Easter Moon
Published on Mar 21, 2008 at 6:56 pm.
3 Comments.
Filed under calendars.
Yesterday was the Vernal Equinox, when the Sun was on the celestial equator. It was overhead at noon on the equator. Many countries mark that as the first day of spring. Today is the Full Moon. And, Sunday will be Easter for most of Christianity. A lot of people have commented that Easter seems to be coming early this year. In fact, it is. March 23 is almost the earliest that Easter can occur. The earliest date possible is March 22. The last time that it was that early, though, was March 22, 1812. But, you’ll have to wait until the year 2285 for it to come that early again. March 23, though, is still pretty early, and besides this year, the next time it comes this early will be in the year 2160. The latest date that Easter can occur is April 25. The last time that Easter was that late was in 1943, during World War II. The next time Easter falls so late will be in 2038, though it will be almost as late, on April 24, in 2011. Making this all the more confusing, these dates only hold for part of Christianity, albeit the vast majority. The Eastern church celebrates its holy days on different dates from the rest of Christianity. Virtually all secular holidays are fixed to specific dates. Some other holy days (such as Palm Sunday, Pentecost, Ash Wednesday, etc.) move around the calendar, but that is often because these dates are computed relative to Easter. So, why does Easter wander around?
The reason for that is that in the early Church, Easter was celebrated in accord with the rules for the Jewish Passover holiday. The Bible records Christ’s resurrection as the Sunday following the Passover. So, Easter Sunday was the Sunday after Passover. The problem, though, is that Passover is computed using the Jewish calendar, and the Jewish calendar is a luni-solar calendar. That is a calendar that is based on the phases of the Moon, with months starting with the New Moon, but also in accord with the Sun, with the year relative to the equinox. The problem is that the month of the passover is difficult to compute far in advance. In fact, it was almost incalculable in the days of the early days of Christianity. As a result, different churches were celebrating Easter on different dates. Setting a uniform set of rules for computing Easter was one of the many tasks of the First Council of Nicaea.
The rules for determining Easter separated the celebration of Easter from the Jewish Passover, but those rules normally result in Easter in being celebrated at about the same time as Passover. Those rules are a bit complex, but they can be summarized as approximately saying that Easter is the Sunday following the Full Moon on or after March 21. This is close to the date of Passover because the Hebrew calendar defines Passover to start on the 15th day of Nisan, the first year of the Hebrew Calendar, which must begin with the last New Moon before the Vernal Equinox. Note that this is about the seventh month of the commonly accepted civil Jewish calendar, which starts near the Autumnal Equinox. The Vernal Equinox is supposed to be about March 21. But, the calendar does not exactly match the motion of the Earth around the Sun, so the Vernal Equinox does not always occur on March 21. This year, for example, the equinox was on March 20 (or March 19 for those living in time zones west of here). Actual determination of the equinox to high precision long ahead of time is possible today, but that was not the case close to 2000 years ago. So, to keep everyone on the same page, the decision was to simply define the Vernal Equinox as March 21 in the ecclesiastical calendar. But, that still didn’t alleviate all confusion. It is sometimes difficult for non-astronomers to determine the exact moment of the Full Moon. If you ask a layman whether the moon is full or not, they will look at it and if it looks mostly full, they will call that Full Moon. But, that means that different people may be off a day or two from the actual Full Moon. So, to make sure, again, that everyone celebrates Easter at the same time, the liturgical moons are defined from tables, not from astronomical observations. Those tables generally give a moon that is within a day of the actual astronomical Full Moon, but not always. But, the Hebrew calendar is much closer to the actual astronomical events, so if a Full Moon occurs between the equinox and March 22, as it did this year, then Easter and Passover are a month apart. This year, Passover, instead of occurring the week before Easter starts April 19, 2008.
But, adding to the complexity is the calendar itself. Most of Christianity uses the Gregorian Calendar. I wrote a little about the Gregorian Calendar at the beginning of last year. The Gregorian Calendar was imposed by Pope Gregory XIII in 1578 in a measure to shift the calendar so that the Vernal Equinox occurs close to March 21 each year. Under the old Julian Calendar, the year was slightly too long, and the equinox drifted away from March 21 by about 3 days every 400 years. It was about ten days off in 1578 when the switch was made. But, long before the Sixteenth Century, there was a schism in the Catholic Church. The Eastern Orthodox Church did not adopt the new calendar imposed by a Roman Catholic pope. In fact, the Eastern Orthodox Church still has not adopted the Gregorian Calendar for ecclesiastical purposes, still using the Julian Calendar. Thus, March 21 in the Julian Calendar occurs almost two weeks after March 21 in the Gregorian Calendar. So, about half the time, as occurs this year, the Eastern Church’s Easter falls a month after the rest of Christianity’s Easter, falling on April 27 this year. Last year was one of the years in which both the Eastern and Western Churches celebrated Easter on the same date, April 8, 2007. The next time that they will fall on the same date will be April 4, 2010.
-Astroprof
Daniel Fischer on March 22, 2008 at 12:38 pm: 1
Great write-up of this (surprisingly complex issue) - so it has become the first link in my own summary (§ 5) of paschal issues.
Brother John on March 24, 2008 at 11:33 am: 2
Excellent! I’m a Capuchin friar, and I’ve learned more from you just now concerning this topic than any ‘in church’ source. Thanks for going into such detail. I’ve forwarded this to one of our friars who’s big into liturgy. He’ll love it I’m sure!
Peace and God’s blessings!
Roland on April 17, 2008 at 6:03 pm: 3
Four small additions and corrections:
1. The Gregorian calendar came into use in 1582, not 1578.
2. The Jews reformed their calendar sometime around the fourth century. They no longer determine when to add an intercalary month by astronomical observation, but by a rule. As a result, their calendar drifts (though not as fast as the Julian calendar). That is why Passover comes so late this year.
3. There is an additional reason why Orthodox Easter falls later than Western Easter: The Orthodox use a 19-year lunar cycle that is out of sync with physical reality. As a result, the Orthodox Paschal full moon falls 3-5 days later than the astronomical full moon. This year, for example, the Orthodox Paschal full moon will fall on 25 April, while the astronmical full moon will fall on 20 April.
4. The Eastern Orthodox still use the Julian calendar for determining the date of Easter, but many of the Orthodox churches now use a calendar nearly identical to the Gregorian calendar for other purposes.