energetically, and consequently there is an increase in the distance
between the earth and the moon. Similarly when the moon happens to
lie on the other side of the earth, so that the earth is interposed
directly between the moon and the sun, the solar attraction exerted
upon the earth is more powerful than the same influence upon the
moon. Consequently in this case, also, the distance of the moon from
the earth is increased by the solar disturbance. These instances
will illustrate the general truth, that, as one of the consequences
of the disturbing influence exerted by the sun upon the earth-moon
system, there is an increase in the dimensions of the average orbit
which the moon describes around the earth. As the time required by
the moon to accomplish a journey round the earth depends upon its
distance from the earth, it follows that among the influences of the
sun upon the moon there must be an enlargement of the periodic time,
from what it would have been had there been no solar disturbing
action.

This was known long before the time of Laplace, but it did not
directly convey any explanation of the lunar acceleration. It no
doubt amounted to the assertion that the moon's periodic time was
slightly augmented by the disturbance, but it did not give any
grounds for suspecting that there was a continuous change in
progress. It was, however, apparent that the periodic time was
connected with the solar disturbance, so that, if there were any
alteration in the amount of the sun's disturbing effect, there must
be a corresponding alteration in the moon's periodic time. Laplace,
therefore, perceived that, if he could discover any continuous change
in the ability of the sun for disturbing the moon, he would then have
accounted for a continuous change in the moon's periodic time, and
that thus an explanation of the long-vexed question of the lunar