The formula for
Easter—"The first Sunday after the first full moon on or
after the vernal equinox"—is identical for both Western
and Orthodox Easters, but the churches base the dates on
different calendars: Western churches use the Gregorian
calendar, the standard calendar for much of the world, and
Orthodox churches use the older, Julian
calendar.
That much is straightforward. But actually calculating these
dates involves a bewildering array of ecclesiastical moons
and paschal full moons, the astronomical
equinox, and the fixed equinox— and
that's in addition to the two different calendar systems.
When Is a Full Moon Full?
The two churches vary on the
definition of the vernal
equinox and the full moon. The Eastern Church sets the date
of Easter according to the actual, astronomical full moon and
the actual equinox as observed along the meridian of Jerusalem,
site of the Crucifixion and Resurrection.
