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2.12.6. How does one calculate Gregorian Easter then?




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This article is from the Calendars FAQ, by Claus Tondering claus@tondering.dk with numerous contributions by others.

2.12.6. How does one calculate Gregorian Easter then?

Look up the Epact in this table to find the date for the Paschal full
moon:

  Epact   Full moon     Epact   Full moon     Epact   Full moon
  -----------------     -----------------     -----------------
     1    12 April        11     2 April        21    23 March
     2    11 April        12     1 April        22    22 March
     3    10 April        13    31 March        23    21 March
     4     9 April        14    30 March        24    18 April
     5     8 April        15    29 March        25    18 or 17 April
     6     7 April        16    28 March        26    17 April
     7     6 April        17    27 March        27    16 April
     8     5 April        18    26 March        28    15 April
     9     4 April        19    25 March        29    14 April
    10     3 April        20    24 March        30    13 April

Easter Sunday is the first Sunday following the above full moon date.
If the full moon falls on a Sunday, Easter Sunday is the following
Sunday.

An Epact of 25 requires special treatment, as it has two dates in the
above table. There are two equivalent methods for choosing the correct
full moon date:

A) Choose 18 April, unless the current century contains years with an
epact of 24, in which case 17 April should be used.

B) If the Golden Number is > 11 choose 17 April, otherwise choose 18 April.

The proof that these two statements are equivalent is left as an
exercise to the reader. (The frustrated ones may contact me for the
proof.)

Example: When was Easter in 1992?

In the previous section we found that the Golden Number for 1992 was
17 and the Epact was 25. Looking in the table, we find that the
Paschal full moon was either 17 or 18 April. By rule B above, we
choose 17 April because the Golden Number > 11.

17 April 1992 was a Friday. Easter Sunday must therefore have been
19 April.

 

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