Primum Mobile
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In medieval Christian cosmology, the Primum Mobile ('first mover') 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 Empyrean, the Primum Mobile was thought to be the source of time.