By Eric Pettifor
I commented to a friend the other day that Easter seemed early this year, and he replied in disgust “Yes, first they move up daylight savings time and now this.”
While the ever-changing date of Easter might seem arbitrary, it is actually carefully calculated. Obviously, it was of great concern to Church fathers for many, many centuries prior to the advent of computers. They referred to it as the Computus:
The canonical rule is that Easter day is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after 21 March (nominally the day of the vernal equinox). For determining the feast, Christian churches settled on a method to define a reckoned “ecclesiastical” full moon, rather than observations of the true Moon as the Jews did at the time. Eastern Orthodox Christians calculate the fixed date of 21 March according to the Julian Calendar rather than the modern Gregorian Calendar, and observe the additional rule that Easter may not precede or coincide with the first day of the Jewish Passover. (Wikipedia: Computus).
Sounds complicated, but today, of course, with our modern computers, it is a simple matter to calculate. Here is the formula:
((19*t+u-w-(u-(u+8)\25)+1)\3)+15)mod30)+(32+2*x+2*y-(19*t+u-w- (u-(u+8)\25)+1)\3)+15)mod30)-z)mod7)-7*(t+11*(19*t+u-w(u- (u+8)\25)+1)\3)+15)mod30)+22*(32+2*x+2*y-(19*t+u-w-(u- (u+8)\25)+1)\3)+15)mod30)-g)mod7)+114)\31
Piece of cake! I bet they would have loved to have had that at the First Council of Nicaea in 325 AD. It could have saved them a lot of debating and agonizing over tables and whatnot — “Oh, let us not quibble, brothers, just use the formula and call it a day.”
Over at God Plays Dice there is a wonderful article which breaks down this formula in a way almost comprehensible to non-mathematicians.
I wrestled with this when I created the Naked Geek calendar. (Warning: Full frontal nudity and css page layout — conservatives, children, and Internet Explorer users, do not click here). Naked geeks are easy, moon phases and Easter are hard (and if that sounds vaguely lewd to you, then you have a naughty mind). If you scroll down past the playgeek of the month, you will see that the math actually works, even for oddball extremes like this Easter.
Speaking of which, I hope yours was happy!