Leaping Lizards

Phill Gross (gross@gateway.mitre.org)
Fri, 8 Jan 88 01:33:52 EST

Since the leaptime issue has hung around until Christmas (for Orthodox
Christians, that is), I thought it would be appropriate to give wider
distribution to a private note I exchanged earlier. (The note below
was originally in response to a comment about "Pope Julian's" calendar.)

> Actually, it was Pope Gregory XIII who, in 1582, revised the Julian calendar
> by lopping out 10 days. He declared that Oct 5th, 1582 would become Oct
> 15th. The extra ten days had accumulated because the year is actually
> 365.2425 solar days long, not 365.25 as Caesar and all our elementary
> school texts claim. (Actually, the year is 365 days, 5 hours, 48 minutes
> and 48 seconds, which is 365.242216 solar days, but who's to quibble?)
> This means that under Caesar's rules, in every century you were adding about
> .75 too many days. By the 1500's, this amounted to approximately the
> aforementioned 10 days. To keep things straight in the future, Gregory
> added the rule that henceforth centesimal years would be leap years only
> when divisible by 400. This cleverly means that you only add 97 days every
> 400 years, instead of 100, which is more like it (.2425 x 400 = 97).
> Of course, Britain and the colonies didn't catch on until September 1752,
> which meant they had to chop out 11 days (try typing `cal 1752' on your local
> Unix box). This plays havoc with the biographers of our founding fathers,
> most of whom were born before the British date switch but died after it.
> Since the Eastern Orthodox and the Roman Catholics had mutually
> excommunicated each other, Russia didn't catch on until after the 1917
> revolution.

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