I’ve been away for the Orthodox Easter, which was last Sunday. I thought it’s a good time to deal with the age-old question: how is the Easter date determined?
Easter is, supposedly, on the Sunday after the first full Moon of the spring, where the spring supposedly starts on the equinox. But people in the Eastern Orthodox countries can see that, more often than not, this is not the case. Acute observers will notice that, sometimes, the Catholic/Protestant Easter is also off, such as in 2019, when the equinox was on 20 March 21:58 UTC, the full Moon was a few hours later, on 21 March 01:43 UTC, and yet Easter was on 21 April.
What happens is that the Church didn’t want to need to ask an astronomer each time. Especially until a few centuries ago, an annual announcement by the head of the Church would be hard to communicate to remote places. So they devised a method with which Easter can be calculated easily by anyone. Here are the details for the Julian Easter (which is the one celebrated by the Eastern Orthodox Church):
It is assumed that the day of equinox is always 21 March (which in the Julian calendar is now off by 13 days, i.e. it is 3 April).
It is assumed that, every 19 years, the Moon phases recur at the same day of the year (the “Metonic cycle”).
A table (with 19 entries) provides the full Moon date for each of these 19 years.
The Gregorian Easter is calculated in a similar manner, but it is more accurate, e.g. it has a provision for the fact that the Metonic cycle is off by roughly one day per century. Interestingly, the Julian calendar as a whole is off by roughly one day per century, and that error partially cancels out the Metonic cycle’s error. As a result, the Julian Easter’s “full Moon” is off by less than a week, which is why sometimes the Gregorian and the Julian Easter coincide.