Here’s a rap on Easter dating protocol I took out of the Mr. Fixit column in the local fishwrapper:
Star Tribune — April, 2001 — Fixit column
Why is Easter on the ‘wrong’ date this year?
Q. Would you please explain why Easter is on the wrong Sunday this year? According to the formula for determining Easter (the first Sunday after the first full moon following the vernal equinox), Easter this year should be April 8, not April 15. At least two of my calendars specify the full moon in April occurring at 10:22 p.m. on Saturday, April 7. So, according to the formula, Easter should be April 8, not 15.
A. You’re right. According to the commonly stated rule, Easter is a week late this year. But the rule — Easter is the first Sunday after the full moon that occurs next after the vernal equinox — is somewhat misleading because Easter Day isn’t determined solely by astronomical rule. There is an ecclesiastical formula that rules, as well.
According to the U.S. Naval observatory, the actual conditions to determine the date for Easter are:
Easter must be on a Sunday.
This Sunday must follow the 14th day of the paschal moon.
The paschal moon is that of which the 14th day (full moon) falls on or next follows the day of the vernal equinox.
The equinox is fixed in the calendar as March 21.
Easter can never occur before March 22 or later than April 25.
There are three major differences to note between the ecclesiastical system and the astronomical system:
The 14th day of the paschal moon is not necessarily identical to the time of astronomical full moon. The ecclesiastical tables do 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. The actual date varies very slightly from year to year.
The date of Easter is a specific calendar date. Easter starts when that date starts for your time zone. Astronomical phenomena occur at a specific date and time all over the Earth at once.
In order for the Easter date to be fixed, and determinable indefinitely in advance, the Roman Catholic Church constructed tables for calculating the time of the paschal moon.
In the ecclesiastical system, the vernal equinox is fixed at March 21 regardless of the actual motion of the sun.
Inevitably, the date of Easter occasionally differs from a date that uses an astronomical full moon and the astronomical vernal equinox. In some cases, this difference may occur in some parts of the world and not in others.
At the U.S. Naval observatory website you can enter the year and it will automatically compute Easter day — for the past or for thousands of years into the future. The site also contains a mathematical formula that you can use to compute the date of Easter.
Incidentally, the date of Easter is different in the eastern (Orthodox) Christian churches. The Julian calendar, which preceded the currently used Gregorian calendar, is the traditional basis for the ecclesiastical calendar. In 1923, Orthodox churches adopted a modified Gregorian calendar and decided to set the date of Easter according to the astronomical full moon for the meridian of Jerusalem. However, these changes have not been universally implemented.