In
the Western Catholic and Protestant church, Easter falls between late
March and late April, in the Orthodox Church, between early April and
early May.
In the 16th century, the Roman church abandoned the Julian calendar in favor of the Gregorian calendar.
As a result, the Catholic church needed to convert the originally Julian-based date of Easter into a Gregorian date.
The
date for celebrating the Easter holiday differs because both churches
use a different system for calculating the date of the vernal equinox
and the first full moon.
Originally,
the Western church decided to set a fixed date, March 21, for the
vernal equinox and then determine the appearance of the full moon
using ecclesiastical calculations.
This
was only true, however, prior to the year 325 A.D. Over the course of
history beginning in 325 with the Council of Niacea, the Western
church decided to establish a more standardized system for determining
the date of Easter.
The
Orthodox church, on the other hand, set the date for Easter based on
the astronomical definitions of the full moon and the equinox, as they
are observed along the meridian of Jerusalem.
Western
Christianity always celebrates Easter on the Sunday immediately
following what is known as the Paschal Full Moon, which is determined
from historical tables and does not correspond to lunar events.
Catholics
used these calculations to establish a table of Ecclesiastical Full
Moon dates. These dates would determine the Holy Days on the
Ecclesiastical calendar. Though modified slightly from its original
form in 1583, the table for determining the Ecclesiastical Full Moon
dates was permanently established and has been used ever since to
determine the date of Easter.
The
Paschal Full Moon is the first Ecclesiastical Full Moon date after
March 21, which also happened to be the date of the vernal equinox in
325. As a result, in Western Christianity, the Easter holiday is
always celebrated on the Sunday immediately following the date of the
Paschal Full Moon, which is why it sometimes falls on the same date as
the Orthodox Easter.
The
Paschal Full Moon can vary as much as two days from the date of the
actual full moon, with dates ranging from March 21 to April 18.
As a result, Easter dates can range from March 22 through April 25 in Western Christianity.
Easter and its related dates are based on a lunar calendar very similar to the Hebrew calendar.
Some
Eastern Orthodox churches not only maintain the date of Easter based
on the Julian calendar, they also use the actual, astronomical full
moon and the actual vernal equinox as observed along the meridian of
Jerusalem.
Due
to the inaccuracy of the Julian calendar, and the 13 days that have
accrued since 325, this sometimes complicates the matter. In order to
stay in line with the vernal equinox, Orthodox Easter cannot be
celebrated before April 3 in the present day Gregorian calendar.
Additionally,
in keeping with the rule established by the First Ecumenical Council
of Niacea, the Eastern Orthodox Church followed the tradition that
Easter must always fall after the Jewish Passover, since the death,
burial and resurrection of Christ happened after the celebration of
Passover.
Eventually,
the Orthodox church came up with an alternative to calculating Easter
based on the Gregorian calendar and Passover, and developed a 19-year
cycle, as opposed to the Western church’s 84-year cycle.
Complicating the matter is the fact that Jesus’ followers never recorded the date of his resurrection.
In sum, Easter Sunday is the Sunday following the Paschal Full Moon date for that particular year.
In
June 325, astronomers approximated astronomical full moon dates for
the Christian church, calling them Ecclesiastical Full Moon dates.
Since
326 the Paschal Full Moon date has always been the Ecclesiastical
Full Moon date after March 21, which was the equinox date in 325.