As it's explained by Dr. Lewis Patsavos of the Holy Cross School of Theology:
The determination of the date of Easter is governed by a computation based
on the vernal equinox and the phase of the moon. According to the ruling of
the First Ecumenical Synod in 325, Easter Sunday should fall on the Sunday
which follows the first full moon after the vernal equinox. If the full moon
happens to fall on a Sunday, Easter is observed the following Sunday. The
day taken to be the invariable date of the vernal equinox is March 21.
Herein lies the first difference in the determination of Easter between the
Orthodox Church and the other Christian Churches. The Orthodox Church
continues to base its calculations for the date of Easter on the Julian
Calendar, which was in use at the time of the First Ecumenical Synod. As
such, it does not take into consideration the number of days which have
since then accrued due to the progressive inaccuracy of the Julian Calendar...
Peter Meyer at Hermetic Systems explains the dilemma in these stark terms:
The average length of a year in the Julian Calendar is 365.25 days (one additional day being added every four years). This is significantly different from the "real" length of the solar year... this error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day. Thus as the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons. This was especially troubling to the Roman Catholic Church because it affected the determination of the date of Easter, which, by the 16th Century, was well on the way to slipping into Summer...
Something had to be done to bring Easter back to Spring, yet try to keep it after Passover. Compromises and upheavals followed. What I find particularly fascinating are the significant calendrical consequences of this split, and how they continued well into the last century:
When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform...
- Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582.
- The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
- New rules for the determination of the date of Easter were adopted.
- The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.
The Gregorian Calendar was adopted in Britain (and in the British colonies) in 1752, with (Wednesday) September 2, 1752, being followed immediately by (Thursday) September 14, 1752...
In many countries the Julian Calendar was used by the general population long after the official introduction of the Gregorian Calendar. Thus events were recorded in the 16th to 18th Centuries with various dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked "O.S." for "Old Style", and those in the Gregorian Calendar were marked "N.S." for "New Style"... Sweden adopted the Gregorian Calendar in 1753, Japan in 1873, Egypt in 1875, Eastern Europe during 1912 to 1919 and Turkey in 1927. Following the Bolshevik Revolution in Russia it was decreed that thirteen days would be omitted from the calendar, the day following January 31, 1918, O.S. becoming February 14, 1918, N.S.
It's neat to think a decree can erase whole weeks from history -- it makes the Orwellian Daylight Saving system seem, well, benign. But why didn't everyone sign onto the Gregorian system?
As Dr. Patsavos further explains,
The need for correction of
the Julian Calendar was well understood in the East and had even led some to
devise a new calendar themselves. Nevertheless, the Julian Calendar remained
in use throughout the Byzantine period and beyond. Despite the efforts of
the emissaries of Pope Gregory to convince the Orthodox to accept the New
(Gregorian) Calendar, the Orthodox Church rejected it. The main reason for
its rejection was that the celebration of Easter would be altered: contrary
to the injunctions of canon 7 of the Holy Apostles, the decree of the First
Ecumenical Synod, and canon 1 of Ancyra, Easter would sometimes coincide
with the Jewish Passover in the Gregorian calendar.
So each side had their priorities, and the differences persisted. Eventually the Orthodox and Western churches developed different "Paschal cycles" of varying years, so that Easter could be calculated with some consistency in perpetuity. Things will line nice up nicely for all Christians again in 2007 and 2010. Next year, however, Orthodox Easter will fall over a month after Gregorian Easter (May 1 vs. March 27).
Most years, we all make do with some confusion, some lengthy explanations, and some groups taking advantage of heavily discounted Easter bunnies. Is it worth it? Unquestionably. Preserving these differences gives us insight into humanity's ancient perceptions of time, space, the seasons, and God. We're far richer for it.