Expert: Ashraf Nisseem - 5/16/2006

Question
what factors do eastern orthodox chuches take into account when establishing the date for Easter?

Answer
In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern churches.

There are three major differences between the ecclesiastical system and the astronomical system.

The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.
The vernal equinox has a precise astronomical definition determined by the actual apparent motion of the Sun as seen from the Earth. It is the precise time at which the apparent ecliptic longitude of the Sun is zero. (Yes, the Sun's ecliptic longitude, not its declination, is used for the astronomical definition.) This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.
The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.
Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.

For example, take the year 1962. In 1962, the astronomical Full Moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox. The ecclesiastical full moon (taken from the tables), however, occured on March 20, before the fixed ecclesiastical equinox at March 21. In the astronomical case, the Full Moon followed its equinox; in the ecclesiastical case, it preceeded its equinox. Following the rules, Easter, therefore, was not until the Sunday that followed the next ecclesiastical full moon (Wednesday, April 18) making Easter Sunday, April 22.

Similarly, in 1954 the first ecclesiastical full moon after March 21 fell on Saturday, April 17. Thus, Easter was Sunday, April 18. The astronomical equinox also occurred on March 21. The next astronomical Full Moon occurred on April 18 at UT=5h. So in some places in the world Easter was on the same Sunday as the astronomical Full Moon.

The following are dates of Easter from 1980 to 2024:


1980  April 6        1995  April 16         2010  April 4

1981  April 19       1996  April 7     2011  April 24

1982  April 11       1997  March 30         2012  April 8

1983  April 3        1998  April 12         2013  March 31

1984  April 22       1999  April 4     2014  April 20

1985  April 7        2000  April 23         2015  April 5

1986  March 30       2001  April 15         2016  March 27

1987  April 19       2002  March 31         2017  April 16

1988  April 3        2003  April 20         2018  April 1

1989  March 26       2004  April 11         2019  April 21

1990  April 15       2005  March 27         2020  April 12

1991  March 31       2006  April 16         2021  April 4

1992  April 19       2007  April 8     2022  April 17

1993  April 11       2008  March 23         2023  April 9