Metonic cycle
The (19-year) Metonic cycle is a lunisolar cycle, as is the (76-year) Callippic cycle. An important example of an application of the Metonic cycle in the Julian calendar is the 19-year lunar cycle insofar as provided with a Metonic structure. In the following century, Callippus developed the Callippic cycle of four 19-year periods for a 76-year cycle with a mean year of exactly 365.25 days.