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Galileo and the Copernican System

Among the various observations made with the telescope, it was the discovery of Venus' phases which strengthened Galileo's Copernican concept. In Ptolemy's architecture, the planet always moved below the Sun, along an epicycle whose centre was on the straight line which joined the Earth to the Sun. The planet would always have to be seen against the light, and it would look like a very thin crescent. However, the complete series of phases observed through the telescope showed the revolution of Venus around the Sun.

In his Dialogue Concerning the Two Chief World Systems, Galileo quoted only Ptolemy's and Copernicus' architectures. Since in Tycho's system all the planets revolved around the Sun, Venus' phases were of no use to discard the geo-heliocentric architecture. Galileo was well acquainted with Tycho's work, and, for the sake of a better defence of Copernican doctrine, he never mentioned a geo-heliocentric system.

In 1632, it was not only the violation of Cardinal Bellarmine's warning by printing the Dialogue that resulted in Galileo being arraigned before the Inquisition. The polemic form of the Dialogue also infuriated Pope Urban VIII (1568-1644). In fact, Galileo was particularly caustic and ironic against Aristotelian philosophers; unfortunately, the Catholic Church was fundamentally Aristotelian.

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