Shea and Artigas - Galileo in Rome

by William R. Shea

  THE MEETING WITH CLAVIUS

  When Galileo called on Clavius at the Roman College in the autumn of 1587, he brought with him an essay on the centers of gravity of solids that was both original and ingenious. Clavius was impressed but raised a number of questions, and the two mathematicians carried on a friendly correspondence after Galileo returned to Florence. Early in 1588, Clavius even promised to send him a copy of his new book on the reform of the calendar as soon as it appeared.

  Galileo now had the Jesuits on his side. They were not the only allies he had cultivated in Rome. He had also managed to win the approval of Cardinal Enrico Caetani, who had recently been papal legate to Bologna and was about to become the pope’s envoy to Paris. The cardinal sent a warm letter of recommendation to the University of Bologna in which he said that he would view it as a personal favor if Galileo was awarded the chair of mathematics.

  Did Galileo discuss Copernicanism with Clavius, Caetani, or other scholars? There is nothing to indicate this in the correspondence, but we know that Galileo had composed in 1586–1587 a manuscript, Treatise on the Sphere, or Cosmography, that he used for his private teaching in Florence and Siena. It is a conventional discussion of climatic geography and spherical astronomy following the thirteenth-century Sphere of Sacrobosco (John Holywood) that had been a standard undergraduate textbook for over three centuries. It contained no discussion of planetary astronomy, but it did outline arguments to show that the Earth was at rest at the center of the universe. So it may be assumed that in 1587 Galileo took the geocentric system for granted. After all, the Earth does not seem to move, and we see the Sun rise in the morning and set in the evening. The Ancients had devised two main astronomical models to account for celestial observations. These went under the names of Aristotle and Ptolemy, and we must say a few words about them.

  TRADITIONAL ASTRONOMY

  The great majority of stars do not appear to change position in relation to one another but form an unchanging pattern in the sky. The Babylonians gave names to the more conspicuous groups of fixed stars, called constellations, which appear to rotate in circles about a point called the pole of the heavens. Those near the pole can be seen to perform a complete circle, and those farther from it dip below the horizon. The time they take to make one complete turn is called a sidereal day. The pole is closely marked by the bright star Polaris, easily found from its relation to the conspicuous constellation Ursa Major.

  There are, however, seven celestial bodies visible to the naked eye whose positions vary in relation to the fixed stars. These are the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The movement of the Moon among the stars is so rapid that it can be noticed in a few hours. That of the planets can be detected if they are observed on successive nights, but the path they follow is not straight, nor is it always covered at the same speed. For instance, the planet Mars can be seen approaching from the west in April, slowing down in June, and then moving backward against the background of fixed stars until mid-August, when it resumes its eastward progression. The apparent stops are called stations, and the backward motions retrogradations. The retrogradations of Mars are always of this general form and duration, but they do not always occur at the same time or in the same part of the sky.

  In the fourth century B.C. in ancient Greece, an astronomer named Eudoxus invented a system to explain how the planets move. Each planet is attached to a sphere whose axis is connected to the inside of another sphere, whose own axis is attached to a third, and so on. The system of Eudoxus gave a rough approximation to the position of the planets, but it suffered from an inherent weakness: It did not allow the distance of the planets to vary, which meant that they could neither approach the Earth nor recede from it. How, then, does one explain the variations in their brightness and apparent size, as well as the fact that solar eclipses are sometimes total and sometimes partial?

  Without departing from the assumption that the Earth was at rest and the Sun in motion, the second-century Alexandrian astronomer Claudius Ptolemy found a better way to explain the apparent path of the planets by placing each planet on a circle, called an epicycle, attached to another circle called the deferent or “carrying” circle. Thus the Ptolemaic system is often described as based on epicycle and deferent. The result is that the planet traces out a curve with a series of loops or cusps. It is clear that this curve, which results from the combination of epicycle and deferent, sometimes brings the planet nearer the center than at other times. Furthermore, when the planet is on the inside of each loop, an observer can see it move with retrograde motion. It is only necessary to choose the relative size of the epicycle and the deferent, and the relative speed of rotation of the two circles in order to make the motion of the planet conform with observation.

  Ptolemy’s systems gave results that are surprisingly good, but he went about his work in what is for us a curious way. He tackled each construction piecemeal; that is, he took up each problem one by one, and dealt with it as though other aspects of the planet’s motion were irrelevant to what he was doing. This raises the question of what Ptolemy was trying to achieve. He was certainly not attempting to devise a unified cosmology. Rather he seems to have assumed that his job as an astronomer was “to save the appearances,” as the phrase went, namely, “to account for the way heavenly bodies appeared,” not to offer a physical explanation of their motion. If a planet showed an irregularity in speed, and another in size, some astronomers took the liberty of explaining the first by an epicyle and the second by two epicycles or vice versa! The question of the reality of these constructions was never raised by Ptolemy.

  Copernicus was dissatisfied with this arbitrary way of doing astronomy and he proposed a radically different system by moving the Sun to the center and locating the Earth among the planets. Galileo had probably heard of this innovation before 1587, but it is only after his first trip to Rome that he will start asking himself whether it really made sense.

  CHAPTER TWO

  The Door of Fame Springs Open

  SECOND TRIP • 29 MARCH-4 JUNE 1611

  When he first went to Rome in 1587, Galileo was an impecunious 23-year-old mathematical student looking for a job. When he returned for the second time in 1611, he was, at 47, a famous professor. The support of Clavius and other mathematicians had enabled him to get a teaching position first at the University of Pisa (1589-1592), and then at the University of Padua (1592-1610). His recent telescopic discoveries had captured the imagination of everyone in Europe, and the grand duke of Tuscany had just made him his official mathematician and philosopher. He had been freed from the drudgery of administrative work and the constraints of teaching, but he now depended entirely on the goodwill of his young patron, Cosimo II. Galileo had taught him mathematics, and Cosimo II was to remain grateful throughout his life to his former tutor, but the radical distinction between monarch and subject was never questioned nor was it in any way ambiguous. Galileo would not have been able to go to Rome in 1611 without the grand duke’s formal acquiescence. In practice this meant that Galileo had to write to the secretary of state, Belisario Vinta, whose position may be compared to that of a prime minister in our modern states. Vinta opened the letters addressed to the Grand Duke, advised him on the appropriate answer, and conveyed his reply to correspondents.

  Galileo suffered from some form of chronic rheumatism, and Florence would have been unbearable for him in the winter were it not for Filippo Salviati, who invited him frequently to his villa a few kilometers outside the city, where the air was more pleasant and the rooms better heated. In the winter of 1610-1611, Galileo was in particularly poor health. If this was the case, why his desire to rush to Rome? He did not much enjoy traveling, and he never left Italy or journeyed beyond a radius of 300 kilometers from his native Tuscany. In order to understand why he felt it was so important to go to Rome we must step back a little and consider the events of 1609–1610 that had completely transformed his position in the academic world.

  In the
summer of 1609 Galileo was still a professor at the University of Padua, where he was only moderately satisfied with his teaching load and less than happy with a salary that was one-quarter of that of Cesare Cremonini, the professor of philosophy. His study of astronomy had led him to believe that the Earth moved around the Sun, and as early as 1597 he had written to the German astronomer Johann Kepler to say that he was also a Copernican. But he did not teach the new system in his public lectures, and the motion of the Earth might have remained a conjecture for him had not something new occurred. The novelty did not descend from the ethereal regions of speculation. It was the mundane outcome of people playing around with convex lenses, in Italy around 1590, in the Netherlands in 1604, and in the whole of Europe by the summer of 1609. The result was a primitive telescope that was sold at fairs. Children and adults alike amused themselves by looking at objects that were bigger but rather hazy. Galileo heard about the device when he made a trip to Venice in July 1609. He did not actually see one of these playthings, but he realized that he could improve upon it by combining a concave lens with a convex one. The result was the opera-glass, which shows objects the right way up and not upside down. Galileo convinced worthy senators in Venice to climb to the top of a tower, whence they were able to see boats coming to port a good two hours before they could be spotted by the naked eye. The strategic advantage of the new instrument was not lost on a maritime power, and Galileo’s salary was increased from 520 to 1,000 florins per year. Unfortunately, after the first flush of enthusiasm, the senators heard the sobering news that the telescope was already widespread throughout Europe, and when the official document was drawn up it stipulated that Galileo would only get his raise at the expiration of his existing contract a year later and that he would be barred, for life, from any increase of salary.

  This incident understandably made Galileo sour. He had not claimed to be the inventor of the telescope, and if the senators had compared his instrument with those made by others they would have found that his own was far superior. Let the Venetian Republic keep the telescope! He would make a better one and offer it to a more enlightened patron. Better still, he would show that much more could be revealed not only on land and sea but beyond the reaches of human navigation. He pointed the telescope to the heavens in November 1609, and, for the first time in history, the human eye had a close-up view of the Moon.

  CELESTIAL NOVELTIES

  Galileo’s reason for examining the Moon was probably to confirm a conjecture already made in antiquity by Plutarch that the light and dark features of the lunar surface are evidence that there are mountains on the Moon. Galileo focused his fifteen-power telescope on the terminator that separates the illuminated portion of the crescent Moon from the dark one, and he noticed that bright points appeared in the dark part close to the terminator. He correctly interpreted these spots as mountain peaks that are struck by the light of the rising Sun, just as happens on Earth. Galileo then turned his telescope on to the stars and found that they popped out from everywhere. In one small corner of the sky, he discovered more than five hundred stars that had never been observed by the human eye. Most spectacular still, the Milky Way resolved itself into a swath of densely packed stars.

  By January 1610, Galileo had considerably improved his telescope and his means of observation. His device now magnified twenty times, and the lenses were fixed at the ends of a tube in such a way that the one with the eyepiece slid up to allow for proper focusing. The instrument was about a meter long and was mounted on a stable base. On the evening of 7 January, Galileo saw three small but very bright stars in the immediate vicinity of Jupiter. The idea that they might be satellites did not occur to him. What struck him was the fact that they were in the unusual configuration of a short straight line. Looking at Jupiter on the next night, he noticed that whereas two had been to the east and one to the west of Jupiter on the previous evening, they were now all to the west of the planet. Again, he did not suspect that they might be in motion but wondered whether Jupiter might not be moving eastward, although the standard astronomical tables indicated that it was moving westward.

  On the ninth, the sky was overcast. On the tenth, he observed two stars to the east of Jupiter. This seemed to dispose of the conjecture that Jupiter might be moving in the wrong direction. On the eleventh, he again saw two stars to the east of Jupiter, but the furthest from the planet was now much brighter. On the twelfth, the third star reappeared to the west of Jupiter. On the thirteenth, a fourth star became visible; three stars were now to the west and one to the east of Jupiter. Why, the stars seemed to behave like satellites of Jupiter! With eagerness, Galileo awaited the evening of the fourteenth to check his hypothesis, but, unfortunately, the sky was again overcast. On the fifteenth, the sky was clear and the four stars reappeared to the west of Jupiter. In Copernican terms, this was the most exciting of Galileo’s discoveries, and he tells us why:

  Here we have a powerful and elegant argument to remove the doubts of those who accept without difficulty that the planets revolve around the Sun in the Copernican system, but are so disturbed to see the Moon alone revolve around the Earth while accompanying it in its annual revolution about the Sun, that they believe that this structure of the universe should be rejected as impossible. Now we have not just one planet revolving around another while both make a large circle around the Sun, but our eyes show us four stars that wander around Jupiter, as does the Moon around the Earth, and these stars together with Jupiter describe a large circle around the Sun in a period of twelve years.

  To those who objected that the Earth could not orbit around the Sun without losing its Moon, Galileo could now point to the skies and show Jupiter circling around a central body (be it the Earth, as Ptolemy believed, or the Sun, as Copernicus argued) while maintaining not just one but four satellites. If Galileo and Copernicus could not explain why the Earth did not shed its Moon, the followers of Ptolemy were equally at a loss to say why Jupiter held on to its satellites. Formerly the challengers, the geocentrists were becoming the challenged!

  THE STARS SPEAK OUT

  Knowing that others were also pointing telescopes to the heavens, Galileo rushed into print on March 13, 1610 with a slim booklet of 56 pages entitled Sidereus Nuncius, w hic h means The Sidere al Message. He named the four satellites of Jupiter Medician Stars in honor of the Medici family and dedicated them to Cosimo II, who had succeeded his father as grand duke of Tuscany a few weeks earlier. In a fulsome letter of dedication to the 20-year old prince, Galileo wrote:

  The Maker of the stars Himself has seemed by clear indications to direct me to assign to these new planets Your Highness’s famous name in preference to all others. For just as these stars, like children worthy of their sire, never leave Jupiter’s side by much, so—and indeed who does not know this?—clemency, kindness of heart, gentleness of manner, splendor of royal blood, nobility in public affairs, and excellency of authority and rule have all fixed their home and habitation in Your Highness.

  When Cosimo was born, Galileo went on, Jupiter occupied the middle of the heavens to “pour forth all his splendor and majesty” and confer upon him his “universal influence and power.” This lavish tribute is typical of the sycophantic style that was becoming common, even necessary, in the baroque age, but the astrological reference should not be too readily dismissed. Galileo and his contemporaries believed that the planets exercised a genuine, although not a compelling, influence on human affairs. The stars did not deprive human beings of their freedom, but it was wise to study what they had to say about one’s chances of success or risks of failure. Jupiter had enormous significance: Cosimo I, who became grand duke in 1569, had filled the Palazzo della Signoria, where he lived and ruled, with frescoes representing Jupiter, the king of the Pantheon.

  Galileo had no doubt that the stars were on his side, and he decided to use them to fulfill his dream of returning to Florence. On 7 May 1610, he wrote to Belisario Vinta to suggest that he be recalled to Florence as court philosophe
r and mathematician. What is singular about this request is the title philosopher that he added to the more usual one of mathematician. Galileo wanted to make it perfectly clear that he saw himself not as someone who merely toyed with numbers but as a scientist (in those days a natural philosopher) who dealt with the real world.

  Vinta immediately took the matter up with the grand duke and by 5 June 1610, he could inform Galileo of the happy outcome. The annual salary that he was offered, one thousand scudi for life, was comparable to the one he had in Padua, but he would no longer have to teach and would be completely free to do research. No sooner had the grand duke signed his appointment on 10 July 1610 than he got, so to speak, his reward. Galileo made a new discovery, as he had promised. On 25 July he noticed that Saturn was composed of three stars. He asked that this be kept a secret until he had published it. The letter suitably impressed his new employer, but Saturn was to become Galileo’s problem child. It seemed to have two close companions that sometimes disappeared, only to reappear in the shape of what he took for handles or ears sticking out on each side of the planet. The puzzle was not solved until 1657, 15 years after Galileo’s death, when Christian Huygens explained that Saturn is surrounded by a ring that is periodically tilted and looks like a handle when viewed with a telescope whose resolving power is weak.