In 1564, Galilei, Galileo was born in Pisa, Italy. His father was a merchandiser who sold cloths. When Galileo was young, he did not only show talent in mathematics, but virtuosity in playing the lute. After he entered medical school, he became a nasty man, because he often criticized some widely accepted beliefs. At last, his father allowed him to quit school. When he was twenty-five, his good friends made use of their power to help Galileo become a mathematics professor at the University of Pisa.

In 1610, Galileo was widely known. Galileo improved his lens-grinding machine and designed a telescope, which could magnify a distant object thirty times. Through his telescope, he discovered four satellites of Jupiter. He also observed the rough surface of the moon, phases of Venus and irregular protrusion near the equator of Saturn. He found that everywhere in the sky is full of stars, but they are invisible to the naked eye. Some discoveries above are evidence of Copernican heliocentric system.

However, his contemporaries only accepted Aristotle's viewpoint about the universe. In Aristotle's point of view, the universe is static and does not change; all celestial objects orbit around the earth. This viewpoint is called the geocentric system. Although Copernicus had proposed the heliocentric system, that is, all celestial objects orbit around the sun, the majority of people did not believe in it. However, Galileo made use of his observations and gave lots of evidence to support the heliocentric system. Since Galileo was highly respectable, there were still some people agreeing to his doubt about the geocentric system.

In 1616, the Inquisition began to pay attention to Galileo's theory. In 1625, after Galileo wrote the book "Dialogue on the Two Chief World Systems", the Inquisition made all their effort to ban the book and wanted to put Galileo on trial. In 1633, the trial took place. In the course of the trial, he was forced to deny his viewpoint and the Copernican system. He also had to confess to bringing up the theory.

After the trial, Galileo returned to Florence. Before he became blind, he finished writing a book on mechanics. In 1642, Galileo died near Florence in Arcetri.