13 April 2017
A Moveable Feast
The milk chocolate Easter algorithm.
by Neil Tidmarsh
Christmas Day? December 25. Saint David’s Day? March 01. Saint Patrick’s Day? March 17. Easter Day? Er… um… well…
We laughed at Homer Simpson when he moaned, after missing out on some sort of Christmas offer, “I’ll never get another chance like that until next Christmas – and who knows when that’s going to come along?” But if he’d been talking about Easter, we might well have nodded our heads in some sort of agreement.
Easter… ok… well, it’s in April, isn’t it? Unless it’s in March. Late March. But it’s usually in April. Isn’t it? Early or mid April. It’s in April this year. This Sunday, in fact. (It is a Sunday, isn’t it, Easter Day? Or is that the Friday? No, that’s Good Friday. The Monday? No, that’s Easter Monday. So the Sunday is Easter Day? Yes, Easter Sunday.) So what date is that, next Sunday? April 16. There you go then.
But Easter Day was March 27 last year. And it was April 05 the year before that. And April 20 in 2014. Why? What’s going on?
“It’s all to do with the lunar calendar” you say.
Er… what?
“The lunar calendar” you repeat. “Where each month is 28 days, a moon’s full cycle. Unlike our modern calendar, a solar calendar, where the sun’s annual cycle is split into 12 months of 30 or 31 days. Hence the variation.”
What?
“The Bible says that Christ’s crucifixion took place on the Jewish Passover” you say. “And the Council of Nicea, in AD325, defined how you work out when that’s going to be; Easter Day, they announced, is the first Sunday after the first full moon after the spring equinox. (The equinox is a solar event, of course).”
Ok… good… that’s… er… simple enough…
“Well, not really, I’m afraid” you say. “For instance, how do you find the spring equinox? We all know it’s March 21 now, it’s marked in our calendars, but centuries ago… And what if the spring equinox itself is on a Sunday? And what if there’s a full moon at midnight when Saturday becomes Sunday? And syncing the lunar calendars and the solar calendars… It was all a massive problem in the early middle ages. Huge confusion. Easter all over the place.”
Not like today, then, eh?
“In the seventh century, King Oswy of Northumbria celebrated Easter on one date, and his queen, Eanfled, celebrated it on another date, weeks later sometimes. It didn’t do much for their marriage.”
No, I guess not. I bet she got really fed up with him eating all the Easter eggs and leaving none for her.
“So the king called together a load of experts at Whitby – priests from the Roman church and the Irish church and the Scottish church and the Frankish church and the Northumbrian church, who all had different ways of working out when Easter was and all celebrated it at different times – to sort it out once and for all, to debate the whole thing and all agree on the single, best way. It was a right rough intellectual punch up, this Synod of Whitby (AD664). They all had their own ‘computus’, you see, their own complicated calculations for working out when Easter was going to fall in the years ahead, algorithms and mathematical tables and everything. Some set lunar limits of XIV to XX, for instance, others set lunar limits of XVI to XXI.”
What?
“Some used a 19-year cycle which could be repeated for up to 532 years before dropping a day because the lunar and solar calendars were out of sync by twenty-four hours. Others used a table of five 19-year lunar cycles which could be adjusted and repeated after 95 years.”
What?
“Ok, I’ll just go over that again. Some used a 19-year cycle which could – ”
No! No, just tell me who won.
“Wilfrid, abbot of Ripon. He argued the Roman method, and clinched the deal by claiming that his mathematical tables had been used by Saint Peter himself, who holds the keys to the kingdom of heaven. So that was sorted at last.”
Phew. What a relief.
“However, in the Eastern Orthodox church, their Easter is still different from ours. They use a different way of working it out. You see, they – ”
No! Stop! Stop right there! That’s quite enough of that! I’ve got hot-cross buns to eat, and Easter eggs, and I can feel a headache coming on…
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