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Other planetary systems were in the air around this time. Many believed that there was no proof that Mars, Jupiter, and Saturn revolve around the Sun, though Galileo's telescopic observations had confirmed this for Mercury and Venus. Thus, men like Francis Bacon, Joseph Blancan, and Charles Malapert supported a Capellan system in which Mercury and Venus orbit the Sun but all the other planets the Earth. The widely respected Giambattista Riccioli suggested that, since Jupiter and Saturn had been shown to have satellites, they were primary planets like the Sun and, hence, orbit the Earth, whereas the others would orbit the Sun.

This battle is described in detail by de Santillana (1961) and all the significant documents which pertain to the affair have been translated in Finocchiaro (1989), a work that also provides a detailed chronology of events.

that one was dealing with a 'true' model of reality. As Galileo wrote in 1615:

I think that in discussions of physical problems we ought to begin not from the authority of scriptural passages, but from sense-experiences and necessary demonstrations.

Thus, Galileo was providing a way to the truth that was an alternative to the Bible, and this was what many members of the Church found so offensive. Galileo is reported to have said that the Bible teaches how to go to heaven, not how the heavens go.

Galileo was not without powerful friends and, although he was told privately to desist from teaching his opinions, he believed still that he could persuade the Catholic leaders that Copernicus' heliocentric universe was not at odds with Scripture. Pope Paul V was having none of it, however, and in 1616 ordered Galileo to be silenced. This gagging order was relaxed 8 years later after the election in 1623 of a more liberal Pope, Urban VIII, who as a cardinal had shown a friendly interest in Galileo's work. Under the new arrangement, Galileo was permitted to discuss the Copernican theory, provided he treated it as an astronomical hypothesis and not a model of reality.

The question of what to do with Copernicus' On the Revolutions was a difficult one for the Church. On the one hand, they would have liked to ban it for its advocacy of a moving Earth, but against this was its usefulness to astronomers. Eventually, they decided, not to proscribe it, but to censor it:

If certain of Copernicus' passages on the motion of the earth are not hypothetical, make them hypothetical; then they will not be against either the truth or the Holy Writ, in a certain sense they will be in agreement with them, on account of the false nature of suppositions, which the study of astronomy is accustomed to use as its special right.

A list of required corrections was issued in 1620. These were fairly small in number and merely toned down some of the passages where Copernicus seemed to be arguing for the truth of his system.25

Following the easing of relations between Galileo and the Church after the election of Pope Urban VIII, Galileo felt sufficiently encouraged to embark on his epoch-making Dialogue Concerning the Two Chief World Systems, the Ptolemaic and the Copernican, which was published in 1632.26 There are three

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