Date of Easter Formula

I am looking for the official, authoritative, defined, and declared formula for calculating the date of Easter currently in use for the Catholic Church.

Also, does anyone know if Eastern Catholics use this formula or a different (or older) one?

Maybe you already have this for the Western Church. It refers to tables (which I believe were computed years, if not centuries, ago. I was hoping for a link to online Church documents, including tables, but this website doesn’t have them. It might be a starting point for further research.

usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/date-easter

I know that after Vatican II, the way Easter was calculated changed. Those of Eastern Catholicism, Tridentine Mass goers, and Orthodox follow the schedule set before Vatican II. They are different. This year was one of the few when they actually match up. I think I know the calculations, but I wish to refrain since I do not have a source.

Pax!

It is the first Sunday after the first full moon after the vernal equinox (first day of Spring).

That was the formula from the Council of Nicea, but don’t we have a new (more advanced) formula following Vatican II?

In general, they follow the same date as the Latin Church.

However, there is provision to follow the Orthodox calculation in certain circumstances–say, for example if the E Catholics are in an isolated area which is heavily Orthodox, they may follow the Orthodox calendar for Easter. This is very rare though. In W Europe, the Americas, Australia, etc. they (ie Catholics) all follow the western date. I’m not sure exactly what happens in heavily Orthodox countries like Russia, but the Catholics are at least permitted to follow the Orthodox–whether they do or not, I can’t say exactly.

No. Vatican II did not change it. The Council left the matter open, and said something about allowing for the possibility of it being changed, but this never happened.

What did happen is that sometime in the mid 1700s, the calendar was adjusted by a few days (about 11) because it was discovered that the calculations were off–it had to do with making century years (1700, 1800, 1900, 2100) NOT leap years, but millenial years (2000, 3000) yes leap years. That’s why there is 14 day difference between the Catholic and Orthodox calendars–the Orthodox didn’t make the adjustment. That makes their March 21 different than ours, and from what I understand, they go by “March 21” rather than the actual astronomical vernal equinox. That’s why Orthodox Easter is usually later, rarely the same, but never earlier.

Isn’t it also the case that the calculation is based on the pascal full moon rather than the astronomical full moon? As I understand it, the pascal full moon may be as many as two days different from the astronomical full moon.

OK You lost me there. That’s something I’ve never heard. I’m interrested. Do you have more info on this?

My ex-sister-in-law is Greek Orthodox. If I remember correctly, she once said they use the same “first Sunday after the first full moon after the vernal equinox” formula, with the added proviso that Easter has to be after Passover.

Is any of this outlined in canon law or some other online Vatican document?

“…after Passover”
I might be confusing Christmas and Easter :blush:

It’s in the Catechism. Here’s a handy link to the Vatican webpage
vatican.va/liturgical_year/easter/2003/catechism_en.html

I simply went to the site and did a search for “date of Easter” (I didn’t have it available offhand!)

I don’t pretend to be an expert on this and I think in reality we are saying the same thing. I’ve just been interested in this topic for some time.

In general, I believe your formula, that Easter is the first Sunday after the first full moon after the vernal equinox is correct and to the best of my knowledge, has been and will continue to be for the foreseeable future.

My point was that the dates for the full moon and for the start of spring used for the computation of Easter are not really determined by current lunar and earth orbits, but rather by tables computed centuries ago. Fortunately the tables are pretty accurate (although as you mention they have had to be adjusted on occasion and the date of the vernal equinox is not necessarily what we think it is.)

I think it’s more that the algorithm mentioned was made to fit the tables than that the tables were computed by the algorithm.

Here are a couple of articles related to the topic.

christianity.about.com/od/faqhelpdesk/qt/whyeasterchange.htm
en.wikipedia.org/wiki/Ecclesiastical_full_moon

This article: en.wikipedia.org/wiki/Computus (with all the usual caveats that apply to Wikipedia) should help. The question you ask, unfortunately, does not have a simple answer, and there is a lot of misinformation out there. Inter gravissimas was the papal bull issued by Pope Gregory XIII in 1582 that promulgated the current calculation of Easter.

Some keywords you could use to search on would be:
Easter, computus, epact, Gregorian calendar, Inter gravissimas, Golden Number, Clavius

The Catholic Encyclopedia (e.g. Newadvent.org) also has pertinent information.

I should also mention this site, which has a lot of good info:
romcal.net/

FrDavid - Thanks for the post. That is what I thought it was, but finding the catechism online USED to be an issue. Not anymore thanks to you! Thanks.

Pax!

Actually, the link I provided wasn’t a direct link to the Catechism–it was to a Vatican webpage from 2003 about Easter which just happened to quote the Catechism about the calculation of Easter.

For the whole Catechism online, the Vatican has it here:
vatican.va/archive/ENG0015/_INDEX.HTM

Here is a link to the document:

Inter gravissimas

The calendar is now perfect, according to Gregory. I think he was being a bit optimistic, but they really did an excellent job - definitely good enough for government work.

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