The Sphere of the World with Commentaries

The sphere, moreover, is divided in two ways: according to substance and according to accident. According to substance, it is divided into spheres—that is, the ninth sphere, which is called the first motion or the primum mobile; and into the sphere of fixed stars, which is named the firmament; and into the seven spheres of the seven planets. Some of these are larger and some smaller, according to how much they approach or recede from the firmament. Hence, among them, the sphere of Saturn is the largest, while the sphere of the Moon is the smallest, as is contained in the following figure.