Computus (Latin for "computation") is the calculation of the date of Easter in terms of the Gregorian calendar or Julian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age.
In principle, Easter falls on the Sunday following the Full Moon that follows the Northern spring equinox (the so-called Paschal Full Moon). However, the vernal equinox and the full moon are not determined by astronomical observation. Instead, the vernal equinox is fixed to fall on 21 March, while the full moon (known as the ecclesiastical full moon) is 14 days after the ecclesiastical equinox. Easter falls on the Sunday after the ecclesiastical full moon. The computus is thus the procedure of determining the first Sunday after the first ecclesiastical full moon that falls on or after 21 March.
The model assumes that 19 tropical years have the same duration as 235 synodic months (modern value: 234.997).
Since the 16th century, there have been differences in the calculation of Easter between the Western and Eastern Churches. Eastern Orthodox Christians use an ecclesiastical full moon that occurs four to five days later than the western ecclesiastical full moon. The Roman Catholic Church since 1583 has been using 21 March under the Gregorian calendar to calculate the date of Easter, while the Eastern Churches continue to use 21 March under the Julian Calendar.