Galileo Galilei

The son of a famous musician and music theorist, Galileo was born near Pisa, Italy. He was educated by monks at Vallombrosa, then entered the university of Pisa in 1581 to study medicine. There, he found his interests changing to philosophy and mathematics and he ended his university career in 1585 without a degree.

Despite his lack of a degree, he tutored privately for a while until he became professor of mathematics at Pisa in 1589. Legend has it that while teaching there, he demonstrated to his students that Aristotle was wrong about speed of fall being related to the weight of an object by dropping two items off the leaning tower.

Contradicting Aristotle was considered almost a heresy by many at the time, and he was not invited back to the university in 1592. That same year, though, he secured a position as chair of mathematics at the University of Padua. He remained there until 1610.

Joining the Establishment

Galileo thrived at Padua where he invented an early calculator called a compass which helped solve complex mathematical problems. He began studying physics, discovering the law of falling bodies as well as the parabolic path of projectiles. He also studied the motion of pendulums, began studying mechanics and material tactile strengths. He professed no interest in Astronomy, though he did favor the Copernican universe model over the Ptolemaic or Arisotalian.
After learning of the invention of the spyglass in Holland, he built a telescope of his own, which had a magnification factor of 20. With this new tool, he found mountains and craters on the moon, discovered that the Milky Way was composed of stars, and discovered the four largest moons of Jupiter. After publishing his findings in The Starry Messenger in March of 1610, he was appointed court mathematician at Florence, freeing him from his teaching duties and giving more time for research. By December, he had observed the phases of Venus, further contradicting Ptolemy and confirming the Copernican theory.

Heresy

Many scientists of the day disputed Galileo's discoveries and theories, often because of their disagreement with Aristotle. In 1614, a priest in Florence denounced him and his followers from a pulpit, causing Galileo to compose an open letter on the irrelevance of the bible in scientific arguments and that scientific positions should never be made articles of Roman Catholic faith.
In 1616, Copernican books were censored by the church and Galileo was told by cardinal Robert Bellarmine to no longer defend the concept that the Earth moves. He made no complaints, continuing his work on determining longitudes at sea, based on his predictions of the positions of Jupiter's satellites. He continued to study falling objects and penned a book on Comets called The Assayer, which was published in 1623.

Banned!

Early in 1616, Copernican books were subjected to censorship by edict, and the Jesuit cardinal Robert Bellarmine instructed Galileo that he must no longer hold or defend the concept that the earth moves. Cardinal Bellarmine had previously advised him to treat this subject only hypothetically and for scientific purposes, without taking Copernican concepts as literally true or attempting to reconcile them with the Bible. Galileo remained silent on the subject for years, working on a method of determining longitudes at sea by using his predictions of the positions of Jupiter's satellites, resuming his earlier studies of falling bodies, and setting forth his views on scientific reasoning in a book on comets, The Assayer (1623; trans. 1957).
In 1630, Galileo received license from Catholic censors to publish a book called Dialogue on the Tides, which discussed both the Ptolemaic and Copernican theories, though they changed the title to Dialogue on Two Chief World Systems. It was published in 1632.

On Trial

Despite holding two licenses for the book, he was summoned to Rome by the Inquisition to stand trial for "grave suspicion of heresy," stating that he had been ordered personally in 1616 to not discuss

Copernicanism, either orally or written. Despite the fact that Galileo was able to produce a certificate signed by the late cardinal Bellarmine stating that he was under no further restrictions than applied to other Catholics, he was compelled in 1633 to abjure and was sentenced to life in prison.

That sentence was swiftly commuted to permanent house arrest, but the Dialogue was ordered to be burned, and the sentence against him was to be read publicly in every university.

A Life Acknowledged

His final book, Discourses Concerning Two New Sciences was published at Leiden in 1638. Discussing and refining his earlier studies of motion and principles of mechanics, it opened a road that was to lead Newton to his law of universal gravitation that linked Kepler's planetary laws with Galileo's mathematical physics. Galileo lost his eyesight before it was published. He died at Arcetri, near Florence, on January 8, 1642.
An investigation into the astronomer's condemnation, calling for its reversal, was opened in 1979 by Pope John Paul II. In October 1992 a papal commission acknowledged the Vatican had made errors, but stopped short of admitting the Church was wrong to convict Galileo on a charge of heresy because of his belief that the Earth rotates round the sun.