We think we know the rule: Easter takes place on “the Sunday following the first full Moon after the spring equinox.” For 2019 the equinox was on March 20 at 5:58 p.m. EDT and the full Moon the same day at 9:43 p.m.; the “Sunday following” was therefore March 24. But a glance at the calendar shows us that this year, Easter is taking place on April 21 instead!
This deviation underscores the fact that the Easter date is determined on the basis of a very ancient method, the ecclesiastical computus, which has very little in common with our modern tools and mathematical methods. The formula invoked above is incorrect.
An ancient definition
Let’s go back 1,700 years. The Fathers of the Church, gathered together for the Council of Nicaea in the year 325, formulated the rule in order to establish the date of the most important Christian holiday: “Easter falls on the Sunday following the 14th day of the Moon that reaches that age on March 21 or immediately afterwards.”
We’re therefore making a mistake when we give a modern spin to this very ancient definition. The ecclesiastical rule, as it happens, involves certain simplifications, the first being the one that sets the spring equinox on March 21. In fact, a stricter calculation shows that in our time, the equinox generally occurs on March 20, and can even happen on the 19th. (And incidentally, in the year 325 the spring equinox took place on…March 20! What were the Fathers of the Church thinking?)
Calculating the phase of the Moon
The same goes for the calculation of the full Moon, which obviously was not as sophisticated as what today’s astronomical knowledge allows. That calculation depended on a simplified representation of the movements of a “fictive” Moon, nevertheless quite close to those of the real Moon. The phase of the Moon on a certain date was given by a number: the age of the Moon, meaning the number of days since the new Moon. Thus, the full Moon happens when the Moon is 14 days of age (whence the expression we find in the rule). The exact time of the full Moon was therefore overlooked, and the concept of time zone was absent.
In general, the date of Easter established according to the ecclesiastical rule coincides with the one determined by an “exact” astronomical calculation, but there are some exceptions, as is the case this year: the last time was in 1981, and the next will be in 2038. In the 21st century, the two methods give discordant results on a total of 10 occasions: to the years 2019 and 2038 already mentioned must be added 2045, 2049, 2057, 2069, 2076, 2089, 2095 and 2096.