computus

Computus is the calculation used to determine the date of Easter Sunday in Western Christianity. The goal is to find the first Sunday after the Paschal Full Moon, which is the first ecclesiastical full moon occurring on or after March 21, the fixed date used for the vernal equinox in the computation. The method combines lunar and solar cycles through calendar rules rather than direct astronomical observation.

Practically, computus employs the 19-year Metonic cycle to approximate the moon’s phases, with epacts indicating the offset of the lunar calendar within the solar year. A solar correction is applied to align the lunar cycle with the solar year. The result is that Easter Sunday can fall on any Sunday between March 22 and April 25 in Western churches.

Historically, computus developed in late antiquity, with contributions from the Alexandrian school and other scholars. In 1582, the Gregorian calendar reform refined the calculation to correct the drift of the Julian calendar and to realign the equinox with March 21. Since then, most Western churches use Gregorian computus, though some communities follow different rules or calendars. Notably, Eastern Orthodox churches often use the Julian calendar and may celebrate Easter on a different date due to alternate temporal rules and calendar systems.