We can now explain a beautiful phenomenon that you will see when the
moon is still quite young. We fancifully describe the old moon as lying
in the new moon’s arms when we observe the faintly illuminated portion
of the rest of that circle, of which a part is the brilliant crescent.
This can only be explained by showing how some light has fallen on the
shadowed side; for nothing which is not itself a source of light can
ever become visible unless illuminated by light from some other body.
[Illustration: FIG. 31.--The Changes in the Moon.]
Let us suppose that there is a man on the moon who is looking at the
earth. To him the earth will appear in the same way as the moon appears
to us, only very much larger. At the time of new moon the bright side
of the earth will be turned directly towards him, so that the man in
the moon will see an earth nearly full, and consequently pouring
forth a large flood of light. Think of the brightest of all the bright
moonlight nights you have ever seen on earth, and then think of a light
which would be produced if you had thirteen moons, all as big and as
bright as our full moon, shining together. How splendid the night
would then be! You would be able to read a book quite easily! Well,
that is the sort of illumination which the lunar man will enjoy under
these circumstances; all the features of his country will be brightly
lighted up by the full earth. Of course, this earth-lighted side of
the moon cannot be compared in brilliancy with the sun-lighted side,
but the brightness will still be perceptible, so that when from the
earth we look at the moon, we see this glow distributed all over the
dark portion; that is, we observe the feebly lighted globe clasped in
the brilliant arms of the crescent. At a later phase the dark part of
the moon entirely ceases to be visible, and this for a double reason:
firstly, the bright side of the earth is then not so fully turned to
the moon, and therefore the illumination it receives from earth-shine
is not so great; and, secondly, the increasing size of the sun-lighted
part of the moon has such an augmented glow that the fainter light is
overpowered by contrast. You must remember that more light does not
always increase the number of things that can be seen. It has sometimes
the opposite effect. Have we not already mentioned how the brightness
of day makes the stars invisible? The moon herself, seen in full
daylight, seems no brighter than a small particle of white cloud.