London _Nature_, in a recent issue, says: From a scientific point of
view, the work done by the tides is of unspeakable importance. Whence is
this energy derived with which the tides do their work? If the tides are
caused by the moon, the energy they possess must also be derived from
the moon. This looks plain enough, but unfortunately it is not true.
Would it be true to assert that the finger of the rifleman which pulls
the trigger supplies the energy with which the rifle bullet is animated?
Of course it would not. The energy is derived from the explosion of
gunpowder, and the pulling of the trigger is merely the means by which
that energy is liberated.
In somewhat similar manner the tidal wave produced by the moon is the
means whereby a part of the energy stored in the earth is compelled to
expend itself in work. Let me illustrate this by a comparison between
the earth rotating on its axis and the fly-wheel of an engine: The fly
wheel is a sort of reservoir, into which the engine pours its power at
each stroke of the piston. The various machines in the mill merely draw
off the power from the store accumulated in the fly-wheel. The earth
is like a gigantic fly-wheel detached from the engine, though still
connected with the machines in the mill. In that mighty fly-wheel a
stupendous quantity of energy is stored up, and a stupendous quantity of
energy would be given out before that fly-wheel would come to rest. The
earth's rotation is a reservoir from whence the tides draw the energy
they require for doing work. Hence it is that though the tides are
caused by the moon, yet whenever they require energy they draw on the
supply ready to hand in the rotation of the earth. The earth differs
from the fly-wheel of an engine in a very important point. As the energy
is withdrawn from the fly-wheel by the machines in the mill, so it is
restored thereto by the power of the steam engine, and the fly runs
uniformly. But the earth is merely the fly-wheel without the engine.
When the work by the tides withdraws energy from the earth, that energy
is never restored. It, therefore, follows that the earth's rotation
must be decreasing. This leads to a consequence of the most wonderful
importance. It tells us that the speed with which the earth rotates on
its axis is diminishing. We can state the result in a manner which has
the merits of simplicity and brevity. The tides are increasing the
length of the day. At present, no doubt, the effect of the tides
in changing the length of the day is very small. A day now is not
appreciably longer than a day a hundred years ago. 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. It must have been at least 50,000,000 years ago.
It may have been very much earlier. This crisis was the interesting
occasion when the moon was born. The length of the day was only a very
few hours. If we call it three hours we shall not be far from the truth.
Purhaps you may think that if we looked back to a still earlier epoch,
the day would become still less, and finally disappear altogether. This
is, however, not the case. The day can never have been much less than
three hours in the present order of things. Everybody knows that the
earth is not a sphere, but there is a protuberance at the equator, so
that, as our school books tell us, the earth is shaped like an orange.
It is well known that this protuberance is due to the rotation of
the earth on its axis, by which the equatorial parts bulge out by
centrifugal force. The quicker the earth rotates the greater is the
protuberance. If, however, the rate of rotation exceeds a certain limit,
the equatorial portion of the earth could no longer cling together. The
attraction which unites them would be overcome by centrifugal force, and
a general break up would occur. It can be shown that the rotation of the
earth, when on the point of rupture, corresponds to a length of the day
somewhere about the critical value of three hours, which we have already
adopted. 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. Some millions of years ago the moon
completed its journey in a week instead of taking twenty-eight days as
at present. Looking back earlier still, we find the month has dwindled
down to a day, then down to a few hours, until at that wondrous epoch
when the moon was almost touching the earth, the moon spun around the
earth once every three hours.
In those ancient times I see our earth to be a noble globe, as it is as
present. Yet it is not partly covered with oceans and partly clothed
with verdure. The primeval earth seems rather a fiery and half-molten
mass, where no organic life can dwell. Instead of the atmosphere which
we now have, I see a dense mass of vapors in which perhaps, all the
oceans of the earth are suspended as clouds. I see that the sun still
rises and sets to give the succession of day and of night, but the
day and the night together only amounted to three hours, instead of
twenty-four. Almost touching the chaotic mass of the earth is another
much smaller and equally chaotic body. Around the earth I see this small
body rapidly rotating, the two revolving together, as if they were bound
by invisible bands. The smaller body is the moon.
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