A completely distinct 84-year cycle, the Insular latercus, was used in the British Isles.
The schematic model that eventually was accepted is the Metonic cycle, which equates 19 tropical years to 235 synodic months.
In 1583, the Catholic Church began using 21 March under the Gregorian calendar to calculate the date of Easter, while the Eastern Churches have continued to use 21 March under the Julian calendar.
Computus (Latin for "computation") is a calculation that determines the calendar date of Easter.
Because the date is based on a calendar-dependent equinox rather than the astronomical one, there are differences between calculations done according to the Julian calendar and the modern Gregorian calendar.
The "computus" is the procedure of determining the first Sunday after the first ecclesiastical full moon falling on or after 21 March, and the difficulty arose from doing this over the span of centuries without accurate means of measuring the precise tropical year.
The synodic month had already been measured to a high degree of accuracy.Easter is the most important Christian feast, and the proper date of its celebration has been the subject of controversy as early as the meeting of Anicetus and Polycarp around 154.According to Eusebius' Church History, quoting Polycrates of Ephesus, churches in the Roman Province of Asia "always observed the day when the people put away the leaven", namely Passover, the 14th of the lunar month of Nisan.Because of these perceived defects in the traditional practice, Christian computists began experimenting with systems for determining Easter that would be free of these defects.But these experiments themselves led to controversy, since some Christians held that the customary practice of holding Easter during the Jewish festival of Unleavened Bread should be continued, even if the Jewish computations were in error from the Christian point of view.In principle, Easter falls on the Sunday following the full moon that follows the northern spring equinox (the paschal full moon).