The commonly stated
rule, that Easter Day is the first Sunday after
the full moon that occurs next after the vernal
equinox, is somewhat misleading because it is
not a precise statement of the actual
ecclesiastical rules.
The actual conditions
to determine the date for Easter are (1) Easter
must be on a Sunday; (2) this Sunday must follow
the 14th day of the paschal moon; (3) the
paschal moon is that of which the 14th day (full
moon) falls on or next follows the day of the
vernal equinox; and (4) the equinox is fixed in
the calendar as March 21. Easter can never occur
before March 22 or later than April 25.
In order that the date
for Easter be incontrovertibly fixed, and
determinable indefinitely in advance, the Church
constructed special tables for calculating the
time of the paschal moon. There are three major
differences to note between the ecclesiastical
system and the astronomical system. (1) The 14th
day of the paschal moon is not necessarily
identical to the time of astronomical full moon.
The ecclesiastical tables do not account for the
full complexity of the lunar motion. (2) The
vernal equinox has a precise astronomical
definition determined by the actual motion of
the Sun. It is the precise time at which the
apparent longitude of the Sun is zero degrees.
The actual date varies very slightly from year
to year. In the ecclesiastical system the vernal
equinox is fixed at March 21 regardless of the
actual motion of the Sun. (3) The date of Easter
is a specific calendar date. Easter starts when
that date starts for your time zone.
Astronomical phenomena occurs at a specific date
and time all over the Earth at once.
Inevitably, the date of
Easter occasionally differs from a date that
uses an astronomical full moon and the
astronomical vernal equinox. In some cases this
difference may occur in some parts of the world
and not in others because two dates separated by
the International Date Line are always
simultaneously in progress on the Earth.
1962 was such a case.
In 1962, the astronomical full moon happened to
occur about six hours after astronomical
equinox. That full moon occured on March 21,
UT=7h 55m. The ecclesiastical full moon,
however, occured on March 20. The ecclesiastical
equinox is fixed to be March 21; therefore, the
ecclesiastical full moon occured before the
ecclesiastical equinox. In the first case, the
full moon followed the equinox; in other case,
it preceeded the equinox. Following the rules,
Easter, therefore, was not until the Sunday that
followed the next ecclesiastical full moon. That
occured on Wednesday, April 18 making Easter
Sunday, April 22.
Similarly, in 1954
Easter was Sunday, April 18 because the first
ecclesiastical full moon after March 21 fell on
Saturday, April 17. However, the first
astronomical full moon after the equinox
happened to fall on April 18 at UT=5h. So in
some places in the world Easter was on the same
Sunday as the astronomical full moon.
The following are dates
of Easter from 1980 to 2024:
1980 April 6 1995 April 16 2010 April 4
1981 April 19 1996 April 7 2011 April 24
1982 April 11 1997 March 30 2012 April 8
1983 April 3 1998 April 12 2013 March 31
1984 April 22 1999 April 4 2014 April 20
1985 April 7 2000 April 23 2015 April 5
1986 March 30 2001 April 15 2016 March 27
1987 April 19 2002 March 31 2017 April 16
1988 April 3 2003 April 20 2018 April 1
1989 March 26 2004 April 11 2019 April 21
1990 April 15 2005 March 27 2020 April 12
1991 March 31 2006 April 16 2021 April 4
1992 April 19 2007 April 8 2022 April 17
1993 April 11 2008 March 23 2023 April 9
1994 April 3 2009 April 12 2024 March 31
Ash Wednesday is 46
days before Easter. Lent - the season that
begins on Ash Wednesday and ends at Easter - is
said to be 40 days long because the six Sundays
that occur in this period are not considered to
be part of Lent.
The date of Easter is
different in the eastern (Orthodox) Christian
churches. The Julian calendar, which preceded
the currently-used Gregorian calendar, is the
traditional basis for the ecclesiastical
calendar. (The Julian calendar was still used in
much of eastern Europe until the early part of
the 20th century.) In a congress held in 1923,
the Orthodox churches adopted a modified
Gregorian calendar and decided to set the date
of Easter according to the astronomical full
moon for the meridian of Jerusalem. However,
these changes have not been universally
implemented, and a variety of practices remain
among the Orthodox churches.
Computing the Date of
Easter
The rule is that Easter
is the first Sunday after the first
ecclesiastical full moon that occurs on or after
March 21.
The lunar cycles used
by the ecclesiastical system are simple to
program. The following algorithm will compute
the date of Easter in the Gregorian calendar
system. All variables are integers and all
remainders from division are dropped. The
algorithm takes the year, y, and yields the
month, m, and day, d, of Easter. The symbol *
means multiply.
c = y / 100
n = y - 19 * ( y / 19 )
k = ( c - 17 ) / 25
i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15
i = i - 30 * ( i / 30 )
i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) )
* ( ( 21 - n ) / 11 ) )
j = y + y / 4 + i + 2 - c + c / 4
j = j - 7 * ( j / 7 )
l = i - j
m = 3 + ( l + 40 ) / 44
d = l + 28 - 31 * ( m / 4 )
For example, using the
year 2010, y=2010, c=2010/100=20,
n=2010-19x(2010/19)=15, etc. with Easter on
April 4, 2010.
