epacts

Epact is a term used in astronomy and the Western Christian liturgical calendar to denote the age of the Moon on January 1, expressed in days. It represents how many days have passed since the Moon’s last new moon within its roughly 29.53-day cycle. The epact for a given year typically ranges from 0 to 29.

In calendar calculations, the epact is used to determine the date of Easter. The computation relies on the Metonic cycle, a 19-year period in which lunar phases recur on roughly the same days of the year, augmented by adjustments to keep the lunar and solar years aligned. In the Gregorian system, century-year rules are applied to correct drift, and the epact feeds into tables that identify the Paschal full moon—the ecclesiastical full moon used to establish Easter Sunday as the first Sunday after that full moon.

The term derives from Latin epacta lunae, meaning the age of the Moon, and it has a long history in medieval and early modern computus, the set of algorithms for calculating Easter. Over time, the practical use of epacts has been superseded by modern computational methods, and they are now mainly of historical or scholarly interest. Still, epact tables and related concepts appear in some liturgical calendars and educational discussions of how Easter dates were historically determined.