As anyone who looks at the Moon from time to time will have noticed, the pattern of dark markings on its surface always looks much the same. This is despite the fact that it is revolving around the Earth. The reason is that it rotates on its axis in the same time that it takes to revolve around the Earth. (The rotation is said to be locked). If the Moon's axis was perpendicular to its orbit and the orbit was circular, then the Moon would present exactly the same face to the earth at all times. However neither of these things is true and the Moon appears to wobble and more than 50% of the surface can be seen from Earth. There are three major reasons for this wobble.
The Moon's axis of rotation is inclined at about 6.5° to the plane of its orbit. The result is that, as it revolves around the Earth, first the north pole is tilted towards us, then the south pole, so we can see just past each pole at one time or the other. This is called Libration in Latitude and amounts to about 6½°.
The moon's orbit is not circular but eliptical, so its distance from the earth varies. When it is at its nearest (perigee it moves more quickly in its orbit than it does when it is at its furthest (apogee). But it rotates on its axis at a constant speed, so when at apogee the rotation moves ahead of that required to keep the same face to the Earth; when at perigee the rotation lags a little behind. This results in our seeing a little beyond the eastern and western limbs. The magnitude of this effect is about 7¾°.
The Earth is not small and it not an infinite distance from the Moon. So the view of an observer on one side of the Earth, where the Moon is rising, will see a slightly different view to someone on the other side of the Earth, where the Moon is setting. Since a single observer will be carried from one point to the other by the rotating Earth in 12 hours, this effect is known as the Diurnal Libration. A similar effect would occur for observers at the Earth's poles. The effect accounts for about 1°.
There is a further very small libration. The mass of the Moon is not centred at the centre of the Moon. There is slightly more mass on the Earth-facing diameter of the Moon than there is on the other two principal diameters. It is this asymmetry that resulted in the locked rotation in the first place. However, the logitudinal libration results in this axis not pointing directly at the Earth all the time, and the Earth's gravitational pull causes a very small physical wobbling of the Moon. This effect is minute (a few hundreths of a degree).
The overall effect of these librations results in about 57% of the Moon's surface being visible to observers on the Earth.
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