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. EDT (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. EDT (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!

Adding additional confusion is that there is also an "ecclesiastical" full moon, determined from ecclesiastical tables and whose date does not necessarily coincide with the "astronomical" full moon, which is based solely on astronomical calculations. In 1981, for example, the full moon occurred on Sunday, April 19, so Easter should have occurred on the following Sunday, April 26. But based on the ecclesiastical full moon it occurred on the same day of the full moon, April 19!

So, in practice, the date of Easter is determined not from astronomical computations, but rather from other formulae such as Epacts and Golden Numbers. In 2008, we are in Epact 22 and the Golden Number is 14.

Since the beginning of the 20th century, a proposal to change Easter to a fixed holiday rather than a movable one has been widely circulated, and in 1963 the Second Vatican Council agreed, provided a consensus could be reached among Christian churches. The second Sunday in April has been suggested as the most likely date.

Changeable weather, too!

Interestingly, the fact that Easter occurs at a time of the year when weather patterns are transitioning from winter to spring, means a wide variation in the type of weather that can be expected, depending upon just when the holiday occurs in a given year. Ask somebody what type of weather immediately comes to mind when Christmas is mentioned, and likely the answer will be cold and snowy. For the Fourth of July: probably sunny and hot.

Yet, Easter can feature both of these extremes!

In 1970, Easter fell on March 29. In that year, a snowstorm hit the Northeastern U.S. In New York City, the famous Easter Parade had to be cancelled, as four inches of snow fell, with as much as a foot of the white stuff in the northern suburbs.

And yet, just six years later, in 1976, Easter fell on April 18, which ended up going down in New York weather annals as the hottest Easter on record. Not only was the 96-degree reading that day the hottest temperature recorded in Central Park that year, it also was the very first (and only time) that New York held the distinction of being the hottest location in the United States!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.