The first inhabitants of this earth saw half the starry sky or 180°, and since then it had revolved through 60°, which together is equal to 240°; therefore there would still remain 120°, or one third of a revolution, to be made to complete the whole 360°, and bring every part of the star sphere into view. Just before this passage Dante had expressed the same idea in another way. This movement proper to the star sphere, he says, will never have an end, for the end of a revolution is to return to the same point; and this heaven will never do so, for since the beginning of the world it has turned through a little more than the sixth part of its whole revolution (60° = ⅛ of 360°), and we are already in the last age of the world, and indeed are awaiting the consummation of the celestial motion.[597] ------------------------------------------------------------------------ [597] _Conv._ II. xv. 108-118. ------------------------------------------------------------------------ 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.