Galilei Galileo

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Date: 2006
Publisher: Gale
Document Type: Biography
Length: 2,147 words
Content Level: (Level 5)
Lexile Measure: 1360L

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About this Person
Born: February 15, 1564 in Pisa, Italy
Died: January 08, 1642 in Arcetri, Italy
Nationality: Italian
Occupation: Astronomer
Other Names: Galilei, Galileo
Updated:Jan. 1, 2006
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Galileo Galilei's astronomical observations with a telescope provided proof that Earth is not the center of the universe.

Galileo Galilei is credited with establishing the modern experimental method. Before Galileo, knowledge of the physical world advanced by scientists and thinkers was for the most part a matter of hypothesis (untested ideas) and guesses. In contrast, Galileo introduced the practice of proving or disproving a scientific theory by conducting tests and observing the results. His desire to increase the precision of his observations led him to develop a number of inventions and make numerous discoveries, particularly in the fields of physics and astronomy.

Discovers principles of the pendulum

The son of Vincenzo Galilei (c. 1520-1591), an eminent composer and music theorist, Galileo (who is commonly known by his first name) was born in Pisa, Italy, on February 15, 1564. He received his early education at a monastery near Florence, and in 1581 he entered the University of Pisa to study medicine. While a student he observed a hanging lamp that was swinging back and forth and noted that the amount of time it took the lamp to complete a single swing remained constant, even as the arc, or curve, of the swing steadily decreased. He later experimented with other suspended objects and discovered that they behaved in the same way, suggesting to him the principle of the pendulum--a suspended body that once set swinging can be used to regulate the movements of a machine, such as a clock. From this discovery he was able to invent an instrument that measured time, which doctors found to be useful for measuring a patient's pulse rate. Dutch mathematician and astronomer Christiaan Huygens (1629-1695) later adapted the principle of a swinging pendulum to build a reliable pendulum clock in 1656.

Establishes scientific reputation

While at the University of Pisa, Galileo listened in on a geometry lesson and was so fascinated with it that he decided to abandon his medical studies and devote himself to mathematics. However, he was unable to complete a degree at the university due to lack of funds. He returned to Florence in 1585, having studied the works of Greek mathematician Euclid (who live in the third century B.C.) and Greek mathematician Archimedes (c. 287-212 B.C.). He expanded on Archimedes's work in hydrostatics (a branch of physics dealing with the amount of pressure in fluids) by creating a hydrostatic balance, a device designed to measure the density, or mass, of objects by weighing them in water. The following year, he published an essay describing his new invention, which earned him a reputation throughout Italy as a talented scientist.

Challenges ideas of Aristotle

In 1592, Galileo was appointed professor of mathematics at Padua University in Pisa, where he began conducting experiments with falling objects. The ancient Greek philosopher Aristotle (384-322 B.C., whose ideas were still considered to be scientific law, had stated that a heavier object should fall faster than a lighter one. It is said that Galileo tested Aristotle's assertion by climbing the leaning tower of Pisa, dropping objects of various weights, and proving once and for all that all objects, no matter what their weight, fall at the same rate. There is no real evidence to prove that Galileo did perform such a demonstration, though a similar experiment had been conducted by Flemish mathematician Simon Stevin (1548-1620) a few years earlier. Whether Galileo conducted this famous experiment or not, his end result was the same--he had successfully disproved the long-standing physics of Aristotle.

Develops early thermometer

In 1593, Galileo invented one of the first measuring devices to be used in science: the thermometer. His thermometer employed a bulb of air that expanded or contracted as temperature changed and in so doing caused the level of a column of water to rise or fall. Though this device was inaccurate because it did not account for changes in air pressure, it was later followed by more accurate instruments that did allow for new breakthroughs in scientific observation.

Studies speed of falling objects

From 1602 to 1609 Galileo studied the motion of pendulums and other objects as they moved along arcs and inclines. Using inclined planes that he built, he concluded that falling objects accelerate at a constant rate. This law of uniform acceleration later helped English mathematician and physicist Isaac Newton (1642-1727) derive the law of gravity.

Attention turns to astronomy

Galileo did not make his first contribution to astronomy until 1604, when a supernova, or a massive explosion of a star, suddenly appeared in the night sky. Galileo reasoned that this object was farther away than the planets and pointed out that this violent event in the sky meant that the "perfect and unchanging heavens" that Aristotle had claimed surrounded Earth were not unchanging after all.

Telescope revolutionizes astronomy

It was a few years after this event that Galileo first heard about the invention of the telescope in 1608 by Hans Lippershey (c. 1570-1619), a Danish spectacle maker. Galileo is often mistakenly given credit for inventing the telescope, partly because of his major improvements to Lippershey's device and partly because he claimed at one point to have done so. When Galileo learned of the invention in mid-1609, he quickly built one himself and made several improvements. His altered telescope could magnify objects at nine power (nine times the power of vision with the naked eye), three times the magnification of Lippershey's model. Galileo's telescope proved to be very valuable for the business of sailing ships, and Galileo was rewarded for his work with a lifetime appointment to the University of Venice.

Publishes The Starry Messenger

Galileo continued to work on the telescope, and by the end of the year he had built one that could magnify at 30 power. He had the idea to turn his telescope up to observe the stars and planets. The discoveries he made with this instrument revolutionized astronomy. Through the telescope, Galileo saw jagged edges on the Moon, which he realized were the tops of mountains. He assumed that the Moon's large dark areas were bodies of water, which he called maria, the Latin name for seas (though we now know there is no water on the Moon). When he observed the Milky Way (the galaxy of which the Sun and Earth are part and which appears as a luminous band of light in the night), Galileo was amazed to discover that the cloudy body was actually made up of individual stars. He published these and other early discoveries in March 1610 in a book entitled Sidereus nuncius ("The Starry Messenger"). The book increased Galileo's fame, and his discoveries--though questioned by some--were praised by many scientists, including the German astronomer Johannes Kepler.

New information about the solar system

Galileo did not stop with this first round of discoveries. Also in 1610 he observed that the planet Venus had light and dark phases (regularly recurring states) like the Moon, and for the same reasons: Venus did not make its own light, but was illuminated by the Sun. Looking at the Sun itself, he saw dark spots on its disc. The position of the spots changed from day to day, allowing Galileo to determine the rate at which the Sun rotated on its axis.

When he looked at Saturn, it looked like an oval. His telescope was not strong enough to make out the planet's famous rings; he concluded that the odd shape was due to some satellites that were very close to the planet. In observing Jupiter, he was able to clearly see that the planet had four moons; he later called them "satellites," a term suggested by Kepler. Galileo named the moons of Jupiter Sidera Medicea ("Medicean stars") in honor of Cosimo de Medici (1519-1574), the Grand Duke of Tuscany, whom Galileo served as "first philosopher and mathematician" after leaving the University of Pisa in 1610. By observing the moons over a few weeks, he was able to see the satellites being eclipsed, or blocked from view, by Jupiter. This brought Galileo to the conclusion that the four moons rotated around Jupiter, and he even calculated the length of time it took each moon to circle the planet. While this idea is now an accepted fact of science, in Galileo's time this observation was revolutionary.

The two views of the universe

Since the second century, the most widely accepted view of the universe was based on the theory of the Alexandrian astronomer Ptolemy, who said that Earth was the center of the universe, and the Sun and all the planets and other heavenly bodies rotated around it. This was the established teaching of the powerful Roman Catholic Church as well. But in the early 1500s, the Polish astronomer Nicolaus Copernicus had suggested that Earth, along with the other planets, circled the Sun. Copernicus had begun compiling evidence for his ideas based on the positions of the planets throughout the year, but his proof was mostly mathematical--he did not have the physical evidence that a telescope could provide. Copernicus's ideas had been banned by the Catholic Church.

Supports Copernican theory

In 1613, Galileo published a book called Letters on Sunspots, in which for the first time he presented evidence for and openly defended the model of the solar system earlier proposed by Copernicus. Presenting his findings on Venus, the Sun, and Jupiter, Galileo showed that not all moving bodies in the universe revolved around Earth. While there was some support even among Church authorities for Galileo's proof of the Copernican theory, the Roman Catholic leadership ultimately determined that it would not change the long-held astronomical teachings of the Church. Thus, in 1616 an order was issued by the Church declaring the Copernican system "false and erroneous," and Galileo was instructed not to support this system.

Following this run-in with the Catholic church, Galileo turned his attention to the less controversial issue of determining a ship's longitude (east-west) position while at sea, which required a reliable clock. (Using an accurate time-keeping system, a person on board a ship could compare the changes in position of a certain heavenly body at the same time every day and calculate how far east or west the ship had traveled.) Galileo thought it possible to measure time by observing eclipses of Jupiter's moons. Unfortunately, this idea was not practical because eclipses could not be predicted with enough accuracy and observing celestial bodies from a rocking ship was nearly impossible.

Book angers Church leaders

Galileo wanted to have the Church's position against the Copernican theory reversed, and in 1624 he traveled to Rome to make his appeal to the newly elected pope, Urban VIII. The pope would not change the Church's ruling, but Galileo was given permission to write about the Copernican system, with the restriction that it would not be given preference over the church-supported Ptolemaic model of the universe.

With this sign of approval from Urban, Galileo wrote his Dialogue Concerning the Two Chief World Systems--Ptolemaic and Copernican, published in 1632. Despite his agreement not to favor the Copernican view, the objections to it in the Dialogue are made to sound unconvincing and even ridiculous. Galileo was soon called to Rome to stand before the Inquisition, a court of the Catholic Church that was designed to uncover and punish people who held beliefs that went against the teachings of the Church. Galileo was accused of violating the original ruling of 1616 forbidding him to promote the Copernican theory. Put on trial for heresy (going against Church belief), he was found guilty and ordered to admit that his ideas were wrong. By this time, Galileo was almost seventy years old, and not wishing a harsh sentence, he did as he was told. According to legend, however, after making the required statement that the Copernican ideas were incorrect, Galileo said quietly, "And yet it moves," referring to the Copernican teaching that Earth rotates on its axis.

Final work published

While the judgment against Galileo included a term of imprisonment, the pope lightened this sentence to house arrest at Galileo's home near Florence. He devoted himself to work in physics, including a study of projectile motion (the movement of a body thrust forward by an external force, for example, a cannonball shot out of a cannon). Although officially forbidden to publish any further works, a book on his work in physics, titled Two New Sciences, was printed in France in 1638.

By the time of his last book's publication, Galileo had become completely blind, perhaps due in part to his many observations of the Sun through a telescope. He died at Arcetri, Italy, just outside of Florence, on January 8, 1642. Even after the scientist's death, the Church refused to allow his work to be recognized. It was not until almost 100 years later that the Church finally gave permission for a monument acknowledging Galileo's accomplishments to be placed at his grave.


  • Asimov, Isaac, Asimov's Biographical Encyclopedia of Science and Technology, Doubleday, 1972, pp. 91-96.
  • Dictionary of Scientific Biography, Volume 5, Scribner, 1972, pp. 237-49.
  • Hitzeroth, Deborah, and Sharon Heerboth, Galileo Galilei, Lucent Books, 1992.
  • MacLachlan, James, Galileo Galilei: First Physicist, Oxford University Press, 1996.
  • The McGraw-Hill Encyclopedia of World Biography, Volume 4, McGraw-Hill, 1973, pp. 289-92.
  • Sis, Peter, Starry Messenger, Farrar Straus Giroux, 1996.


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Source Citation   

Gale Document Number: GALE|K2641500077