Any of several effects that alter precisely which hemisphere of the Moon's surface is visible from the Earth. Despite the fact that the Moon's rotation and orbital periods are equal, so that the Moon very nearly keeps the same face towards the Earth all the time, a total of 59 per cent of the Moon's surface can be viewed from the Earth at some time or other as a result of libration. Physical libration is a real irregularity in the Moon's rotation; the larger effect is geometrical libration, in both latitude and longitude. Libration in latitude results from the Moon's orbit being inclined to the ecliptic by an angle of 5° 9'. The elliptical shape of the Moon's orbit means that its orbital velocity is not constant, and this produces libration in longitude of 7° 45'. Additionally, diurnal libration is a small effect that results from observing the Moon at different times of day.