Perhaps Galileo ought to have recognized that his teaching of Copernican theory -- that the Sun, not Earth, was the center of the universe -- would not be tolerated by the leaders of the Roman Catholic church. After all, the Church, feeling threatened by the spread of Protestantism through Europe at the end of the sixteenth century, had revived the Inquisition, a tribunal to protect against heresy that was first assembled in the thirteenth century. Already, a monk named Giordano Bruno had been convicted of heresy for stating his belief in a Sun-centered universe; he was burnt to death in 1600.

The observations Galileo made from 1609 to 1610 with the telescope he had built provided proof, he thought, that Copernican theory was correct. Brought to Rome in 1615 to testify about his views, Galileo was formally warned not to teach this theory as fact. He accepted this warning and managed to disguise any defense of Copernican theory in his published works until 1632, when after many years of work, he published Dialogue on the Two Great Systems of the World. Accused of advocating Copernican theory in his astronomical findings, thus violating the Church's earlier decree, Galileo was sentenced by the Inquisition to house arrest.

The Church's main concern was not with the science behind Galileo's theories. In fact, many members of the council of theologians entrusted to protect the Church were actually scientists and mathematicians who agreed with his conclusions. What they were unable to tolerate, however, was Galileo's publishing his theories as fact, even after an explicit order from the pope not to do so. Though he was nearing 70 at the time of his sentencing and on the verge of blindness, Galileo continued his work. He completed a book that for the first time summarized his earlier work on mechanics -- the study of forces acting on bodies in motion and at rest -- which laid the foundations for classical physics.