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- European astronomers of the sixteenth century encountered two competing systems of mathematical astronomy. On the one hand, in the ancient system of Ptolemy, the earth was the immobile center of the universe. Constrained by Aristotelian physical principles to move in uniform circular motion, the moon, sun, and seven known planets orbited the earth. “Absolutely all phenomena, are in contradiction,” Ptolemy writes, “to any of the alternate notions that have been propounded.” On the other hand, the new theory of Nicolaus Copernicus which, while still committed to uniform circular motion, argued that by placing the sun at the center instead, the apparent retrograde motion of the planets could be accounted for with greater mathematical simplicity and elegance.
Copernican theory encountered resistance on theological and philosophical grounds. Other opponents pointed to the evidence of the senses: the earth was perceptibly not in motion. But what about the predictive power of Copernican theory? Whatever its philosophical or physical flaws, was it at least superior to the Ptolemaic system in calculating the motions of celestial objects?