Galileo Galilei (1564-1642) was born in Pisa in Tuscany, Italy. The son of a court musician, Galileo's family was in what would be roughly equivalent to what today would be called the upper middle class. He is most widely remembered for three things: his trial of 1632, his physics, and his telescopic discoveries. He was also a teacher of mathematics. This, and his ability to repair certain instruments, were his main sources of income before the Grand Duke's patronage. He was an advocate of Copernacanism, and his observations supported this theory.
Galileo's approach to physics was entirely new at his time. Although he was extremely anti-Aristotelian, he thought of motion in an Aristotelian way. Indeed, Galileo originally believed that motion was an essential property of bodies (Essentialism) and the impetus theory. Later, Galileo switched to a non-essentialist theory of motion in which he states that the motion of an object tells nothing of the inner nature of the object. Motion is simply a state of a body, rather than an essential property. He worked with projectile motion, dealt with inertia, discovered parabolic trajectory, and was known for using an inclined plane to slow acceleration. In explaining projectile motion, Galileo believed that an object could participate in more than one motion at the same time, again denying Aristotelian thought. These theories and observations are contained in his book, De Motu. He had a strong aversion to Aristotelian physics, and did his best to find any theory that would disprove it. This involved the acceptance of Copernican astronomy (since removing the earth from the center of the universe shattered Aristotle's views) and the denial of some of Aristotle's physical rules.
One of Galileo's classic experiments invovles two iron balls tied together with string. According to Aristotelian physics, an object of twice the weight should fall twice as quickly. Galileo took two iron balls of equal weight and tied them together. Since they form one complete unit of double the weight of the original, the two balls tied together should fall twice as fast. But, by observation, they clearly do not. Galileo then responded that no matter the size of the objects, they will fall at the same speed so long as they are made of the same material.
Galileo also utilized a new method of doing science, which was referred to as the "hypothetico-deductive method." It consisted of using mathematics to make a prediction, and then testing the prediction with physical experiment, often repeated. However, by limiting oneself to a prediction a priori, as Galileo's method did, one could easily be tempted to make the data fit the desired observation. This result was often the case with Galileo as the experiments one conducts will never match the perfection of mathematical theory.
Though Galileo did not invent the telescope (it is thought to have originated in the Netherlands), he greatly improved it. Before Galileo came across the instrument, it was a novelty item used mostly for parlor tricks. With his improved telescope, he observed sunspots, the phases of Venus, and discovered four moons of Jupiter, as well as Saturn's rings (although he did not realize that they were rings). In addition, Galileo studied the surface of the moon and revealed it was mountainous. This moutainous terrain contradicted the Aristotelian theory that the celestial realm was perfect and uniform.
Most of his discoveries, as well as his improvements to the telescope, according to Robert Westfall, were made for practical reasons and economic gain. Galileo was nothing if not an opportunist, and he encountered his fair share of luck. As luck would have it, he practically stumbled upon the moons of Jupiter, as they happened to be in a perfect position for observation when Galileo was scouting around for something to report. While his reasons for his works are morally questionable to some, Galileo's improvements of the telescope allowed him to present a new powerful tool that could be used in sailing, navigation, and the military. This new invention, and naming the four moons of Jupiter after the late Duke's four sons, earned him favor and allowed him to finally gain a position as court mathemetician and philosopher after courting the Duke for years.
Galileo also contributed to evidential support of Copernicanism by his observation of the period and phases of Venus. His observations of Venus contradicted the predictions of the Ptolemaic system, supporting instead the Copernican prediction. However, even this pursuit of Copernicanism may have been fueled by a perceived slight, as Westfall argues, in a letter written by one of Galileo's previous students.
"Hence just from seeing the falling stone graze the tower, you could not say for sure that it described a straight and perpendicular line, unless you first assumed the earth to stand still. ... Then here, clear and evident, is the paralogism of Aristotle and of Ptolemy--. They take as known that which is intended to be proved."
-Galileo, from Dialogue Concerning the Two Chief World Systems
Richard S. Westfall, "Science and Patronage: Galileo and the Telescope," Isis, Vol. 76, No. 1 (Mar., 1985), pp. 11-30
Essentialism- belief that objects have inherent properties that determine how they will behave (especially with regards to motion)
Non-Essentialism- The idea that how an object behaves does not reveal any inner properties within the thing itself. (especially with regards to motion)