Libration

This simple picture is only approximately true since over time, slightly more than half (about 59% in total) of the Moon's surface is seen from Earth because of libration.

[1] Lunar libration arises from three changes in perspective because of the non-circular and inclined orbit, the finite size of the Earth, and the orientation of the Moon in space.

These oscillations can be expressed with trigonometric series that depend on the lunar moments of inertia A < B < C.[6] The sensitive combinations are β = (C – A)/B and γ = (B – A)/C.

[8][9] Modern values are: The fluid core can cause a fourth mode with a period around four centuries.

[11] The free librations are expected to damp out in times very short compared to the age of the Moon.

The lunar phases and librations in 2019 in the Northern Hemisphere at hourly intervals, with music, titles, and supplemental graphics
Over one lunar month more than half of the Moon's surface can be seen from the surface of the Earth.
Simulated views of the Moon over one month, demonstrating librations in latitude and longitude . Also visible are the different phases, and the variation in visual size caused by the variable distance from the Earth.
Theoretical extent of visible lunar surface (in green) due to libration, compared to the extent of the visible lunar surface without libration (in yellow). The projection is the Winkel Tripel projection . Mare Orientale , just outside the yellow region, is brought into visibility from Earth by libration.
Diurnal libration of the moon as actually observed from beginning to end of a single night. The two angles are created by the different position of the observer with respect to the Moon because of the rotation of the Earth over a few hours.
Animation showing the changing position of the Moon due to libration, in relation to a fictitious red position on perfectly circular orbit.
Full moon at opposite librations