The Primum Mobile exists because two exhaustive systems needed a joint. Aristotle's physics required that every motion have a mover, and that the chain terminate in something unmoved; astronomy required a single daily rotation sweeping the whole heaven westward while the planets crept eastward on their own slower courses. The solution was a final sphere: starless, invisible, outermost, whose one function is to receive motion first and distribute it inward, the first movable pressed against the unmoving.
The scholastics placed it with care. Below it, the fixed stars; beyond it, nothing movable at all, only the Empyrean, which is not a sphere but a state. Its motion is the fastest in the universe, since it makes the full circuit daily while dragging everything with it, and the swiftness was given a theological reading that Dante perfected: it moves fastest because it desires most, each part of it longing to touch the divine presence that surrounds it, and time itself, he says, has its roots in this vessel, all clocks below being merely its subdivisions. The sphere also solved an astronomical embarrassment: the slow drift of the equinoxes, precession, could be assigned to the starry sphere only if something else supplied the daily motion, so the ninth sphere was, in the technical astronomy, precession's bookkeeping as much as theology's hinge. It is the old cosmology's most abstract invention, a wheel with nothing on it, turning because God is on the other side, and everything else turns because it does.