His Life

Galileo Galilei is born in Pisa on February 15. He is the first child of Vincenzo Galilei of Florence, a music teacher, and Giulia degli Ammannati of Pescia.

Galileo studies Greek, Latin, and logic at the Benedictine monastery of Santa Maria di Vallombrosa and considers becoming a monk until his father expresses displeasure at the idea.

Galileo begins his studies in September at the University of Pisa, where he studies medicine and mathematics. Though he is a diligent medical student, mostly to satisfy his father's wish that he become a doctor, Galileo prefers mathematics.

Galileo, now 21, leaves the University of Pisa without a degree after four years of study. He spends the next four years giving private lessons in mathematics in Florence and Siena.

Galileo takes a teaching position at the University of Pisa. He refuses to wear the standard academic regalia, a black robe, dismissing the sartorial tradition as pretentious and cumbersome. University officials repeatedly impose fines on him for this transgression.

Galileo's father dies at 70, and Galileo becomes the primary financial provider for his family, which includes his mother, his married sister Virginia (whose dowry requires regular payments), his 16-year-old brother Michelangelo, and his unmarried sister Livia. Three other siblings died during childhood.

In December, Galileo becomes chair of the mathematics department at the University of Padua in the Republic of Venice. He gives lectures on geometry and astronomy in addition to private lessons on Euclid, cosmography, and other subjects.

Galileo develops his theory of the tides, asserting that they ebb and flow in relation to the Earth's diurnal and annual movements. His theory, though elegantly conceived, is incorrect (see His Big Mistake).

Galileo invents a geometric and military compass, which has a commercial use as a pocket calculator. He hires a full-time instrument maker to mass-produce the compass, publishes a companion manual to the instrument, and gives lessons on its use.

Galileo, 36, begins a relationship with 22-year-old Marina Gamba of Venice.

In August, Galileo and Marina Gamba's first daughter, Virginia, is born out of wedlock.

In August, Livia, Galileo and Marina Gamba's second child, is born almost exactly one year after her sister.

Galileo conducts experiments with a pendulum on the measurement of time increments (see His Experiments: Pendulum). He explains his findings in a letter to Santorio Santorio, a doctor friend in Venice, who then successfully uses a pendulum to measure his patients' pulses.

Marina Gamba gives birth to Vincenzio, Galileo's only son, in August.

In May, Galileo learns of the invention of telescopic lenses in the Netherlands, which can be used to see objects at a distance. Within a month, he creates his own three-powered telescope (see His Telescope).

Throughout the summer and fall Galileo continues to work on his telescope and begins to observe the night sky through it. He presents an eight-powered telescope to the Senate in Venice and is awarded tenure at the University of Padua.

From November 30 to December 19, Galileo observes the moon through his telescope.

On January 7, Galileo sees three bright stars near Jupiter; six days later he spies a fourth. Within a week he determines these are Jupiter's satellites.

In Rome, Jesuit mathematicians at the Collegio Romano certify Galileo's celestial discoveries, which include Saturn, sunspots, and the satellites of Jupiter, among other things.

Galileo publishes Bodies That Stay Atop Water or Move Within It in Florence.

Galileo publishes his Sunspot Letters. Virginia and Livia Galilei, Galileo's daughters, enter the Convent of San Matteo in Arcetri. They both take the habit within a year.

Tommaso Caccini, a Dominican friar, delivers a sermon in Florence in which he denounces as heretics Galileo and others who subscribe to the Copernican view of the heavens (that the Earth revolves around the sun and not the other way around). Shortly thereafter, one of Caccini's superiors sends Galileo a written apology. Later this year, Caccini is deposed by the Roman Inquisition.

In January, Galileo writes about his theory of the tides, arguing that it proves the movement of the Earth and the central position of the sun. He addresses his writing to Cardinal Alessandro Orsini.

Pope Paul V orders Robert Cardinal Bellarmine, the so-called "hammer of the heretics," to warn Galileo against defending Copernican theory.

Marina Gamba dies in February. She and Galileo were never married and never lived under the same roof.

Galileo's mother dies in September at the age of 82.

In February, Roman censors give permission for Galileo's book The Assayer to be printed. The book serves as a retort to Orazio Grassi, a teacher of mathematics, on the subject of comets, including their weight and composition, and meditates on the primacy of experimental science over the opinions of the popular majority.

Galileo travels to Rome, where he has audiences with Pope Urban VIII and several cardinals. The Pope grants Galileo permission to address Copernican theory in his writing on the condition that he only lend it the weight of a hypothesis.

Galileo finishes his work in April for Dialogue Concerning Two Chief World Systems, which includes his treatise on the tides. It is published two years later.

Pope Urban VIII suspends distribution of Galileo's Dialogue and appoints a commission to examine the book. The case is referred to the Inquisition, and Galileo is summoned from Florence to Rome.

In April, the Inquisition formally interrogates Galileo, who has been detained in the building of the Inquisition for several weeks. Galileo agrees to plead guilty in order to receive a lenient sentence, and on April 30 he confesses that he advocated Copernican theory too vigorously in the Dialogue. He agrees to modify his opinions in his next work.

In June, the Pope orders Galileo imprisoned indefinitely under house arrest. Galileo makes his way back to his villa in Arcetri, near Florence, where he spends the remainder of his life under house arrest.

Galileo begins work on his Discourse Concerning Two New Sciences.

Galileo's daughter Virginia, known as Sister Maria Celeste, dies in the Convent of San Matteo in April.

Galileo, in failing health for several years, loses his eyesight. He petitions the Inquisition to be freed for medical reasons. His request is denied but in March the Inquisition gives Galileo permission to attend religious services on holidays.

Discourse Concerning Two New Sciences is published in Holland.

Galileo conceives of a pendulum-controlled clock.

Galileo dies in Arcetri on January 8. Isaac Newton is born in England on December 25.