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.

My JavaScript computing the date of Easter:

 Year: 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020      Date of Easter: From    to

Another JavaScript

More details about the date of Easter

 Web Links Easter Dating Method (Astronomical Society of South Australia) The Date of Easter (Royal Observatory Greenwich, Information Leaflets) The Date of Easter (U. S. Naval Observatory) Wann ist Ostern ? (Physikalisch-Technischen Bundesanstalt) Kalender - Computus (Die Grundlagen des julianischen und gregorianischen Kalenders) Herrad of Landsberg (Wikipedia) Calendar Converter

Updated: 2010, May 09