Galileo Life Story and Contributions To Science


Who is Galileo Galilei? What did Galileo do? Information on Galileo biography, life story, works and contributions to science and astronomy.

Galileo; (1564-1642), Italian astronomer, physicist, and mathematician, who initiated the scientific revolution of the 17th century in Italy. Galileo linked physics and astronomy with mathematics rather than with traditional philosophy. While he is best known for his writings on astronomy and for his conflict with authority over the issue of the freedom of scientific inquiry, his Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638) constitutes his major contribution to science. In it he correctly defined uniform acceleration, set forth the laws of falling bodies, developed the mathematical theory of projectile motion, and expressed numerous fruitful ideas about sound, heat, and light, the relation of mathematics to physics, the role of experiment, and the problems of infinitesimals in the analysis of matter and of motion.

Early Life and Work.

Galileo Galilei was born in Pisa on Feb. 15, 1564. His father, Vincenzio Galilei, was a Florentine patrician of slender means who taught music and wrote against the prevailing abstract, numerical theories of harmony. Galileo was educated by a private tutor and by the Camaldolese monks of Vallombrosa and in 1581 entered the University of Pisa as a medical student. Two years later he began studying mathematics under a family friend, Ostilio Ricci. Galileo left the university without a degree in 1585, having developed little interest in medicine. He began to apply mathematics to physics, producing some theorems on the centers of gravity of solid bodies and a treatise on the hydrostatic balance. He became interested in the uniform beating of pendulums and the speeds of descent of bodies in air and in water. In 1589 he was appointed to the chair of mathematics at Pisa, after an unsuccessful attempt to obtain a similar post at Bologna, and spent the next two decades as a university professor.

Galileo Life Story and Contributions To Science

Physics before Galileo was treated as a branch of Aristotelian philosophy and not as an experimental science. Heavy bodies were supposed to fall at speeds proportional to their weights, seeking to reach their natural place, which was the center of the universe. Thrown bodies supposedly were kept in motion either fcy some property of the air or by a temporary force put into them by the thrower. Medieval attempts to apply mathematics to motion, though highly ingenious, remained abstract and did not give rise to a new physics that was separate from philosophy. In the 16th century, Archimedes’ works were published, giving mathematical laws for static problems, but problems of motion were still not solved mathematically.


About 1590, Galileo wrote a treatise on motion in which he disputed nearly every assumption of Aristotelian physics. He held the view that bodies composed of the same material fall with the same speed through a given medium regardless of their weights. In support he used arguments based on the principle of Archimedes. He offered a new proof for equilibrium on inclined planes, reducing this to the law of the lever, and then tried to deduce from it the speeds of bodies moving on inclined planes.

Galileo disputed Aristotle’s division of all motions into “natural” and “forced,” asserting that there are also “neutral” motions exemplified by rotating spheres and motions along horizontal planes. For projectile motions he adopted the medieval idea of an “impressed force” that wasted away with motion, whereas Aristotle had held the view that projectiles were moved by the medium. Galileo polished this anti-Aristotelian treatise for publication but never published it, probably because he found that he could not reconcile his proposed rules for motion on inclined planes with observation, having neglected acceleration in his early studies.

By the time his first contract expired, in 1592, Galileo had offended his colleagues by disputing against Aristotle and in other ways. It is said that he demonstrated from the Leaning Tower of Pisa, in view of students and professors, that speed and weight were not related in the way Aristotle believed. Later that year Galileo moved to the University of Padua, where freedom of opinion was protected by the Venetian government. There he wrote a treatise on mechanics for his private pupils, which was widely circulated in manuscript copies, and he gave lectures on geometry and astronomy.

Galileo was not much interested in astronomy in his early years. To satisfy the curriculum, designed chiefly to teach medical students the elements of astronomy for use in medical astrology, he based his lectures on a medieval astronomical treatise and, in alternate years, on Ptolemaic planetary theory. In 1597, however, he received copies of a Copernican book published by Kepler. In the Copernican system of astronomy, the earth is removed from its traditional stationary position at the center of the universe and is treated as a planet that rotates on its axis daily and revolves around the sun annually. Galileo wrote to Kepler that he preferred that theory (though not openly) because it enabled him to explain some terrestrial phenomena—this meant the tides, which later became his main argument for the earth’s double motion. In the same year he developed a useful mathematical instrument, the proportional compass, which he manufactured for sale to augment his income. Soon afterward he became interested in heat and devised a crude thermometer. Through the work of the English physicist William Gilbert he also became interested in magnetism.

At Padua, Galileo had a mistress, Marina Gamba, who bore him two daughters and a son. Galileo never married, perhaps because of the heavy financial obligations put on him by the death of his father in 1591. He was hard pressed to pay the dowries of two of his sisters, getting no help from his younger brother, Michelangelo, a musician who himself required financial aid from Galileo.

Astronomical Work.

Galileo’s first known activity in astronomy occurred in 1604-1605, when a supernova aroused wide interest and speculation. He used it in public lectures to attack Aristotle’s doctrine of the immutability of the heavens, which was supported by the Padfean professors of philosophy.

In 1606, Galileo privately printed his first book, an account of his proportional compass. This was quickly plagiarized, leading to a polemic in 1607 against the offenders. Galileo’s main interest was still in mechanics and the laws of motion, on which he was preparing to publish a work in mid-1609. However, hearing that in Holland an instrument had been invented to make distant objects appear closer, he applied himself at once to duplicate the invention. He succeeded so well that late in August 1609 he was able to present to the Venetian senate a 9-power telescope, three times as effective as its early rivals. In return he received a lifetime professorship with a large increase in salary.


Galileo continued to improve his telescope and early in 1610 made one of 30 power. With it he discovered the mountainous surface of the moon, many new stars, and four of Jupiter’s satellites. He published these discoveries in his book Sidereus nuncius (Starry Messenger, 1610), creating a sensation. The proof provided by Jupiter’s satellites that not all heavenly bodies revolved about the earth and the discovery that the moon was not perfectly spherical reawakened Galileo’s interest in Copernicus, and he began an attack on Ptolemy and Aristotle. Knowing that university teaching of the old doctrines could no longer satisfy him, and being homesick for Florence, he pressed for employment by the Grand Duke of Tuscany as court mathematician and philosopher. He was granted the position and in September 1610 left Padua for Florence, where he remained for the rest of his life.

By the end of 1610, Galileo observed the rings of Saturn, which his telescope was too weak to show distinctly, and saw the phases of Venus, which convinced him that its orbit encircled the sun rather than the earth. Early in 1611 he took his telescope to Rome and showed church dignitaries his discoveries, including the phenomenon of sunspots. While in Rome he was elected to the Accademia dei Lincei, the first true scientific society, founded in 1603. The academy provided Galileo means of communication among scientists that more than replaced those he had lost by leaving the university.

Galileo Galilei

On his return to Florence, Galileo became involved in a controversy about the laws of floating bodies, and in 1612 he published a book refuting Aristotle and supporting Archimedes on this subject. The book was attacked by four professors at Pisa and Florence, and an informal league against Galileo was formed. Meanwhile a German Jesuit, Christoph Schemer, had published three letters on sunspots, mistakenly supposing them to be tiny planets. Galileo refuted this view in his Letters on Sunspots, published at Rome in 1613 by the Accademia dei Lincei. In this book, Galileo came out for the first time openly for Copernicus. This led to opposition on religious grounds by certain priests and professors. Galileo composed a long letter to his friend and pupil Benedetto Castelli, a Benedictine monk, arguing that Biblical authority should not be invoked against scientific theories. In 1614 he was attacked from the pulpit in Florence, and a copy of his letter to Castelli was sent to Rome by a Dominican preacher for action by the Inquisition. Galileo became alarmed both for his own safety and for the freedom of scientific inquiry. He expanded his previous letter into a famous defense of free science, now known as the Letter to the Grand Duchess Christina, which was widely circulated in 1615 and finally published in 1636.

Conflict with Rome.

Late in 1615, Galileo went to Rome and argued publicly for Copernicus. This action angered Pope Paul V, who appointed a church commission to examine the theory of the earth’s motion. The commission held the theory to be contrary to the Bible and possibly heretical. Late in February 1616 the Pope ordered Robert Cardinal Bellarmine to admonish Galileo to abandon the Copernican system. If Galileo resisted, the Commissary of the Inquisition was instructed to threaten him with imprisonment were he ever to teach it again orally or in writing. The only record of the ensuing proceeding indicates that Galileo did not resist Bellarmine’s admonition, but that the Commissary was present and added the Pope’s threat anyway. Thus Galileo was silenced for a time with regard to astronomy.

On his return to Florence, Galileo applied himself to the possible practical uses of the telescope for problems of navigation and to the noncontroversial studies of motion and mechanics on which he had been engaged before 1610. He published nothing until 1619, when he entered a controversy over three comets of 1618 by having a pupil, Mario Guiducci, make his views known in a book that was critical of a Jesuit at Rome, Orazio Grassi. Grassi replied under a pen name, attacking Galileo directly and intimating that he still held the forbidden Copernican view. Galileo replied in his own name in 1623 with the Assayer (II saggiatore), in which he set forth the principles by which he believed scientific research should be guided but avoided giving support to Copernicus.

As Galileo’s new book was being printed at Rome by the Accademia dei Lincei, Galileo’s old friend Maffeo Barberini was elected pope, taking the name Urban VIII. The book was dedicated to him, and he enjoyed it greatly. In 1624, Galileo visited Rome and tried to induce the new Pope to rescind the edict of 1616 under which the work of Copernicus had been banned “until corrected.” Urban did not do this but gave Galileo permission to write a book comparing the old and new astronomies, provided both were treated hypothetically and impartially. Over the next several years, Galileo wrote his Dialogue Concerning the Two Chief World Systems, published in 1632.

This celebrated book was not so much an astronomical treatise as a sustained attack on the ancient idea that the earth is composed of a totally different kind of matter from that of the heavens. To reconcile the earth’s motion with man’s experience in everyday life, Galileo set forth the concept of the relativity of motion, the idea of inertia, and the notion of composition of independent motions. His Dialogue also included the law of uniform acceleration and its application to falling bodies, though these were not developed mathematically as in his final book.

This strategy meant a rejection not only of the old astronomy but also of the old physics and a large part of the prevailing philosophy. In conducting his battle against so large a body of opinion, Galileo shrewdly concentrated on a relatively small number of essential points, ignoring many details that are important to astronomy, physics, or philosophy. Thus it remains doubtful what his exact views were concerning the principle of inertia, the shapes of planetary orbits, the nature of comets, and many other matters of scientific importance. It is clear, however, that he advocated explanations based not on authority and argument, but on observation, analogy, and discovery of mathematical laws. On that basis he attempted to demonstrate in his Dialogue that the ancient separation of earth and heavens was unnecessary and often misleading, whereas a unified science of physics and astronomy was possible and afforded enlightenment.

Such a unification is not possible if the earth ( or any other body ) is given a special and privileged position with respect to the entire universe. In that case, the earth and terrestrial bodies would have one set of laws, and everything else would have another set. That was the situation in Aristotelian physics and Ptolemaic astronomy. Treating the earth as a planet, as Copernicus did, opened the way to a single set of laws for terrestrial and celestial physics. Galileo understood this even though he had not discovered the laws themselves. The result was that his Dialogue, though it pretended to be impartial, was in fact strongly biased in favor of the Copernican system. In it he attempted a mechanical explanation of the tides, based on the double motion of the earth, asserting that no physical explanation of the tides was possible for an absolutely stationary earth. This went beyond the treatment of the two systems as mere mathematical hypotheses, which was all that Galileo had permission to debate in his book.

Galileo encountered a good deal of trouble in obtaining from church officials a license to print the Dialogue, but once this was done he did not expect any further difficulties. In the autumn of 1632, however, five months after the book was published, he was suddenly ordered to Rome to appear before the Inquisition. It is evident that in reviewing his old file, the Inquisition uncovered the memorandum of 1616 with its indication that he had been personally ordered never to teach the Copernican system again, orally or in writing. Thus it appeared that he was guilty of a breach of a personal instruction and had concealed this information from the Pope when he obtained permission to write his book. The Pope, who had been his friend for many years, now became his implacable foe. Galileo protested that the Dialogue had been printed only with the official license and after long examination, but even the Grand Duke could not prevail against the Pope’s order for him to stand trial.


At the trial, Galileo produced an affidavit from Cardinal Bellarmine, who had meanwhile died, attesting that he had not received any personal order but only the admonition not to defend or hold the forbidden view. Galileo argued that he had not held or defended the Copernican view of the earth’s motion since that time and that he did not remember any order against teaching it orally or in writing. The impasse was resolved when Galileo was induced to admit that he had gone too far in his book with arguments for the earth’s motion that were not adequately offset by counterarguments. He was then sentenced to life imprisonment, and the Dialogue was ordered burned. The sentence was ordered to be read in all university cities, and all Galileo’s books were forbidden to be printed or reprinted.

Galileo’s sentence was quickly commuted to house arrest, first in care of the archbishop of Siena, an old friend of Galileo’s, and then in his own villa at Arcetri in the hills above Florence. Galileo, at first crushed by the severity of the sentence, quickly recovered under the encouragement of the archbishop, who set him to work again on noncontroversial physics. This he put into dialogue form, continuing the banned book but avoiding astronomy. The book was smuggled out of Italy and printed in 1638 at Leiden as the Discourses and Mathematical Demonstrations Relating to Two New Sciences.

In 1638, Galileo became blind. The Pope refused him permission to visit Florence, even to see doctors. The Grand Duke, however, visited Galileo at Arcetri as did such distinguished foreign travelers as Thomas Hobbes and John Milton. Galileo’s character and attitude are clearly revealed in a statement in a letter to Nicholas Fabri de Peiresc, who had written that he was working to obtain a pardon for him. Galileo replied that he could hardly hope for a pardon, since only the guilty could be pardoned, whereas those unjustly condemned could expect only that their accusers would continue to try to make it appear that the crime was worse than supposed, in order to justify themselves. But he said Peiresc’s own efforts turned his misfortune into good fortune by revealing to him the friendship and goodwill of persons who otherwise would never have had occasion to makes themselves known to him.

Galileo died at Arcetri on Jan. 8, 1642. The Grand Duke wished to erect a proper tomb for him, but was warned from Rome to do nothing that might offend the Inquisition; so Galileo was buried without pomp in the family church, the cathedral of Santa Croce. It was not until the following century that his remains were moved to a more fitting place in that cathedral, with a fine epitaph composed by his loyal last pupil, Vincenzio Viviani, who in 1654 composed the first biography of Galileo, published in 1717.

Galileo Galilei

Galileo’s Influence.

Galileo had become known throughout Europe in 1610 as the result of his discoveries with the telescope. Many astronomers as well as philosophers at first disputed his claims as mere illusions of the instrument. But with the support of Kepler, the validity of these discoveries was soon established. Galileo did not take part in the published polemics about them. Observers in England, Germany, and France were soon engaged in verifying and extending the discoveries. As to their interpretation, Galileo was prevented from fully expressing his views until 1632, when his Dialogue appeared. This book was written in Italian instead of Latin because Galileo despaired of convincing professors and philosophers at home or abroad and addressed himself instead to the educated layman. But a Latin translation appeared in Strasbourg in 1635 and was reprinted in Lyon in 1641 and in London in 1663. Meanwhile an English translation had been published in 1661. Thus the ideas in the book were read throughout Europe despite their suppression in Italy.

Galileo was influential outside Italy largely through discussion of the new scientific ideas in the Dialogue rather than from his more technical physics of the Two New Sciences, which was not translated into English until 1665 or into Latin until 1699. By the latter date, Sir Isaac Newton’s work had supplanted Galileo’s, and the law of gravitation had completed the mathematical unification of physics and astronomy.

Galileo’s work is undoubtedly responsible to some degree for the rise of experimentation in physics and for the replacement of verbal with mathematical laws. Galileo seems to have advocated experiment only as a means of corroborating laws he had discovered and not as a means of discovering the laws themselves. The latter approach became established in science long after Galileo’s death. Few before Galileo, however, described experiments to test precise mathematical rules, as did Galileo and his followers.

Galileo considered mathematics to be the only completely reliable form of logic available to the human mind. Hence, when a mathematical law could be found for observed events, it should be trusted. If it did not fit precisely, that was an indication that the investigator had not yet found how to balance his books. The fault was neither in nature nor in mathematics but in poor accounting. This is far from the Platonic idea, often attributed to Galileo, that the world of sense is but a poor copy of some real universe that is entirely geometrical. Pure mathematics did not interest Galileo very much, though he made one important contribution to it—the concept of one-to-one correspondence.

Apart from his purely scientific contributions, Galileo’s importance historically lies in his battle for freedom of inquiry unhampered by authority and tradition. Although he lost his own freedom in that struggle, Galileo swiftly became a symbol to others who opposed authority. But in conducting his campaign he neither opposed the church nor questioned its authority. Rather, he foresaw the consequences to the church of an unwise exercise of authority against scientific theories and argued only against the application of church power beyond the domain of theology.

Personal Traits and Interests.

Galileo was of medium stature, stoutly built, and is said to have had reddish brown hair. His friends included mechanics, noblemen, monks, cardinals, professors, artists, and men in every walk of life. His pupil Vincenzio Viviani said that he was quick to anger but quickly pacified.


Galileo’s wit is apparent both in his writings and in surviving anecdotes. Applied against his opponents, it frequently took the form of irony and sometimes of cutting sarcasm. Galileo’s polemics are marked by a habit of improving the opposing arguments before destroying their very basis and by an uncommon ability to invoke commoplace events and easily made observations as evidence against sophisticated theories.

Galileo’s knowledge of music and painting was extensive. He is said to have known yergil by heart, and he lectured on Dante and wrote criticisms of Tasso and Ariosto. To Ariosto he acknowledged a debt for his own clarity of style. The painter Lodovico Cigoli attributed to Galileo his instruction in perspective, and Galileo himself once remarked that he had considered painting as a career.

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