April 2006 Feature: The Date of Easter

The date of Easter seems to be relatively straight forward. It happens on the first Sunday after the first Full Moon in spring. Of course, I should specify that this means a Northern Hemisphere spring, because south of the equator, Easter happens on the first Sunday after the first Full Moon in autumn.
Spring in the Northern Hemisphere (autumn in the Southern Hemisphere) begins at the instant of the March equinox. This year's spring or vernal equinox happens on March 20, at 18:26 (6:26 p.m.) Universal Time, or 1:26 p.m. Eastern Standard Time.
The first Full Moon after the March equinox falls on Thursday, April 13, at 16:40 (4:40 p.m.) UT -- or at 12:40 p.m. Eastern Daylight Time (11:40 a.m. EST). That places the first Sunday after the first springtime Full Moon on April 16. Thus, Easter Sunday is on this date.
Should this April Full Moon have fallen on Sunday, April 16, then Easter would have taken place on the following Sunday, April 23. That's because Easter falls on the first Sunday after the first Full Moon in spring. For reasons that are not altogether clear to me, the Eastern Orthodox Church celebrates Easter on April 23.
For more on the discrepancy between Easter dates in Eastern and Western Christiandom, you are welcome to click here and here. The remainder of this article will refer to the date of Easter by the Western tradition -- though in some years, the date of Easter is the same for both.
Regardless of reckoning, Easter is a moveable feast, because the date of the first Full Moon in spring changes yearly. Generally, the Full Moon happens 11 days earlier in the following year. Sure enough, in 2007, the first Full Moon in spring occurs on April 2 (a Monday). Therefore, next year's Easter will fall on the Sunday thereafter, on April 8, 2007 -- and what's more, it'll be this date for both the Western and Eastern Church.

Ecclesiastical versus Astronomical Easter

Alas, now for a disclaimer on the date of Easter: seven times in the 21st century Easter will not fall on the first Sunday after the first Full Moon in spring. These years are 2038, 2049, 2069, 2076, 2089, 2095 and 2096 (page 365, Mathematical Astronomy Morsels by Jean Meeus). We will investigate why the date of Easter in the year 2038 doesn't follow this rule.
In 2038, the equinox falls on Saturday, March 20, and the Full Moon falls on Sunday, March 21. Therefore, the first Sunday after the first Full Moon in spring falls on Sunday, March 28, 2038. Clearly, the March equinox preceeds the March Full Moon.
By ecclesiastical rules, however, the equinox is fixed on March 21, and the date of the first Full Moon in spring -- the Paschal Full Moon -- is derived from ecclesiastical tables, not astronomical almanacs. In other words, the Paschal Full Moon may or may not coincide with the date of the astronomical Full Moon.
By ecclesiastical reckoning, the March Full Moon in 2038 preceeds the March equinox, so the first Full Moon in spring takes place in April. By ecclesiastical rules, Easter falls on Sunday, April 25, 2038 -- though astronomically, the date of Easter is March 28, 2038.
In years when the dates of an ecclesiastical Easter and an astronomical Easter are in conflict, an ecclesiastical Easter trumps an astronomical Easter.

copyright 2006 by Bruce McClure

March 2006 Feature * May 2006 Feature

For your information. . .
The Date of Easter
Easter Dates
Equinox Dates
Full Moon Dates
Eastern Orthodox Calendars: Old and New