Epact

The epact (Latin: epactae, from Ancient Greek: ἐπακται ἡμεραι (epaktai hēmerai) = added days) used to be described by medieval computists as the age of a phase of the Moon in days on 22 March;[1] in the newer Gregorian calendar, however, the epact is reckoned as the age of the ecclesiastical moon on 1 January.[2] Its principal use is in determining the date of Easter by computistical methods. It varies (usually by 11 days) from year to year, because of the difference between the solar year of 365–366 days and the lunar year of 354–355 days.[3]

  1. ^ Bede the Venerable (1999) [725], "Lunar Epacts", The Reckoning of Time, Translated Texts for Historians, vol. 29, translated by Wallis, Faith, Liverpool: Liverpool University Press, p. 131, ISBN 0-85323-693-3, The epacts noted in the 19-year cycle specifically stand for the age of the moon on the 11th kalends of April [22 March], the beginning of the Paschal feast.
  2. ^ Richards, E. G. (2012), "Calendars" (PDF), in Urban, S. E.; Seidelman, P. K. (eds.), Explanatory Supplement to the Astronomical Almanac, Mill Valley, CA: University Science Books, pp. 599–601, ISBN 978-1-891389-85-6, archived from the original (PDF) on 2019-04-30, retrieved 2017-01-24, The Epact of a year … is the age in days (0 to 29) of the ecclesiastical moon on the first day of the year (January 1).
  3. ^ Latin text and French translation of the Second Canon of the Gregorian calendar