Biography of Galileo Galilei, Renaissance Philosopher and Inventor

An engraving of Galileo Galilei

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Galileo Galilei (February 15, 1564–January 8, 1642) was a famous inventor, mathematician, astronomer, and philosopher whose inventive mind and stubborn nature ran him into trouble with the Inquisition.

Fast Facts: Galileo Galilei

  • Known For: Italian Renaissance philosopher, inventor, and polymath who faced the wrath of the Inquisition for his astronomical studies
  • Born: February 15, 1564 in Pisa, Italy
  • Parents: Vincenzo and Giulia Ammannati Galilei (m. July 5, 1562)
  • Died: January 8, 1642 in Arcetri, Italy
  • Education: Privately tutored; Jesuit monastery, University of Pisa
  • Published Works: "The Starry Messenger"
  • Spouse: None; Marina Gamba, mistress (1600–1610)
  • Children: Virginia (1600), Livia Antonia (1601), Vincenzo (1606)

Early Life

Galileo was born in Pisa, Italy on February 15, 1564, the oldest of seven children of Giulia Ammannati and Vincenzo Galilei. His father (c. 1525–1591) was a gifted lute musician and wool trader and wanted his son to study medicine because there was more money in that field. Vincenzo was attached to the court and was often traveling. The family was originally named Bonaiuti, but they had an illustrious ancestor named Galileo Bonaiuti (1370–1450) who was a physician and public officer in Pisa. One branch of the family broke off and began calling itself Galilei ("of Galileo"), and so Galileo Galilei was doubly named after him.

As a child, Galileo made mechanical models of ships and watermills, learned to play the lute to a professional standard, and showed an aptitude for painting and drawing. Originally tutored by a man named Jacopo Borghini, Galileo was sent to the Camaldlese monastery at Vallambroso to study grammar, logic, and rhetoric. He found the contemplative life to his liking, and after four years he joined the community as a novice. This was not exactly what his father had in mind, so Galileo was hastily withdrawn from the monastery. In 1581 at the age of 17, he entered the University of Pisa to study medicine, as his father wished.

The University of Pisa

At age 20, Galileo noticed a lamp swinging overhead while he was in a cathedral. Curious to find out how long it took the lamp to swing back and forth, he used his pulse to time large and small swings. Galileo discovered something that no one else had ever realized: the period of each swing was exactly the same. The law of the pendulum, which would eventually be used to regulate clocks, made Galileo Galilei instantly famous.

Except for mathematics, Galileo was soon bored with the university and the study of medicine. Uninvited, he attended the lecture of court mathematician Ostilio Ricci—who had been assigned by the Duke of Tuscany to teach the court attendants in math, and Galileo was not one of those. Galileo followed up the lecture by reading Euclid on his own; he sent a set of questions to Ricci, the content of which greatly impressed the scholar.

Galileo's family considered his mathematical studies subsidiary to medicine, but when Vincenzo was informed that their son was in danger of flunking out, he worked out a compromise so that Galileo could be tutored in mathematics by Ricci full-time. Galileo's father was hardly overjoyed about this turn of events because a mathematician's earning power was roughly around that of a musician, but it seemed that this might yet allow Galileo to successfully complete his college education. The compromise didn't work out, for Galileo soon left the University of Pisa without a degree.

Becoming a Mathematician

After he flunked out, Galileo started tutoring students in mathematics to earn a living. He did some experimenting with floating objects, developing a balance that could tell him that a piece of gold, for example, was 19.3 times heavier than the same volume of water. He also started campaigning for his life's ambition: a position on the mathematics faculty at a major university. Although Galileo was clearly brilliant, he had offended many people in the field and they would choose other candidates for vacancies.

Ironically, it was a lecture on literature that would turn Galileo's fortunes. The Academy of Florence had been arguing over a 100-year-old controversy: what were the location, shape, and dimensions of Dante's Inferno? Galileo wanted to seriously answer the question from the point of view of a scientist. Extrapolating from Dante's line that the giant Nimrod's "face was about as long/and just as wide as St. Peter's cone in Rome," Galileo deduced that Lucifer himself was 2,000 arm-lengths long. The audience was impressed, and within the year, Galileo had received a three-year appointment to the University of Pisa, the same university that never granted him a degree.

The Leaning Tower of Pisa

When Galileo arrived at the University, some debate had started up on one of Aristotle's "laws" of nature: that heavier objects fell faster than lighter objects. Aristotle's word had been accepted as gospel truth, and there had been few attempts to actually test Aristotle's conclusions by actually conducting an experiment.

According to legend, Galileo decided to try. He needed to be able to drop the objects from a great height. The perfect building was right at hand—the Tower of Pisa, which was 54 meters (177 feet) tall. Galileo climbed to the top of the building carrying a variety of balls of varying sizes and weights and dumped them off the top. They all landed at the base of the building at the same time (legend says that the demonstration was witnessed by a huge crowd of students and professors). Aristotle was wrong.

It might have helped the junior member of the faculty if Galileo had not continued to behave rudely toward his colleagues. "Men are like wine flasks," he once said to a group of students, "Look at…bottles with the handsome labels. When you taste them, they are full of air or perfume or rouge. These are bottles fit only to pee into!" Perhaps not surprisingly, the University of Pisa chose not to renew Galileo's contract.

The University of Padua

Galileo Galilei moved on to the University of Padua. By 1593, he was desperate and in need of additional cash. His father had died, so Galileo was now head of his family. Debts were pressing down on him, most notably the dowry for one of his sisters, which was to be paid in installments over decades. (A dowry could be thousands of crowns, and Galileo's annual salary was 180 crowns.) Debtor's prison was a real threat if Galileo returned to Florence.

What Galileo needed was to come up with some sort of device that could make him a tidy profit. A rudimentary thermometer (which, for the first time, allowed temperature variations to be measured) and an ingenious device to raise water from aquifers found no market. He found greater success in 1596 with a military compass that could be used to accurately aim cannonballs. A modified civilian version that could be used for land surveying came out in 1597 and ended up earning a fair amount of money for Galileo. It helped his profit margin that the instruments were sold for three times the cost of manufacture, he offered classes on how to use the instrument, and the actual toolmaker was paid dirt-poor wages.

Galileo needed the money to support his siblings, his mistress (21-year-old Marina Gamba), and his three children (two daughters and a boy). By 1602, Galileo's name was famous enough to help bring in students to the University, where Galileo was busily experimenting with magnets.

Building a Spyglass (Telescope)

During a vacation to Venice in 1609, Galileo Galilei heard rumors that a Dutch spectacle-maker had invented a device that made distant objects seem near at hand (at first called the spyglass and later renamed the telescope). A patent had been requested, but not yet granted. The methods were being kept secret because it was obviously of tremendous military value for Holland.

Galileo Galilei was determined to attempt to construct his own spyglass. After a frantic 24 hours of experimentation, working only on instinct and bits of rumors—he had never actually seen the Dutch spyglass—he built a three-power telescope. After some refinement, he brought a 10-power telescope to Venice and demonstrated it to a highly impressed Senate. His salary was promptly raised, and he was honored with proclamations.

Galileo's Observations of the Moon

If he had stopped here and become a man of wealth and leisure, Galileo Galilei might be a mere footnote in history. Instead, a revolution started when, one fall evening, the scientist trained his telescope on an object in the sky that all people at that time believed must be a perfect, smooth, polished heavenly body—the moon.

To his astonishment, Galileo Galilei viewed a surface that was uneven, rough, and full of cavities and prominences. Many people insisted that Galileo Galilei was wrong, including a mathematician who insisted that even if Galileo was seeing a rough surface on the Moon, that only meant that the entire moon had to be covered in invisible, transparent, smooth crystal.

Discovery of Jupiter's Satellites

Months passed, and his telescopes improved. On January 7, 1610, he turned his 30-power telescope toward Jupiter and found three small, bright stars near the planet. One was off to the west, the other two were to the east, all three in a straight line. The following evening, Galileo once again took a look at Jupiter and found that all three of the "stars" were now west of the planet, still in a straight line.

Observations over the following weeks led Galileo to the inescapable conclusion that these small "stars" were actually small satellites that were rotating around Jupiter. If there were satellites that didn't move around the Earth, wasn't it possible that the Earth was not the center of the universe? Couldn't the Copernican idea of the sun resting at the center of the solar system be correct?

Galileo Galilei published his findings in a small book titled "The Starry Messenger." A total of 550 copies were published in March 1610, to tremendous public acclaim and excitement. It was the only one of Galileo's writings in Latin; most of his work was published in Tuscan.

Seeing Saturn's Rings

There continued to be more discoveries via the new telescope: the appearance of bumps next to the planet Saturn (Galileo thought they were companion stars; the "stars" were actually the edges of Saturn's rings), spots on the Sun's surface (though others had actually seen the spots before), and seeing Venus change from a full disk to a sliver of light.

For Galileo Galilei, saying that the Earth went around the Sun changed everything since he was contradicting the teachings of the Catholic Church. While some of the church's mathematicians wrote that his observations were clearly correct, many members of the church believed that he must be wrong.

In December 1613, one of the scientist's friends told him how a powerful member of the nobility said that she could not see how his observations could be true since they would contradict the Bible. The woman quoted a passage in Joshua in which God causes the sun to stand still and lengthen the day. How could this mean anything other than that the sun went around the Earth?

Charged With Heresy

Galileo was a religious man and agreed that the Bible could never be wrong. However, he said, the interpreters of the Bible could make mistakes, and it was a mistake to assume that the Bible had to be taken literally. That was one of Galileo's major mistakes. At that time, only church priests were allowed to interpret the Bible or define God's intentions. It was absolutely unthinkable for a mere member of the public to do so.

Some of the church clergy started responding, accusing him of heresy. Some clerics went to the Inquisition, the Catholic Church court that investigated charges of heresy, and formally accused Galileo Galilei. This was a very serious matter. In 1600, a man named Giordano Bruno was convicted of being a heretic for believing that the Earth moved about the sun and that there were many planets throughout the universe where life—living creations of God—existed. Bruno was burned to death.

However, Galileo was found innocent of all charges and was cautioned not to teach the Copernican system. Sixteen years later, all that would change.

The Final Trial

The following years saw Galileo work on other projects. With his telescope he watched the movements of Jupiter's moons, recorded them as a list, and then came up with a way to use these measurements as a navigation tool. He developed a contraption that would allow a ship captain to navigate with his hands on the wheel, but the contraption looked like a horned helmet.

As another amusement, Galileo started writing about ocean tides. Instead of writing his arguments as a scientific paper, he found that it was much more interesting to have an imaginary conversation, or dialogue, between three fictional characters. One character, who would support Galileo's side of the argument, was brilliant. Another character would be open to either side of the argument. The final character, named Simplicio, was dogmatic and foolish, representing all of Galileo's enemies who ignored any evidence that Galileo was right. Soon, he wrote up a similar dialogue called "Dialogue on the Two Great Systems of the World." This book talked about the Copernican system.

Inquisition and Death

"Dialogue" was an immediate hit with the public, but not, of course, with the church. The pope suspected that he was the model for Simplicio. He ordered the book banned and also ordered the scientist to appear before the Inquisition in Rome for the crime of teaching the Copernican theory after being ordered not to do so.

Galileo Galilei was 68 years old and sick. Threatened with torture, he publicly confessed that he had been wrong to have said that the Earth moves around the Sun. Legend then has it that after his confession, Galileo quietly whispered, "and yet, it moves."

Unlike many less famous prisoners, he was allowed to live under house arrest in his house outside of Florence and near one of his daughters, a nun. Until his death in 1642, he continued to investigate other areas of science. Amazingly, he even published a book on force and motion although he had been blinded by an eye infection.

The Vatican Pardons Galileo in 1992

The Church eventually lifted the ban on Galileo's Dialogue in 1822—by that time, it was common knowledge that the Earth was not the center of the Universe. Still later, there were statements by the Vatican Council in the early 1960s and in 1979 that implied that Galileo was pardoned and that he had suffered at the hands of the church. Finally, in 1992, three years after Galileo Galilei's namesake had been launched on its way to Jupiter, the Vatican formally and publicly cleared Galileo of any wrongdoing.


  • Drake, Stillman. "Galileo at Work: His Scientific Biography." Mineola, New York: Dover Publications Inc., 2003.
  • Reston, Jr., James. "Galileo: A Life." Washington DC: BeardBooks, 2000. 
  • Van Helden, Albert. "Galileo: Italian Philosopher, Astronomer and Mathematician." Encyclopedia Britannica, February 11, 2019.
  • Wootton, David. Galileo: "Watcher of the Skies." New Haven, Connecticut: Yale University Press, 2010.
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Bellis, Mary. "Biography of Galileo Galilei, Renaissance Philosopher and Inventor." ThoughtCo, Feb. 16, 2021, Bellis, Mary. (2021, February 16). Biography of Galileo Galilei, Renaissance Philosopher and Inventor. Retrieved from Bellis, Mary. "Biography of Galileo Galilei, Renaissance Philosopher and Inventor." ThoughtCo. (accessed March 22, 2023).