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• Born: 15 February 1564
• Birthplace: Pisa, Italy
• Died: 8 January 1642
• Best Known As: The inventor of the astronomical telescope

Galileo was a great Italian scientist who helped unlock many secrets of astronomy and natural motion. Galileo's achievements include: building the first high-powered astronomical telescope; inventing a horse-powered pump to raise water; showing that the velocities of falling bodies are not proportional to their weights; describing the true parabolic paths of cannonballs and other projectiles; coming up with the ideas behind Newton's laws of motion; and confirming the Copernican theory of the solar system. Because he believed that the planets revolved around the sun, and not the Earth, Galileo was denounced as a heretic by the church in Rome. He faced the Inquisition and was forced to renounce those beliefs publicly, though later research, of course, proved him to be correct. His works include Sidereus Nuncius (The Sidereal Messenger, 1610), Il saggiatore (The Assayer, 1623), and Dialogo sopra i due massimi sistemi del mondo tolemaico, e copernicano (Dialogue Concerning the Two Chief World Systems, 1632).

The Vatican officially recognized the validity of Galileo's work in 1993... Galileo was a professor of mathematics at the University of Padua from 1592-1610... Others had invented very low-power telescopes before Galileo, but he refined and improved the idea so greatly that he is generally considered the inventor of the modern telescope.

Galileo Galilei

Library of Congress

[b. Galileo Galilei, Pisa (Italy), February 15, 1564, d. Arcetri (Italy) January 8, 1642]

Most historians consider Galileo (usually known by his first name only) as the first scientist of the Scientific Revolution. His greatest fame is for discoveries in astronomy (moons of Jupiter, phases of Venus, and much more), but his influence on physics is pervasive; the observation that all bodies fall at the same speed in a vacuum is just one of Galileo's ideas that led to the laws of motion and eventually to relativity theory. Galileo also contributed to the study of mathematical infinity. His influence comes not only through his persuasive and popular books about the solar system, kinematics, and materials, but also as a result of his inventions (the astronomical telescope and the thermometer), his correspondence, and his pupils and assistants. Galileo promoted Copernican views as early as 1604 and did not stop when in 1616 the church declared such ideas to be heresy. As a result, Galileo was put before the Inquisition and informed that he must recant or be tortured. He recanted, but spent the last years of his life under house arrest, during which time he wrote and published his most influential work on physics, Dialogue on Two New Sciences.

The Italian scientist Galileo Galilei (1564-1642) 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."

Further Reading

Galileo's chief works are available in excellent translations: Dialogue concerning the Two Chief World Systems (translated by Stillman Drake, 1953); Dialogues concerning Two New Sciences (translated by H. Crew and A. de Salvio, 1914; repr. 1952); and The Discoveries and Opinions of Galileo (edited and translated by Stillman Drake, 1957), which contains The Starry Messenger, the Letters on Sunspots, the Letter to Grand Duchess Christina, and the Assayer.

Stillman Drake also wrote Galileo Studies: Personality, Tradition, and Revolution (1970), which discusses Galileo and 16th-century science. An excellently written, relatively short biography is James Brodrick, Galileo: The Man, His Work, His Misfortunes (1965). Giorgio de Santillana, The Crime of Galileo (1955), and Jerome J. Langford, Galileo: Science and the Church (1966), treat Galileo's condemnation and trial. His philosophy of science is the principal consideration in Ludovico Geymonat, Galileo Galilei (1965). A Galileo bibliography of some 2,000 entries, covering the period 1940-1965, is in Galileo: Man of Science (1968), edited by Ernan McMullin, a volume of essays commemorating the four-hundredth anniversary of Galileo's birth.

For more information on Galileo (Galilei), visit Britannica.com.

(1564-1642) Italian scientist. Although Galileo's distinction belongs to the history of physics and astronomy rather than philosophy, his mature philosophy and methodology of science, particularly as derived from the Dialogue Concerning the Two Chief World Systems (1632) and the Dialogues Concerning Two New Sciences (1638), have been much debated. Galileo unquestionably holds that science based on observation is the true source of knowledge of the physical world, as opposed to traditional authority and philosophical speculation. He also advocates a becoming modesty concerning what we know about nature, in opposition to the dogmatic certainties of much late medieval thought. But within science the relative roles of mathematics, a priori reasoning, pure observation, and model-building are not so clear, and Galileo has been seen as an example of Platonistic rationalism as well as of Aristotelian naturalism. Particular doctrines for which he is known in philosophy include the distinction between primary and secondary qualities, and the relativity of motion. The conception of the world associated with modern science is frequently referred to as the Galilean world view.

(Galileo Galilei) (găl'ĭlē'ō; gälēlĕ'ō gälēlĕ'ē), 1564–1642, great Italian astronomer, mathematician, and physicist. By his persistent investigation of natural laws he laid foundations for modern experimental science, and by the construction of astronomical telescopes he greatly enlarged humanity's vision and conception of the universe. He gave a mathematical formulation to many physical laws.

Contributions to Physics

His early studies, at the Univ. of Pisa, were in medicine, but he was soon drawn to mathematics and physics. It is said that at the age of 19, in the cathedral of Pisa, he timed the oscillations of a swinging lamp by means of his pulse beats and found the time for each swing to be the same, no matter what the amplitude of the oscillation, thus discovering the isochronal nature of the pendulum, which he verified by experiment. Galileo soon became known through his invention of a hydrostatic balance and his treatise on the center of gravity of solid bodies. While professor (1589–92) at the Univ. of Pisa, he initiated his experiments concerning the laws of bodies in motion, which brought results so contradictory to the accepted teachings of Aristotle that strong antagonism was aroused. He found that bodies do not fall with velocities proportional to their weights, but he did not arrive at the correct conclusion (that the velocity is proportional to time and independent of both weight and density) until perhaps 20 years later. The famous story in which Galileo is said to have dropped weights from the Leaning Tower of Pisa is apocryphal. The actual experiment was performed by Simon Stevin several years before Galileo's work. However, Galileo did find that the path of a projectile is a parabola, and he is credited with conclusions foreshadowing Newton's laws of motion.

Contributions to Astronomy

In 1592 he began lecturing on mathematics at the Univ. of Padua, where he remained for 18 years. There, in 1609, having heard reports of a simple magnifying instrument put together by a lens-grinder in Holland, he constructed the first complete astronomical telescope. Exploring the heavens with his new aid, Galileo discovered that the moon, shining with reflected light, had an uneven, mountainous surface and that the Milky Way was made up of numerous separate stars. In 1610 he discovered the four largest satellites of Jupiter, the first satellites of a planet other than Earth to be detected. He observed and studied the oval shape of Saturn (the limitations of his telescope prevented the resolving of Saturn's rings), the phases of Venus, and the spots on the sun. His investigations confirmed his acceptance of the Copernican theory of the solar system; but he did not openly declare a doctrine so opposed to accepted beliefs until 1613, when he issued a work on sunspots. Meanwhile, in 1610, he had gone to Florence as philosopher and mathematician to Cosimo II de' Medici, grand duke of Tuscany, and as mathematician at the Univ. of Pisa.

Conflict with the Church

In 1611 he visited Rome to display the telescope to the papal court. In 1616 the system of Copernicus was denounced as dangerous to faith, and Galileo, summoned to Rome, was warned not to uphold it or teach it. But in 1632 he published a work written for the nonspecialist, Dialogo...sopra i due massimi sistemi del mondo [dialogue on the two chief systems of the world] (tr. 1661; rev. and ed. by Giorgio de Santillana, 1953; new tr. by Stillman Drake, 1953, rev. 1967); that work, which supported the Copernican system as opposed to the Ptolemaic, marked a turning point in scientific and philosophical thought. Again summoned to Rome, he was tried (1633) by the Inquisition and brought to the point of making an abjuration of all beliefs and writings that held the sun to be the central body and the earth a moving body revolving with the other planets about it. Since 1761, accounts of the trial have concluded with the statement that Galileo, as he arose from his knees, exclaimed sotto voce, “E pur si muove” [nevertheless it does move]. That statement was long considered legendary, but it was discovered written on a portrait of Galileo completed c.1640.

After the Inquisition trial Galileo was sentenced to an enforced residence in Siena. He was later allowed to live in seclusion at Arcetri near Florence, and it is likely that Galileo's statement of defiance was made as he left Siena for Arcetri. In spite of infirmities and, at the last, blindness, Galileo continued the pursuit of scientific truth until his death. His last book, Dialogues Concerning Two New Sciences (tr., 3d ed. 1939, repr. 1952), which contains most of his contributions to physics, appeared in 1638. In 1979 Pope John Paul II asked that the 1633 conviction be annulled. However, since teaching the Copernican theory had been banned in 1616, it was technically possible that a new trial could find Galileo guilty; thus it was suggested that the 1616 prohibition be reversed, and this happened in 1992. The pope concluded that while 17th-century theologians based their decision on the knowledge available to them at the time, they had wronged Galileo by not recognizing the difference between a question relating to scientific investigation and one falling into the realm of doctrine of the faith.

Bibliography

See biography by L. Geymonat (tr. 1965); studies by G. de Santillana (1955), S. Drake (1970, 1978, and 1980), and W. R. Shea (1973); G. de Santillana, The Crime of Galileo (1955, repr. 1976); M. A. Finocchiaro, Galileo and the Art of Reasoning (1980).

Galileo Galilei (1564–1642), Italian scientist. Born in Pisa, Galileo was the eldest of the six or seven children of Vincenzio Galilei, a merchant and music theorist, and Giulia Ammannati. He spent his childhood in Pisa and Florence; in the fall of 1581, upon his father's advice, he enrolled at the University of Pisa as a student of medicine. Not enthusiastic about this discipline, within two years he had begun to study Euclidean and Archimedean works privately and left the university in 1585 without a degree. He offered both public and private lessons in mathematics for the next three years and sought, unsuccessfully, to obtain a professorial chair at Bologna in 1588. His various meditations on and experiments with mechanics, metrology, and musical consonance, and his participation in a Florentine academy in this period, helped him secure the chair in mathematics at the University of Pisa in the fall of 1589.

By late 1592 Galileo had won a more prestigious post in mathematics at the University of Padua, and it was here that he undertook significant work in optics and catoptrics, magnetism, tidal theory, mechanics, and instrumentation. This last area was crucial to his financial well-being: in order to meet the demands incumbent upon him as the eldest son, and to supplement his professorial salary, Galileo offered private lessons to students in Padua, many of whom were eager to learn the various uses of a calculating instrument of his design. Galileo's extant writings in mechanics in these same years likewise reflect a strong interest in combining classical problems with actual devices for lifting, lowering, and guiding solid bodies and fluids.

Galileo may have become an adherent of the heliocentric world system posited by Nicolaus Copernicus (1473–1543) in the mid-1590s: so he asserted in 1597 in a letter to the German astronomer Johannes Kepler (1571–1630), discoverer of the laws of planetary motion. Certain conjectures regarding tidal theory reflect a cautious interest in the hypothesis of a mobile Earth, for tides were explained as a product of the globe's annual and diurnal motions, with variations in periodicity deriving from the particular shape of any large body of water. One might also infer Galileo's discreet support of the Copernican system through the attention he devoted in this period to speculative arguments derived from mechanics. The arena in which cosmogony and mechanics intersected was in a quantified approach to a myth mentioned in Plato's Timaeus involving the "creation point," or the place or places from which the Divine Architect originally dropped the various planets. These bodies, after falling toward the sun, would each reach and remain in the orbits to which they had been assigned. Scholars have suggested that around 1602–1604 Galileo did attempt to combine his still evolving understanding of the law of falling bodies and of the way such bodies behave when diverted into uniform orbital motion, with Kepler's estimated periods of revolution for Saturn, Mars, and Jupiter.

By the fall of November 1604 Galileo's attention was on the heavens, for the appearance of a new star seemed to offer strong evidence against Aristotelian conventions regarding an immutable world beyond the Moon. But his most explicitly Copernican conjectures concern the Moon; between 1605 and 1607 he and several of his closest associates had observed the ashen light reflected onto that body by Earth at the beginning and end of each lunar cycle. The rough and opaque body of Earth was, in other words, like other planets, tolerably bright; the corollary was, for some, that Earth likewise participated in "the dance of the stars." In this period Galileo was also engaged in more studies of motion and hydrostatics, and involved with additional work in magnetism.

By spring or summer 1609, Galileo was making celestial observations with the aid of a telescope at least three times more powerful than a prototype from The Hague. By November of that year, he had developed a telescope that magnified twenty times, and it was with this instrument that he undertook his observations of the lunar body. His Starry Messenger of 1610 shows that the telescope confirmed his earlier naked-eye impressions of both a rough lunar surface and of the ashen light, and that it allowed him to present certain of the Moon's features, most notably its peaks, valleys, and craters, in terms of their terrestrial counterparts. He used the shadows cast by a particular mountain on the Moon to calculate the average height of such formations. On the basis of these observations of the Moon's similarity to Earth, Galileo proposed a thoroughgoing revision of the Ptolemaic conception of the cosmos, and he promised to deliver such arguments in his System of the World, the forerunner to the eventual Dialogue concerning the Two Chief World Systems of 1632.

The greatest discoveries in the Starry Messenger lay in its final section, a description of the positions of the satellites of Jupiter from 7 January until 2 March 1610, when the treatise went to press. In these brief observations and in the spare diagrams that accompanied them, Galileo presented the orbital movements of four satellites, or Medici stars, whose very existence was new to virtually all of his audience. The fact that Jupiter had moons strongly suggested to him that Earth was neither unique nor central nor motionless: satellites revolving about a celestial body clearly did not prevent its movement.

By the end of 1610, Galileo, newly appointed as mathematician and philosopher at the court of the grand duke of Tuscany, had interpreted the phases of Venus as a confirmation of Copernican claims, and perhaps more importantly, evidence against the models of both Ptolemy and and the Danish astronomer Tycho Brahe (1546–1601), who posited that the five planets revolved around the Sun, which in turn revolved around Earth; Kepler obligingly published his letters on the matter in his Dioptrice of 1611. Galileo had some notion of sunspots by spring 1611, but his systematic study of the phenomena appears to date only to early 1612, when he had learned of the observations of several friends, and of the treatise of an eventual enemy, the Swabian Jesuit Christoph Scheiner (1573–1650). Galileo took immediate issue with Scheiner's initial conjecture that the spots were actually small stars orbiting and partially eclipsing the solar body, and he did not hesitate to expose both the Jesuit astronomer's ignorance of Galileo's recent findings concerning Venus, and the weakness of Scheiner's geometrical proofs. Because he saw no reason to subscribe to the Aristotelian fiction of the changeless heavens, Galileo's three letters on the subject offered the more consistent (though inaccurate) explanation of the sunspots as enormous masses of dark clouds constantly produced on the solar surface and moving uniformly over it before vanishing forever.

Galileo's next writing, the Letter to the Grand Duchess Christina, was of little scientific importance, for it neither offered new observations nor announced novel astronomical hypotheses, and was published only in 1636 in a Latin translation. In terms of the sort of interpretation it offered—a brilliant analysis of the Old Testament verse Joshua 10:12 as compatible with a heliocentric universe and incompatible with a geocentric one—the Letter was among the boldest and most ill-advised moves of Galileo's career. His confidence in his reading, for all of its economy, appears to have been misplaced, and by early 1615 a complaint had been lodged with the Inquisition. In a meeting whose general tenor and purpose are still the subject of debate, Galileo met with Robert Cardinal Bellarmine in February 1616, but was not asked to abjure his Copernican beliefs. The Edict of 1616 formally prohibited books attempting to reconcile Scripture and the hypothesis of a mobile Earth, and stipulated that Copernicus's On the Revolutions of the Heavenly Spheres was suspended until such passages could be struck through. While Galileo appears not to have seen the edict as of particular concern to him, rivals immediately recognized its impact on the astronomer's career.

The controversy between Galileo and the Jesuit astronomer Orazio Grassi ranged from the fall of 1618, when three comets emerged, to 1626, when Grassi published his third and final work on the phenomena. Galileo's principal discussion of the comets, the Assayer, appeared in 1623. Although Galileo could no longer openly defend Copernicanism, and did not have an accurate explanation of the comets, he recognized flaws in many of Grassi's arguments, particularly in the implicit support that Grassi gave to the Tychonic world system. The Assayer contains important discussions of the usefulness of parallax and of the causes of telescopic magnification of distant bodies, several of Galileo's clearest formulations of his own methodology, and some of the most caustic and amusing moments of any scientific controversy.

The synthesis of Galileo's decades of astronomical observations, speculation, and revision, the Dialogue concerning the Two Chief World Systems, Ptolemaic and Copernican, was published in Florence in 1632. Divided into four days of exchanges between the learned Salviati, the cultured Sagredo, and the tireless Aristotelian Simplicio, the Dialogue examines and discards traditional arguments distinguishing the motions, substance, and final purpose of celestial and terrestrial bodies, discusses the experimental and logical evidence for Earth's diurnal and annual movements, presents the particulars of the orbits and telescopic appearance of the other planets, draws on the emergent science of magnetism as well as upon observations of the new stars of 1572 and 1604, the fixed stars, Moon spots, and sunspots, and concludes with an ample discussion of Galileo's theory of tides. The tempo and variety of the Dialogue are surely part of its enormous appeal: the speakers move easily from minute calculations to the most abstruse philosophical speculations without losing sight of their goal of assessing the two chief world systems. But to suggest, as Galileo did, that the work involves equally qualified opponents, or recognizes the merits of aspects of both views, or presents Copernicanism as merely hypothetical, is to err: Simplicio is overmatched from the outset, a rather inept spokesman for the Ptolemaic position throughout, and effectively silenced by his companions in the last pages of the Dialogue.

Summoned to Rome to account for his publication, Galileo recanted on 22 June 1633. Although depressed and humiliated by this turn of events, he soon focused on the Two New Sciences Pertaining to Mechanics and Local Motions. Published in Leiden in 1638, his last great work is in dialogue form, and again involves Salviati, Sagredo, and Simplicio. The product of a warring age, it is set in Venice's arsenal, the site of the republic's shipbuilding and munitions production. It has as one focus the "supernatural violence" with which projectiles are fired, presents the legendary burning mirrors of antiquity as plausible weapons, discusses at length notions of impact and resistance, is dedicated to a member of the noblesse d'épée, and refers to the battlefield death of one of Galileo's former students and fellow experimenters. That said, the Two New Sciences also attend to nonmilitary matters such as the void, the speed of light, the principle of the balance, musical intervals, the role of scale in very large structures or animals, uniformly accelerated or natural motion, and the Platonic "creation point." The true fight, as Galileo's dedication and several asides suggest, is for the reestablishment of his scientific and ethical reputation, and despite the burden of illness and old age, the stricture of house arrest, and his renunciation of cosmological issues, the victory was his.

Bibliography

Primary Sources

Galilei, Galileo. Dialogue concerning the Two Chief World Systems. Translated by Stillman Drake. 2nd rev. ed. Berkeley, 1967.

——. Discourse on the Comets. In The Controversy on the Comets of 1618. Translated by Stillman Drake and C. D. O'Malley. Philadelphia, 1960.

——. Sidereus Nuncius or the Sidereal Messenger. Translated and with an introduction, commentary, and notes by Albert Van Helden. Chicago, 1989.

——. Two New Sciences. Translated with an introduction and notes by Stillman Drake. Madison, Wis., 1974.

Secondary Sources

Biagioli, Mario. Galileo, Courtier: The Practice of Science in the Culture of Absolutism. Chicago, 1993.

Drake, Stillman. Essays on Galileo and the History and Philosophy of Science. Selected and introduced by N. M. Swerdlow and T. H. Levere. 3 vols. Toronto, 1999.

——. Galileo at Work: His Intellectual Biography. Chicago, 1978.

Redondi, Pietro. Galileo: Heretic. Translated by Raymond Rosenthal. Princeton, 1987.

—EILEEN A. REEVES

(gal-uh-lee-oh, gal-uh-lay-oh)

An Italian scientist of the late sixteenth and early seventeenth centuries; his full name was Galileo Galilei. Galileo proved that objects with different masses fall at the same velocity. One of the first persons to use a telescope to examine objects in the sky, he saw the moons of Jupiter, the mountains on the moon, and sunspots.

Authorities of the Roman Catholic Church forced Galileo to renounce his belief in the model of the solar system proposed by Nicolaus Copernicus. Galileo had to assert that the Earth stands still, and the sun revolves around it. A famous legend holds that Galileo, after making this public declaration about a motionless Earth, muttered, “Nevertheless, it does move.”

Quotes:

"Doubt is the father of invention."

"You cannot teach a man anything; you can only help him find it within himself."