Easter is this Sunday, March 31. But Orthodox Easter, celebrated by most branches of the Eastern Orthodox Church (including the Russian Orthodox Church) and some Oriental Orthodox churches, is May 5 this year, a full five weeks after Easter as celebrated by Western Christian churches. (As a footnote, you may listen to Rimsky-Korsakov’s Russian Easter Overture at this site.) The five-week difference has happened recently in 2002, 2005, and 2008. But in 2001, 2004, 2007, 2010, and 2011, Western and Orthodox Easter fell on the same day. And in 2000, 2003, 2006, 2009, and 2012, Orthodox Easter was one week after Western Easter. What’s up with that?
First, let’s go over the basic difference between the Julian and Gregorian calendars. The Julian calendar, introduced in 45 BC, is easy: the year is 365 days long, except that we have a leap year every four years. After some confusion caused by off-by-one errors, apparently the proper sequence of leap years was reestablished by AD 4 or AD 8. (Good thing 4 and 8 happen to be divisible by 4: otherwise we might have leap years that occur on non-multiple-of-four years, which would be more confusing! Of course the numbering that established those years as being 4 and 8 came around much later, and was based on a totally different criterion.)
The Julian calendar had the advantage that it made the average year length 365.25 days rather than 365, which is closer to the astronomic truth. Without leap years, the years would be too short, so every four years the solstice would have a calendar day that seems a day later; instead of June 21, in 120 years we’d have it at July 21; and roughly 600 years after that, the summer solstice would be in December. All havoc would break loose. Dogs and cats would be living together. Also, it would make it hard to interpret old agricultural texts that tell you to do particular things on particular days.
Trouble is, the year isn’t quite 365.25 days long; the tropical year, which is the most relevant one for seasonal purposes, is more like 365.24219 days. So if you use the Julian calendar, your year is 0.00781 days too long, so after 400 Julian years, you’ll be 3.124 days off. Call it 3 days off, though let’s still remember, in the back of our heads, that it’s not exactly 3. So if a “true” calendar (where you’re constantly resetting the calendar to keep the solstices around the same days) and a Julian calendar were synchronized on January 1, 2000, then by the “true” date of January 1, 2400, the Julian calendar (with the longer years) would read something like December 29, 2399. The Julian guys would celebrate the New Year three days later than the “true” guys. Every 400 years, you get about 3 days more wrong, so the Julian dates seem to creep later and later relative to a “true” calendar. Before, the 365-day year had the problem that the date of the solstice was creeping later and later fairly rapidly (by a day every roughly 4 years); the Julian calendar overcorrects this slightly, so the date of the solstice creeps earlier and earlier fairly slowly (by, on average, a day every 128 years).
The Gregorian calendar fixes this problem by removing 3 days every 400 years. In practice, this is easy to implement: take the century years 2000, 2100, 2200, and 2300, and remove the leap days in 2100, 2200, and 2300. So of the century years, the only ones that remain leap years are 1600, 2000, 2400, etc., the ones divisible by 400. Now the average year length is 365.2425, which is off from 365.24219 by 0.00031 days, so the years are still too long, but by a much smaller margin, so the date of the solstice will creep earlier and earlier much more slowly (by, on average, a day every 3226 years). (See here for a more precise discussion of accuracy, which also discusses more technical points like the precession of the equinoxes.)
Now it’s one thing to figure out a calendar like the Gregorian one, which is more accurate and fairly easy to implement; but how do you actually get it adopted? The Gregorian reform was proposed by Pope Gregory XIII, and Catholic countries mostly adopted it in or shortly after 1582. By then, the error was 10 days; to prevent the backward creep of the date of astronomical phenomena, it was necessary to move the calendar date forward in one swell foop. The first places to adopt were Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy; there, September 4, 1582 was immediately followed by September 15, 1582. France adopted the calendar later in 1582, and other places adopted in 1583. The Protestant countries mostly adopted the new calendar over the course of the 18th century; England did it in 1752, by which time the error was 11 days.
The Orthodox countries of Eastern Europe lagged behind, though, and by 1918, the error was 13 days. In 1918, the new Soviet government adopted the Gregorian calendar; January 31, 1918 was followed by February 14, 1918. But the Russian Orthodox church didn’t change calendars, and, as the other Orthodox countries adopted the new calendar in the 1910s and 1920s, neither did their churches. (Some churches have adopted the Revised Julian calendar or Milankovic calendar, which is different than the Gregorian calendar, but is temporarily aligned with it.) So most Orthodox churches continue to follow the old calendar. Since 2000 was a leap year for both the Julian and Gregorian calendars, the error continues to be 13 days, and will continue to be 13 days until 2100. Thus, the date that the Orthodox churches consider to be December 25 is what we call January 7.
O.K., understanding this calendar issue is an important first step toward understanding the difference in the dates of Easter, but doesn’t quite get you there: after all, 13 days doesn’t sound much like the 0, 7, or 35 days’ difference that has prevailed in recent years. We’ll get closer to that in a subsequent post.