Abstract
Phillip Nothaft’s important study of the medieval antecedents of the Gregorian reform reflects the progress of the study of calendrical astronomy since George Coyne, Michael Hoskin, and Olaf Pedersen’s Gregorian Reform of the Calendar (1983). One indicator of the activity in this field is the detailed reference list: 43 pages of work on the history of astronomy, history of computus, history of the calendar, and selected historical topics, which the author notes was cut by 40% during publication. Among them are many editions of previously unedited texts which a community of active researchers have produced in the last three decades.
Drawing on these sources, Nothaft tells a richly detailed story of how medieval scholars, astronomers, and churchmen dealt with the astronomy and mathematics of the calendar, focusing on the millennium between the adoption of the 19-year luni-solar cycle for computing the dates of Easter and other movable feasts in the fifth century and the Gregorian reform of 1582. The most valuable contribution of this study is that it fills the gaps in the big picture, using heretofore unstudied fourteenth- and fifteenth-century investigations into the ecclesiastical calendar at the papal court and church councils to connect the Early and High Middle Ages to the widely studied Gregorian reform itself, which is only treated cursorily.
Since Nothaft focuses on medieval investigations that led to the reform of the 19-year Easter cycle, he leaves some gaps in his study of computus. The alternative 84-year cycle, which was used for a time in Rome and Ireland, appears only briefly in the context of the Irish acceptance of the Dionysian form of the 19-year cycle. Although the protagonists in his story include figures familiar to students of medieval computus as well as obscure clerics whose works only survive in a few manuscript copies, his attention to scholars who actively investigated calendric astronomy leads him to downplay authors of medieval textbooks. For example, Rabanus Maurus and his widely-read De computo are only mentioned in a few footnotes to provide examples of common medieval computistical ideas.
In this short review, I can only mention a few themes in Nothaft’s story. He frequently shows the varied interplay between the demands of ritual, practicality, and astronomy. The roles of church councils and popes and their interactions with the scholarly community become central to the later portion of his story. I will focus, however, on the astronomical content.
Nothaft’s detailed examination of the sources demonstrates how computists distinguished different aspects of their discipline, which they called common computus, natural computus, and philosophical computus. All these aspects dealt with the interacting lunar and solar components of the ecclesiastical calendar. Correction of the solar calendar was conspicuously manifested by Pope Gregory’s notorious deletion of 10 days from the calendar in October 1582. Yet for medieval scholars, whose primary concern was the date of Easter, the less noticeable correction of the lunar cycle was more important.
As early as the twelfth century, observations had indicated problems with both the solar and lunar components. The vernal equinox observed by “the moderns” fell as early as the 16th of March, while computistical practice placed it on the 21st and Roman tradition placed it on the 25th. Similar problems were apparent for the lunar components; eclipses were observed more than 2 days before the new moon as reckoned using the common computus. Yet, at this stage, there was little willingness to abandon traditional methods.
The arrival of Arabic astronomy and the study of the Jewish calendar provided medieval computists with new data for computing the time of new moons, which they compared with their own values derived from the traditional 19-year cycle. Arabic astronomy also brought complications in the form of theoretical models proposing an access and recess of the place of the equinox and hence of the length of the tropical year and geometric models which introduced noticeable variations into the length of the lunar month. While the former provided data for calendric reform, Nothaft makes the crucial point that doubts about the constancy of the solar year remained an enduring obstacle to reform.
Nothaft’s closing survey of the Gregorian reform shows how it applied twofold corrections to both the solar year and the lunar cycle, removing the accumulated error with one-time corrections to the year and the lunar cycle and preventing further errors through regular applications of “equations” that skipped the leap year three times in 400 years and adjusted the lunar cycle eight times in 2500 years.
This essential volume reworks the well-established framework of medieval computus, providing a richer understanding of the place of the calendar in medieval astronomy. It belongs in every library that collects in medieval history or the history of science.