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This chapter seeks to explore the meaning of impartiality for Kepler, and the ways in which he understood it as a virtue that applied to mathematicians in particular. It does so by focusing on Kepler’s engagement with the disputes over the Gregorian calendar reform and the acceptance of the Gregorian calendar throughout the Holy Roman Empire. Kepler highlighted impartiality as a view from everywhere, one that implied tolerance. He used a dialogue he wrote on the question of calendar reform in order both to suggest the manner in which the fissures in Christendom could be repaired, and also to offer a specific model for a kind of churchly unity, one that did not depend on absolute agreement on points of doctrine between the confessions.

This chapter begins with an account of the calendar-reform initiative spearheaded in 1411–17 by Cardinal Pierre d’Ailly at the Councils of Rome and Constance, followed by an in-depth look at the repeated efforts towards a new calendrical legislation made at the Council of Basel in the years 1434–40, which saw the matter debated by a specially created commission or task force. The final part continues the story into the second half of the fifteenth century, highlighting in particular the role of print technology in the dissemination of calendrical and astronomical knowledge. Special attention is given to the activities of the astronomer Johannes Regiomontanus, whose premature death in 1476 prevented him from assisting Pope Sixtus IV in preparing a reform of the ecclesiastical calendar.

This chapter offers a bird’s-eye view of the 100 years of debate that followed upon Regiomontanus’s death and culminated in the Gregorian Reform of 1582, focusing in particular on the time of the Fifth Lateran Council (1512–17) and the work carried out in the 1570s by a commission of experts convened by Pope Gregory XIII, which came to favour an intricate scheme for an astronomically accurate and freely adjustable calendar. Some attention is paid to the extent to which Copernican heliocentric astronomy may have influenced, or was influenced by, the ongoing discussions surrounding the calendar reform. At the same time, the key argument of this chapter is that the breakthrough achieved in the sixteenth century rested to a very large extent on premises, concepts, and insights first formulated during the preceding medieval centuries.

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 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.

The fourth chapter centres on the Lutheran mayor Bartholomäus Scultetus (1540–1614) who introduced the Gregorian Calendar to Lusatia and the Bohemian lands. Other Lutheran territories, most notably Saxony, refused to accept the more accurate calendar on religious grounds. Scultetus, however, advocated for the calendar and exchanged letters with Catholic dignitaries, praising the benefits of a calendar reform. He dedicated multiple works to Catholics, was friends with some of them and even included woodcuts of his Catholic friends or their coat of arms in his works. Other examples of this cross-confessional exchange include a monk who was one of the most popular godfathers in Zittau until the 1540s or the peaceful negotiations between Lutheran town councils and Franciscan monks regarding new town schools. Scultetus and other councillors also engaged in the creation of a Reformation memory, but without a clear shape of Lutheranism, these histories did not follow a unified pattern.