epact

astronomy

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

June 26, 2026

Quick Summary

The epact is a period added to harmonize the lunar calendar with the solar calendar. This concept is crucial for determining the date of Easter, which is based on both the Moon's phases and the vernal equinox.12

The epact system, derived from the Greek word for "to intercalate," indicates the age of the Moon on the first day of the year. This allows for the approximate calculation of the Moon's age on any given day. While earlier methods like the Metonic cycle had inaccuracies, the epact system, with lunar and solar corrections, provided a more precise way to align the calendar with astronomical events.2

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

calendar
The date of Easter
Called the epact—the word is derived from the Greek epagein, meaning “to intercalate”—this was again a system of numbers concerned with the Moon’s phases, but now indicating the age of the Moon on the first day of the year, from which the age of the Moon on…
Read More
Called the epact—the word is derived from the Greek epagein, meaning “to intercalate”—this was again a system of numbers concerned with the Moon’s phases, but now indicating the age of the Moon on the first day of the year, from which the age of the Moon on…
Read More