Full moon heralds early Easter

— -- Friday, March 21 brings us the first full moon of the new spring season, the vernal equinox having already occurred on March 20 at 1:49 a.m. ET (or on March 19 if you live in the Mountain, Pacific or Alaskan-Hawaii time zones).

The official moment that the moon will turn full on March 21 is 2:40 p.m. ET (though in reality it's never actually full).

The first full moon of spring is sometimes referred to as the Paschal full moon; the moon that is used to set the date of Easter in a given year. This year, if you have not already noticed, Easter is going to arrive unusually early. If you're 50 years old or younger, the earliest Easter in your lifetime came on March 26 (in 1967, 1978 and 1989). In 1951, Easter fell on March 25; in 1940, March 24.

But in 2008, Easter will arrive on March 23. So early in fact, that Palm Sunday, which is observed on the Sunday before Easter, was celebrated this year on the day before Saint Patrick's Day; a calendrical oddity.

The last time that Easter fell this early in the calendar was 1913. And before that, in 1856.

Which leads one to ask the question, exactly just how is the date of Easter determined?

Equinox and full moon are the keys

Traditionally, Easter is observed on the Sunday after the Paschal full moon. If the Paschal moon occurs on a Sunday, Easter is the following Sunday.

Following these rules, we find that Easter can fall as early as March 22 and as late as April 25. Pope Gregory XIII decreed this in 1582 as part of the Gregorian calendar.

As we already have noted, in 2008, the Paschal full moon occurs on Friday, March 21. So according to the current ecclesiastical rules Easter is to be celebrated two days later, on Sunday, March 23.

Interestingly however, these rules also state that the vernal equinox is fixed on March 21, even though at European longitudes from the years 2008 through 2101 it actually will occur no later than March 20.

Hence, there can sometimes be discrepancies between the ecclesiastical and astronomical versions for dating Easter. In the year 2038, for instance, the equinox falls on March 20 with a full moon the next day, so astronomically speaking, Easter should fall on March 28 of that year. In reality, however, as mandated by the rules of the Church, Easter in 2038 will be observed as late as it can possibly come, on April 25!