"Cosmos" is a term used synonymously with "universe". The kosmos of the ancient Greeks and Christian Middle Ages consisted of Earth at the center of a series of concentric, revolving spheres to which the Moon, Sun, planets, and stars were fixed. In the scheme of Aristotle and Ptolemy, which prevailed until the Copernican Revolution, only one kosmos was allowed with nothing beyond. According to the school of atomism, however, there were many kosmoi, distinct and separate from our own, each with a world, possibly inhabited, at its heart.