[86] Imbriani, Dante a Padova.
[87] “Torno a Ravenna e de lì non mi parto (I am going back to Ravenna, and shall not leave it again), is a line in the Acerba which Cecco d’ Ascoli puts into the mouth of Dante, as though from a letter written to himself from the divine poet at the time” (about the year 1319). Dante and Giovanni del Virgilio, by Wicksteed and Gardner, p. 84.
[88] Inf. xxxi. 136-141.
[89] Conv. II. xiii. 22-26.
[90] Conv. III. ix. 146-157.
[91] Conv. II. xv. 73-77.
[92] Conv. II. iii. 36-52, V. N. xxx.
[93] Averroës, in his commentary on Aristotle’s De Cœlo, says that the ancients believed the eighth, or starry, heaven, to be the outermost, but that Ptolemy assumed a ninth, “because he said that he had discovered a slow motion along the signs of the zodiac in the fixed stars.” Albertus Magnus, in his De Cœlo et Mundo, Book II., says also that the ancients, including Aristotle, believed that there were only eight heavens, but that Ptolemy, so far as he can understand, believed in ten, on philosophical not mathematical grounds (compare Conv. II. iii. 40, 41). Albertus accepted the theory of “trepidation,” and thought this was the only movement which ought to be assigned to the star sphere; there remained, therefore, two motions, which affect all the planetary spheres and the star sphere, for which two more spheres must be assumed, a ninth sphere for precession, and a tenth, the primum mobile, for the diurnal motion. Outside all was the Empyrean. Dante never mentions trepidation, and evidently did not believe in it: he needed only nine moving spheres, therefore, but counts the Empyrean as a tenth heaven.
[94] Conv. II. xiv. 198-202. Ibid. 249-253.
[95] “Ptolemy says in the book above cited.”