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Elusive Easter
When is Easter?
mwls.com


Easter is one of the trickiest holidays when it comes to planning training and it can catch out even the most wary. Originally Easter was associated by the early Church with the Jewish Passover but the exact date has been a matter of controversy for centuries. In 325 AD a method of determining the date was agreed at the Council of Nicaea. This definition, which is still used today is (wait for it):

‘The first Sunday after the first full moon after or on the vernal equinox; and that if the full moon should occur on a Sunday and thereby coincide with the Passover festival, Easter should be commemorated on the Sunday following.’

Let’s look at the reasoning behind this statment. According to the New Testament: the feast of the Passover (The Last Supper) fell on a Thursday, Christ was crucified the following day (Good Friday) and rose again on the third day (Easter Sunday).

This would have been reasonably straight forward if it wasn’t for the fact that the feast of the Passover occurs on the day of the first full moon after the spring equinox. This means that Passover falls on different days from year to year. Christians of Jewish origin were quite comfortable with this, but those of Gentile origin wanted to celebrate the Resurrection on — well — Easter Sunday.

All this led to the curious situation of different parts of the Church celebrating the Resurrection on different days: those in the East observed Easter according to the date of the Passover festival and those in the West celebrated Easter on a Sunday. This in turn led to Council of Nicaea ruling and definition given above. Although the spring equinox can fall between March 19 and 21 in the northern hemisphere, the official Church spring equinox is always March 21. I should also mention at this stage that, although no-one has ever seen one, an 'ecclesiastical moon' whose motion differs slightly from the real one is used for the calculations.

All this means Easter can fall on any Sunday between 22 March and 25 April and, as you can imagine, this made plenty of work for astronomers.

Because the Earth goes round the Sun in 365.2422 days, the lunar month is 29.53 days, and the Julian calendar in use at that time was drifting out of synchronisation with the real year, calculating the date for Easter was a bit of an astronomical nightmare in the fourth century.

The adoption of the Gregorian calendar in 1582 eliminated much of the difficulty in fixing the date of Easter. In 1752 the Gregorian calendar was also adopted in Great Britain and Ireland and, since that time, Easter has been celebrated on the same day in the Western part of the Christian world. The Eastern churches did not adopt the Gregorian calendar and, in most years, they commemorate Easter Sunday on a different date to the West. Occasionally the dates coincide — the most recent times being 1980, 1984, 1987, 1990, 2001, 2004 and 2007. The next coincidence is 2010 when Easter Sunday will occur on Gregorian calendar April 4 2010 which is the same day as the Julian (Eastern) calendar Easter date March 22 2010. For a complete table of the coincidental dates please see Table 2 on http://www.smart.net/~mmontes/Eastdiff.html.

Like most problems, working out the date for Easter for a particular year benefits form breaking the problem down into smaller problems. In this case, the two consituent problems are:
  • the dates of the Sundays between 22 March and 25 of April and
  • the dates of the Full Moon between 22 March and 25 April.

The 'day of the week' (and hence the dates of the Sundays) can be worked out by referring to the tables in our Day of the Week article.

Working out the phases of the Moon (and hence the dates of the full Moon) is much more difficult and an introduction to the subject is given in our article on The Moon and its Phases. In 2001, the first Full Moon after the 21 March falls on Sunday, 8 April so Easter Sunday will fall a week later on 15 April.

If you would like to calculate the date for Easter Sunday yourself, you could try the following method:

Divide
By
QuotientRemainder
The year x
19
a
The year x
100
b
c
b
4
d
e
b + 8
25
f
b – f + 1
3
g
19a + b – d – g + 15
30
h
c
4
i
k
32 + 2e + 2i – h – k
7
l
a + 11h + 22l
451
m
h + l – 7m + 114
31
n
o


Then n is the month of the year and o + 1 is the day of the month on which Easter Sunday falls.

For example, to find the date of Easter Sunday 2002:

Divide
By
QuotientRemainder
2002 (x)
19
7 (a)
2002
100
20 (b)
2 (c)
(b=20)
4
5 (d)
0 (e)
(b=20) + 8
25
1 (f)
(b=20) – (f=1) + 1
3
6 (g)
19(a=7) + (b=20) – (d=5) – (g=6) + 15
30
7 (h)
(c=2)
4
0 (i)
2 (k)
32 + 2(e=0) + 2(i=0) – (h=7) – (k=2)
7
2 (l)
(a=7) + 11(h=7) + 22(l=2)
451
0 (m)
(h=7) + (l=2) – 7(m=0) + 114
31
3(n)
30(o)


Therefore Easter Sunday 2002 fell on the 31st (o + 1) day of March (n).

No wonder moves are being made to have Easter on a fixed date. In fact, in 1928 a Bill was passed by the British Parliament which would allow the Church of England to ‘fix’ Easter to be the Sunday after the second Saturday in April. This would have the effect of restricting Easter between 9 April and 15 April. However, this Bill can only take effect after it has been agreed by the World Council of Churches. Assuming that this agreement may still be sometime in coming, the table below gives the expected dates for Easter Sunday up to 2016.

200708 April
200823 March
200912 April
201004 April
201124 April
201208 April
201331 March
201420 April
201505 April
201627 March


To make things easier for you, we have written a dates program which calculates the date of Easter, other moveable holidays, fixed holidays, and the day of the week for any date.



 
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