By Erik Derr ( | First Posted: Mar 29, 2013 01:50 PM EDT
Tags Easter

(Photo : Creative Commons/Erik Derr)

Easter Sunday is almost here. The candy's been bought, the eggs have either been colored or will be soon, the Peeps have been thrown in the freezer. Plans for the feast have been set, the times everyone has to get to church for Sunday services are circled on the calendar --- and yet, big questions remain:

Why is Easter on March 31 this year, when it was on April 8 in 2012, it will fall on April 20 in 2014 and on April 5 in 2015? Why does the date of Easter Sunday always vary?

First and foremost, Easter is always different because the ancient Egyptians and Hebrews used different calendars. The Egyptians used a calendar based on the sun's cycle and which was passed to the Romans and then Christians to eventually become the modern world's standard. The Jews, on the other hand, followed a calendar based on the phases of the moon.

As a general rule, Easter falls on the first Sunday after following the first full moon after 21 March --- but not always.

Second, Easter is the time when Christians celebrate the resurrection of Jesus, and according to the gospels, he was killed three days before he rose from the dead, around the same time as the Jewish Passover.

Christians decided to coincide the Easter celebration with Passover, which is fixed by the first full moon following the vernal equinox - the spring day when night and day are exactly the same length.

The problem arises because a solar year (the length of time it takes the earth to move round the sun) is 365 days, 5 hours, 49 minutes, and 12 seconds, while a lunar year is 354.37 days. So, calculating one against another and when one attempts to reconcile the two, it's a one-way trip to Headacheville.

There have in fact been innumerable efforts to solve the disparities between the two calendars, but nothing has so far stuck.

Third, there are dozens of ways to figure of the date or Easter Sunday.

In the 2nd century, a Roman named Hippolytus devised an eight-year cycle. A century after that, 84-year tables were invented. Then there was also a ruling by the First Council of Nicaea in 325 that Easter should be celebrated on the Sunday after the first full moon following spring equinox ---although that directive was ignored for several more centuries

Also complicating matters is the fact the spring equinox, though it occurred on 20 March, during that First Council of Nicaea, also varies. So, the old church opted to get around that by redefining what a full moon actually is.

Fourth, there are many different types of moons. An astronomical full moon, as is an astronomical equinox, is not a day but a moment in time - which means it can be observed on different days, depending on where you are in the world

As a result, the church decided that the Paschal Full Moon would be an ecclesiastical full moon, not be an astronomical moon. Easter was therefore defined as the Sunday after the first ecclesiastical full moon after March 20 --- and that date was the appointed vernal equinox, regardless of whether it was or not.

That means Easter may fall on March 22, like it did in 1761 and 1818, or as late as April 25 --- but that won't happen again until 2038. The most common date for Easter Sunday is April 19.

But, you ask, can't the date for Easter be fixed by modern astronomy? Astronomers have tried to reconcile the lunar and solar cycles since the time of ancient Greece and the simple answer to that is: no.

How about if everyone just comes together and settles on a specific date? Well, governments and churches have tried to do that many times. Some have suggested Easter fall on the second Sunday of April every year. The World Council of Churches in 1997 even suggested swapping the current equation-based system with direct astronomical observations. Again, strategizing about something is one thing, actually implementing something worldwide --- like affixing the date Easter is celebrated --- is another thing altogether.

As a result, it appears the Easter holiday will always move from year to year. Enjoy your Peeps.

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