Galileo Galilei was born in the same year as Shakespeare (1564), and died in the same year that Newton was born (1642). He was a mathematician who was interested in the motions of objects. One of his most important contributions was the invention of an astronomical telescope. Galileo publicly claimed that his observations through his telescope gave absolute proof that the Copernican solar system was the correct model. This claim was unfounded; his telescope could only refine measurements, but it offered no "proof" of Copernicus's theory. Although the Church had banned Copernicus's work, the authorities were kinder to Galileo, and they only warned him that he must discuss the Copernican view as a theory, and not as a fact.

When Galileo's old friend Cardinal Barberini became Pope, he allowed Galileo to write a book about how the planetary motions appear to be. However, Galileo was a devious character, and he mocked the church and general public opinion in his books, "Two New Sciences" and "Two World Systems". The books are a dialogue between the dull-witted Simplicio, who represents public and Church opinion (and supports the Earth-centred hypothesis), Salviati, an intelligent man who represents Galileo's views, and Sagredo, a wise and pragmatic man who is persuaded that Salviati's (Galileo's) views are correct. Needless to say, Galileo was forced to renounce his opinion that the Earth moved, and he was removed from public life by the Church, as a result of his impudence.

Another important contribution of Galileo's was the discovery of some laws of falling bodies. Galileo stated that:

1. In the absence of resistance, all bodies will fall with the same speed.

2. During equal time intervals, a falling body increases its speed by equal increments.

Galileo reached these conclusions by doing experiments with balls rolling down inclined planes, and dropping objects of different mass from the same height. He used a water clock to measure time intervals. A water clock involves measuring the amount of water that has poured through a small hole in a given time interval, where the size of the hole limits (and keeps constant) the flow of water. Galileo's work on falling bodies formed a basis for a lot of Newton's work.

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