A frequently overlooked aspect of the knowledge transfer from Arabic into Latin in the twelfth century is the introduction of the Islamo-Arabic calendar, which confronted Western computists with a radically different scheme of lunar reckoning that was in some ways superior to the 19-year lunar cycle of the Roman Church. One of the earliest sources to properly discuss this new system and compare it to the old one is the anonymous Collatio Compoti Romani et Arabici, found in a manuscript from Tewkesbury Abbey, Gloucestershire. This article contains the first edition and translation of this previously unknown text, preceded by an analysis of its content and sources. As will be argued, the text was written in the second quarter of the twelfth century as a reaction to the astronomical tables of al-Khwārizmī, recently translated by Adelard of Bath, as well as to eclipse observations that had exposed the flaws of the ‘Roman’ computation.
Isidore of Seville, Etymologies 3.58–59, ed. G. Gasparotto and J.-Y. Guillaumin (Paris, 2009), 121; Isidore of Seville, De natura rerum 20–21, ed. Jacques Fontaine (Bordeaux, 1960), 247–253; Macrobius, Commentarii in Somnium Scipionis 1.15.11, ed. James Willis, 2nd ed. (Stuttgart, 1970), 62; Martianus Capella, De nuptiis Philologiae et Mercurii 8.869–870, ed. James Willis (Stuttgart, 1983), 329.
L.C. Bethmann, ed., “Chronicon S. Andreae Castri Cameracesii,” in MGH Scriptores, vol. 7, ed. Georg Heinrich Pertz (Hannover, 1846), 326–550, at 550: “Quae res cum omnium mentes admiratione simul ac stupore concuteret, quidam quasi prudentiores eclipsim solis ex oppositione lunae dicebant accidisse. Quod nequaquam fieri posse, ratio patet – solem scilicet eclipsim a luna, nisi in interlunio, pati posse – : cum constet, ea die lunam 27am fuisse. Quidam vero probabilius asserebant, hoc signum tenebrarum aliquid novi prodigii presignare. Dictum est multo post a redeuntibus de Hierusalem, ipsa die, ea hora tenebrarum, quadringentos quinque minus milites de templo a Sarracenis trans flumen esse peremptos. Nec mirum, si in suorum membrorum occisione placuerit Deo tenebras mundo inducere, in cuius morte solem et totum mundum constat obtenebratum fuisse.” On ‘miraculous’ eclipses in medieval chronicles, see Umberto dall’Olmo, “Eclypsis naturalis ed Eclypsis prodigialis nelle cronache medioevali,” Organon,15 (1979), 153–166; Robert Bartlett, The Natural and the Supernatural in the Middle Ages (Cambridge, 2008), 51–70.