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Treebeard's Stumper Answer
11 Apr 97

Christmas is always on December 25, but Easter is a movable feast that falls on a different date each year in a complicated way. Easter is the first Sunday after the first full moon after the spring equinox. This year the equinox was on Thursday, March 20. The next full moon was on the following Sunday, March 23. (This was also the eclipse we couldn't see.) And so Easter was on the next Sunday after that, March 30. The equinox is always March 20 or 21. So what are the earliest and latest possible dates for Easter?

Oops. Easter is the first Sunday after the first full moon after or on the equinox. And the equinox is always figured as March 21, regardless of astronomy and geography. If the equinox is on a full moon Saturday, then Easter is the next day, March 22. The worst case is when the full moon is the day before the equinox. The next full moon is 29 days later on April 19. If this happens to be a Sunday, then we have to wait 7 more days for a very late Easter Sunday on April 26. This chart shows how complicated this is, plotting days after the equinox for each year from 1900 to 2100.

Easter graph

I wrote a simple BASIC program that illustrates an algorithm to figure Easter. Check out The Ecclesiastical Calendar Page for more information.

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