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For almost two thousand years the Universe of Ptolemy and Aristotle dominated western thinking about the nature of the cosmos. But in the sixteenth century the Polish astronomer Nicolaus Copernicus (1473-1543) proposed a new idea in a book called On the Revolutions of the Heavenly Bodies, that was published as Copernicus lay on his deathbed.
In his book, Copernicus suggested rather timidly that the Sun, not the Earth, was the center of the Solar System. Such a sun-centered model is called a heliocentric system. The ordering of the planets known to Copernicus in this new system is illustrated in the adjacent figure, which we recognize as the modern ordering of those planets.