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...