How Is The Lent/easter Calendar Determined Every Year?

[LinaSt.Cecilia2772]

I heard that it had to do with certain moon cycles, but I could be wrong.

[cappie]

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:

Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox;
this particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon); and the vernal equinox is fixed as March 21.

Resulting in that Easter can never occur before March 22 or later than April 25. The Gregorian dates for the ecclesiastical full moon come from the Gregorian tables. Therefore, the civil date of Easter depends upon which tables - Gregorian or pre-Gregorian - are used. The western (Roman Catholic and Protestent) Christian churches use the Gregorian tables; many eastern (Orthodox) Christian churches use the older tables based on the Julian Calendar.

There are three major differences between the ecclesiastical system and the astronomical system.

The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.

The vernal equinox has a precise astronomical definition determined by the actual motion of the Sun. It is the precise time at which the apparent longitude of the Sun is zero degrees. This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.

The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.

Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.