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All three systems are able to explain the positions of Venus, but the Ptolemaic system cannot explain the phases.

In the Ptolemaic system, with Venus always between the Earth and the Sun – travelling on an epicycle on a deferent with the Earth as its centre – an observer on Earth would never see the face of Venus anywhere near fully illuminated. This figure shows Venus in three positions.

In the Copernican system, below, in which both Venus and the Earth orbit the Sun, Venus has almost a full set of phases, as the Moon does.

In Tycho Brahe’s system, in which Venus orbits the Sun while the Sun orbits the Earth, Venus has the same set of phases it has in the Copernican system, left.

From August till October of 1610, Venus would have appeared as a blurry disc through Galileo’s telescope. In October he would have seen the disc flatten to a lozenge. Galileo knew then that Venus was shining by reflected light from the Sun, not by its own light. From November till January, Venus would have waned to a crescent in the same manner the Moon does. Galileo was aware that in the Ptolemaic system it would have been impossible for Venus to have nearly a
full range
of phases, even if its epicycle had been miscalculated and was actually on the other side of the Sun from Earth. To put it bluntly, Galileo could not have seen what he saw if Ptolemaic astronomy had been correct. In the Copernican system, and in Tycho Brahe’s system (let us hasten to admit), it was what one would expect to see.

Finally Galileo had found persuasive observational evidence that Ptolemaic astronomy was inferior to Copernican astronomy. To Galileo’s mind this was actually not the first evidence he had found. Six years earlier, in 1604, he had first sided publicly with Copernican theory, announcing in a series of lectures that the nova seen that year, later known as Kepler’s Star because Kepler wrote a book about it, provided evidence that some of Ptolemy’s arguments were invalid. Since no texts of his lectures survive, it’s a mystery what Galileo thought that evidence was. The phases of Venus are a different matter. Clearly this discovery was a serious setback for Ptolemaic astronomy.

When Galileo published
Sidereus Nuncius
in 1610, though most Catholic believers undoubtedly thought the Earth was the
centre
of the universe and assumed that scripture supported this view, the Catholic Church had no official policy regarding the arrangement of the cosmos. Contrary to modern popular legend, it had succeeded in staying clear of the debate ever since the appearance of
De revolutionibus
, continuing a centuries-old practice of tolerating diversity when it came to cosmological arguments. Nicholas of Cusa had proposed a moving Earth before Copernicus, and he was a cardinal and papal legate. The Church hadn’t criticized or condemned him. Giordano Bruno, a scholar strongly influenced by Cusa, had been burned at the stake primarily because of his heretical religious views, not because of his scientific ideas – though those didn’t help. Two of Copernicus’s strongest supporters had been powerful Church officials. Now, in the wake of Galileo’s discoveries, many among the Catholic hierarchy appear to have been hoping the Church could continue to make no official pronouncement on this matter, and some among them were particularly anxious that it should not take a fundamentalist stand on the interpretation of scriptural passages having to do with cosmology. The quote often attributed to Galileo – ‘Scripture teaches how to go to heaven, not how the heavens go’ – actually was not from him but from one Cardinal Baronis. The general, vague state of truce seemed to be that if all parties could avoid saying that any scientific arrangement of the universe
or
scriptural cosmological statements should,
or should not
, be treated as literal truth, no one would be taken to task and everything would continue to go smoothly. That was a truce that was violated on all sides but nevertheless continued to prevail for a time.

As news of Galileo’s discoveries spread, the most outspoken and dogmatic reactions came from conservative astronomers in the universities, who still continued to lisp rote ‘truths’, insisting that the authority of Aristotle and Ptolemy must not be questioned. Why bother to observe nature or look through a telescope when Aristotle and Ptolemy had already given the answers? Galileo’s battle with these men was not only over the
arrangement
of the universe. His way of doing science, his insistence on examining and testing nature to learn about it, was to these intransigent scholars foolishness at best, scientific heresy at worst.

However, not all who opposed Galileo were so lacking in intelligence and valid arguments. Even modern astronomers and historians of science admit that Galileo’s case for Copernican astronomy was not as open and shut as he insisted it was. Some of Galileo’s contemporaries argued, correctly, that his ‘evidence’ was not ‘proof’. Jesuit scholars pointed out that while Copernican theory was capable of explaining Galileo’s discoveries, Tycho’s theory could explain them equally as well without changing the centre of the universe. Many astronomers, picking up on what they thought was Copernicus’s preface to
De revolutionibus
, were willing to accept the Copernican arrangement as an excellent hypothetical model that ‘saved the appearances’, while not making decisions whether or not it actually represented reality.

Galileo himself did not have a great many personal supporters, and his scientific views were not the only reason. He had never learnt to curb his arrogant tongue. He was careless of fragile egos and didn’t suffer even the most reasoned opposition graciously. In fact he had a tendency to think of anyone who disagreed with him as an enemy. Vitriolic, insulting statements that he made in letters and in person didn’t make him popular among his colleagues.

Galileo actually bears a large share of the blame for making the Church the arena where the most visible clash between the two theories took place. Remarkably, given the Galileo ‘legend’, he didn’t at all oppose the idea that the Church should exercise authority in scientific matters. Quite the contrary: he was impatient and scornful of its reluctance to face up to the issue. Modern scholars would consider it healthy that the Church had no official position about the design of the universe. Galileo did not. What was important to him was that Church
authority
weigh in on
his
side. He was supremely confident that that would happen.

Though Church officialdom continued to be stubborn about involving itself publicly in this squabble, behind closed doors in the upper echelons of power it seems reasonably certain that the question was not being ignored and that it was considered a delicate matter. Several intellectuals among the Church leaders had concluded that Sun-centred astronomy was preferable to Earth-centred astronomy. If that was the case, the Holy Office would be wise to support Earth-centred astronomy; it eventually would have to. Some thought it best to continue the hands-off policy and let matters drift in that direction, if they did, on their own. Nearly all agreed that the Church should proceed slowly and with caution. The Catholic Church had long felt a duty to protect its members, particularly the less well-educated among them, from ideas that threatened to undermine simple faith, however much that simple faith might seem
too
simple to more sophisticated Catholics. The belief systems of most parishioners evidently included the Aristotelian/Ptolemaic arrangement of the universe, though how devoutly attached to that they were is difficult to judge. Arguably this was one reason a gradual shift seemed far preferable to any sudden move. It was much less likely to shock and confuse. But also – quite apart from any concern it might have had for the spiritual well-being of its members – given the tenor of the times and the ongoing struggle with Protestantism, Church leaders could ill afford to risk any major unsettling of the flock.

In this setting, Galileo was the proverbial bull in the china shop. He had too much zeal for his cause to recognize that forcing the issue was not in his own best interest. He was impatient. He knew that powerful people were agreeing with him. Surely others could not fail to see that on both scientific and scriptural grounds, Copernican theory was correct and acceptable.

In 1611, Galileo paid a visit to Rome and was received with
enormous
respect and friendliness. But two years later he made a serious error. He wrote letters to his former student Castelli and to the mother of the Duke of Tuscany, stating that Copernicanism should be treated as fact and commenting that it would be unwise to interpret passages of scripture in such a way as to force them to support interpretations of nature that might later prove obviously wrong. (It seems Galileo thought that only a ‘forced’ interpretation of scripture could support Ptolemaic astronomy.) Some in high Church circles had been voicing similar sentiments, recognizing that this was not a contest between literal and metaphoric reading but a matter of
which
metaphoric reading. More damagingly, Galileo also said in his letters that scripture had not gone into more sophisticated scientific detail which
could
be taken literally because it was written to be understood by ‘common people who are rude and ignorant’.

Galileo’s letter got passed around and soon some were interpreting it (or deliberately misinterpreting it) to say that Galileo was questioning the validity of scripture. When pressed, a committee of the Holy Office looked into the matter and ruled that Galileo’s letter was
not
heretical. However, he received a letter from the powerful Jesuit Cardinal Roberto Bellarmine, advising him that until there was definite proof that the Earth moved, that idea ought to be treated as hypothetical and scripture interpreted in the more commonly accepted way. Bellarmine was one of those who believed that that was how Copernicus himself had regarded his model.

In 1616, Galileo was once again in Rome pushing his campaign for Copernicanism. This time the Holy Office, increasingly beleaguered by Galileo’s enemies, sent a survey of sorts to leading university scholars regarding the centrality of the Sun and the motion of the Earth. It isn’t known who drew up the mailing list, but the replies were overwhelmingly anti-Galileo: he was scientifically wrong and philosophically heretical.

Galileo’s opponents won a qualified victory in this round. The diary of one Giovanfrancesco Buonamici, a diplomat from Tuscany, states that two cardinals opposed the Pope’s inclination to declare Copernicanism contrary to the faith. Bellarmine was one of them. The other was Cardinal Maffeo Barberini, a good friend of Galileo. In the end the judgement was that Copernicanism was ‘contrary to Holy Scripture and cannot be defended or held’. However, that judgement was not given the stamp of papal authority, and this meant that Copernicanism was not officially heresy. That seems a trivial legal distinction, as does another, that Galileo had been
informed
of the Church’s decision and
admonished
to abandon Copernican views until he had unassailable proof, but
not
punished or forbidden to teach Copernicanism. However, Galileo recognized this last point as so crucial that he asked for and received a letter from Bellarmine, who had delivered the Church’s verdict to him, making the details clear. Galileo did leave off campaigning for Copernican theory for a while and turned to other work, biding his time and watching for a better political moment to resume that effort.

In 1623, that moment seemed to have arrived. Liberal Catholics rejoiced as Cardinal Barberini became the new Pope Urban VIII. Barberini and Galileo had feasted often together and enjoyed discussing science and philosophy. Barberini, as a cardinal, had successfully opposed the papal decree that would have declared Copernicanism heretical. Some accounts have it that now, in 1623, Barberini told Galileo that as Pope he could no longer chat off the record with Galileo and express his own opinions freely. Galileo asked to be allowed to write a book on
both
systems, and he took Barberini’s response, whatever that actually was, as encouragement.

By this time, in spite of the 1616 stalemate in Italy, Copernican astronomy – with Kepler’s elliptical orbits and his three laws, Galileo’s discoveries, and the Rudolphine tables soon to be published – was very near to holding its winning hand. But if
Galileo
thought the Church was ready to put its cards on the table and support Sun-centred astronomy, he was mistaken.