Although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures--advertisement posters, for instance--in which stars appear _within_ the horns of a crescent moon! Can it be that there are to-day educated persons who believe that the moon is a thing which _grows_ to a certain size and then wastes away again; who, in fact, do not know that the entire body of the moon is there all the while? When the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. This appearance is popularly known as the "old moon in the new moon's arms." The dark part of her surface must, indeed, be to some degree illumined, or we should not be able to see it at all. Whence then comes the light which illumines it, since it clearly cannot come from the sun? The riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. He would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sunlight. The lunar landscape all around would, therefore, be bathed in what to _him_ would be "earthlight," which of course takes the place there of what _we_ call moonlight. If, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. It is considered, indeed, to be some twenty times brighter. It is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth. The ancients were greatly exercised in their minds to account for this "earthlight," or "earthshine," as it is also called. Posidonius (135-51 B.C.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. It was not, however, until the fifteenth century that the correct solution was arrived at. [Illustration: One side of the moon only is ever presented to the earth. This side is here indicated by the letters S.F.E. (side facing earth). By placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth. FIG. 15.--The Rotation of the Moon on her Axis.]