Abstract

Some twenty years after the Gregorian calendar reform, towards the end of his life, François Viète published his own calendar proposal. This treatise contains a sharp attack against the Jesuit scholar Clavius, the mathematical mind behind the reform. Understandably enough, Clavius prepared a negative reply. Viète heard of it and exploded in a fit of rage, ``I demonstrated that you are a false mathematician [... ], and a false theologian.'' Sadly, Clavius' rejection, added as a chapter to his monumental apology of the Gregorian reform, appeared when Viète had already passed away.Viète seriously believed that the true aim of the Gregorian reform has been betrayed and he was furious about some logical inconsistencies which he claimed to have found in Clavius' calendar. Clavius apparently confused solar day and epactal day (or ``tithi''), the thirtieth part of a lunar month. This is the very core of Viète's attack against Clavius whom he accused of having introduced a false lunar period (``falsa periodus lunaris''). But his own work has some logical inconsistencies too. For instance, he reproaches Clavius for having introduced lunar months of 31 days which, indeed, are unrealistic. Grievously, his own rules can likewise give rise to lunations of unnatural lengths.In order to understand these subtle twists reader and author must work largely through both Clavius and Viète's methods of Easter reckoning. The fruit of all those efforts might be an insight into Viète's clear mathematical thinking. His calendar, however, was never considered.