In medieval Christian cosmology, the Primum Mobile ('first mover') or Crystalline Heaven is the outermost sphere of Heaven. Medieval astronomers extrapolated from the observed motion of the planets that the Earth was surrounded by concentric rotating spheres, each of which contained a planet. The outermost visible sphere contained the fixed stars, and it was conjectured that outside that was a final invisible sphere, the Primum Mobile, which also rotated. Since it was the furthest moving sphere before the timeless Empyrean, the Primum Mobile was thought to be the source of time.
Since all planetary motion was ascribed to the rotation of vast spheres around a fixed Earth, calculations for the rotation speed of the Primum Mobile resulted in astronomically fast speeds. Dante used this as a literal representation of the power (or energy) provided by close exposure to God's divine love - the further into space you go, the closer to God you are, so the more energy you receive from Him. However, all theories of this kind were eventually rejected when Galileo, Copernicus and others found better explanations for the apparent movement of the heavens.