Home The Computus Engine What is Computus?

What is Computus?

written by John March 22, 02008

Codex 250, Burgerbibliothek Bern, Switzerland

As Easter approaches it struck me that I’ve never really explained the meaning of “computus” in Computus Engine. It’s derivation is obviously Latin but its origin is fascinating. It might surprise you to learn that computers have been around since medieval times. The original “computers” were Christian monks who reckoned the date of Easter by performing a tricky astronomical calculation called “computus”.

Easter is the most important feast in the Christian calendar and getting the date correct was very important to the church. After many years of debate, schism and calendar reform the Catholic church finally decreed that Easter falls on:

the first Sunday, after the first full moon, after the vernal equinox.

If you’re familiar with astronomy, astrology or even Paganism for that matter, then you’ll probably know what the ‘vernal equinox’ is: The proportion of nighttime to daytime fluctuates throughout the year. There are two points in this annual cycle where the duration of night and day are equal. Each point is called an ‘equinox’ meaning ‘equal night’. The first is in the Spring and is often called the ‘vernal equinox’.

The vernal equinox this year is on the 20th of March. The first full moon after this date is today, the 22nd of March. And the first Sunday after that is tomorrow, the 23rd of March. So Easter Sunday is very early this year. So early in fact that it has caused some controversy and confusion over the date of St Patrick’s Day. Even more bizarre though is the chronology caused by the date of Passover (reckoned in the Jewish calendar and held this year at the end of April) which has put the crucifixion before the last supper.

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