THE’epact it is a concept of ecclesiastical computation traditionally used to determine the date of the Easter and other movable feasts of the liturgical calendar.
📌 Definition:
The epact indicates the number of days by which the solar year exceeds the lunar year. In other words, it is used to know which phase of the moon we are in. January 1st of a given year.
🔬 Technical detail
- The solar year lasts approximately 365 days.
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The lunar year, based on 12 lunar months, lasts approximately 354 days.
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The difference is 11 days.
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Each year, the epact increases by 11 (modulo 30). When it exceeds 30, it is “carried forward” and the counting continues.
📖 Historical use
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The epact was used in the medieval ecclesiastical calendars to calculate the ecclesiastical new moons, necessary to determine the date of Easter (which falls on Sunday following the first full moon after the spring equinox).
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With the transition from Julian calendar to the a an Gregorian (1582), the calculation of the epacts was corrected by introducing more complex rules, to maintain alignment with the real lunar cycle.
👉 In summary: the epact is a conventional number which connects the solar calendar to the lunar one and is fundamental for the calculation of Christian movable feasts.