Just as in the Muslim world the interest in astronomy was tied to the requirements of the faith – times of prayer, appearance of the new Moon, orientation of the mosque – so too in the Christian. By the tenth century the Latin West had two well established traditions of practical astronomy. Ecclesiastical computus (calculation) required rudimentary arithmetical calculations to determine the dates of Easter and its related feasts. Furthermore, simple observations of the Sun and stars enabled monastic communities to determine the times for both daytime and night-time prayers.

By the middle of the first millennium one of the most important issues for the early church was the date of Easter. The Christian calendar featured both solar and lunar festivals. Christmas on 25 December – the medieval date of the winter solstice (astronomically December 21) – was solar. Easter, on the other hand, tied to the Jewish Passover, was lunar. Jesus of Nazareth and his early followers were Jews, and his execution occurred in Jerusalem during the Passover celebration. The Last Supper (the final communal meal for Jesus and his followers) was a Passover feast (on Maundy Thursday). Jesus was crucified on Friday and, according to the early accounts, rose from the grave on Sunday. To retain the New Testament chronology, therefore, Easter had to be celebrated after Passover. In the lunisolar calendar of the Jews, Passover occurred on the first full moon after the vernal equinox (21 March). Thus the early church decided that Easter should be celebrated on the first Sunday after the first day of Passover. For the medieval scholar, calculating the date of Easter was a complicated calendrical (and astronomical) problem. Determining the exact moment of the equinox and integrating the two years – the 365. day solar and the 355 day lunar – was extraordinarily difficult. So important, in fact, was this calculation that it acquired its own name – the computus (calculation). In 525 ce Dionysius Exiguous (c. 470–545), a Romanian monk, published a table establishing the dates of Easter for the next ninety-five years but at that time the Christian calendar employed the Era of Diocletian. Named after an emperor who had persecuted the early church, the era was solar – its epoch 1 January 284.