Why does the professor discuss the epicycle theory?A. To explain why early astronomers started measuring the velocity of the planets.B. To explain why the astronomer Aristarchus rejected the geocentric model.C. To show how early astronomers explained the apparent backward motion of some planets.D. To show that early astronomers believed that all planets moved in their own orbits around the Sun.
古代人认为地球是宇宙的中心，其它行星都在按照同一方向绕着地球转，但是天文学家观察到有的行星在逆行。“the epicycle theory” 就是古人对这个现象的一个解释，他们认为行星在围绕地球转的同时也会在自己的一个小圈上旋转。所以 C 选项正确。
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