Calculating the Date of Easter

The God Plays Dice blog has an entertaining post on how the date of Easter is calculated. Wikipedia has all the messy details of course, but the blog makes a good introduction to the topic. "Easter is the date of the first Sunday after the first full moon on or after March 21... [T]he cycle of Easter dates repeat themselves every 5,700,000 years. The cycle of epacts (which encode the date of the full moon) in the Julian calendar repeat every nineteen years. There are two corrections made to the epact, each of which depend[s] only on the century; one repeats (modulo 30, which is what matters) every 120 centuries, the other every 375 centuries, so the [p]air of them repeat every 300,000 years. The days of the week are on a 400-year cycle, which doesn't matter because that's a factor of 300,000. So the Easter cycle has length the least common multiple of 19 and 300,000, which is 5,700,000 [years]."

Metric School Terms

In the UK the academic year is split according to the date of Easter. I recall hearing about an effort to move to a "metric" system which doesn't depend on Easter. This suddenly makes a lot of sense...

Re:Metric School Terms

My school (south-west UK) seems to have detached term times from Easter. This is Easter weekend at the moment, so we get the Friday and Monday off as they are bank holidays, but the two week long "Easter break" isn't for another two weeks yet.

Re:Metric School Terms

In the UK school is split into three terms ... in the middle of each, you get a week off, and between them, you get two weeks off. Except over the summer when it's six weeks.

So there's more holiday through the year, but the summer vacation is shorter.

(This is probably because we don't have as much summer.)

Re:Metric School Terms

Yeah, those of us above school age also get a statutory 28 days paid holiday. Which seems a lot compared to the US 11 or 12(?)

I think 11 or 12 days is about what Americans in the professional class wind up getting on average, but *statutorily* we get somewhere between jack and shit.

To take it to the extreme the French are forced to work at most on 35 hours and get four weeks but have to take them in August.

So basically, if you want to invade France make sure to do it in August. That way, they won't notice until they come back from vacation :-).

Only this year.

Ah! The March of progress.

Next year it will be April again though.

Re:Metric School Terms

Absolutely - the Anglo-Saxons had a lot to say about the dating of Easter. See http://faculty.virginia.edu/OldEnglish/aelfric/detemp.html [virginia.edu] for an original text on the subject if you're wildly interested. Melvyn Bragg's novel "Credo" dramatises the Synod of Whitby and gives a sense of exactly how serious an issue this was for people. Since Easter is the major Christian feast, it was a matter of orthodoxy to date it correctly. Interesting to think that being bad at math could make you a heretic!

Spring equinox

I've always thought that it is more fun to say the date of Easter is "the first Sunday after the first full moon after the Spring Equinox," rather than March 21st.

It sounds so much more Pagan my way.

Re:Spring equinox

Only problem is, your way isn't always right, because the date of Easter is always calculated from March 21st even if (as this year) the northern hemisphere spring equinox doesn't fall on that date.

In Perl

sub GetEasterDate {
  my($year)=@_;
  # http://www.smart.net/~mmontes/nature1876.html
  my $a=$year%19;
  my $b=int($year/100);
  my $c=$year%100;
  my $d=int($b/4);
  my $e=$b%4;
  my $f=int(($b+8)/25);
  my $g=int(($b-$f+1)/3);
  my $h=(19*$a+$b-$d-$g+15)%30;
  my $i=int($c/4);
  my $k=$c%4;
  my $l=(32+2*$e+2*$i-$h-$k)%7;
  my $m=int(($a+11*$h+22*$l)/451);
  my $month=int(($h+$l-7*$m+114)/31);
  my $p=($h+$l-7*$m+114)%31;
  my $day=$p+1;
  return (0,0,0,$day,$month-1,$year-1900);
};

Recommended Reading

I won't hesitate to recommend the book 'Marking Time' by Duncan Steel - it's a great book about the history and evolution of calendars. The date of easter is a particularly interesting question and Duncan goes as far as to explain how the date of Easter was at the core of an English plan to attack the legitimacy of the Catholic church and how this plan was what triggered Britain's first attempts to colonize America, great stuff.

Stupid *nix Tricks

cal 9 1752

Calendars are funny things.

Catholic Easter != Orthodox Easter

There are millions of people who did not celebrate Easter today (23 March 2008) because they will be celebrating on 27 April 2008 (yep, 5 weeks later ... this is an unusual year). Orthodox Easter is computed to always fall after Passover (because, recall, the Last Supper was a Passover Seder).

Here's a web site that is more, um, shall we say, enlightened: http://www.assa.org.au/edm.html [assa.org.au]

One of the main differences between the calculations for Roman Catholic Easter and Eastern Orthodox Easter is in which calendar (Gregorian or Julian) is used. Use Google. It's actually quite interesting because of all the history and politics involved. It's not just simple (eg, exactly when is the moon full? over which point on the earth?) as one might think.

Re:Why would

Just because the date (and what it commemorates) is meaningless to you, is it really necessary to cast all those who do care about it as irrational?

Re:Why would

Do you ever cast UFO believers as irrational?

Re:Why would

In his defense, the picture tagged with the story is pi.

Re:Why would

Historical significance, for one. The history of time-keeping and astronomy are intimately tied to the need to celebrate religious events; this goes back much before Christianity. It's really a very neat subject, and it's really fascinating how much math developed simply out of a need to know when and how to throw a party for the gods.

Re:Why would

I don't care about the myths - the chocolate bunnies and eggs are good enough for me.

Save the earth! It is the *only* planet with chocolate!

Re:how is it...

Calculating the Date of Easter Finds Possible Cure For Cancer

There, now it's an official Science article.

Re:how is it...

Your grasp of astronomical chronology far exceeds mine, then. I'm not a Christian and have no interest in the holiday per se, but thought this article was a fascinating piece of science history, and certainly learned more science from the underlying astronomy and the computation thereof than I would have gotten from any ten Roland Piquepaille rehashings of press releases he doesn't understand.

Re:how is it...

Arguably it is a math article to the interested christians on /.

Methinks many families that profess no especial religion nonetheless buy their children bunny figures, chocolate, and disgusting gelatin chicks in the springtime. These sort of articles, besides showing Christians when their religious day falls, also explain when to expect such mechandise in your local stores.

Don't forget about Mardi Gras!

Mardi Gras (Fat Tuesday, Shrove Tuesday, Pancake Day, etc.) is the day before Ash Wednesday, which is the first day of Lent, which begins 40 days (excluding Sundays) before Easter. So, once you've calculated the date of Easter, subtract 47 to get the date of Mardi Gras [wikipedia.org].

Jewish, not Pagan, and especially not Druid

The date of Easter is approximately "The Sunday in Passover", because (unlike Christmas, for which there's no recorded time of year for the original event, and therefore the holiday was set to rip off Roman pagan holidays, though some modern Yuletide customs were adopted from northern Europeans) the events being celebrated at Easter happened in conjunction with the Passover holiday, and there are symbolic and theological connections to Passover in addition to just the date. Since the Jews have a lunar calendar and the Romans used a solar calendar, it was somewhat difficult for the Romans to reconcile the two, and they weren't willing to use the obvious method ("ask some Jews when Passover is each year").

If you want to say that Passover's date is set at spring pagan holiday time, you'll need to argue with your rabbi or maybe Lehrhaus Judaica about whether your druids are at all the same kinds of pagans as Caananites were. And if you want to say that the name "Easter" and the bunnies and and eggs and marshmallow chickens are ripped off from Germanic spring fertility goddess stuff, you'll have a tough time getting anybody to argue the other side except maybe some atheists who'll say that the Germanic fertility goddess folks ripped that off from nature, which provided the bunnies and eggs, or from the chemical industry who brought us marshmallow peeps.

Re:how is it...

Even worse... there are Christian women on /.

Seriously, do you assume that all Christians are no-brain idiots who think dinosaur skeletons are an atheist conspiracy? Donald Knuth is a Lutheran, Gregor Mendel was an Augustinian monk, Copernicus was a priest, as was Georges Lemaitre. Lord Kelvin and Max Planck were committed Christians, Arthur Stanley Eddington was a Quaker... There are more here: http://en.wikipedia.org/wiki/List_of_Christian_thinkers_in_science [wikipedia.org] (Not all of those in this list were Christians throughout their lives, but the ones I've named were/are.)

Re:So what day did Jesus die on?

I find it quite amusing that the birth of Jesus is pretty much set in stone (at least if I believe that day to be Christmas), but the date of his death (or resurrection) isn't.

Yes, it's set in stone on the wrong date. Shepherds were living outside with their flocks when Jesus was born, yet they wouldn't be doing this in December. It's too cold in Israel. In addition, Jesus died on Nisan 14 (the first full moon after the vernal equinox)... not on a Friday year after year.

Annual celebrations are arbitrary anyway.

Yes, it's set in stone on the wrong date.

Right. Because we have to celebrate everything in exact intervals of one earth-sun-revolution, and only whole-number interval offsets from the time of the original event.

There's no such thing as the 'right' and 'wrong' date. An event happens. Choosing to celebrate that event once a year (where "year" is the amount of time it takes the earth to go around the sun once) is arbitrary in the first place. It would be just as 'right' to celebrate it every 12 moon-earth revolutions, or 2 mercury-sun revolutions.

If you're already going to base your celebration intervals on the convenience of how often one ball of rock revolves around one ball of gas because you happen to live on said ball of rock, you might as well always celebrate something on the 259th day of the year, or the 4th time the 4th day of the week falls in the 11th month of the year, or the 1st 7th day of the week following the vernal equinox.

Getting bent out of shape because the commemoration/celebration of an event doesn't have the same calendar date as the original event - especially when the original event occured in a time period where the calendar you're using didn't even exist - seems pretty silly. Especially when you're celebrating the birth/death of the son of God.

Re:666 !!!

I bet next people will believe that this guy's mother was somehow a virgin, and not just spouting the same lies that every young, newly sexually active woman says when confronted by her parents.

I seriously doubt that Mary went around saying that she became pregnant despite being a virgin, for two reasons. First, everyone would have read between the lines and assumed Jesus was the product of infidelity, then as now. Saying that Yahweh was the real father makes you look like you're not just loose, you're also batshit insane. The cover story would have been that Joseph was the father.

What's far more likely is that the virgin birth is a later addition to the story of Jesus. In comic book terminology, this is a retroactive alteration of the continuity, or "retcon". "Hm... how do we explain the origin of Jesus' amazing superpowers? How is he able to walk on water, cure leprosy, and feed multitudes using a single loaf of bread, if he's just some average Jew? It's just not plausible, our audience will never buy it. I KNOW! We have a special "Origins of Jesus" issue in the Bible, where we reveal that ACTUALLY, Jesus is the son of God! Now, the fact that he has these amazing superpowers makes sense!"

It's exactly like how Marvel went back and created a backstory to explain the origins of the super-powers of the X-men. In the case of Marvel, alien visitors altered the DNA of ancient humans which resulted in mutants like Wolverine. In the case of the Catholic Church, a super-powerful being impregnates Jesus' mom. It's a really ancient theme. If you recall many of Greek heroes, such as Hercules, had gods for parents, which explained why they were so powerful. Achilles was more like the Incredible Hulk, in that exposure to magic (the waters of the River Styx in the case of Achilles, gamma rays in the case of the Hulk) give them their powers. But Odysseus is like Batman- he doesn't have any superpowers, he's just clever.