Moon libration. Composite image of two views of the moon illustrating its libration. Libration is small oscillation of the moon about its mean position. There are three components of libration, diurnal, latitudinal and longitudinal. The combination of these three means that it is possible to see around 59% of the surface of the moon from Earth. In the image at left, a greater proportion of the western side (left) of the moon's surface is visible than in the image at right. The difference in size of the two images is due to the change in distance of the moon from Earth during its orbit.