Metonic cycle
noun
Me·ton·ic cycle
meˈtänik-
: a period of 19 years after the lapse of which the phases of the moon return to a particular date in the calendar year:
a
: one of the 19-year periods reckoning from June 27, 432 b.c. that were used in determining lengths of years and the placing of the intercalary month in the ancient Greek calendar
b
: one of the 19-year periods reckoning from 1 b.c. that are used in determining the date of Easter in the Gregorian calendar
Word History
Etymology
Meton, 5th century b.c. Greek astronomer + English -ic