metonic cycle

A metonic cycle is the approximately 19-year period after which the phases of the moon recur on the same days of the solar year.

Named for the Greek astronomer Meton, who calculated it in 432 BCE, this cycle has been fundamental to calendar systems that must reconcile lunar months with solar years: The Hebrew and Chinese calendars both rely on the metonic cycle to determine when to add leap months. (Capitalized Metonic cycle is more common.)