It is, therefore, impossible for us to suppose a day much
shorter than three hours.
Let us leave the earth for a few minutes and examine the past history
of the moon. We have seen the moon revolve around the earth in an
ever-widening orbit, and consequently the moon must, in ancient times,
have been nearer the earth than it is now. No doubt the change is slow.
There is not much difference between the orbit of the moon a thousand
years ago and the orbit in which the moon is now moving. But when we
rise to millions of years, the difference becomes very appreciable.
Thirty or forty millions of years ago the moon was much closer to the
earth than it is at present; very possibly the moon was then only half
its present distance. We must, however, look still earlier, to a certain
epoch not less than fifty million of years ago. At that epoch the moon
must have been so close to the earth that the two bodies were almost
touching. Everybody knows that the moon revolves now around the earth
in a period of twenty-seven days. The period depends upon the distance
between the earth and the moon. In earlier times the month must have
been shorter than our present month.
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