Lagrange explained it as due to an oscillatory change
in the earth's orbit, thus fully exonerating the moon,
and seeming to demonstrate the absolute stability of
our planetary system, which the moon's misbehavior
had appeared to threaten.

This highly satisfactory conclusion was an orthodox
belief of celestial mechanics until 1853, when Professor
Adams of Neptunian fame, with whom complex analyses
were a pastime, reviewed Laplace's calculation,
and discovered an error which, when corrected, left
about half the moon's acceleration unaccounted for.
This was a momentous discrepancy, which at first no
one could explain. But presently Professor Helmholtz,
the great German physicist, suggested that a key
might be found in tidal friction, which, acting as a perpetual
brake on the earth's rotation, and affecting not
merely the waters but the entire substance of our
planet, must in the long sweep of time have changed its
rate of rotation. Thus the seeming acceleration of the
moon might be accounted for as actual retardation of
the earth's rotation--a lengthening of the day instead
of a shortening of the month.

Again the earth was shown to be at fault, but this
time the moon could not be exonerated, while the
estimated stability of our system, instead of being
re-established, was quite upset. For the tidal retardation
is not an oscillatory change which will presently
correct itself, like the orbital wobble, but a