Editing Celtic Christianity (section)

===Easter calculation===
{{main article|Easter controversy|computus}}{{anchor|Computus}}
[[Easter]] was originally dated according to [[Hebrew calendar]], which [[Hebrew calendar#New year|tried to place Passover]] on the first full moon following the [[Spring equinox (Northern Hemisphere)|Spring equinox]] but did not always succeed. In his ''[[Life of Constantine]]'', [[Eusebius]] records that the [[First Council of Nicaea]] (325) decided that all Christians should observe a common date for Easter separate from the Jewish calculations, according to the practice of the [[bishops of Rome]] and [[bishop of Alexandria|Alexandria]].<ref name=conlet>{{Citation| author  = Constantine| author-link = Constantine the Great| chapter = Letter on the Keeping of Easter to those not present at Nicaea| date = 325| editor  = Eusebius of Caesaria| editor-link = Eusebius of Caesaria| title  = The Life of Constantine| volume = III| publisher = Signature Books| at = §18–20| publication-date = 1996| isbn = 1-56085-072-8|chapter-url=http://www.ccel.org/ccel/schaff/npnf214.vii.x.html}}</ref> Calculating the proper date of Easter (''computus'') then became a complicated process involving a [[lunisolar calendar]], finding the first Sunday after an idealized Passover on the first full moon after the equinox.

Various tables were drawn up, aiming to produce the necessary alignment between the [[solar year]] and the [[moon phases|phases]] of the [[ecclesiastical full moon|calendrical moon]]. The less exact [[octaeteris|8-year cycle]] was replaced by (or by the time of) [[Augustalis (bishop)|Augustalis]]'s treatise "[[De ratione Paschae|On the measurement of Easter]]", which includes an 84-year cycle based on [[Metonic cycle|Meton]]. This was introduced to Britain, whose clerics at some point modified it to use the [[Julian calendar]]'s original equinox on 25 March instead of the Nicaean equinox, which had already drifted to 21 March. This calendar was conserved by the Britons and Irish<ref>{{harvnb|Wormald|2006|p=224 n. 1}}</ref> while the Romans and French began to use the [[Victorius of Aquitaine|Victorian]] cycle of 532 years. The Romans (but not the French) then adopted the still-better work of [[Dionysius Exiguus|Dionysius]] in 525, which brought them into harmony with the [[Church of Alexandria]].

In the early 600s Christians in Ireland and Britain became aware of the divergence in dating between them and those in Europe. The first clash came in 602 when a synod of French bishops opposed the practices of the monasteries established by [[Columbanus|St&nbsp;Columbanus]]; Columbanus appealed to Pope Gregory I but received no answer and finally moved from their jurisdiction. It was a primary concern for St&nbsp;Augustine and his mission, although [[Oswald of Northumbria|Oswald]]'s flight to [[Dál Riata]] and eventual restoration to his throne meant that Celtic practice was introduced to [[Northumbria]] until the 664 [[Synod of Whitby|synod in Whitby]]. The groups furthest away from the [[Gregorian mission]] were generally the readiest to acknowledge the superiority of the new tables: the bishops of southern Ireland adopted the continental system at the [[Synod of Mag Léne]] ({{circa|lk=no|630}}); the {{circa|lk=no|697}} [[Council of Birr]] saw the northern Irish bishops follow suit. The [[Iona Abbey|abbey at Iona]] and its satellites held out until 716,<ref>{{harvnb|John|2000|p=34}}</ref> while the Welsh did not adopt the Roman and Saxon ''computus'' until induced to do so around 768 by Elfodd, "archbishop" of Bangor.