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Dionysius Exiguus appended to his 95‐year table a set of rules (argumenta) for working with the data. Of the sixteen rules included in the manuscripts, only the first ten derive genuinely from Dionysius, with the possible exception of the seventh rule. In his examples, Dionysius uses what he calls the present year—the consulship of Probus, AD 525. The rules are clearly of Alexandrian origin. Dionysius has adapted them to the Roman calendar, but some of them are explicable only as formulae based on the Alexandrian calendar.

The introduction to this book sets the stage with some framing remarks on the significance of computus in the context of medieval culture and on the ‘calendar problem’ as confronted by medieval scholars. It provides several examples to highlight the importance of calendrical reckoning to the life of the Latin Church and gives some indication of how and why the growing inaccuracies in the date of Easter came to be regarded as a major scandal that could only be removed by a calendar reform. It continues with a discussion of previous research on the topics covered by the book before concluding with a summary of its eight chapters.

This chapter begins with a succinct account of the development of lunar cycles for the purpose of Easter reckoning in late antiquity and the controversies these cycles generated up to the end of the eighth century, most notably in Britain and Ireland. After a look at the politics and arguments behind these controversies, the discussion turns towards the gradual emergence of a standardized ecclesiastical calendar during the early medieval period. While this calendar was still being constructed, the astronomical handicaps of the underlying 19-year cycle had already started to cause discrepancies between predicted and observable new and full moons. One reaction to this problem was the development of a completely new approach to lunar reckoning in the guise of the computus naturalis, which attempted to predict the precise time of the new moon by extracting the length of the mean synodic month and reckoning forward from observed eclipses.

This chapter follows the story of calendar reform from the end of the twelfth to the end of the thirteenth century, showing how during this period the problem was popularized via two different categories of computus text: the pedagogically oriented ‘vulgar’ or ecclesiastical computus and the astronomically refined ‘philosophical’ computus. Authors whose contributions to the debate are looked at in greater detail include Alexander Neckam, John of Sacrobosco, Robert Grosseteste, Robert Holcot, Campanus of Novara, Giles of Lessines, and Roger Bacon, who is well known for having directed a reform appeal to Pope Clement IV (1265–8). Attention is also paid to an obscure group of Franciscan scholars active in the 1270s to 1290s, who are noteworthy for their knowledge of the Jewish calendar, and to an anonymous treatise of 1276, which contains the first fully developed proposal to restore the Roman calendar.

This chapter focuses on the first serious effort made within the medieval Latin Church to correct the calculation of Easter by legislative means. This effort took place at the court of Pope Clement VI in Avignon, who invited skilled astronomers such as Jean des Murs and Firmin de Beauval to assist him in a planned reform of the Golden Number. The chapter explores the background to this papal initiative and the contributions made by its various protagonists, focusing in particular on a recently discovered Expositio kalendarii novi written by the monk Johannes de Termis in 1345. It also takes a closer look at the parallel discussions that took place in the Byzantine East, where the prospect of a calendar reform was first raised by Nicephorus Gregoras in 1324.