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Tycho proposed a model of the solar system where all planets but Earth move around the sun while the sun and the moon move around the earth.

I wonder how this model could explain the retrograde motion. Thanks!

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up vote 1 down vote accepted

You can see the effect in this video

The Earth is the yellow dot in the middle. The planets shown appear to be Mercury, Mars, and Jupiter. The inner planets go retrograde when they are in front of the Sun and going in the opposite direction, because their apparent radial motion is greater. The outer planets go retrograde when they are opposite the Sun, because in effect they are being pulled along while in their slow orbit of the Sun. Since the motion that dominates is the Sun's, and they are on the opposite Sun of the Earth from the Sun, they appear to be in retrograde motion.

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