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18-5 How is the date of the Orthodox Easter calculated




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This article is from the Bulgaria FAQ, by Dragomir R. Radev radev@tune.cs.columbia.edu with numerous contributions by others.

18-5 How is the date of the Orthodox Easter calculated

(by Konstantin Zahariev), last updated: 06-Apr-1996
The URLs are below. For those of you who do not have an easy access to the
Web, here is a quote from from http://cssa.stanford.edu/~marcos/ortheast.html:

Between AD326 and AD1582, Christianity determined Easter using an algorithm
approved by a Church Council in AD325, with the equinox defined as March 20
(the date in the Julian calendar of the equinox in AD325). From AD1054 (when
the Orthodox and Catholic Churches split) through AD1582 both the Catholic and
Orthodox Churches celebrated Easter on the same date, still using the
algorithm from AD325. The Julian Calendar was used by the European (and
Christian) communities until the Gregorian reform of 1582.

Since AD1582 October (when the Gregorian Calendar was adopted by much of
Catholic Europe), the Orthodox Easter usually falls on dates different than
the Western Christian Easter, although apparently the Churches are discussing
using the same formula to determine Easter - probably a formula different than
that currently used by either Church.

The Orthodox Easter is determined in the Julian Calendar, and is never
before or on the Jewish Feast of Passover (14 Nisan in the Hebrew Calendar).
>From 1900 until 2099 the Eastern Easter will fall one, four, or five
weeks after the Western Easter - and rarely on the same date.

Alex Kochergin (Kochergin_alex@mm.ssd.lmsc.lockheed.com) has sent the
following information about the Eastern Easter: It has a cycle that
(in the Julian Calendar) repeats itself every 532 (19x28) years (since the
Julian Solar calendar repeats every 28 years and the Metonic Lunar cycle is
19 years). Because of the strict concordance with the lunar cycle, the
Eastern Easter occurs only after Passover. The Gregorian Easter (on the
other hand) does not track Passover. For example: in 1997, Passover is
22 April; Western Easter is three weeks EARLIER (30 March) and the Eastern
Easter is the Sunday following Passover (27 April).

Alex also provided the following algorithm derived by the German mathematician
Gauss. This algorithm calculates the number of days AFTER March 21 (Julian)
that Easter occurs (Note: It is a much simpler calculation than the Western
Easter. RMD(x,y) = remainder when x is divided by y.)

R1=RMD(Year,19)
R2=RMD(Year,4)
R3=RMD(Year,7)
RA=19*R1+16
R4=RMD(RA,30)
RB=2*R2+4*R3+6*R4
R5=RMD(RB,7)
RC=R4+R5

The number RC ranges from 1 to 35 which corresponds to March 22 to April 25
in the Julian Calendar (currently April 4 to May 8 on the Gregorian). The
Julian Calendar is now 13 days behind the Gregorian, and will be until
March 1, 2100 when it will be 14 days behind the Gregorian Calendar.

For more details on the Orthodox Easter date rules, check out
http://cssa.stanford.edu/~marcos/ortheast.html
http://cssa.stanford.edu/~marcos/ec-cal.html
http://www.ozemail.com.au/~jahisco/BSS/easter.htm

For the Catholic Easter date rules, see
http://www.ast.cam.ac.uk/RGO/leaflets/easter/easter.html

For the Hebrew calendar:
http://www-syntim.inria.fr/htbin/air/hebrewcalendar2.pl

Finally, for you Matlab users, I could not resist but write the above algorithm
as a Matlab program -
http://maelstrom.seos.uvic.ca/people/zahariev/easter_date.m

It accepts any year between 1583 and 2099.

 

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