OF ASPECT, ROTATION, AND LIBRATION. 191

tions in latitude and longitude. It remains thatwe should consider what is the actual extent of themoon’s surface which these librations bring into viewin addition to that which is seen when the meancentre is at the actual centre of the lunar disc. Inmaking the inquiry, we must take into account anotherlibration, called the diurnal libration, which dependson the circumstance, that owing to the earth’s rotation,the place of the observer is shifted with respect tothe line joining the centres of the earth and moon.This form of libration might very well be made thesubject of a separate investigation, which would, how-ever, be more tedious than profitable, because theextent and nature of the diurnal libration varies indifferent latitudes and at different seasons. On thispoint, I shall content myself with remarking that ifwe imagine an observer placed at the centre of themoon’s visible disc, a line drawn from him to anystation on the earth would be carried by the earth’srotation along a latitude-parallel, and the angle whichit made at any moment with a line joining the centresof the earth and moon would correspond to the

was carried from O to O'. Thus points M and N are at their meanplace, and points F and G are shifted by an arc equal to O O' fromtheir mean position. The point which in its mean position wouldbe at F is behind the disc, and the po nt which in its mean positionwould he at G has advanced on the disc as to g (G g being anarc equal to 0 O', but foreshortened). It is obvious that whereverO' may be (except at 0), two points only, and those both on theedge of the disc, are in their mean positions (as M and N in thecase illustrated by fig. 77).