The date of Easter Sunday, the great Christian festival celebrating the resurrection of Jesus, is determined by an amalgam of astronomy, mathematics and pragmatism, an algorithm that determines the date in accordance with the motions of the sun and moon.
The ecclesiastical equinox is fixed on March 21st, which generally – though not always – coincides with the sun passing northwards across the celestial equator. The first full moon on or after March 21st is taken as the Easter full moon, which this year is on Thursday, April 2nd. Thus, April 5th is Easter Sunday.
The first council of Nicaea, in AD 325, decreed that Easter should be celebrated on the same day throughout Christendom. The computation of the appropriate date was a formidable mathematical challenge through the Middle Ages.
In one form or another, the Alexandrian Easter reckoning, endorsed by Cyril, Bishop of Alexandria in AD 418, was to play a dominant role. By the tenth century, after many twists and turns, the Alexandrian Easter table had been adopted throughout the western world. This continued until the drift of March 21st from the observed equinox led to the calendrical reform of Pope Gregory XIII in 1582.
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Around AD 457, the Roman church changed from their 84-year table, the Romana Supputatio, to the 532-year Easter table devised by Victorius of Aquitaine. Victorius adapted the Alexandrian table to accommodate Pope Leo’s ideas on Easter.
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Within a year, St Patrick in turn adapted Victorius’ table by removing all his accommodations of Pope Leo ideas, thereby restoring all the Alexandrian Easter dates, and brought this table to Ireland. However, until AD 625 Victorius’ table was used in Rome.
In that year, Pope Honorius abandoned Victorius’ adjustments and adopted the Alexandrian table as tabulated by Dionysius Exiguus about a century earlier.
Honorius required all Christians to observe the Alexandrian dates and, in particular, he exhorted the Irish to get into step with Rome. At the Synod of Mag Léne about AD 632, Patrick’s tabulation of the Alexandrian reckoning was formally adopted by the southern Irish churches.
The southern adoption of Patrick’s table provoked sharp criticism from the monks of Iona. To address this, the scholar Cummian wrote a letter c. AD 632 to the abbot of Iona. This letter, a copy of which was found by Archbishop James Ussher in 1632, justifies the decision of the Synod of Mag Léne to adopt the Alexandrian reckoning. Iona remained out of step with Rome for a further 90 years before eventually coming into line around AD 716.
There has been significant progress in recent years in understanding early Easter dates used in Ireland. Dan McCarthy, emeritus fellow of Trinity College, has examined the letter of Cummian and has noted some striking consequences.
He concludes that, since the monks of southern Ireland were using the table brought by Patrick, they were avoiding the anomalies introduced by Victorius. Thus, far from being behind Rome, Patrick’s disciples in Ireland were far ahead, using a table that determined the Easter date in agreement with the Alexandrian table later adopted by the universal church.
This conclusion is consistent with the assertion of the Venerable Bede, that the monks of southern Ireland had “long before learned to observe Easter according to canonical custom”.
McCarthy’s results have been published in Peritia, the journal of the Medieval Academy of Ireland.
Peter Lynch is emeritus professor at the School of Mathematics & Statistics, University College Dublin. He blogs at thatsmaths.com
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