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Gerard of Silteo (Sileto)

Gerard of Silteo (Sileto)

(fl thirteenth century)


Gerard was a Dominican friar about whom little is known except that he composed Summa de astris. He is cited twice by Dominican chroniclers as from Silteo and once as from Sileto, but neither location is known, J. Quétif and J.Échard list him as belonging to a German province (Scriptores ordinis, II, 918), whereas M. Grabmann refers to him as an Italian (Mittelalterliches Geistesleben II, 397). One manyscript of his work is preceded by a letter from the Dominican Gerard of Feltre, who is identified as the author of the work, to another Dominican called John, noted marginally (and in a different hand) as John of Vercelli, master general of the order from 1264 to 1283; perhaps Silteo or Sileto was a hamlet near Feltre, in the Italian Alps east of Trent and north of Padua. An early list gives the date 1291 for Gerard, which is possibly the year of his death.

The Summa de astris is divided into three parts, the first of which is converned with astronomy, the second with astrology, and the third with a critical refutation of astrological excesses. Part 1 comprised twenty-three distinctions and deals with classical topics of medieval astronomy at approximately the level of the Rudimenta of al-Farghānī and the Sphere of Sacrobosco; the chief source cited is the Arab astrologer Abû Ma’shar. The final distinction, dealing with comets, has been edited and analyzed by Thorndike, who notes that Gerard saw and described the comet of 1264 (probably writing soon after that date) and that he otherwise borrowed extensively from Albertus Magnus. Generally, Gerard’s discussion of the comet of 1264 is inferior to that of another Dominican, Giles (Aegidius) of Lessines; there seems to be no connection between the two. Gerard’s criticisms of judicial astrology “may have had some influence on the subsequent attacks upon astrology by Nicolas Oresme, Henry of Hesse, and Pico della Mirandola” (Thorndike, p. 187).


Lynn Thorndike, Latin Treatises on Comets Between 1238 and 1368 A. D. (Chicago, 1950), pp. 185-195, apart from the Latin text of the section of Summa de astris on comets, lists the distinction headings of the entire work. For biographical details, see Jacques Quétif and Jacques Échard, Scriptores ordinis praedictorum 2 vols. (Paris, 1719-1721; repr. New York, 1959), I, 725b; and Martin Grabmann Mittelalterliches Geistesleben, 3 vols. (Munich, 1925-1956), II (1936), 397.

William A. Wallace, O.P.

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