Cosmology was not an independent discipline during the late Middle Ages but an undifferentiated part of the broad domain of natural philosophy. Throughout the Middle Ages, the creation of the world from nothing was an article of faith. In Aristotle's Meteorology, he speaks clearly of a world that is divided into two radically different parts. Medieval natural philosophers had to choose between Aristotle's system of purely concentric orbs, which assumed that the Earth was at rest in the geometric center of the world. Each planet had its own total concentric orb, at least eight orbs from the Moon to the fixed stars were required for the celestial bodies. Since uniform circular motion was the only kind of motion deemed appropriate for celestial bodies, natural philosophers were expected to identify the cause of such motions. The dimensions were measured only from the Earth to the sphere of the fixed stars.