Galileo's observations with a telescope challenged several of the Aristotelian assumptions. Prior to this time all of the observations of the stars and planets were were made with the naked eye.
Galileo was born on 15 February, 1564 in Pisa which was part of the Duchy of Tuscany. He was the first of six children. His father Vincenzo was a music teacher and an accomplished player of the lute (a stringed instrument). Galileo also became a very good lute player. In his teen years Galileo was sent to a monastery for his education. Galileo was attracted to the contemplative life style there and became a novice, intending to join the order. His father didn't approve and withdrew his son. In 1581 Galileo was enrolled at the University of Pisa to study medicine as his father desired.
In 1582–1583 Ostilio Ricci, the mathematician of the Tuscan court, taught a course at the university on Euclid's elements which Galileo attended. Galileo began studying the elements on his own and asked many penetrating questions. Ricci attempted to persuade Vincenzo that his son should pursue a career in mathematics. Vincenzo was not convinced, but did allow his son to be tutored in mathematics while continuing his education in medicine. This didn't work out well and in 1585 Galileo dropped out of college without completing his degree.
For the next few years Galileo taught mathematics privately in Florence and later in Siena. In 1587 Galileo traveled to Rome to meet with Clavius who was professor of mathematics at the Jesuit Collegio Romano there. He showed the professor some methods he had developed for calculating the center of gravity of various solids and Clavius was very impressed. The two remained in correspondence after Galileo left Rome. Galileo also corresponded regularly withe another mathematician Guidobaldo del Monte. Through the influence of Clavius and Guidobaldo, Galileo was appointed to the chair of mathematics at the University of Pisa in 1589. He held this position for three years. While at the university he wrote De Motu a series of essays on motion which he never published. One important idea contained in De Motu was the use of inclined planes to test theories of falling bodies at slower speeds.
In 1591 Galileo's father Vincenzo died and Galileo had to assume the financial support of the family. His salary at the university was not sufficient, so he sought another place of employment. Again through the influence of Clavius and Guidobaldo he was appointed professor of mathematics at the University of Padua at three times the salary. At Padua his duties were mainly to teach Euclidean geometry and standard (geocentric) astronomy to medical students. He taught there for eighteen years.
In 1604 he gave three public lectures concerning the appearance of a new star (now known as Kepler's Supernova). Using a parallax argument he showed that the new star could not be in the lunar region close to the earth. This contradicted the Aristotelian assertion that all changes in the heavens must occur in the region between the earth and the moon. Galileo had also questioned other Aristotelian assumptions. In particular, he didn't believe that the speed of falling objects was proportional to their weight. As a young boy he had observed that hailstones of different sizes appeared to fall at the same rate. It is not sure when Galileo became a believer in the theory of Copernicus, but in a 1598 letter to Kepler he stated that he was a Copernican. However, he didn't state that publicly until much later.
A working telescope was first unveiled in the Netherlands in 1608. Initially it was used for military applications. In 1609 Galileo began using the telescope for astronomical observations. He made a number of refinements of the instrument and eventually achieved a magnification of 32x. His observations with the telescope challenged many of Aristotle's assumptions. His observations of the moon revealed an uneven (mountainous) surface much as we have on the earth. Figure 6 shows drawings he made of the moon surface from his observations.
He also observed objects he correctly identified as moons revolving around the planet Jupiter. This challenged the view that everything revolved around a stationary earth. Here was an example of objects revolving around another object that was itself moving. He also observed that the Milky Way was made up of many tiny stars. In 1610 he published a booklet called the Starry Messenger (Sidereus Nuncius) describing his initial telescopic observations. Shortly afterwards he was appointed the Chief Mathematician and Philosopher to the Grand Duke of Tuscany.
Galileo was also the first to observe sun spots through a telescope (see appendix). This challenged the assumed perfection of the superlunar region. He also observed that the planet Venus went through a complete set of phases like those of the moon. This was consistent with the sun-centered model of Copernicus (1473–1543), but was not consistent with Ptolemy's model. In Ptolemy's model Mercury and Venus had orbits between the earth and the sun. Since they always appear close to the sun, the centers of their epicycles must revolve around the earth at the same rate as the sun does. Thus, Ptolemy's model would never show a wide range of phases. There is always a significant portion of the dark side of Venus facing the earth. You can see this from Figure 7.
Figure 8 shows the phases of Venus in the Copernican model (without epicycles). Here an observer on the earth will see a wide range of phases. Galileo's observations were repeated by Jesuit astronomers, and they confirmed what he saw.
Galileo viewed his observations as a confirmation of the sun-centered Copernican model. Although his observations did pose a serious threat to the model of Aristotle and Ptolemy, there was another earth-centered model that produced accurate results and was also consistent with Galileo's observations. This was the model proposed by the Danish astronomer Tycho Brahe.