The Lunisolar Cycle (cyclus decennovennalis, Metonic Cycle)
The Metonic cycle is an approximate common multiple of the tropical year and the synodic month:
19 tropical years = 19*365.242 days = 6939,60 days
235 synodic months = 235*29.53 days = 6939,55 days
Easter table according to Herrad of Landsberg (c.1130 - July 25, 1195) valid for the Julian Calendar,
using the Golden Numbers and the Concurrents (concurrente, epactae solaris).
The concurrent is the number of the weekday of March 24 counted from Sunday: Sunday=1, Monday=2, etc.
The concurrent will advance by one each year. However, the insertion of leap-year day will advance the concurrent by two every fourth year.
Example: compute the date of Easter of the year 2011
year 2011: golden number = 2012 mod 19 = 17
March 24 Julian date is Wednesday, concurrent = 4
reading the table the Julian date of easter is April 11 ( = April 24 Gregorian Calendar).
Computus Paschalis
Julian Calendar
The date of Easter is the first Sunday after the first ecclesiastical full moon on or after March 21.
The ecclesiastical full moon is not necessarily the same as the astronomical full moon.
We start in the year 2011: the Julian spring full moon date (terminus paschalis) is Monday, April 5, the Julian Easter Date is Sunday, April 11.
As the lunar year (354 days) is 11 days shorter than the tropical solar year (365 days), the next spring full moon is 11 days earlier, on Saturday March 25 in 2012.
The procedure is the same for the next years of the cycle, except for the cycle numbers 3, 6, 8, 11, 14, 17, going forward by 30-11=19 days, and cycle number 0,
going forward by 29-11=18 days (saltus lunae):
[Moon applet]
| Epact | Full Moon |
Epact | Full Moon | Epact | Full Moon |
| 1 | April 11 | 11 |
April 2 | 21 | March 23 |
| 2 | April 11 | 12 | April 1 | 22 | March 22 |
| 3 | April 10 | 13 | March 31 | 23 | March 21 |
| 4 | April 9 | 14 | March 30 | 24 | April 18 |
| 5 | April 8 | 15 | March 29 | 25 | April 17 or 18 |
| 6 | April 7 | 16 | March 28 | 26 | April 17 |
| 7 | April 6 | 17 | March 27 | 27 | April 16 |
| 8 | April 4 | 18 | March 26 | 28 | April 15 |
| 9 | April 4 | 19 | March 25 | 29 | April 14 |
| 10 | April 3 | 20 | March 24 | 30 | April 13 |
| 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. If the Golden Number is > 11 choose 17 April, otherwise choose 18 April. |
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