Question:

In the recent Q&A on the date of Easter, the question as posed in the article asked why it was sometimes in March and sometimes in April. The answer which says a lot of good things like that the Puritans not observing Easter, does not actually address the question as posed. While I'm no expert, I thought it was always the first Sunday after the first full moon after the vernal equinox. Perhaps that just got lost in translation?

Answer:

Concerning the date of Easter, I think the answer as posted does answer the question as posed ("Some years Easter is in March and other years it is in April. Why is this?"):

The Western church uses the Gregorian calendar.... The Gregorian calendar is 13 days ahead of the Julian calendar, which, coupled with differing definitions of a full moon and an equinox, accounts for the date disparity.

Even if we ignore Eastern Orthodox and Puritan churches, the question of the date on which to celebrate Easter is still a complicated one.

Let's go back to your statement:

... I thought [the date for Easter] was always the first Sunday after the first full moon after the vernal equinox.

The World Council of Churches Web Web site seems to agree with you, in a discussion of "frequently asked questions about the date of Easter":

Q. So how is the date of Easter calculated?
A. The Council of Nicaea established [in A.D. 325] that the date of Easter would be the first Sunday after the full moon following the vernal equinox.

So, yes, the date for Easter does seem to be, as you say, "always the first Sunday after the first full moon after the vernal equinox." That definition is the reason that Easter is "movable," i.e., that it does nor occur on the same date each year. Thus in one year, Easter may be in March; in another year, Easter may be in April.

But there is another problem: In defining the date for Easter, people (and churches) speak of "the first full moon" or "the vernal equinox" in an inexact or even incorrect way:

One web site provides examples of right and wrong definitions:

This is right:
Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date for the year.

This is wrong:
Easter Sunday is the Sunday following the full moon after the Vernal Equinox.

That web site explains why the second statement is inadequate or wrong:

Sadly, many definitions of Easter on the Internet and in Encyclopaedias and Almanacs are misleading, ambiguous and just plain wrong!....

I think that almost everyone reading this would assume that "full moon" refers to an astronomical full moon date. An astronomical full moon (AFM) occurs at one instant in time, and therefore occurs on 2 dates around the world (consider countries either side of the international dateline!). Again, countries do not celebrate different Easter dates based upon their own full moon dates!

Astronomical Full Moons dates are not directly related to Easter dates. Easter is based upon Paschal Full Moon (PFM) dates, and each PFM is the particular Ecclesiastical Full Moon (EFM) date after March 20. EFM dates are approximated astronomical full moon dates....

In June 325 A.D. astronomers approximated astronomical full moon dates for the Christian church, calling them Ecclesiastical Full Moon (EFM) dates. From 326 A.D. the PFM date has always been the EFM date after March 20 (which was the equinox date in 325 A.D.)....

Vernal means spring, and countries in the Southern hemisphere have opposite seasons to those in the Northern hemisphere. Of course, Easter is not celebrated in September in the southern hemisphere! Most astonomers interpret "Vernal Equinox" to mean the March Equinox, but even that is ... wrong ....

The equinox is not related to Easter! March 20 is the critical date for determining all Easters, and March 20 was the equinox date in 325 AD when the definition of Easter date was agreed. In our current Gregorian calendar, the March Equinox is one of 5 dates from March 18 to 22.

That particular Web site's author refers to an article on the Web site of the Astronomical Society of South Australia as being "the most authoritative article on Easter dating" he has seen which can be consulted for a complete (and correct) explanation." Here, again, is their definition:

Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date for the year.... See Christian Prayer Books for proof of this concise definition.

Further, this definition is accurate for each of the 3 different implementations of Easter Sunday:

* From 326 AD in the Julian calendar (no longer used)
* From 1583 AD or later in the Julian calendar, converted to the Gregorian calendar (e.g., Orthodox Christian churches)
* From 1583 AD or later with PFM dates revised for the Gregorian calendar (e.g., Western cultures, also used for public holidays)

To Summarise ...

* Easter Sunday date is the Sunday after the Paschal Full Moon (PFM) date

* The PFM is the first Ecclesiastical Full Moon (EFM) date after March 20....

* EFMs are approximated astronomical full moon dates, not actual astronomical full
* The PFM date is an EFM date that estimates an astronomical full moon date....

* Therefore, Easter Sunday occurs around the time of an astronomical full moon, but the astronomical full moon has nothing to do with determining Easter Sunday date.

* Because the Gregorian March Equinox date is one of 5 dates from March 18 to 22, and Easter Sunday dates range from March 22 to April 25, Easter sometimes occurs around the time of the March equinox. But again, the March Equinox has nothing to do with determining Easter Sunday date.