In a 2nd Century treatise on mathematics and astronomy, Claudius Ptolemy claimed to have examined 800 years of scholarship in order to come up with a model that could predict astrological phenomena such as the placement of constellations in the night sky. Ptolemy believed in a geocentric view of the universe, meaning that the earth was at the universe's center, and the sun, stars and planets all revolved around the earth.
Nicolaus Copernicus was a Polish Renaissance mathematician and astronomer who developed a model that placed the sun, rather than the earth, at the center of the universe. This is called heliocentricity. In 1543, Copernicus published "On the Revolutions of Heavenly Spheres," shortly before his death. He waited until his deathbed, fearing reprisals from the Catholic Church that disputed his claims of a heliocentric universe.
Galileo proved Copernicus' theory through the use of his improved telescope. He discovered that the moon was not a smooth surface and that it reflects light from the sun. He also discovered the moons of Jupiter and the phases of Venus. Galileo's work in his 1610 treatise "Starry Messenger," created an uproar and he was condemned in the Catholic Inquisition in 1633. It is said that at the time of Galileo's death, he looked up to the sky and said of the earth, "Epper si muove," or "And still, it moves."
Between 1609 and 1619, Johannes Kepler developed the laws of planetary motion, which described the way that planets moved in an elliptical orbit. The name "planet" comes from the original Greek word for "wanderer." Because planets move in an elliptical orbit, they were one factor that could not be explained by Copernicus' theory of heliocenticity before being explained by Kepler.
Knowledge of astronomy changed dramatically during the Renaissance. After about 1500 years of belief in the Ancient Greek view of a geocentric universe, important thinkers such as Nicolaus Copernicus, Johannes Kepler, Tycho Brahe and Galileo Galilei revolutionized astronomical thought. After this point, scientists began to look at the earth as a small part of a much larger universe, rather than the center of all things.