Nicolaus Copernicus, (1473–1543), was one of the greatest astronomers that first introduced the theory of heliocentric system. Copernicus rejected the Ptolemy's geocentric model, sustaining the idea that only a heliocentric model could respect the concept of uniform circular motion of all celestial bodies, introduced by Aristotle. Copernicus's heliocentric concept was very questionable, at that time, however, Copernicus became the leader of the Scientific Revolution.
Galileo Galilei is also known as one of the greatest astronomers of all time. He discovered the Jupiter's moons, having as support the Copernicus' heliocentric model. His contributions to astronomic field gained the full acceptance of Copernicus' heliocentric theory. Galileo is also known for his great findings, the use of telescope to study celestial bodies and the universe, being the notable to many. The discovery of the motion of sunspots made for Galileo to conclude that the Sun was revolving on an axis. Galileo used his telescope to prove that Venus, just like Moon, went through a complete set of phases, this late discovery holding only in Copernican system.