Computus

Computus (Latin for "computation") is the method for calculating the date of Easter. In the early 4th century there was confusion about when Christian Easter or the Resurrection should be celebrated.

 History 
 Background 
At the Arles Council, in 314, all of Christianity was obliged to celebrate Easter on the same day. This date was to be assigned by the Pope, who would send epistles to all the churches in the world with the necessary instructions. However, not all the congregations followed those precepts.
 Nicaea Council 
In the First Council of Nicaea, in 325, this question was resolved.

It was established that the Resurrection Easter had to be celebrated fulfilling some norms:
 Easter had to be celebrated on Sunday.
 It could not coincide with the Jewish festival of Passover, celebrated independently which day of the week it was.
 Christians could not celebrate Easter twice the same year. This was because the new year started in the spring equinox, so the celebration of Easter before the real equinox was prohibited.

However, there still were discussions between the Roman Church and the Alexandrian Church, while the Nicaea 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 

Let's define 5 variables, a, b, c, d, and e; and two constants, M and N. For the years between 1900 and 2100, M and N take the values 24 and 5, respectively (see table below). We'll call A the year that we want to calculate the Easter date.

a is the remainder of the division , or technically according to the modular arithmetic we should say ,

b is the remainder of the division ,

c is the remainder of the division ,

d is the remainder of the division ,

e is the remainder of the division .

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 
 Mosshammer, Alden A. The Easter Computus and the Origins of the Christian Era. Oxford: Oxford University Press, 2008. .

Category:Easter
Category:4th-century works