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From: Stephen Hayes
To: All
Date: 2004-03-27 02:51:58
Subject: Calendar

[part 5 of 5]

4.   THE DATE OF EASTER

The date of Easter determines the dates of various other movable feasts in the
christian calendar.  Methods for calculating the date of Easter have
historically been the subjects of major disputes among Christians.  In England
the decision was taken at the Synod of Whitby in AD 644 to follow the Catholic
method.  The methods given here are derived from various sources.

4.1  EASTER IN THE WEST

The Catholic church and most Protestants calculate the date of Easter as:

"The Sunday following the first full moon on or after 21st March"

so that the earliest possible Easter Sunday is 22nd March - when 21st March is
a Saturday and is the day of a full moon the following day, Sunday 22nd March
is Easter Sunday.  All the other movable feasts may be calculated from this.

a)   The following (partial) program in Pascal illustrates one method.

     TYPE yr   =    1583..4200;
          mn   =    (march,april);
          dy   =    1..31;

     PROCEDURE easter    (y: yr;   VAR d: dy;     VAR m: mn);
          {
          This procedure computes the date of Easter in the year AD `yr'
          returning day `dy' and month `mn'.  The algorithm is from Knuth's
          "Fundamental Algorithms", where Aloysius Lillius of
Naples and the
          German Christopher Clavius are acknowledged.  It applies to any year
          from AD 1582 (1583?) through AD 4200.
          }
     VAR  gold:     1..19;         {the "golden number"}
          cent:     16..43;        {century}
          epact:    -29..30;       {the "epact" specifies full moon}
          sun:      1950..3750;    {March (-sun) MOD 7 is a Sunday}
          full:     14..55;        {date of full moon}
          x:        2..11;         {correction for non-leap century years}
          z:        1..10;         {synch. with moon's orbit}

     BEGIN
      gold     :=   (y MOD 19) + 1;
      cent     :=   (y DIV 100) + 1;
      x        :=   ((3 * cent) DIV 4) - 12;
      z        :=   (((8 * cent) + 5) DIV 25) - 5;
      sun      :=   (((5 * y) DIV 4) - x) - 10;
      epact    :=   ((11 * gold) + 20 + Z - X) MOD 30;
      IF (epact < 0)
       THEN
        epact  :=   epact + 30;
      IF (epact = 25) AND (gold > 11) OR (epact = 24)
       THEN
        epact  :=   epact + 1;
      full     :=   44 - epact;
      IF (full < 21)
       THEN
        full   :=   full + 30;
      {
      "first Sunday following first full moon occuring on or after
March 21"
      }
      full     :=   full + 7 - (sun + full) MOD 7;
      IF (full > 31)
       THEN
        BEGIN
         m     :=   april;
         d     :=   full - 31;
        END
       ELSE
        BEGIN
         m     :=   march;
         d     :=   full;
        END;
     END       {easter};

b)   Read the notes in the front of the Church of England's Book of Common
     Prayer to find a "pencil and paper" method for a more
restricted range of
     years.

4.2  EASTER IN THE EAST

The Orthodox and some other eastern christians calculate the date of Easter
differently.

5    NOTE FOR GENEALOGISTS

It is common nowadays (as in the past) for (nominally) Christian marriages to
take place on a Saturday.  If the subject of your research married on some
other day of the week, try to seek out why.  Were they simply in a hurry?  Was
it a feast day or other public holiday?  Did they have some reason to avoid
publicity?  (Elopement?)  Was it by Banns or by Licence?

It is also common nowadays (as in the past) for baptisms to take place on a
Sunday.  If the subject of your research was baptised on some other day of the
week, try to seek out why.  Was it a sickly child in imminent danger of death?
Did the child survive?  Was it the baptism of an adult who, for some reason,
had never been baptised?  Convert from Judaism to avoid persecution or poverty
or to gain social or political position?  Unbelieving parents but child, when
adult, became believer?  No point looking for later marriage, or any other
history, of child which died young.  No point looking for parents marriage two
or three years before baptism of adult.

[end]
 

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