From: http://www.smart.net/~mmontes/ortheast.html


The Date of Orthodox Easter: A variation of Gauss' algorithm

Alex also provided the following algorithm that is based on the
algorithm derived by the German mathematician Gauss, the principal
simplification is that substitutions have been made for the case of
Julian calendars and Orthodox Easters. 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.

The Date of Orthodox Easter: An algorithm based on Oudin's Algorithm

Another simple algorithm is listed in the Calendar FAQ by Claus
Tondering. It is based on Oudin's algorithm, and is also simple and
elegant.

Copyright and disclaimer
------------------------
        This document is Copyright (C) 1996 by Claus Tondering.
        E-mail: c-t@pip.dknet.dk.
        The document may be freely distributed, provided this
        copyright notice is included and no money is charged for
        the document.

        This document is provided "as is". No warranties are made as
        to its correctness.

2.9.6. Isn't there a simpler way to calculate [Orthodox] Easter?
-----------------------------------------------------
This is an attempt to boil down the information given in the previous
sections (the divisions are integer divisions, in which remainders are
discarded): 

[Note: 22%7=1 ; 22/7=3, so % returns the remainder, and / neglects the
remainder.] 

G = year % 19
I = (19*G + 15) % 30
J = (year + year/4 + I) % 7
L = I - J
EasterMonth = 3 + (L + 40)/44
EasterDay = L + 28 - 31*(EasterMonth/4)

[Note: Orthodox Easter is then EasterDay of EasterMonth in the Julian
       Calendar. You will need to add the correct offset to obtain the 
       date in the Gregorian Calendar. From Julian Mar 1, 1900, to Julian
       Feb 29, 2100, the correction is to add 13 days to the Julian date to
       obtain the Gregorian date.]

This algorithm is based in part on the algorithm of Oudin (1940) and
quoted in "Explanatory Supplement to the Astronomical Almanac",
P. Kenneth Seidelmann, editor.

People who want to dig into the workings of this algorithm, may be
interested to know that
    G is the Golden Number-1
    I is the number of days from 21 March to the Paschal full moon
    J is the weekday for the Paschal full moon (0=Sunday, 1=Monday,
      etc.)
    L is the number of days from 21 March to the Sunday on or before
      the Pascal full moon (a number between -6 and 28)

Using these algorithms, I have made tables for the date of Orthodox
Easter, tabulated in both the Julian and Gregorian calendars for
AD 1875-2124.

Carter's algorithm, Butcher's algorithm, and Oudin's algorithm are
algorithms for the Western Churches. 

