Two world systems

Two world systems

. Among Galileo’s most compelling arguments in favor of a Sun-centered solar system, previously advocated by Copernicus, was the fact that Venus displays phases like those of the Moon. In the long-held, Earth-centered Ptolemaic system (left) Venus always lies between the Earth and Sun, so it can never appear fully illuminated when viewed from the Earth. But in the Copernican system (right), Venus can show the whole range of illuminated phases.

Copyright 2010, Professor Kenneth R. Lang, Tufts University