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Computus ( Latin for computation) is calculation of the date of Easter in the Christian 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.The canonical rule is that Easter Sunday is the first Sunday after the 14th day of the lunar month (the Full Moon) that falls on or after 21 March (the day of the vernal equinox; see also Ostara). For determining the feast, the Christian churches settled on a method to define a reckoned "ecclesiastic" Moon, rather than observations of the true Moon like the Jews did.
1 History
Easter has been considered the most important Christian feast. Accordingly, the proper date of its celebration has been a cause of much controversy, at least as early as the meeting (c. 154) of Anicetus, bishop of Rome and Polycarp, bishop of Smyrna. The problem is that the passion and resurrection of Jesus Christ occurred during the Jewish feast of PassoverPassover also known as Pesach or Pesah ( pesa , is an eight-day Jewish holiday (seven days in Israel) that commemorates the exodus and freedom of the Israelites from Egypt; it is also observed by some Christians to commemorate the deliverance from sin by, which they celebrate according to their lunisolar calendarA lunisolar calendar is a calendar whose date indicates both the moon phase and the season. This is normally done by having a year that corresponds to a cycle of seasons and a month which corresponds to a lunation so that the day of month indicates the mo. The Christians in late-roman times had to redefine this for the Julian calendarThe Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita''. It was chosen after consultation with the Alexandrian astronomer Sosigenes and was probably designed to approximate the tropical year, known sinc that was used at that time, which is a solar calendarA solar calendar is a calendar whose dates indicate the season and so are synchronized to the declination of the sun. The mean calendar year approximates the tropical year. The following are solar calendars: Gregorian calendar Julian calendar Coptic calen.
At the First Council of NicaeaThe First Council of Nicaea which took place during the reign of the emperor Constantine in 325, was the first ecumenical (from Greek oikumene "worldwide") conference of bishops of the Christian Church. The participating bishops were given free travel to in 325Events May 20 First Council of Nicaea first "Ecumenical Council" of the Christian Church: The Nicene Creed is formulated, the date of Easter is discussed. Gladiatorial combat is outlawed in the Roman Empire. The Church of the Nativity is built in Bethlehe, it was agreed that the Christians should use a common method to establish the date, independent from the Jews. Also they decided to celebrate it always on the dies Domini, Sunday, which was the day of the week on which Jesus was resurrected, and which has been the Christian holy day of the week for this reason (the QuartodecimansQuartodecimanism ("fourteenism") was the practice of fixing the date of Pascha (Easter) to the 14th day of Nisan in the Jewish Calendar. It was one of several methods of fixing the date of the Pascha holiday. Quartodecimism was popular among Christians in wished to follow the Jews and always celebrate it on the 14th day of the Jewish month of Nisan, whatever day of the week that might be). However, they made few decisions that were of practical use as guidelines for the computation, and it took several centuries before a common method was accepted throughout Christianity.
The method from Alexandria became authoritative. It was based on the epacts of a reckoned moon according to the 19-year cycle. This was first used by bishop Anatolius of Laodicea (in present-day Turkey) c. 277. The Alexandrians may have derived their method from a similar calendar, based on the Egyptian civil solar calendar, and used at that time by the influential Jewish community there; it may have survived in the Ethiopian computus. In Constantinople several computists were active over the centuries after Anatolius (and after the Nicaean Council), but their Easter dates coincided with those of the Alexandrians. A variation of the Alexandrian method, based on some interpretation of the edicts of the Council of Nicaea, was used in Rome by Dionysius Exiguus (who translated the edicts into Latin). Dionysius introduced the Christian Era (counting years from the birth of Christ) when he published new Easter tables in 525. This replaced a method based on a less accurate 84-year cycle introduced in Rome by one Augustalis in the 3rd century, and which was used in the British Isles in the Anglo-Saxon kingdom of Northumbria until the Synod of Whitby in 664, and by the Celtic Church as late as 768. Annianus of Alexandria around 410, and Victorius of Aquitania in Rome in 457 had devised a method which had a cycle of 532 years (28 × 19). This was later popularized by Bede. A simplified method, using Golden Numbers, was introduced by Abbo of Fleury in 988. This remained in use in Western Europe until the Gregorian calendar reform, which was mostly designed by Aloysius Lilius.
