Libration

Libration describes the apparent oscillating movements of the Moon allowing a little more than half its surface to be observed. Optical libration due to variations in the orbital velocity of the Moon (longitudinal libration), the inclination of the equator of the Moon in its orbital plane (latitudinal libration) and the motion of the terrestrial observer because of the rotation of the Earth about its axis (diurnal libration), is differentiated from physical libration - which is much smaller - due to variations in the rotation of the Moon about its axis.