In the fully developed
Aristotelian system, the
spherical circular Earth is at the center of the universe, and all other heavenly bodies are attached to 47–55 transparent, rotating
spheres discs surrounding the Earth, all concentric with it. (The number is so high because several
spheres discs are needed for each planet.) These
spheres discs, known as crystalline
spheres discs, all moved at different uniform speeds to create the revolution of bodies around the Earth. They were composed of an incorruptible substance called
aether.