In the Dark

As as Astronomist I am often asked “How do they calculate the date of Easter?”, so here goes.

The simple answer is that Easter Sunday is on the first Sunday after the first full Moon on or after the Vernal equinox. The Vernal Equinox took place this year on March 20th and the first full moon after that was on April 2nd.

I say “simple” answer above because it isn’t quite how the date of Easter is reckoned for purposes of the liturgical calendar.

For a start, the ecclesiastical calculation of the date for Easter – the computus – assumes that the Vernal Equinox is always on March 21st, while in reality these days it is more frequently 20th March, like this year.

On top of that there’s the issue of what reference time and date to use. The equinox is a precisely timed astronomical event but it occurs at different times and possibly on different days in different time zones. Likewise the full Moon. In the ecclesiastical calculation the “full moon” does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, as determined from tables (see below). It may differ from the date of the actual full moon by up to two days.

There have been years (1974, for example) where the official date of Easter does not coincide with the date determined by the simple rule given above. The actual rule is a complicated business involving Golden Numbers and Metonic cycles and whatnot.

Here is excerpt from the Book of Common Prayer that shows Anglicans how to determine the date of Easter for any year up to 2199:

The calculations are based on the approximately 19-year metonic cycle., which is why the above table will not work indefinitely

For this year we find that (2026+1) ÷19=106 with a remainder of 13 (106 × 19 being 2014). The Golder Number for this year is therefore 13, or XIII in the Table. This gives the date of the Paschal Full Moon, which occured this year on 2nd April, which is indeed the day in the centre column next to XIII in the left-hand column in the table. The Sunday Letter is determined by the remainder of (2026+506+6)÷7, which is 4, so this year’s Sunday Letter is D. The date of Easter Sunday is given by the entry in the centre column next to the first occurrence of D in the right-hand column after the Golden Number XIII appears in the left-hand column, i.e. April 5th. I hope this clarifies the situation.

This entry was posted on April 3, 2026 at 6:28 pm and is filed under History, The Universe and Stuff with tags astronomy, Book of Common Prayer, computus, Easter, Equinox, Golden Numbers, Sunday Letters. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.