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# Panet retrograde motion - Earth-center and Sun-center

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How can planet retrograde motion be explained by the Earth-center hypothesis and by the Sun-center hypothesis.

##### Solution Summary

In 699 words, this solution discusses planetary motion as opposed to star motion, and how this can be explained using both the Earth-center and Sun-center hypotheses.

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As you know, to an observer on earth, the entire sky appears to rotate from East to West during the course of a night (actually during the day too, but the only celestial object we can see during the day is the sun), which is really due to the fact that the Earth is rotating.

Now imagine that, at the same time every night for a year--say, at midnight--you were to go outside and make a perfect sketch of every object you could see in the sky: stars and planets.

What you would soon notice is that, while the stars appear to be in the same position relative to one another *every night*, the planets actually change their positions from night to night. That is, the planets appear to *move* relative to the stars, and this motion is fast enough to become noticeable after only a few days of observations in most cases.

What we mean by "retrograde motion" is this: *most* of the time, this relative motion of the planets with relative to the fixed stars is West to ...

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