The number of days by which the solar year exceeds the lunar year, used in calculating Easter and other lunar-based holidays.
From Medieval Latin 'epacta,' from Greek 'epaktós' meaning 'brought in' or 'added,' derived from 'epágein' (to bring in). The word originally referred to the extra days that accumulate between solar and lunar calendars.
Ancient astronomers discovered that the sun and moon don't sync up perfectly—the solar year is about 11 days longer than 12 lunar months—so they invented the epact to keep religious calendars from drifting. This mathematical solution helped priests predict Easter for over 1,500 years before we had computers!
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