Innumerate star-gazers

In The Making of the Middle Ages (1953) Richard Southern recalled two scholars, one of them the master of the monastic school in Cologne, the other of that in Li ge, worrying in 1025 over the meaning of the term "interior angles" in The Consolation of Philosophy of Boethius . What could more vividly reveal the poverty of mathematical and scientific learning in the Latin West in the early Middle Ages? Stephen McCluskey reminds us, however, that it was one of the same pair of masters who, in their exchange of letters, "bubbled over with enthusiasm" on having recently acquired an astrolabe from Islamic Spain. He sees a paradox here. If astronomical knowledge was at such a low level, how can we explain the eagerness with which scholars reached out for the treasures of Greek and Islamic astronomy when the opportunities came their way in the 11th century?

It can be resolved, according to him, only by ceasing to focus exclusively on the qualitative quadrivium astronomy of the cathedral schools, taught from Boethius, Cassiodorus, Isidore, and Martianus Capella. There were several other astronomical traditions, well established by the 9th century, but largely ignored by previous historians of astronomy. Each of these "astronomies" fulfilled a particular need and had a social base in the support of an elite group, with a level of observational practices and theoretical sophistication adapted to that need.

The first of these astronomies, had a predominantly local importance. It aimed at determining the dates of saint's days and depended on simple horizon observations. The other two astronomies served monastic communities. One was concerned with the times of prayers by day or night by observing the sun or the constellations. The other fixed the all-important date of Easter and hence set up the ecclesiastical calendar. It used the computus and required knowledge of the rudiments of arithmetic and of the lunar and solar periods. By the 9th century, the computus was also taught in monastic schools of the Carolingian educational revival, together with a qualitative treatment of ancient geometric astronomy, as part of the artes liberales. It is the interaction of these various astronomies that set the stage for the reception of Islamic astronomy by the 10th century.

The worries of those early 11th-century masters concerning elementary geometrical terms in Boethius assume a very different significance in the perspectives opened up by McCluskey. They are not to be seen as marking the abysmally low level of astronomical and mathematical knowledge in the early 10th century. Rather, they were the culmination of a process in which the interaction of these different astronomies made scholars far more conscious of the limitations of the arithmetical computus, while the revived quadrivium teaching of the Carolingian educational renewal gave them a tantalising glimpse of the lost geometrical astronomy of the ancients, with a far superior predictive power.

After surveying the heritage of astronomical practices available in the Latin West from the prehistoric traditions and the classical and Christian ones in the early Middle Ages, McCluskey studies the detailed history of the four traditions, constituting the distinct but interconnected "astronomies" that he has identified. The concluding part examines the "harvest" garnered from the fusion of the various traditions, which set the scene for the encounter with Islamic astronomy, and ends with the rebirth of Ptolemaic astronomy in the 13th century.

Besides providing a novel view of early-medieval astronomical knowledge in the Latin West, McCluskey furnishes a highly successful example of an approach that integrates the history of scientific knowledge with cultural history. "Fixing the calendar" ceases to be a dry technical problem and emerges as vital to establishing a distinct Christian culture, in competition with paganism and amid the ruins of the urban context in which the astronomical enterprise had flourished in classical antiquity.

McCluskey's study ought to serve as an inspiring exemplar for charting the history of other sorts of scientific knowledge and in other periods.

P. M. Rattansi is emeritus professor of the history and philosophy of science, University College London.