In astronomy, heliocentrism is the theory that the sun is at the center of the Solar System. The wor came from the Greek (ήλιος Helios = sun and κέντρον kentron = center). Historically, heliocentrism is opposed to geocentrism and currently to modern geocentrism, which places the earth at the center. (The distinction between the Solar System and the Universe was not clear until modern times, but extremely important relative to the controversy over cosmology and religion.) Although many early cosmologists such as Aristarchus speculated about the motion of the Earth around a stationary Sun, it was not until the 16th century that Copernicus presented a fully predictive mathematical model of a heliocentric system, which was later elaborated by Kepler and defended by Galileo, becoming the center of a major dispute.