Galileo Galilei
By Isabella Romeo


Galileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and astronomical observations, and support for Copernicanism. Galileo was born in Pisa, Italy on February 15th, 1564. Galileo was the son of Vincenzio Galilei and Giulia degli Ammannati. He was named after his ancestor, Galileo Buonaiuti, who was a doctor of medicine. Galileo was the first of six children, although some historians believe he was the first of seven ( The Galilei family belonged to the nobility, but was not extravagantly rich. Their house was always filled with music. Galileo’s father, Vincenzio, was a music teacher. His father even taught him to play the lute. In 1572, Galileo and his family moved to Florence.

In his new home in Florence, Galileo was taught by a private tutor. Then, when he was nearly thirteen years old, Galileo was sent away to study at the monastery school of Vallombrosa. He decided that he wanted to become a monk, but his father was unhappy with that idea. Galileo was pulled from the monastery school and sent to another school. Galileo was very clever, and not just at his school lessons. He was very good at finding out how things around him worked. He seemed to look at the world in his own particular way. Galileo returned to Pisa in 1581 and started to study at the University of Pisa, where his father hoped he would go into the field of medicine. Galileo wanted to make his father happy, so he learned all the old theories about the human body and diseases. Many of these ideas were not based on science, though. Galileo started to questions what everyone else took for granted.

While at the University of Pisa, Galileo began his study of the pendulum. According to legend, he watched a suspended lamp swing back and forth in the cathedral of Pisa (“The Galileo Project”). In 1602, Galileo made his most notable discovery about the pendulum. He discovered that the period (the time in which a pendulum swings back and forth) does not depend on the arc of the swing (the isochronisms) ( This discovery would lead to Galileo’s further study of the time intervals and the development of his idea for a pendulum clock.


While at the University of Pisa, Galileo learned of the Ancient Greek scientist, Aristotle. He was very interested in Aristotle’s theory of physics, and questioned the Aristotelian approach to physics. Aristotelians believed that heavier objects fall faster through a medium than lighter ones (The Galileo Project). Galileo eventually disproved this idea by declaring that all objects, regardless of their density, fall at the same rate in a vacuum. To prove his theory correct, Galileo performed various experiments in which he dropped objects from a certain height. One of his most famous experiments was when he conducted an experiment on top of the Leaning Tower of Pisa. He dropped two iron balls from the top of the tower; one of heavier mass and the other was of lighter mass. Galileo proved that objects of unequal mass fall at the same rate. This contradicted Aristotle's theory that an object of heavier mass will fall faster than a lighter object. Some onlookers of this experiment thought that Galileo was involved in witchcraft. After a few more experiments, scholars and professors started to believe Galileo. Galileo documented his discoveries in his book, De Motu, which means “on motion” (
Leaning Tower of Pisa

Galileo was not really interested in medicine, he enjoyed mathematics. He became inspired by Ostilio Ricci, chief mathematicians to the Grand Duke of Tuscany. In the summer of 1583, Ricci visited the Galileo household in Florence to try to persuade Vincenzio Galilei that Galileo had a special gift. Vincenzio Galilei insisted that Galileo should stick to medical studies, though. Galileo left the university in 1585 without completing his degree. Galileo became a teacher to help support his family. In 1589, he was offered a job at the University of Pisa as Professor of Mathematics. The job was not well paid, though. In 1591, Galileo’s father died. This was a time for Galileo to think about the future. As the eldest son, he was now responsible for the Galilei family (Galileo: The Genius who Faced the Inquisition, p. 25).

Luckily, in 1592, Galileo found a new job that was well paid. He was appointed Professor of Mathematics at the University of Padua. While teaching there, he frequently visited a place called the Arsenal, where Venetian ships were docked and loaded. Galileo had always been interested in mechanical devices. Naturally, during his visits to the Arsenal, he became fascinated by nautical technologies, such as the sector and shipbuilding. In 1593, he was presented with the problem involving the placement of oars in galleys. He treated the oar as a lever and correctly made the water the fulcrum. A year later, he patented a model for a pump. His pump was a device that raised water by using only one horse. Galileo was extremely interested in the movement of the Earth, oceans, tides, the planets, stars, and the sun. He began to think about the ideas of the Polish astronomer Nicolaus Copernicus. Copernicus claimed that the Earth was not the center of the universe but rather, it traveled around the sun.

To enjoy himself, Galileo often traveled from Padua to Venice, the chief city of the Republic of Venice. It was there that Galileo met Marina Gamba, an attractive woman fourteen years younger than he was. By 1600, Marina was expecting a baby, and she moved from Venice to Padua to be near Galileo. On August 13, she gave birth to a girl that they named Virginia. A second daughter, Livia, was born in 1601. In 1606, Galileo and Marina Gamba had a son named Vincenzio. Galileo and Marina Gamba were not of the same social class so they never married. When Galileo left Padua in 1610 to take a position at the Court of the Medici family, Marina Gamba married another man named Giovanni Bartoluzzi. Galileo’s two daughters, Virginia and Livia became nuns and entered the convent of San Matteo at Arcetri.

Galileo's First Daughter

In 1610, Galileo was appointed Chief of Mathematicians and Philosopher to the new Grand Duke of Tuscany. He was also made Chief Mathematician to the University of Pisa. It was an honorary role that did not involve teaching. Galileo invented many mechanical devices other than the pump, such as the hydrostatic balance. But perhaps his most famous invention was the telescope. Galileo made his first telescope in 1609, modeled after telescopes produced in other parts of Europe that could magnify objects three times. He created a telescope later that same year that could magnify objects twenty times. With this telescope, he was able to look at the moon, discover the four satellites of Jupiter, observe a supernova, verify the phases of Venus, and discover sunspots. His discoveries proved the Copernican system which states that the earth and other planets revolve around the sun ( Galileo was proud to be a member of the Lyncean Academy, the world’s first international scientific society.
Galileo's Telescope

Galileo's belief in the Copernican system eventually got him into trouble with the Catholic Church. The Inquisition was a permanent institution in the Catholic Church charged with the eradication of heresies. A committee of consultants declared to the Inquisition that the Copernican proposition that the Sun is the center of the universe was a heresy. Because Galileo supported the Copernican system, he was warned by Cardinal Bellarmine, under order of Pope Paul V, that he should not discuss or defend Copernican theories. In 1624, Galileo was assured by Pope Urban VIII that he could write about Copernican theory as long as he treated it as a mathematical proposition (Galileo: The Genius who Faced the Inquisition, p. 38)

The Inquisition

Galileo’s health became increasingly poor since he had moved back to Florence. In 1619, Marina Gamba died in Venice. Galileo then managed to get full rights under the law to his son. The following year, Galileo’s mother, Guilia, died. Galileo continued to work, trying to steer clear of trouble. In 1618, three comets were sighted in the night sky, and they caused much discussion. Galileo criticized the Jesuit astronomer Orazio Grassi for his opinions on a comet’s orbit.

In 1624, Galileo began to write a new book. It was called the Dialogue of the Two Principal Systems of the World. Its aim was to contrast the ideas of Aristotle and Ptolemy with those of Copernicus. Galileo had been ordered not to put Copernicus’ ideas down as facts, so instead, his book took the form of a discussion between two people. Galileo had thought that he was on safe ground. He even discussed his project with leading church figures. The book was published in 1632 and became a success. The argument in the book was won by the supporter of Copernicus. To people in the church, this was a step too far. Galileo was called to Rome in 1633 to face the Inquisition again. On June 22, dressed in a white robe, Galileo knelt before the judges and promised to give up his beliefs of Copernicus (Galileo: The Genius who Faced the Inquisition, p. 45). Galileo was found guilty of heresy in his book, and was sent to his home near Florence where he was to be under house arrest for the remainder of his life.

Galileo Galilei was kept under house arrest until he died on January 8, 1642, at the age of 77. During the years before his death, he had taken on a new project. He created a new book discussing the mathematics of structures. It was called Discourses and Mathematical Demonstrations Concerning Two New Sciences and Pertaining to Mechanics and Local Motions. He was forbidden to publish his book in Italy, but he secretly arranged for his book to be published at Leiden in Holland, a Protestant country beyond the reach of the Inquisition. Before his death, Galileo had become completely blind. His eyes had been permanently damaged by years of observing the sun. The years after Galileo’s death saw many of his theories proved. Galileo lived in an exciting age in which modern science, mathematics, medicine, and astronomy were being born. He was someone who tried to separate science from tradition and superstition. Galileo did not give up, though, and his important work and the truth came out in the end.

Summary of Mathematical and Scientific Accomplishments

Over four and a half centuries ago, an Italian physicist and astronomer named Galileo set the Scientific Revolution in motion, bringing modern science out of ancient natural philosophy. Galileo Galilei’s had many mathematical and scientific accomplishments. Although he originally intended to study medicine when he entered university, he changed his course of study to philosophy and mathematics. Galileo discovered the law of parabolic fall. This led to studies on the motions of projectiles. Another result of his studies was that he realized Aristotle’s formulation of the principle of inertia was incorrect. His study of the swing of the pendulum led to his idea for a pendulum clock. Galileo also made his first telescope modeled after telescopes produced in other parts of Europe. Later, he created a telescope that could magnify objects twenty times. He used his telescope to make discoveries in outer space that supported the Copernican system.

Some of his astronomical achievements include discovering a
full set of phases of the planet Venus. This helped prove the heliocentric concept of the solar system first developed by Copernicus. While observing Saturn, he identified the rings as planets in their own right. Thinking it was a triple-planetary system, he noticed that the rings would appear and disappear depending on the time in which he was observing them. Galileo also was one of the first people to observe sunspots. Using his telescopes, he was able to identify that the Moon had mountains and craters, dispelling the belief that it was a perfect sphere. Galileo also found that the Milky Way was actually close-packed stars rather than some sort of nebula. He was able to locate a number of stars previously unseen with the naked eye. According to his notes, he observed the planet Neptune in 1612, but did not recognize it as a planet. He thought it was simply another dim star.

Explanation of the Law of Parabolic Fall

The Law of Parabolic Fall states that the distance traveled by a falling object is directly proportional to the square of the time it takes to fall.

Example: A stone falling for twice as long as another stone will travel four times the distance.
Example: A ball falling for three seconds will travel nine times the distance traveled by a ball falling for one second.

The formula for the law of fall is:


In the time period before Galileo, scientists thought that force causes speed. Galileo showed that force causes acceleration. Galileo reached the conclusion that objects fall on the surface of the Earth at a constant acceleration, and that the force of gravity which causes all objects to move downward is a constant force. In other words, constant force does not lead to constant speed but to constant acceleration. Galileo claimed that an object in motion will continue its motion, so long as no factor disturbs that motion. This is called the principle of inertia (

Contrary to the claims of the natural philosophers of his period, Galileo claimed that an object will not attain a certain speed which will remain constant but will continue accelerating until it comes into contact with the ground. This claim is of course true so long as one ignores air resistance, which can be very significant for certain objects and at high speeds.

An example of the Law of Parabolic Fall is:

A falling ball, in the 2nd second of its fall, will travel a distance three times as great as the distance it traveled during the 1st second; in the 3rd second it will travel a distance five times that of the 1st second; in the 4th second it will travel a distance seven times that of the 1st second.


If during the 1st second, the ball traveled a distance of 10 cm, it will travel a distance of 30 cm during the 2nd second, of 50 cm during the 3rd second, 70 cm during the 4th second, and so on.



Most Interesting Fact

The most interesting fact that was found was about Galileo and the Inquisition. One of the most important events in Galileo’s life was when he battled with the Catholic Church. Galileo believed in Copernicanism, the theory that the Earth and other planets revolved around the sun. Galileo was found guilty of heresy in court and sentenced to be under house arrest. He also had to denounce the theory of Copernicanism in front of the Catholic Church, and was forbidden to publish his works. What was found interesting was that in 1990, a number of influential church figures started to make speeches about Galileo and his persecution. On February 15, Cardinal Ratzinger questioned the politics of the Church during Galileo’s time, but did not make a formal apology. Pope John Paul II expressed regret for the actions taken by the Church during Galileo’s time. He did this on October 21, 1992. Lastly, Pope Benedict CVI praised Galileo for his contributions to the fields of astronomy and physics.

This fact was found fascinating because the Catholic Church never made a formal apology. The Church probably does not want to admit it was in the wrong in Galileo's time, because that would be a sign of weakness against science in today’s time.

The Inquisition