Epact

Epact is the age of the Moon in days on January 1 of a given year. In the ecclesiastical calendar, the epact is used to determine the date of Easter by locating the paschal full moon and the first Sunday after it. The concept originated in the computus, the medieval method for calculating Easter, and derives from the Latin word epacta, meaning the age or matured state of the Moon. The calculation relies on the Metonic cycle, a 19-year period after which lunar phases recur on the same days of the year, with adjustments to keep pace with the solar year.

Practically, the epact is represented as a number from 0 to 29, indicating how old the Moon is on January 1. This value, together with the Golden Number and other computed quantities, helps determine the ecclesiastical new moon and the paschal full moon, from which Easter is derived as the first Sunday after that full moon. The Gregorian calendar reform refined the computation by introducing century-based corrections to keep the epact aligned with actual lunar phases over long spans.

In modern practice, epacts remain a component of traditional Easter algorithms and are often tabulated for years; they also appear in astronomical contexts as the Moon’s age on a given date, independent of Easter. The term is sometimes discussed in historical treatments of calendar systems and the computus, illustrating how lunar cycles were integrated into annual dating systems.

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