Orbits in Space: Aristotle proposed that the Sun, moon, planets, and stars consisted of a fifth elemental
substance whose natural motion was to follow perfect and eternally moving circles. He proposed that the
planets were located on transparent spheres, nested one inside the other, from the sphere that carried the
"fixed stars" to the innermost sphere that carried the Moon. The motive force behind all planetary motion
was the Primum Mobile, the outermost sphere, which was continuously powered by a divine force.
Revising this theory by adding and adjusting spheres set the agenda for successive generations of
astronomers for nearly 2,000 years