Galileo is credited with establishing the modern experimental method. Before Galileo, knowledge of the physical world that was advanced by scientists and thinkers was for the most part a matter of hypothesis and conjecture, having some basis in logic and observation but often wholly deriving from religious authority or simply irrational prejudice. In contrast, Galileo introduced the practice of proving or disproving a scientific theory by conducting tests and observing the results. By this procedure he made numerous significant discoveries, particularly in the fields of physics and astronomy.
The son of a musician, Galileo was born in Pisa. He received his early education at a monastery near Florence, and in 1581 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 an oscillation remained constant, even as the arc of the swing steadily decreased. He later experimented with other suspended objects and discovered that they behaved in the same way, suggesting to him that the principle of the pendulum might have an application in regulating clocks.
While at the University of Pisa, Galileo listened in on a geometry lesson and afterward abandoned his medical studies to 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, and the following year published an essay describing his invention of the hydrostatic balance, which determined the specific gravity of objects by weighing them in water. With this invention Galileo gained a scientific reputation throughout Italy.
In 1589 Galileo obtained a professorship in mathematics at the University of Pisa, where he remained for the next eighteen years. During this time Galileo conducted his most important scientific research and published treatises documenting his work. Among these was an untitled treatise in which he discredited Aristotle's contention that heavier objects fell at a faster rate than lighter ones, an idea that until that time had been universally accepted. Galileo advanced theoretical proof that falling bodies accelerate or decelerate uniformly with time, a principle that was later called the law of uniformly accelerated motion.
In 1609 Galileo first learned of the telescope, which had recently been invented by a Dutch lens-grinder, Hans Lipperhey. Improving on the original design, Galileo constructed a telescope with a magnifying power of 32, enabling him to make a number of important astronomical discoveries.
With his telescope Galileo found that the moon was not a perfectly smooth sphere, as was previously thought, but irregular in its surface. He also observed that the Milky Way was composed of individual stars. Galileo's study of Jupiter resulted in his discovery of its four moons, which he later called " satellites," a term suggested by the German astronomer Johannes Kepler. To the moons of Jupiter, Galileo gave the name Sidera Medicea ("Medicean stars") in honor of Cosimo de Medici, the Grand Duke of Tuscany, whom Galileo served as "first philosopher and mathematician" after leaving the University of Pisa in 1610. Training his telescope on Venus, Galileo discovered that this planet exhibited phases much like the moon, and for the same reason: Venus did not produce its own light but was illuminated by the sun. In addition Galileo noted that Saturn was encompassed by rings, which his telescope allowed him to see only as "protuberances" on either side of the planet, and documented his observation of sunspots.
In 1613 Galileo published a book in which for the first time he presented evidence for and openly defended the model of the solar system earlier proposed by the Polish astronomer Nicholas Copernicus, who argued that the Earth, rather than being positioned at the center of the universe, as in the Ptolemaic design, was only one of several galactic bodies that orbited the sun. While there was some support even among ecclesiastical authorities for Galileo's proof of the Copernican theory, the Roman Catholic hierarchy ultimately determined that a revision of the long-held astronomical doctrines of the church was unnecessary. Thus, in 1616 a decree was issued by the church declaring the Copernican system "false and erroneous," and Galileo was enjoined from supporting it. For several years thereafter Galileo led a retiring existence at his home near Florence.
Galileo wanted to have the edict against the Copernican theory revoked, and in 1624 travelled to Rome to make his appeal to the newly elected pope, Urban VIII. The pope would not revoke the edict but did give Galileo permission to write about the Copernican system, with the provision that it would not be given preference to the church-sanctioned Ptolemaic model of the universe.
With Urban's imprimatur, Galileo wrote his Dialogue Concerning the Two Chief World Systems--Ptolemaic and Copernican, which was 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. Summoned to Rome to stand before the Inquisition, Galileo was accused of violating the original proscription of 1616 forbidding him to promote the Copernican theory. Put on trial for heresy, he was found guilty and ordered to recant his errors. At some point during this ordeal Galileo is supposed to have made his famous statement: "And yet it moves," referring to the Copernican doctrine of the Earth's rotation on its axis.
While the judgement against Galileo included a term of imprisonment, the pope commuted this sentence to house arrest at Galileo's home near Florence. There Galileo continued to work, although he was forbidden to publish any further works.
Galileo died in 1642. Exactly 350 years later, in 1992, Pope John Paul II formally exonerated Galileo, relieving him posthomously of the church's proscriptions.
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