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Galileo Galilei (1564–1642) was a great Italian astronomer (a scientist specializing in the study of matter in outer space), mathematician, and physicist (a scientist specializing in the study of matter, energy, and their interactions) best known for his contributions to modern experimental science. His persistence in investigating and testing natural laws laid the foundation for the modern experimental method, which is based on conducting tests and observing results. Prior to this innovation, scientific theory was based purely on assumption and supposition.
In order to conduct accurate tests Galileo developed a number of important inventions, including the hydrostatic balance (an instrument for measuring the density of objects) in 1586 and the thermometer (a device for measuring temperature) in 1593. Galileo also expanded upon the basic telescope (an instrument for viewing distant objects) to make it into a highly functional tool. He was the first to use the telescope to study the skies, enabling him to make several important discoveries. For instance, in 1610 he found that the Moon shines with light that is reflected from the Sun and that the Moon's surface is mountainous. He also discovered sunspots (dark areas that occasionally appear on the Sun) and learned that Jupiter has four large satellites (moons). Galileo is credited with determining that the Milky Way (the Earth' galaxy) is made up of countless stars. He was able to estimate the period of rotation for each of Jupiter's moons (which he named the "Medicean stars"), and he observed the phases of Venus.
Galileo made other important contributions to science at a relatively young age. When he was just nineteen years old he used his heartbeat to time the oscillations (movements backward and forward) of a swinging lamp in the cathedral at Pisa, Italy. He also discovered that the time for each swing is the same, even if the distance of the swing varies. He went on to experiment with this concept and proved important theories about arcs (arched or curved lines) and inclines (sloped surfaces). At the age of twenty-five he also published a study of the center of gravity (physical force) in solid substances. He concluded that a falling object gains speed at a constant rate, a discovery that would help English physicist Isaac Newton (1642–1727) formulate the law of gravity.
Galileo pursued scientific truth throughout his life and encountered some adversity as a result. His use of the telescope led him to endorse the somewhat controversial theory of Polish astronomer Nicholas Copernicus (1473–1543; also known as Mikolaj Kopernik), who claimed that the Sun—not the Earth—is at the center of the universe. At the time this revelation was a threat to the Roman Catholic Church (a Christian religion based in Rome, Italy, and headed by a pope), which taught that the Earth is the center of the universe. In 1632 Galileo published a work in which he supported Copernicus's theory. As a result of his work, he was summoned to Rome, where he was tried by the Inquisition, a court that imprisoned or killed Catholics who were suspected of not following the teachings of the church. In 1633 Galileo was forced to sign a document stating that he rejected all beliefs and written works that held the Sun to be the central body of the universe. After the trial he was placed under guard at a house in Siena. Later he was allowed to live at Arcetri near Florence, Italy, where he continued to write and experiment despite old age and blindness. In 1979 Pope John Paul II asked that Galileo's conviction be overturned, but since the Copernican theory had been banned from the church's teachings in 1616 this was a technical impossibility. Finally, in 1992 the ban was reversed and Galileo's name was officially cleared by the church.
Further Information: Galilei, Galileo. Dialogues Concerning Two New Sciences: Galileo Galilei. Translated by Henry Crew, and Alfonso de Salvio. Mineola, N.Y.: Dover Publications, 1990; Galileo Galilei —A Biography. [Online] Available http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Galileo. html, November 8, 2000; "Galileo." The Internet Encyclopedia of Philosophy. [Online] Available http://www.utm.edu/research/iep/g/galileo.htm, November 8, 2000.
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