epacta

Epacta is a term used in calendrical astronomy and the computation of the Christian Easter. In Latin, epacta refers to the plural form describing the age of the Moon on January 1 of a given year. The concept was central to the medieval and early modern computus, the set of algorithms used to determine the date of Easter.

The epacta value represents the Moon’s age in days on January 1, typically ranging from about 0 to 29 days, reflecting the roughly 29.53-day synodic month. Because lunar and solar years do not align, epacta values are used together with the year’s position in the Metonic cycle (the 19-year cycle that nearly synchronizes solar and lunar calendars) to locate the ecclesiastical Paschal full moon. Once the Paschal full moon is known, Easter Sunday is determined as the first Sunday after that moon.

Historically, epacta tables guided calendar-makers in computing dates of Easter and other movable feasts. The practice peaked in medieval and early modern Europe, where scholars adjusted epacta values to account for leap years and long-term drift between the lunar and solar cycles. In modern times, Easter is calculated algorithmically with accurate astronomical data, and the term epacta is primarily of historical interest within the study of computus and the history of calendars. Related concepts include the Metonic cycle, the Paschal moon, and the dominical and golden numbers used in traditional Easter computations.