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Galileo Galilei  (15 February 1564 – 8 January 1642) was an Italian astronomer, and instrument maker. He ran into conflict with the Catholic Church of his day, and was put on trial for heresy by the Inquisition.
Some people believe that Galileo was the first person to build a telescope. This is not true, but he was the first person to publish his observations through a telescope of objects in the sky, like the Moon, stars and other planets. He discovered that the Milky Way is made of many stars, that the Moon has hills, and he found four moons around Jupiter, now called the Galilean moons. Later scientists found many others. He also discovered sunspots, which are dark areas of the Sun. He saw that the planet Venus has light and dark phases just like the Moon. This helped people to know that the Sun is at the center of the Solar System, as Nicolaus Copernicus had said.
Galileo also studied natural forces, and other things that are now called principles of physics. A legend says that he climbed the Leaning Tower of Pisa, and dropped cannonballs of different weights, to see which would strike the ground first. Even though their weights were not the same, they hit the ground at the same time. Galileo found that objects fall to the ground at the same rate, unless things like wind resistance change the rate. This went against the views of Aristotle, an ancient philosopher whose theory was different. Galileo's findings were ignored by most people, and Aristotle's view was still accepted as correct until Isaac Newton proved Galileo was right. This also led to Newton creating his Law of Gravity.
Galileo also tried to determine the speed of light. He climbed a hill, and had an assistant climb another hill, both carrying lanterns with closed shutters. He then opened the shutter of his lantern. His assistant opened his own shutter when he saw Galileo's lantern. Galileo then measured the time it took for his assistant's shutter to open. Knowing the time difference, and the distance between the hills, he tried to estimate the speed of light. However, this did not work.
Trial for heresy
Galileo came to accept the findings of Copernicus, that the Sun was the center of the then-known universe, and not the Earth. Because he promoted this and other ideas, he came to the notice of the Committee of Propaganda, the dreaded Inquisition. The Church taught that the Earth stood still, while everything in the sky moved around it. The Inquisition ruled that other theories could only be discussed as possibilities, not facts.
Galileo later defended his views in his most famous work, Dialogue Concerning the Two Chief World Systems, published in 1632. The book was in the form of conversations between three men. The man representing the Church's point of view was called 'Simplicio'. At this, the Inquisition took action. He was arrested and put on trial. They found him "vehemently suspect of heresy". They reminded him of the fate of Giordano Bruno, who had been burnt at the stake for heresy. Bruno's heresy was to believe the Earth went round the Sun, and that there were many other stars. The Inquisition forced Galileo to recant (say he was wrong) under the threat of execution, and to withdraw his works from publication. Galileo spent the rest of his life under house arrest.
In time, more and more of Galileo's findings were accepted as true. Late in the 20th century, Pope John Paul II called Galileo the "father of modern physics", and made a public apology for how the Church treated him earlier.
Galileo's main publications
- Siderius nuncius (starry messenger). Venice 1610. Discovery of 'new worlds' with the telescope.
- Dialogo sopra i due massimi sistemi del mondo, Tolemaico et Copernicano. Florence, 1632. The famous 'dialogue between two world systems'.
- Discorsi et demonstrazioni mathematiche, intorno a due nuove scienze. (discourses and mathematical demonstrations relating to two new sciences) Leiden 1638. This, on the mathematics of motion, was not such a controversial subject, and was published in Holland, out of the Inquisition's territory.
- Giordano Bruno
- Nicholas Copernicus
- Johannes Kepler
- List of science books#17th century
- Speed of light
- Michelson–Morley experiment
- pronounced Gal-lee-lay-oh Gal-eel-ay
- Evans I.O. 1962. Inventors of the World. Warne. p. 37–50.
- Gay, Peter 1966. The practical philosophers, in Age of Enlightenment. Time-Life Books. pp. 16, 18.
- Sharratt, Michael 1994. Galileo: decisive innovator. Cambridge University Press. pp. 1–2. .
- Gamow, George 1988. One, two, three – infinity: facts and speculations of science. Courier Dover, N.Y.
- Finocchiaro (1997), p. 47.
- Hilliam (2005), p. 96.
- Drake, Stillman 2001. Galileo: a very short introduction..
- Brodrick, James S.J. (1965, c1964). Galileo: the man, his work, his misfortunes. London: Chapman.