Galileo Galilei

Galileo Galilei (15 February 1564 - 8 January 1642) was an Italian mathematician, astronomer, and physicist whose ideas and observations made him the initiator of the scientific revolution. He is considered to be one of the scientists who suffered the most in trying to make science as accurate as possible. The scientific discoveries Galileo brought him into direct conflict with the teachings of the Catholic church and he was condemned for heresy. He is most known for his developement of the telescope, the discovery of the moons of Jupiter, his support of heliocentrism, and his experiments with gravity.
A 1639 portrait of Galileo by Justus Sustermans


  1. Life
  2. Astronomy
  3. Physics
  4. Galileo vs. the Catholic Church
  5. Written Works
  6. References


Galileo was born in Pisa, Italy on February 15, 1564. His father, Vincenzio Galilei, taught music. Galileo was taught by a private tutor, along with monks. In his early life, Galileo seriously considered becoming a priest, but instead in 1581 he entered the University of Pisa as a medical student. However, he left just four years later when he became uninterested in the topic. Instead, Galileo was more interested in the centers of gravity, hydrostatic balance, and pendulums. Galileo privately taught mathematics in Sienna in starting in 1585 until he was appointed to be the chair of mathematics at the University of Pisa, a job which he held for about 20 years. However Galileo argued against many of original ideas of Aristotle and theories and this made many of his colleagues angry. So Galileo moved to the University of Padua, where freedom of opinion was protected. There he gave lectures about geometry and astronomy. Galileo had a mistress, Marina Gamba, along with two daughters and a son at Padua. However, he never married and when his father died in 1591 and had to then pay the dowries of his sisters and financially help his brother. After that, he went to Florence, where he stayed for the rest of his life, to take a job as a court mathematician.

Galileo was very interested in artisanship, or craftsmanship, and often went to shipyards to view construction techniques and increase his knowledge of construction. His crafts were, of course, his telescopes which he developed until the lenses were too large and distorted the image. Galileo was the first person to truly understand the telescope as a scientific tool, not just an entertaining creation. Thus, Galileo began looking at the sky and recording data, which allowed him to be able to support the idea of a heliocentric (sun-centered) universe. He also noticed that the sky was very different from the old theories; the moon had mountains like the earth, Jupiter had its own moons and there were many more stars than the naked eye could see or have imagined. The public was so impressed with his astronomical findings that poets wrote poems about him, artists painted his telescopes and numerous individuals asked him to look through the telescopes he developed. However, other scientists and the Church were less convinced and he spent the last years of his life trying to prove his work and stay away from the punishment of an offended Church.


Galileo is famous for his work with telescopes because of the ways he developed them used them. After coming across the telescope, he immediately set himself to the task and came out with a 9-power telescope, which was, in fact, three times stronger than the rivals. Galileo continued to work on his telescope and by 1610 had produced a 30-power telescope. Even though only ten of his telescopes were strong enough to show the universe in accurate detail, he was able to observe mountainous surface of the moon, new stars, and some of the satellites around Jupiter. The discovery of these satellites proved that not everything in space revolves around the Earth. Before Galileo found the moons of Jupiter, the theory was that Earth could not be a planet when it was the only one that had a moon. The discovery of the moons of Jupiter allowed the Earth to also be named a planet and therefore, possibly not the center of the universe. In the same year he noted the rings of Saturn and the phases of Venus, showing its orbit around the Sun. Galileo observed the Sun carefully and when he noticed sun spots and how they changed frequently, he was able to conclude that the Sun, like all the other planets, is imperfect and just like the Earth, rotates on an axis.

Galileo started out only teaching astronomy for the reason to teach students medical astronomy. He received the Copernican book in 1597 and became interested in the subject. This book stated that the Earth rotates and revolves around the sun just like the other planets. Galileo liked this theory because it explained the pattern of the tides. From there he became interested in magnetism. The most important discovery of Galileo involving astronomy was developing the telescope. The observations he made from the astronomical telescope led him to believe that all planets revolve around the sun. Galileo was able to produce a book of all his discoveries he had made so far in March of 1610 called The Starry Messenger. The book was written in Latin because Galileo hoped to spread his observations to foreign countries and varied groups of people. The distribution across the globe of his findings was phenomenal, considering transportation and communication was so slow at the time. The Starry Messenger was reprinted in Germany as well as translated into Chinese within 5 years of its release.


Galileo became interested in physics when in 1582 he experimented with the swing of a pendulum and how fast it swung depending on its weight; at the time, he was 18 years old. Physics before the time of Galileo was just the philosophy Aristotle and it was rarely experimented with or challenged. This philosophy was that bodies fell at a speed depending on their weight to go to the center of the universe - which was, at the time the center of the Earth. Aristotle also thought that thrown bodies were only kept in motion by force. In 1590, Galileo wrote a treatise disproving the previous philosophy. It stated that bodies of the same material will fall at the same speed, no matter what the size is. This theory led him to various tests, where he tested the law of lever and the speeds of bodies on inclined planes.

The major contribution of Galileo to physics was correctly defining uniform acceleration, constituting the laws of falling bodies, developing the theory of projectile motion, and the role of experiments. By the time he had proved his thesis, he had been thinking about the laws of physics for more than 30 years. Galileo built off of the previous mathematical laws, but he was the first to solve the laws of motion. He was able to go beyond the Greeks knowledge of geometrical space and weight and was able to understand motion, something the Greeks were never clear about. Also, he solved a problem that had vexed many scientists around him, Galileo developed one of the first thermometers. Galileo wrote his Dialogue Concerning the Two Chief World Systems - Ptolemaic & Copernican in order to answer the questions that scholars had not only about astronomy but the laws of physics as well. Physics stated that all matter falls to earth because the earth is the center of the universe. Galileo had to help them understand that just because things fell to earth, it did not mean that the earth was necessarily the center of the universe.

Galileo used mathematics in both physics and astronomy instead of traditional methods. He not only created the mathematical theory of projectile motion, he also contributed a great deal in the relation of mathematics to physics.In 1606, Galileo wrote a book on the invention of his proportional compass. However, his main interest still centered on mechanics and the laws of motion.

Galileo vs. the Catholic Church

Galileo faces the inquisitors
Galileo faces the inquisitors
Throughout his life, the scientific work of Galileo brought him into conflict with the Catholic Church. Not only did he anger the Aristotelians in disproving the physics Aristotle, but he upset the Church by publishing his findings through his telescope which disproved the theory of the Church. Because the Church had power over knowledge at the time, The Church felt threatened and decided that his findings were not only against their model of the universe, but also against the Bible, and that made Galielo a heretic. It was because of this that Galileo had so many problems with the Church during his life. In 1614, Father Thomas Caccini angrily rang out against mathematics and the idea of an earth-centered world in a sermon directed in the direction of Galileo. Word spread fast and as people began finding that Galileo was contradicting the Bible, chaos and confusion began. In 1615, Niccolo Lorino who had previously argued with Galileo, sent a written complaint about him to the inquisition. After having his views declared absurd by a committee of consultants, Galileo was called to the residence of Cardinal Bellarmine, who warned Galileo not to defend the Copernican theory because it had been announced to be "in error."

When his friend and follower Maffeo Cardinal Barberini (Urban VIII) was elected after Pope Paul V and Gregory XV died in 1624, Galileo tempted his friend to allow him to once again publicly support his theory. The Pope, Urban VIII, had to be careful in his views and the theory of Galileo was supposedly against the Bible. Therefore, the Pope would not withdraw the 1616 prohibition of Galileo's fight for the Copernican theory. He instead suggested that Galileo write literature to guide his new theory. Galileo followed this suggestion, but soon his luck would change again.

Galileo started one of his soon to be literature masterpieces when he was 60 years old. Even though frequent illnesses slowed him down, he published his book, Dialogue Concerning the Two Chief World Systems, after 5 years of work. In 1630 after his Dialogue Concerning the Two Chief World Systems was published, requests to ban the book started arriving in Rome. The supporters of Galileo went to his friend, Pope Urban VIII. Little did they know, Pope Urban thought the Aristotelian character in his book was supposed to be a representation of the Pope himself and was furious with Galileo. Galileo was summoned to Rome by the Inquisition after the paper he had signed that said he was not allowed to "hold, teach, or defend it (the Copernican theory) in any way whatsoever, verbally or in writing" was found. Galileo argued the fact that the part about him not being able to write about the Copernican theory was not in the original copy. However, Galileo was in real trouble now because no one was brave enough to stand by him.

This supposed proof was enough for the inquisitors, and the Inquisition found him guilty of ignoring his prohibition. In the words of the inquisitors: "That the Sun is the centre of the universe and doth not move from his place is a proposition absurd and false in philosophy, and formerly heretical; being expressly contrary to Holy Writ: That the Earth is not the centre of the universe nor immoveable, but that it moves, even with a diurnal motion, is likewise a proposition absurd and false in philosophy, and considered in theology ad minus erroneous in faith." He was interrogated for 18 days, but only being threatened with torture did he recant his belief of the Copernican Theory. Galileo, now an old man, spent the remaining years of his life under closely monitored house arrest, until his death in 1642.

Written Works

Dialogue Concerning the Two Chief World Systems - Ptolemaic and Copernican (1632)
Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638)
On Motion (1589)
Treatise on the Sphere: or Cosmographia (1597)
Dialogue of Cecco di Ronchitti da Bruzene with regard to the New Star (1605)
Considerations of Alimberto Mauri on Some Places in the Discourse of Lodovico Delle Colombe about the Star (1606)
History and Demonstrations about Sunspots and their Properties (1613)
Letter to the Grand Duchess Christina (1615)
The Assayer (1623)
Mechanics (1634)
Sidereus nuncius (1610)


1. Bixby, William. "Galileo's Fight to Reconcile Sceince and Faith." The Scientific Revolution. Ed. Mitchell Young. Farmington Hills, MI: Greenhaven P, 2006. 73-82.

2. Bolles, Edmund Blair, ed. Galileo's Commandment: an Anthology of Great Science Writings. New York: W.H. Freeman, 1997. 97-104,167-176, 415-419

3. Fermi, Laura, and Gilberto Bernardini. "Galileo Makes Remarkable Discoveries with His Telescope." The Scientific Revolution.Farmington Hills, MI: Greenhaven P, 2006. 65-72.

4. "Galileo." Encyclopedia Britannica. 15th ed. 2002.

5. Galileo Timeline. 1995. The Galileo Project. <>

6. Stillman, Drake. "Galileo." Encyclopedia Americana. Deluxe Library Edition ed. 1989.

7. The Sector. 1995. The Galileo Project. 12 Sept. 2007. <>

8. Wudka, Jose. Galileo and the Inquisition. 12 Sept. 2007. <>