Galileo Galilei was a 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 religion and freedom of thought, particularly with science, in Western society.

Galileo Galilei was born in Pisa on February 15, 1564. His parents were Vincenzo Galilei and Guilia Ammannati. His father, Vincenzo Galilei, belonged to a noble family of straitened fortune, and had gained some distinction as a musician and mathematician. The boy at an early age manifested his aptitude for mathematical and mechanical pursuits, but his parents, wishing to turn him aside from studies which promised no substantial return, destined him for the medical profession.

In 1581, Galileo was sent to the University of Pisa to study medicine. While a student at the university, Galileo discovered that he had a talent for mathematics. He was able to persuade his father to allow him to leave the university to become a tutor in mathematics. He later became a professor of mathematics.

In the year 1582-83 Ostilio Ricci, who was the mathematician of the Tuscan Court and a former pupil of Tartaglia, taught a course on Euclid's Elements at the University of Pisa which Galileo attended. During the summer of 1583 Galileo was back in Florence with his family and Vincenzo encouraged him to read Galen to further his medical studies. However Galileo, still reluctant to study medicine, invited Ricci (also in Florence where the Tuscan court spent the summer and autumn) to his home to meet his father. Ricci tried to persuade Vincenzo to allow his son to study mathematics since this was where his interests lay. Certainly Vincenzo did not like the idea and resisted strongly but eventually he gave way a little and Galileo was able to study the works of Euclid and Archimedes from the Italian translations which Tartaglia had made. Of course he was still officially enrolled as a medical student at Pisa but eventually, by 1585, he gave up this course and left without completing his degree.

As a professor of astronomy at University of Pisa, Galileo was required to teach the accepted theory of his time that the sun and all the planets revolved around the Earth. Later at University of Padua he was exposed to a new theory, proposed by Nicolaus Copernicus, that the Earth and all the other planets revolved around the sun. Galileo's observations with his new telescope convinced him of the truth of Copernicus's sun-centered or heliocentric theory.

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.

The Catholic Church, which was very powerful and influential in Galileo's day, strongly supported the theory of a geocentric, or Earth-centered, Universe. After Galileo began publishing papers about his astronomy discoveries and his belief in a heliocentric, or Sun-centered, Universe, he was called to Rome to answer charges brought against him by the Inquisition (the legal body of the Catholic Church). Early in 1616, Galileo was accused of being a heretic, a person who opposed Church teachings. Heresy was a crime for which people were sometimes sentenced to death. Galileo was cleared of charges of heresy, but was told that he should no longer publicly state his belief that Earth moved around the Sun. Galileo continued his study of astronomy and became more and more convinced that all planets revolved around the Sun. In 1632, he published a book that stated, among other things, that the heliocentric theory of Copernicus was correct. Galileo was once again called before the Inquisition and this time was found guilty of heresy. Galileo was sentenced to life imprisonment in 1633. Because of his age and poor health, he was allowed to serve his imprisonment under house arrest. Galileo died on January 8, 1642.


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