Ask A Scientist© Astronomy Archive

Question:  I would like some help in finding out about the golden number for
predicting or calculating the full moon and Easter. The Encyclopedia Britannica
says to find it by adding 1 to the calendar year (AD) and dividing it by 19, the 
remainder being the golden number. If there is no remainder, the number is 19. O nce the number has
been calculated, how does one use it? What does one do with it to arrive at the
full moon date?
julie e larsen

Answer:  I read the Britannica write-up and it is not at all clear what you're
supposed to do. I got the following from an insert that came with my desk
calendar:
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Easter falls on the first Sunday following the arbitrary Paschal Full Moon,
which does not necessarily coincide with a real or astronomical full moon.
The Paschal Full Moon is calculated by adding 1 to the remainder obtained by
dividing the year by 19 and applying the following table:

   1 - Apr 14    5 - Mar 31    9 - Apr 16   13 - Apr  2   17 - Apr 17
   2 - Apr  3    6 - Apr 18   10 - Apr  5   14 - Mar 22   18 - Apr  7
   3 - Mar 23    7 - Apr  8   11 - Mar 25   15 - Apr 10   19 - Mar 27
   4 - Apr 11    8 - Mar 28   12 - Apr 13   16 - Mar 30
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The date you get from the above table is this so-called Paschal Full Moon; it
may be a day or two off from the astronomical full moon.

So for this year the "key" is 1; Apr 14 is a Friday, so Easter is Apr 16. The
article doesn't say what the range of years is to which this method applies; I
would hesitate to apply it except for years in the 19th,
20th, and 21st centuries.
Rcwinther