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a collection of rules for determining the date of Easter.
The rules set down certain conditions relative to the movement of the earth around the sun (Easter should be soon after the vernal equinox), the revolution of the moon around the earth (Easter is connected with the full moon), and a seven-day week (the holiday falls on a Sunday). Consequently, the date of Easter varies from year to year. Since Easter is not calculated by the various religions according to the same rules, the days when it is celebrated do not coincide. The mathematical theory of the paschal cycle was developed by K. Gauss.