The article introduces a previously unknown fourteenth-century treatise on computus and calendrical astronomy entitled Expositio kalendarii novi, whose author proposed elaborate solutions to the technical flaws inherent in the calendar used by the Roman Church. An analysis of verbal parallels to other contemporary works on the same topic makes it possible to establish that the Expositio was produced in the context of a calendar reform initiative led by Pope Clement VI in 1344/45 and that this anonymous text is probably identical to a great and laborious work' on the calendar that the monk Johannes de Termis prepared for the pope around this time. Its author strove to make an original contribution by extracting new astronomical parameters from both ancient and contemporary data, which made him arrive at an estimate of the length of the tropical year that was independent of the then-current Alfonsine Tables. With its suggestion to remove eleven days from the Julian calendar and to correct the calendar through modified leap-year rhythms and periodically adjusted sequences of lunar epacts, the proposal enshrined in the Expositio exhibits some remarkable similarities to the Gregorian reform of the calendar promulgated in 1582. Although its influence on the latter must remain a matter of speculation, the newly discovered text sheds a revealing light on the history of medieval calendar reform debates and on the mathematical sciences practiced at the Avignon court of Clement VI.
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