The problems of Christian unity and predicting the date of the full moon to determine Easter endured and were even exacerbated by reform based on limited astronomical data.
By the 8th century, there was a chance that, contrary to the ruling of the Nicene Council, Easter might precede or coincide with Passover. Easter had to follow the Vernal Equinox, but the Vernal Equinox no longer fell on March 21. It was now 3 days earlier. Calculations, at the time, were made using the Julian calendar in which a 3-year cycle of 365 days is followed by a leap year with 366.
It was up to the 16th century Pope Gregory XIII to straighten the Julian calendar and normalize the calculation of Easter in the West. His new cycle was 5,700,000 years long.
But the Orthodox Church resisted the Gregorian calendar and never changed their calculation of Pascha. Instead of Vernal Equinox, they based calculations on March 21, regardless of its relationship to the equinox. The Orthodox Church continued to follow the decree of the Nicene Council using the Julian calendar:
"Easter Day... is the first Sunday after the Full Moon which happens upon or next after the twenty first of March; and if the Full Moon happen upon a Sunday, Easter Sunday is the Sunday after.... The Eastern Church still observes the rule laid down by the Council of Nicea (A.D. 325) and now disregarded by the Western Church, that the Christian Easter shall never precede or coincide with Jewish Passover, but must always follow it. Easter cannot fall earlier than April 5 or later than May 8. The Full Moon used for the purposes of the Easter reckoning is the fourteenth day of a Lunar Moon reckoned according to the ancient Ecclesiastical computation, and not the real Astronomical Full Moon."By 2001 the Western and Eastern Orthodox churches had hoped to reconcile their Easter dates using advances in astronomical information. Using the Gregorian calendar dates, the Orthodox Easter is still no "earlier than April 5 or later than May 8." Although the dates still differ, the 15th of April in 2001 was the correct date according to both calendars, and frequently the dates now correspond.
* The Service Book of the Holy Orthodox Catholic Apostolic Church.
"According to the proposal [of the World Council of Churches], churches would continue to follow the current formula to calculate the date of Easter, but with the assistance of the most accurate astronomical scientific knowledge available. This would overcome the previous division, under which both traditions insisted upon retaining their old methods for calculating the date, even though they are not always completely faithful to formula laid down by the early church."
- A 1997 move for a common Easter date
Arian Controversy and the Council at Nicea
Early Church Trivia
URL = www.piar.hu/councils/ecum01.htm] First Council of Nicea
Dates of Orthodox Easter
[URL = aa.usno.navy.mil/faq/docs/passover.php] Dates of Passover
* [URL = <http://thepetrovics.net/sts_peter_paul/pascha.html>]