axial motion, and _AB_ is again perpendicular to _ME_. (3) _Parallactic Libration._--While an observer at the centre of the earth would get the same view of the moon, whether she were on the eastern horizon, in the zenith, or on the western horizon, an observer on the surface of the earth does not get exactly the same view in these three cases. When the moon is on the eastern horizon, an observer on the surface of the earth would see a little farther around on the western limb of the moon than when she is in the zenith, and not quite so far around on the eastern limb. On the contrary, when the moon is on the western horizon, an observer on the surface of the earth sees a little farther around on the eastern limb of the moon than when she is in the zenith, and not quite so far around on her western limb. [Illustration: Fig. 120.] This will be evident from Fig. 120. _E_ is the centre of the earth, and _O_ a point on its surface. _AB_ is a line drawn through the centre of the moon, perpendicular to a line joining the centres of the moon and the earth. This line marks off the part of the moon turned towards the centre of the earth, and remains essentially the same during the day. _CD_ is a line drawn through the centre of the moon perpendicular to a line joining the centre of the moon and the point of observation. This line marks off the part of the moon turned towards _O_. When the moon is in the zenith, _CD_ coincides with _AB_; but, when the moon is on the horizon, _CD_ is inclined to _AB_. When the moon is on the eastern horizon, an observer at _O_ sees a little beyond _B_, and not quite to _A_; and, when she is on the western horizon, he sees a little beyond _A_, and not quite to _B_. _B_ is on the western limb of the moon, and _A_ on her eastern limb. Since this libration is due to the point from which the moon is viewed, it is called _parallactic_ libration; and, since it occurs daily, it is called _diurnal_ libration. [Illustration: Fig. 121.] 107. _Portion of the Lunar Surface brought into View by Libration._--The area brought into view by the first two librations is between one-twelfth and one-thirteenth of the whole lunar surface, or nearly one-sixth of the hemisphere of the moon which is turned away from the earth when the moon is at her state of mean libration. Of course a precisely equal portion of the hemisphere turned towards us during mean libration is carried out of view by the lunar librations.