B00B7H7M2E EBOK, page 9
At Padua, Galileo still taught Ptolemaic astronomy. However, though he lacked observational evidence to support a change of allegiance – and he was almost always careful about waiting for that before making announcements or going into print – by the mid-1590s he was personally convinced that Copernican astronomy was correct. In 1597 (12 years before he first looked through a telescope) he wrote a letter to that effect to a friend at Pisa, and it was also at this time that the exchange of letters with Kepler took place concerning Kepler’s book Mysterium. Galileo had joined the Copernican camp, but he declined to say so publicly.
In the late spring of 1609, the same year that Kepler published Astronomia Nova with his first and second laws, Galileo, in Padua, heard of an invention from Holland currently on view nearby in Venice – a tube with lenses arranged in it so that it made objects in the distance look closer. Apparently Galileo didn’t hurry off to examine this wonder in person. A few days later he heard a report of another such instrument from a friend in Paris. His interest aroused, Galileo started pondering what arrangement of lenses would produce the reputed effect. Venice and its nearby islands were centres of expert glass-making, so there was no difficulty obtaining the lenses he needed to build his own improved ‘perspicillum’.
The use of lenses for eye-glasses was nothing new. That had begun as early as the 13th century. There were probably also telescopic devices before the 17th century. However, one of the first pieces of documentable evidence of such an instrument identifies the maker as Jän Lippershey, a lens-grinder from the Dutch island of Walcheren. He presented it to Dutch authorities in October of 1608, eight or nine months before it came to Galileo’s attention.
It was obvious that such an instrument would be useful for sighting ships and distant features of the landscape. One brochure in the autumn of 1608 also pointed out its advantage for ‘seeing stars which are not ordinarily in view, because of their smallness’. Sir William Lower, a Welshman, looked at the Moon through a telescope earlier than Galileo did and thought it looked similar to a tart: ‘here some bright stuff, there some dark, and so confusedly all over’.
Clearly, the familiar story that Galileo invented the telescope is untrue. By the time he knew of its existence it had already been on sale in Paris and probably elsewhere for several months. A second piece of fiction is that he tried to pass it off as his own invention. However, Galileo did proceed immediately to make better capital out of it than anyone else was doing. On this occasion, and perhaps several others (it isn’t always clear precisely where he got his ideas), Galileo displayed a talent for seeing the unrealized potential of another person’s thought or invention and carrying it forward so rapidly and enthusiastically that he was halfway over the horizon before its originator had left the starting line. That isn’t plagiarism, but it did, in the case of the telescope at least, result in Galileo getting the popular credit while (his letters show) he was actually giving fair credit to others.
Galileo, a master of self-promotion, presented his own improved version of the tube with lenses to the Senate in Venice, hustled some of them up to the top of the Campanile, and showed them that it was possible to look out to sea and spot ships that wouldn’t be visible to the naked eye until two hours later. The military and commercial advantages of such an instrument were obvious to rulers of a major city-state whose prosperity rested on trade by sea. Galileo received a permanent appointment at the University of Padua and a hefty increase in salary.
But Galileo had uses in mind for his ‘perspicillum’ other than spotting ships, providing curious occasional glimpses of the Moon and stars, and securing university tenure at Padua. He set about making systematic astronomical observations, recording them, and using his fine mathematical skills to draw conclusions about what they meant.
In the autumn of 1609, Galileo turned one of his new instruments on the Moon. Although the ancient Greeks had described the Moon as ‘earthy, with mountains and valleys’, conventional wisdom in Galileo’s day had it that it was perfectly smooth and spherical. Both Greek and Hellenistic astronomers and medieval scholars understood that the Moon shines by reflected sunlight, not by its own light. Through his telescope Galileo watched sunrise on the Moon’s surface and saw isolated bright dots in the dark portion expand and join with one another. Reminded of what he had observed when sunrise strikes mountain peaks on Earth, he speculated that the separate bright spots must be peaks and ridges, lit first by the Sun’s rays before these could penetrate to the lower areas of the Moon’s surface. It occurred to him that by studying the shadows of these features he might measure the heights of the peaks and ridges. Galileo arrived at an estimated height of four to five miles. Modern measurement of the particular range of lunar mountains that he studied has them no higher than 18,000 feet. Never mind that discrepancy. The more significant point was that the Moon was not, as many had supposed, smooth.
Aiming his telescope at things more distant than the Moon, Galileo began to make further discoveries. In January of 1610, using an instrument whose lenses he had ground himself with great care, he discovered three pinpricks of light near Jupiter, neatly lined up with the planet. Galileo watched, mystified and then increasingly excited, as the little stars and Jupiter exchanged positions in their line-up and varied in brightness over the course of several nights. See Figure 3.7.
Galileo concluded that this remarkable heavenly quadrille ‘ought to be observed henceforward with more attention and precision’. Before long he found that there are four rather than three stars; that the stars move within a narrow range, always in line with Jupiter and with one another; that they stay with the planet when its motion becomes retrograde; that when they are furthest from Jupiter they are never closely packed together, but when they are nearer to Jupiter they are sometimes closely packed. The implication of this last was that if the stars are circling Jupiter, the orbits in which they move are not all the same. If the stars were following one another in the same track, it’s likely they would sometimes line up so as to seem (from our vantage point) to cluster when they are furthest from the star.
Galileo reasoned that these could only be satellites, ‘planets never seen from the beginning of the world up to our own time’, orbiting Jupiter in the same way the Moon orbits the Earth. The deeper importance of what he had discovered also didn’t escape him. Never again would it be possible to suppose that there was only one body that was the centre of all motion in the universe.
Galileo, being Galileo, soon found a way to capitalize on his discovery. He rushed into print with a book called Sidereus Nuncius, translated Starry Message (the quotations in Figure 3.7 come from that book), calling on all astronomers to equip themselves with good instruments and turn them on Jupiter. He dedicated his book not to just any local nobleman, but to the powerful Grand Duke Cosimo II de’ Medici, of Tuscany, who had once been his pupil. He decided to name his discovery the Cosmican Stars – in honour of Cosimo – but soon thought better of that. It would sound too much like ‘cosmic’ and the significance of the name would be missed. He settled on the Medicean Stars. There were, after all, four stars and four Medici brothers.
Figure 3.7
(The large disc in this picture is Jupiter. Galileo assumed the three little ‘stars’ were part of the background of fixed, distant stars – though they made him ‘somewhat wonder’.)
(Jupiter seemed to have passed up the three stars, and Galileo ‘became afraid lest the planet might have moved differently from the calculation of astronomers’.)
(Galileo waited ‘with the most intense longing’.)
(Galileo decided the third star must be hidden by the planet and it also occurred to him, ‘changing from doubt to surprise – that the interchange of position belonged not to Jupiter but to the stars’.)
(Galileo noticed that one of the two visible stars was larger than it had been before and quite a bit larger than the other. He ‘decided unhesitatingly, that there are three stars in the heavens moving around Jupiter’.)
Galileo meanwhile hadn’t been neglecting other stars and planets and had found that, though his instrument transformed the planets into discs, the stars still looked like points of light. Furthermore there were astounding numbers of them that had never been seen before. The Milky Way, he discovered, is ‘nothing else but a collection of innumerable stars . . . many of them are tolerably large and extremely bright, but the number of smaller ones is quite beyond determination’. There was far, far more to the universe than anyone on the face of the Earth had ever supposed. Galileo hastily added some pages in the middle of his book to report these discoveries.
A copy of Sidereus Nuncius reached Kepler in Prague. He also heard about the discovery of Jupiter’s satellites through his friend Wackher von Wackenfels. Galileo asked Kepler for his opinion and the reply came in the form of a long letter that was later published as Conversation with the Starry Messenger. In it, Kepler discussed Galileo’s discoveries and theories and expressed his agreement. Galileo wrote back, ‘I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.’ Galileo did not, however, respond to Kepler’s rather broad hint that he would enjoy owning one of Galileo’s telescopes, though he was sending them as gifts to many influential people. Galileo actually understood the principles of the telescope no better than Kepler, perhaps not as well, though Kepler never built one. When Kepler was working with Tycho’s data he had to study optics to learn how to eliminate errors due to the smearing (‘refraction’) of light as it passes through the Earth’s atmosphere. Kepler did, for a short while, have one of Galileo’s telescopes on loan from a mutual acquaintance.
Galileo’s self-marketing scheme was successful. His book appeared in March 1610 – quick publishing indeed – and by late summer he had accepted the Grand Duke’s offer of a job and moved back to Florence. He was now a celebrated and well-placed scientist and astronomer.
About the time Galileo must have been unpacking his equipment in Florence, the planet Venus came into a good position for viewing in the evening sky. Galileo examined the planet and the area around it, searching in vain for companions like the ones he had found around Jupiter. In a letter in mid-November to Cosimo’s brother Giuliano, ambassador in Prague, he wrote that there seemed to be no satellites around any of the planets except Jupiter. However, his study of Venus was to yield other significant results. Although most scholars give the credit to Galileo, there is some question whether it was he or his former student Benedetto Castelli who at this juncture remembered a suggestion that Copernicus had made in De revolutionibus, that Venus might supply important evidence in the case against an Earth-centred universe. It was certainly Galileo who proceeded to find the evidence.
When we visited the amusement park in Chapter 2 and studied the moving lights, there was one possibility we failed to consider. Imagine once again the entire park plunged into darkness, with glowing lights attached to the heads of only a few of the riders. As we try to figure out what carnival rides might produce this pattern of movement – and what the park would look like in daylight – we should bear in mind that some of the pinpoints of light we see might not be light sources at all, but instead be shining by reflected light. Perhaps a bauble that is not a light source itself, on the head of one of the carousel horses, is reflecting the light cast from a nearby horseman. How would we know the difference?
Do any of the lights have ‘phases’ like the Moon? Do any of them sometimes appear as a distorted disc, a half, or a crescent? If we find that, might it indicate that, like the Moon, this is not a light source but a reflection of light coming from elsewhere? If so, perhaps we could use its ‘waxing and waning’ as a clue to its position and motion, and to the position and motion of the source of its light.
It was this line of reasoning that Galileo used in 1610, when he studied the planet Venus through his telescope. In Ptolemaic astronomy, Venus always lay between the Earth and the Sun. For that reason, if Venus sheds no light of its own but only shines with reflected sunlight, observers on the Earth should never see the face of Venus anywhere near fully lit. In other words, it should never be the near equivalent of a full Moon. See Figure 3.8.
Figure 3.8
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.
