Monday, March 28, 2005

When is Easter?

Kyndall read this kid's book that explained Easter's date to be the following formula:

The first Sunday after the first Full Moon after the first Day of Spring (March 21).

The tie to astrological activity made me uncomfortable (I am not confusing astronomy and astrology - I realize a Full Moon can be an innocent, natural event). Fortunately, Kyndall did some reasearch and found that this was ALMOST right; the correct formula is:

The first Sunday after the first ECCLESIASTICAL Full Moon after the first Day of Spring (March 21).

Here's an interesting note about that:

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.

For example, take the year 1962. In 1962, the astronomical Full Moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox. The ecclesiastical full moon (taken from the tables), however, occured on March 20, before the fixed ecclesiastical equinox at March 21. In the astronomical case, the Full Moon followed its equinox; in the ecclesiastical case, it preceeded its equinox. Following the rules, Easter, therefore, was not until the Sunday that followed the next ecclesiastical full moon (Wednesday, April 18) making Easter Sunday, April 22.

Similarly, in 1954 the first ecclesiastical full moon after March 21 fell on Saturday, April 17. Thus, Easter was Sunday, April 18. The astronomical equinox also occurred on March 21. The next astronomical Full Moon occurred on April 18 at UT=5h. So in some places in the world Easter was on the same Sunday as the astronomical Full Moon.

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