In astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek (ήλιος Helios = sun and κέντρον kentron = center). Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical antiquity. It was not until the 16th century that a fully predictive mathematical model of a heliocentric system was presented, by mathematician and astronomer Nicolaus Copernicus. In the following century, this model was elaborated and expanded by Johannes Kepler and supporting observations made using a telescope were presented by Galileo Galilei.