What does epact mean?
An epact is the number of days by which a solar year exceeds twelve lunar months, and more precisely — in church chronology — the age of the moon on a given day of the year, traditionally counted from 1 January. Because twelve lunar months fall roughly eleven days short of the solar year, the epact advances each year and cycles over a nineteen-year period, making it indispensable to computus, the medieval discipline of calculating the dates of movable feasts such as Easter. The word belongs to the specialist vocabulary of astronomy, chronology, and liturgy, and appears mainly in scholarly, historical, or antiquarian contexts rather than everyday speech. Though rare, it survives wherever calendars and the moon must be reconciled, and it carries an aura of monastic learning and mathematical precision. For anyone interested in how Western civilization kept its feasts aligned with both sun and moon, the epact is one of the key terms of the trade.
nounThe number of days by which the solar year exceeds twelve lunar months; specifically, in ecclesiastical reckoning, the age of the moon on 1 January, used in computing the date of Easter.
- Astronomy/chronology: the excess of the solar year over twelve lunar months.
- Ecclesiastical computus: the age of the moon on a given date (traditionally 1 January), expressed as a number used in calculating the movable feasts of the church calendar.
"Medieval computists relied on the epact to determine each year's golden number and the date of Easter."
"Because the solar year runs about eleven days longer than twelve lunations, each successive year's epact increases, cycling through the calendar over nineteen years."
"The clerk of the calendar dutifully recorded the epact beside the golden number, as his predecessors had done for centuries."
Regular plural; used when referring to the epact values of multiple years or multiple calendrical traditions.
"The table listed the epacts for all nineteen years of the Metonic cycle."
Without this obscure little word, Christmas and Easter would never line up with the moon — medieval monks built entire tables of 'epacts' just to keep Easter in its place.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of epact
The word derives from Greek epaktē (hēmera), meaning '(brought-in) day' — an intercalary day added to reconcile the lunar and solar calendars. It comes from epagein, 'to bring in', formed from epi- ('on, upon') plus agein ('to lead, drive'), a root also visible in words like pedagogy, synagogue, and stratagem. English adopted the term via Latin epacta in early scientific and ecclesiastical writing, with attestations from around the late sixteenth to seventeenth century.
How epact is actually used
A technical term confined to astronomy, chronology, and liturgical computation (computus); it is rare outside scholarly or historical writing and would strike most readers as arcane. The plural is regular (epacts).
Easily confused with epact
An epoch is a fixed point or period marking the start of an era, whereas an epact is the small surplus of days by which a solar year outstrips twelve lunar months.