Even in a thousand
years the change in the length of the day is only a fraction of a
second. But the importance arises from the fact that the change, slow
though it is, lies always in one direction. The day is continually
increasing. In millions of years the accumulated effect becomes not only
appreciable, but even of startling magnitude.
The change in the length of the day must involve a corresponding change
in the motion of the moon. If the moon acts on the earth and retards the
rotation of the earth, so, conversely, does the earth react upon the
moon. The earth is tormented by the moon, so it strives to drive away
its persecutor. At present the moon revolves around the earth at a
distance of about 240,000 miles. The reaction of the earth tends to
increase this distance, and to force the moon to revolve in an orbit
which is continually growing larger and larger. As thousands of years
roll on, the length of the day increases second by second, and the
distance of the moon increases mile by mile. A million years ago the
day, probably, contained some minutes less than our present day of
twenty-four hours. Our retrospect does not halt here; we at once project
our view back to an incredibly remote epoch which was a crisis in the
history of our system.
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