Easter date differences

Created on 2020-04-12 07:22

Published on 2020-04-12 08:20

Many of my colleagues in Germany, Netherlands and US are surprised by the fact that there are two Easter dates therefore two disjoint sets of vacation days. One for the western, Catholic world, and one for the eastern, Orthodox world. The explanation for this is in fact very interesting and comprises a lot of mathematics and history.

The Catholic Easter is computed as the first Sunday after the first full moon on or after the March Equinox. Seems simple but in fact this is just a little bit more complicated. The synodal moon period is 29.530588 days hence there is not a constant day of month for occurrence, the first full moon Sunday after March 20/21 could be as late as April 18.

The western world started using Gregorian calendar from 1528 but the Orthodox countries kept using the Julian calendar that is 13 days behind. The formula is basically the same but there is the Julian offset to be counted in. Moreover Orthodox Church has a third condition, given by the 1st Ecumenical Council of Nicaea held in 325, that states that the Easter should be after the Jewish Passover (Passover is on Nissan 14th, date computed on a lunar calendar). As Passover is one week long and is computed similarly to the Catholic Easter this can push the Orthodox Easter date to May. The difference between the two can be up to a month. For example if in 2020 the Catholic Easter is on April 12 and Orthodox Easter is on April 19, in 2024 the dates will be be March 31 and May 5. However, sometimes the two dates could be equal, last time in 2017 and next time in 2025.

Now for the geeks, computing Easter date is a very interesting coding kata. Implementations rely on moon phase determination (math) and the Gregorian/Julian conversions (math and history).

Java and .NET provide pretty good mechanisms and there is a plethora of libraries for other languages. The calculation of Easter date was called "Computus" in Latin (https://en.wikipedia.org/wiki/Computus) and might be the etymon for Computer, hence the Computer Science has a very interesting origin. The wiki page offers quite a lot of information and starting points (Gauss algorithm, tabular representations, ...) so during self-isolation boredom one can try something interesting. As a side note, the Julian calendar seems to be related to the works of St. Dionysius Exiguus, a monk born in present-day Romania, so probably the bugs in the Julian calendar are also his fault.