Galileo Galilei was one of the most influential scientists and astronomers ever to live. With a single idea, a single stroke of genius, he turned the world of astronomy upside down. This did have its consequences though. The powerful Catholic Church punished Galileo's views and deemed them heretical and untrue. He was forced to renounce his beliefs. Yet, his goal was accomplished. The printing press spread his works and his views became accepted.
Galileo did not actually invent the telescope. He merely took a magnifying tool and pointed it at the heavens. However, this was revolutionary in itself. Almost instantly Galileo noticed many more stars than were previously known to exist. It was then that he began to have doubts about the church and its astronomical teachings. Galileo also noticed and proved the motions of the planets, which were mathematically proven by Kepler as well. With these observations, he violated church doctrine by introducing a system in which the stars and planets were not in fixed positions and the earth was not the center of the universe.
Galileo also conducted many physical experiments. He noticed that a large ball and a small ball of the same shape would fall and hit the ground at the same time.