Technical Chronology and Computus Naturalis in Twelfth-Century Lotharingia: A New Source

Recent research has shown that the use of astronomy as a chronological problem-solving tool has deep roots in the scholarly practices of the Latin Middle Ages, as is manifest from the writings of Marianus Scotus, Gerland, and other "critical computists" of the eleventh and twelfth centuries. This essay enlarges the existing picture by introducing a hitherto unknown epistolary treatise of the mid-twelfth century. Written in Lotharingia in 1144, this poorly preserved work documents an attempt to reconstruct the timeline of world history via techniques that had emerged within the Latin computus tradition prior to the advent of Greco-Arabic astronomy. Drawing on the so-called computus naturalis, the author performed lunar computations on the basis of observed eclipses and used them to propose new dates for the Creation and the Last Supper. He also relied on a revised estimate of the time and date of the vernal equinox, placing it on 16 March, and gave a detailed account of how this result can be derived from solar observations with a meridian line. By virtue of these methodological features, the Chronology of 1144 represents an important transitional stage between the "critical computists" and the use of eclipse-based dating methods by later scholars such as Giles of Lessines (fl. 1260).