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VMs: Re: Random discoveries in the library

Jorge Stolfi writes:

>>Hmmm... Well, the usual [Easter] algorithm is based on a *19* year cycle. 
It uses other divisors, but mostly to account for leap years and the 
Gregorian change....

The 19 year metonic cycle (named after the Greek astronomer Meton, but known 
to the Babylonians. Mahzor katan, small cycle, in Hebrew) returns the Moon to 
the same phase on the same solar year date, with an error of an hour or two. 
It's used to calculate the dates of Passover and Easter, each using an 
algorithm that crashes if it sets the holiday before the vernal equinox.

To return a solar calendar date to the same day of the week requires a 28 
year cycle (omitting the Gregorian correction. Hebrew mahzor gadol, great 
cycle). Thus Easter and Passover date cycles repeat every 19 x 28 = 532 years.

Bob Richmond
Knoxville, Tennessee