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Galilei, Galileo

Galilei, Galileo

(b. Pisa, Italy, 15 February 1564; d. Arcetri, Italy, 8 January 1642)

physics, astronomy.

The name of Galileo is inextricably linked with the advent, early in the seventeenth century, of a marked change in the balance between speculative philosophy, mathematics, and experimental evidence in the study of natural phenomena. The period covered by his scientific publications began with the announcement of the first telescopic astronomical discoveries in 1610 and closed with the first systematic attempt to extend the mathematical treatment of physics from statics to kinematics and the strength of materials in 1638. The same period witnessed Kepler’s mathematical transformation of planetary theory and Harvey’s experimental attack on physiological dogma. Historians are divided in their assessment of this widespread scientific revolution with respect to its elements of continuity and innovation, both as to method and as to content. Of central importance to its understanding are the life and works of Galileo, whose personal conflict with religious authority dramatized the extent and profundity of the changing approach to nature.

Early Years. Galileo’s father was Vincenzio Galilei, a musician and musical theorist and a descendant of a Florentine patrician family distinguished in medicine and public affairs. He was a member of the Florentine Camerate, a cultural group which included musicians whose devotion to the revival of Greek music and monody gave birth to opera. It was headed by giovanni Bardi, who sponsored Vincenzio’s musical studies under Gioseffo Zarlino at Venice around 1561. In 1562 he married Giulia Ammannati of Pescia, with whom he settled at Pisa. Galileo was the eldest of seven children. His brother Michelangelo became a professional musician and spent most of his life abroad. Two of his sisters, Virginia and Livia, married and settled in Florence. Of the other children no record survives beyond that of their births.

Galileo was first tutored at Pisa by one Jacopo Borghini. Early in the 1570’s, Vincenzio returned to Florence, where he resettled the family about 1575. Galileo was then sent to school at the celebrated monastery of Santa Maria at Vallombrosa. In 1578 he entered the order as a novice, against the wishes of his father, who removed him again to Florence and applied unsuccessfully for a scholarship on his behalf at the University of Pisa. Galileo resumed his studies with the Vallombrosan monks in Florence until 1581, when he was enrolled at the University of Pisa as a medical student.

The chair of mathematics appears to have been vacant during most of Galileo’s years as a student at Pisa. His formal education in astronomy was thus probably confined to lectures on the Aristotelian De Caelo by the philosopher Francesco Buonamici. Physics was likewise taught by Aristotelian lectures, given by Buonamici and Girolamo Borro. As a medical student, Galileo may have received instruction from Andrea Cesalpino. His interest in medicine was not great; he was instead attracted to mathematics in 1583, receiving instruction from Ostilio Ricci outside the university. Ricci, a friend of Galileo’s father and later a member of the Academy of Design at Florence, is said to have been a pupil of Niccolò Tartaglia. Galileo’s studies of mathematics, opposed at first by his father, progressed rapidly; in 1585 he left the university without a degree and returned to Florence, where he pursued the study of Euclid and Archimedes privately.

From 1585 to 1589 Galileo gave private lessons in mathematics at Florence and private and public instruction at Siena. In 1586 he composed a short work, La bilancetta, in which he reconstructed the reasoning of Archimedes in the detection of the goldsmith’s fraud in the matter of the crown of Hieron and described an improved hydrostatic balance. During the same period he became interested in problems of centers of gravity in solid bodies. During a visit to Rome in 1587, he made the acquaintance of the Jesuit mathematician Christoph Klau (Clavius). In 1588 he was invited by the Florentine Academy to lecture on the geography of Dante’s Inferno treated mathematically. In the same year he applied for the chair of mathematics at the University of Bologna, seeking and obtaining from Guidobaldo del Monte an endorsement based on his theorems on the centers of gravity of paraboloids of revolution. The chair was awarded, however, to Giovanni Antonio Magini, probably on the basis of his superiority in astronomy, a subject in which Galileo appears to have shown little interest up to this time.

While Galileo was residing in Florence, his father was engaged in a controversy with Zarlino over musical theory. To destroy the old numerical theory of harmony, Vincenzio performed a series of experimental investigations of consonance and its relation to the lengths and tensions of musical strings. These he embodied in a published polemic of 1589, the Discorso intorno all’opere di messer Gioseffo Zarlino da Chioggia, and two unpublished treatises that survive among Galileo’s papers. It is probable that Galileo’s interest in the testing of mathematical rules by physical observations began with the musical experiments devised by his father during these years.

Professorship at Pisa. In 1589, on the recommendation of Guidobaldo, Galileo gained the chair of mathematics at the University of Pisa. The philosopher Jacopo Mazzoni, who came to Pisa at the same time, and Girolamo Mercuriale, professor of medicine, were close friends of the young mathematician. Luca Valerio, a Roman mathematician noted particularly for his later treatise on centers of gravity, met Galileo on a visit to Pisa and later corresponded with him. With other professors at Pisa, however, Galileo’s relations were not so cordial, chiefly because of his campaign to discredit the prevailing Aristotelian physics to the advantage of his mathematical chair. His alleged demonstration at the Leaning Tower of Pisa that bodies of the same material but different weight fall with equal speed—if actually performed—was clearly not an experiment but a public challenge to the philosophers.

During Galileo’s professorship at Pisa, he composed an untitled treatise on motion against the Aristotelian physics, now usually referred to as De motu. Its opening sections developed a theory of falling bodies derived from the buoyancy principle of Archimedes, an idea previously published by Giovanni Battista Benedetti in 1553–1554 and again in 1585. In the same treatise, Galileo derived the law governing equilibrium of weights on inclined planes and attempted to relate this law to speeds of descent. The result did not accord with experience—as Galilieo noted—which may be the principal reason for his having withheld the treatise from publication. The discrepancy arose from his neglect of acceleration, a phenomenon that he then considered to be evanescent in free fall and that he accounted for by a Hipparchian theory of residual impressed force. In order to reconcile that theory with fall from rest, Galileo introduced a conception of static forces closely allied to Newton’s third law of motion. Equality of action and reaction, together with the idea of virtual velocities, pervades much of Galileo’s physics. From his earliest demonstrations of equilibrium on inclined planes, Galileo limited the action of tendencies to motion to infinitesimal distances, unlike his ancient and medieval predecessors. In so doing, he was able to relate vertical fall to descent along circular arcs and tangential inclined planes, an achievement that was to provide him with the key to many phenomena after he recognized the essential role of acceleration.

In his De motu, Galileo undertook to destroy the Aristotelian dichotomy of all motions into natural and forced motions. He did this by introducing imaginary rotations of massive spheres. Rotations of homogeneous spheres, or of any sphere having its geometric center or its center of gravity at the center of the universe, he declared to be “neutral” motions, neither natural nor forced. Motions on the horizontal plane, or on imaginary spheres concentric with the earth’s center, were likewise neutral—a conception that led Galileo to his restricted concept of inertia in terrestrial physics. His discussion of spheres in De motu shows further that in 1590 Galileo had not yet abandoned the geocentric astronomy, but suggests that he saw no difficulty in the earth’s rotation as assumed in the semi-Tychonic astronomy.

Vincenzio Galilei died in 1591, leaving Galileo, as eldest son, with heavy domestic and financial responsibilities. Galileo’s position at Pisa was poorly paid; he was out of favor with the faculty of philosophy and he had offended Giovanni de’ Medici by criticizing a scheme for the dredging of the harbor of Leghorn. His disrespectful attitude toward the university administration is reflected in a jocular poem he composed against the wearing of academic robes. Thus, at the end of his three-year contract, Galileo had no hope of strengthening his position at Pisa and little promise even of reappointment. Once more with the aid of Guidobaldo, he moved to the chair of mathematics at Padua. The rival candidate was again Magini, whose hostility toward Galileo after this defeat became extreme.

Professorship at Padua. The atmosphere at Padua was propitious in every way to Galileo’s development. He quickly made the acquaintance of free and erudite spirits, in such men as G. V. Pinelli and Paolo Sarpi. Among his students were Gianfrancesco Sagredo and Benedetto Castelli. A conservative professor, Cesare Cremonini, became his personal friend while staunchly opposing his anti-Aristotelian views. Padua was a gathering point of the best scholars in Italy and drew students from all over Europe. Under the Venetian government, the university enjoyed virtually complete freedom from outside interference.

Galileo lectured publicly on the prescribed topics: Euclid, Sacrobosco, Ptolemy, and the pseudo-Aristotelian Questions of Mechanics. Privately he gave instruction also on fortification, military engineering, mechanics, and possibly also on astronomy, although we lack concrete evidence of his having become deeply interested in that subject much before 1604. He composed several treatises for the use of his students. One, usually known as Le mecchaniche, survives in three successive forms, dating probably from 1593, 1594, and about 1600. In this treatise, besides developing further his treatment of inclined planes, he utilized as a bridge between statics and dynamics the remark that an infinitesimal force would serve to disturb equilibrium. This move, although itself not unobjectionable, removed serious existing obstacles (which had been raised on logical grounds by Guidobaldo and Simon Stevin) from the mathematical analysis of dynamic problems. Galileo’s treatise, before it was first published in a French translation by Marin Mersenne in 1634, circulated widely in manuscript, and an English manuscript translation was made in 1626. Its authorship was not always known to readers even in Italy, because Galileo’s treatises composed for his students were invariably supplied in copies bearing no title or signature.

In May 1597 Galileo wrote to his former colleague at Pisa, Jacopo Mazzoni, defending the Copernican system against a mistaken criticism. In August of the same year he received copies of the Mysterium cosmographicum, the first book by Johannes Kepler, to whom he wrote expressing his sympathies with Copernicanism. Kepler replied, urging him to support Copernicus openly, but Galileo allowed this correspondence to languish. His preference for Copernicus at this time seems to have had a mechanical rather than an astronomical basis; he wrote to Kepler that it afforded an explanation of physical effects not given by its rivals. This referred to a tidal theory of Galileo’s in which the double motion of the earth was invoked to account for the periodic disturbance of its water. The first notation concerning this theory occurs in the notebooks of Sarpi in 1595. Galileo wrote a treatise on it early in 1616, and wished to make it the central theme of his Copernican Dialogue of 1632, considering the tides to offer a compelling argument for the double motion of the earth.

It was also in 1597 that Galileo began the production—for sale—of a mathematical instrument, the sector or proportional compass. The idea for this instrument probably came to him from Guidobaldo, whose knowledge of it may in turn have been derived from Michel Coignet. Galileo transformed it from a simple device of limited use to an elaborate calculating instrument of varied uses and of great practical utility by adding to it a number of supplementary scales. He employed a skilled artisan to produce it (and other mathematical instruments) in his own workshop and wrote a treatise on its use for engineers and military men.

During his residence at Padua, Galileo took a Venetian mistress named Marina Gamba, by whom he had two daughters and a son. The elder daughter, Virginia, who was born in 1600, later became Galileo’s chief solance in life. The vivacity of her mind and the sensitivity of her spirit—as well as her many impositions on her father’s good nature—are evident in the letters that Galileo received and treasured. Both she and her sister Livia were entered in a nunnery near Florence at an early age, Virginia taking the name Maria Celeste. Livia, who took the name Arcangela, was of a peevish disposition and frail health. The son, Vincenzio, was later legitimized. After periods of estrangement from his father, Vincenzio became reconciled with him in his last years but did not long survive him. Marina Gamba remained at Venice when Galileo returned to Florence, and shortly afterward she married.

Early Work on Free Fall. Toward the end of 1602, Galileo wrote to Guidobaldo concerning the motions of pendulums and the descent of bodies along the arcs and chords of circles. His deep interest in phenomena of acceleration appears to date from this time. The correct law of falling bodies, but with a false assumption behind it, is embodied in a letter to Sarpi in 1604. Associated with the letter is a fragment, separately preserved, containing an attempted proof of the correct law from the false assumption. No clue is given as to the source of Galileo’s knowledge of the law that the ratios of spaces traversed from rest in free fall are as those of the squares of the elapsed times. The law is algebraically derivable from the medieval mean-degree theorem known as the Merton rule, but Galileo’s false assumption in 1604 contradicts the specific association of speed and time that is always found in medieval derivations of that theorem. Moreover, Galileo’s faulty demonstration invoked no single instantaneous velocity as a mean or representative value; instead, it proceeded by comparison of ratios between infinite sets of instantaneously varying velocities. It is probable either that he observed a rough 1, 3, 5, . . . progression of spaces traversed along inclined planes in equal times and assumed this to be exact, or that he reasoned (as Christian Huygens later did) that only the oddnumber rule of spaces would preserve the ratios unchanged for arbitrary changes of the unit time. From this fact, the times-squared law follows immediately. Galileo’s derivation of it from the correct definition of uniform acceleration followed only at a considerably later date.

The appearance of a supernova in 1604 led to disputes about the Aristotelian idea of the incorruptibility of the heavens, in which Galileo took an active part. He delivered three lectures to overflow crowds at Padua and prepared to publish an astronomical work; he did not do so, however, and only a short fragment of the manuscript survives. Lodovico delle Colombe, who published a theory of new stars at Florence, suspected Galileo of having written a pseudonymous attack on him, and it is certain that Galileo’s ideas are reflected in still another pseudonymous work, published in rustic dialect at Padua in 1605, which ridiculed the professors of philosophy. In 1606, however, Galileo’s attention was diverted from this dispute by the plagiarism of his proportional compass by Simon Mayr (or Marius, in the Latinized form used for publication), a German then at Padua, and Mayr’s pupil Baldassar Capra. Galileo had privately printed a small edition of his treatise on the use of the compass in that year; Mayr and Capra produced a Latin book on the construction and use of the same instrument, claiming that Galileo had stolen it from them. Mayr had returned to Germany, so Galileo brought his action against Capra. The book was suppressed and Capra was expelled from the university. In the following year Galileo published a full account of the case in his first publicly circulated printed work, the Difesa . . . contro alle calunnie & imposture di Baldessar Capra.

Early in 1609, Galileo began the composition of a systematic treatise on motion in which his studies of inclined planes and of pendulums were to be integrated under the law of acceleration, known to him at least since 1604. In the composition of his treatise, he became aware that there was something wrong with his attempted derivation of 1604, which had assumed proportionality of speed to space traversed. Accordingly, he introduced in its place two propositions drawn from mechanics, which he submitted for criticism to Valerio. Galileo received Valerio’s reply in July 1609, just after his attention had again been diverted from mechanics, this time by news of the invention of the telescope.

The Telescope. A Dutch lens-grinder, Hans Lipperhey, had applied in October 1608 to Count Maurice of Nassau for a patent on a device to make distant objects appear closer. Sarpi, whose extensive correspondence (maintained for theological and political reasons) kept him currently informed, learned of this device within a month. Somewhat skeptical, he applied for further information to Jacques Badovere (Giacomo Badoer), a former pupil of Galileo’s then at Paris. In due course the report was confirmed. Galileo heard discussions of the news during a visit to Venice in July 1609, learned from Sarpi that the device was real, and probably heard of the simultaneous arrival at Padua of a foreigner who had brought one to Italy. He hastened back to Padua, found that the foreigner had left for Venice, and at once attempted to construct such a device himself. In this he quickly succeeded, sent word of it to Sarpi, and applied himself to the improvement of the instrument. Sarpi, who had meanwhile been selected by the Venetian government to assess the value of the device offered for sale to them by the stranger, discouraged its purchase. Late in August, Galileo arrived at Venice with a nine-power telescope, three times as effective as the other. The practical value of this instrument to a maritime power obtained for him a life time appointment to the university, with an unprecedented salary for the chair of mathematics. The official document he received, however, did not conform to his understanding of the terms he had accepted. As a result, he pressed his application for a post at the Tuscan court, begun a year or two earlier.

Galileo’s swift improvement of the telescope continued until, at the end of 1609, he had one of about thirty power. This was the practicable limit for a telescope of the Galilean type, with plano-convex objective and plano-concave eyepiece. He turned this new instrument to the skies early in January 1610, with startling results. Not only was the moon revealed to be mountainous and the Milky Way to be a congeries of separate stars, contrary to Aristotelian principles, but a host of new fixed stars and four satellites of Jupiter were promptly discovered. Working with great haste but impressive accuracy, Galileo recited these discoveries in the Sidereus nuncius, published at Venice early in March 1610.

His sudden fame assisted Galileo in his negotiations at Florence. Moreover, the new discoveries made him reluctant to continue teaching the old astronomy. In the summer of 1610, he resigned the chair at Padua and returned to Florence as mathematician and philosopher to the grand duke of Tuscany, and chief mathematician of the University of Pisa, without obligation to teach.

Galileo’s book created excitement throughout Europe and a second edition was published in the same year at Frankfurt. Kepler endorsed it in two small books, the Dissertatio cum Nuncio Sidereo, published before he had personally observed the new phenomena, and the Narratio de observatis a se quatuor Jovis satellitibus, published a few months later. Other writers attacked the claimed discoveries as a fraud. Galileo did not enter the controversy but applied himself to further observations. He discovered, later in 1610, the oval appearance of Saturn and the phases of Venus. His telescope was inadequate to resolve Saturn’s rings, which he took to be satellites very close to the planet. The phases of Venus removed a serious objection to the Copernican system, and he saw in the satellites of Jupiter a miniature planetary system in which, as in the Copernican astronomy, it could no longer be held that all moving heavenly bodies revolved exclusively about the earth.

Early in 1611 Galileo journeyed to Rome to exhibit his telescopic discoveries. The Jesuits of the Roman College, who had at first been dubious, confirmed them and honored Galileo. Federico Cesi feted Galileo and made him a member of the Lincean Academy, the first truly scientific academy, founded in 1603. The pope and several cardinals also showed their esteem for Galileo.

Controversies at Florence. Shortly after his return to Florence, Galileo became involved in a controversy over floating bodies. In that controversy an important role was played by Colombe, who became the leader of a group of dissident professors and intriguing courtiers that resented Galileo’s position at court. Maffeo Barberini—then a cardinal but later to become pope—took Galileo’s side in the dispute. Turning again to physics, Galileo composed and published a book on the behavior of bodies placed in water (Discorso . . . intorno alle cose che stanno ub su l’acqua, o in quella si muovono), in support of Archimedes and against Aristotle, of which two editions appeared in 1612. Using the concept of moment and the principle of virtual velocities, Galileo extended the scope of the Archimedean work beyond purely hydrostatic considerations.

While this work was in progress, Galileo received from Marcus Welser of Augsburg a short treatise on sunspots that Welser had published pseudonymously for the Jesuit Christoph Scheiner, asking Galileo’s opinion of it. Galileo replied in three long letters during 1612, demolishing Scheiner’s conjecture that the spots were tiny planets. He asserted also that he had observed sunspots much earlier and had shown them to others at Rome early in 1611. This set the stage for deep enmity of Scheiner toward Galileo, which, however, did not take active form at once.

Galieo’s Letters on Sunspots was published at Rome in 1613 under the auspices of the Lincean Academy. In this book Galileo spoke out decisively for the Copernican system for the first time in print. In the same book he found a place for his first published mention of the concept of conservation of angular momentum and an associated inertial concept. During its composition he had taken pains to determine the theological status of the idea of incorruptibility of the heavens, finding that this was regarded by churchmen as an Aristotelian rather than a Catholic dogma. But attacks against Galileo and his followers soon appeared in ecclesiastical quarters. These came to a head with a denunciation from the pulpit in Florence late in 1614.

In December 1613 it had happened that theological objections to Copernicanism were raised, in Galileo’s absence, at a court dinner, where Galileo’s part was upheld by Benedetto Castelli. Learning of this, Galileo wrote a long letter to Castelli concerning the inadmissibility of theological interference in purely scientific questions. After the public denunciation in 1614, Castelli showed this letter to an influential Dominican priest, who made a copy of it and sent it to the Roman Inquisition for investigation. Galileo then promptly sent an authoritative text of the letter to Rome and began its expansion into the Letter to Christina, composed in 1615 and eventually published in 1636. Galileo argued that neither the Bible nor nature could speak falsely and that the investigation of nature was the province of the scientist, while the reconciliation of scientific facts with the language of the Bible was that of the theologian.

The book on bodies in water drew attacks from four Aristotelian professors at Florence and Pisa, while a book strongly supporting Galileo’s position appeared at Rome. Galileo prepared answers to his critics, which he turned over to Castelli for publication in order to avoid personal involvement. Detailed replies to two of them (Colombe and Grazia), written principally by Galileo himself, appeared anonymously in 1615, with a prefatory note by Castelli implying that he was the author and that Galileo would have been more severe.

Late in 1615 Galileo went to Rome (against the advice of his friends and the Tuscan ambassador) to clear his own name and to prevent, if possible, the official suppression of the teaching of Copernicanism. In the first, he succeeded; no disciplinary action against him was taken on the basis of his letter to Castelli or his Copernican declaration in the book on sunspots. In the second objective, however, he failed. Pope Paul V, irritated by the agitation of questions of biblical interpretation— then a bone of contention with the Protestants—appointed a commission to determine the theological status of the earth’s motion. The determination was adverse, and Galileo was instructed on 26 February 1616 to abandon the holding or defending of that view. No action was taken against him, nor were any of his books suspended. A book by the theologian Paolo Antonio Foscarini reconciling the earth’s motion with the Bible was condemned, and the work of Copernicus and a commentary on Job by Diego de Zuñiga were suspended pending the correction of a few passages. One contemporary document, bound into the proceedings but of uncertain reliability, states that Galileo was also ordered never to discuss the forbidden doctrine again. If such an order was given, it was in contravention of certain specific instructions of the pope and had no legal force.

Returning to Florence, Galileo took up a practical and noncontroversial problem, the determination of longitudes at sea. He believed that this could be solved by the preparation of accurate tables of the eclipses of the satellites of Jupiter, which were of frequent occurrence and could be observed telescopically from any point on the earth. As a practical matter, the eclipses could neither be predicted with sufficient accuracy nor observed at sea with sufficient convenience to make the method useful.

It is probable that Galileo also returned during this period to his mechanical investigations, interrupted in 1609 by the advent of the telescope. A Latin treatise by Galileo, De motu accelerato, which correctly defines uniform acceleration and much resembles the definitive text reproduced in his final book, seems to date from this intermediate period, and copies of many of his propositions in kinematics exist in the handwriting of Mario Guiducci, who studied under Galileo at this time.

In 1618 three comets attracted the attention of Europe and became the subject of many pamphlets and books. One such book was printed anonymously by Orazio Grassi, the mathematician of the Jesuit Roman College. Galileo was bedridden at the time, but he discussed his views on comets with Guiducci, who then delivered lectures on them to the Florentine Academy and published them over his own name. In these lectures, which were largely dictated or corrected by Galileo, the anonymous Jesuit was subjected to criticism. The result was a direct attack on Galileo by Grassi, under the pseudonym of Lotario Sarsi, published in 1619.

Galileo replied, after much delay, with one of the most celebrated polemics in science, Il saggiatore (the Assayer). It was addressed to Virginio Cesarini, a young man who had heard Galileo debate at Rome in 1615–1616 and had written to him in 1619 to extol the method by which Galileo had opened to him a new road to truth. Since he could no longer defend Copernicus, Galileo avoided the question of the earth’s motion; instead, he set forth a general scientific approach to the investigation of celestial phenomena. He gave no positive theory of comets, but developed the thesis that arguments from parallax could not be decisive concerning their location until it was first demonstrated that they were concrete moving objects rather than mere optical effects of solar reflection in seas of vapor. No such proof appeared to him to be available. In the course of his argument, Galileo distinguished physical properties of objects from their sensory effects, repudiated authority in any matter that was subject to direct investigation, and remarked that the book of nature, being written in mathematical characters, could be deciphered only by those who knew mathematics.

The Saggiatore was printed in 1623 under the auspices of the Lincean Academy. Just before it emerged from the press, Maffeo Barberini became pope as Urban VIII. The academicians dedicated the book to him at the last minute. Cesarini was appointed chamberlain by the new pope, who had long been Galileo’s friend and was a patron of science and letters. Galileo journeyed to Rome in 1624 to pay his respects to Urban, and secured from him permission to discuss the Copernican system in a book, provided that the arguments for the Ptolemaic view were given an equal and impartial discussion. Urban refused to rescind the edict of 1616, although he remarked that had it been up to him, the edict would not have been adopted.

Dialogue on the World Systems. The Dialogue Concerning the Two Chief World Systems occupied Galileo for the next six years. It has the literary form of a discussion between a spokesman for Copernicus, one for Ptolemy and Aristotle, and an educated layman for whose support the other two strive. Galileo thus remains technically uncommitted except in a preface which ostensibly supports the anti-Copernican edict of 1616. The book will prove, he says, that the edict did not reflect any ignorance in Italy of the strength of pro-Copernican arguments. The contrary is the case; Galileo will add Copernican arguments of his own invention, and thus he will show that not ignorance of or antagonism to science, but concern for spiritual welfare alone, guided the Church in its decision.

The opening section of the Dialogue critically examines the Aristotelian cosmology. Only those things in it are rejected that would conflict with the motion of the earth and stability of the sun or that would sharply distinguish celestial from terrestrial material and motions. Thus the idea that the universe has a center, or that the earth is located in such a center, is rejected, as is the idea that the motion of heavy bodies is directed to the center of the universe rather than to that of the earth. On the other hand, the Aristotelian concept of celestial motions as naturally circular is not rejected; instead, Galileo argues that natural circular motions apply equally to terrestrial and celestial objects. This position appears to conflict with statements in later sections of the book concerning terrestrial physics. But uniform motion in precise circular orbits also conflicts with actual observations of planetary motions, whatever center is chosen for all orbits. Actual planetary motions had not been made literally homocentric by any influential astronomer since the time of Aristotle. Galileo is no exception; in a later section he remarked on the irregularities that still remained to be explained. Opinion today is divided; some hold that the opening arguments of the Dialogue should be taken as representative of Galileo’s deepest physical and philosophical convictions, while others view them as mere stratagems to reduce orthodox Aristotelian opposition to the earth’s motion.

Important in the Dialogue are the concepts of relativity of motion and conservation of motion, both angular and inertial, introduced to reconcile terrestrial physics with large motions of the earth, in answer to the standard arguments of Ptolemy and those added by Tycho Brahe. The law of falling bodies and the composition of motions are likewise utilized. Corrections concerning the visual sizes and the probable distances and positions of fixed stars are discussed. A program for the detection of parallactic displacements among fixed stars is outlined, and the phases of Venus are adduced to account for the failure of that planet to exhibit great differences in size to the naked eye at perigee and apogee. Kepler’s modification of the circular Copernican orbits is not mentioned; indeed, the Copernican system is presented as more regular and simpler than Copernicus himself had made it. Technical astronomy is discussed with respect only to observational problems, not to planetary theory.

To the refutation of conventional physical objections against terrestrial motion, Galileo added two arguments in its favor. One concerned the annual variations in the paths of sunspots, which could not be dynamically reconciled with an absolutely stationary earth. Geometrically, all rotations and revolutions could be assigned to the sun, but their conservation would require very complicated forces. The Copernican distribution of one rotation to the sun and one rotation and one revolution to the earth fitted a very simple dynamics. The second new argument concerned the existence of ocean tides, which Galileo declared, quite correctly, to be incapable of any physical explanation without a motion of the earth. His own explanation happened to be incorrect; he argued that the earth’s double motion of rotation and revolution caused a daily maximum and minimum velocity, and a continual change of speed, at every point on the earth. The continual variation of speed of sea basins imparted different speeds to their contained waters. The water, free to move within the basins, underwent periodic disturbances of level, greatest at their coasts; the period depended on sizes of basins, their east-west orientations, depths, and extraneous factors such as prevailing winds. In order to account for monthly and annual variations in the tides, Galileo invoked an uneven speed of the earthmoon system through the ecliptic during each month, caused by the moon’s motion with respect to the earth-sun vector; for annual seasonal effects, he noted changes of the composition of rotational and revolutional components in the basic disturbing cause.

The Dialogue was completed early in 1630. Galileo took it to Rome, where it was intended to be published by the Lincean Academy. There he sought to secure a license for its printing. This was not immediately granted, and he returned to Florence without it. While the matter was still pending, Federico Cesi died, depriving the Academy of both effective leadership and funds. Castelli wrote to Galileo, intimating that for other reasons he would never get the Roman imprimatur and advising him to print the book at Florence without delay. Negotiations ensued for permission to print the book at Florence. Ultimately these were successful, and the Dialogue appeared at Florence in March 1632. A few copies were sent to Rome, and for a time no disturbance ensued. Then, quite suddenly, the printer was ordered to halt further sales, and Galileo was instructed to come to Rome and present himself to the Inquisition during the month of October.

The Trial of Galileo. The background of the action is fairly clear. Several ecclesiastical factions were hostile to the book but at first produced only shallow pretexts to suppress it. More serious charges were lodged against Galileo when Urban was persuaded that his own decisive argument against the literal truth of the earth’s motion—that God could produce any effect desired by any means—had been put in the mouth of the simpleminded Aristotelian in the dialogue as a deliberate personal taunt by Galileo. Next, a search of the Inquisition files of 1616 disclosed the questionable document previously mentioned, which contained a specific threat of imprisonment for Galileo if he ever again discussed the Copernican doctrine in any way. Urban, having known nothing of any personal injunction at the time Galileo sought his permission to write the book, assumed that Galileo had deceitfully concealed it from him. The case was thereafter prosecuted with vindictive hostility. Galileo, who had either never received a personal injunction or had been told that it was without force, was unaware of any wrongdoing in this respect.

Confined to bed by serious illness, he at first refused to go to Rome. The grand duke and his Roman ambassador intervened stoutly in his behalf, but the pope was adamant. Despite medical certificates that travel in the winter might be fatal, Galileo was threatened with forcible removal in chains unless he capitulated. The grand duke, feeling that no more could be done, provided a litter for the journey, and Galileo was taken to Rome in February 1633.

The outcome of the trial, which began in April, was inevitable. Although Galileo was able to produce an affidavit of Cardinal Bellarmine to the effect that he had been instructed only according to the general edict that governed all Catholics, he was persuaded in an extrajudicial procedure to acknowledge that in the Dialogue he had gone too far in his arguments for Copernicus. On the basis of that admission, his Dialogue was put on the Index, and Galileo was sentenced to life imprisonment after abjuring the Copernican “heresy.” The terms of imprisonment were immediately commuted to permanent house arrest under surveillance. He was at first sent to Siena, under the charge of its archbishop, Ascanio Piccolomini. Piccolomini, who is said to have been Galileo’s former pupil, was very friendly to him. Within a few weeks he had revived Galileo’s spirits—so crushed by the sentence that his life had been feared for—and induced him to take up once more his old work in mechanics and bring it to a conclusion. While at Siena, Galileo began the task of putting his lifelong achievements in physics into dialogue form, using the same interlocutors as in the Dialogue.

Piccolomini’s treatment of Galileo as an honored guest, rather than as a prisoner of the Inquisition, was duly reported to Rome. To avoid further scandal, Galileo was transferred early in 1634 to his villa at Arcetri, in the hills above Florence. It was probably on the occasion of his departure from Siena that he uttered the celebrated phrase “Eppur si muove,” apocryphally said to have been muttered as he rose to his feet after abjuring on his knees before the Cardinals Inquisitors in Rome. The celebrated phrase, long considered legendary, was ultimately discovered on a fanciful portrait of Galileo in prison, executed about 1640 by Murillo or one of his pupils at Madrid, where the archbishop’s brother was stationed as a military officer.

Galileo was particularly anxious to return to Florence to be near his elder daughter. But she died shortly after his return, in April 1634, following a brief illness. For a time, Galileo lost all interest in his work and in life itself. But the unfinished work on motion again absorbed his attention, and within a year it was virtually finished. Now another problem faced him: the printing of any of his books, old or new, had been forbidden by the Congregation of the Index. A manuscript copy was nevertheless smuggled out to France, and the Elzevirs at Leiden undertook to print it. By the time it was issued, in 1638, Galileo had become completely blind.

Two New Sciences. The title of his final work, Discourses and Mathematical Demonstrations Concerning Two New Sciences (generally known in English by the last three words), hardly conveys a clear idea of its organization and contents. The two sciences with which the book principally deals are the engineering science of strength of materials and the mathematical science of kinematics. The first, as Galileo presents it, is founded on the law of the lever; breaking strength is treated as a branch of statics. The second has its basis in the assumption of uniformity and simplicity in nature, complemented by certain dynamic assumptions. Galileo is clearly uncomfortable about the necessity of borrowing anything from mechanics in his mathematical treatment of motion. A supplementary justification for that procedure was dictated later by the blind Galileo for inclusion in future editions.

Of the four dialogues contained in the book, the last two are devoted to the treatment of uniform and accelerated motion and the discussion of parabolic trajectories. The first two deal with problems related to the constitution of matter; the nature of mathematics; the place of experiment and reason in science; the weight of air; the nature of sound; the speed of light; and other fragmentary comments on physics as a whole. Thus Galileo’s Two New Sciences underlies modern physics not only because it contains the elements of the mathematical treatment of motion, but also because most of the p′roblems that came rather quickly to be seen as problems amenable to physical experiment and mathematical analysis were gathered together in this book with suggestive discussions of their possible solution. Philosophical considerations as such were minimized.

The book opens with the observation that practical mechanics affords a vast field for investigation. Shipbuilders know that large frameworks must be strongly supported lest they break of their own weight, while small frameworks are in no such danger. But if mathematics underlies physics, why should geometrically similar figures behave differently by reason of size alone? In this way the subject of strength of materials is introduced. The virtual lever is made the basis of a theory of fracture, without consideration of compression or stress; we can see at once the inadequacy of the theory and its value as a starting point for correct analysis. Galileo’s attention turns next to the problem of cohesion. It seems to him that matter consists of finite indivisible parts, parti quante, while at the same time the analysis of matter must, by its mathematical nature, involve infinitesimals, parti non quante. He does not conceal—but rather stresses—the resulting paradoxes. An inability to solve them (as he saw it) must not cause us to despair of understanding what we can. Galileo regards the concepts of “greater than,” “less than,” and “equal to” as simply not applicable to infinite multitudes; he illustrates this by putting the natural numbers and their squares in one-to-one correspondence.

Galileo had composed a treatise on continuous quantity (now lost) as early as 1609 and had devoted much further study to the subject. Bonaventura Cavalieri, who took his start from Galileo’s analysis, importuned him to publish that work in order that Cavalieri might proceed with the publication of his own Geometry by Indivisibles. But Galileo’s interest in pure mathematics was always overshadowed by his concern with physics, and all that is known of his analysis of the continuum is to be found among his digressions when discussing physical problems.

Galileo’s parti non quante seem to account for his curious physical treatment of vacua. His attention had been directed to failure of suction pumps and siphons for columns of water beyond a fixed height. He accounted for this by treating water as a material having its own limited tensile strength, on the analogy of rope or copper wire, which will break of its own weight if sufficiently long. The cohesion of matter seemed to him best explained by the existence of minute vacua. Not only did he fail to suggest the weight of air as an explanation of the siphon phenomena, but he rejected that explanation when it was clearly offered to him in a letter by G. B. Baliani. Yet Galileo was not only familiar with the weight of air; he had himself devised practicable methods for its determination, set forth in this same book, giving even the correction for the buoyancy of the air in which the weighing was conducted.

Phenomena of the pendulum occupy a considerable place in the Two New Sciences. The relation of period to length of pendulum was first given here, although it probably represents one of Galileo’s earliest precise physical observations. Precise isochronism of the pendulum appears to have been the one result he most wished to derive deductively. In discussing resistance of the air to projectile motion, he invoked observations (grossly exaggerated) of the identity of period between two pendulums of equal length weighted by bobs of widely different specific gravity. He deduced the existence of terminal constant velocity for any body falling through air, or any other medium, but mistakenly believed increase of resistance to be proportional to velocity.

Like the pendulum, the inclined plane plays a large role in Galileo’s ultimate discussion of motion. The logical structure of his kinematics, as presented in the Two New Sciences, is this: He first defines uniform motion as that in which proportional spaces are covered in proportional times, and he then develops its laws. Next he defines uniform acceleration as that in which equal increments of velocity are acquired in equal times and shows that the resulting relations conform to those found in free fall. Postulating that the path of descent from a given height does not affect the velocity acquired at the end of a given vertical drop, he describes an experimental apparatus capable of disclosing time and distance ratios along planes of differing tilts and lengths; finally, he asserts the agreement of experiment with his theory. The experiments have been repeated in modern times, precisely as described in the Two New Sciences, and they give the results asserted. Following these definitions, assumptions, and confirmation by experiment, Galileo proceeds to derive a great many theorems related to accelerated motion.

In the last section Galileo deduces the parabolic trajectory of projectiles from a composition of uniform horizontal motion and accelerated vertical motion. Here the concept of rectilinear inertia, previously illustrated in the Dialogue (“Second Day”), is mathematically applied but not expressly formulated. This is followed by additional theorems relating to trajectories and by tables of altitude and distance calculated for oblique initial paths. Because of air resistance at high velocities, the tables assumed low speeds and hence were of no practical importance in gunnery. But like Galileo’s theory of fracture, they opened the way for rapid successive refinements at the hands of others.

Last Years. Galileo lived four years, totally blind, beyond the publication of his final book. During this time, he had the companionship of Vincenzio Viviani, who succeeded him (after Evangelista Torricelli) as mathematician to the grand duke and who inherited his papers. Viviani wrote a brief account of Galileo’s life in 1654 at the request of Leopold de’ Medici, which, despite some demonstrable errors, is still a principal source of biographical information, in conjunction with the voluminous correspondence of Galileo that has survived and with the autobiographical passages in his works. Near the end of his life, Galileo was also visited by Torricelli, a pupil of Castelli and the ablest physicist among Galileo’s immediate disciples. Galileo’s son, Vincenzio, also assisted in taking notes of his father’s later reflections, in particular the design of a timekeeping device controlled by a pendulum.

Galileo died at Arcetri early in 1642, five weeks before his seventy-eighth birthday. The vindictiveness of Urban VIII, who had denied even Galileo’s requests to attend mass on Easter and to consult doctors in nearby Florence when his sight was failing, continued after Galileo’s death: The grand duke wished to erect a suitable tomb for Galileo but was warned to do nothing that might reflect unfavorably on the Holy Office. Galileo was buried at Santa Croce in Florence, but nearly a century elapsed before his remains were transferred, with a suitable monument and inscription, to their present place in the same church.

Sources of Galileo’s Physics. The habitual association of Galileo’s name with the rapid rise of scientific activity after 1600 makes the investigation of his sources a matter of particular interest to historians of science.

All agree that Archimedes was a prime source and model for Galileo, who himself avowed the fact. The work of Aristotle and the pseudo-Aristotelian Questions of Mechanics were likewise admitted inspirations to Galileo, although often only as targets of criticism and attack. The astronomy of Copernicus and the magnetic researches of William Gilbert were obvious and acknowledged sources of his work. Beyond these, there is little agreement.

Among sixteenth-century writers, Galileo probably drew chiefly on Niccolò Tartaglia, Girolamo Cardano, and Guidobaldo del Monte. Parallels between his early unpublished work and that of Benedetti are very striking, but the establishment of a direct connection is difficult. As with the case of Stevin, the parallels in thought may result from the Archimedean revival and a common outlook rather than from early and direct knowledge of Benedetti’s work.

Similarly, a direct influence of medieval writers on Galileo, although widely accepted by most historians, is still largely conjectured on the basis of specific parallels. The statics of Jordanus de Nemore was widely known in Italy after 1546, when Tartaglia published in Italian and endorsed the “science of weights” as necessary to an understanding of the balance; yet all subsequent writers (at least in Italy) condemned it in favor of the Archimedean approach. Writings of the Merton school, published repeatedly in Italy up to about 1520, continued to be discussed thereafter at Paris and in Spain. Galileo’s reasoning about acceleration, after his recognition of its importance around 1602, invariably proceeded by comparison of ratios, whereas medieval writers adopted a mean speed as representative of uniformly changing velocities. Medieval impetus theory, which Galileo adopted at first for the explanation of projectile motion, had no place in the concept of neutral motions that led him eventually to an inertial terrestrial physics. A connection of Galileo’s own physical thought with medieval sources may yet be convincingly established, but at present this has not been done.

Experiment and Mathematics. The role of experiment in Galileo’s physics was limited to the testing of preconceived mathematical rules and did not extend to the systematic search for such rules. It is probable that his use of experiment had its roots in the musical controversy conducted by his father rather than in philosophical considerations of method. Appeal to experiment in his published works was resorted to by Galileo chiefly as a means of confuting rival theories, as in the dispute over bodies in water and in his rejection of proportionality of speed to space traversed in free fall.

It is difficult to find older sources for Galileo’s attitude toward mathematics, which was strikingly modern. He considered mathematics to enjoy a superior certainty over logic. Where a mathematical relation could be found in nature, Galileo accepted it as a valid description and discouraged further search for ulterior causes. He attributed discrepancies between mathematics and physical events to the investigator who did not yet know how to balance his books. Galileo did not adopt the traditional Platonist view that our world is a defective copy of the “real” world, and he derided philosophical speculation about a world on paper.

The Influence of Galileo. Except with respect to the acceptance of Copernican astronomy, Galileo’s direct influence on science outside Italy was probably not very great. After 1610 he published his books in Italian and made little effort to persuade professional scholars either at home or abroad. His influence on educated laymen both in Italy and abroad was considerable; on university professors, except for a few who were his own pupils, it was negligible. Latin translations of his Dialogue appeared in Holland in 1635, in France in 1641, and in England in 1663; but the only Latin translation of the Two New sciences was published in 1700, long after Newton’s principia had superseded it.

Between Galileo and Newton, science was Cartesian rather than Galilean. Indirectly, Galileo’s science exerted some influence in France through Marin Mersenne, Pierre Gassendi, and Nicholas Fabri de Peiresc; in Germany through Kepler; and in England through John Wilkins and John Wallis. Descartes, who repudiated Galileo’s approach to physics because of its neglect of the essence of motion and physical causation, did not mention him in any published work. Newton seems not to have read Galileo’s Two New Sciences, at least not before 1700, but knew his Dialogue as early as 1666. Aware of his achievements in physics only indirectly, Newton, in the Principia, mistakenly credited Galileo with a derivation of the laws of falling bodies from the law of inertia and the force-acceleration relationship.

Within Italy, Galileo had a strong following both in scientific and nonscientific circles. His ablest pupil, Castelli, was the teacher of Torricelli and Cavalieri, both of whom also had personal acquaintance with Galileo. His last pupil, Viviani, did much to extend Galileo’s influence in the succeeding generation, editing the first collection of his works in 1655–1656. But by that time physics and astronomy had both progressed well beyond the point where Galileo had left them.

Outside scientific circles, Galileo’s influence was strongly felt in the battle for freedom of inquiry and against authority. English translations of his Dialogue and Letter to Christina, published in 1661, carried this influence outside academic circles. John Milton cited the fate of Galileo in his Areopagitica. French writers during the Enlightenment also made Galileo a symbol of religious persecution.

Personal Traits. Galileo was of average stature, squarely built, and of lively appearance and disposition. Viviani remarks that he was quick to anger and as quickly mollified. His unusual talents as a speaker and as a teacher are beyond question. Among those who knew him personally, even including adversaries, few seem to have disliked him. Many distinguished men became his devoted friends, and some sacrificed their own interests in his support at crucial periods. On the other hand, there were many contemporary rumores discreditable to Galileo, and demonstrable slanders occur in letters of Georg Fugger, Martin Horky, and others. Pugnacious rather than belligerent, he refrained from starting polemic battles but was ruthless in their prosecution when he answered an attack at all. His friends included artists and men of letters as well as mathematicians and scientists; cardinals as well as rulers; craftsmen as well as learned men. His enemies included conservative professors, several priests, most philosophers, and those scientists who had publicly challenged him and felt the bite of his sarcasm in return.

Caution and daring both had a place in Galileo’s personality. His reluctance to speak out for the Copernican system until he had optical evidence against the rival theories is evidence of scientific prudence rather than of professorial timidity. Once convinced by his own eyes and mind, he would not be swayed even by the advice of well-informed friends who urged him to proceed with caution. In the writings he withheld from publication, as in his surviving notes, many errors and wrong conjectures are to be found; in his published works, very few. He was as respectful of authority in religion and politics as he was contemptuous of it in matters he could investigate for himself. It is noteworthy that before his Copernican stand was challenged by an official Church edict, he had composed and submitted to the authorities a carefully documented program, based on positions of Church fathers, that would have obviated official intervention against his science—a program that was in fact adopted by a pope nearly three centuries later as theologically sound.


I. Original Works. All works by Galileo and virtually all known Galilean correspondence and manuscripts are contained in Le opere di Galileo Galilei, Antonio Favaro, ed., 20 vols. (Florence, 1890–1909); repr. with some additions (Florence, 1929–1939; 1965). English translations of Galileo’s principal works are listed below. Following the translator’s name are the English book title, the abbreviated original title of each work included, and date of first ed. or approximate date of composition.

T. Salusbury, Mathematical Collections and Translations. I (London, 1661; repr. 1967), Lettera a Madama Cristina (ca. 1615) and Dialogo (Florence, 1632); II (London, 1665; repr. 1967), La bilancetta (ca. 1586); Le meccaniche (ca. 1600); Discorso . . . intorno alle cose che stanno in su l’acqua (Florence, 1612); and Discorsi (Leiden, 1638).

T. Weston, Mathematical Discourses Concerning Two New Sciences (London, 1730; 2nd ed. 1734); Discorsi (Leinden, 1638).

E. Carlos, The Sidereal Messenger (London, 1880; repr. 1959); Sidereus nuncius (Venice, 1610).

H. Crew and A. De Salvio, Dialogues Concerning Two New Sciences (New York, 1914; repr. n.d.); Discorsi (Leiden, 1638).

G. de Santillana, ed., Dialogue on the Great World Systems (Chicago, 1953), the Salusbury trans.: Dialogo (Florence, 1632).

S. Drake, Dialogue Concerning the Two Chief World Systems (Berkeley, Cal., 1953; rev. 1967); Dialogo (Florence, 1632).

S. Drake, Discoveries and Opinions of Galileo (New York 1957); Sidereus nuncius (Venice, 1610); Lettere sulle macchie solari (Rome, 1613); Lettera a Madama Cristina (ca. 1615); and Il saggiatore (Rome, 1623).

I. Drabkin and S. Drake, Galileo on Motion and on Mechanics (Madison, Wis., 1960); De motu (ca. 1590), and Le meccaniche (ca. 1600).

S. Drake and C. D. O’Malley, The Controversy on the Comets of 1618 (Philadelphia, 1960): Discorso sulle comete (Florence, 1619), and Il saggiatore (Rome, 1623).

S. Drake, ed., Galileo on Bodies in Water, (Urbana, Ill., 1960), the Salusbury trans.: Discorso (Florence, 1612).

L. Fermi and G. Bernadini, Galileo and the Scientific Revolution (New York, 1961): C. S. Smith, trans., La bilancetta (ca. 1586).

S. Drake and I. Drabkin, Mechanics in Sixteenth-Century Italy (Madison, Wis., 1969): Dialogus de motu (ca. 1589).

II. Secondary Literatrure, Nearly 6,000 titles relating to Galileo are listed in the following bibliographies: Bibliografia Galileiana, 1568–1895, A. Carli and A. Favaro, eds. (Rome, 1896); Bibliografia Galileiana, Primo Supplemento, 1896–1940. G. Boffito, ed. (Rome, 1943); “Bibliografia Galileiana, 1940–1964,” in Galileo, Man of Science, E. McMullin, ed. (New York, 1967); E. Gentili, Bibliografia Galileiana fra i due centenari (1942–1964) (Varese, 1966).

Selected biographies are A. Banfi, Galileo Galilei (Milan, 1948); J. Fahie, Galileo: His Life and Works (London, 1903); A. Favaro, Galileo e lo studio di Padova (Florence, 1883); and Galileo Galilei e Suor Maria Celeste (Florence, 1891); K. von Gebler, Galileo Galilei and the Roman Curia (London, 1879), English trans., Mrs. G. Sturge; L. Geymonat, Galileo Galilei (Milan, 1957), English trans; S. Drake (New York, 1965); T. Martin, Galilée (Paris, 1868); L. Olschki, Galilei und seine Zeit, (Hallem 1927; repr., Vaduz, 1965); M. Allen-Olney, The Private Life of Galileo (London, 1870); P. Paschini, Vita e opere di Galileo Galilei, (Rome, 1965); F. Reusch, Der Process Galilei’s und die Jesuiten (Bonn, 1879); G. de Santillana, The Crime of Galileo (Chicago, 1955); F. Taylor, Galileo and the Freedom of Thought (London, 1938); E. Wohlwill, Galilei und sein Kampf (Hamburg-Leipzig, 1909, 1926).

Fundamental to the study of Galileo’s scientific work are the publications of A. Favaro listed in G. Favaro, Bibliografia Galileiana di A. Favaro (Venice, 1942); A. Koyré, Études Galiléennes (Paris, 1939; repr., Paris, 1966); and M. Clavelin, La philosophic naturelle de Galilée (Paris, 1968).

Collections of modern Galilean studies include Nel terzocentenario della morte di Galileo Galilei (Milan, 1942); M. Kaplon, ed., Homage to Galileo (Cambridge, Mass., 1965); Nel quarto centenario della nascita di Galileo (Milan, 1966); C. Golino, ed., Galileo Reappraised (Berkeley, Cal., 1966); Atti del Symposium Internazionale . . . “Galileo nella storia e nella filosofia della scienza” (Vinci, 1967); C. Maccagni, ed., Saggi su Galileo Galilei (Florence, 1967–); E. McMullin, ed., Galileo: Man of Science (New York, 1967); Galilée, Aspects de sa vie et de son oeuvre, preface by Suzanne Delorme (Paris, 1968); and S. Drake, Glileo Studies (Ann Arbor, Mich., 1970).

Separate articles and monographs are listed in the bibliographies cited above.

Stillman Drake

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Galilei, Galileo


(b. Pisa, Italy, 15 February 1564; d. Arcetri, Italy, 8 January 1642),

physics, astronomy, philosophy of science. For the original article on Galileo see DSB, vol. 5.

Since the publication of Stillman Drake’s DSB entry on Galileo in 1972, Galilean studies have come a long way. New translations have made Galileo’s works accessible to a large, multinational reading public. New critical editions of his texts, in many cases complemented by a sophisticated apparatus and explanatory comments, have helped to provide a more profound appraisal of his splendid prose. Above all, a host of studies on nearly every aspect of Galileo’s life and accomplishments have deepened scholars’ understanding of his eminent intellectual legacy. As a rough indication as of 2007 of the number of studies on Galileo published after Drake’s DSB article, suffice it to say that the International Galilean Bibliography (edited by the Institute and Museum of History of Science in Florence) counts more than 6,100 post-1972 records.

Thanks to this vast number of studies scholars are in a position to assess from a sounder historical standpoint the many facets of Galileo’s achievements. Even though the results of these studies do not significantly affect the excellent outline provided by Drake, subsequent works have added pieces of information that deserve to be taken into account in order to achieve a more comprehensive appraisal of Galileo’s accomplishments.

Editions . A noteworthy feature of Galilean studies since the 1970s is the publication of new, often outstanding editions of Galileo’s works. Of special value are the Sidereus nuncius edited by Isabelle Pantin (with French translation and very detailed notes) and the critical editions of the Dialogue on the Two Chief Systems, the Discourse on the Comets, and The Assayer prepared by Ottavio Besomi and Mario Helbing. All these editions couple philological exactitude with extensive and accurate commentaries. Of great interest for Galilean scholars is the edition of the proceedings of Galileo’s trial by Father Sergio Pagano, which adds to the known documentation some materials not included in the Edizione Nazionale (National Edition) of Galileo’s works.

Readers in English have increased opportunities to read Galileo’s texts thanks to improved or fresh translations. In 1974 Stillman Drake replaced the old version of the Two New Sciences by Henry Crew and Alfonso De Salvio (first issued in 1914) with a subsequent, more careful one. Further, in 1977 William Wallace translated from the Latin Galileo’s Early Notebooks(the so-called Juvenilia), and in 1989 Albert Van Helden edited a remarkable English Sidereus nuncius (The Sidereal Messenger). In the same year, Maurice Finocchiaro translated the most relevant documents pertaining to the “Galileo affair,” comprising the theological letters to Benedetto Castelli, Piero Dini, and Grand Duchess Cristina, as well as the Discourse on the Tides (1616) and the Reply to Francesco Ingoli (1624). Eight years later, in 1997, Finocchiaro also published a large collection of excerpts from Galileo’s Dialogue, which he complemented with explanatory notes.

Editorial work has also been actively engaged with Galileo’s unpublished texts. This has been the case notably with the logical notes of Manuscript 27 of the Galilean Collection in the National Library of Florence, which were integrally edited by William F. Edwards and William Wallace in 1988 under the title Tractatio de praecognitionibus et praecognitis and Tractatio de demonstratione. Wallace also provided an English translation of these treatises, emphasizing their importance for the development of Galileo’s scientific methodology.

Another remarkable edition concerns the Galilean Manuscript 72, whose contents show the evolution of Galileo’s thought on mechanics from his early years in Padua to the printing of Two New Sciences in 1638. Under the title Galileo Galilei’s Notes on Motion, the Central National Library (Florence), the Institute and Museum of History of Science (Florence), and the Max Planck Institute for the History of Science (Berlin) have carried out an innovative project, publishing on the Internet an electronic reproduction (accompanied by transcriptions and apparatus) of this manuscript. The Institute and Museum of History of Science in Florence has also edited a Web archive, Galileothek@, which offers texts of all of Galileo’s works as well as images, bibliographical records, lexicographical and thematic indexes, sections devoted to experiments, and a detailed Galilean chronology, along with powerful tools for searching and navigating through the various repositories.

Finally, it should be added that a project of updating the masterful twenty-volume National Edition of Galileo’s Works (Opere), edited by Antonio Favaro and published between 1890 and 1909, was initiated in 2006. This project, anticipated to be completed by 2010, includes the publication of several volumes devoted to all those Galilean materials (works, letters, documents) that were unknown to Favaro and accordingly were not included in his edition.

Jesuit Sources . One of the most interesting debates surrounding Galileo concerns the sources of the above-mentioned notes on logic (Galilean Manuscript 27) and of the treatises edited by Favaro under the title Juvenilia. A few scholars (Alistair Crombie, Adriano Carugo, and William Wallace) have argued that these texts are based on works of Jesuit authors. As the specific sources of Galileo’s Juvenilia they name Franciscus Toletus’s commentaries (1573 and 1575) on Aristotle’s Physics and De generatione et corruptione, Benedictus Pererius’s textbook De communibus omnium rerum naturalium principiis et affectionibus(1576), and the commentary on Sacrobosco’s Sphere by the distinguished Jesuit astronomer Christopher Clavius (1581). More controversial is the identification of the precise texts said to have inspired the notes on logic of Galileo’s Manuscript 27. Crombie and Carugo held that Galileo relied on a printed book (Ludovico Carbone’s Additamenta ad commentaria D. Francisci Toleti in Logicam, 1597), while Wallace has maintained that the real source of these Galilean comments on Aristotle’s Posterior Analytics was a manuscript reportatio of the logic course offered at the Collegio Romano by the Jesuit professor Paolo Della Valle during the academic year 1587–1588. Though no copy of this manuscript is extant, Wallace holds that its contents were plagiarized by Ludovico Carbone, a circumstance that would account for the resemblances between Galileo’s Manuscript 27 and Carbone’s Additamenta.

On these grounds, Wallace has emphasized the crucial role played by Aristotelian logic and methodology for Galileo’s achievements, adding that the alleged strong epistemological continuity perceptible in Galileo’s scientific evolution is the result of his unwavering reliance on the Aristotelian demonstrative method he learned from the Jesuit commentaries. Wallace’s conclusion is in fact quite bold, arguing that “Galileo’s methodology was already spelled out in the treatises he appropriated from the Collegio Romano” (1992, p. xvi). Although it is not possible to provide here a detailed survey of Wallace’s arguments, it must be observed that, apart from its pronounced conjectural character—there is no compelling evidence of Galileo’s use of the Collegio Romano’s materials—Wallace’s reconstruction obscures Galileo’s vehement anti-Aristotelian polemic, which actually forms a substantial part of his accomplishments.

As antidote to such a “pan-logical” view of Galileo’s epistemology, one should also bear in mind what Galileo claims in the “Second Day” of the Two New Sciences: “It seems to me that logic teaches how to know whether or not reasonings and demonstrations already discovered are conclusive, but I do not believe that it teaches how to find conclusive reasonings and demonstrations” (Drake trans., 2nd edition, 1989, p. 133).

Galilean Manuscript 72 . As a central contribution to modern science, Galileo’s theory of motion has always attracted much scholarly attention. In the last decades of the twentieth century a more precise assessment of the development of Galileo’s views on this matter became possible thanks to more careful studies of the scraps of Manuscript 72.

It is known that Galileo had planned to write a treatise on motion prior to his discoveries with the telescope in the 1609–1610 period. In May 1610 he wrote to the secretary of the Grand Duke of Tuscany, Belisario Vinta, that he was about to bring to completion “three books on local motion, an entirely new science, no one else, ancient or modern, having discovered some of the very many admirable properties that I demonstrate to exist in natural and forced motions” (Opere, Edizione Nazionale a cura di A. Favaro, Florence: Giunti Barbera, 1890–1909, repr. 1968, X, pp. 351–52). However, because Galileo embarked on different scientific pursuits and became involved in a number of scientific disputes, he was unable to bring out his “new science of motion” before 1638, when he published the Two New Sciences. Manuscript 72 constitutes a kind of filing cabinet in which Galileo saved the drafts of the theorems that he was to include in his Two New Sciences, along with numerous textual fragments, drawings, and calculations related to his mechanical research. Because they cover a period of nearly forty years, the materials of the codex are of the utmost importance for a more precise appraisal of Galileo’s route to his final theory of motion. For this reason, Galileo scholarship has paid increasing attention to Manuscript 72.

A remarkable result of these studies concerns clues in Manuscript 72 that indicate Galileo carried out an extensive experimental program. Several diagrams and calculations contained in the codex seem to provide evidence that, since the earliest years of the seventeenth century, Galileo performed experiments by rolling balls down planes inclined at small angles to the horizontal and by studying the swings of pendulums of different lengths. Although scholars have proposed different interpretations and chronologies of its contents, consensus exists that several folios of Manuscript 72 record experimental data. This evidence strongly reinforces the thesis that an important part of Galileo’s accomplishments in mechanics was rooted in experimentation.

Thus, while in his entry for the DSB in 1972 Drake wrote that “the role of experiment in Galileo’s physics was limited to the testing of preconceived mathematical rules and did not extend to the systematic search for such rules” (p. 247), seven years later, in 1979, he argued that the contents of Manuscript 72 bear out the conclusion that “Galileo found the law of free fall by experiment, or rather by the making of very careful measurements” (1979, p. x).

Nevertheless, it is still difficult to ascertain whether Galileo resorted to experiments merely to confirm the results he had already obtained via mathematical reasoning or whether the experimentation itself played a role in obtaining the results. At any rate, relying on careful survey of the contents of Manuscript 72, one can confidently assume that experimental practice was an essential constitutive element of Galileo’s “new science of motion.”

Atomism and the Eucharist . In section 48 of The Assayer(1623), Galileo set forth a theory of knowledge based on a sharp distinction between “objective” and “subjective” qualities. According to this view, whereas features such as shape, size, position, motion, and number are qualities intrinsic to real things, impressions such as colors, tastes, smells, or tactile properties do not exist in the objects themselves but only in the sentient subject experiencing them. For this reason, sensible qualities were characterized by Galileo as “mere names,” qualities that “reside only in the consciousness” and that would be “wiped away and annihilated” once human sensibility is removed. Behind sensible qualities are the true components of the real world, atoms, whose impinging on the sense organs produces sensory impressions. Hence, for example, the sensation of heat stems from the motion of a “multitude of minute particles” that penetrate human bodies; “their touch as felt by us when they pass through our substance is the sensation we call ‘heat.’” (trans. in Drake, Discoveries and Opinions of Galileo, New York: Anchor Books, 1957, p. 277). Galileo’s stance was clearly rooted in the tradition of ancient atomism, whose most distinguished representatives, such as Democritus and Lucretius, had already stated similar views.

Two documents discovered in the Archives of the Congregation for the Doctrine of the Faith (formerly Holy Office) show that Galileo’s atomistic theory was brought to the attention of the Inquisitorial authorities, most likely before the trial of 1633. The first document was found in 1982 by Pietro Redondi and is usually referred to as “G3,” from the code appearing on the top of its first page. G3 is a denunciation of the atomism of The Assayer. The anonymous author protested that Galileo’s interpretation of sensible qualities clashed with the Catholic doctrine of the Eucharist, according to which, after consecration in the Mass, bread and wine become the body and blood of Jesus Christ. This transformation is understood as transubstantiation because it concerns the

substances of bread and wine, whereas their “accidents,” or apparent qualities (color, odor, exterior shape), remain unchanged by virtue of a divine miracle. The author of G3 remarked that, according to the terms of Galileo’s argument, it would be impossible to separate the accidental properties of bread and wine from their own substances. Indeed, because those accidental properties are regarded as “mere names” and as nonexistent outside human sensory perception, on the basis of Galileo’s theory one would be obliged to conclude that “in the Sacrament there are substantial elements of the bread and the wine, which is an error condemned by the Sacred Council of Trent.”

Redondi dated G3 to 1624 and attributed it to Orazio Grassi, the Jesuit mathematician against whom Galileo had written The Assayer. Redondi also connected the document to the trial of 1633, suggesting that the charge of Copernicanism that motivated the trial was a stratagem devised by Pope Urban VIII (a former friend of Galileo) in order to avoid having the scientist face the more serious accusation of Eucharistic heresy.

Redondi’s ascription of G3 to Grassi has been proved to be mistaken, and his thesis concerning the “true” (although disguised) reasons of the trial has been generally rejected by scholars. Nevertheless, Redondi’s book triggered a fresh wave of interest in the Galileo affair and renewed investigations into its cultural and political context.

Another document, similar to G3, was discovered by Mariano Artigas in 1999 and has been carefully studied by Artigas himself along with Rafael Martinez and William Shea. This document is placed in the same volume as G3, the volume EE of the collection “Acta et Documenta,” where it occupies sheet 291. For this reason it has been called “EE 291.”

Like G3, EE 291 is anonymous (it is in Latin while the former is in Italian), and it equally develops a criticism of the theory of sensible qualities expounded in The Assayer, which it deems incompatible with the doctrine of the Eucharist. The author of this document has been identified as the Jesuit Melchior Inchofer, who probably was a member of the commission appointed by the pope in the summer of 1632 to examine Galileo’s Dialogue. Inchofer, a firm opponent of Copernicanism, could have written EE 291 in order to worsen Galileo’s position by adding a further charge against him. Thus, the discoveries of G3 and EE 291, besides providing valuable pieces of information on previously unknown episodes of Galileo’s life, also opened a new chapter of investigation concerning the difficult relationship between atomism and Eucharistic doctrine.

The Role of Patronage . It is well known that Galileo spent a great part of his mature life, from 1610 until his death in 1642, at the Medici court as mathematician and philosopher of the Grand Duke of Tuscany. Furthermore, even before his return to Florence from Padua (in the autumn of 1610), Galileo had to deal with several patrons in order to promote his career and to obtain academic positions. Indeed, the practice of relying on the support of influential patrons was quite normal at the time. As Richard S. Westfall remarked: “Patronage was perhaps the most pervasive institution of preindustrial society” (1985, p. 29); hence: “the system of patronage […] was a feature of 17th century life as distinctive as scientific technology is in the 20th century” (1984, p. 200).

For this reason, the last decades have witnessed a growing interest in re-interpreting Galileo’s life and achievement in the light of the patronage culture. Richard S. Westfall focused on the role played by patronage in Galileo’s relationship with the Accademia dei Lincei (1984) and the Jesuit order (1988) as well as in the controversial episode of the discovery of the phases of Venus (1985), while Frederick Hammond provided a fascinating outline of the connection between Pope Barberini’s system of patronage and the Galileo affair.

But the most comprehensive study on this matter is certainly Mario Biagioli’s seminal book, Galileo Courtier. Biagioli argues that “Galileo’s courtly role was integral to his science” (1993, p. 1), because “the court contributed to the cognitive legitimation of the new science by providing venues for the social legitimation of its practitioners” (1993, p. 2). Actually, courtly patronage being “the social world of Galileo’s science” (1993, p. 4), the latter was involved in a process of self-fashioning, aimed to work out a fresh social and intellectual image, best fitted to courtly codes and rules. In this process, Galileo “used the resources he perceived in the surrounding environment to construct a new socio-professional identity for himself, to put forward a new natural philosophy, and to develop a courtly audience for it” (1993, p. 5). In short, Biagioli views Galileo “not only as a rational manipulator of the patronage machinery but also as somebody whose discourse, motivations, and intellectual choices were informed by the patronage culture in which he operated throughout his life” (1993, p. 4).

Biagioli’s detailed account (based on detailed documentation from primary and secondary sources) mainly concerns Galileo’s experience at the Medici court, spanning from 1610 to just after the 1633 trial. The core of his interpretation relies on the assumption that the social legitimation Galileo acquired in the courtly milieu assured the cognitive legitimation of his theories.

While innovative and appealing, Biagioli’s historio-graphical proposal runs the strong risk of being sometimes unreliable and implausible. For example, Biagioli views Galileo’s Copernican commitment as an outcome of a strategy based on the logic of patronage. As he puts it: “Copernicanism was the ‘natural’ choice for someone such as Galileo who aspired to a higher socioprofessional status, while the court was the social space that could best legitimize such an unusual socioprofessional identity” (1993, p. 226). Hence, “the increasing commitment to Copernican astronomy that Galileo developed in those years [i. e. after 1609–1610] may have resulted also from the patronage dynamics that pushed him to defend his discoveries and produce even more of them” (1993, p. 91).

This seems an oversimplified account of the motivations that drove Galileo to embrace the Copernican theory, because it completely ignores the theoretical reasons behind his choice, which were rooted in the interplay between astronomical arguments and the principles of Galileo’s “new science of motion.” Indeed, by reducing the cognitive acceptance of science to its social legitimation, Biagioli tends to obscure the autonomy of scientific debate. Consequently, he often disregards the multifaceted complexity of history, failing to recognize that ideas follow often their own paths, connected to, but not always dependent on, social features.

The Galileo Affair Revisited . On 10 November 1979, on the occasion of the one hundredth anniversary of the birth of Albert Einstein, before a plenary session of the Pontifical Academy of Sciences, Pope John Paul II delivered an address on the “deep harmony that unites the truths of science with the truth of faith” (L’Osservatore Romano, English week edition, November 26, 1979, pp. 9–10). In his speech John Paul II dealt with the trial and condemnation of Galileo, frankly admitting that the scientist “had to suffer a great deal at the hands of men and organs of the Church.” John Paul II expressed the hope that “theologians, scholars and historians, animated by a spirit of sincere collaboration, will study the Galileo case more deeply and, in loyal recognition of wrongs from whatever side they come, will dispel the mistrust that still opposes, in many minds, a fruitful concord between science and faith.”

As a consequence of this wish, in July 1981, the Vatican constituted a study commission divided into various sections (exegetical, cultural, scientific-epistemological, and historical-juridical). The commission met several times, held a few conferences, and issued a significant number of publications. Its work was declared to be concluded on 31 October 1992, at an audience given by the pope at a plenary session of the Pontifical Academy. On that occasion the pope underlined Galileo’s mistake in not presenting the Copernican system as a hypothesis, because “it had not been confirmed by irrefutable proofs” (trans. in Fantoli, 2003, p. 370). Yet John Paul II acknowledged that Galileo’s views on scriptural interpretation were sounder than those put forth by the theologians of his epoch. The pope also claimed that the Galileo affair resulted from a “tragic mutual incomprehension” that would have poisoned the subsequent relationship between faith and science, creating the myth of the Church’s opposition to the free search for truth. He concluded that “the clarifications furnished by recent historical studies enable us to state that this sad misunderstanding now belongs to the past.”

John Paul II’s words were of the highest importance, marking a break with the Church’s long-held attitude toward Galileo by honestly recognizing the errors committed by the Catholic Church. Nevertheless, some of the arguments put forth by John Paul II suggested a defensive strategy not consonant with that “loyal recognition of wrongs from whatever side they come.” It is misleading to blame Galileo for his refusal to consider Copernicanism as a hypothesis while emphasizing his alleged inability to provide definitive evidence in support of the Copernican theory. Indeed, Galileo did not regard Copernicanism as a purely mathematical expedient to predict celestial events. According to his view, in fact, a system of the world should account for the true structure of the universe. At the same time, Galileo was firmly convinced of having good reasons in support of Copernicanism, because observations and theoretical explanations (not only his mistaken theory of tides, but also his new science of motion) confirmed to him that the arguments for the Earth’s motion were much stronger than those against it.

Still, it must be remembered that Galileo was not condemned for the inadequacy of his scientific or epistemological position but for exegetical considerations pertaining to the clash between heliocentrism and several passages of the Bible. The epistemological concerns raised by John Paul II were never addressed by the Roman inquisitors, who only focused on the theological consequences of Galileo’s Copernicanism.

In conclusion, the Galileo affair is by no means a closed question and continues to be a promising field for historical investigation. Many of its most obscure facets are as of 2007 still in need of clarification, and it also deserves to be carefully and constantly pondered for its worth as a significant memento. As Annibale Fantoli has observed, the Galileo affair “remains, and should remain, ‘open’, as a severe lesson of humility to the Church at all levels and as a warning, no less rigorous, not to wish to repeat in the present or in the future errors similar to those which have brought about [such a] heavy burden” (2003, p. 373).


The most useful and complete bibliographical resource in the field of Galilean studies is the International Galilean Bibliography, edited by the Institute and Museum of History of Science in Florence. Covering the entire reception of Galilean work, it lists more than 17,000 records. Available from


Galileo’s Early Notebooks: The Physical Questions. Translated from the Latin with historical and paleographical commentary by William A. Wallace. Notre Dame, IN: University of Notre Dame Press, 1977.

I documenti del processo di Galileo Galilei. A cura di Sergio M. Pagano. Vatican City: Pontificia Academia Scientiarum, 1984.

Tractatio de praecognitionibus et praecognitis and Tractatio de demonstratione. Transcribed from the Latin autograph by William F. Edwards, with an introduction, notes, and commentary by William A. Wallace. Padua: Antenore, 1988.

The Galileo Affair: A Documentary History. Edited and translated with an introduction and notes by Maurice A. Finocchiaro. Berkeley: University of California Press, 1989.

Sidereus nuncius, or, The Sidereal Messenger. Translated with introduction, conclusions, and notes by Albert Van Helden. Chicago: University of Chicago Press, 1989.

Two New Sciences Including Centers of Gravity and Force of Percussion. Translated with introduction and notes by Stillman Drake. Madison: University of Wisconsin Press, 1974; 2nd ed., Toronto: Wall & Thompson, 1989.

Discorsi e dimostrazioni matematiche intorno a due nuove scienze, attinenti alla meccanica ed i movimenti locali. A cura di Enrico Giusti. Turin, Italy: Einaudi, 1990.

Galileo’s Logical Treatises: A Translation, with Notes and Commentary, of His Appropriated Latin Questions on Aristotle’s Posterior Analytics. By William A. Wallace. Dordrecht-Boston: Kluwer, 1992.

Sidereus nuncius/Le messager céleste. Texte, traduction et notes établis par Isabelle Pantin. Paris: Les Belles Lettres, 1992.

Galileo on World Systems: A New Abridged Translation and Guide. By Maurice A. Finocchiaro. Berkeley: University of California Press, 1997.

Dialogo sopra i due massimi sistemi del mondo tolemaico e copernicano. Edizione critica e commentata a cura di Ottavio Besomi e Mario Helbing. Padua: Antenore, 1998.

Discorso delle comete. Edizione critica e commentata a cura di Ottavio Besomi e Mario Helbing. Padua: Antenore, 2002.

Le mecaniche. Edizione critica e saggio introduttivo di Romano Gatto. Florence: Olschki, 2002.

Il Saggiatore. Edizione critica e commentata a cura di Ottavio Besomi e Mario Helbing. Padua: Antenore, 2005.

Galileo Galilei’s Notes on Motion. Electronic representation of Galilean Manuscript 72. Available from


Artigas, Mariano, Rafael Martinez, and William R. Shea. “New Light on the Galileo Affair?” In The Church and Galileo, edited by Ernan McMullin. Notre Dame, IN: University of Notre Dame Press, 2005.

Beretta, Francesco. “Le procès de Galilée et les archives du Saint-Office: Aspects judiciaires et théologiques d’une condamnation célèbre.” Revue des sciences philosophiques et théologiques 88 (1999): 441–490.

———. “The Documents of Galileo’s Trial: Recent Hypotheses and Historical Criticism.” In The Church and Galileo, edited by Ernan McMullin. Notre Dame, IN: University of Notre Dame Press, 2005.

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

———. Galileo’s Instruments of Credit: Telescopes, Images, Secrecy. Chicago: University of Chicago Press, 2006.

Blackwell, Richard J. Galileo, Bellarmine, and the Bible. Notre Dame, IN: University of Notre Dame Press, 1991.

Bucciantini, Massimo. Contro Galileo: Alle origini dell’affaire. Florence: Olschki, 1995.

———. Galileo e Keplero: Filosofia, cosmologia e teologia nell’età della Controriforma. Turin, Italy: Einaudi, 2003.

Camerota, Michele, and Mario Helbing. “Galileo and Pisan Aristotelianism: Galileo’s De motu antiquiora and the Quaestiones de motu elementorum of the Pisan Professors.” Early Science and Medicine 5 (2000): 319–365.

Camerota, Michele. Galileo Galilei e la cultura scientifica nell’età della Controriforma. Rome: Salerno, 2004.

Carugo, Adriano, and Alistair C. Crombie. “The Jesuits and Galileo’s Ideas of Science and of Nature.” Annali dell’Istituto e Museo di Storia della Scienza di Firenze 8, no. 2 (1983): 3–67.

Clavelin, Maurice. Galilée copernicien: Le premier combat (1610–1616). Paris: Albin Michel, 2004.

Coyne, George V., SJ. “The Church’s Most Recent Attempt to Dispel the Galileo Myth.” In The Church and Galileo, edited by Ernan McMullin. Notre Dame, IN: University of Notre Dame Press, 2005.

Drake, Stillman. Galileo at Work: His Scientific Biography. Chicago: University of Chicago Press, 1978.

———. Galileo’s Notes on Motion. Florence: Istituto e Museo di Storia della Scienza, 1979.

———. Essays on Galileo and the History and Philosophy of Science. Selected and introduced by Noel M. Swerdlow and Trevor H. Levere. 3 vols. Toronto: University of Toronto Press, 1999.

Fantoli, Annibale. Galileo, for Copernicanism and for the Church. 3rd rev. and enl. ed. Translated by George V. Coyne. Vatican City: Vatican Observatory Publications, 2003.

Feldhay, Rivka. Galileo and the Church: Political Inquisition or Critical Dialogue? Cambridge, U.K.: Cambridge University Press, 1995.

Finocchiaro, Maurice A. Retrying Galileo, 1633–1992. Berkeley: University of California Press, 2005.

Galilaeana: Journal of Galilean Studies. 2004– . Promoted by the Institute and Museum of History of Science in Florence and edited by Massimo Bucciantini and Michele Camerota. Published annually and specifically devoted to investigating all aspects of the life, work, scientific findings, and fortunes of Galileo Galilei.

Galluzzi, Paolo. Momento: Studi galileiani. Rome: Edizioni dell’Ateneo & Bizzarri, 1979.

———, ed. Novità celesti e crisi del sapere. Florence: Giunti Barbera, 1984.

Hammond, Frederick. “The Artistic Patronage of the Barberini and the Galileo Affair.” In Music and Science in the Age of Galileo, edited by Victor Coelho. Dordrecht and Boston: Kluwer, 1992.

Machamer, Peter, ed. The Cambridge Companion to Galileo. Cambridge, U.K.: Cambridge University Press, 1998.

McMullin, Ernan, ed. The Church and Galileo. Notre Dame, IN: University of Notre Dame Press, 2005.

Montesinos, José, and Carlos Solís, eds. Largo campo di filosofare: Eurosymposium Galileo 2001. La Orotava, Spain: Fundación Canaria Orotava de Historia de la Ciencia, 2001.

Naylor, Ronald H. “Galileo’s Theory of Motion: Processes of Conceptual Change in the Period 1604–1610.” Annals of Science 34 (1977): 365–392.

———. “Galileo’s Theory of Projectile Motion.” Isis 71 (1980): 550–570.

Palmerino, Carla Rita, and J. M. M. H. Thijssen, eds. The Reception of Galilean Science of Motion in Seventeenth-Century Europe. Dordrecht and Boston: Kluwer, 2004.

Pesce, Mauro. L’ermeneutica biblica di Galileo e le due strade della teologia cristiana. Rome: Edizioni di Storia e Letteratura, 2005.

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

Renn, Jürgen. “Proofs and Paradoxes: Free Fall and Projectile Motion in Galileo’s Physics.” In Exploring the Limits of Preclassical Mechanics, edited by Peter Damerow et al. New York and Berlin: Springer, 1992.

Renn, Jürgen, ed. Galileo in Context. Cambridge, U.K.: Cambridge University Press, 2001.

Shea, William R., and Mariano Artigas. Galileo in Rome: The Rise and Fall of a Troublesome Genius. Oxford: Oxford University Press, 2003.

Stabile, Giorgio. “Linguaggio della natura e linguaggio della Scrittura in Galilei: Dalla Istoria sulle macchie solari alle lettere copernicane.” Nuncius 9 (1994): 37–64.

Wallace, William A. “Galileo and the Accademia dei Lincei.” In Novità celesti e crisi del sapere, edited by Paolo Galuzzi. Florence: Giunti Barbera, 1984.

———. Galileo and His Sources: The Heritage of the Collegio Romano in Galileo’s Science. Princeton, NJ: Princeton University Press, 1984.

———. Galileo’s Logic of Discovery and Proof: The Background, Content, and Use of His Appropriated Treatises on Aristotle’s Posterior Analytics. Dordrecht and Boston: Kluwer, 1992.

———, ed. Reinterpreting Galileo. Washington, DC: Catholic University of America Press, 1986.

Westfall, Richard S. “Science and Patronage: Galileo and the Telescope.” Isis 76 (1985): 11–30.

———. “Galileo and the Jesuits.” In Metaphysics and Philosophy of Science in the Seventeenth and Eighteenth Centuries: Essays in Honour of Gerd Buchdahl, edited by Roger S. Woolhouse. Dordrecht and Boston: Kluwer, 1988.

Wisan, Winifred L. “The New Science of Motion: A Study of Galileo’s De motu locali.” Archive for the History of Exact Sciences 13 (1974): 103–306.

Michele Camerota

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Galileo Galilei

Galileo Galilei

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. □

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Galileo Galilei (1564–1642)


GALILEO GALILEI (15641642), 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 (14731543) in the mid-1590s: so he asserted in 1597 in a letter to the German astronomer Johannes Kepler (15711630), 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 16021604 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 (15461601), 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 (15731650). 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 offereda brilliant analysis of the Old Testament verse Joshua 10:12 as compatible with a heliocentric universe and incompatible with a geocentric onethe 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.

See also Astronomy ; Brahe, Tycho ; Copernicus, Nicolaus ; Kepler, Johannes ; Optics ; Scientific Instruments .


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

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Galileo Galilei

Galileo Galilei

The condemantion of Galileo by the Roman Catholic Church in 1633 is one of the most dramatic incidents in the long history of the relations between science and religion. Galileo claimed in his Dialogue on the Two Chief World Systems, published the year before, that the sun-centered system of Copernicus was not only a convenient mathematical device for calculating the position of the planets but that it was the physical truth. This appeared to many Christians to run counter to statements in the Bible where the sun is described as mobile and the earth as stationary.

The clash between scientific truth and biblical revelation could have been avoided if Galileo, who had no decisive proof that the earth moves, had been more cautious and if theologians, who tended to be dogmatic, had not assumed that the Bible was to be interpreted literally whenever it mentioned natural events.

Early life

Galileo Galilei was an Italian astronomer, physicist, and natural philosopher. He was born in Pisa on February 15, 1564, and died in Arcetri on January 8, 1642. Galileo studied at the University of Pisa where he became Professor of Mathematics in 1589. Three years later he moved to the University of Padua where he taught elementary astronomy, mathematics, and physics. Medical students made up the majority of his audience, and he also lectured on fortification and military engineering to young noblemen.


The first indication of Galileo's commitment to the Polish astronomer Nicolaus Copernicus (14731543) appeared in a letter that Galileo wrote to his former colleague at Pisa, Jacopo Mazzoni, in 1597. In August of that year he received a copy of Johannes Kepler's Mysterium Cosmographicum, in which the heliocentric theory of the solar system was vindicated on mathematical and symbolic grounds. After reading the preface, Galileo wrote to Kepler (15711630) to voice his approval of the view that the earth is in motion, but also to express his fear of making his position known to the public at large.

Around 1602, Galileo began making experiments with falling bodies in conjunction with his study of the motion of pendulums. He first expressed the law of freely falling bodies, namely the fact that speed increases as the time squared, in 1604, but claimed to have derived it from the assumption that speed is proportional to distance (whereas, as he later realised, speed is proportional to the square root of the distance). In the autumn of 1604, the appearance of a supernova gave him the opportunity to argue that heavenly matter is not unchangeable.

In July 1609, after hearing that a Dutchman had invented a device to make distant objects appear nearer, Galileo built one himself and gave a demonstration of his telescope from the top of the Campanile of San Marco in Venice. The practical value for sighting ships at a distance impressed the Venetian authorities who confirmed Galileo's appointment for life and raised his salary from 520 to 1,000 florins, an unprecedented sum for a professor of mathematics. Galileo never quite mastered the optics of his combination of a plano-convex objective and a plano-concave eyepiece (an opera glass), but he succeeded in producing a twenty-power telescope, which he turned to the sky in 1610. What he saw is reported in the Sidereus Nuncius (The Starry messenger), which appeared in March 1610. The work was to revolutionize astronomy. The moon was revealed as covered with mountains, new stars appeared as out of nowhere, the Milky Way dissolved into a multitude of starlets and, more spectacular still, four satellites were found orbiting around Jupiter. This was particularly important since, if Jupiter was revolving around a central body with four attendant planets, it could no longer be objected that the earth could not carry the moon around the sun. Jupiter's satellites were not a decisive argument for Copernicanism, but they removed a major obstacle to having it seriously entertained by astronomers.

The Grand Duke of Tuscany, Ferdinand, died in January 1609 and was succeeded by his son, Cosimo II. Galileo had wanted to return to Florence for some time and he realised that his newly-won fame might assist him in effecting a change of residence. He christened the satellites of Jupiter Medicean stars in honour of Cosimo and, in July 1610, he was appointed Mathematician and Philosopher of the Grand Duke of Tuscany. Soon thereafter he discovered that Venus has phases like the moon, and that sunspots move across the surface of the sun.

Theological objections

In December 1613, theological objections were raised at a dinner at the court of the Grand Duke in Pisa. Galileo was absent but his disciple Benedetto Castelli defended his views when questioned by Christina of Lorraine, the Grand Duchess of Tuscany and the mother of the Grand Duke. Galileo felt that the matter was important enough to write a long letter to Castelli, dated December 21, 1613, in which he argued that the heliocentric system was not at variance with the Christian faith. On the fourth Sunday of Advent 1614, a Dominican friar, Tommaso Caccini, inveighed against the Copernican system from the pulpit of the church of Santa Maria Novella in Florence. Another Dominican, Nicolo Lorini, denounced Galileo to the Inquisition. Galileo then wrote a long letter to Christina of Lorraine, where he developed the view that God speaks through the book of nature as well as through the book of Scripture, and that the Bible teaches people how to go to heaven, not how the heavens go. In 1615, Cardinal Robert Bellarmine wrote a letter stating that in the absence of a conclusive proof for the motion of the earth, Galileo and astronomers should content themselves with speaking hypothetically. The Cardinal added that should such a proof become available then the passages in the Bible that seem to say that the earth is at rest would have to be reinterpreted. In 1616, Copernicus's On the Revolutions of the Heavenly Spheres was placed on the list of proscribed books and Galileo was privately, but nonetheless officially, warned not to teach orally or in writing that the earth revolves around the sun.

The debate on the comets and Galileo's trial

In 1618 great excitement was generated over the appearance, in rapid succession, of three comets. Galileo thought that they were merely optical phenomena caused by refraction in the atmosphere and he wrote a Discourse on the Comets to criticise the account of Father Orazio Grassi (15831654), a professor of mathematics at the Collegio Romano, who claimed the comets were real bodies beyond the moon. Grassi published a rejoinder, to which Galileo replied. The result was bitter enmity between himself and the Jesuits.

What changed Galileo's Copernican fortune was the election of Cardinal Maffeo Barberini to the Roman Pontificate in 1623. The following spring Galileo journeyed to Rome, and the new Pope, Urban VIII (16231644), granted him no less than six audiences. Galileo returned to Florence feeling that he could now write about the motion of the earth. In January 1630 his long awaited Dialogue on the Two Chief World Systems was ready for publication and the manuscript was sent to Rome where a friend, Giovanni Ciampoli, played a vital role in securing permission to print the book. Ciampoli exceeded his powers and was largely responsible for Galileo's subsequent trouble.

The Dialogue had gone to press in Florence in June 1631. The publisher had decided to print a thousand copies, a large edition for the time, and the work was not completed until February 1632. Copies did not reach Rome until the end of March or early April. Pope Urban VIII created a commission to investigate the licensing of the Dialogue. In the file on Galileo at the Holy Office the commission found an unsigned memorandum of 1616 stating that he had been enjoined not to teach that the earth moves. The commission concluded that Galileo had disobeyed a formal order of the Holy Office, and Galileo was summoned to Rome, arriving, after much delay, on February 13, 1633. Despite his vigorous denial, Galileo was judged to have contravened the orders of the Church. On the morning of June 22, 1633, he was taken to a hall in the convent of Santa Maria Sopra Minerva in Rome and was made to kneel while the sentence condemning him to imprisonment was read out aloud. Still kneeling, Galileo formally adjured his error. He was allowed to leave for Siena and later, in 1634, to return to Florence, where he was confined to his house in Arcetri.

Later years and modern assessment

Galileo sought comfort in work, and within two years he completed the Discourse on Two New Sciences, the book on which his lasting fame as a scientist rests. In this work Galileo studied the structure of matter and the strength of materials, and explained motion in the light of the timessquared law of falling bodies and the independent composition of velocities. Together these laws enabled him to give an accurate description of the parabolic path of projectiles. When he cast about for a publisher, he came up against a new problem: the Church had issued a general prohibition against printing or reprinting any of his books. Galileo's manuscript was sent to the Protestant Louis Elzevier in Holland, where it appeared in 1638. Galileo became blind in that year, and he remained under house arrest until his death on January 8, 1642, five weeks before his seventy-eighth birthday.

In contemporary times, the Roman Catholic Church has recognized that the trial of Galileo rested on a misunderstanding of the moral authority of the Church. This was clearly expressed by Pope John Paul II in 1983 at a commemoration of the 350th anniversary of the publication of the Dialogue on the Two Chief World Systems. The Pope declared that divine revelation does not involve any particular scientific theory of the universe, and that the Holy Spirit does not guarantee our human explanations of the physical constitution of reality. Galileo had made exactly that point in his letter to Christina of Lorraine.

See also Astronomy; Christianity, Roman Catholic, Issues in Science and Religion; Cosmology; Gravitation; Mathematics; Science and Religion, Models and Relations


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william r. shea

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Galilei, Galileo (1564-1642)

Galilei, Galileo (1564-1642)

Italian mathematician and astronomer

Galileo Galilei is credited with establishing the modern experimental method. Before Galileo, knowledge of the physical world that was advanced by scientists and thinkers was for the most part a matter of hypothesis and conjecture. In contrast, Galileo introduced the practice of proving or disproving a scientific theory by conducting tests and observing the results. His desire to increase the precision of his observations led him to develop a number of inventions and discovery, particularly in the fields of physics and astronomy .

The son of Vincenzo Galilei (c.15201591), an eminent composer and music theorist, Galileo was born in Pisa. He received his early education at a monastery near Florence, and in 1581, entered the University of Pisa to study medicine. While a student he observed a hanging lamp that was swinging back and forth, and noted that the amount of time it took the lamp to complete an oscillation remained constant, even as the arc of the swing steadily decreased. He later experimented with other suspended objects and discovered that they behaved in the same way, suggesting to him the principle of the pendulum. From this discovery he was able to invent an instrument that measured time, which doctors found to be useful for measuring a patient's pulse rate, and Christiaan Huygens later adapted the principle of a swinging pendulum to build a pendulum clock.

While at the University of Pisa, Galileo listened in on a geometry lesson and afterward abandoned his medical studies to devote himself to mathematics. However, he was unable to complete a degree at the university due to lack of funds. He returned to Florence in 1585, having studied the works of Euclid and Archimedes. He expanded on Archimedes' work in hydrostatics by creating a hydrostatic balance, a device designed to measure the density of objects. The following year, he published an essay describing his new invention, which determined the specific gravity of objects by weighing them in water . With the hydrostatic balance, Galileo gained a scientific reputation throughout Italy.

In 1592, Galileo was appointed professor of mathematics at Padua University in Pisa, where he conducted experiments with falling objects. Aristotle had stated that a heavier object should fall faster than a lighter one. It is said that Galileo tested Aristotle's assertion by climbing the leaning tower of Pisa, dropping objects of various weights, and proving conclusively that all objects, regardless of weight, fall at the same rate.

Some of Galileo's experiments did not turn out as expected. He tried to determine the speed of light by stationing an assistant on a hill while he stood on another hill and timed the flash of a lantern between the hills. He failed because the hilltops were much too close together to make a measurement.

In 1593, Galileo invented one of the first measuring devices to be used in science: the thermometer. Galileo's thermometer employed a bulb of air that expanded or contracted as temperature changed and in so doing caused the level of a column of water to rise or fall. Though this device was inaccurate because it did not account for changes in air pressure, it was the forerunner of improved instruments.

From 1602 to 1609, Galileo studied the motion of pendulums and other objects along arcs and inclines. Using inclined planes that he built, he concluded that falling objects accelerate at a constant rate. This law of uniform acceleration later helped Isaac Newton derive the law of gravity.

Galileo did not make his first contribution to astronomy until 1604, when a supernova abruptly exploded into view. Galileo postulated that this object was farther away than the planets and pointed out that this meant that Aristotle's "perfect and unchanging heavens" were not unchanging after all. Ironically, Galileo's best-known invention, the telescope , was not his creation after all. The telescope was actually invented in 1608 by Hans Lippershey , a Danish spectacle maker. When Galileo learned of the invention in mid-1609, he quickly built one himself and made several improvements. His altered telescope could magnify objects at nine-power, three times the magnification of Lippershey's model. Galileo's telescope proved to be very valuable for maritime applications, and

Galileo was rewarded with a lifetime appointment to the University of Venice.

He continued his work, and by the end of the year he had built a telescope that could magnify at 30-power. The discoveries he made with this instrument revolutionized astronomy. Galileo saw jagged edges on the Moon , which he realized were the tops of mountains. He assumed that the Moon's large dark areas were bodies of water, which he called maria ("seas"), though we now know there is no water on the Moon. When he observed the Milky Way, Galileo was amazed to discover Jupiter, which resulted in his discovery of its four moons; he later called them "satellites," a term suggested by the German astronomer Johannes Kepler . Galileo named the moons of Jupiter, Sidera Medicea ("Medicean stars") in honor of Cosimo de Medici, the Grand Duke of Tuscany, whom Galileo served as "first philosopher and mathematician" after leaving the University of Pisa in 1610. Also, with repeated observation, he was able to watch the moons as they were being eclipsed by Jupiter and from this he was able to correctly estimate the period of rotation of each of the moons.

In 1610, Galileo outlined planetary discoveries in a small book called Siderus Nuncius ("The Sidereal Messenger"). Venus, seen through the telescope, exhibited phases like the Moon, and for the same reasons: Venus did not produce its own light but was illuminated by the Sun .

Saturn was a mystery: Galileo's 30-power telescope was at the limit of its ability to resolve Saturn, and the planet appeared to have three indistinct parts. When Galileo looked at the Sun, he saw dark spots on its disc. The position of the spots changed from day to day, allowing Galileo to determine the rotational rate of the Sun.

In 1613, Galileo published a book in which for the first time he presented evidence for and openly defended the model of the solar system earlier proposed by the Polish astronomer Nicholas Copernicus, who argued that Earth, rather than being positioned at the center of the universe, as in the Ptolemaic design, was only one of several galactic bodies that orbited the Sun. While there was some support even among ecclesiastical authorities for Galileo's proof of the Copernican theory, the Roman Catholic hierarchy ultimately determined that a revision of the long-held astronomical doctrines of the church was unnecessary. Thus, in 1616, a decree was issued by the church declaring the Copernican system "false and erroneous," and Galileo was ordered not to support this system.

Following this run-in with the Catholic Church and the inquisition that forced his adherence to the Copernican theory of the solar system, Galileo focused on the problem of determining longitude at sea, which required a reliable clock. Galileo thought it possible to measure time by observing eclipses of Jupiter's moons. Unfortunately, this idea was not practical for eclipses could not be predicted with enough accuracy and observing celestial bodies from a rocking ship was nearly impossible.

Galileo wanted to have the edict against the Copernican theory revoked, and in 1624, traveled to Rome to make his appeal to the newly elected pope, Urban VIII. The pope would not revoke the edict but did give Galileo permission to write about the Copernican system, with the provision that it would not be given preference to the church-sanctioned Ptolemaic model of the universe.

With Urban's imprimatur, Galileo wrote his Dialogue Concerning the Two Chief World SystemsPtolemaic and Copernican, which was published in 1632. Despite his agreement not to favor the Copernican view, the objections to it in the Dialogue are made to sound unconvincing and even ridiculous. Summoned to Rome to stand before the Inquisition, Galileo was accused of violating the original proscription of 1616 forbidding him to promote the Copernican theory. Put on trial for heresy, he was found guilty and ordered to recant his errors. At some point during this ordeal Galileo is supposed to have made his famous statement: "And yet it moves," referring to the Copernican doctrine of Earth's rotation on its axis.

While the judgment against Galileo included a term of imprisonment, the pope commuted this sentence to house arrest at Galileo's home near Florence. Although he was forbidden to publish any further works, he devoted himself to his work on motion and parabolic trajectories, arriving at theories that were later refined by others and made an important impact on gunnery. Galileo died blind at the age of 78.

See also Gravity and the gravitational field

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Galileo (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 known 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.


See biographies by L. Geymonat (tr. 1965), J. L. Heilbron (2010), and D. Wooton (2010); 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).

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Galileo Galilei

Galileo Galilei

Italian Astronomer, Physicist, and Mathematician 15641642

Galileo Galilei is a pivotal figure in intellectual and scientific history. His ideas and activities were integral to the Scientific Revolution, which resulted in world-changing advances in science and technology, and in fundamental changes in the way reality is perceived.

Galileo was born in Pisa, Italy, in 1564. In 1581, he entered the University of Pisa, where his father wanted him to study medicine. But Galileo was interested in mathematics and philosophy, and he left the university without a degree. In 1589, he taught mathematics at the university, but lost his job by challenging Aristotelian teachings held by the university and the Catholic Church. However, he immediately became professor of mathematics at the University of Padua.

Galileo was among the first to perceive that the natural world acts in a regular manner that can be interpreted and understood mathematically. Applying this approach, he developed the concept of acceleration and discovered the law of falling bodies, explaining the movement of projectiles, pendulums, and objects moving on an inclined plane.

Galileo accepted the Sun-centered model of the solar system that had been proposed by Copernicus. This model was in opposition to the Earth-centered model of Ptolemy that was accepted by scholars and the Catholic Church. Soon after the first telescope was invented, Galileo built his own version in 1609 and improved its magnification power. He was the first to use a telescope to study the heavens, obtaining, through these investigations, proof of the Copernican system. He discovered sunspots, valleys and mountains on the Moon, satellites circling Jupiter, and the phases of Venus.

In 1610, Galileo published his observations and interpretations in The Starry Messenger, refuting Aristotle and Ptolemy and supporting Copernicus. Opposition came immediately from scholars and churchmen, who accused him of heresy.

In 1612, Galileo published a book on hydrostatics based on observations, measurements, and mathematical analysis. He was again attacked by churchmen and university scholars for not adhering to the accepted Aristotelian approach. He openly argued that physical evidence and mathematical proofs should not be made dependent on interpretations of scripture but that such interpretations should be subject to change when new evidence becomes available. Not surprisingly, the Catholic Church issued an edict in 1616 banning Copernicanism and censored Galileo's writings.

Undaunted, in 1632 Galileo published Dialogue on the Two Chief World Systems contrasting the planetary models of Ptolemy and Copernicus, with clear preference for Copernicus. He was called to Rome and tried for heresy. Convicted, he was forced to publicly retract his ideas and placed on permanent house arrest. His works were banned, but this order was essentially ignored outside of Italy. His ideas spread rapidly, gaining support throughout Europe.

Galileo continued his work, and his last book, Discourses Concerning Two New Sciences, was published in Leiden in 1638. This classic volume presented a mechanical mathematical physics that eventually led to the development of what would be called Newtonian physics. Galileo died in 1642, the year of Isaac Newton's birth. In 1992, the Catholic Church rescinded its 1633 conviction of Galileo as a heretic350 years after his death.

see also Newton, Sir Isaac; Solar System Geometry, History of.

J. William Moncrief


Gamow, George. The Great Physicists from Galileo to Einstein. New York: Dover Publications, Inc., 1988.

Santillana, Giorgio de. The Crime of Galileo. Chicago: University of Chicago Press, 1955.


Galileo believed that the development of new ideas and understanding was stifled by blind adherence to the authority of the Catholic Church and the writings of the ancient Greeks. Rather than simply accepting the statements of authorities, he believed investigators should rely on their own observations, measurements, and calculations.

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Galilei, Galileo

Galilei, Galileo

Italian Astronomer, Mathematician, and Physicist 1564-1642

Galileo Galilei (commonly known as Galileo) was a founder of modern physics and modern astronomy. He was born in Pisa, Italy, in 1564, and was a professor from 1592 through 1610 at Padua, which was part of the Venetian Republic. While in Pisa, he noticed a chandelier swinging in the cathedral and developed the physical law that shows that pendulums of the same length swing in the same time interval. Using a pendulum for timing, he experimentally worked out how objects accelerate while falling. In these experiments, he rolled objects down an inclined plane; the traditional story that he dropped weights from the Leaning Tower of Pisa was a myth.

In 1609 Galileo heard of a device that existed that could magnify distant objects. Using his experimental abilities, he ground lenses and assembled a telescope. He demonstrated its possibilities for aiding commerce by showing Venetian nobles that they could see ships approaching farther out than ever before. Starting that same year, Galileo also turned his telescope toward the sky. He subsequently discovered that the Moon had mountains and craters on it, that Jupiter had moons orbiting it, and that Venus went through a complete set of phases. These observations indicated that Greek philosopher Aristotle's (384-322 B.C.E.) view of the universe as unchanging and perfect was not true, and Galileo endorsed Polish astronomer Nicholas Copernicus's (1473-1543) idea that the Sun instead of Earth is the center of the solar system. Galileo's book Sidereus nuncius (The starry messenger; 1610) brought his discoveries to a wide audience.

Soon Galileo discovered sunspots, showing that the Sun is not a perfect body. But a controversy with a Jesuit astronomer over who discovered sunspots set the Roman Catholic Church against him. In 1616 the Church's Inquisition warned him against holding or defending Copernicus's ideas. To get his agreement, they showed him instruments of torture.

Galileo was relatively quiet until his book Dialogo sopra i due massimi sistemi del mondo (Dialogue on the two great world systems) was published in 1632. It was written in his native Italian instead of the scholarly Latin, to spread his discussion widely. The Inquisition then convicted him of teaching Copernicanism and sentenced him to house arrest. But even under those conditions, and the blindness that came on, he continued his scientific work. He died in Florence in 1642. In 1992 Pope John Paul II agreed that Galileo was correct to endorse Copernicanism, though Galileo was not pardoned.

see also Astronomy, History of (volume 2); Copernicus, Nicholas (volume 2); Jupiter (volume 2); Moon (volume 2); Religion (volume 4); Saturn (volume 2); Venus (volume 2).

Jay Pasachoff


Machamer, Peter, ed. The Cambridge Companion to Galileo. Cambridge, UK: Cambridge University Press, 1998.

MacLachlan, James H. Galileo Galilei: First Physicist. New York: Oxford UniversityPress, 1987.

Reston, James, Jr. Galileo: A Life. New York: Harper Collins, 1994.

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Galileo (1564–1642) ( Galileo Galilei) Italian physicist and astronomer. According to legend, Galileo observed that a hanging lamp in Pisa Cathedral took the same time to complete one oscillation however long the swing, and he suggested the pendulum could be used for timekeeping. He later studied falling bodies, and disproved Aristotle's view that they fall at different rates according to weight. He also discovered the parabolic flight path of projectiles. In 1609, Galileo used one of the first astronomical telescopes to discover sunspots, lunar craters, Jupiter's major satellites, and the phases of Venus. In Sidereus Nuncius (1610), he supported Copernicus' heliocentric theory that the Earth moves around the Sun. Galileo was forbidden by the Roman Catholic Church to teach that this system represented physical reality but, in Dialogue on the Two Great World Systems (1632), he defied the Pope by making his criticism of Ptolemy's system even more explicit. As a result, Galileo was brought before the Inquisition and forced to recant.

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Galileo A NASA spacecraft, launched on 15 October 1989 from the shuttle Atlantis on an international deep-space mission, that passed and photographed Venus on 9 February 1990, for a gravity assist. On its journey through the asteroid belt, Galileo encountered Gaspra on 29 October 1991, and Ida on 28 August 1993, where it discovered the satellite Dactyl. It reached Jupiter in December 1995, then entered an orbit that brought it into repeated encounters with the Galilean satellites. The spacecraft comprises an orbiter and a probe, released in July 1995, as Galileo was still approaching Jupiter, which penetrated the atmosphere and returned data for 61.4 minutes, by which time it had descended to a level where the pressure was 24 bars, 140 km below the 1 bar pressure altitude.

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Galileo Galilei

Galileo Galilei (1564–1642), Italian astronomer and physicist, one of the founders of modern science. He discovered the constancy of a pendulum's swing, formulated the law of uniform acceleration of falling bodies, and described the parabolic trajectory of projectiles. He applied the telescope to astronomy and observed craters on the moon, sunspots, Jupiter's moons, and the phases of Venus. His acceptance of the Copernican system was rejected by the Catholic Church, and under threat of torture from the Inquisition he publicly recanted his heretical views.
Galilean moons the four largest satellites of Jupiter (Callisto, Europa, Ganymede, and Io), discovered by Galileo in 1610 and independently by the German astronomer Simon Marius (1573–1624).

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Galileo, the seventeenth-century astronomer and mathematician, was the first to see the moons of Jupiter. Using a telescope that he developed, Galileo observed four moons orbiting the planet. This discovery was not well received by the scientific community, as most believed that all objects in the heavens revolved around the Earth. Galileo also plotted the phases of Venus and observed sunspots.

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Galileo Space probe to Jupiter, launched in October 1989. The probe passed the asteroids Gaspra and Ida in October 1991 and August 1993 respectively, photographing both. Galileo went into orbit around Jupiter in 1995. In 2003, Galileo's mission ended and it was destroyed in Jupiter's atmosphere.

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GalileoBulawayo, Galileo, Mayo, Montevideo

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