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Easter and the Calendar
Astronomical Applications Department of the U.S. Naval Observatory.
The usual statement, that Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox, is not a precise statement of the actual ecclesiastical rules. The full moon involved is not the astronomical Full Moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical Moon.
The ecclesiastical rules are:
See Scriptures and Easter