Computus (Latin for “computation”) is the calculation of the date of Easter in terms of, first, the Julian and, later, the Gregorian calendar. The name has been used for this procedure since the early Middle Ages, as it was considered the most important computation of the age.
Following the First Council of Nicaea, the date for Easter was completely divorced from the Jewish calendar and its computations for Passover. Thereafter, in principle, Easter fell 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 were not determined by astronomical observation. Instead, the vernal equinox was fixed to fall on the 21st day of March, while the full moon (known as the ecclesiastical full moon) was fixed at 14 days after the beginning of the ecclesiastical lunar month (known as the ecclesiastical new moon). Easter thus falls on the Sunday after the ecclesiastical full moon. 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 solar or lunar years.
The model that was worked out 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. The Roman Catholic Church since 1583 has been using 21 March under the Gregorian calendar to calculate the date of Easter, while the Eastern Orthodox continued and continue to use 21 March under the Julian Calendar. The Catholic and Protestant denominations thus use an ecclesiastical full moon that occurs four to five days earlier than the eastern one.
Main article: Easter controversy
Easter is 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. 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. The rest of the Christian world at that time, according to Eusebius, held to “the view which still prevails”, of always fixing Easter on Sunday. Eusebius does not say how the Sunday was decided. Other documents from the 3rd and 4th centuries, however, reveal that the customary practice was for Christians to consult their Jewish neighbors to determine when the week of Unleavened Bread would fall, and to set Easter on the Sunday that fell within that week.
By the end of the 3rd century, however, some Christians had become dissatisfied with what they perceived as the disorderly state of the Jewish calendar. The chief complaint was that the Jewish practice sometimes set the 14th of Nisan before the spring equinox. This is implied by Dionysius, bishop of Alexandria in the mid-3rd century, who stated that “at no time other than the spring equinox is it legitimate to celebrate Easter” (Eusebius, Church History 7.20); and by Anatolius of Alexandria (quoted in Eusebius, Church History 7.32) who declared it a “great mistake” to set the Paschal lunar month when the sun is in the twelfth sign of the zodiac. And it was explicitly stated by Peter, bishop of Alexandria that “the men of the present day now celebrate [Passover] before the [spring] equinox…through negligence and error.” Another objection to using the Jewish computation may have been that the Jewish calendar was not unified. Jews in one city might have a method for reckoning the Week of Unleavened Bread different from that used by the Jews of another city. 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.
At the First Council of Nicaea in 325, it was agreed that the Christians should use a common method to establish the date, independent from the Jewish method. 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 Christendom. The process of working out the details generated still further controversies.
The method from Alexandria became authoritative. In its developed form it was based on the epacts of a reckoned moon according to the 19-year cycle (a.k.a. the Metonic Cycle). Such a cycle was first proposed by Bishop Anatolius of Laodicea (in present-day Syria), c. 277. Alexandrian Easter tables were composed by Bishop Theophilus about 390 and within the bishopric of Cyril about 444. 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. Churches on the eastern frontier of the Byzantine Empire deviated from the Alexandrians during the 6th century, and now celebrate Easter on different dates from Eastern Orthodox churches four times every 532 years. The Alexandrian computus was converted from the Alexandrian calendar into the Julian calendar in Rome by Dionysius Exiguus, though only for 95 years. Dionysius introduced the Christian Era (counting years from the Incarnation of Christ) when he published new Easter tables in 525.
Dionysius’s tables replaced earlier methods used by the Church of Rome. The earliest known Roman tables were devised in 222 by Hippolytus of Rome based on 8-year cycles. Then 84-year tables were introduced in Rome by Augustalis near the end of the 3rd century. These old tables were used in Northumbria until 664, and by isolated monasteries as late as 931. A modified 84-year cycle was adopted in Rome during the first half of the 4th century. Victorius of Aquitaine tried to adapt the Alexandrian method to Roman rules in 457 in the form of a 532-year table, but he introduced serious errors. These Victorian tables were used in Gaul (now France) and Spain until they were displaced by Dionysian tables at the end of the 8th century.
In the British Isles Dionysius’s and Victorius’s tables conflicted with older Roman tables based on an 84-year cycle. The Irish Synod of Mag Léne in 631 decided in favor of either the Dionysian or Victorian Easter and the northern English Synod of Whitby in 664 adopted the Dionysian tables. The Dionysian reckoning was fully described by Bede in 725. They may have been adopted by Charlemagne for the Frankish Church as early as 782 from Alcuin, a follower of Bede. The Dionysian/Bedan computus remained in use in Western Europe until the Gregorian calendar reform, and remains in use in most Eastern Churches, including most Eastern Orthodox Churches and Oriental Orthodox Churches. Churches beyond the eastern frontier of the former Byzantine Empire use an Easter that differs four times every 532 years from this Easter, including the Assyrian Church of the East.
The Gregorian Easter has been used since 1583 by the Roman Catholic Church and was adopted by most Protestant churches between 1753 and 1845. German Protestant states used an astronomical Easter based on the Rudolphine Tables of Johannes Kepler between 1700 and 1774, while Sweden used it from 1739 to 1844. This astronomical Easter was one week before the Gregorian Easter in 1724, 1744, 1778, 1798, etc.