Galileo

A

Discoveries with the Telescope

In 1609 Galileo heard that a telescope had been invented in the Netherlands. In August of that year he constructed a telescope that magnified about 10 times and presented it to the doge of Venice. Its value for naval and maritime operations resulted in the doubling of his salary and the assurance of lifelong tenure as a professor.

By December 1609 Galileo had built a telescope of 20 times magnification, with which he discovered mountains and craters on the Moon. Not only did this contradict the Aristotelian idea that heavenly bodies must be perfectly spherical; it also indicated that a heavenly body could be much more like the Earth than had hitherto been imagined. Galileo also saw that the Milky Way was composed of stars, and he discovered four satellites circling Jupiter. It was therefore undeniable that at least some heavenly bodies move round a centre other than the Earth, a finding that did not prove that Copernicus had been right, but did fit in well with the Copernican system of the universe. Galileo published these findings in March 1610 in a book called The Starry Messenger. He astutely used his new fame to secure an appointment for which he had been angling for some time, that of court mathematician and philosopher at Florence; he was thereby freed from teaching duties and had time for research and writing. By December 1610 he had observed the phases of Venus, which are a natural consequence of the Copernican system, which has Venus circling the Sun within Earth’s own orbit. The Ptolemaic arrangement, by contrast, had Venus moving on an epicycle, a circle whose centre moved around the Earth but was tied to the Earth-Sun line, and it could not reproduce the phases. Ptolemaic astronomers had to concede that Venus orbits the Sun rather than the Earth, while still insisting that the Sun moves around the Earth. Galileo naturally took the discovery to be a strong confirmation of Copernicanism.

Traditionalist professors of philosophy scorned Galileo’s discoveries because Aristotle had held that only perfectly spherical bodies could exist in the heavens and that nothing new could ever appear there. So comets, for instance, had to be assigned to the world of change below the Moon and treated as meteorological phenomena. (It is a curiosity that, in a controversy over the comets of 1618, Galileo, who did as much as anyone to bridge the artificial gap between Earth and the heavens, was nevertheless willing to treat the comets as sublunar.)

Galileo also disagreed with professors at Florence and Pisa about hydrostatics, and he published a book on floating bodies in 1612. Four printed attacks on this book followed, rejecting Galileo’s physics. Aristotelians took shape to be the key to explaining why bodies float, whereas Galileo relied on the relative densities of the floating object and the medium in which it floated. Despite some embarrassment caused by the fact that he did not understand surface tension any more than his opponents, Galileo had the better of the argument, an argument he considered it useless to pursue with adversaries who were ignorant of elementary mathematics. In 1613 he published a work on sunspots (see Sun: Sunspots) and predicted victory for the Copernican theory.

IV

Conflict with the Church

A Pisan professor, in Galileo’s absence but in the presence of his pupil Castelli, told the Medici (the ruling family of Florence as well as Galileo’s employers) that belief in a moving Earth was contrary to Scripture. Galileo immediately wrote a pamphlet for private circulation, Letter to Castelli, sketching his views on the relation of Scripture and science. In December 1614 a Florentine Dominican denounced “Galileists” from the pulpit, and early in 1615 the Florentine Dominican convent of San Marco sent criticisms of Galileists to the Inquisition in Rome. Galileo enlarged his Letter to Castelli into a Letter to the Grand Duchess Cristina on the correct use of biblical passages in scientific arguments, holding that the interpretation of the Bible should be adapted to increasing knowledge and warning against the danger of treating any scientific opinion as an article of Roman Catholic faith. This remarkable work of amateur theology was not published in Italy in his lifetime and had little influence on the course of events.

Early in 1616 Copernican books were subjected to censorship by the Roman Congregation of the Index of Forbidden Books, after the Jesuit cardinal Robert Bellarmine had instructed Galileo that he must no longer hold or defend the opinion that the Earth moves. Following a long tradition that hypotheses in astronomy were merely instruments or calculating devices, Cardinal Bellarmine had previously advised him to treat this subject only hypothetically and for scientific purposes, without taking Copernican concepts as literally true or attempting to reconcile them with the Bible. The public ruling of 1616 similarly laid down that Catholics could use Copernicanism as a calculating device, but could not say that it was the true system of the universe. Galileo remained silent on the subject for years, working on a method for determining longitude at sea by using his predictions of the motions of Jupiter’s satellites, resuming his earlier studies of falling bodies, and skilfully setting forth his views on scientific reasoning in a book on comets, The Assayer (1623), which is a classic of polemical writing.

A

The Trial of Galileo

In 1624 Galileo began a book he wished to call Dialogue on the Tides, in which he discussed the Ptolemaic and Copernican hypotheses in relation to the physics of tides. In 1630 the book was licensed for printing by Roman Catholic censors at Rome, but they altered the title to Dialogue on the Two Chief World Systems. Because of the prevalence of plague in central Italy, it was published at Florence in 1632. Despite the book’s having two official licences, Galileo was summoned to Rome by the Inquisition to stand trial for “grave suspicion of heresy”. Although he had made considerable efforts to conform to the letter of the ruling of 1616, Galileo had clearly written a pro-Copernican book. He had occasionally also slipped up by explicitly treating Copernicanism as “probable”, meaning that, though it was yet unproven, sooner or later it could well be shown to be true. Such a position was incompatible with the ruling of 1616, as was pointed out at his trial: Catholics were allowed to use Copernicanism as a helpful calculating device, provided that they did not treat it as having any truth in it.

Galileo’s legal position was worsened by the presence in his file of a written but unsigned report that in 1616 he had been personally ordered not to discuss Copernicanism either orally or in writing. Cardinal Bellarmine had died, but Galileo produced a certificate signed by the cardinal, which gave no indication that Galileo had been subjected to any greater restriction than applied to any Roman Catholic under the 1616 edict. No signed document contradicting this was ever found. Galileo was compelled in 1633 to abjure and was sentenced to life imprisonment (swiftly commuted to permanent house arrest). The Dialogue was prohibited and the sentence against him was to be read publicly in every university.

V

Galileo’s Impact on Thought

The condemnation of Galileo did have some effect on universities and colleges in those countries where the Catholic Church was able to exercise control of teaching and publication, though the permission to treat Copernicanism as a useful, though false, calculating device meant that heliocentric ideas could always be made familiar to students. The ideas contained in the Dialogue could not be repressed and Galileo’s own scientific reputation remained high, both in Italy and abroad, especially after the publication of his final and greatest work.

This was the Discourses Concerning Two New Sciences, published at Leiden in 1638. It reviews and refines Galileo’s earlier studies of motion and, in general, the principles of mechanics. The book opened a road that was to lead Newton to the law of universal gravitation, which linked the planetary laws discovered by Galileo’s contemporary Johannes Kepler with Galileo’s mathematical physics. Galileo became blind before it was published, and he died at Arcetri, near Florence, on January 8, 1642.

Galileo’s most valuable scientific contribution was his part in transforming physics from a plausible framework erected on casual observations of complex everyday experiences into a method whereby selected experiences were so simplified that their underlying structures or patterns became tractable in geometrical terms and so susceptible to precise measurement (see Experiment). So, for instance, the law of falling bodies disregards the resistance of the medium and concentrates solely on the relationship between distance fallen and time elapsed in a vacuum. If this simplified law proves to be only approximate, then the approach is repeated to find what refinement is needed to account for how an actual body falls—for example, through air.

Galileo abandoned the key Aristotelian ideas according to which rest is a natural state and only motion needs explanation, and got so near to understanding the nature of inertial motion that Newton credited him with the discovery. More widely influential, however, were The Starry Messenger and the Dialogue on the Two Chief World Systems, which opened new vistas in astronomy. He was an outstanding popularizer of his own work and is recognized as a master of Italian prose.

Galileo’s lifelong struggle to free scientific inquiry from restriction by philosophical and theological interference is also remembered as a major contribution to the development of science. Since the full publication of Galileo’s trial documents in the 1870s, entire responsibility for Galileo’s condemnation has customarily been placed on the officials of the Roman Catholic Church. A fuller picture would include the role of the professors of philosophy who first persuaded theologians to link Galileo’s science with heresy, though the responsibility for the ruling of 1616 and for the condemnation of Galileo must remain with the officials of the Church and their advisers.

An investigation into the astronomer’s condemnation was opened in 1979 by Pope John Paul II. A papal commission, set up in 1982, produced several scholarly publications related to the trial. In October 1992 the commission acknowledged the error of the Church’s officials. In a speech accepting the report Pope John Paul, alluding to Galileo’s views on Scripture and science, said that Galileo, “a sincere believer, showed himself to be more perceptive in this regard than the theologians who opposed him”.