The computus: how the Church learned to calculate the date of Easter

Easter is the only major Christian feast whose date changes every year according to the Moon. It can fall between 22 March and 25 April, and its calculation, called in Latin the computus, was for centuries one of the main applications of mathematics and astronomy in the West. In medieval monasteries the computus was learned like grammar: without it one could not know when to celebrate the most important feast of the liturgical year, nor the movable feasts that depend on it.

The rule and its origins

The rule we know states that Easter is celebrated on the Sunday after the first full moon of spring, meaning the full moon that falls on or immediately after 21 March. The Council of Nicaea in 325 is traditionally linked to this rule, but its canons contain no rule of calculation: we know from a letter of the emperor Constantine that the council wanted a single date and independence from the Jewish reckoning.

An often misunderstood point is that the full moon of the computus is not the one observed in the sky but an ecclesiastical moon calculated from tables. The equinox, too, is fixed by convention on 21 March, the date of the Alexandrian tradition. This is why Easter can sometimes differ from the Sunday that would follow the real astronomical full moon, and why the calculation holds even where the sky is overcast.

Alexandria, Rome and Dionysius Exiguus

In the early centuries Alexandria and Rome used different tables, and the date of Easter could differ. The Alexandrian Church, strong in its astronomical school, adopted the nineteen-year lunar cycle; Rome long followed eighty-four-year cycles and then, from 457, the tables of Victorius of Aquitaine. Discrepancies between the two traditions caused disputes and long correspondence between bishops and popes.

In 525 the monk Dionysius Exiguus, working in Rome, prepared an Easter table based on the Alexandrian method. In doing so he introduced an innovation destined to last: instead of counting years from the era of Diocletian, as was done in Egypt, he counted them from the incarnation of Christ. From that choice comes the Christian era, the Anno Domini we still use today to date events.

Bede and the British Isles

In the British Isles the Churches of Irish tradition followed a different cycle and celebrated Easter on dates that differed from the Roman ones. The conflict was debated at the Synod of Whitby in 664, where King Oswiu of Northumbria chose the Roman usage. Bede the Venerable tells the story in his Ecclesiastical History of the English People, completed in 731, with a liveliness that still makes the dispute readable.

Bede himself wrote in 725 the De temporum ratione, the most complete treatise on the computus produced in the Latin Middle Ages. In it he explains the phases of the Moon, the tides, which he links to the Moon's motion, the nineteen-year cycle and the construction of Easter tables, which he extends to 1063. The work was copied in a great many manuscripts and became the standard handbook of the monastic schools.

The computus shows how present the Moon was in medieval culture, not as an object of superstition but as a problem of calculation. The tables, cycles and rules devised to fix a single date left traces everywhere: in the calendars of missals, in library codices, even in the way we count years. When the system showed its limits, a papal reform was needed to correct it.