Like the Sun, the Moon moves eastward relative to the stars but at a faster rate, completing its motion in one month. The apparent motion of the Moon relative to the Sun produces the cycle of lunar phases as well as both lunar and solar eclipses. Ancient Greek mathematicians devised ways of estimating the distances and sizes of the Sun and Moon from observational data, including the phenomenon of parallax. The planets, too, appear to move relative to the stars. They generally move eastward relative to the stars but occasionally they halt their eastward motion and move westward (in retrograde motion) before resuming their normal eastward trek. The planets can be classified into two groups, inferior and superior, each of which displays certain characteristics of motion.