If the premises be granted, the conclusions are correct. As regards sun, moon, and planets, they are correct even from the point of view of modern knowledge. For if the diurnal motion (that is Earth’s rotation) were to cease, while her other motions, and those of the rest of the solar system remained as at present, her motion round the sun would cause it to rise and set once in a year, and the appearance of moon and planets also would be as described. The vastly distant stars would have no appreciable motion, save that caused by precession, for if the earth revolved round the sun without turning on her axis, the same stars would always remain on the meridian at any given place. Precession, however, would not have the result which Dante supposed. He considered it to be a rotation of the star sphere, but we know that it is merely a nodding. Hence at any given place a star at the pole would describe, in the course of centuries, a small circle in the sky, and those at the equator would move in short lines first north and then south, while stars between would move in ellipses which become narrower as they approach the equator. All round the horizon, therefore, there would be slight very slow changes, bringing a few fresh stars into view, and causing others to disappear. The whole amount of the sky actually seen would vary but little, even if the world should last much longer than Dante expected.
Dante evidently took his value of precession from Alfraganus, for he was one of the few Arab astronomers who accepted Ptolemy’s erroneous figure.
In the Paradiso, as the poet mounts with Beatrice from sphere to sphere he mentions the number of each one in its order. The moon is “la prima stella,”[598] Mercury “il secondo regno,”[599] Venus “il terzo ciel,”[600] the spirits met in the sun are “la quarta famiglia,”[601] Mars is “questa quinta soglia,”[602] and “più levato”[603] than the last heaven, Jupiter is the “stella sesta,”[604] Saturn “il settimo splendore.”[605] The starry heaven is alluded to as “la spera ottava.”[606] The Primum Mobile is “il ciel velocissimo,”[607] “il maggior corpo.”[608] The Empyrean, “il ciel ch’ è pura luce,”[609] is called “l’ultima spera,”[610] but this has no limits and no movement, and therefore no poles:—
“ ... In quella sola E ogni parte là dove sempr’ era; Perchè non è in loco, e non s’impola.”[611]
In Conv. II. iv. 35-39 it is thus described:—
“Questo è il sovrano edificio del mondo, nel quale tutto il mondo s’inchiude; e di fuori dal quale nulla è; ed esso non è in luogo, ma formato fu solo nella Prima Mente, la quale li Greci dicono Protonoe.”[612]
We must remember, however, that the spheres are sometimes counted in the other direction, from the outermost to the innermost or lowest, and therefore this “ultima spera”[613] is elsewhere spoken of as “il primo cielo,”[614] “il primo giro,”[615] “cœlum primum.”[616]
2. THE SPHERES AND THE FOUR ELEMENTS.
The pure region of the spheres, “il paese sincero,” is immortal as the spirits themselves;[617] but below the lowest celestial sphere (“la celestial c’ha men salita” Par. iv. 39) all is mortal and transitory, as the Greeks and the Latin poets had said. This is expressed in the Letter to Can Grande, when Dante contrasts the spheres (cœlum) and the elements, and says: “illud incorruptibile, illa vero corruptibilia sunt.”[618] It is also implied in Virgil’s address to Beatrice:
“O donna di virtù, sola per cui L’umana specie eccede ogni contento Da quel ciel che ha minor’ li cerchi sui.”[619]