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Nicholaus Copernicus (the Latin version of Koppernigk) was a Polish church official whose passion was astronomy, and who actually performed some observations. By that time, all sorts of corrections had to be made to fit the motion of the planets to Ptolemy's ideas.
Copernicus proposed an alternative theory that the Earth was a planet orbiting the Sun, and that all planets moved in circles, one inside the other. Mercury and Venus had the smallest circles, smaller than that of the Earth, and therefore their position in the sky was always near the Sun's.
That made it easy to estimate their distances from the Sun in terms of the Earth-Sun distance. Mars, Jupiter and Saturn moved in bigger circles, and they moved more slowly, so that whenever the Earth overtook them, they seemed to move backwards.