Period (modern)
Io
1d 18h 30m
1d 18h 29m
Europa
3d 13h 20m
3d 13h 18m
Ganymede
7d 4h 0m
7d 4h 0m
Callisto
16d 18h 0m
16d 18h 5m
The accuracy of Galileo’s measurements testifies to his careful observations and precise timekeeping.*
Galileo dedicated Siderius Nuncius to his former pupil Cosimo II di Medici, now the grand duke of Tuscany, and he named the four companions of Jupiter the “Medicean stars.” This was a calculated compliment. Galileo had a good salary at Padua, but he had been told that it would not again be increased. Also, for this salary he had to teach, taking time away from his research. He was able to strike an agreement with Cosimo, who named him court mathematician and philosopher, with a professorship at Pisa that carried no teaching duties. Galileo insisted on the title “court philosopher” because despite the exciting progress made in astronomy by mathematicians such as Kepler, and despite the arguments of professors like Clavius, mathematicians continued to have a lower status than that enjoyed by philosophers. Also, Galileo wanted his work to be taken seriously as what philosophers called “physics,” an explanation of the nature of the Sun and Moon and planets, not just a mathematical account of appearances.
In the summer of 1610 Galileo left Padua for Florence, a decision that turned out eventually to be disastrous. Padua was in the territory of the republic of Venice, which at the time was less under Vatican influence than any other state in Italy, having successfully resisted a papal interdict a few years before Galileo’s departure. Moving to Florence made Galileo much more vulnerable to control by the church. A modern university dean might feel that this danger was a just punishment for Galileo’s evasion of teaching duties. But for a while the punishment was deferred.
5. In September 1610 Galileo made the fifth of his great astronomical discoveries. He turned his telescope on Venus, and found that it has phases, like those of the Moon. He sent Kepler a coded message: “The Mother of Loves [Venus] emulates the shapes of Cynthia [the Moon].” The existence of phases would be expected in both the Ptolemaic and the Copernican theories, but the phases would be different. In the Ptolemaic theory, Venus is always more or less between the Earth and the Sun, so it can never be as much as half full. In the Copernican theory, on the other hand, Venus is fully illuminated when it is on the other side of its orbit from the Earth.
This was the first direct evidence that the Ptolemaic theory is wrong. Recall that the Ptolemaic theory gives the same appearance of solar and planetary motions seen from the Earth as the Copernican theory, whatever we choose for the size of each planet’s deferent. But it does not give the same appearance as the Copernican theory of solar and planetary motions as seen from the planets. Of course, Galileo could not go to any planet to see how the motions of the Sun and other planets appear from there. But the phases of Venus did tell him the direction of the Sun as seen from Venus—the bright side is the side facing the Sun. Only one special case of Ptolemy’s theory could give that correctly, the case in which the deferents of Venus and Mercury are identical with the orbit of the Sun, which as already remarked is just the theory of Tycho. That version had never been adopted by Ptolemy, or by any of his followers.
6. At some time after coming to Florence, Galileo found an ingenious way to study the face of the Sun, by using a telescope to project its image on a screen. With this he made his sixth discovery: dark spots were seen to move across the Sun. His results were published in 1613 in his Sunspot Letters, about which more later.
There are moments in history when a new technology opens up large possibilities for pure science. The improvement of vacuum pumps in the nineteenth century made possible experiments on electrical discharges in evacuated tubes that led to the discovery of the electron. The Ilford Corporation’s development of photographic emulsions allowed the discovery of a host of new elementary particles in the decade following World War II. The development of microwave radar during that war allowed microwaves to be used as a probe of atoms, providing a crucial test of quantum electrodynamics in 1947. And we should not forget the gnomon. But none of these new technologies led to scientific results as impressive as those that flowed from the telescope in the hands of Galileo.
The reactions to Galileo’s discoveries ranged from caution to enthusiasm. Galileo’s old adversary at Padua, Cesare Cremonini, refused to look through the telescope, as did Giulio Libri, professor of philosophy at Pisa. On the other hand, Galileo was elected a member of the Lincean Academy, founded a few years earlier as Europe’s first scientific academy. Kepler used a telescope sent to him by Galileo, and confirmed Galileo’s discoveries. (Kepler worked out the theory of the telescope and soon invented his own version, with two convex lenses.)
At first, Galileo had no trouble with the church, perhaps because his support for Copernicus was still not explicit. Copernicus is mentioned only once in Siderius Nuncias, near the end, in connection with the question why, if the Earth is moving, it does not leave the Moon behind. At the time, it was not Galileo but Aristotelians like Cremonini who were in trouble with the Roman Inquisition, on much the same grounds that had led to the 1277 condemnation of various tenets of Aristotle. But Galileo managed to get into squabbles with both Aristotelian philosophers and Jesuits, which in the long run did him no good.
In July 1611, shortly after taking up his new position in Florence, Galileo entered into a debate with philosophers who, following what they supposed to be a doctrine of Aristotle, argued that solid ice had a greater density (weight per volume) than liquid water. The Jesuit cardinal Roberto Bellarmine, who had been on the panel of the Roman Inquisition that sentenced Bruno to death, took Galileo’s side, arguing that since ice floats, it must be less dense than water. In 1612 Galileo made his conclusions about floating bodies public in his Discourse on Bodies in Water.22
In 1613 Galileo antagonized the Jesuits, including Christoph Scheiner, in an argument over a peripheral astronomical issue: Are sunspots associated with the Sun itself—perhaps as clouds immediately above its surface, as Galileo thought, which would provide an example (like lunar mountains) of the imperfections of heavenly bodies? Or are they little planets orbiting the Sun more closely than Mercury? If it could be established that they are clouds, then those who claimed that the Sun goes around the Earth could not also claim that the Earth’s clouds would be left behind if the Earth went around the Sun. In his Sunspot Letters of 1613, Galileo argued that sunspots seemed to narrow as they approach the edge of the Sun’s disk, showing that near the disk’s edge they were being seen at a slant, and hence were being carried around with the Sun’s surface as it rotates. There was also an argument over who had first discovered sunspots. This was only one episode in an increasing conflict with the Jesuits, in which unfairness was not all on one side.23 Most important for the future, in Sunspot Letters Galileo at last came out explicitly for Copernicus.
Galileo’s conflict with the Jesuits heated up in 1623 with the publication of The Assayer. This was an attack on the Jesuit mathematician Orazio Grassi for Grassi’s perfectly correct conclusion, in agreement with Tycho, that the lack of diurnal parallax shows that comets are beyond the orbit of the Moon. Galileo instead offered a peculiar theory, that comets are reflections of the sun’s light from linear disturbances of the atmosphere, and do not show diurnal parallax because the disturbances move with the Earth as it rotates. Perhaps the real enemy for Galileo was not Orazio Grassi but Tycho Brahe, who had presented a geocentric theory of the planets that observation could not then refute.
In these years it was still possible for the church to tolerate the Copernican system as a purely mathematical device for calculating apparent motions of planets, though not as a theory of the real nature of the planets and their motions. For instance, in 1615 Bellarmine wrote to the Neapolitan monk Paolo Antonio Foscarini with both a reas
surance and a warning about Foscarini’s advocacy of the Copernican system:
It seems to me that Your Reverence and Signor Galileo would act prudently by contenting yourselves with speaking hypothetically and not absolutely, as I have always believed Copernicus to have spoken. [Was Bellarmine taken in by Osiander’s preface? Galileo certainly was not.] To say that by assuming the Earth in motion and the Sun immobile saves all the appearances better than the eccentrics and epicycles ever did is to speak well indeed. [Bellarmine apparently did not realize that Copernicus like Ptolemy had employed epicycles, only not so many.] This holds no danger and it suffices for the mathematician. But to want to affirm that the Sun really remains at rest at the world’s center, that it turns only on itself without running from East to West, and that the Earth is situated in the third heaven and turns very swiftly around the Sun, that is a very dangerous thing. Not only may it irritate all the philosophers and scholastic theologians, it may also injure the faith and render Holy Scripture false.24
Sensing the trouble that was gathering over Copernicanism, Galileo in 1615 wrote a celebrated letter about the relation of science and religion to Christina of Lorraine, grand duchess of Tuscany, whose wedding to the late grand duke Ferdinando I Galileo had attended.25 As Copernicus had in De Revolutionibus, Galileo mentioned the rejection of the spherical shape of the Earth by Lactantius as a horrible example of the use of Scripture to contradict the discoveries of science. He also argued against a literal interpretation of the text from the Book of Joshua that Luther had earlier invoked against Copernicus to show the motion of the Sun. Galileo reasoned that the Bible was hardly intended as a text on astronomy, since of the five planets it mentions only Venus, and that just a few times. The most famous line in the letter to Christina reads, “I would say here something that was heard from an ecclesiastic of the most eminent degree: ‘That the intention of the Holy Ghost is to teach us how one goes to heaven, not how heaven goes.’ ” (A marginal note by Galileo indicated that the eminent ecclesiastic was the scholar Cardinal Caesar Baronius, head of the Vatican library.) Galileo also offered an interpretation of the statement in Joshua that the Sun had stood still: it was the rotation of the Sun, revealed to Galileo by the motion of sunspots, that had stopped, and this in turn stopped the orbital motion and rotation of the Earth and other planets, which as described in the Bible extended the day of battle. It is not clear whether Galileo actually believed this nonsense or was merely seeking political cover.
Against the advice of friends, Galileo in 1615 went to Rome to argue against the suppression of Copernicanism. Pope Paul V was anxious to avoid controversy and, on the advice of Bellarmine, decided to submit the Copernican theory to a panel of theologians. Their verdict was that the Copernican system is “foolish and absurd in Philosophy, and formally heretical inasmuch as it contradicts the express position of Holy Scripture in many places.”26
In February 1616 Galileo was summoned to the Inquisition and received two confidential orders. A signed document ordered him not to hold or defend Copernicanism. An unsigned document went further, ordering him not to hold, defend, or teach Copernicanism in any way. In March 1616 the Inquisition issued a public formal order, not mentioning Galileo but banning Foscarini’s book, and calling for the writings of Copernicus to be expurgated. De Revolutionibus was put on the Index of books forbidden to Catholics. Instead of returning to Ptolemy or Aristotle, some Catholic astronomers, such as the Jesuit Giovanni Battista Riccioli in his 1651 Almagestum Novum, argued for Tycho’s system, which could not then be refuted by observation. De Revolutionibus remained on the Index until 1835, blighting the teaching of science in some Catholic countries, such as Spain.
Galileo hoped for better things after 1624, when Maffeo Barberini became Pope Urban VIII. Barberini was a Florentine and an admirer of Galileo. He welcomed Galileo to Rome and granted him half a dozen audiences. In these conversations Galileo explained his theory of the tides, on which he had been working since before 1616.
Galileo’s theory depended crucially on the motion of the Earth. In effect, the idea was that the waters of the oceans slosh back and forth as the Earth rotates while it goes around the Sun, during which movement the net speed of a spot on the Earth’s surface along the direction of the Earth’s motion in its orbit is continually increasing and decreasing. This sets up a periodic ocean wave with a one-day period, and as with any other oscillation, there are overtones, with periods of half a day, a third of a day, and so on. So far, this leaves out any influence of the Moon, but it had been known since antiquity that the higher “spring” tides occur at full and new moon, while the lower “neap” tides are at the times of half-moon. Galileo tried to explain the influence of the Moon by supposing that for some reason the Earth’s orbital speed is increased at new moon, when the Moon is between the Earth and the Sun, and decreased at full moon, when the Moon is on the other side of the Earth from the Sun.
This was not Galileo at his best. It’s not so much that his theory was wrong. Without a theory of gravitation there was no way that Galileo could have correctly understood the tides. But Galileo should have known that a speculative theory of tides that had no significant empirical support could not be counted as a verification of the Earth’s motion.
The pope said that he would permit publication of this theory of tides if Galileo would treat the motion of the Earth as a mathematical hypothesis, not as something likely to be true. Urban explained that he did not approve of the Inquisition’s public order of 1616, but he was not ready to rescind it. In these conversations Galileo did not mention to the pope the Inquisition’s private orders to him.
In 1632 Galileo was ready to publish his theory of the tides, which had grown into a comprehensive defense of Copernicanism. As yet, the church had made no public criticism of Galileo, so when he applied to the local bishop for permission to publish a new book it was granted. This was his Dialogo (Dialogue Concerning the Two Chief Systems of the World—Ptolemaic and Copernican).
The title of Galileo’s book is peculiar. There were at the time not two but four chief systems of the world: not just the Ptolemaic and Copernican, but also the Aristotelian, based on homocentric spheres revolving around the Earth, and the Tychonic, with the Sun and Moon going around a stationary Earth but all other planets going around the Sun. Why did Galileo not consider the Aristotelian and Tychonic systems?
About the Aristotelian system, one can say that it did not agree with observation, but it had been known to disagree with observation for two thousand years without losing all its adherents. Just look back at the argument made by Fracastoro at the beginning of the sixteenth century, quoted in Chapter 10. Galileo a century later evidently thought such arguments not worth answering, but it is not clear how that came about.
On the other hand, the Tychonic system worked too well for it to be justly dismissed. Galileo certainly knew about Tycho’s system. Galileo may have thought his own theory of the tides showed that the Earth does move, but this theory was not supported by any quantitative successes. Or perhaps Galileo just did not want to expose Copernicus to competition with the formidable Tycho.
The Dialogo took the form of a conversation among three characters: Salviati, a stand-in for Galileo named for Galileo’s friend the Florentine nobleman Filippo Salviati; Simplicio, an Aristotelian, perhaps named for Simplicius (and perhaps intended to represent a simpleton); and Sagredo, named for Galileo’s Venetian friend the mathematician Giovanni Francesco Sagredo, to judge wisely between them. The first three days of the conversation showed Salviati demolishing Simplicio, with the tides brought in only on the fourth day. This certainly violated the Inquisition’s unsigned order to Galileo, and arguably the less stringent signed order (not to hold or defend Copernicanism) as well. To make matters worse, the Dialogo was in Italian rather than Latin, so that it could be read by any literate Italian, not just by scholars.
At this point, Pope Urban was shown the unsigned 1616 order of the Inquisition to Galileo, perhaps by enemies
that Galileo had made in the earlier arguments over sunspots and comets. Urban’s anger may have been amplified by a suspicion that he was the model for Simplicio. It didn’t help that some of the pope’s words when he was Cardinal Barberini showed up in the mouth of Simplicio. The Inquisition ordered sales of the Dialogo to be banned, but it was too late—the book was already sold out.
Galileo was put on trial in April 1633. The case against him hinged on his violation of the Inquisition’s orders of 1616. Galileo was shown the instruments of torture and tried a plea bargain, admitting that personal vanity had led him to go too far. But he was nevertheless declared under “vehement suspicion of heresy,” condemned to eternal imprisonment, and forced to abjure his view that the Earth moves around the Sun. (An apocryphal story has it that as Galileo left the court, he muttered under his breath, “Eppur si muove,” that is, “But it does move.”)
Fortunately Galileo was not treated as roughly as he might have been. He was allowed to begin his imprisonment as a guest of the archbishop of Siena, and then to continue it in his own villa at Arcetri, near Florence, and near the convent residence of his daughters, Sister Maria Celeste and Sister Arcangela.27 As we will see in Chapter 12, Galileo was able during these years to return to his work on the problem of motion, begun a half century earlier at Pisa.
Galileo died in 1642 while still under house arrest in Arcetri. It was not until 1835 that books like Galileo’s that advocated the Copernican system were removed from the Index of books banned by the Catholic church, though long before that Copernican astronomy had become widely accepted in most Catholic as well as Protestant countries. Galileo was rehabilitated by the church in the twentieth century.28 In 1979 Pope John Paul II referred to Galileo’s Letter to Christina as having “formulated important norms of an epistemological character, which are indispensable to reconcile Holy Scripture and science.”29 A commission was convened to look into the case of Galileo, and reported that the church in Galileo’s time had been mistaken. The pope responded, “The error of the theologians of the time, when they maintained the centrality of the Earth, was to think that our understanding of the physical world’s structure was, in some way, imposed by the literal sense of the Sacred Scripture.”30
To Explain the World: The Discovery of Modern Science Page 19
