Yesterday was nearly Easter

As as Astronomist I am often asked “How do they calculate the date of Easter?”, to which my answer is usually “Look it up on Wikipedia!“.

The simple answer is that Easter Sunday is on the first Sunday after the first full Moon on or after the Vernal equinox. The Vernal Equinox took place this year on March 20th and the more observant among you will have noticed that yesterday was (a) Sunday and (b) a Full Moon. Yesterday was not Easter Sunday because the rule says Easter is on the first Sunday after the first full Moon on or after the Vernal equinox, which does not include a Full Moon on the first Sunday on or after the vernal equinox. Accordingly Easter 2021 is next Sunday 4th April. If the Full Moon had happened on Saturday, yesterday would have been Easter Sunday.

That is just as well really because next weekend is when the holidays and sporting events have been arranged.

I say “simple” answer above because it isn’t quite how the date of Easter is reckoned for purposes of the liturgical calendar.

For a start the ecclesiastical calculation of the date for Easter – the computus – assumes that the Vernal Equinox is always on March 21st, while in reality it can be a day or two either side of that. This year it was on March 20th.

On top of that there’s the issue of what reference time and date to use. The equinox is a precisely timed astronomical event but it occurs at different times and possibly on different days in different time zones. Likewise the full Moon. In the ecclesiastical calculation the “full moon” does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, as determined from tables (see below). It may differ from the date of the actual full moon by up to two days.

There have been years (1974, for example) where the official date of Easter does not coincide with the date determined by the simple rule given above. The actual rule is a complicated business involving Golden Numbers and Metonic cycles and whatnot.

I’m grateful to Graham Pointer on Twitter for sending this excerpt from the Book of Common Prayer that sheweth how to determine the date of Easter for any year up to 2199:

I don’t care what happens after that as I’ll be retired by then. If you apply this method to 2021 you will find it is an 8C. Next year will be a 9B. Further calculations are left as an exercise to the reader.

10 Responses to “Yesterday was nearly Easter”

  1. Anton Garrett Says:

    The ironic thing is that Easter, the Christian festival which celebrates the rising from the dead of Jesus of Nazareth at Passover, no longer coincides with the Jewish Passover feast in most years. I’ve forgotten exactly why, but I think the church claims that the Jews have changed how they calculate Passover since. There is plenty of contention about it between the Eastern Orthodox churches and the Latin-derived churches too; some Orthodox still use the Julian calendar rather than the Gregorian. (The Orthodox call it ‘Pascha’, from Passover, a better name than Easter which comes from the Anglo-Saxon name of the relevant month – which Bede said is derived from an ancient pagan goddess called Eostre. ‘Easter’ entered English Bibles via Tyndale’s translation – Luther had done the same in his German translation – and it persisted in the King James Bible in one place, at Acts 12:4.)

    The Synod of Whitby in AD664, about whether the celtic churches, which had been in place since before the Roman withdrawal, would subjugate themselves to Rome as its missionaries were demanding, centred on the date of Easter. (They did.) But no calendar is demanded in the New Testament (Colossians 2:16), so no Christian leader should press calendric tradition on believers who prefer none. These are matters of private conscience, not apostasy.

    And don’t get me started on the timings of Holy Week! Sabbath just means “religious holiday”, and Passover counts as one of those in addition to the weekly day off work. So which Sabbath did Jesus rise upon as described in the gospel accounts?

    • telescoper Says:

      I recall being invited by Greek friends to Easter barbecues (of lamb) on the date of Greek Orthodox Easter, quite a lot later than the Western Church dates (and usually better weather).

      • The dates are the same, one week apart, 4 weeks apart, or 5 weeks apart, with Orthodox never being earlier.

        Some countries till use the Julian calendar for Easter but the Gregorian for Christmas, while some use the Julian for both.

  2. Carl Friedrich Gauß, noted astronomer, mathematician, and man about town, also put some effort into computing the Easter date.

  3. Anton Garrett Says:

    I’m grateful to Graham Pointer on Twitter for sending this excerpt from the Book of Common Prayer that sheweth how to determine the date of Easter for any year up to 2199

    Microsoft needs an autocorrect option to 17th century English!

    • telescoper Says:

      When I was young I was in the local church choir and I remember reading quite a lot of the old-style Book of Common Prayer during the vicar’s interminable sermons. I found the slightly archaic language very appealing, especially the Psalms, and was very unhappy when we were eventually given a modern English version that made everything seem so banal.

      • Anton Garrett Says:

        The Psalms in BCP are actually Miles Coverdale’s translation, not Tyndale’s or the committee version in the King James Bible. Everybody calls BCP the “1662 Prayer Book” but that is when it was REinstated as mandatory following Oliver Cromwell’s experiment with protestant religious freedom. Most of the prayers in BCP are also from the previous century, having been written by Thomas Cranmer (made in Cambridge, burnt in Oxford…)

        I’d have a mongrel Bible today so far as translations go. The Psalms would be on of the 16th/17th century translations because they are poetry. For Paul’s letters you want the simplest and clearest modern English.

      • telescoper Says:

        I have a King James Bible (not original), which is occasionally useful for crosswords.

  4. During the early 1970s the Home Office had an odds-and-sods department responsible for, amongst other things, dealing with rambling correspondence and also the date of Easter. Maybe they still do. Now I’ve far too much time on my hands it would be very tempting to send Ms Patel a long hand-written letter detailing how T. V. Bede had misunderstood, or misrepresented, the Celtic bishops’ argument.

    • Anton Garrett Says:

      Without doubt one of Bede’s agendas was (unfortunately IMHO) to endorse the decision at Whitby.

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