Libration. The moon rotates on her axis in the same time as she revolves in her orbit; but, although the former motion is perfectly uniform, the latter is not. Hence, instead of seeing always exactly the same portion of her surface, we have two narrow strips extending between her poles on her eastern and western sides alternately shown to us. This phenomenon is called the moon's libration in longitude. Since the moon's axis is not quite perpendicular to the plane of her orbit, at different times we see different bits of the surface round her poles. This is the libration in latitude. The fact that we are on the surface, and not at the centre, of the earth causes the diurnal libration; for we see a little more of one limb when the moon is on the horizon, and a little more of the other when she is high in the heavens. On account of these librations we are able to get some knowledge of rather more than half the moon's surface.