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The belief that that was the way the universe worked was accepted for a very long time. Copernicus proposed a slightly different view of the universe, but it wasn't until Kepler came along in the late 1500's - early 1600's that the way we view the cosmos was changed forever. Kepler was born in Germany in 1571. He was a very religious man and believed that God created the universe and that God created man in his image. Since man was created in His image, Kepler believed that man should be able to understand how the Universe worked through mathematics. Kepler at first believed that the planetary orbits had something to do with the five perfect shapes Euclid had described in his work, he spent a long time studying this theory of his but it was to avail. However, later Kepler was invited to work with Tycho Brache, a leading astronomer of the day. Tycho had made numerous accurate observations of the stars and planets, but he needed a mathematician to put all his work together. Kepler and Brache were colleagues but not friends and Tycho only showed Kepler some of his work not all of it. The job which Brache actually gave Kepler, was to explain the orbit of Mars, which he had a lot of data on. After studying Mars for a long time Kepler came to the conclusion that Aristotle and Copernicus were incorrect in saying that the planets revolved about the sun in circular paths. (Actually to clarify that a little, Aristotle thought the universe was geocentric and so did Tycho Brache; but Kepler believed in the Copernican theory, that the Universe was heliocentric.) In fact he found out that the planets orbited about the sun on an ecliptic path. That means that the planets orbits were not perfect circles, they were eclipses, which is a slightly squashed circle. After realizing this he developed his famous Three Laws of Planetary Motion. .
The orbit of planets are ellipses, with the sun at one center of the ellipse.
