Orthodox Easter: What’s up with that? — Part 2

Yesterday, I posted about the difference between the Gregorian calendar that we use in the West and the Julian calendar that’s still used by most Eastern Orthodox churches. There’s a 13-day difference between them for the period between 1900 and 2100, so any given day (e.g. October 12) shows up 13 days later on the Julian calendar than on the Gregorian calendar. So someone with a Julian calendar would celebrate October 12 on a day that shows up as October 25 on our Gregorian calendars.

Now, step 2 in understanding Easter: how is Easter defined? As an initial matter, note that Easter Sunday is the day when Jesus Christ rose from the dead, and the Gospels say that happened when he came to Jerusalem to celebrate Passover. So in principle, you could think that Easter happens at Passover — more precisely, on 14 Nisan, the day before Passover starts on 15 Nisan. (See also various verses in chapter 19 of the Gospel according to John.) Now the Jewish calendar is a complicated thing, but the bottom line is that 15 Nisan is always on a full moon after the spring equinox. How do they guarantee that this happens? Intercalation, that’s how. Because Jewish months are lunar, the folks in charge of the calendar stick in a whole nother month, called Adar II, before Nisan just to ensure that Passover is on a full moon after the equinox.

So why not tie Easter to Passover and celebrate it whenever 14 Nisan would fall in the Jewish calendar? The Christians who believed you should do this were called Quartodecimans, or “fourteeners” if you will. Various Christian communities followed the “14 Nisan” rule and just asked their local Jews when Passover started, but after controversies in the second and third centuries, Christians ended up settling the matter at the Council of Nicaea in 325.

The Nicene rule separated the Christian computation method from the Jewish one. In the first place, there was a concern over being in spring: though these days, 15 Nisan is always a full moon after the spring equinox, sometimes Easter ended up being celebrated before the equinox, apparently, said the bishop of Alexandria, “through negligence and error” on the part of the Jewish calendrical authorities. (It’s not necessarily negligence and error, because you could also choose to date the start of spring, as some Jews did, as the time the barley ripens. If you did that, Passover could sometimes fall before the equinox.)

In the second place, there was a concern over uniformity: apparently, not all Jewish communities at the time calculated their months in exactly the same way. This might actually be the same concern, if some communities used the equinox and others used the barley, or if barley ripened at different times in different places.

In the third place, reported Emperor Constantine: “It was . . . declared improper to follow the custom of the Jews in the celebration of this holy festival, because, their hands having been stained with crime, the minds of these wretched men are necessarily blinded.”

So Christian churches basically try and create their own “Christian Nisan” — figure out the full moon after the spring equinox — but also add on an extra rule, which is that Easter should fall on a Sunday. So that’s where we get the rule that Easter is the Sunday after the full moon after the spring equinox. (In principle, that still means that Western Easter should fall within Passover, but since Hillel II’s reforms in the 4th century, the Jewish calculations for Nisan are based on a formula and not on astronomy — see Gauss’s formula for the date of Pesach. Thus, in 2008, Western Easter fell on March 23, while Passover didn’t start until April 20.)

Given this formula, it’s not at all obvious why Christians should differ as to the date of Easter. We can observe the spring equinox using astronomy. We can observe the full moon using astronomy (though that might differ by a day or two depending on where one is in the world). And everyone has the same Sunday. So why should there be any (significant) difference?

The answer is that “Sunday” really does mean “Sunday,” but “full moon” doesn’t necessarily mean “full moon,” and “spring equinox” doesn’t necessarily mean “spring equinox.” The equinox rule is the biggest factor in the East-West date divergence. For purposes of calculating Easter, we use March 21 instead of the true date of the equinox, which could be March 19 or March 20. (The complexities behind “full moon” will be in a later post.) So we immediately see how the Western and Eastern churches can differ: March 21 is considered to fall on a different day depending on your calendar, and March 21 in the Julian calendar is what we in the West would call April 3.

Sometimes there’s no full moon between March 21 and April 3, so the relevant full moon for Easter-computation purposes is the same. For instance, in 2011, there were full moons on March 19 and April 18, so both calendars celebrated Easter on (Gregorian) April 24, the Sunday after (Gregorian) April 18. But sometimes there is a full moon between March 21 and April 3, so the relevant full moons will be about a month off. For instance, in 1997, the full moons were March 24 and April 27, so the Western churches celebrated Easter on March 30, the Sunday after March 24, while the Eastern churches celebrated Easter on April 27 itself (which happened to be a Sunday).

This explains an initial potential four-week difference between Western and Orthodox Easter. Stay tuned for why the difference could also be one week or five weeks.

UPDATE: Sorry, Christians out there, I accidentally wrote that Easter is when Jesus was crucified, not rose from the dead. Mea maxima culpa.