nal position at
_A_, and becomes once more new moon, with "full earth" shining upon it.

Now it is evident that in consequence of the peculiar law of the moon's
rotation its days and nights are each about two of our weeks, or
fourteen days, in length. That hemisphere of the moon which is in the
full sunlight at _A_, for instance, is buried in the middle of night at
_C_. The result is different than in the case of Mercury, because the
body toward which the moon always keeps the same face directed is not
the luminous sun, but the non-luminous earth.

It is believed that the moon acquired this manner of rotation in
consequence of the tidal friction exercised upon it by the earth. The
tidal attraction of the earth exceeds that of the sun upon the moon
because the earth is so much nearer than the sun is, and tidal
attraction varies inversely as the cube of the distance. In fact, the
braking effect of tidal friction varies inversely as the sixth power of
the distance, so that the ability of the earth to stop the rotation of
the moon on its axis is immensely greater than that of the sun. This
power was effectively applied while the moon was yet a molten mass, so
that it is probable that the moon has rotated just as it does now for
millions of years.

As was remarked a little while ago, the moon traveling in an elliptical
orbit about the earth has a libratory movement which, if represented in
our picture, would cause the cross to swing now a little one way and now
a little the other, and thus produce an apparent pendulum motion of the
earth in the sky, similar to that of the sun as seen from Mercury. But
it is not necessary to go into the details of this phenomenon. The
reader, if he chooses, can deduce them for himself.

But we may inquire a little into the effects of the long days and nights
of the moon. In consequence of the extreme rarity of the lunar
atmosphere, it is believed that the heat of the sun falling upon it
during a day two weeks in length, is radiated away so rapidly that the
surface of the lunar rocks never rises above the freezing temperature
of water. On the night side, with no warm atmospheric blanket such as
the earth enjoys, the temperature may fall far toward absolute zero, the
most merciful figure that has been suggested for it being 200 deg. below
the zero of our ordinary thermometers! But there is much uncertainty
about the actual temperature on the moon, and different experiments, in
the attempt t