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According to his calculations the heavens were divided into nine circles, because he observed seven planets, calculating seven distant orbits through which they moved. The circle outside of the seventh was thought to be studded with the fixed stars. In order to make this theory clear the ancient astronomers used as an illustration the different layers of an onion - thus the curving surface of each layer adheres to the curve of each succeeding layer. Outside the eighth circle there was thought to be a ninth through which these colossal bodies were given the power to march in their destined course. They further believed this ninth sphere of action to be devoid of any solar bodies and that it moved through an invisible power, thus causing the motion of all the other stars within its radius. A motion of the outward layer of the onion moves all the inside layers with it. This, in brief, was the system of ancient astronomy.
(113:3)
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