Galileo Galilei: The First Modern Scientist
Galileo Galilei's life, Pisan and Paduan years, telescopic discoveries, Sidereus Nuncius, controversy with Jesuits, the Dialogue, trial, and his role in founding modern physics.
Galileo Galilei: The First Modern Scientist
Of all the figures of the Scientific Revolution, none has a more secure grip on the popular imagination than Galileo Galilei. The dramatic image — the old man, half-blind, in his house in Arcetri, surrounded by the trappings of a civilization that rejected his discoveries — has been endlessly reproduced. The trial before the Inquisition in 1633, the abjuration, the legendary muttered “and yet it moves,” the reconciliation of scripture and science, the brave scientist versus the dogmatic church: these are the materials of which myths are made. The reality was less simple, and more interesting. Galileo was a devout Catholic, a courtier, a polemicist, a brilliant experimentalist, an aggressive self-promoter, and a man who managed, more or less single-handedly, to dismantle the Aristotelian physics of the heavens. The standard image of him as a “modern scientist” is partly anachronistic, because there were no modern scientists yet, and partly accurate, because Galileo did establish the prototype of the work that would eventually be called scientific. This page follows his life and work in some detail. The three articles treat specific aspects: What Did Galileo Discover With His Telescope? covers the Sidereus Nuncius; Why Was Galileo Put on Trial? covers the events from 1610 to 1633; and How Did Galileo Contribute to Physics? covers the mechanics. For the broader context, see the page on the key figures of the Scientific Revolution and the page on the telescope and the microscope in the discoveries section.
Early Life and the Pisan Years
Galileo Galilei was born in Pisa on 15 February 1564, the same year as Shakespeare and a few days before the death of Michelangelo. His father, Vincenzo Galilei, was a musician, music theorist, and member of the Florentine Camerata, a group interested in reviving the music of the ancient Greeks. Vincenzo was an experimentalist; he tested, for instance, whether strings of equal length but different thickness produced the same pitch as strings of the same length and thickness under different tensions, and he engaged in long arguments with the defenders of the old, unexamined tradition. Galileo credit his father with instilling in him “a disdain for the merely verbal” and a taste for hands-on inquiry. Galileo was educated at the monastery of Vallombrosa, near Florence, and then at the University of Pisa, where he matriculated as a medical student in 1580. He never completed the medical degree; his interests drifted to mathematics and natural philosophy, then to motion. Around 1583, while still nominally a medical student, he made his first recorded discovery: that the time taken for a pendulum to complete a swing depends on the length of the pendulum, not on the amplitude of the swing (or so the story goes; modern reconstructions show that the isochronism of the pendulum holds only for small swings, but Galileo’s claim was correct as a first approximation). The young man had also, by the tradition, dropped objects of different weights from the Leaning Tower of Pisa to show that they fell at the same rate; the story is probably apocryphal, but the idea was Galileo’s. In 1589 Galileo was appointed to the chair of mathematics at the University of Pisa. He was then twenty-five. He was not, in modern terms, a member of the senior faculty; the chair of mathematics was a junior post, well below the chairs of philosophy and medicine, and Galileo’s salary was lower than the philosophers’. He spent the three Pisan years writing on motion, attacking Aristotelian claims about falling bodies, and acquiring a reputation for argumentativeness. He left Pisa in 1592, having made more enemies than friends, for a more prestigious post at the University of Padua.
The Paduan Years (1592–1610)
The University of Padua, in the Republic of Venice, was the leading Italian university of the period. Galileo was appointed to the chair of mathematics in 1592, and he remained there for eighteen years — the longest continuous appointment of his life, and his most productive period. Padua was a tolerant city, governed by a republic that looked with suspicion on both papal and Spanish power, and Galileo’s patrons among the Venetian patricians gave him a kind of intellectual freedom he would not have enjoyed in Pisa. In Padua, Galileo worked on a remarkable range of topics. He wrote on mechanics, on the strength of materials, on the nature of motion, on the design of military instruments, and on the geometry of the heavens. He gave private lectures on fortification and military engineering, which supplemented his modest university salary. He had a long relationship with a Venetian woman, Marina Gamba, by whom he had two daughters and a son. He was, by all accounts, a popular and effective teacher. It was in Padua that Galileo did most of the work that would make his name. He refined his arguments against the Aristotelian physics of motion; he worked out the law of falling bodies (more or less correctly) and the parabolic trajectory of projectiles; he developed the concept of relative motion; and he constructed an early thermoscope, a primitive thermometer. He also built and sold geometric and military compasses, an early version of a calculating device that combined the functions of a straightedge, a divider, and a sector. The sales of the compasses provided a substantial additional income, and Galileo was, throughout his life, a man who minded his finances carefully. By 1609, when Galileo heard of a Dutch device that made distant objects appear closer, he was ready. The story is told in the article on his telescopic discoveries. Briefly, within a few months of hearing about the device, Galileo had built his own telescope, improved it to a magnification of about twenty or thirty, and turned it on the heavens. The results were the most consequential series of astronomical observations since the invention of the instrument.
The Sidereus Nuncius and Its Aftermath (1610–1611)
In March 1610, Galileo published his telescopic observations in a small, fast-printed book titled Sidereus Nuncius — the “Starry Messenger.” The book was dedicated to Cosimo II de’ Medici, the Grand Duke of Tuscany, and it announced the discovery of four new “Medicean stars,” the moons of Jupiter. It also reported the existence of mountains on the Moon, of an immense number of stars invisible to the naked eye, and of a strange appearance of Saturn (which Galileo interpreted, in his small telescope, as a triple planet). The book was an instant sensation. Six hundred copies were printed, and they sold out within a year. The telescopic discoveries were politically important, and Galileo was a master of political choreography. He named the moons of Jupiter the “Medicean stars” to honor the Medici, his hoped-for patrons, and he sent copies of the Sidereus Nuncius to leading astronomers and politicians across Europe. By the summer of 1610 he was negotiating for a position at the Medici court in Florence, and in the autumn he was back in Tuscany, appointed “First Philosopher and Mathematician” to the Grand Duke, with no teaching obligations and a much larger salary. The discoveries raised immediate objections. Some scholars refused to look through the telescope; others looked and saw nothing. The Aristotelian philosophers at the universities objected that the heavens could not be imperfect (mountains on the Moon?), and that no new celestial bodies could exist, since Aristotle had specified that there were exactly seven planets. The theologians objected that the new findings, if true, seemed to undermine the scriptural picture of a perfect, unchanging cosmos. Galileo defended his discoveries vigorously, in letters and in a series of essays he collected under the title Istoria e dimostrazioni intorno alle macchie solari (1613), the “Letters on Sunspots.”
The Controversy with the Jesuits (1611–1615)
The opposition to Galileo’s work was not, in the first instance, Catholic. It came from the Aristotelian establishment at the Italian universities, and it was, for several years, mostly an academic affair. The more serious opposition began in 1611, when Galileo visited Rome and demonstrated his telescope to a group of Jesuit mathematicians at the Collegio Romano. The Jesuits, who ran the most rigorous astronomy program in Catholic Europe, were initially impressed. They confirmed Galileo’s observations of the moons of Jupiter and the phases of Venus (the latter had been observed by Galileo, but he had not yet published the results). But within a few years, the Jesuit astronomers, including Christopher Scheiner and Orazio Grassi, would be Galileo’s most persistent and effective opponents. The dispute with the Jesuits was technical and bitter. Grassi, a mathematician and astronomer, argued in a 1619 treatise that the comets of 1618 were beyond the Moon, in the celestial realm, and that they moved in straight lines. Galileo, in his Il Saggiatore (1623), the “Assayer,” ridiculed Grassi’s arguments and the Aristotelian method generally, and proposed that the comets were an optical illusion caused by the bending of light in the vapors around the Earth. The book is one of the masterpieces of Italian scientific prose. It is also an exhibition of Galileo’s gift for invective. (Grassi’s response, under the pseudonym Lotario Sarsi, was equally pointed.) The other Jesuit target was the Copernican theory. By the early 1610s, Galileo had become an open advocate of heliocentrism, and his opponents were aware of the implications. The Jesuit theologians at the Collegio Romano had, on the whole, been cautious; they had confirmed the telescopic observations, but had not endorsed the cosmology. The Dominican friar Niccolò Lorini, in 1614, preached a sermon in Florence on the text “Ye men of Galilee, why stand ye gazing up into heaven?” — a clear reference to Galileo’s Copernicanism. Galileo’s friends were alarmed. The Pope, Paul V, was a stern and conservative man. The climate was not friendly.
The First Inquisition Crisis (1615–1616)
In 1615, a Carmelite friar named Paolo Antonio Foscarini published a tract arguing that the heliocentrism of Copernicus was not, properly interpreted, contrary to scripture. Foscarini’s tract was denounced to the Roman Inquisition. In December 1615, Galileo wrote his famous “Letter to the Grand Duchess Christina,” defending the view that scripture should be read in light of scientific demonstration, and arguing that the heliocentric theory was such a demonstration. The matter was referred to the Holy Office. In February 1616, a committee of eleven consultants, including several Jesuits, found that the heliocentrism of Copernicus was philosophically “foolish and absurd” and theologically “erroneous in faith.” The Index, on 5 March 1616, placed De Revolutionibus on the list of books forbidden until corrected. The Holy Office also summoned Galileo, on 25 February 1616, and gave him a private injunction, in the presence of witnesses, that he could “no longer hold, teach, or defend” the Copernican doctrine. Galileo accepted the injunction. He was not, in 1616, tried or condemned. He was warned. The exact terms of the warning are disputed, and the question of what precisely Galileo was told not to do would become a central issue in the trial of 1633. For the full story, see the article on Why Was Galileo Put on Trial?.
The Dialogue and the Second Trial (1632–1633)
The years after 1616 were a strange interlude. Galileo was, on the whole, careful in public. He returned to his work on motion, on the strength of materials, and on a number of smaller projects. In 1623 his old friend Maffeo Barberini was elected Pope, taking the name Urban VIII. Urban had been, before his election, a sympathetic reader of Galileo’s work, and he had even written a poem praising Galileo’s telescopic discoveries. In 1624 Galileo visited Rome and was received by the Pope in six long audiences. The Pope is reported to have told Galileo that he should write about Copernicanism, but only as a hypothesis, not as physical truth. Galileo took this as permission to proceed. In 1632, after seven years of writing, Galileo published the Dialogue Concerning the Two Chief World Systems, in Italian, in Florence. The book took the form of a four-day conversation among three characters: Salviati, a Copernican; Sagredo, an intelligent layman; and Simplicio, a defender of the Ptolemaic system. The book was a brilliant exposition of the case for the Copernican theory, written in a clear, vigorous Italian that reached an audience well beyond the universities. But it also contained a fatal flaw: the character of Simplicio, the loser of every argument, was widely understood to be a caricature of the Pope himself. (The name Simplicio was also the name of a sixth-century Aristotelian commentator; the pun may not have been intentional, but it did not help.) The book was read eagerly in Rome. The Pope was furious. Galileo was summoned to appear before the Inquisition in Rome. He arrived in February 1633, in poor health, and was interrogated over the course of several months. On 22 June 1633, he founded “vehemently suspect of heresy” and was forced to recite an abjuration, declaring that he “abjured, cursed, and detested” the Copernican doctrine. The famous words “and yet it moves” — E pur si muove — are probably apocryphal. Galileo was sentenced to formal imprisonment, immediately commuted to house arrest, first at the Medici embassy in Rome and then at his villa in Arcetri, near Florence, where he remained for the rest of his life. The details of the trial and abjuration are described in Why Was Galileo Put on Trial?.
The Two New Sciences and the Final Years (1634–1642)
In his last years, blind and ailing, Galileo produced the Discourses and Mathematical Demonstrations Concerning Two New Sciences (1638), often called the Two New Sciences and considered his masterwork of physics. The book was written as a conversation among the same three characters as the Dialogue, and it was published in Leiden in the Dutch Republic, because Galileo could not get a license to print it in Italy. The “two new sciences” were the science of strength of materials and the science of motion. The first, a study of why beams break under load, was the first systematic treatment of what be called engineering mechanics. The second was a careful mathematical treatment of uniformly accelerated motion, including the famous law that the distance fallen is proportional to the square of the time. The book also contained a discussion of the parabolic trajectory of projectiles and a defense of the principle that, in the absence of resistance, a body in motion continues in uniform motion. This principle — the law of inertia — was the precursor of Newton’s first law. Galileo’s contributions to physics are surveyed in How Did Galileo Contribute to Physics?. Galileo died in Arcetri on 8 January 1642, the year Newton was born. He was seventy-seven years old. The Pope refused to allow a public monument.
Galileo’s Place in the History of Science
The standard assessment of Galileo — that he was the first “modern scientist” — needs to be qualified. He was not, in the modern sense, a professional scientist. He was a university professor, a court mathematician, an instrument-maker, and a polemicist. He did not work within a community of professional peers, because none yet existed. The Accademia dei Lincei, of which Galileo was a member from 1611, was one of the first scientific societies, and it was a small and largely honorary body. The Royal Society, the Académie Royale, and the other “republics of letters” came later. What Galileo did establish was the prototype. He combined, in a way that no one had quite done before, careful observation, controlled experiment, mathematical analysis, and aggressive public defense. He was, in the famous phrase of the historian of science Alexandre Koyré, “the father of the experimental method” — though this, too, needs to be qualified, because Galileo did not run controlled experiments in the modern sense, and his most famous “experiments” (the inclined plane, the pendulum) were more often demonstrations than systematic studies. He was also a brilliant popularizer, who wrote in Italian and reached a wide audience, rather than in Latin and addressing only the learned. The combination of empirical detail, mathematical argument, and literary skill is the inheritance of modern science, and Galileo is its prototype.