he crescent; after which her motion carries her into the
neighbourhood of the sun, and she is once more new, and lost to our
sight in the solar glare. Following this she draws away to the east of
the sun again, and the old order of phases repeat themselves as before.

The early Babylonians imagined that the moon had a bright and a dark
side, and that her phases were caused by the bright side coming more and
more into view during her movement around the sky. The Greeks, notably
Aristotle, set to work to examine the question from a mathematical
standpoint, and came to the conclusion that the crescent and other
appearances were such as would necessarily result if the moon were a
dark body of spherical shape illumined merely by the light of the sun.

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 i