14, 17,19, arelong (13-month) years ofMetonic cycle. This incorporates two less accurate subcycles,which 8 years = 99 lunations (an Octaeteris),11 years = 135 lunations.

The 19-year cyclealso close (to somewhat more than halfday)255 draconic months, soalsoan eclipse cycle, which lasts onlyabout 4 or 5 recurrenceseclipses. The implicit 11-year cycleclose146.5 draconic months,isbetter eclipse cycle called tritos.