The Computus

St Emmeram smallThe recent Anglican conference at Lambeth led to some controversial decisions, of which the most surprising, perhaps, was an agreement to work with other churches worldwide to fix the date of Easter. It was almost as surprising that the initiative for this startling break with tradition originated with the Coptic Church, not one usually associated with radical change. The archbishop of Canterbury rather sweetly suggested that any decision was unlikely to be applied in less than five years, because the calendar industry had probably already got stocks in for 2020. I dispute this: how can they possibly know already who the hot seminarians to feature in Calendario Romano (available ‘in all kioscos and souvenir store of Rome and Venice’), let alone the cutest puppies and kittens, will be in five years’ time?

Get your Calendario from this kiosco.

Get your Calendario from this kiosco!

But, as Him Indoors (who really ought to know this sort of thing, given his upbringing) asked: why is Easter not fixed when Christmas is? It has long been argued that the choice of 25 December for the day of Christ’s Nativity (itself the only day of the year on which a Mass was could be said during the hours of darkness) was a pragmatic borrowing by fourth-century Christians of the existing Roman feast of Dies Natalis Solis Invicti (the birthday of the Unconquered Sun), instituted by the Emperor Aurelian (270–5) to coincide with the winter solstice.

Sol Invictus, from a late Roman stela.

Sol Invictus, from a late Roman stela.

That Christ is the ‘Sun of righteousness’ prophesied by Malachi (4:2) made this takeover even more appropriate.

However, another equally ancient argument has the conception of Christ take place on 25 March (Annunciation, Lady Day): nine months after 25 March is 25 December (QED), though this is surely a chicken and egg argument? (25 March was also the first day of the year in England until 1752, and remains a Quarter Day.)

Detail from Van Eyck's 'Annunciation'. Washington, D.C., National Gallery of Art.

Detail from Van Eyck’s ‘Annunciation’. Washington, D.C., National Gallery of Art.

But 25 March was, according to one line of reasoning, the day not only of the conception of Christ, but also (33 years later) of his crucifixion: and thus fixed. However, the crucifixion was known from the Bible to have taken place on the eve of the Jewish feast of the Passover, which of course is a moveable feast in modern terms, because it depends on the lunar calendar.

Rogier van der Weyden's 'Crucifixion' triptych. Vienna: Kunsthistorisches Museum.

Rogier van der Weyden’s ‘Crucifixion’ triptych. Vienna: Kunsthistorisches Museum.

The feast begins on the 14th day of the month of Nisan. According to the church historian Eusebius (c. 260–340), the church in Asia followed the tradition of celebrating Good Friday on the second day of Passover, and Easter two days later, regardless of the day of the week (apparently the Christians asked their Jewish neighbours when they were putting away their leavened bread to ascertain the date); while Irenaeus, among others, argued strongly that Easter Day should always be celebrated on a Sunday. Victor I (pope from c. 189 to 199), ‘who presided over the church of Rome’, promptly excommunicated the churches in Asia, but Irenaeus, living up to his name, begged Victor not to take such a harsh line.

There followed decades of controversy and the complicated calculations known as the computus.

Diagrammatic form of the computus, from the library of the monastery of St Emmeram, Regensburg.

Diagrammatic form of the computus, from the library of the monastery of St Emmeram, Regensburg.

The problem with Passover was that it sometimes came before the spring equinox (fixed at 21 March), while Easter should/could not. Paschal full moons, ecclesiastical new moons, synodic months, the Metonic cycle (or Enneadecaeteris, the nineteen-year period over which the solar year and lunar months end up in synch, and which the Antikythera Mechanism could calculate) and many other astronomical, mathematical and ecclesiastical concepts were thrown into a complex brew from which emerged the authoritative statement of the Council of Nicaea (325) that Easter will fall on the Sunday following the full moon that follows the (northern) spring equinox.

The Antikythera Mechanism, a Greek astronomical calculator from the second century BCE.

The Antikythera Mechanism, a Greek astronomical calculator from the second century BCE.

Unsurprisingly, this did not end controversy. Churches in different areas did or did not adopt the system, and those that did (Alexandria and Rome, for example) used different calendars. One of the more heroic figures in the effort to reconcile all this and produce a consistent calendar across the remains of the Roman empire was Dionysius Exiguus (c. 470–c. 544) from Tomis in Scythia, who also calculated the year of Christ’s birth and instituted the system of BC/AD which in the form of BCE/CE is still among us, even though he almost certainly got the year wrong. (The epithet ‘Exiguus’ refers to his humility rather than his stature, apparently.)

A modern icon of Dionysius Exiguus.

A modern icon of Dionysius Exiguus, writing out his theory.

In England, it was not until the Synod of Whitby in 664 that the decision of Nicaea was adopted, as against the ‘Celtic’ tradition which had flourished at and spread from Iona and had remained largely untouched by decisions made in Rome. (Another controversial point settled at Whitby was that monks should be tonsured in the Roman style, and not the Celtic…)

St Thomas Aquinas modelling the Roman tonsure, by Carlo Crivelli. London: The National Gallery.

St Thomas Aquinas modelling the Roman tonsure, by Carlo Crivelli. (London: The National Gallery.) Opinions differ as to how the Celtic tonsure looked.

Then, of course, came the Great Schism of 1054 between the eastern and western Churches, and the 1582 Gregorian reform of the calendar (driven inter alia by the problem that the actual vernal equinox had by now drifted to 10-11 March, while the ecclesiastical computations required that it stay on 21 March). This meant that various Christian churches were again celebrating Easter on different days, and of course they still do: western and Orthodox Easter occasionally coincide (as they did in 2011 and will again in 2017), but they can be as much as a month apart: 27 March (western) and 1 May (Orthodox) this year, for example.

Easter Day: Titian's 'Noli me tangere'. (London: The National Gallery.)

Easter Day: Titian’s ‘Noli me tangere’. (London: The National Gallery.)

Meanwhile, under the Gregorian calendar, the earliest possible date for Easter Sunday is 22 March (it happened in 1761 and 1818, apparently, but will not occur again until 2285), while the latest is 25 April, which last happened in 1943 and is next due in 2038. The fixing of Easter has long been regarded as desirable by the travel industry, the world of education (as the spring holiday is still tied to the Easter weekend, and this makes school and university terms of uneven lengths and examinations difficult to schedule), and, I assume, the catering and confectionery industries ­– although I spotted the first hot cross buns in my local Sainsbury’s on 7 January, and the (to me) most disgusting Easter pseudo-symbol of all – the Cadbury’s Crème Egg – is on sale between January and April.

Coming to a High Street near you...

Coming to a High Street near you…

I’m assuming that the archbishop’s modest proposal will be opposed by traditionalists of various hues. I am a bit torn: I can see the practical sense, but my conservative side regrets that all that millennium-old work and intellectual effort of the computus – all the factors which turn the date of Easter into an astronomical/ecclesiastical Schleswig-Holstein question – should be cast aside to simplify the arrangement of package holidays.


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5 Responses to The Computus

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  4. Dane says:

    Has the diagram labelled “Diagrammatic form of the computus, from the library of the monastery of St Emmeram, Regensburg.” Been dated? If so when was it created?


    • This manuscript is now known as the Munich Computus, because it was acquired by the Bayerische Staatsbibliothek in the nineteenth century. It is itself an early ninth-century copy (and believed to be the only surviving one) of a text first written in Ireland about 720.


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