(Recall the similar phenomenon in the case of Venus.)
The moon seen from the earth is now in the phase called full moon.
Another lapse of seven days, and the moon is at _D_, in the phase called
third quarter, while the earth, viewed from the cross on the moon, which
is still pointed directly at it, appears again in the shape of a huge
half-moon.
During the next seven days the moon returns to its original 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.
Pages:
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182