Guido Bonatti left us an astrological compilation, entitled *Decem continens tractatus astronomiae*, which survives in a large number of codices and which was printed three times: in 1491, in 1506 and in 1550. The book was written shortly after 1277 when the author, who died in 1296 or 1297, was already advanced in age; one may assume therefore that it reflected more the knowledge of astronomy in Italy in the first half of the XIIIth century than in the times when the old author drafted the text that has come down to us.

In Bonatti’s treatise the section devoted to mathematical astronomy is rather short. He lays out basic ideas regarding the equator, the ecliptic, the altazimuth coordinates, Ptolemy’s system of deferents and epicycles and illustrates how these can explain the phenomena of station and retrogradation of the planets. Bonatti ends the section: "*I will not develop these arguments in any greater detail seeing how a complete review can be found in the Alfraganus*" (18).

The book of Alfraganus (Ahmad ibn Muhammad ibn Kathîr al-Farghânî) (IX cen.) mentioned by Bonatti is the book known as *Elementa Astronomiae*, in the 1134 translation from Arabic by the Spaniard Johannes de ;Luna (XII cen), or as *Liber aggregationis scientiae stellarum et de principiis coelestium motuum* in the translation by Gerard of Cremona (1114-1187) in the second half of the XIIth century.

The work, written in the IXth century, is no more than a compendium of Ptolemy’s *Matematikè Suntaxis*, better known as *Almagest*, dating from the IInd century AD. The summary presented in the work of the great Alexandrian astronomer has no astrological section and appears simple, accurate and elementary (19). We know that Dante Alighieri studied at Bologna in 1287 and it is believed his knowledge of astronomy came from this very book, in the translation of Gerard of Cremona (20), cited by the same Dante in the *Convivio* (II,6) with the name *libro de l’Aggregazion de le Stelle*. In the Inferno (XX,118-120) Dante recalls the figure of Guido Bonatti, placing him amongst magicians, astrologers and sooth-sayers who slowly proceeded backwards "*along the hollow vale*",

"...so that the countenance

Was from the reins averted: and because

None might before him look, they were compell’d

To’ advance with backward gait."

The first lecturer in astronomy at the University of Bologna that we have a record of is Bartolomeus de Parma (XIII-XIV cen.) who began teaching somewhere around 1280 and was still teaching in 1297. In that year he did in fact write a *Tractatus Spherae* intended to combine the classical excursus of Sacrobosco with more "physical" (in the aristotelian sense) and astrological considerations (21).

Given the evidence offered us indirectly by Dante and the composition date of this comment, it is fair to say that, before 1297, the teaching of the mathematical part of astronomy was based on the treatise of Alfraganus and that therefore the treatise of Sacrobosco had only recently come into use.

This is confirmed by the fact that we have at least one other early comment on Sacrobosco written for the students of Bologna, the comment by Cecco d’Ascoli (1285?-1327) in 1322 (22). The *Tractatus de sphaera mundi*, written around 1240 for the students of the University of Paris by the Englishman Johannes de Sacrobosco (end XII cen.-1244 or 1256) - literal Italian translation of the original name John of Holywood - was the first book of astronomy written by a western author: it was not much more than a summary of the work by Ptolemy and the Arabic astronomers, but for those times it was enormously influential. It would remain the most popular treatise of elementary astronomy for almost four centuries, as witness the plethora of comments made on it, substituting once and for all the ancient works of Pliny and Martianus Capella.

- - B. Boncompagni: 1851,
*Della vita e delle opere di Guido Bonatti*, Roma, p. 22. - - G. Bonatti: 1506,
*Decem continens tractatus astronomiae*, Venezia, f28v. - - Alfragano: 1910,
*Libro dell'aggregazione delle stelle*, (trad. di Gherardo da Cremona), Città di Castello. - - P. Duhem: 1916,
*Le système du monde*, IV, p. 222, Paris. - - E. Narducci: 1885,
*I primi due libri del Tractatus Sphaerae di Bartolomeo da Parma*, Roma. - - Chicus Esculanus: 1499 e 1518,
*Commentarius in J. de Sacro Bosco Spheram mundi*, Venetiis.