The first of these is that discovered by Laplace,
though its value was overestimated by him, in which the perturbations
of the earth by the planets indirectly affect the motion of the
moon. The remaining part of the acceleration of our satellite is
apparent rather than real, it is not that the moon is moving more
quickly, but that our time-piece, the earth, is revolving more
slowly, and is thus actually losing time. It is interesting to note
that we can detect a physical explanation for the apparent checking
of the earth's motion which is thus manifested. The tides which ebb
and flow on the earth exert a brake-like action on the revolving
globe, and there can be no doubt that they are gradually reducing its
speed, and thus lengthening the day. It has accordingly been
suggested that it is this action of the tides which produces the
supplementary effect necessary to complete the physical explanation
of the lunar acceleration, though it would perhaps be a little
premature to assert that this has been fully demonstrated.
The third of Professor Adams' most notable achievements was connected
with the great shower of November meteors which astonished the world
in 1866. This splendid display concentrated the attention of
astronomers on the theory of the movements of the little objects by
which the display was produced.
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