(Keep in mind that there was not a great deal of real accurate data regarding the positions of the planets at the time. ![]() For example, the planet Mars had a retrograde or. Eudoxus was the first person to devise a model that could explain the retrograde motion of the planets in the sky. In this paper I will show that this historical hypothesis is not as firmly founded as it is usually believed to be.īrightness change Deferent Epicycle Homocentric spheres.Ĭopyright © 2015 Elsevier Ltd. When the motion of the planet is direct, it is margi and when the motion is retrograde, it is vakra. So, it is usually affirmed that the main reason for the rejection of Eudoxus' homocentric spheres in favor of the epicycle and deferent system was that the first cannot explain the manifest planetary increase of brightness during retrograde motion, while the second can. Now, while according to the Eudoxian model the planet is always equidistant from the earth, according to the epicycle and deferent system, the planet changes its distance from the earth, approaching to it during retrograde motion, just as observed. The end of September and the beginning of October bring six planets (Mercury, Jupiter, Saturn, Uranus, Neptune, and Pluto) into retrograde motion. It is natural to interpret a change of brightness, i.e., of apparent size, as a change in distance. Nevertheless, there is another explanandum: during retrograde motion the planets increase their brightness. ![]() At least in a qualitative way, both models could explain the retrograde motion, the most challenging phenomenon to be explained using circular motions. The planet appears brightest during retrograde motion. In Ancient Greek two models were proposed for explaining the planetary motion: the homocentric spheres of Eudoxus and the Epicycle and Deferent System. Retrograde Motion occurs when for a brief period of time a planet (Mars most dramatic example) appears to move backwards (westward) against the background of stars when observed from earth.
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