Robert Grosseteste (1168/75-1253), Bishop of Lincoln from 1235-1253, is widely recognized as one of the key intellectual figures of medieval England and as a trailblazer in the history of scientific methodology. Few of his numerous philosophical and scientific writings circulated as widely as the Compotus, a treatise on time reckoning and calendrical astronomy apparently written during a period of study in Paris in the 1220s. Besides its strong and long-lasting influence on later writers, Grossteste's Compotus is particularly noteworthy for its innovatory approach to the theory and practice of the ecclesiastical calendar-a subject of essential importance to the life of the Latin Church. Confronting traditional computistical doctrines with the lessons learned from Graeco-Arabic astronomy, Grosseteste offered his readers a critical and reform-oriented take on the discipline, in which he proposed a specific version of the Islamic lunar as a substitute for the failing nineteen-year cycle the Church still employed to calculate the date of Easter. This new critical edition of Grosseteste's Compotus contains the Latin text with an en-face English translation. It is flanked by an extensive introduction and chapter commentary, which will provide valuable new insights into the text's purpose and disciplinary background, its date and biographical context, its sources, as well as its reception in later centuries.

The Gregorian calendar reform of 1582, which provided the basis for the civil and Western ecclesiastical calendars still in use today, has often been seen as a triumph of early modern scientific culture or an expression of papal ambition in the wake of the Counter-Reformation. Much less attention has been paid to reform's intellectual roots in the European Middle Ages, when the reckoning of time by means of calendrical cycles was a topic of central importance to learned culture, as impressively documented by the survival of relevant texts and tables in thousands of manuscripts copied before 1500. For centuries prior to the Gregorian reform, astronomers, mathematicians, theologians, and even Church councils had been debating the necessity of improving or emending the existing ecclesiastical calendar, which throughout the Middle Ages kept losing touch with the astronomical phenomena at an alarming pace. Scandalous Error is the first comprehensive study of the medieval literature devoted to the calendar problem and its cultural and scientific contexts. It examines how the importance of ordering liturgical time by means of a calendar that comprised both solar and lunar components posed a technical-astronomical problem to medieval society and details the often sophisticated ways in which computists and churchmen reacted to this challenge. By drawing attention to the numerous connecting paths that existed between calendars and mathematical astronomy between the Fall of Rome and the end of the fifteenth century, the volume offers substantial new insights on the place of exact science in medieval culture.