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In the 16th century, inconsistencies of the Ptolemaic system were discussed in several Universities. Such institutions were the environment within which supporters of the need for introducing radical changes finished their studies.
In On the Revolutions of the Heavenly Spheres, Nicolaus Copernicus (1473-1543) explained the new structure of the World which he had conceived, to restore the uniform circularity of the planetary motions violated by Ptolemy. Copernicus positioned the Sun at the immovable centre of the Universe, and transformed the Earth into a planet. This exchange was revolutionary from many points of view, but the new World system was not completely innovative. Copernicus preserved the crystalline spheres of the Aristotelian tradition, and much of the geometrical apparatus used by the ancient astronomers.
A different construct of the uniform circularity of planetary motion was attempted by Tycho Brahe (1546-1601). A supporter of the geocentric doctrine for physical and theological reasons, Tycho built improved astronomical instruments and, using them, systematically observed the planets for more than 20 years. In the middle of this long-term program, he sketched a geo-heliocentric World system. The Earth was immovable at the centre of the Universe, the Sun revolved around the Earth, and the planets revolved around the Sun. Even this system was revolutionary: since some planetary orbits intersected each other, the crystalline heavenly spheres could not exist.
None of the new World systems appeared preferable to the Ptolemaic one. Reliability in making calculations was considered the only reason to prefer one planetary model to another, until Galileo Galilei (1564-1642) introduced the astronomical telescope. The new instrument displayed the morphology of the known planets, revealed new heavenly bodies, and enabled the observer to recognize the true World system. In his Dialogue Concerning the two Chief World Systems Galileo preferred the Copernican system, was sarcastic about the Aristotle-Ptolemaic system, and did not mention Tycho's version. This polemical position increased anti-Copernican reaction of the Catholic Church.
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The Heliocentric Theory