Calculating Easter

Most people know that Easter moves around a bit. They look it up on the Calendar and leave it at that. But why?

The Short Answer

If you just want a list of Easter's for this century, The Astronomical Society of South Australia has a list here. But you still won't know why.

The Story

It turns out there is quite a story behind this movable holiday. The calculation of Easter has had broad reaching effects that most people are blissfully unaware of.

Good Friday and Easter, are the days Christians recognize the Crucifixion of Jesus Christ and the Resurrection. The Bible tells us that this was a Friday because the Sabbath began that evening. It also tells us that was during Passover because Pilate offers to release Jesus as is the custom during Passover. Easter, also called Pascha, follows on the Sunday. Easter Monday is a remnant of week long celebration and is not just a "bank holiday". There is also some debate if the Last Supper was a Passover meal or not.

At this point you might be tempted to think that Good Friday should be the first Friday in Passover or that it should be on the same day each year. If only it had been that easy.

Now since Passover is based on the Hebrew calendar which is Lunisolar (a hybrid of Lunar and Solar events) the Moon is involved. This is why it moves around on a solar calendar. Anytime you have overlapping calendar systems be prepared for some complication.

Complications did ensue. Some people thought it should be on the same date in the year and others thought Easter should be on a Sunday. This led to disagreements, including the excommunication of the Quartodecimans. And while it seems that folks later made up, this may have been one of several precursors to later disagreements that split the the church.

Easter is defined as the first Sunday after the first fourteenth day of the the Paschal Full Moon that is on or after March 21 and the vernal equinox. However, just to complicate things this full moon doesn't always correspond to a real full moon. Although it should be within a day or two.

The multiple references to 14 (quartodecima) arise from the original dating of Easter after the 14th day of Nisan on the Hebrew calendar. Nisan falls in the March-April time frame.

So Easter can fall anytime from March 22nd to April 25th on Christian calendars. And yes, I meant calendars plural. While the west uses the Gregorian calendar, Eastern churches mostly use the Julian calendar. The result is that Eastern churches celebrate Easter from April 4 to May 8 on the Gregorian calendar. Despite this, Easter sometimes falls on the same day as in 2001, 2004, 2007, 2010, and 2011.

The main differences between the two calendars is in the way they handle leap years and leap days. The Julian calendar has too many and consequently that calendar gets ahead of the seasons. Pope Gregory XIII advised by Clavius and Kepler reformed the calendar to bring the Equinox and Easter back in line with the start of spring.

Now before I wander off topic with these last notes, please consider that I am not one to get hung up on literal interpretations of the Bible. As Jesus said, give to Caesar what is his and to God what is God's. I choose to rely on science and faith for the things they each do best. And, I think, mixing them up is unwise.

Of Other Things Easter Related

The Biblical account of the Crucifixion speaks of three hours of darkness, earthquakes, and the Moon turning blood red. Some people believe that the darkness is an eclipse and have used that to try and get an accurate date for this event. However, the facts don't support this. Daytime (Solar) eclipses last only minutes and they happen on the new moon. Lunar eclipses happen on full moons and turn the moon red but they happen at night.

Another problem is that the year and date of Christ's birth aren't really known exactly and may vary from 8 BC to 6 AD. Some people have tried to fit astronomical events to the birth of Christ, but there is simply too much uncertainty. The determination of AD or Anno Domini (the year of our lord and not "after death") was calculated by a monk named Dionysius Exiguus. Prior dates became Before Christ. Ironically, Dionysius didn't set out to determine when Christ was born but was calculating tables of Easter dates.

BC/AD have become widely accepted outside of Christian culture but they are often referred to as BCE and CE (Common Era).

Dates BC/AD cause mathematical problems. Consider, is there a year zero? Since the Romans had no number for zero, the Early Christians wouldn't have had one either. Mind you Dionysius almost coined it! One of the few sciences that actually has to accurately deal with spanning BC/AD is Astronomy. And for ease of calculation, astronomers have their own years which use integer notation. We are living in +2008, 1 BC is 0, and 2 BC is -1.

There is an old myth that you can make an egg stand on end at the moment of the Vernal Equinox. You can. But you can also do it other times as well. A couple of articles and a video were published, here and here.

Exactly how did we get from the Biblical events of Easter to Rabbits and Eggs? Wikipedia has some ideas, here.

And while I remember, folks from Purdy's Chocolates were handing out free samples at the Ontario Science Center during March break. Yum!

Update:

• Astroprof posted a discussion of the Easter Moon and the difficulties of calculating Easter.
• Additional links about Easter and astronomy can be found over at Cosmos4u at the bottom of the article.
• The Toronto Star published an account of why Easter is so early. Nice photo of the Cathedral of Strasbourg's astronomical clock too. See here.