h that fundamental part of astronomy known as "celestial
mechanics" lies outside the scope of this work, and we therefore pass
over in silence the immense labours of Plana, Damoiseau, Hansen,
Delaunay, G. W. Hill, and Airy in reconciling the observed and
calculated motions of the moon, there is one slight but significant
discrepancy which is of such importance to the physical history of the
solar system, that some brief mention must be made of it.

Halley discovered in 1693, by examining the records of ancient eclipses,
that the moon was going faster then than 2,000 years previously--so much
faster, as to have got ahead of the place in the sky she would otherwise
have occupied, by about two of her own diameters. It was one of
Laplace's highest triumphs to have found an explanation of this puzzling
fact. He showed, in 1787, that it was due to a very slow change in the
ovalness of the earth's orbit, tending, during the present age of the
world, to render it more nearly circular. The pull of the sun upon the
moon is thereby lessened; the counter-pull of the earth gets the upper
hand; and our satellite, drawn nearer to us by something less than an
inch each year,[954] proportionately quickens her pace. Many thousands
of years hence the process will be reversed; the terrestrial orbit will
close in at the sides, the lunar orbit will open out under the growing
stress of solar gravity, and our celestial chronometer will lose instead
of gaining time.

This is all quite true as Laplace put it; but it is not enough. Adams,
the virtual discoverer of Neptune, found with surprise in 1853 that the
received account of the matter was "essentially incomplete," and
explained, when the requisite correction was introduced, only half the
observed acceleration.[955] What was to be done with the remaining half?
Here Delaunay, the eminent French mathematical astronomer, unhappily
drowned at Cherbourg in 1872 by the capsizing of a pleasure-boat, came
to the rescue.[956]

It is obvious to anyone who considers the subject a little attentively,
that the tides must act to some extent as a friction-brake upon the
rotating earth. In other words, they must bring about an almost
infinitely slow lengthening of the day. For the two masses of water
piled up by lunar influence on the hither and farther sides of our
globe, strive, as it were, to detach themselves from the unity of the
terrestrial spheroid, and to follow the movements of the moon. The moon,