Copernicus proposed a model of the solar system in which the Sun is at the center and the planets orbit around it in circles. Actually to make his model agree reasonably well with observations he was forced to make his planets move on circles ("epicycles") which themselves moved around on circles which had the Sun at the center ("deferents"). He observed that, although his model did not agree any better with observations than did the system of Ptolemy, nor was it appreciably simpler, it did have more explanatory power. It explained why Mercury and Venus were always observed in the same part of the sky as the Sun, because their orbits lay between the orbit of the Earth and the Sun. It gave an unambiguous order for the planets, based on the lengths of their years, so that Mercury was closed, then Venus, Earth, Mars, Jupiter and Saturn furthest away. This greater explanatory power greatly appealed to later astronomers such as Kepler. Kepler wanted to explain many features of Copernicus' system, such as why there was a gap between Mars and Jupiter, why there were 6 planets and so on. But his lasting success came with three laws which described how the planets moved in their orbits. The first law says that planets move in an ellipse around the Sun (an eccentric circle), with the Sun at one of the two focii of the ellipse (at a focus). The second law, known as the
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