To Explain the World: The Discovery of Modern Science (26 page)

4. The most dramatic and important discovery reported in
Siderius Nuncius
was made on January 7, 1610. Training his telescope on Jupiter, Galileo saw that “three little stars were positioned near him, small but very bright.” At first Galileo thought that these were just another three fixed stars, too dim to have been seen before, though he was surprised that they seemed to be lined up along the ecliptic, two to the east of Jupiter and one to the west. But on the next night all three of these “stars” were to the west of Jupiter, and on January 10 only two could be seen, both to the east. Finally on January 13, he saw that four of these “stars” were now visible, still more or less lined up along the ecliptic. Galileo concluded that Jupiter is accompanied in its orbit with four satellites, similar to Earth’s Moon, and like our Moon revolving in roughly the same plane as planetary orbits, which are close to the ecliptic, the plane of the Earth’s orbit around the Sun. (They are now known as the four largest moons of Jupiter: Ganymede, Io, Callisto, and Europa, named after the god Jupiter’s male and female lovers.)
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This discovery gave important support to the Copernican theory. For one thing, the system of Jupiter and its moons provided a miniature example of what Copernicus had conceived to be the system of the Sun and its surrounding planets, with celestial
bodies evidently in motion about a body other than the Earth. Also, the example of Jupiter’s moons put to rest the objection to Copernicus that, if the Earth is moving, why is the Moon not left behind? Everyone agreed that Jupiter is moving, and yet its moons were evidently not being left behind.

Though the results were too late to be included in
Siderius Nuncius
, Galileo by the end of 1611 had measured the periods of revolution of the four Jovian satellites that he had discovered, and in 1612 he published these results on the first page of a work on other matters.
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Galileo’s results are given along with modern values in days (d), hours (h), and minutes (m) in the table below:

The accuracy of Galileo’s measurements testifies to his careful observations and precise timekeeping.
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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.
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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.
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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 reassurance 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.
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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.
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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.”
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