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.
Leap days extend both the solar and lunar year, so they do not affect epact calculations for any other dates.
By 1582 it was noted (for example, in the text of the bull Inter gravissimas itself) that the new and full moons were at that point occurring "four days and something more" sooner than the old lunar cycle indicated.
The discovery of the epact for computing the date of Easter has been attributed to Patriarch Demetrius I of Alexandria, who held office from 189–232 AD.
[7](pp 40–45) As early as the fourth century we see Easter computus using the epact and the nineteen-year Metonic cycle in Alexandria, and subsequent computistical tables were influenced by the structure of the Alexandrian calendar.
Under the influence of Dionysius Exiguus and later, of Bede, the Alexandrian Easter Tables were adopted throughout Europe where they established the convention that the epact was the age of the Moon on 22 March.
The first was the increasing error of computistical techniques, which led to the introduction of a new Julian epact around 1478 AD, to be used for practical computations of the phase of the Moon for medical or astrological purposes.
The first was the Lilian epact, developed by Aloisius Lilius as an element of the ecclesiastical computations using the Gregorian calendar.