On Determining the Date of Easter

I will sing unto the LORD, for he hath triumphed gloriously: the horse and his rider hath he thrown into the sea (Exodus 15.1)

Easter has its roots in the Jewish Festival of Passover, an eight-day feast; which commemorates the journey of the Israelites, out of slavery in Egypt and to the Promised Land. We will hear this account (Exodus 14) at the Easter Vigil. In order to facilitate their remarkable escape across the Red Sea, they would no doubt have depended on the light of a full moon. They would have also needed to harvest barley to bake bread, the earliest possible opportunity being spring, of which they consumed in haste unleavened. Passover therefore was celebrated on the coming of the first full moon after the spring equinox (14th day of the month Nisan of the Hebrew calendar).

Easter Window, Chester Cathedral

The early Christians continued to use the Jewish method for determining the date of the Passover, placing the Crucifixion on the 15th day of Nisan (see John 19:14), meaning that Easter would not necessarily be celebrated on a Sunday!; however a diversity of local practice evolved with the evangelisation of the Gentiles.

The Council of Nicaea, A.D. 325 attempted to resolve this controversy, by ruling that Easter should be celebrated on the first Sunday after the first full moon following the spring equinox and the calculation would be carried out independently from the Hebrew calendar. Indeed it took centuries for calculations to converge; indeed Celtic Christians continued to follow their own method for determining Easter as late as the 7th Century and did not adopt the method used by the Latin Church until the Synod of Whitby in A.D. 664.

A further controversy occurred in 1582 when it was realised that the earth doesn’t quite take 365 1⁄4 days to orbit the sun as was previously assumed. Pope Gregory XIII therefore reformed the Julian calendar as a result by making the century years non-leap years (unless divisible by 400). Due to the political and religious turmoil at that time, the new calendar took many centuries to be accepted. Britain did not adopt the Gregorian Calendar until 1752 and even today the Eastern Orthodox Churches continue to use the original Julian Calendar.

When Great Britain finally adopted the Gregorian Calendar, the then Astronomer Royal, the Rev’d James Bradley simplified the tables produced in 1582 and his method is used to compile the tables in the Book of Common Prayer.

Calculating the date of Easter is rather complicated because calendars have to work in whole numbers of days and the number of days taken for the Earth to go round the Sun and for the Moon to go round the Earth most certainly are not! For those readers who enjoy playing around with numbers and are armed with a pocket calculator (or a knowledge of long division!) we present a simplified method of calculation.

  1. First determine the “Golden Number”. Divide the year by 19, take the remainder and then add one to it. The Golden number for this year (2021) is 8.
  2. Now multiply the Golden number by 11 (gives 88)
  3. Take this result away from 575* (gives 487)
  4. Divide it by 30 and take the remainder (gives 7). This remainder is the number of days that the first full moon occurs after March 21st (7 days).
  5. If the full moon is calculated to be 28 days after the spring equinox (and the a golden number greater than 11) or is 29 days after (regardless of the golden number), then the number of days is reduced by one. This year this adjustment is not applicable because the Pascal Full Moon is only 7 days after March 21st, so the Paschal Full Moon occurs on March 28th (Sunday)
  6. Easter Sunday is the first Sunday after the Paschal Full Moon – April 4th

This simplification is reliable for the remainder of this century (the number 575 in step 3 actually contains minor solar and lunar corrections which periodically need adjusting but not before then!). The calculations are shown in full on-line on http://almanac.oremus.org/easter/bradley.html

This article was written by Jonathan Wilson and was originally published in St Wilfrid’s parish magazine, Network, in April 2011. The steps in the calculation have been amended to bring it up to date!

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