Computus

Adapted from Augustus De Morgan’s “Rule for determining Easter Day of the Gregorian Calendar in any year of the new style,” A Budget of Paradoxes I pp366-7. I reckon that the best thing we got out of Easter, culturally speaking, was the ability to use algorithms. For instance, Dionysius Exiguus (Dennis the Short to his mates) was one of the first Western scholars to use something like the concept of zero in his Easter tables.

I	Add one to the year you are given.	2011
II	Divide the year by four, rounded down.	502
III	From the centuries, take sixteen (if you can)	4
IV	And divide that by four, rounded down.	1
V	Add I, II and IV, then take away III,	2510
VI	Then take that value modulo seven	4
	(Divide by seven and keep the remainder)
VII	Subtract that from seven again:	3
	This is the dominical number.
VIII	Take I mod nineteen (if it’s zero, nineteen)	16
	This is the year’s golden number.
	Now take seventeen from the centuries,	3
IX	Over twenty-five (chuck the remainder)	0
	Take IX and 15 from the centuries	5
X	Over three (and chuck the remainder)	1
	To VIII, add ten times (VIII minus one)	166
XI	Take that sum modulo thirty	16
	Add XI, X and IV and then take away III,	14
	(If it’s large enough, modulo thirty)	14
	If it be twenty-four, make it twenty-five;
	If twenty-five, and if VIII is more than eleven
	Make it twenty-six instead;
	If it’s zero, set it to thirty.
XII	The result is the epact; a good Scrabble word,	14
	The age of the moon on New Years’ Day.
	If the epact is less than twenty-four,
XIII(b)	Subtract it from forty-five (write that down)	31
	Then subtract the epact from twenty-seven
	Divide that by seven and keep the remainder:
XIV(b)	If it’s zero, change it to seven.	6
	If the epact is higher than twenty-three,
XIII(b)	Subtract it from seventy-five instead
	Then subtract the epact from fifty-seven,
	Divide that by seven and keep the remainder:
XIV(b)	If it’s zero, change it to seven.
	Then add XIII to VII (the dominical number).	34
	If XIV’s more than VII, add seven more.	41
XV	And then take away what you got for XIV.	35
	If the result is below thirty-two,
	Easter Sunday’s in March, and that’s the date,
	Otherwise, it’s in April – subtract thirty-one.	4