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In ancient times it was already known that, unlike the Sun and the Moon, planets trace out complex motions in comparison with stars in the sky (not by chance, the word “pianet”, from the Greek, means “errant”). Even their direction of motion seems to be reversed, for a certain time, and then its direction is restored (apparent retrograde motion). That is why the  geocentric models, whose highest expression will be Ptolemy’s model, had to introduce various corrections to the originary idea, according to which planets simply orbit along circular trajectories whoseLEGGI TUTTO