Galileo Galilei

Galileo Galilei was one of the central figures of the Scientific Revolution, the period in the sixteenth and seventeenth centuries when European thinkers built new ways of understanding nature. He is remembered for three big achievements. He made some of the first and most important observations of the sky with a telescope. He created a mathematical science of motion that later physicists, including Isaac Newton, built upon. And he argued publicly that the Earth moves around the Sun, which led to his famous trial.

The Stanford Encyclopedia of Philosophy notes that from the seventeenth century onward many people have called Galileo the “father” of modern science. It also warns that Galileo has been used as a symbol for many different causes—as a hero of experiment, of mathematics, or of reason against religion—and that historians try to understand the real person behind these images.

He came from a modest background. His father, Vincenzo Galilei, was a court musician and a writer on music theory. Galileo left the University of Pisa without a degree, yet by the time he died he was one of the most famous people in Europe.

Galileo was born in Pisa on 15 February 1564. His family moved to Florence in 1572. As a boy he studied for a time with monks at Vallombrosa and even thought about becoming a monk himself. In 1580 his father sent him to study medicine at the University of Pisa, but Galileo became fascinated by mathematics instead, studying with Ostilio Ricci, a teacher linked to the Tuscan court.

After leaving university he tutored mathematics privately in Florence and Siena. In 1586 he wrote a short work on a hydrostatic balance he had invented, which first brought him to the attention of scholars. With help from the Jesuit mathematician Christoph Clavius and the engineer Guidobaldo del Monte, he became lecturer in mathematics at Pisa in 1589. In 1592 he moved to a better-paid chair at the University of Padua, in the Republic of Venice, where he stayed for eighteen years.

In Padua he taught, took private students, and ran a workshop where hired craftsmen made instruments he designed, such as a calculating “geometric and military compass.” He had a long relationship with Marina Gamba, with whom he had two daughters and a son. The daughters later became nuns in a convent at Arcetri; the elder, Virginia, took the name Maria Celeste and wrote him many letters.

In 1609 Galileo heard about a spyglass recently invented in the Netherlands. He worked out how it must function and made his own. While the Dutch instruments magnified two or three times, Galileo soon built one that magnified about twenty times. By late 1609 he was using it to study the sky—not the first person to do so, since the Englishman Thomas Harriot had looked at the Moon a few months earlier, but the first to publish a detailed study.

On 7 January 1610 he saw three small “stars” in a line near Jupiter; over the following nights he found four, moving back and forth. He concluded that they were moons orbiting Jupiter. (The German astronomer Simon Marius observed them independently at almost the same time, and the names we use today—Io, Europa, Ganymede and Callisto—come from him.) Galileo also saw that the Moon has mountains and valleys and that the Milky Way is made of countless stars.

He rushed these findings into print in a short Latin book, Sidereus Nuncius (The Starry Messenger), published on 13 March 1610, with more than seventy drawings. He named Jupiter’s moons the “Medicean stars” after the Medici rulers of Tuscany. The gesture worked: later that year he became Chief Mathematician and Philosopher to the Grand Duke of Tuscany and moved to Florence. In 1611 he was welcomed in Rome by the Jesuit astronomers of the Collegio Romano, who confirmed his observations, and he joined the Accademia dei Lincei, one of the first scientific societies.

Long before the telescope, Galileo was studying motion. The traditional physics of Aristotle said that heavy things fall because it is their nature to go down, and that light things such as fire naturally rise. Galileo came to believe that there is only one principle—heaviness—and that things rise only when heavier things push them up, as in water.

He used inclined planes to slow down falling motion so that it could be measured, and he studied pendulums. Around 1601 he recognized that a pendulum of a given length takes the same time for each swing. From his experiments he found the law of free fall: a body falling from rest covers a distance proportional to the square of the time. After two seconds it has fallen four times as far as after one second.

A famous story, told by his pupil Vincenzo Viviani, says Galileo dropped balls of different weights from the Leaning Tower of Pisa to show that they fall together. Most historians think this was at most a thought experiment or a demonstration that may not have happened as described, though a few, such as Stillman Drake, have argued it did take place. What is certain is that in Two New Sciences Galileo’s spokesman states that in a medium without resistance all bodies would fall with the same speed.

Galileo had privately told Johannes Kepler in 1597 that he believed in the Copernican system, but he said so publicly only in his Letters on Sunspots (1613). His telescope discoveries weakened the old picture of a perfect, unchanging heaven. The Moon looked like the Earth, Jupiter had moons as the Earth does, sunspots showed that the Sun changes and rotates, and later in 1610 he observed that Venus shows a full set of phases like the Moon—which meant Venus circles the Sun, something impossible in Ptolemy’s arrangement.

These observations did not strictly prove that the Earth moves: Tycho Brahe’s system, in which the planets circle the Sun while the Sun circles the Earth, could also explain the phases of Venus. Galileo’s most important contribution was therefore in physics. Opponents argued that if the Earth moved, a stone dropped from a tower would land far behind it, and birds and clouds would be left behind. Galileo answered with what we now call inertia and the relativity of motion: bodies on the Earth share its motion and keep it, just as objects on a smoothly sailing ship move with the ship, so experiments on Earth cannot reveal its motion.

Galileo also offered what he thought was a positive proof: the tides, which he explained as sloshing caused by the combination of the Earth’s daily rotation and yearly orbit. This argument was wrong. It predicted one main tide a day at fixed times, and Galileo dismissed the Moon’s role, which Kepler and others had already suspected. The Stanford Encyclopedia notes that the weakness of this “proof” did not help his case.

In December 1613 the Grand Duchess Christina of Tuscany raised biblical objections to the Earth’s motion with his student Benedetto Castelli. Galileo replied in a Letter to Castelli, which he expanded in 1615 into the Letter to the Grand Duchess Christina. He argued that Scripture and nature both come from God and cannot truly contradict each other; the Bible speaks in ways ordinary people can understand and teaches faith and morals, so when a truth about nature is demonstrated, interpreters should adjust their reading of Scripture.

Cardinal Robert Bellarmine, a leading theologian of the Roman Inquisition, took a different view. He accepted that Scripture could be reinterpreted if the Earth’s motion were truly demonstrated, but he did not think astronomical models could provide such proof, and he regarded them as calculating devices rather than descriptions of reality. In 1615 he wrote that the Copernican system could not be defended without “a true physical demonstration.”

In February 1616 theologians advising the Inquisition judged the Sun-centred view to be contrary to Scripture. In March 1616 the Congregation of the Index suspended Copernicus’s book “until corrected.” Galileo was summoned to Bellarmine and admonished not to hold or defend the doctrine. Whether he also received a stricter written injunction “not to hold, teach, or defend it in any way whatever” is one of the most disputed questions of the whole affair, because the document that says so would be used against him in 1633.

In 1623 Galileo published The Assayer, a polemic about comets that contains his famous statement that the book of nature “is written in the language of mathematics.” The same year his friend and admirer Cardinal Maffeo Barberini became Pope Urban VIII. Encouraged, Galileo began a book comparing the Ptolemaic and Copernican systems. The pope asked him to present arguments on both sides and not to advocate heliocentrism.

The Dialogue Concerning the Two Chief World Systems was printed in Florence in 1632. It is written in lively Italian as a conversation over four days between Salviati, who speaks for the Copernican view, Sagredo, an intelligent layman, and Simplicio, a defender of Aristotle whose name also suggests “simpleton.” Although it claimed to be neutral, it clearly favoured Copernicus, and some readers thought Simplicio mocked the pope’s own arguments. The Inquisition soon banned its sale and summoned Galileo to Rome.

After hearings from April 1633, on 22 June Galileo was made to kneel in the convent of Santa Maria sopra Minerva while his sentence was read. He was found “vehemently suspect of heresy” for holding that the Sun is at the centre and the Earth moves, and he was made to abjure—to formally reject and curse—these views. His sentence of imprisonment was commuted to house arrest, first with the Archbishop of Siena and then, from December 1633, at his villa at Arcetri. The legend that he muttered “Eppur si muove” (“And yet it moves”) has no evidence from the time.

At Arcetri Galileo suffered a heavy blow when his daughter Maria Celeste died on 2 April 1634. He nevertheless completed his last and, many scientists think, most important book: Discourses and Mathematical Demonstrations Relating to Two New Sciences. The two sciences are the strength of materials and the science of local motion—including uniformly accelerated motion and the parabolic path of projectiles. The manuscript had to be taken out of Italy and was published by the Elzevir firm in Leiden, in the Dutch Republic, in 1638.

He went completely blind in 1638. Even so, he designed an escapement for a pendulum clock, although the first working pendulum clock was built by Christiaan Huygens in the 1650s. His young assistant and pupil Vincenzo Viviani stayed with him and later collected his papers. He died at Arcetri on 8 January 1642.

Because he had been condemned, the grand duke’s plan for a grand tomb in the Basilica of Santa Croce in Florence was dropped after protests from Rome. His remains were moved to a monument in the main body of the church only in 1737. In 1741 Pope Benedict XIV allowed the publication of his complete works, with a mildly censored Dialogue; the Dialogue itself remained on the Index until 1835.

The Galileo affair is often told as the classic example of a “war” between science and religion. Historians today see it as more complicated. Galileo was a sincere Catholic and a friend of the pope; many churchmen, including Jesuit astronomers, had admired his work; and his opponents included university philosophers as well as theologians. Political circumstances mattered: the Counter-Reformation, the Thirty Years’ War, and tensions inside Urban VIII’s court, as well as Galileo’s dependence on the Medici. Mario Biagioli has studied how court patronage shaped Galileo’s career and style.

The legal questions are also debated. In 1616 an internal commission had declared the Copernican thesis heretical, but this was not proclaimed publicly; the Church’s first public statement that Copernicanism was heretical came in Galileo’s own sentence. The 1616 note recording a strict injunction, found in the files in 1633, made Galileo look like a repeat offender, and scholars disagree about whether it reflects what actually happened. Pietro Redondi proposed in 1983 that the Copernican charge covered up a more dangerous accusation of atomism touching on the Eucharist; this thesis has found little support.

The Church’s later reassessment was gradual. The general ban on heliocentric books was lifted in 1758, and Copernicus’s and Galileo’s books were dropped from the Index in 1835. In 1979 Pope John Paul II called for a fresh study of the case; a commission worked from 1981, and on 31 October 1992 the pope acknowledged that Galileo’s judges had erred, saying that the theologians had failed to distinguish between the Bible and its interpretation. Some historians found the 1992 statement vague and incomplete.

What exactly made Galileo’s science new is itself a long-running debate. Alexandre Koyré and others read him as a mathematical Platonist who reasoned from ideal geometry and thought experiments. Thomas Settle and other historians have stressed that real experiments and measurement mattered to him, and Stillman Drake’s Galileo at Work remains a classic account of his scientific practice. Others connect him to the Archimedean tradition of simple machines, to the practical knowledge of engineers and artisans, or to late-scholastic ideas about demonstration. The philosopher Paul Feyerabend provocatively argued that Galileo followed no fixed method at all, and other scholars have studied his rhetoric as a key to his success.

The Stanford Encyclopedia stresses a unity running through his career: an effort to replace Aristotle’s separate heavenly and earthly elements with a single kind of matter whose motions can be described mathematically. His early studies of balances, floating bodies and inclined planes, his telescope discoveries showing that heaven and Earth are alike, and the Two New Sciences all belong to this project. Notably, his principle of inertia was not yet Newton’s: Galileo often thought of undisturbed motion as circular around the Earth, which is why scholars speak of “circular inertia.”

The sources for Galileo are unusually rich: his printed books, manuscripts and a large correspondence, including Maria Celeste’s letters, survive, many of them preserved after his death by Viviani. The trial documents were published and studied in detail, notably by Maurice Finocchiaro. Viviani’s early biography is valuable but idealizing, and some of the best-known anecdotes—the swinging lamp in Pisa Cathedral, the Tower of Pisa, “Eppur si muove”—come from later tradition rather than contemporary evidence.