Okay, it's not just me...

[jducoeur]

One thing that has always mystified me is, "When the heck is Easter?". The date has always seemed -- well, arbitrary to me. But I figured that that was just because I was raised in a secular Jewish household, and it just didn't impact our lives enough to talk about it.

Maybe not so much, though. Thanks to Gundormr for pointing out this article on the subject. The main thrust of the article is about the fact that the very early Easter this year has screwed up British families' holiday plans, but it also includes this delicious formula, showing how Easter is calculated:

((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

No, I don't know what the variables are, but if this is correct, it's pretty insanely complex regardless. This wikipedia article discusses the rules in broad strokes, and this long and detailed one actually gives the algorithms. (The formula above looks like a variation of the "Meeus/Jones/Butcher Gregorian algorithm".) *Surely* the entire concept of the date of Easter must have been invented by geeks, as a way to pass the time before the invention of the slide rule.

Footnote: he also sent along this picture, of the only mechanical watch in the world sophisticated enough to calculate the date of Easter. (Which, given how much high-end watchmakers love to put in every imaginable gadget, tells you something about how messy the problem is.)

Comments

[tpau]

i thought easterused to be the first sunday after the paschal moon (whichit stil lsi for Orthodox christians) and the catholics and protestants went tothe first sunday afte rhte vernal equinox model?

[lady-cassia.livejournal.com]

It's the first Sunday after the first full moon after the first day of spring.

[tpau]

right i forgot the 14 day offset thing. (i.e. it is 14 days after the paschal moon, which um makes it in general the first sunday after passover, jsut nto this year)

[tafkad.livejournal.com]

Yep. First Sunday after the first full moon after the first day of Spring. Easy. Once you know that, the rest is nonsense, claptrap and rudeness in the form of mockery.

You could also just look at a monthly calendar. If you don't see it in March, check April.

[meranthi.livejournal.com]

Reminds me of when I wanted to figure out when I'd get another 3 paycheck February. :) (Leap year with the 29th falling on a Friday) Not for a very very very long time was about as far as I got.

[its-just-me.livejournal.com]

Ow my eyes! Ow my eyes! Ow my eyes!

[calygrey.livejournal.com]

My sentiments precisely!

[its-just-me.livejournal.com]

No the thing is, and I have a hard time getting people to belive me - formulae tends to cause my dizziness and painful headaches upon impact.

[calygrey.livejournal.com]

Pity them! I believe you. It's gives me heart palpitations everytime I look at -it-.

Seems like a no brainer; who wouldn't rather look at a nice beach scene, nature scene, nude scene, than a fire's-of-hell math problem. Of course pictures affect us physically.

[jducoeur]

Really? My apologies, then, but that's kind of fascinating. Some kind of synaesthesia, almost...

[its-just-me.livejournal.com]

Oh no need to apologize. I'm perfectly able to allow it to pass it by : )

[dlevey.livejournal.com]

Ya see, Passover is really much easier. It's 15 Nisan, every year. And guaranteed to be a full moon each time.

[umbran.livejournal.com]

Easier, yes, but not better. One does not get chocolate bunnies for Passover.

[dlevey.livejournal.com]

Why not? As long as they're not made with corn syrup, what's the problem?

A number of years ago, my wife was in the Godiva store. They were, for some reason, making a big thing about the fact that their chocolate was kosher (though not, I presume, kosher for Passover). In this particular store they were really pushing the Easter-specific kosher candy. She just *had* to ask the person behind the counter why they'd do that. "Oh, I'm sure there's a good reason" the young woman said helpfully, so my wife asked what that might be. "Um, uh..., well... uh, I guess you're right," she replied.

[fairdice.livejournal.com]

Well sure — except for the part where you have to know when the new year starts. Historically, the answer was "Okay, it's Adar now, and next month with be Nisan: are we going to be ready for a spring harvest holiday in two weeks? Hmm, no? Okay, let's have a leap month this year!"

Nowadays instead we add a leap month 7 times every 19 years, which is much simpler that the Easter formula... and that explains why Easter is "the Sunday after the Passover seder most years, but occasionally a month off of that."

[alexx-kay.livejournal.com]

I remember an entertaining class at a Carolingian University given by antoniseb, the gist of which was that knowing complex details of astronomy was important because it let you know when Easter was.

[cellio]

I thought it was the Sunday following the first full moon following the vernal equinox. Yeah, there could be edge cases, but that should be rare.

(But wouldn't it have been easier to just say "first Sunday after Pesach" and be done with it? Or would that have been too difficult politically for an early church that was trying to separate itself from Judaism and reach out to the gentiles?)

[baron-steffan.livejournal.com]

Chill. Easter is the first Sunday after the first full moon after the vernal equinox.

Wasn't that easy %^) ?

[hudebnik.livejournal.com]

Even if you knew what the variables meant, you couldn't use that formula because it's gibberish. For one thing, it has 13 more right-parentheses than left-parentheses. And I don't know what the "\" operator means; is that the Microsoft version of division? The formula is also written to look much longer and more complicated than it really is: the sub-formula

((((19*t+u-w-(u-(u+8)\25)+1)\3)+15)mod 30)

appears four times (assuming that the one time there's no "-" after the "w" is a typo). If we give this sub-formula a name like, say, R, the whole thing becomes

R + (32+2*x+2*y-R-z)mod 7) - 7*(t+11*R)+22*(32+2*x+2*y-R-g)mod 7)+114)\31

which is still sorta long and complicated, but much less so.

Realistically, the way Easter was originally "calculated" was by observing the vernal equinox, then observing the next full moon, then waiting for Sunday -- no math involved at all. If you want to know when Lent starts (6 weeks and 4 days earlier), you have to do a little calculation, but still only based on this year's interaction of equinoxes and moons. The heavy lifting comes in when you want to predict Easter for some other year. I've seen formulae for that, and I don't recall them looking nearly this ugly.