Libration in latitude is due to the Moon's axis being slightly inclined relative to the Earth's. Each of the lunar poles will appear to be alternately tipped slightly toward the terrestrial observer over a roughly four week cycle.

Diurnal libration is due to the observer being on the surface of the Earth, up to four thousand miles to one side of the Earth-Moon axis, a significant proportion of the centre-to-centre distance. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to west and then to east.

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west oscillation.

Although the Moon always presents the same face towards the Earth, due to its rotation and revolution being locked to the same period, the combined effect of these different librations allows us over time to see some 59% of its surface.

António Cidadão has created a spectacular 133K animation illustrating libration

Inconstant Moon