For many of us it’s just one of those bits of knowledge that we absorbed somewhere along the way: Easter falls on different dates for the Latin and Greek Orthodox churches. We know it, and it never occurs to us to wonder about it, until someone asks the question – a child, maybe, or a Jeopardy contestant.
The Pope endorses his scientific advisors’
Detail from Gregory XIII tomb
by Camillo Rusconi, St. Peter’s Basilica, Rome
Why does Easter fall on different dates in the Latin Church and the Greek Orthodox Church? And which one’s the “right” one? Until the latter part of the 16th century, all Christians celebrated Easter on the same date. So what happened? The answer is found in a fascinating little slice of history.
Ancient calendars were based on the lunar cycle until around 45 BC, when Julius Caesar decided that his vast Roman Empire should adopt a new calendar, one based upon the earth's revolution around the sun. By giving up a link with the moon, the Julian calendar – so-called in honor of Caesar -- gained about three days every 400 years. By the 16th century, this error had accumulated to 10 days, and the discrepancy between the calendar dates of the solstices and the actual occurrence of the solstices had become a real concern to the Roman Church.
The long-standing formula for dating Easter depended upon the lunar cycle and the vernal equinox. Easter was being celebrated on the wrong date!
A solution challenged astronomers and mathematicians until 1582. Pope Gregory XIII endorsed their recommendations to modify the calendar and the new Gregorian Calendar changed leap-year rules to synchronize with the solar year -- to an accuracy of about 1 day in 2500 years!
To adjust for the discrepancy that had accumulated since 45 BC, the year 1582 was shortened by 10 days. The days between October 4th and 15th were abolished. So that the last day of the Julian calendar, October 4, 1582, was followed by the first day of the Gregorian calendar, October 15! Roman Catholics and Protestants use the Gregorian calendar, while Greek Orthodox Christians base their liturgical dates on a much older calendar.
The 16th Century was the era of the Protestant Reformation, and papal decrees carried little weight with non-Catholic Christians, so Gregory’s calendar reform was not soon adopted in Protestant countries. By the time Britain adopted it in 1752, an eleventh day had to be dropped. Russia didn’t adopt it until the 20th C. That’s why the October Revolution of 1917 actually occurred in November!
The Greek Orthodox Church has never endorsed the Gregorian Reform of the Calendar, so Orthodox Easter (and Christmas) falls some days later than the same celebrations in the Latin Church.