The Ninth Sphere

Like all spheres above the sphere of the fixed stars the ninth sphere was transparent and lacked a celestial body. It does not appear in all models of the cosmos, and was often identified with the crystalline sphere, which then took on the role of initiating cosmic motion (a model of ten, rather than eleven spheres). The movement of the ninth sphere was itself thought to be initiated by an unmoved mover, the Prime Mover.