Of God and His Creatures

The eighth sphere of solid crystal carried all the fixed stars, set in it like stones in a ring. Beyond that was a ninth: and the tenth and outermost sphere was the primum mobile, the daily rotation of which from east to west carried round the inferior spheres. This is called the Ptolemaic system, but the eight spheres already figure in Plato, Rep. x, 616d, 617.

Of God and His Creatures: 3.57