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Galileo Galilei (February 15, 1564 - January 8, 1642), was an Italian astronomer, philosopher, and physicist; who is commonly associated with the "Scientific Revolution". He has been referred to as the "father" of astronomy and, along with Bacon, was one of the pioneers of the modern scientific method. Galilei was born in Pisa and his career coincided with that of Kepler. The work of Galilei is considered to be a significant break from that of Aristotle; particularly, Galilei placed emphasis on the quantity, rather than the quality.
Scientific achievements Galileo was one of the first astronomers to use the telescope. He acquired a 10x telescope and then made an 20x one. He published his initial observations in March 1610 as Sidereus Nuncius (Sidereal Messenger). Galileo is credited with the discovery of Jupiter's 4 largest satellites. He noted that Venus exhibited "phases" like Luna. Galilei's work supported Copernicus' theory of heliocentrism. In addition, he was the first to report lunar mountains.
It has been argued that Galilei discovered sunspots; although, there is evidence that Chinese astronomers discovered this phenomenon. Galilei also argued that Luna is not a perfect sphere; an argument based upon the occultation of stars[?].
Galilei studied balls, rolling down inclined planes, and concluded that falling objects are accelerated independent of their mass, and that objects retain their velocity unless a force acts upon them. Galilei's work, in dynamics, was further developed by Kepler's and Newton's. In addition, Galilei noted that a pendulum's swings always take the same amount of time, independent of amplitude, a discovery which had an impact upon the manufacture of clocks. .
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 lantern. Galileo concluded that the speed of light was too high to be measured.
Galileo wrote several books which were circulated outside of Italy. He created sketches of various inventions, such as; a candle and mirror combination to reflect light through a building), an automatic tomato picker, a pocket comb/eating utensil, and a ballpoint pen.
Jupiter's Moons Galilei discovered Jupiter's 4 largest satellites in 1610. He determined that these moons were orbiting the planet since they would occasionaly disappear; something he attributed to their movement behind Jupiter. He made additional observations, in regards to this topic, in 1620.
Galilei was a devout Catholic, yet his writings on Copernican heliocentrism disturbed the Catholic Church, which believed in a geocentric model of the solar system. The church argued that heliocentrism was in direct contradiction with the Bible and the highly revered ancient writings of Aristotle and Plato. For his insights, Galilei was threatened with death at the stake and would eventually face several years of house arrest after recanting his claims.
The geocentric model was generally accepted at the time not only for scriptural reasons. By the time of the controversy, the Catholic Church had in fact abandoned the Ptolemaic model for the Tychonian model in which the Earth was at the center of the Universe, the Sun revolved around the Earth and the other planets revolved around the Sun. This model is geometrically identical to the Copernican model and has the extra advantage that it predicts no parallax of the stars. Yet, Galilei's arguments were most fiercely fought on the religious level. The late-nineteenth- and early twentieth century historian Andrew Dickson White wrote from an anti-clerical perspective:
The war became more and more bitter. The Dominican Father Caccini preached a sermon from the text, "Ye men of Galilee, why stand ye gazing up into heaven?" and this wretched pun upon the great astronomer's name ushered in sharper weapons; for, before Caccini ended, he insisted that "geometry is of the devil," and that "mathematicians should be banished as the authors of all heresies." The Church authorities gave Caccini promotion. Father Lorini proved that Galileo's doctrine was not only heretical but "atheistic," and besought the Inquisition to intervene. The Bishop of Fiesole screamed in rage against the Copernican system, publicly insulted Galileo, and denounced him to the Grand-Duke. The Archbishop of Pisa secretly sought to entrap Galileo and deliver him to the Inquisition at Rome. The Archbishop of Florence solemnly condemned the new doctrines as unscriptural; and Paul V, while petting Galileo, and inviting him as the greatest astronomer of the world to visit Rome, was secretly moving the Archbishop of Pisa to pick up evidence against the astronomer. But by far the most terrible champion who now appeared was Cardinal Bellarmin, one of the greatest theologians the world has known. He was earnest, sincere, and learned, but insisted on making science conform to Scripture. The weapons which men of Bellarmin's stamp used were purely theological. They held up before the world the dreadful consequences which must result to Christian theology were the heavenly bodies proved to revolve about the Sun and not about the Earth. Their most tremendous dogmatic engine was the statement that "his pretended discovery vitiates the whole Christian plan of salvation." Father Lecazre declared "it casts suspicion on the doctrine of the incarnation." Others declared, "It upsets the whole basis of theology. If the Earth is a planet, and only one among several planets, it can not be that any such great things have been done specially for it as the Christian doctrine teaches. If there are other planets, since God makes nothing in vain, they must be inhabited; but how can their inhabitants be descended from Adam? How can they trace back their origin to Noah's ark? How can they have been redeemed by the Saviour?" Nor was this argument confined to the theologians of the Roman Church; Melanchthon, Protestant as he was, had already used it in his attacks on Copernicus and his school.  (http://www.santafe.edu/~shalizi/White/astronomy/war)
In 1616 the Inquisition warned Galileo not to hold or defend the hypothesis asserted in Copernicus' On the Revolutions, though it has been debated whether he was admonished not to 'teach in any way' the heliocentric theory.
Despite his continued insistence that his work in the area was purely theoretical, despite his strict following of the church protocol for publication of works (which required prior examination by church censors and subsequent permission), and despite his close friendship with Maffeo Barberini who later became Pope Urban VIII and presided throughout the ordeal, Galileo was forced to recant his views repeatedly and was put under life-long house arrest (1633-1642).
The Inquisition had rejected earlier pleas by Galilei to postpone or relocate the trial because of his ill health. At a meeting presided by Pope Urban VIII, the Inquisition decided to notify Galilei that he either had to come to Rome or that he would be arrested and brought there in chains. Galileo arrived in Rome for his trial before the Inquisition on February 13, 1633. After two weeks in quarantine, Galilei was detained at the comfortable residence of the Tuscan ambassador, as a favor to the influential Grand Duke Ferdinand II de' Medici. In April 1633 he was formally interrogated by the Inquisition. He was not imprisoned in a dungeon cell, but detained in a room in the offices of the Inquisition for 22 days.
On June 22, 1633, the Roman Inquisition started its trial against Galilei, who was then 69 years old and pleaded for mercy, pointing to his "regrettable state of physical unwellness". Threatening him with torture, imprisonment and death on the stake, the show trial forced Galileo to "abjure, curse and detest" his work and to promise to denounce others who held his prior viewpoint. Galileo did everything the church requested him to do. (The idea that he muttered Eppur si muove! - "But it moves anyway!" - is a legend.) That the threat of torture and death Galileo was facing was a real one had been proven by the church in the earlier trial against Giordano Bruno, who was burned at the stake in 1600 for holding a naturalistic view of the Universe.
Galileo was sentenced to prison but because of his advanced age was allowed to serve his term under house arrest at his villas in Arcetri and Florence (http://www.lucidcafe.com/library/96feb/galileo) Because of a painful hernia, he requested permission to consult physicians in Florence, which was denied by Rome, warning that further such requests would lead to imprisonment. Under arrest, he was forced to recite penitentiary psalms regularly, and his social contacts were highly restricted, but he was allowed to continue his less controversial research and publish under strict rules of censorship. He went totally blind in 1638 (his petition to the Inquisition to be released was rejected, but he was allowed to move to his house in Florence where he was closer to his physicians). His Dialogue was put on the Index librorum prohibitorum, a black list of banned books, until 1822. 
According to Andrew Dickson White and many of his colleagues, Galileo's experiences demonstrate a classic case of a scholar forced to recant a scientific insight because it offended powerful, conservative forces in society: for the church at the time, it was not the scientific method that should be used to find truth -- especially in certain areas -- but the doctrine as interpreted and defined by church scholars, and this doctrine was defended with torture, murder, deprivation of freedom, and censorship.
More recently, the viewpoints of White and his colleagues have become less-generally accepted by the academic community, partially because White wrote from a perspective that Christianity is a destructive force. This attitude can also be seen in the works of Bertolt Brecht, whose play about Galileo is one of the chief sources for popular knowledge on the scientist. Moreover, deeper examination of the primary sources for Galileo and his trial shows that claims of torture and deprivation were likely exagerrated. Dava Sobel's Galileo's Daughter offers a different set of insights into Galileo and his world, in large part through the private correspondence of Maria Celeste, the daughter of the title, and her father.
In 1992, 359 years after the Galileo trial, Pope John Paul II issued an apology, lifting the edict of Inquisition against Galileo: "Galileo sensed in his scientific research the presence of the Creator who, stirring in the depths of his spirit, stimulated him, anticipating and assisting his intuitions."