Libration of lunar orbit, illustration. The phenomenon of libration is where the apparent diameter and tilt of the Moon varies during its orbit around the Earth. At top, the light cones show the differences in the areas of the Moon's surface visible from different sides of the Earth (diurnal libration). At centre, the variation in the areas of lunar surface that are visible due to the Moon's orbit is shown (libration in longitude): the eastern limb is more visible at perigee (closest approach) and the western limb is more visible at apogee (furthest approach). At right, the variation due to the position of the Moon above or below the Earth's equator is shown (libration in latitude). The Moon's north and south poles are visible when the Moon lies below and above the Earth respectively. Libration is the combination of all three effects.