How is Easter Sunday Date Determined?

Joined: March 29th, 2003, 4:05 pm

April 6th, 2007, 10:35 am #1

How is Easter Sunday Date Determined?

SUMMARY
Easter Sunday is the Sunday following the Paschal Full Moon (PFM)
date for the year. (Paschal is pronounced "PAS-KUL", not "pas-
chal"). See Christian Prayer Books for proof of this concise
definition.

In June 325 A.D. astronomers approximated astronomical full moon
dates for the Christian church, calling them Ecclesiastical Full
Moon (EFM) dates. From 326 A.D. the PFM date has always been the
EFM date after March 20 (which was the equinox date in 325 A.D.)

From 1583, each PFM date differs from an Astronomical Full Moon
(AFM) date usually by no more than 1 date, and never by more than 3
dates. (Each AFM is a two-dates event due to world time zones.
Each PFM is a one-date event world-wide).

HISTORY
Easter Sunday is the date of the annual celebration of Christ's
resurrection. The aim of the Easter Dating Method is to maintain,
for each Easter Sunday, the same season of the year and the same
relationship to the preceding astronomical full moon that occurred
at the time of his resurrection in 30 A.D.

This was achieved in 1583 A.D. using skill and common-sense by Pope
Gregory the 13th, and his astronomers and mathematicians,
predominantly Lilius and Clavius, by introducing their new larger
(revised) PFM Gregorian dates table. This replaced the (original)
326 A.D. "19 PFM dates" table in the Julian calendar.

Easter Sunday, from 326 A.D., is always one of the 35 dates March 22
to April 25.

From 31 A.D. to 325 A.D. Easter Day was celebrated either:
(a) on or just after the first day of the Jewish Passover (no matter
on which day of the week that Easter Day occurred), or
(b) on a Sunday close to or on the first Passover Day.
Both of these methods existed continuously throughout this period.

From 326 A.D. to 1582 A.D. Easter Sunday date was based on the
Julian calendar in use at that time. It became defined as the
Sunday following the Paschal Full Moon date for the year, using a
simple "19 PFM dates" table. Precise information on this subject
can be found on pages 415 to 425 of the Explanatory Supplement to
the 1961 Astronomical Ephemeris.

The Julian calendar was replaced by the Gregorian calendar in
October 1582 to re-align March 20 (and therefore Easter) with the
seasons by removing 10 dates October 5 to 14, 1582. This
replacement did not occur until later in many countries e.g. in
September 1752 in England. See GM Arts Easter Date Calculations
for more information. ENGLISH Easter Sunday dates for 1583 to 1752
can be calculated using information near the end of this Easter
Dating Method document.

The Gregorian calendar very closely maintains the alignment of
seasons and calendar dates by having leap years in only 1 of every 4
century years, namely, those divisible exactly by 400. One
additional February 29 date will need to be removed in about 4140
A.D., therefore Easter calculations will need to use the changed
Days of Week of PFM dates when the exact year for this removal is
decided.

From 326 A.D., the Easter Sunday Date for any given year is NOT
determined by the March Equinox date for that year. March 20 (not
March 21) is the most common Gregorian Equinox date from 1583 to
4099 A.D.

Historically, references to March 21 have caused mistakes in
calculating Easter Sunday dates. March 20 has become the important
date in recent Easter dating methods. Despite frequent references
to March 21, this date has no special significance to any recent
Easter dating methods.

The present method describing the Easter Dating Procedure can be
found in Christian Prayer Books. This procedure has been
dramatically simplified by Ronald W. Mallen, Adelaide, South
Australia. See Finding Easter Sunday Dates with a Calculator for a
clear and unique explanation of this procedure.

ORTHODOX CHURCHES
Orthodox churches became fully autonomous in 1054 A.D., and
celebrate their Easter always on the basis of the Julian calendar
and the "19 PFM dates" table. The Julian calendar date Thursday
October 4, 1582 was followed by the Gregorian calendar date Friday
October 15, 1582. The 10 dates October 5 to 14 were removed.

Consequently, their Easter Sunday dates are identical up to 1582,
then from 1583 onwards often differ from those of Western churches.

In some years the Orthodox Easter Sunday occurs on the same day as
the Western Easter Sunday. For example, this occurred in 1990
because the Western Easter Sunday date of (Gregorian calendar) April
15, 1990 is the same as the Orthodox Easter Sunday date of (Julian
calendar) April 2, 1990. In most years, Orthodox Easter follows
Western Easter by one or more weeks.

To determine the Orthodox Easter Sunday date, it is first necessary
to find the Julian Easter Sunday date, then to add the number of
days which have been "skipped" in the Gregorian calendar. See
Finding Orthodox Easter Sunday Dates with a Calculator for a simple
explanation of this procedure.

Special thanks to http://www.assa.org.au/edm.html#Method
The American Astronomical Society web site.
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