In order to give a coherent form to his Commedia, Dante had to set his journey through the afterlife in an organised universe, and given that a fictitious universe would not have given him much resonance in the intellectual spheres, a factual account of the universe was necessary.
Dante’s astronomical knowledge is first and foremost a by-product of the Greco-Roman mathematician and astronomer Claudius Ptolemy’s work.15 However, Dante cites the Almagest (Ptolemy’s treatise on the motion of starts and planetary paths) incorrectly on three occasions in his Convivio (1304-1307), which suggests he may not actually have read the book. The source he does undoubtedly have access to though, is Alfraganus’ Elementa Astronomica (otherwise known as al-Farghānī’s Kitāb fīl-Harakāt al-Samāwiya wa Jawāmi’ Ilm al-Nujūm) since “nearly all [of his] astronomical data appear to have been taken from him, and his very expressions are sometimes repeated.”16 Indeed, Dante’s Paradiso features twenty-four spirits that are likened to twenty-four brilliant stars (Para. 8.1-13). These numbers come from the Elementa’s chapter 19, where we find an enumeration of the 15 first-magnitude stars, plus the 9 second-magnitude stars (i.e. next in brightness) from the Benet Naax and Alfarcatein Arab constellations.
Dante’s structure of the Paradiso is also modelled on Arabic scientific accounts of the universe, with the number of spheres of heaven amounting to 9. The Greeks, including Aristotle, believed that the outermost “sky” was the 8th heaven. It is only after Averroës’ (Ibn Rushd) commentary on Aristotle’s De Caelo that we see the appearance of a 9th heaven in cosmological discourses.17