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Nicolaus Copernicus originally studied canon law and medicine in Italy in preparation for a career in the Catholic Church. While in Italy he became interested in astronomy, which he then pursued in his spare time while working as a church administrator in Frauenberg, Poland. In a brief essay, Commentariolis (Little Commentary), he introduced his heliocentric system, in which the Sun is at the center of the universe, with all the planets and stars revolving around it in circular orbits. Copernicus was trying to account for the movements of the planets that, in the days prior to the invention of telescopes, were visible to the unaided eye and that did not fit the older Earth-centered, or geocentric, model of the universe of the Greek astronomer Ptolemy. Copernicus published a longer, more complete account of his theory, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), in 1543, just before he died. His heliocentric model of the universe was disputed by most astronomers of the time, as well as by the Church. After Copernicus's death, the few defenders of his ideas included Johannes Kepler and Galileo Galilei. De revolutionibus was on the Catholic Church's Index of Forbidden Books from 1616 to 1835. Theoretical support for Copernicus's system was provided almost 150 years after the publication of De revolutionibus by Sir Isaac Newton's theory of universal gravitation. His other great accomplishment was his proposal that the Earth rotates once daily on its own axis.