American Scientist

Astronomies and Cultures in Early Medieval Europe. Stephen C. McCluskey. xiv + 235 pp. Cambridge University Press, 1998. $54.95.

Astronomical practices of the 2nd through 13th centuries in Europe, roughly the period described in this work by the science historian Stephen C. McCluskey, are rarely taught to astronomers and their students today. After all, this period was the Dark Ages (starting in late 5th century), when scientific learning advanced little. Nevertheless, the ecclesiastical and civil authorities had important needs to determine time, date and season, and if they did not sponsor original investigation, they strongly encouraged practical astronomy. The Roman Church emphasized the prediction of astronomical events by calculation, to combat astrology and pagan religious practices. If the movements of the stars and planets could be predicted, then they did not hold sway over man on earth. Pagan festivals whose dates had been determined by celestial events were replaced by Christian feasts whose dates were fixed in the Julian calendar. Manuscripts based on the discoveries of antiquity were preserved as precious user manuals, but the underlying theory of the celestial mechanisms was of little concern. For some users, events were predicted by arithmetical calculations, even when the corresponding geometry on the sky was not understood.

One of the common daily practices of this era was that of the sky-watching monk who, by following the stars through the night, determined when that pre-dawn hour was reached at which the whole monastery needed to be awakened for the day's first prayers. (Whether anyone threw a shoe at the astronomer who awakened him is unrecorded.) In computing the dates of festivals each year, different monasteries followed rules derived from different, poorly understood authorities, often with discordant results. Others worked on long-range problems.

By Charlemagne's time, says McCluskey, the Irish monk Dungal of St. Denis claimed "for astronomers the ability to predict eclipses and other celestial events not only a month in advance, or a year, or twenty years, or a hundred or a thousand, but over fifteen thousand years." Dungal's actual calculations, however, were not very accurate. By this time, in the early 9th century, interest was revived in the stars and constellations themselves, but works of the period simply described them. Unlike Ptolemy's work of six and a half centuries earlier, the descriptions were not accompanied by celestial coordinates. The Carolingian works in this sense were closer to a beginner's guide to the sky than to a celestial reference manual.

Despite numerous scholarly footnotes that mark almost every page, McCluskey succeeds in presenting a very readable if serious account, reminding us of, or introducing, as the case may be, such historic characters as Martin of Braga, who condemned a pagan custom of observing the Moon and stars "to determine the time to build a house or plant trees or consort with one's wife." Today's astrophysicists universally condemn astrological practices but have made little progress in influencing society. You are more likely to find a horoscope in your local daily newspaper than an account of astronomical discoveries. Also remembered is Queen Eanfled, who observed the Paschal fast in Northumbria while King Oswiu, whose monks followed a different rule, was already feasting. This marked inconsistency led to improvements by Bede of Yarrow (approximately 672–735 A.D.) who, according to Alcuin of York, "wrote with marvelous clarity a book on time, containing the courses, places, times, and laws of the stars."

Toward the end of the period covered by Astronomies and Cultures in Early Medieval Europe, Latin Christendom rediscovered the science of astronomy as it was known to the ancients through contact with Islamic scholars. From then on, the history of astronomy is familiar to many. But for the centuries covered by McCluskey's book, it is an essential guide, with a comprehensive bibliography and a fascinating, if modest, set of monochrome illustrations.—Stephen P. Maran, NASA's Goddard Space Flight Center