Galileo Galilei

Galileo Galilei was born in Pisa on February 15, 1564, the first of six or seven children of Vincenzo Galilei and Giulia Ammannati ^[1]. His father was a lutenist and music theorist who had studied the mathematics of musical tuning and was known for challenging received authority in his own field, an attitude the son would inherit. The family belonged to the minor Florentine nobility but had little money, and Vincenzo supplemented his income through the wool trade.

The family moved to Florence around 1574, and Galileo received part of his early education at the monastery of Vallombrosa nearby ^[1]. He briefly considered a religious vocation, which his father discouraged. In 1581 he enrolled at the University of Pisa to study medicine, a career chosen for its earning potential rather than the boy's inclinations ^[2].

At Pisa the direction of his life changed. He attended lectures on Euclid given by Ostilio Ricci, a mathematician attached to the Tuscan court, and persuaded his reluctant father to let him abandon medicine for mathematics ^[2]. He left the university in 1585 without a degree, returning to Florence to teach privately and to begin studies of motion and of the centers of gravity of solids that earned him notice among Italian mathematicians.

In 1589 Galileo obtained the chair of mathematics at the University of Pisa, a poorly paid but respectable post ^[2]. There he composed an unpublished treatise, De motu, attacking Aristotelian accounts of falling bodies. The famous story that he dropped weights from the Leaning Tower of Pisa comes from a biography written by his disciple Vincenzo Viviani and cannot be confirmed from contemporary records, though it captures his method of testing claims against observation ^[3].

His father died in 1591, leaving Galileo responsible for his mother and siblings, including dowries for his sisters. Needing a better salary, he secured the mathematics chair at the University of Padua in 1592, within the tolerant Republic of Venice, where he remained for eighteen years ^[1]. He later described this period as the happiest of his life. At Padua he taught geometry, mechanics, and astronomy, ran a workshop producing instruments, and invented a geometric and military compass, a calculating device that sold well among engineers and officers ^[2].

During these years he worked out, largely in private, the mathematics of accelerated motion. Through experiments with balls rolling down inclined planes he established that falling bodies gain speed uniformly with time and that the distance covered grows with the square of the time elapsed, results he would publish only decades later ^[3]. He also concluded that projectiles follow parabolic paths, a finding with obvious military value.

Word of a Dutch spyglass reached Galileo in 1609. Working from descriptions alone, he ground his own lenses and built instruments of steadily increasing power, presenting an eight-powered version to the Venetian Senate, which rewarded him with a salary increase ^[1]. By the winter of 1609 to 1610 he had a telescope magnifying about twenty times, and he turned it toward the night sky.

What he saw overturned centuries of assumption. The Moon was not the smooth crystalline sphere of Aristotelian cosmology but a rough body with mountains and valleys, whose heights Galileo estimated from the shadows they cast ^[4]. The Milky Way resolved into countless individual stars. Most striking of all, in January 1610 he found four points of light moving around Jupiter: satellites of another planet, proof that not everything in the heavens circled the Earth ^[4]. He announced these discoveries in March 1610 in a short book, Sidereus Nuncius (Starry Messenger), naming the moons the Medicean Stars in honor of the Medici family ^[4].

The gambit worked. Cosimo II de' Medici appointed him chief mathematician and philosopher to the Grand Duke of Tuscany, freeing him from teaching duties ^[1]. Further observations followed: the phases of Venus, which showed that Venus orbits the Sun and could not be reconciled with the traditional Ptolemaic arrangement, the strange appendages of Saturn, and dark spots moving across the face of the Sun ^[2]. In 1611 he traveled to Rome, where the Jesuit astronomers of the Collegio Romano confirmed his observations and the Lincean Academy elected him a member.

Galileo's discoveries strengthened his conviction that Copernicus had been right in placing the Sun at the center of the planetary system. Critics answered that a moving Earth contradicted passages of Scripture. In a 1615 letter to the Grand Duchess Christina of Lorraine, Galileo argued that the Bible teaches how to go to heaven, not how the heavens go, and that scriptural interpretation should yield to demonstrated natural knowledge ^[5]. The argument did not persuade the authorities. In 1616 the Congregation of the Index declared the Copernican doctrine formally heretical in its Sun-centered claim, and Cardinal Robert Bellarmine warned Galileo not to hold or defend it ^[5].

For several years Galileo turned to other work, including a bitter dispute over the nature of comets that produced Il Saggiatore (The Assayer) in 1623, a polemical masterpiece in which he described the universe as a book written in the language of mathematics ^[2]. That same year his admirer Maffeo Barberini became Pope Urban VIII, and Galileo believed the climate had changed. With what he took to be papal permission to treat the rival world systems hypothetically, he spent years composing his Dialogue Concerning the Two Chief World Systems, published in Florence in 1632 ^[5].

The Dialogue presented arguments for and against the motion of the Earth through three speakers, but readers had little doubt where the author stood, and the defender of the old cosmology, named Simplicio, voiced an argument favored by the pope himself. Urban felt betrayed. The book was withdrawn, and Galileo, then sixty-eight and in poor health, was summoned to Rome. In 1633 the Inquisition tried him on suspicion of heresy for defending a condemned doctrine. Shown the instruments of torture but never subjected to them, he formally abjured the Copernican opinion on June 22, 1633, and was sentenced to imprisonment, commuted almost at once to house arrest ^[5]. The story that he muttered 'and yet it moves' after recanting is a later legend with no contemporary evidence ^[3].

Galileo never married. During his Padua years he formed a lasting relationship with Marina Gamba, a Venetian woman with whom he had three children: two daughters, Virginia and Livia, and a son, Vincenzo ^[1]. Because the daughters were considered unmarriageable owing to their illegitimate birth, Galileo placed both in the convent of San Matteo at Arcetri, near Florence, while they were still girls. Vincenzo was later legitimated and studied law.

The elder daughter, who took the religious name Suor Maria Celeste, became her father's closest confidante. More than one hundred of her letters to him survive, full of practical care, requests for supplies for the poor convent, and steady affection during his trial and its aftermath ^[6]. Her death in April 1634, only months after Galileo returned from Rome in disgrace, was a blow from which friends said he never fully recovered ^[6].

Contemporaries described Galileo as sociable, quick in argument, and fond of good wine, music, and literature. He knew Dante and Ariosto well, lectured on the geography of Dante's Inferno as a young man, and wrote with a directness and wit unusual in the learned Latin culture of his day, publishing much of his work in Italian to reach readers beyond the universities ^[2].

After the trial Galileo lived under house arrest, first briefly with the Archbishop of Siena and then, from late 1633, at his villa Il Gioiello in Arcetri, in the hills above Florence and close to his daughters' convent ^[1]. Visitors required permission, and he was forbidden to publish. He nevertheless returned to the physics of his youth and completed his most enduring scientific book.

The manuscript of Discourses and Mathematical Demonstrations Relating to Two New Sciences was smuggled out of Italy and printed by Louis Elzevir at Leiden in the Netherlands in 1638 ^[3]. In it Galileo set out his mature results on the strength of materials and on motion: the law of falling bodies, the parabolic trajectory of projectiles, and a treatment of uniform and accelerated motion that gave later physicists, including Isaac Newton, an essential foundation ^[3].

By the time the book appeared Galileo was completely blind, his sight destroyed after years of eye trouble. He continued to work by dictation, assisted by his son and by young disciples, among them Vincenzo Viviani and Evangelista Torricelli, who lived with him in his final months ^[2]. He also proposed using the eclipses of Jupiter's moons as a celestial clock for finding longitude at sea, and late in life he sketched an application of the pendulum to regulating clocks. Galileo died at Arcetri on January 8, 1642, at the age of seventy-seven ^[1].

Any Galileo Galilei biography must reckon with two intertwined stories: the science and the trial. On the scientific side his influence is hard to overstate. He insisted that claims about nature be tested by measurement and experiment, expressed in mathematics, and judged against observation rather than ancient authority. His work on motion fed directly into Newton's mechanics, and his telescopic discoveries opened astronomy to instruments rather than the unaided eye ^[3]. For these reasons he is often described as a founder, some say the father, of modern observational science.

The trial made him a symbol. For later generations the confrontation between Galileo and the Inquisition became shorthand for the tension between free inquiry and institutional authority, a reading that historians have complicated but never dislodged. The Church itself revisited the case: the Dialogue came off the Index of Forbidden Books in 1835, and in 1992 Pope John Paul II acknowledged, after a thirteen-year study commission, that the theologians of Galileo's day had erred in condemning him ^[7].

Asking who was Galileo Galilei today brings answers from far beyond the history of science. NASA named its Jupiter orbiter Galileo, and the spacecraft studied the very moons he discovered between 1995 and 2003 ^[8]. Europe's satellite navigation system also bears his name. Three fingers and a vertebra, removed when his remains were moved to a monumental tomb in the Florentine basilica of Santa Croce in 1737, are preserved as relics, two of the fingers at the Museo Galileo in Florence ^[9]. Among the most repeated Galileo Galilei facts is that the 1633 sentence confined a man whose measurements of rolling balls and moving points of light still frame how science is done, which remains the most durable of all Galileo Galilei achievements.