Galileo Galilei: Life, Telescope Discoveries and the Scientific Method

Who Was Galileo Galilei: A Biography

Galileo Galilei was born in Pisa on February 15, 1564, the son of Vincenzo Galilei, a court musician and composer, and Giulia Ammannati. In 1580 he enrolled in medicine at the University of Pisa, but left the course after a short time to pursue the study of mathematics instead.

An academic career followed quickly: Galileo held the chair of mathematics at the University of Pisa from 1589 to 1592, then moved to the University of Padua, where he taught starting that same year.

The Telescope and the Discoveries That Changed Astronomy

Having learned of a telescope that had just been invented, Galileo built an improved version of it in 1609 and used it for celestial observations regarded as extraordinary. With that instrument he found that Jupiter is accompanied by four moons, known today as the Galilean moons. Watching how sunlight falls on the Moon, he concluded that its surface is mountainous and uneven. In 1610 he also observed that Venus shows phases, a finding that points to the planet orbiting the Sun.

Galileo rushed these discoveries into print in March 1610, in the Sidereus Nuncius (the “Starry Messenger” or “Sidereal Messenger”).

Galileo’s Scientific Method

According to the Stanford Encyclopedia of Philosophy, Galileo had little patience for a style of natural philosophy that relied on books alone, without direct observation. He designed and built experimental devices to test his ideas, among them inclined planes and pendulums.

Real-world example: to study motion, Galileo built inclined planes and pendulums, and used those devices to put his ideas to the test.

Galileo described the universe as a book written in the language of mathematics, legible only to those who learn its characters. Historians of science commonly treat “Galilean science” as close to a stand-in for the modern scientific method itself, a reading the Stanford Encyclopedia of Philosophy records as well established among scholars in the field.

Support for Copernicanism and the Trial

Galileo’s support for the Copernican system built up over several stages.

Year	Milestone
1597	Confides privately to Kepler, in a letter, that he believes in the heliocentric system
1604	Begins defending it in public, drawing on his own astronomical observations
1610	Observes the phases of Venus, a clue favoring the planet’s orbit around the Sun
1612	The first public opposition to his views appears
1616	Cardinal Bellarmine warns him: he may present Copernicanism only as a hypothesis
1632	Publishes the Dialogue Concerning the Two Chief World Systems in Florence
1633	The Holy Office tries him, convicts him and forces him to recant

The Dialogue Concerning the Two Chief World Systems favors Copernican theory without stating it explicitly, according to the Stanford Encyclopedia of Philosophy. In June 1633 Galileo was convicted of “vehement suspicion of heresy”: the prison term set by the sentence became permanent house arrest right away. He was taken to the church of Santa Maria sopra Minerva, in Rome, and ordered to kneel while his sentence was read aloud.

Final Years and Legacy

After the trial, Galileo spent his remaining years in Arcetri, under house arrest. There he completed the Discourses and Mathematical Demonstrations Relating to Two New Sciences, smuggled out of Italy and published in the Netherlands by the Elzevir family in 1638. He died in early 1642 (January 8, according to Museo Galileo); because of his conviction, the location of his grave stayed largely unmarked until he was reinterred in 1737.

The Stanford Encyclopedia of Philosophy calls him one of the central figures — if not the central figure — of the seventeenth-century scientific revolution in any history of science: a judgment about his place in that history. Readers who want to follow the thread forward from Galileo can see how Kepler’s laws describe planetary motion around the Sun, how Newton’s laws of motion generalize the physics of motion, what we mean today by the scientific method, how the solar system Galileo observed through his telescope is put together, and the cultural setting of the Renaissance in which he came of age.