<P> Nicolaus Copernicus' major theory of a heliocentric model was published in De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), in 1543, the year of his death, though he had formulated the theory several decades earlier . Copernicus' ideas were not immediately accepted, but they did begin a paradigm shift away from the Ptolemaic geocentric model to a heliocentric model . The Copernican revolution, as this paradigm shift would come to be called, would last until Isaac Newton's work over a century later . </P> <P> Johannes Kepler published his first two laws about planetary motion in 1609, having found them by analyzing the astronomical observations of Tycho Brahe . Kepler's third law was published in 1619 . The first law was "The orbit of every planet is an ellipse with the Sun at one of the two foci ." </P> <P> On 7 January 1610 Galileo used his telescope, with optics superior to what had been available before . He described "three fixed stars, totally invisible by their smallness", all close to Jupiter, and lying on a straight line through it . Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars . On 10 January Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter . Within a few days he concluded that they were orbiting Jupiter: Galileo stated that he had reached this conclusion on 11 January . He had discovered three of Jupiter's four largest satellites (moons). He discovered the fourth on 13 January . </P> <P> His observations of the satellites of Jupiter created a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the principles of Aristotelian Cosmology, which held that all heavenly bodies should circle the Earth, and many astronomers and philosophers initially refused to believe that Galileo could have discovered such a thing . </P>

Who believed the sun was the center of the solar system