Since the twelfth century, Europeans had adopted a cosmology based on Aristotelian physics and popularized by the Egyptian astronomer Ptolemy (c. 90–168).46 The Earth was firmly at the center of the universe, encased like an onion in eight spherical shells composed of an invisible substance called ether. These spheres revolved in a uniform manner around the Earth, and embedded in the ether of each of the first seven spheres was one of the heavenly bodies: Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. The fixed stars occupied the eighth sphere at the outermost rim of the universe and
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