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Galileo Galilei
1564 - 1642

The Italian scientist Galileo Galilei is renowned for his epoch-making contributions to astronomy, physics, and scientific philosophy.

Galileo was born in Pisa on Feb. 15, 1564, the first child of Vincenzio Galilei, a merchant and musician and an abrasive champion of advanced musical theories of the day. The family moved to Florence in 1574, and that year Galileo started his formal education in the nearby monastery of Vallombrosa. Seven years later he matriculated as a student of medicine at the University of Pisa.

In 1583, while Galileo was at home on vacation, he began to study mathematics and the physical sciences. His zeal astonished Ostilio Ricci, a family friend and professor at the Academy of Design. Ricci was a student of Nicolò Tartaglia, the famed algebraist and translator into Latin of several of Archimedes' works. Galileo's life-long admiration for Archimedes started, therefore, as his scientific studies got under way.

Galileo's new interest brought to an end his medical studies, but in Pisa at that time there was only one notable science teacher, Francisco Buonamico, and he was an Aristotelian. Galileo seems, however, to have been an eager disciple of his, as shown by Galileo's Juvenilia, dating from 1584, mostly paraphrases of Aristotelian physics and cosmology. Because of financial difficulties Galileo had to leave the University of Pisa in 1585 before he got his degree.

Early Work

Back in Florence, Galileo spent 3 years vainly searching for a suitable teaching position. He was more successful in furthering his grasp of mathematics and physics. He produced two treatises which, although circulated in manuscript form only, made his name well known. One was La bilancetta (The Little Balance), describing the hydrostatic principles of balancing; the other was a study on the center of gravity of various solids. These topics, obviously demanding a geometrical approach, were not the only evidence of his devotion to geometry and Archimedes. In a lecture given in 1588 before the Florentine Academy on the topography of Dante's Inferno, Galileo seized on details that readily lent themselves to a display of his prowess in geometry. He showed himself a perfect master both of the poet's text and of the incisiveness and sweep of geometrical lore.

Galileo's rising reputation as a mathematician and natural philosopher (physicist) gained him a teaching post at the University of Pisa in 1589. The 3 years he spent there are memorable for two things. First, he became exposed through reading a work of Giovanni Battista Benedetti to the "Parisian tradition" of physics, which originated during the 14th century with the speculations of Jean Buridan and Nicole Oresme at the University of Paris. This meant the breakaway point in Galileo's thought from Aristotelian physics and the start of his preoccupation with a truly satisfactory formulation of the impetus theory. Second, right at the beginning of his academic career, he showed himself an eager participant in disputes and controversies. With biting sarcasm he lampooned the custom of wearing academic gowns. The most he was willing to condone was the use of ordinary clothes, but only after pointing out that the best thing was to go naked.

The death of Galileo's father in 1591 put on his shoulders the care of his mother, brothers, and sisters. He had to look for a better position, which he found in 1592 at the University of Padua, part of the Venetian Republic. The 18 years he spent there were, according to his own admission, the happiest of his life. He often visited Venice and made many influential friends, among them Giovanfrancesco Sagredo, whom he later immortalized in the Dialogue as the representative of judiciousness and good sense.

In 1604 Galileo publicly declared that he was a Copernican. In three public lectures given in Venice, before an overflow audience, he argued that the new star which appeared earlier that year was major evidence in support of the doctrine of Copernicus. (Actually the new star merely proved that there was something seriously wrong with the Aristotelian doctrine of the heavens.) More important was a letter Galileo wrote that year to Father Paolo Sarpi, in which he stated that "the distances covered in natural motion are proportional to the squares of the number of time intervals, and therefore, the distances covered in equal times are as the odd numbers beginning from one." By natural motion, Galileo meant the unimpeded fall of a body, and what he proposed was the law of free fall, later written as s = 1/2 (gt2), where s is distance, t is time, and g is the acceleration due to gravity at sea level.

In 1606 came the publication of The Operations of the Geometrical and Military Compass, which reveals the experimentalist and craftsman in Galileo. In this booklet he went overboard in defending his originality against charges from rather insignificant sources. It was craftsmanship, not theorizing, which put the crowning touch on his stay in Padua. In mid-1609 he learned about the success of some Dutch spectacle makers in combining lenses into what later came to be called a telescope. He feverishly set to work, and on August 25 he presented to the Venetian Senate a telescope as his own invention. The success was tremendous. He obtained a lifelong contract at the University of Padua, but he also stirred up just resentment when it was learned that he was not the original inventor.

Astronomical Works

Galileo's success in making a workable and sufficiently powerful telescope with a magnifying power of about 40 was due to intuition rather than to rigorous reasoning in optics. It was also the intuitive stroke of a genius that made him turn the telescope toward the sky sometime in the fall of 1609, a feat which a dozen other people could very well have done during the previous 4 to 5 years. Science had few luckier moments. Within a few months he gathered astonishing evidence about mountains on the moon, about moons circling Jupiter, and about an incredibly large number of stars, especially in the belt of the Milky Way. On March 12, 1610, all these sensational items were printed in Venice under the title Sidereus nuncius (The Starry Messenger), a booklet which took the world of science by storm. The view of the heavens drastically changed, and so did Galileo's life.

Historians agree that Galileo's decision to secure for himself the position of court mathematician in Florence at the court of Cosimo II (the job also included the casting of horoscopes for his princely patron) reveals a heavy strain of selfishness in his character. He wanted nothing, not even a modest amount of teaching, to impede him in pursuing his ambition to become the founder of new physics and new astronomy. In 1610 he left behind in Padua his common-law wife, Marina Gamba, and his young son, Vincenzio, and placed his two daughters, aged 12 and 13, in the convent of S. Matteo in Arcetri. The older, Sister Maria Celeste as nun, was later a great comfort to her father.

Galileo's move to Florence turned out to be highly unwise, as events soon showed. In the beginning, however, everything was pure bliss. He made a triumphal visit to Rome in 1611. The next year saw the publication of his Discourse on Bodies in Water. There he disclosed his discovery of the phases of Venus (a most important proof of the truth of the Copernican theory), but the work was also the source of heated controversies. In 1613 Galileo published his observations of sunspots, which embroiled him for many years in bitter disputes with the German Jesuit Christopher Scheiner of the University of Ingolstadt, whose observations of sunspots had already been published in January 1612 under the pseudonym Apelles.

First Condemnation

But Galileo's real aim was to make a sweeping account of the Copernican universe and of the new physics it necessitated. A major obstacle was the generally shared, though officially never sanctioned, belief that the biblical revelation imposed geocentrism in general and the motionlessness of the earth in particular. To counter the scriptural difficulties, he waded deep into theology. With the help of some enlightened ecclesiastics, such as Monsignor Piero Dini and Father Benedetto Castelli, a Benedictine from Monte Cassino and his best scientific pupil, Galileo produced essays in the form of letters, which now rank among the best writings of biblical theology of those times. As the letters (the longest one was addressed to Grand Duchess Christina of Tuscany) circulated widely, a confrontation with the Church authorities became inevitable. The disciplinary instruction handed down in 1616 by Cardinal Robert Bellarmine forbade Galileo to "hold, teach and defend in any manner whatsoever, in words or in print" the Copernican doctrine of the motion of the earth.

Galileo knew, of course, both the force and the limits of what in substance was a disciplinary measure. It could be reversed, and he eagerly looked for any evidence indicating precisely that. He obeyed partly out of prudence, partly because he remained to the end a devout and loyal Catholic. Although his yearning for fame was powerful, there can be no doubt about the sincerity of his often-voiced claim that by his advocacy of Copernicanism he wanted to serve the long-range interest of the Church in a world of science. The first favorable sign came in 1620, when Cardinal Maffeo Barberini composed a poem in honor of Galileo. Three years later the cardinal became Pope Urban VIII. How encouraged Galileo must have felt can be seen from the fact that he dedicated to the new pope his freshly composed Assayer, one of the finest pieces of polemics ever produced in the philosophy of science.

The next year Galileo had six audiences with Urban VIII, who promised a pension for Galileo's son, Vincenzio, but gave Galileo no firm assurance about changing the injunction of 1616. But before departing for Florence, Galileo was informed that the Pope had remarked that "the Holy Church had never, and would never, condemn it [Copernicanism] as heretical but only as rash, though there was no danger that anyone would ever demonstrate it to be necessarily true." This was more than enough to give Galileo the necessary encouragement to go ahead with the great undertaking of his life.

The Dialogue

Galileo spent 6 years writing his Dialogue concerning the Two Chief World Systems. When the final manuscript copy was being made in March 1630, Father Castelli dispatched the news to Galileo that Urban VIII insisted in a private conversation with him that, had he been the pope in 1616, the censuring of Copernicanism would have never taken place. Galileo also learned about the benevolent attitude of the Pope's official theologian, Father Nicolò Riccardi, Master of the Sacred Palace. The book was published with ecclesiastical approbation on Feb. 21, 1632.

Its contents are easy to summarize, as its four main topics are discussed in dialogue form on four consecutive days. Of the three interlocutors, Simplicius represented Aristotle, Salviati was Galileo's spokesman, and Sagredo played the role of the judicious arbiter leaning heavily toward Galileo. The First Day is devoted to the criticism of the alleged perfection of the universe and especially of its superlunary region, as claimed by Aristotle. Here Galileo made ample use of his discovery of the "imperfections" of the moon, namely, of its rugged surface revealed by the telescope. The Second Day is a discussion of the advantages of the rotation of the earth on its axis for the explanation of various celestial phenomena. During the Third Day the orbital motion of the earth around the sun is debated, the principal issues being the parallax of stars and the undisturbed state of affairs on the surface of the earth in spite of its double motion. In this connection Galileo gave the most detailed account of his ideas of the relativity of motion and of the inertial motion. Bafflingly enough, he came to contradict his best-posited principles when he offered during the Fourth Day the tides as proof of the earth's twofold motion. The inconsistencies and arbitrariness that characterize his discourse there could not help undermine an otherwise magnificent effort presented in a most attractive style.

Second Condemnation

The Dialogue certainly proved that for all his rhetorical provisos Galileo held, taught, and defended the doctrine of Copernicus. It did not help Galileo either that he put into the mouth of the discredited Simplicius an argument which was a favorite with Urban VIII. Galileo was summoned to Rome to appear before the Inquisition. Legally speaking, his prosecutors were justified. Galileo did not speak the truth when he claimed before his judges that he did not hold Copernicanism since the precept was given to him in 1616 to abandon it. The justices had their point, but it was the letter of the law, not its spirit, that they vindicated. More importantly, they miscarried justice, aborted philosophical truth, and gravely compromised sound theology. In that misguided defense of orthodoxy the only sad solace for Galileo's supporters consisted in the fact that the highest authority of the Church did not become implicated, as the Catholic René Descartes, the Protestant Gottfried Wilhelm von Leibniz, and others were quick to point out during the coming decades.

The proceedings dragged on from the fall of 1632 to the summer of 1633. During that time Galileo was allowed to stay at the home of the Florentine ambassador in Rome and was detained by the Holy Office only from June 21, the day preceding his abjuration, until the end of the month. He was never subjected to physical coercion. However, he had to inflict the supreme torture upon himself by abjuring the doctrine that the earth moved. One hundred years later a writer with vivid imagination dramatized the event by claiming that following his abjuration Galileo muttered the words "Eppur si muove (And yet it does move)."

On his way back to Florence, Galileo enjoyed the hospitality of the archbishop of Siena for some 5 months and then received permission in December to live in his own villa at Arcetri. He was not supposed to have any visitors, but this injunction was not obeyed. Nor was ecclesiastical prohibition a serious obstacle to the printing of his works outside Italy. In 1634 Father Marin Mersenne published in French translation a manuscript of Galileo on mechanics composed during his Paduan period. In Holland the Elzeviers brought out his Dialogue in Latin in 1635 and shortly afterward his great theological letter to Grand Duchess Christina. But the most important event in this connection took place in 1638, when Galileo's Two New Sciences saw print in Leiden.

Two New Sciences

The first draft of the work went back to Galileo's professorship at Padua. But cosmology replaced pure physics as the center of his attention until 1633. His condemnation was in a sense a gain for physics. He had no sooner regained his composure in Siena than he was at work preparing for publication old, long-neglected manuscripts. The Two New Sciences, like the Dialogue, is in the dialogue form and the discussions are divided into Four Days. The First Day is largely taken up with the mechanical resistance of materials, with ample allowance for speculations on the atomic constitution of matter. There are also long discussions on the question of vacuum and on the isochronism of the vibrations of pendulums. During the Second Day all these and other topics, among them the properties of levers, are discussed in a strictly mathematical manner, in an almost positivist spirit, with no attention being given to "underlying causes." Equally "dry" and mathematical is the analysis of uniform and accelerated motion during the Third Day, and the same holds true of the topic of the Fourth Day, the analysis of projectile motion. There Galileo proved that the longest shot occurred when the cannon was set at an angle of 45 degrees. He arrived at this result by recognizing that the motions of the cannonball in the vertical and in the horizontal directions "can combine without changing, disturbing or impeding each other" into a parabolic path.

Galileo found the justification for such a geometrical analysis of motion partly because it led to a striking correspondence with factual data. More importantly, he believed that the universe was structured along the patterns of geometry. In 1604 he could have had experimental verification of the law of free fall, which he derived on a purely theoretical basis, but it is not known that he sought at that time such an experimental proof. He was a Christian Platonist as far as scientific method was concerned. This is why he praised Copernicus repeatedly in the Dialogue for his belief in the voice of reason, although it contradicted sense experience. Such a faith rested on the conviction that the world was a product of a personal, rational Creator who disposed everything according to weight, measure, and number.

This biblically inspired faith was stated by Galileo most eloquently in the closing pages of the First Day of the Dialogue. There he described the human mind as the most excellent product of the Creator, precisely because it could recognize mathematical truths. This faith is possibly the most precious bequest of the great Florentine, who spent his last years partially blind. His disciple Vincenzio Viviani sensed this well as he described the last hours of Galileo: "On the night of Jan. 8, 1642, with philosophical and Christian firmness he rendered up his soul to its Creator, sending it, as he liked to believe, to enjoy and to watch from a closer vantage point those eternal and immutable marvels which he, by means of a fragile device, had brought closer to our mortal eyes with such eagerness and impatience."






Galileo Galilei, (February 15, 1564 - January 8, 1642) was an Italian philosopher, physicist and astronomer. He has been called the father of modern astronomy, and along with Bacon was one of the pioneers of the scientific method. He was born in Pisa.

Scientific achievements

Galileo was one of the first people to use a telescope to observe the sky. He acquired a 10x telescope and promptly made an improved 20x one. He published his initial telescopic observations in March 1610 in a short treatise entitled Sidereus Nuncius (Sidereal Messenger). Galileo discovered the four largest satellites of Jupiter. He also observed that the planet Venus exhibited a full set of "phases" like the Moon. Both of these discoveries lent support to the heliocentric model of the solar system developed by Copernicus. Galileo was able to see lunar mountains, sunspots, and a "mass of innumerable stars".

He was the first westerner to report sunspots (there is evidence that Chinese astronomers had already observed them). He also argued from the occultation of stars (visible only through a telescope) that the Moon is not a perfect sphere.

His experimental work in dynamics paved the way for Kepler's and Newton's laws of motion, and he is often credited with being one of the first scientists to fully exploit the experimental method and to insist on a mathematical description of the laws of nature. His study of balls rolling down inclined planes convinced him that falling objects are accelerated independent of their mass, and that objects retain their velocity unless a force acts on them.

Galileo also described that a pendulum's swings always take the same amount of time, independent of the amplitude, a discovery which made later precise clocks possible.

In the early 1600s, Galileo and an assistant tried to measure the speed of light. They stood on different hilltops, each holding a shuttered lantern. Galileo would open the shutter; as soon as his assistant saw the flash, he would open his lantern. Galileo concluded that the speed of light is too high to be measured by human reactions.

Galileo wrote several long books which were circulated widely, at least outside of Italy, although several of Galileo's inventions exist today only in his notes and drawings. He created sketches of imaginary devices such as a candle and mirror combination to reflect light through an entire home, an automatic tomato picker, a pocket comb that doubled as an eating utensil, and what appeared to be a crude form of ballpoint pen.

Church Controversy

Galileo was a devout Catholic, yet his writings on the Copernican model of the Universe (incorporating a heliocentric, or sun-centered solar system) disturbed the Church, which held to an Earth-centered theory of the Universe. Church fathers argued extensively that any other view would contradict scripture.

The geocentric model was generally accepted at the time not only for scriptural reasons. By the time of the controversy, the Catholic Church had in fact abandoned the Ptolemaic model for the Tychonian model in which the Earth was at the center of the Universe, the Sun revolved around the Earth and the other planets revolved around the Sun. This model is geometrically identical to the Copernican model and has the extra advantage that it predicts no parallax of the stars. Yet, Galilei's arguments were most fiercely fought on the religious level. The late-nineteenth- and early twentieth century historian Andrew Dickson White wrote from an anti-clerical perspective:

The war became more and more bitter. The Dominican Father Caccini preached a sermon from the text, "Ye men of Galilee, why stand ye gazing up into heaven?" and this wretched pun upon the great astronomer's name ushered in sharper weapons; for, before Caccini ended, he insisted that "geometry is of the devil," and that "mathematicians should be banished as the authors of all heresies." The Church authorities gave Caccini promotion. Father Lorini proved that Galileo's doctrine was not only heretical but "atheistic," and besought the Inquisition to intervene. The Bishop of Fiesole screamed in rage against the Copernican system, publicly insulted Galileo, and denounced him to the Grand-Duke. The Archbishop of Pisa secretly sought to entrap Galileo and deliver him to the Inquisition at Rome. The Archbishop of Florence solemnly condemned the new doctrines as unscriptural; and Paul V, while petting Galileo, and inviting him as the greatest astronomer of the world to visit Rome, was secretly moving the Archbishop of Pisa to pick up evidence against the astronomer.


But by far the most terrible champion who now appeared was Cardinal Bellarmin, one of the greatest theologians the world has known. He was earnest, sincere, and learned, but insisted on making science conform to Scripture. The weapons which men of Bellarmin's stamp used were purely theological. They held up before the world the dreadful consequences which must result to Christian theology were the heavenly bodies proved to revolve about the Sun and not about the Earth. Their most tremendous dogmatic engine was the statement that "his pretended discovery vitiates the whole Christian plan of salvation." Father Lecazre declared "it casts suspicion on the doctrine of the incarnation." Others declared, "It upsets the whole basis of theology. If the Earth is a planet, and only one among several planets, it can not be that any such great things have been done specially for it as the Christian doctrine teaches. If there are other planets, since God makes nothing in vain, they must be inhabited; but how can their inhabitants be descended from Adam? How can they trace back their origin to Noah's ark? How can they have been redeemed by the Saviour?" Nor was this argument confined to the theologians of the Roman Church; Melanchthon, Protestant as he was, had already used it in his attacks on Copernicus and his school. In 1616 the Inquisition warned Galileo not to hold or defend the hypothesis asserted in Copernicus' On the Revolutions, though it has been debated whether he was admonished not to 'teach in any way' the heliocentric theory.

Despite his continued insistence that his work in the area was purely theoretical, despite his strict following of the church protocol for publication of works (which required prior examination by church censors and subsequent permission), and despite his close friendship with Maffeo Barberini who later became Pope Urban VIII and presided throughout the ordeal, Galileo was forced to recant his views repeatedly and was put under life-long house arrest (1633-1642).

The Inquisition had rejected earlier pleas by Galilei to postpone or relocate the trial because of his ill health. At a meeting presided by Pope Urban VIII, the Inquisition decided to notify Galilei that he either had to come to Rome or that he would be arrested and brought there in chains. Galileo arrived in Rome for his trial before the Inquisition on February 13, 1633. After two weeks in quarantine, Galilei was detained at the comfortable residence of the Tuscan ambassador, as a favor to the influential Grand Duke Ferdinand II de' Medici. In April 1633 he was formally interrogated by the Inquisition. He was not imprisoned in a dungeon cell, but detained in a room in the offices of the Inquisition for 22 days.

On June 22, 1633, the Roman Inquisition started its trial against Galilei, who was then 69 years old and pleaded for mercy, pointing to his "regrettable state of physical unwellness". Threatening him with torture, imprisonment and death on the stake, the show trial forced Galileo to "abjure, curse and detest" his work and to promise to denounce others who held his prior viewpoint. Galileo did everything the church requested him to do. (The idea that he muttered Eppur si muove! - "But it moves anyway!" - is a legend.) That the threat of torture and death Galileo was facing was a real one had been proven by the church in the earlier trial against Giordano Bruno, who was burned at the stake in 1600 for holding a naturalistic view of the Universe.

Galileo was sentenced to prison but because of his advanced age was allowed to serve his term under house arrest at his villas in Arcetri and Florence. Because of a painful hernia, he requested permission to consult physicians in Florence, which was denied by Rome, warning that further such requests would lead to imprisonment. Under arrest, he was forced to recite penitentiary psalms regularly, and his social contacts were highly restricted, but he was allowed to continue his less controversial research and publish under strict rules of censorship. He went totally blind in 1638 (his petition to the Inquisition to be released was rejected, but he was allowed to move to his house in Florence where he was closer to his physicians). His Dialogue was put on the Index librorum prohibitorum, a black list of banned books, until 1822.

According to Andrew Dickson White and many of his colleagues, Galileo's experiences demonstrate a classic case of a scholar forced to recant a scientific insight because it offended powerful, conservative forces in society: for the church at the time, it was not the scientific method that should be used to find truth -- especially in certain areas -- but the doctrine as interpreted and defined by church scholars, and this doctrine was defended with torture, murder, deprivation of freedom, and censorship.

More recently, the viewpoints of White and his colleagues have become less-generally accepted by the academic community, partially because White wrote from a perspective that Christianity is a destructive force. This attitude can also be seen in the works of Bertolt Brecht, whose play about Galileo is one of the chief sources for popular knowledge on the scientist. Moreover, deeper examination of the primary sources for Galileo and his trial shows that claims of torture and deprivation were likely exagerrated. Dava Sobel's Galileo's Daughter offers a different set of insights into Galileo and his world, in large part through the private correspondence of Maria Celeste, the daughter of the title, and her father.

In 1992, 359 years after the Galileo trial, Pope John Paul II issued an apology, lifting the edict of Inquisition against Galileo: "Galileo sensed in his scientific research the presence of the Creator who, stirring in the depths of his spirit, stimulated him, anticipating and assisting his intuitions."











This web page was last updated on: 21 December, 2008