The Sphere of the World with Commentaries

...that the firmament moves from the east to the west.

ON THE ROUNDNESS OF THE HEAVEN.

That the heaven is round, there is a triple reason: likeness, convenience, and necessity. Likeness: since the sensible world was made in the likeness of the archetypal world, in which there is neither beginning nor end. Hence, in likeness to this, the sensible world has a round form, in which it is not possible to assign a beginning nor an end. Convenience: because of all isoperimetric bodies, the sphere is the greatest; of all forms, also, the round one is the most capacious. Since, therefore, it is the greatest and round, it is for that reason most capacious; hence, since the world contains all things, such a form was useful and convenient for it. Necessity: since if the world were of another form than round—namely, three-sided, four-sided, or multi-sided—two impossibilities would follow: namely, that some place would be empty, and a body without a place, both of which are false, as is evident in raised and circumvolved angles.

Likewise, as Alfraganus says, if the heaven were flat, some part of the heaven would be closer to us than another—that, namely, which would be above us. Therefore, a star existing there would be closer to us than one existing at rising or setting. But those things that are closer to us appear larger. Therefore, the sun or another star existing in the middle of the heaven ought to appear larger than one existing at rising or setting—the contrary of which we see to occur. For the sun or another star appears larger existing in the east or west than in the middle of the heaven. But since the truth of the matter is not so, the cause of this appearance is that in winter or rainy weather, certain vapors rise between our sight and the sun or another star. And since those vapors are a diaphanous body, they disperse our visual rays, so that they do not comprehend the thing in its natural and true magnitude.