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]."

Spring equinox

[wile_e_wonka]

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:Why would

[Wuhao]

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:Metric School Terms

[Corsix]

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:Why would

[BlindRobin]

Agreed, the history of time-keeping is a very interesting and important subject, however, an arcane method of determining the date for a specific holiday belongs in the category of 'curious minutiae' and is in and of it self just an obscure exercise, except for the devout adherents to it's attending myths.

Re:Metric School Terms

[acroyear]

could be worse. In the early 600s, Easter as calculated by Patrick's Irish/Celtic church was on a different day some years than the Roman church. In one particularly odd incident, the King of Northumbria celebrated Easter on a different day from his wife.

The Council of Whitby resolved this, supposedly.

Re:So what day did Jesus die on?

[pyite]

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.

Re:666 !!!

[flyingsquid]

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.

Re:how is it...

[Stormwatch]

Hey, it's not like religious people are all irrational! In fact, except for fanatics, it's like they have two brains - one to deal with day by day matters, and one for the church things. Which is a good thing, actually. But when you think about... well, it's quite a freakish notion.

Re:how is it...

[wickerprints]

I'm not Christian and don't observe Easter, but I am a mathematician, and even I found the calculation interesting. In particular, I was interested to see the variety of algorithms used, as well as their relationship to astronomy.

One should not forget that astronomy--and much of science in general--historically were motivated by religious belief, not just in Western Judaeo-Christian cultures, but all cultures. That this is no longer the case speaks to the power of rational thought over pre-rational mythologies; but it is also a disservice to apply a revisionist view towards the origins of science--which was born from our innate human desire to not merely accept the mechanisms of nature, but to understand it.

In Vim Script

[Anonymous Coward]

Only valid between 1583 and 4899

function! s:EasterSunday(year, return_value)

                if a:year 4089
                                return 0
                endif
                let a = a:year / 100
                let b = a:year % 100
                let c = (3 * (a + 25)) / 4
                let d = (3 * (a + 25)) % 4
                let e = (8 * (a + 11)) / 25
                let f = (5 * a + b) % 19
                let g = (19 * f + c - e) % 30
                let h = (f + 11 * g) / 319
                let j = (60 * (5 - d) + b) / 4
                let k = (60 * (5 - d) + b) % 4
                let m = ( 2 * j - k - g + h) % 7
                let n = ( g - h + m + 114) / 31
                let p = ( g - h + m + 114) % 31

                if a:return_value == 1
                                let easterday = p + 1
                                return easterday
                else
                                let eastermonth = n
                                return eastermonth
                endif
endfunction

hmm

[thatskinnyguy]

Is it just me or does it seem like anything posted having to do with politics or religion turns into a mod point black hole?

Recommended Reading

[szyzyg]

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.

Re:how is it...

[Phroggy]

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].

Curious

[OeLeWaPpErKe]

Just because I'm curious : where does it say that (not that it changes anything about the meaning of the bible if it does indeed say that, but I'm curious nonetheless) ?

Besides, religion isn't irrational : this [economist.com] article gives a few hints on why (note : if you know a bit of stuff about the differences between religions you'll find that while the arguments presented are not about one single religion, they do exclude a lot of religions, in short the article makes a lot of sense when interpreted to a christian context, and specifically compares this christian(-oriented*) belief system to atheism, it states that atheist societies exist for about 20 years while christian communities generally survive for 150 years, with a number of them being older than any reliable records (about 200 years that is))

* -oriented because of 2 facts :
1) some members of other religions are "cryptochristian", ie they believe and practice the principles of christianity, even when in direct contradiction with their stated religion
2) some christians ... (I'm sure you can fill this in, this seems to be a smaller group though)

Catholic Easter != Orthodox Easter

[pz]

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:Annual celebrations are arbitrary anyway.

[2short]

Well said. My wife and I celebrate our anniversary on the Saturday before Thanksgiving, because, well, because that's when we got married. If I mention this, some people think it's very strange. They immediately ask "Well, but what was the date?" and are horrified I don't know off the top of my head. Heck, we picked the day because it was a Saturday, why commemorate it mid-week?

Jewish, not Pagan, and especially not Druid

[billstewart]

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.

In PHP

[JoeCommodore]

$unixdatenum = easter_date($year);

Okay, but...

[Jane Q. Public]

One still has to take into account what is really going on when the word 'virgin' is used, based on the context of the writing. Even if the translation is literal, a writer might assume that a young woman is 'virginal' when she is, in fact, not. At any given time, not many people have that actual knowledge about an individual, and so it is usually nothing but an assumption, in any time or place. And given the actual record of history, that assumption is probably wrong more often than right.