In medieval and Renaissance astronomy, the Primum Mobile, or "first moved," is the outermost moving sphere in the geocentric model of the universe. Astronomers believed that the seven naked-eye planets (including the Moon and the Sun) were carried around the spherical Earth on invisible orbs. Beyond them was the sphere of fixed stars, the Primum Mobile, and the Empyrean.
The total number of celestial spheres was not fixed. In this 16th-century illustration, the firmament (sphere of fixed stars) is eighth, a "crystalline" sphere (posited to account for the reference to "waters . . . above the firmament" in Genesis 1:7) is ninth, and the Primum Mobile is tenth. Outside all is the Empyrean, the "habitation of God and all the elect."