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Galileo Galilei

Galileo Galilei (Pisa, February 15, 1564Arcetri, January 8, 1642), was a Tuscan astronomer, philosopher, and physicist who is closely associated with the scientific revolution. His achievements include improving the telescope, a variety of astronomical observations, the first law of motion, and supporting Copernicanism effectively. He has been referred to as the "father of modern astronomy," as the "father of modern physics," and as "father of science." His experimental work is widely considered complementary to the writings of Francis Bacon in establishing the modern scientific method. Galileo's career coincided with that of Johannes Kepler. The work of Galileo is considered to be a significant break from that of Aristotle. In addition, his conflict with the Roman Catholic Church is taken as a major early example of the conflict of authority and freedom of thought, particularly with science, in Western society.


Early career

Galileo was born in Pisa, Italy.

He attended the University of Pisa, but was forced to "drop out" for financial reasons. However, he was offered a position on its faculty in 1589 and taught mathematics. Soon after, he moved to the University of Padua, and served on its faculty teaching geometry, mechanics, and astronomy until 1610. During this time he explored science and made many landmark discoveries.

Experimental science

In the pantheon of the scientific revolution, Galileo takes a high position because of his pioneering use of quantitative experiments with results analyzed mathematically. There was no tradition of such methods in European thought at that time; the great experimentalist who immediately preceded Galileo, William Gilbert, did not use a quantitative approach. (However, Galileo's father, Vincenzo Galilei, had performed experiments in which he discovered what may be the oldest known non-linear relation in physics, between the tension and the pitch of a stretched string.) Galileo also contributed to the rejection of blind allegiance to authority (like the Church) or other thinkers (such as Aristotle) in matters of science and to the separation of science from philosophy or religion. These are the primary justifications for his description as "father of science."

In the 20th century some authorities challenged the reality of Galileo's experiments, in particular the distinguished French historian of science Alexandre Koyré . The experiments reported in Two New Sciences to determine the law of acceleration of falling bodies, for instance, required accurate measurements of time, which appeared to be impossible with the technology of 1600. According to Koyré, the law was arrived at deductively, and the experiments were merely illustrative thought experiments.

Later research, however, has validated the experiments. The experiments on falling bodies (actually rolling balls) were replicated using the methods described by Galileo (Settle, 1961), and the precision of the results was consistent with Galileo's report. Later research into Galileo's unpublished working papers from as early as 1604 clearly showed the reality of the experiments and even indicated the particular results that led to the time-squared law (Drake, 1973).


In 1600, astronomers were engaged in a great debate between the Copernican system (the planets revolved around the Sun) and the geocentric system (the planets and Sun revolved around Earth). In 1604, Galileo announced his support for the Copernican school of thought, but he lacked the means to reinforce the opinion.

Although the popular idea of Galileo inventing the telescope is inaccurate, he was one of the first people to use the telescope to observe the sky. Based on sketchy descriptions of telescopes invented in the Netherlands in 1608, Galileo made one with about 8x magnification, and then made improved models up to about 20x. On August 25, 1609, he demonstrated his first telescope to Venetian lawmakers. His work on the device also made for a profitable sideline with merchants who found it useful for their shipping businesses. He published his initial telescopic astronomical observations in March 1610 in a short treatise entitled Sidereus Nuncius (Sidereal Messenger).

It was on this page that Galileo first noted an observation of the of . Galileo published a full description in in March .
It was on this page that Galileo first noted an observation of the moons of Jupiter. Galileo published a full description in Sidereus Nuncius in March 1610.

On January 7, 1610 Galileo discovered Jupiter's four largest satellites (moons): Io, Europa, Ganymede, and Callisto. He determined that these moons were orbiting the planet since they would occasionally disappear; something he attributed to their movement behind Jupiter. He made additional observations of them in 1620. (Later astronomers overruled Galileo's naming of these objects, changing his Medicean stars to Galilean satellites.) The demonstration that a planet had smaller planets orbiting it was problematic for the orderly, comprehensive picture of the geocentric model of the universe, in which everything circled around the Earth.

Galileo noted that Venus exhibited a full set of phases like the Moon. Because the apparent brightness of Venus is nearly constant, Galileo reasoned that Venus could not be circling the Earth at a constant distance. By contrast, the heliocentric model of the solar system developed by Copernicus would neatly account for the steady brightness by reason of the much greater distance from the Earth at the time of "full Venus," when the two planets were on opposite sides of the Sun such that Venus' illuminated hemisphere faced the Earth.

Galileo was one of the first Europeans to observe sunspots, although there is evidence that Chinese astronomers had done so before. The very existence of sunspots showed another difficulty with the perfection of the heavens as assumed in the older philosophy. And the annual variations in their motions, first noticed by Francesco Sizzi , presented great difficulties for either the geocentric system or that of Tycho Brahe. A dispute over priority in the discovery of sunspots led to a long and bitter feud with Christoph Scheiner; in fact, there can be little doubt that both of them were beaten by David Fabricius and his son Johannes.

He was the first to report lunar mountains and craters, whose existence he deduced from the patterns of light and shadow on the Moon's surface. He even estimated the mountains' heights from these observations. This led him to the conclusion that the Moon was "rough and uneven, and just like the surface of the Earth itself", and not a perfect sphere as Aristotle had claimed.

Galileo observed the Milky Way, previously believed to be a cloud, and found it to be a multitude of stars, packed so densely that they appeared to be clouds from Earth. He also located many other stars too distant to be visible with the naked eye.

Galileo observed the planet Neptune in 1611, but took no particular notice of it; it appears in his notebooks as one of many unremarkable dim stars.

Galileo's theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes, was a precursor of the Classical mechanics developed by Sir Isaac Newton. He was a pioneer, at least in the European tradition, in performing rigorous experiments and insisting on a mathematical description of the laws of nature.

One of the most famous stories about Galileo is that he dropped balls of different masses from the Leaning Tower of Pisa to demonstrate that their velocity of descent was independent of their mass (excluding the limited effect of air resistance). This was contrary to what Aristotle had taught: that heavy objects fall faster than lighter ones, in direct proportion to weight. Though the story of the tower first appeared in a biography by Galileo's pupil Vincenzo Viviani, it is now generally believed to be false. However, Galileo did perform experiments involving rolling balls down inclined planes, which proved the same thing: falling or rolling objects (rolling is a slower version of falling) are accelerated independently of their mass.

He determined the correct mathematical law for acceleration: the total distance covered, starting from rest, is proportional to the square of the time. (This law is regarded as a predecessor to the many later scientific laws expressed in mathematical form.) He also concluded that objects retain their velocity unless a force -- often friction -- acts upon them, refuting the accepted Aristotelian hypothesis that objects "naturally" slow down and stop unless a force acts upon them. This principle was incorporated into Newton's laws of motion (1st law).

Galileo also noted that a pendulum's swings always take the same amount of time, independently of the amplitude. While Galileo believed this equality of period to be exact, it is only an approximation appropriate to small amplitudes. It is good enough to regulate a clock, however, as Galileo may have been the first to realize. (See Technology below.)

In the early 1600s, Galileo and an assistant tried to measure the speed of light. They stood on different hilltops, each holding a shuttered lantern. Galileo would open his shutter, and, as soon as his assistant saw the flash, he would open his shutter. At a distance of less than a mile, Galileo could detect no delay in the round-trip time greater than when he and the assistant were only a few yards apart. While he could reach no conclusion on whether light propagated instantaneously, he recognized that the distance between the hilltops was perhaps too small for a good measurement.


Galileo made a few contributions to what we now call technology as distinct from pure physics, and suggested others. This is not the same distinction as made by Aristotle, who would have considered all Galileo's physics as techne or useful knowledge, as opposed to episteme, or philosophical investigation into the causes of things.

In 15951598, Galileo devised and improved a "Geometric and Military Compass" suitable for use by gunners and surveyors. This expanded on earlier instruments designed by Niccolo Tartaglia and Guidobaldo del Monte . For gunners, it offered, in addition to a new and safer way of elevating cannons accurately, a way of quickly computing the charge of gunpowder for cannonballs of different sizes and materials. As a geometric instrument, it enabled the construction of any regular polygon, computation of the area of any polygon or circular sector, and a variety of other calculations.

About 16061607 (or possibly earlier), Galileo made a thermometer, using the expansion and contraction of air in a bulb to move water in an attached tube.

In 1608, Galileo was the first to use a refracting telescope as a instrument to observe stars, planets or moons.

In 1610, he used a telescope as a compound microscope, and he made improved microscopes in 1623 and after. This appears to be the first clearly documented use of the compound microscope.

In 1612, having determined the orbital periods of Jupiter's satellites, Galileo proposed that with sufficiently accurate knowledge of their orbits one could use their positions as a universal clock, and this would make possible the determination of longitude. He worked on this problem from time to time during the remainder of his life; but the practical problems were severe. The method was first successfully applied by Giovanni Domenico Cassini in 1681 and was later used extensively for land surveys; for navigation, the first practical method was the chronometer of John Harrison.

In his last year, when totally blind, he designed an escapement mechanism for a pendulum clock. The first fully operational pendulum clock was made by Christiaan Huygens in the 1650s.

He created sketches of various inventions, such as a candle and mirror combination to reflect light throughout a building, an automatic tomato picker, a pocket comb that doubled as an eating utensil, and what appears to be a ballpoint pen.

Galileo Galilei
Galileo Galilei

Church controversy

Galileo was a practicing Catholic, yet his writings on Copernican heliocentrism disturbed some in the Catholic Church, who believed in a geocentric model of the solar system. They argued that heliocentrism was in direct contradiction of the Bible, at least as interpreted by the church fathers, and the highly revered ancient writings of Aristotle and Plato (especially among the Dominican order, facilitators of the Inquisition).

The geocentric model was generally accepted at the time for several reasons. By the time of the controversy, the Catholic Church had largely abandoned the Ptolemaic model for the Tychonian model in which the Earth was at the center of the Universe, the Sun revolved around the Earth and the other planets revolved around the Sun. This model is geometrically equivalent to the Copernican model and had the extra advantage that it predicted no parallax of the stars, an effect that was impossible to detect with the instruments of the time. In the view of Tycho and many others, this model explained the observable data of the time better than the geocentric model did. (That inference is valid, however, only on the assumption that no very small effect had been missed: that the instruments of the time were absolutely perfect, or that the Universe could not be much larger than was generally believed at the time. As to the latter, belief in the large, possibly infinite, size of the Universe was part of the heretical beliefs for which Giordano Bruno had been burned at the stake in 1600.)

An understanding of the controversies, if it is even possible, requires attention not only to the politics of religious organizations but to those of academic philosophy. Before Galileo had trouble with the Jesuits and before the Dominican friar Caccini denounced him from the pulpit, his employer heard him accused of contradicting Scripture by a professor of philosophy, Cosimo Boscaglia, who was neither a theologian nor a priest. The first to defend Galileo was a Benedictine abbot, Benedetto Castelli, who was also a professor of mathematics and a former student of Galileo's. It was this exchange that led Galileo to write the Letter to Grand Duchess Christina. (Castelli remained Galileo's friend, visiting him at Arcetri near the end of Galileo's life, after months of effort to get permission from the Inquisition to do so.)

However, real power lay with the Church, and Galileo's arguments were most fiercely fought on the religious level. The late nineteenth and early twentieth century historian Andrew Dickson White wrote from an anti-clerical perspective:

The war became more and more bitter. The Dominican Father Caccini preached a sermon from the text, "Ye men of Galilee, why stand ye gazing up into heaven?" and this wretched pun upon the great astronomer's name ushered in sharper weapons; for, before Caccini ended, he insisted that "geometry is of the devil," and that "mathematicians should be banished as the authors of all heresies." The Church authorities gave Caccini promotion.
Father Lorini proved that Galileo's doctrine was not only heretical but "atheistic," and besought the Inquisition to intervene. The Bishop of Fiesole screamed in rage against the Copernican system, publicly insulted Galileo, and denounced him to the Grand-Duke. The Archbishop of Pisa secretly sought to entrap Galileo and deliver him to the Inquisition at Rome. The Archbishop of Florence solemnly condemned the new doctrines as unscriptural; and Paul V, while petting Galileo, and inviting him as the greatest astronomer of the world to visit Rome, was secretly moving the Archbishop of Pisa to pick up evidence against the astronomer.
But by far the most terrible champion who now appeared was Cardinal Robert Bellarmine, one of the greatest theologians the world has known. He was earnest, sincere, and learned, but insisted on making science conform to Scripture. The weapons which men of Bellarmin's stamp used were purely theological. They held up before the world the dreadful consequences which must result to Christian theology were the heavenly bodies proved to revolve about the Sun and not about the Earth. Their most tremendous dogmatic engine was the statement that "his pretended discovery vitiates the whole Christian plan of salvation." Father Lecazre declared "it casts suspicion on the doctrine of the incarnation." Others declared, "It upsets the whole basis of theology. If the Earth is a planet, and only one among several planets, it can not be that any such great things have been done specially for it as the Christian doctrine teaches. If there are other planets, since God makes nothing in vain, they must be inhabited; but how can their inhabitants be descended from Adam? How can they trace back their origin to Noah's ark? How can they have been redeemed by the Saviour?" Nor was this argument confined to the theologians of the Roman Church; Melanchthon, Protestant as he was, had already used it in his attacks on Copernicus and his school. (White, 1898; online text)

In 1616, the Inquisition warned Galileo not to hold or defend the hypothesis asserted in Copernicus's On the Revolutions, though it has been debated whether he was admonished not to "teach in any way" the heliocentric theory. When Galileo was tried in 1633, the Inquisition was proceeding on the premise that he had been ordered not to teach it at all, based on a paper in the records from 1616; but Galileo produced a letter from Cardinal Bellarmine that showed only the "hold or defend" order. The latter is in Bellarmine's own hand and of unquestioned authenticity; the former is an unsigned copy, violating the Inquisition's own rule that the record of such an admonition had to be signed by all parties and notarized. Leaving aside technical rules of evidence, what can one conclude as to the real events? There are two schools of thought. According to Stillman Drake, the order not to teach was delivered unofficially and improperly; Bellarmine would not allow a formal record to be made, and assured Galileo in writing that the only order in effect was not to "defend or hold". According to Giorgio di Santillana, however, the unsigned minute was simply a fabrication by the Inquisition.

In 1623 Pope Gregory XV died, and Galileo's close friend Maffeo Barberini became Pope Urban VIII. The new Pope gave Galileo vague permission to ignore the ban and write a book about his opinions, so long as he did not openly support his theory. Galileo consented, and set to work writing his masterpiece, Dialogue Concerning the Two Chief World Systems (often shortened to Dialogues). It involved an argument between two intellectuals, one geocentric, the other heliocentric, and a layman, neutral but interested. Although it presented the Church's point of view, the geocentrist was depicted foolishly, while the heliocentrist often dominated the argument and convinced the neutral member in the end.

The Dialogues were published in 1632 with the approval of Catholic censors. It was applauded by intellectuals but nevertheless aroused the Church's ire. Despite his continued insistence that his work in the area was purely theoretical, despite his strict following of the church protocol for publication of works (which required prior examination by church censors and subsequent permission), and despite his close friendship with the Pope (who presided throughout the ordeal), Galileo was summoned to trial before the Roman Inquisition in 1633.

The Inquisition had rejected earlier pleas by Galileo to postpone or relocate the trial because of his ill health. At a meeting presided by Pope Urban VIII, the Inquisition decided to notify Galileo that he either had to come to Rome or that he would be arrested and brought there in chains. Galileo arrived in Rome for his trial before the Inquisition on February 13, 1633. After two weeks in quarantine, Galileo was detained at the comfortable residence of the Tuscan ambassador, as a favor to the influential Grand Duke Ferdinand II de' Medici . When the ambassador reported Galileo's arrival and asked how long the proceedings would be, the Pope replied that the Holy Office proceeded slowly, and was still in the process of preparing for the formal proceedings. In the event, having responded to the urgent demands of the Inquisition that he must report to Rome immediately, Galileo was left to wait for two months before proceedings would begin.

On April 12, 1633, Galileo was brought to trial, and the formal interrogation by the Inquisition began. During this interrogation Galileo stated that he did not defend the Copernican theory, and cited a letter of Cardinal Bellarmine from 1615 to support this contention. The Inquisition questioned him on whether he had been ordered in 1616 not to teach Copernican ideas in any way (see above); he denied remembering any such order, and produced a letter from Bellarmine saying only that he was not to hold or defend those doctrines.

He was then detained for eighteen days in a room in the offices of the Inquisition (not in a dungeon cell). During this time the Commissary General of the Inquisition, Vincenzo (later Cardinal) Maculano , visited him for what amounted to plea bargaining, persuading Galileo to confess to having gone too far in writing the book. In a second hearing on April 30, Galileo confessed to having erred in the writing of the book, through vain ambition, ignorance, and inadvertence. He was then allowed to return to the home of the Tuscan ambassador. On May 10, he submitted his written defense, in which he defended himself against the charge of disobeying the Church's order, confessed to having erred through pride in writing the book, and asked for mercy in light of his age and ill health.

A month later (June 21), by order of the Pope, he was given an examination of intention, a formal process that involved showing the accused the instruments of torture. At this proceeding, he said, "I am here to obey, and have not held this [Copernican] opinion after the determination made, as I said."

On June 22, 1633, the Inquisition held the final hearing on Galileo, who was then 69 years old and pleaded for mercy, pointing to his "regrettable state of physical unwellness". Threatening him with torture, imprisonment, and death on the stake, the show trial forced Galileo to "abjure, curse and detest" his work and to promise to denounce others who held his prior viewpoint. Galileo did everything the church requested him to do, following (so far as we can tell) the plea bargain of two months earlier. He was convicted and sentenced to life imprisonment.

Although ten Cardinal Inquisitors had heard the case, the sentence carried out on June 22 bears the signature of only seven; one of the three missing was Cardinal Barberini, the Pope's nephew. It is generally held that this indicates a refusal to endorse the sentence. The seven who signed, however, were those who were present at that day's proceedings; Cardinals Barberini and Borgia in particular, were attending an audience with the Pope on that day. Analysis of the Inquisition's records has shown that the presence of only seven of ten Cardinals was not exceptional; hence the inference that Barberini was protesting the decision may be doubted.

That the threat of torture and death Galileo was facing was a real one had been proven by the church in the earlier trial against Giordano Bruno, who was burned at the stake in 1600 for holding a naturalistic view of the Universe.

The tale that Galileo, rising from his knees after recanting, said "E pur si muove!" 1 (But it does move!) cannot possibly be true; to say any such thing in the offices of the Inquisition would have been a ticket to follow Bruno to the stake. But the widespread belief that the whole incident is an 18th century invention is also false. A Spanish painting, dated 1643 or possibly 1645, shows Galileo writing the phrase on the wall of a dungeon cell. Here we have a second version of the story, which also cannot be true, because Galileo was never imprisoned in a dungeon; but the painting shows that some story of "Eppur si muove"1 was circulating in Galileo's time. In the months immediately after his condemnation, Galileo resided with Archbishop Ascanio Piccolomini of Siena, a learned man and a sympathetic host; the fact that Piccolomini's brother was a military attaché in Madrid, where the painting was made some years later, suggests that Galileo may have made the remark to the Archbishop, who then wrote to his family concerning the event, which later became garbled in re-telling.

Galileo was sentenced to prison, but because of his advanced age (and/or Church politics) the sentence was commuted to house arrest at his villas in Arcetri and Florence 2. Because of a painful hernia, he requested permission to consult physicians in Florence, which was denied by Rome, which warned that further such requests would lead to imprisonment. Under arrest, he was forced to recite penitentiary psalms regularly, and he was forced to reject house guests, but he was allowed to continue his less controversial research and the social-contact punishment was not enforced very well.

Publication was another matter. His Dialogue had been put on the Index Librorum Prohibitorum, the official black list of banned books, where it stayed until 1822 (Hellman, 1998). Though the sentence announced against Galileo mentioned no other works, Galileo found out two years later that publication of anything he might ever write had been quietly banned. The ban was effective in France, Poland, and German states, but not in the Netherlands.

Placed under house-arrest, Galileo would, in 1638, be allowed to move to his home near Florence. Though by then totally blind, he continued to teach and write. He died at his villa in Arcetri, just north of Florence, in 1642.

According to Andrew Dickson White, in A History of the Warfare of Science with Theology in Christendom (III.iii), 1896, Galileo's experiences demonstrate a classic case of a scholar forced to recant a scientific insight because it offended powerful, conservative forces in society: for the church at the time, it was not the scientific method that should be used to find truth—especially in certain areas— but the doctrine as interpreted and defined by church scholars, and White documented how this doctrine was defended by the Church with torture, murder, deprivation of freedom, and censorship. In a less polemical frame, this has remained the mainstream view among the historians of science.

The viewpoints of White and similar-minded colleagues were never accepted by the Catholic community, partially because White's final analysis depicted Christianity as a destructive force. A fierce expression of this critical attitude can also be seen in Bertolt Brecht's play about Galileo, a source for popular ideas about the scientist. Moreover, deeper examination of the primary sources for Galileo and his trial shows that claims of deprivation were likely exaggerated. Dava Sobel's biography Galileo's Daughter offers a different set of insights into Galileo and his world, in large part through the private correspondence of Maria Celeste, the daughter of the title, and her father.

In 1992, 359 years after the Galileo trial, Pope John Paul II issued an apology, lifting the edict of Inquisition against Galileo: "Galileo sensed in his scientific research the presence of the Creator who, stirring in the depths of his spirit, stimulated him, anticipating and assisting his intuitions." After the release of this report, the Pope said further that "... Galileo, a sincere believer, showed himself to be more perceptive in this regard [the relation of scientific and Biblical truths] than the theologians who opposed him."

Galileo's family

Although a devout Catholic, Galileo fathered three children out of wedlock. All were the children of Galileo and Marina Gamba.

  • Virginia (b. 1600) see Maria Celeste, Galileo's eldest child was his most beloved, and inherited her father's sharp mind.
  • Livia
  • Vincenzio

Galileo's writings

Writings on Galileo


  • Drake, Stillman (1973). "Galileo's Discovery of the Law of Free Fall". Scientific American v. 228, #5, pp. 84-92.
  • Drake, Stillman (1978). Galileo At Work. Chicago: University of Chicago Press. ISBN 0-226-16226-5
  • Drake, Stillman. Discoveries and Opinions of Galileo. ISBN 0-385-09239-3
  • Fantoli, Annibale (2003). Galileo—For Copernicanism and the Church, third English edition. Vatican Observatory Publications. ISBN 88-209-7427-4
  • Hellman, Hal (1988). Great Feuds in Science. Ten of the Liveliest Disputes Ever. New York: Wiley.
  • Lessl, Thomas, "The Galileo Legend". New Oxford Review, 27-33 (June 2000).
  • Newall, Paul (2004). "The Galileo Affair."
  • Settle, Thomas B. (1961). "An Experiment in the History of Science". Science, 133:19-23.
  • White, Andrew Dickson (1898). A History of the Warfare of Science with Theology in Christendom. New York 1898.

Named after Galileo

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