Computus

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Dionysius Exiguus invented the Anno Domini era to calculate the Easter date.

The Computus is a calculation method of the Easter date. In the early 4th century there was confusion about when Christian Easter or the Resurrection should be celebrated.

History[change | change source]

Background[change | change source]

At the Arles Council, in 314, was obliged to all the Christianity to celebrate Easter the same day, and that this date was going to be assigned by the Pope, that was going to send epistles to all the churches in the world with the necessary instructions. However, not all the congregations followed those precepts.

Nicaia Council[change | change source]

In the Nicaia Council, in 325, this subject was solved.

Was established that the Resurrection Easter had to be celebrated fulfilling some norms:

  • The Easter had to be celebrated in Sunday.
  • It can not coincide with the Jewish Easter, celebrated independendently which day of the week it was.
  • The Christians can not celebrate Easter twice the same year. This is explained because the new year started in the springlike equinox, so was prohibited the celebration of Easter before the real equinox.

However, there still were discussions between the Roman Church and the Alexandrian Church, while the Nicaia Council said that the Alexandria Church was right, they had to calculate the date (in Alexandria), then communicate it to Rome. The Roman Church had to spread the computus to the rest of the Christianity.

Calculation[change | change source]

Dates for Easter
1982–2022
In Gregorian dates
Year Western Eastern
1982 April 11 April 18
1983 April 3 May 8
1984 April 22
1985 April 7 April 14
1986 March 30 May 4
1987 April 19
1988 April 3 April 10
1989 March 26 April 30
1990 April 15
1991 March 31 April 7
1992 April 19 April 26
1993 April 11 April 18
1994 April 3 May 1
1995 April 16 April 23
1996 April 7 April 14
1997 March 30 April 27
1998 April 12 April 19
1999 April 4 April 11
2000 April 23 April 30
2001 April 15
2002 March 31 May 5
2003 April 20 April 27
2004 April 11
2005 March 27 May 1
2006 April 16 April 23
2007 April 8
2008 March 23 April 27
2009 April 12 April 19
2010 April 4
2011 April 24
2012 April 8 April 15
2013 March 31 May 5
2014 April 20
2015 April 5 April 12
2016 March 27 May 1
2017 April 16
2018 April 1 April 8
2019 April 21 April 28
2020 April 12 April 19
2021 April 4 May 2
2022 April 17 April 24


Let's define 5 variables, a, b, c, d, and e. In addition to two constants, M and N, so the years between 1900 and 2100 take the values 24 and 5, respectively. We'll call A the year that we want to calculate the Easter date.

a is the remainder of the division \frac{A}{19}, or technically according to the modular arithmetic we should say A\ mod \ 19,
b is the remainder of the division \frac{A}{4},
c is the remainder of the division \frac{A}{7},
d is the remainder of the division \frac{19a+M}{30},
e is the remainder of the division \frac{2b+4c+6d+N}{7}.

If d + e < 10, the Easter date will be in March (d + e + 22). If opposed (d + e > 9), will be in April (d + e − 9).

There are 2 exceptions:

  • If the obtained date is April 26, the Easter date will be April 19, not April 26.
  • If the obtained date is April 25, with d = 28, e = 6 and a > 10, then the Easter date will be April 18.

The values for M and N for years before 1900 or after 2100 can be obtained from the following table:

Years M N
1583-1699 22 2
1700-1799 23 3
1800-1899 23 4
1900-2099 24 5
2100-2199 24 6
2200-2299 25 0

Further reading[change | change source]