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. 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 21is 2:40 p.m. EDT (though in reality it's never actuallyfull).

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

But in 2008, Easter will arrive on March 23. So early infact, that Palm Sunday, which is observed on the Sunday before Easter, wascelebrated this year on the day before Saint Patrick's Day; a calendricaloddity.

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

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

Equinox and full moon are the keys

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

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

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

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

Hence, there can sometimes be discrepancies between theecclesiastical and astronomical versions for dating Easter. In the year 2038, forinstance, the equinox falls on March 20 with a full moon the next day, soastronomically speaking, Easter should fall on March 28 of that year. Inreality, however, as mandated by the rules of the Church, Easter in 2038 willbe 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 necessarilycoincide with the "astronomical" full moon, which is based solely onastronomical calculations. In 1981, for example, the full moon occurred onSunday, April 19, so Easter should have occurred on the following Sunday, April26. But based on the ecclesiastical full moon it occurred on the same day ofthe full moon, April 19!

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

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

Changeable weather, too!

Interestingly, the fact that Easter occurs at a time of theyear when weather patterns are transitioning from winter to spring, means awide variation in the type of weather that can be expected, depending upon justwhen the holiday occurs in a given year. Ask somebody what type of weatherimmediately comes to mind when Christmas is mentioned, and likely the answerwill 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 snowstormhit the Northeastern U.S. In New York City, the famous Easter Parade had to becancelled, as four inches of snow fell, with as much as a foot of the whitestuff in the northern suburbs.

And yet, just six years later, in 1976, Easter fell on April18, which ended up going down in New York weather annals as the hottest Easteron record. Not only was the 96-degree reading that day the hottest temperaturerecorded 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 UnitedStates!

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 otherpublications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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Joe Rao
Skywatching Columnist

Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.