Calculating Easter

April 6, 2010

We mentioned in passing that Easter here in Cyprus does not always coincide with Easter in other parts of the world.  A moral obligation to explain why reared its ugly head but a large glass of crisp white wine dealt with that.

You see, it’s all to do with the first Sunday after the full moon after the vernal equinox.  And the Julian and Gregorian calendars and the missing 13 days.  Oh, and the Nicaea Council of 325AD.  And Passover, that’s important too.  You can see the appeal of the white wine in the face of all that surely?

Happily someone at the Cyprus Mail took on the challenge and published a useful explanation.  Read carefully, there may be a test later on.

THIS YEAR, like next year, Easter falls on the same day for both Orthodox and Western Christians.

Next year Easter Sunday falls on April 24. The problem in identifying when precisely Easter occurs perhaps stems from the time of the Apostles of Christ, who did not actually record the date of his resurrection.

This then left room for approximations, historical research and logical deduction: when exactly did the Resurrection occur, and when would it best be celebrated? These speculations and logical discourses, by means of ecumenical councils, became codified into Church lore over the centuries.

Both Orthodox Christians and the rest of the Christian world, including Catholics and Protestants, define the date in the same way as the first Sunday after the full moon following the first Vernal Equinox.

The equinox is an astronomical event, being the two days in the year when the Sun crosses the equator – as viewed from Earth – and with night and day being of equal length. However, the early Church fathers convened the Conference of Nicaea in 325AD, during which they decided to calculate the date by means of reference to pre-formulated tables instead of waiting to see when the days of the year were the same length.

These tables were formulated using the Julian calendar in place at the time, which was originally designed by Julius Caesar, the Roman emperor.

Caesar’s calendar was actually quite accurate: it erred from the real solar calendar by only 11½ minutes a year. After centuries, though, even a small inaccuracy like this adds up. By the 16th century, it had put the Julian calendar behind the solar one – the actual astronomical position of the solar system – by ten days.

In 1582, Pope Gregory XIII ordered the advancement of the calendar by ten days and introduced a new corrective device to curb further error.

If somewhat inelegant, this system is undeniably effective, and is still in official use for most countries globally. The Gregorian calendar year differs from the solar year by only 26 seconds—accurate enough for most mortals, since this only adds up to one day’s difference every 3,323 years.

Following the schism between the Orthodox and Catholic Churches, the former ended up retaining the Julian system, whilst the latter based their calculations on dates from the Gregorian calendar.

The approximate 13-day difference currently experienced between the two calendars is expected to continue to add up as the centuries progress.

On top of the calendrical differences, a number of canonical theological considerations factor into the process. The most pertinent of these, again stemming from Nicaea, is the tradition that Easter must always fall after the Jewish Passover, since the death, burial and resurrection of Christ happened after the celebration of Passover.

This, in turn, factors the Jewish lunar calendar into the considerations determining the date of Easter.

You can see why the thought of explaining that brings on a headache?

Next year, 2011, the Western and Eastern calculations coincide once more.  To balance that out in 2013 they are far apart once more with the Western date falling in March whilst here in Cyprus we won’t celebrate Easter until May.  The same is true again in 2016.



  1. I’ve very much enjoyed reading through your blog. I especially liked the post you did about confusing road signs!

    • Glad you enjoyed it, and nice to see you here :-)

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