The old Greek astronomers thought that all the heavenly bodies moved about the earth in circles, while the latter body remained stationary. Soon they discovered that this could not be true of the planets, and to account for the apparent differences they assumed that each planet moved in a small circle of its own, and that the center of this circle moved about the earth. This small circle, the orbit of the planet, was known as the epicycle, and the path through which the center of the epicycle passed was called the deferent.