Galileo Galilei was born in Pisa, Tuscany, on February 15th, 1564. He was the eldest of the four surviving children of Vicenzo Galilei, a well-known lutenist, and his wife, Guilia Ammannati. When Galileo was around 8 years old, his father moved the family to Florence, and at a monastery in the neighbouring town of Vallombrosa the teenaged Galileo began his academic career.
In 1581, Galileo entered Pisa University, initially at his father's urging to study medicine, but had a change of heart and switched to mathematics and philosophy. However, in 1585, Galileo left Pisa University without completing his degree, and for a while earned a living by giving private lessons in mathematical subjects in Florence and Siena. During this period he also began studying motion.
In 1588, Galileo applied for the chair of mathematics at Bologna University, but was unsuccessful. In spite of his failure, he was invited that same year to give a lecture at the Florentine Academy, and won the patronage of Guidobaldo del Monte, nobleman and author of various works on mechanics.
Through patronage and his favorable reputation, Galileo obtained the chair of mathematics at Pisa University in 1589. While at Pisa University, Galileo proved, by dropping objects of different weights from Pisa's Leaning Tower, that the rate of fall of a heavy object is not proportional to its weight, as claimed by Aristotle. Subsequently, Galileo made his findings known in an essay entitled De Motu (On Motion). However, his anti-Aristotelian views were not well received by his colleagues and, in 1592, his contract at Pisa University was not renewed. In spite of this, his supporters were able to secure for him the chair of mathematics at Padua University where he remained until 1610.
1609 was a pivotal year for Galileo. Continuing his research in motion, he discovered the law of falling bodies: that the distance fallen by an object is proportional to the square of the elasped time. He also determined that the trajectory of a projectile is a parabola. Both discoveries contradicted Aristotelian physics.
In the spring of 1609, he heard that in the Netherlands an instrument had been invented which made distant objects appear near. Gathering all of the information he could about the 'Dutch perspective glass', he was able, through trial and error, to make his own three-powered instrument. In August, he presented an eight-powered spyglass to the Senate of the Venetian Republic. By the autumn, Galileo himself was using a twenty-powered instrument to study the heavens, and in December drew sketches of the Moon's phases.
In January, 1610, Galileo made a discovery which made him question even more the then accepted geocentric theory of the universe (that Man and the world were God's unique creation and that the Sun, Moon and stars revolved around the the Earth). While studying Jupiter, Gallileo was startled to see four smaller 'stars' (the moons now known as Io, Europa, Callisto and Ganymede) orbiting the planet. He described his amazing discoveries in a booklet entitled Sidereus Nuncius (The Starry Messenger), dedicated to Cosimo II de Medici, Grand Duke of Tuscany, and named the moons the 'Medicean Stars'. As a reward, Cosimo II made Galileo his court mathematician and philosopher.
Through further astronomical observations, Galileo discovered that the planet Venus goes through phases similar to those of the Moon. For Galileo, this was additional proof that the Sun-centred model of the universe postulated by Copernicus was correct. In 1613, the division between Galileo's theories and the accepted dogma of the scientific and religious communities became the focus of vindictive contention when his enemies circulated an altered copy of a letter Galileo had written to one of his students on the question of reconciling Copernican theories with Biblical passages.
The doctored copy of Galileo's letter was sent to the Inquisition, and Galileo was forced to go to Rome to present the true version of his letter in defence of his reputation and the Copernican model of the universe. He presented his case publically by publishing an expanded version of his letter. But the eventual outcome of the debate was the Church's official declaration, in 1615, that Copernican theories were heretical, and Galileo was warned not to 'hold or defend' them.
In 1623, Galileo published a treatise entitled Il Saggiatore (The Assayer) in which he argued that the universe around us could only be understood through mathematical principles. In his words:
Around the same time that Il Saggiatore was being printed, a supporter of Galileo, Mafeo Cardinal Barberini, became Pope Urban VIII. Following a number of interviews with the new Pope in 1624, Galileo was given permission to write about the theories of the universe, on condition that Copernican theory was treated as hypothetical only.
In 1630, Galileo finished his Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano (Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican) which was finally published in Florence in 1632. However, it was apparent to all those who read the work that Galileo, while paying lip-service to the geocentric universe, was in fact arguing the case for Copernicus' sun-centred model.
As a result, Galileo was ordered to appear before the Inquisition in Rome in 1633. After being put on trial, he was judged to be vehemently heretical and was sentenced to spend the rest of his life under house-arrest, after having first being made to renounce publically his heliocentric beliefs. Nevertheless, with the help of a young student, Vincenzo Viviani, Galileo continued his work on motion, as well as on the strength of materials. He died on January 8th, 1642, at his villa in Arcetri, near Florence.
In spite of his unsuccessful confrontation with the Church of his day, Galileo Galilei is considered to be one of the great men of science and astronomy. He is remembered today not only for discoveries about the laws of motion and Jovian moons, but especially for the experiments and rational observations that he used to validate theories, and to discard unfounded beliefs, about the universe which surrounds us. Albert Einstein called Galileo the father of modern science.