computus

The term computus refers to a medieval system of tables used for calculating astronomical events and movable dates within the Christian calendar. This system was crucial for determining the date of Easter, the most important feast in the Christian church, which in turn dictated the timing of other movable feasts.12

The computus was based on reconciling lunar and solar reckonings. Early attempts to create a stable calendar, like the Julian calendar, had inaccuracies that caused them to drift out of sync with the seasons. The need for a precise method to calculate Easter led to the development of complex systems, eventually refined in the Gregorian calendar. These calculations involved understanding cycles like the Metonic cycle for lunar phases and the solar cycle for seasons.234