Sunday, April 08, 2012

Why Western, Orthodox Easter Fall On Different Days

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.