2.12.6. How does one calculate Gregorian Easter then?
Description
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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