Darwin
A third hypothesis of the Moon"s origin is due principally to
Darwin. He and Poincare have shown that a great rotating mass
of fluid matter, such as the Earth-Moon could be assumed to
have been, by cooling, contracting and increasing rotation
speed, would, under certain conditions thought to be
reasonable, become unstable and eventually divide into two
bodies revolving around their common center of mass, at first
with their surfaces nearly in contact. Here would begin to act
a tide-raising force which must have played, according to
Darwin"s deductions, a most important part in the further
history of the Earth and Moon. The Earth would produce enormous
tides in the Moon, and the Moon much smaller tides in the
Earth. Both bodies would contract in size, through loss of
heat, and would try to rotate more and more rapidly. The two
rotating bodies would try to carry the matter in the tidal
waves around with the rest of the materials in the bodies, but
the pull of each body upon the wave materials in the other
would tend to slow down the speed of rotation. The tidal
resistance to rotation would be slight if the bodies at any
time were attenuated gaseous masses, for the friction within
the surface strata would be slight. Nevertheless, there would
eventually be a gradual slowing down of the Moon"s rotation, a
gradual slowing down of the Earth"s rotation, and a slow
increase in the distance between the two bodies. In other
words, the Moon"s day, the Earth"s day and our month would
gradually increase in length. Carried to its logical
conclusion, the Moon would eventually turn the same face to the
Earth, the Earth would eventually turn the same face to the
Moon, and the Earth"s day and the Moon"s day would equal the
month in length. The central idea in this logic is as old as
Kant: in 1754 he published an important paper in which he said
that tidal interactions between Earth and Moon had caused the
Moon to keep the same face turned toward us, that the Earth"s
day was being very slowly lengthened, and that our planet would
eventually turn the same face to the Moon. Laplace, a
half-century later, proposed the action of such a force in
connection with the explanation of lunar phenomena, and
Helmholtz, just 100 years after Kant"s paper was published,
lent his support to this principle; but Sir George Darwin has
been the great contributor to the subject. His popular volume,
'The Tides,' devotes several chapters to the effects of tidal
friction upon the motions of two bodies in mutual revolution.
We must pass over the difficult and complicated intermediate
steps to Darwin"s conclusions concerning the Earth and Moon,
which are substantially as follows: the Earth and Moon were
originally much closer together than they now are: after a very
long period of time, amounting to hundreds of millions of
years, the Moon will revolve around the Earth in 55 days
instead of in 27 days as at present; and the Moon and Earth
will then present the same faces constantly to each other. The
estimated period of time required, and the final length of day
and month, 55 days, are of course not insisted upon as accurate
by Darwin.
