# Happy Computus

A table from Sweden to compute the date of Easter 1140-1671 according to the Julian calendar. Notice the runic writing.

Computus (Latin for computation) is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age.

The canonical rule is that Easter day is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after 21 March (nominally the day of the vernal equinox). For determining the feast, Christian churches settled on a method to define a reckoned “ecclesiastical” full moon, rather than observations of the true Moon. Eastern Orthodox Christians calculate the fixed date of 21 March according to the Julian Calendar rather than the modernGregorian Calendar, and use an ecclesiastical full moon that occurs four to five days later than the western ecclesiastical full moon.

In modern language, this definition is best described as: Easter is the Sunday following the Paschal Full Moon date. The Paschal Full Moon date is the Ecclesiastical Full Moon date following 20 March and, for the years 1900 to 2199, can be found in Tabular methods.

We here at Deskarati find it easier to pop this equation into Excel (Cell A1 needs to be the year) – Happy Easter

=DOLLAR((“4/”&A1)/7+MOD(19*MOD(A1,19)-7,30)*14%,)*7-6

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### 2 Responses to Happy Computus

1. alfy says:

When working in Tonbridge years ago I lodged with a lady in her seventies. Connie had only been educated up to elementary standard (i.e. left school at 14) but she still had a keen intelligence, which she described modestly as, “my bump of curiosity”. Had she been born 50 years later she would certainly have gone to university, as I once said to her.

She asked me to find out how Easter was calculated. The College reference works were no more informative than this posting. In the end we gave up on trying to understand it, as it is almost impossible to put it into simple, plain English.

Basically, it is the problem of matching a solar cycle of 365.25 days from which we get the calendar, to a lunar cycle of 28 and a bit days. There is no reason why one astronomical cycle should match any other one, but human beings have been trying to do this for literally millennia.
The matching is done by creating a 19 year cycle and each year is given a number. Hence the occurrence of the two 19s in the formula. The final result should ensure that the date of Good Friday occurs close to a full moon.

One outcome of the problem of calculating Easter was that in the medieval period simple tables of the correct dates of Easter were prepared by experts for the next few years. These were hand copied and distributed all over the medieval world to priests, whose education at that time was very limited. This worked well and was a very convenient system. Because the relatively large vellum books had space, priests and monks took to recording important events for each particular year.

These became known to scholars in later years as “the Easter Annals”. They have enormous value over written histories because they give a clear timeline between one event and another. They also record events which a medieval historian might consider unimportant but which may be of enormous interest to a modern historian, or scientist. For example, the recurrence of plagues, or catastrophic weather events.

One of the best pieces of evidence for the existence of a real historical person called “Arthur”, later described incorrectly as “King Arthur” comes from a set of Welsh Easter Annals where his activities are described briefly and simply for particular years without any romantic or magical associations. These were contemporary records uncontaminated by any future assumptions. The Annals recorded bald facts and left later centuries to interpret them.

2. Steve B says:

I thought I would re-check the Excel Formula – and I still found that it does not work

I had to make a couple of chenges to get it to do so. Thus, I would suggest that the correct version should be:
=DOLLAR((4/A1)/7+MOD(19*MOD(A1,19)-7,30)*14%,)*7-6