How is the date of Easter calculated, and might the method change after 2001?

The date of Easter is an important one in the Christain calendar, but
the date of its observance has been the source of controversy over the
years.  In A.D. 325, the First Ecumenical Council of Nicaea decided
that it should be celebrated as the first Sunday following the first
vernal full moon.  However, astronomical techniques have never been the
ultimate basis for calculating when this occurred.  It is also
desirable to have Easter celebrated on the same day around the world,
and to have the date easily calculatable arbitrarily far in the future.
So various methods have been used to determine the date for Easter over
the years, all based on tables and assuming that the vernal equinox
falls on a certain day.  (Different churches sometimes used different
formulas at different times, and I know that at least one time one
church's formula included a provision that their Easter could not fall
on the same Easter as another church.)

If you have followed rec.puzzles for any length of time, you probably
know that there was a change from the Julian calendar to the Gregorian
calendar.  The major motivation for this calendar reform was because
the date of the vernal equinox used by the Easter formulas no longer
corresponded to the actual vernal equinox, which messed up the date for
Easter.  Long-time readers also know that different countries adopted
the Gregorian calendar at different times.  You should also know that
many Eastern Orthodox churches have not yet adopted the Gregorian
calendar, and they base the celebration of their holidays on the Julian
calendar.  This, combined with different methods for calculating the
date of Easter, means that most years, Easter is celebrated on
different days around the world.

Last year, at a meeting of the World Council of Churches in Aleppo, a
proposal was made to reform the calculation of Easter using modern
astronomical techniques.  Their seems to be a good deal of support for
this.  If the churches agree, the new formula could be adopted after
2001.  Both methods agree on a 2001 April 15 date for Easter, and the
symbolism of a worldwide celebration of Easter beginning in the third
millenium of the Christian Era is rather nifty.

I don't know exactly what the new formula would be, however.

-- Randall M! Gee <gee@math.berkeley.edu>, May 1998