Kepler and Galileo
Kepler was an avid supporter of the Copernican system, and he was determined to discover why it could not account for what astronomers observed. In 1600, Kepler became an associate of the Danish astronomer Tycho Brahe, who had spent years making detailed records of planetary motions. Brahe died in 1601, but Kepler continued to study Brahe's data, trying to make them fit the idea of circular orbits. Finally, Kepler realized that circular orbits were simply not correct, and it was then that he found the answer. The planets, he proclaimed in 1609, move in elliptical (oval) paths.
A contemporary of Kepler's, the Italian astronomer and physicist Galileo Galilei, advanced celestial observations with an important new tool: the telescope. Using a telescope to view the heavens, Galileo saw that the moon is covered with mountains and craters and that Venus, like the moon, goes through regular phases, changing in appearance from crescent shaped to a round disk.
The stars were a particular surprise to Galileo. They not only apparently numbered in the millions but also varied tremendously in their range of brightness. Most stars were much too faint to be seen by the unaided eye. This finding indicated that some stars were considerably more distant than others, rather than all being equally far away, as most people had assumed since at least the time of Aristotle. The perceived universe was growing ever larger.
Galileo's observations greatly strengthened the Copernican hypothesis. For example, the fact that Venus goes through phases like those of the moon indicated that Venus circles the sun. Although absolute proof that the Earth was not the center of the universe was still lacking, Galileo became an outspoken critic of all who expressed doubts about the Copernican system. As a result, angry church authorities in Rome brought Galileo to trial in 1633 and forced him to renounce his belief in Copernicanism. Their victory, however, was nearly the last gasp in efforts to preserve the notion of an Earth centered cosmos.