Ptolemy. Ptolemy was an astronomer and mathematician. He believed that the Earth was the center of the Universe. The word for earth in Greek is geo, so we call this idea a "geocentric" theory. This flawed view of the Universe was accepted for many centuries.
Copernicus. Well over a thousand years later, Nicolaus Copernicus came up with a radical way of looking at the Universe. His heliocentric system put the Sun (helio) at the center of our system. His ideas, including the revelation that the Earth rotates on its axis, were too different for most of the scholars of his time to accept.
Galileo. He had his own ideas on how motion really worked, and devised a telescope that could enlarge objects up to 20 times. He was able to use this telescope to prove the truth of the Copernican system of heliocentrism. He published his observations which went against the established teaching of the Church. He was brought to trial and, although he made a confession of wrong-doing, he was still imprisoned for life.
Extracted from http://kids.librarypoint.org/node/2280/