The Movable Date of Easter

I am providing here a link to a site where you can enter a year and get the dates for Easter and the dependent festival days of the church year. That site also explains in detail the reason for the somewhat confusing algorithm. To click on the link means that you will be leaving Emmanuel's site. You can click on your back button to return or better yet, put us in your Favorites file! Go to the Easter Date Calculator.

The most concise explanation of determining the date of Easter that I have been able to find follows here, rather than link to the site, I have cut and pasted the information so that it won't disappear in the event that the author should decide to eliminate the page in the future. The work of Lair Gauche follows:

Many people know the definition of Easter as "Easter Sunday falls on the first Sunday after the Full Moon on, or after, the March Equinox".  But this definition hides a number of problems and can require quite accurate knowledge of the orbit of the moon.  The first problem is the need to define a longitude on earth for which Sunday is determined.  For example, at the instant of Full Moon it may be Sunday in Sydney but still Saturday in London giving rise in certain circumstances to a different date for Easter.  A second problem arises from the variabilities in the orbit of the moon which is perturbed by the sun and the major planets.  The exact orbit of the moon was not known precisely until recent times and it would be difficult if the date of Easter changed as our knowledge of the moon's orbit changed.

To obtain consistency in the date of Easter, the Church at the Council of Nicea decided to define Easter with respect to an imaginary moon - known as the "ecclesiastic moon".  Also, the date of equinox was fixed at March 21 even though it can vary slightly from this date.  With this definition, the date of Easter can be determined in advance without further astronomical knowledge.  But the sequence of dates varies significantly from year-to-year with Easter Sunday being as early as March 22 and as late as April 25. In fact, the exact sequence of Easter dates repeats itself approximately every 5,700,000 years in our Gregorian calendar.

The Eastern Orthodox churches decided not to follow the above definition and they determine Easter from the real full moon at the longitude of Jerusalem.  Hence, there is sometimes a difference in the date of Easter between churches.

The following is an algorithm by which the date of Easter may be calculated.

   Divide                      by       Quotient     Remainder

the year x                     19          -             a
the year x                    100          b             c
b                               4          d             e
b + 8                          25          f             -
b - f + 1                       3          g             -
19*a + b - d - g + 15          30          -             h
c                               4          i             k
32 + 2*e + 2*i - h - k          7          -             l
a + 11*h + 22*l               451          m             -
h + l - 7*m + 114              31          n             p

Then n is the number of the month (3 = March; 4 = April) and p + 1 is the day of that month upon which Easter Sunday falls. The symbol * means the product of the two numbers.

Sources: Astronomical Algorithms, Jean Meeus; Man and the Stars, Robert Hanbury Brown.