The date of Easter   The simple standard definition of Easter is
that it is the first Sunday after the Full Moon that occurs on
or after the vernal equinox. If the full moon falls on a Sunday
then Easter is the next Sunday.  Unfortunately this simple
definition is not strictly speaking correct. The vernal equinox
used is not the true equinox but an artificial one always
assumed to be on 21 March. The full moon used is not the true
full moon but an artificial construct based on the Metonic
cycle (below).  The reasons for this are that the method is then
independent of longitude on the Earth and is thus independent of
time zone. It also allows the date of Easter to be calculated in
advance regardless of the actual motion of the Earth around the
Sun.  The method quoted here is valid for the determination of
the date of Easter in Western Christian churches; the date used
by the Eastern churches can be one, four or five weeks later.
The Metonic cycle  The Metonic cycle of 19 years is one in which
the phases of th