interval between the position in which the moon would have been found
two thousand years ago if there had been no acceleration, and the
position in which the moon was actually placed, amounts to about a
degree, that is to say, to an arc on the heavens which is twice the
moon's apparent diameter.

If no other bodies save the earth and the moon were present in the
universe, it seems certain that the motion of the moon would never
have exhibited this acceleration. In such a simple case as that
which I have supposed the orbit of the moon would have remained for
ever absolutely unchanged. It is, however, well known that the
presence of the sun exerts a disturbing influence upon the movements
of the moon. In each revolution our satellite is continually drawn
aside by the action of the sun from the place which it would
otherwise have occupied. These irregularities are known as the
perturbations of the lunar orbit, they have long been studied, and
the majority of them have been satisfactorily accounted for. It
seems, however, to those who first investigated the question that the
phenomenon of the lunar acceleration could not be explained as a
consequence of solar perturbation, and, as no other agent competent
to produce such effects was recognised by astronomers, the lunar
acceleration presented an unsolved enigma.

At the end of the last century the illustrious French mathematician
Laplace undertook a new investigation of the famous problem, and was
rewarded with a success which for a long time appeared to be quite
complete. Let us suppose that the moon lies directly between the
earth and the sun, then both earth and moon are pulled towards the
sun by the solar attraction; as, however, the moon is the nearer of
the two bodies to the attracting centre it is pulled the more