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Galileo Galilei

gal' i lA O, Italian physicist and astronomer who "discovered gravity"


Galileo Galilei was born in Pisa, Italy, on February 15, 1564. He showed unusual skill in building toys as a child, played the lute and the organ well, and won a reputation for his excellent paintings. His father, a merchant and musician, taught Galileo music, but encouraged him to become a doctor. Galileo studied medicine and the philosophy of Aristotle at the University of Pisa.

Young Scientist

Galileo made his first important scientific discovery while he was still a university student. While at a service in Pisa Cathedral, his attention was caught by the swaying of the chandeliers above his head. Struck by their steady rhythm, he designed a simple pendulum in order to investigate the regularity of its swing. Using his own pulse beats to measure the swing, he discovered that each swing took the same amount of time regardless of the length of the arc. He then suggested that pendulums might therefore be used to regulate the measurement of time, an idea that was later applied by Christiaan Huygens.

left: a model constructed from Galileo's design for a pendulum clock
right: copy of Galileo's design for an escapement or pendulum clock

model constructed from Galileo's design for a pendulum clockcopy of Galileo's design for a pendulum clock

Galileo left the university in 1585 due to lack of funds, and abandoned medicine for research in mathematics. During this time, he invented the hydrostatic balance, which is used to find the specific gravity of objects by weighing them in water.

Galileo returned to the University of Pisa at the age of 25 as professor of mathematics. During this period, he is credited with discovering the law of falling bodies. Reasoning that gravity pulls all bodies to earth with the same acceleration, regardless of their weight, Galileo is said to have dropped two unequal weights from the top of the Leaning Tower of Pisa to prove his theory. According to a much-told story, a crowd of students, professors, and priests looked on as both weights struck the ground at about the same time. Whether the story is entirely accurate or not, there is no doubt that Galileo did openly dispute the long-held theory, originally proposed by Aristotle, that heavy bodies fall faster than lighter ones, and that Galileo was forced to leave the university because of his views.

Astronomical Discoveries

In 1592, Galileo became professor of mathematics at the University of Padua, where he remained for 18 years. In 1597, he invented the sector, a type of compass still used by draftsmen. Beginning in 1609, he built many telescopes and sold them throughout Europe. He also made larger and more powerful telescopes than had been made before.

Galileo's first important astronomical observations were of the Moon, and he once again found himself opposing the teachings of Aristotle. He discovered that the moon was not a smooth sphere shining by its own light, but that its surface was actually marked by valleys and mountains and that it showed only the light it reflected. He also studied the Milky Way and found that our galaxy is a mass of stars "so numerous as to be almost beyond belief."

one of Galileo's views of the Moon
one of Galileo's views of the Moon

In 1610, Galileo discovered the four brightest satellites of Jupiter. He named these the Medicean stars, after the Medici family, who ruled the province of Tuscany, where he was born. That same year, he observed the peculiar form of Saturn, the rings of which would later be recognized by Christiaan Huygens.

These discoveries added support to the theory put forward by Nicolas Copernicus, that the Earth moves around the Sun. But they also brought him extreme abuse. Many churchmen and follwers of Aristotle opposed Galileo. But Cosimo II, a member of the Medici family and Grand Duke of Tuscany, became his patron and invited Galileo to serve as his personal mathematician in Florence and the University of Pisa.

In Florence, Galileo detected the phases of Venus and a slight phase of Mars. In Rome, he used one of his telescopes to show Pope Paul V and other high church officials what he had discovered. In spite of these demonstrations, a dispute followed between churchmen and scientists. The Church also bitterly opposed Galileo's report on sunspots.

In the 1620's, Galileo published a paper outlining the basic ideas of what is today known as the scientific method. He proposed that the results of experiments should form the basis of mathematical formulations of new theories, and that these theories should themselves be tested by further experiment. He also argued that the tradition of treating mathematics and science as separate disciplines should be discarded.

The Inquisition

In 1632, Galileo published his masterpiece, A Dialogue on the Two Principal Systems of the World. The Holy Office, or Inquisition, immediately called him to appear before it. After a long trial, Church officials forced him to say that he no longer believed in the Copernican theory, and sentenced him to an indefinite prison term. Instead of imprisoning him, however, they confined him to his villa in Florence.

Cristiano Banti's 1857 painting "Galileo Facing the Roman Inquisition"
Galileo facing the Inquisition

Galileo spent his last years writing on the laws of force and motion. Dialogues on the Two New Sciences, published in 1638, summed up his life's work on motion, acceleration, and gravity, and furnished a basis for the three laws of motion laid down by Sir Isaac Newton in 1687.

Galileo died on January 8, 1642, and was buried in the Church of Santa Croce in Florence. Fifty years after his death, the city erected a monument at the church in his honor.

In 1992, Pope John Paul II formally proclaimed that the Roman Catholic Church had erred in condemning Galileo.

The Galileo Project

Christiaan Huygens
Leaning Tower of Pisa
Hans Lippershey
Nicolas Copernicus
Sir Isaac Newton
Pope John Paul II

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This page was last updated on 05/25/2017.