has been no external
interference, then it cannot be doubted that the moon and the earth were
once in immediate proximity. We can, indeed, calculate the period in
which the moon must have been revolving round the earth. The nearer the
moon is to the earth the quicker it must revolve; and at the critical
epoch when the satellite was in immediate proximity to our earth it
must have completed each revolution in about three or four hours.

This has led to one of the most daring speculations which has ever been
made in astronomy. We cannot refrain from enunciating it; but it must be
remembered that it is only a speculation, and to be received with
corresponding reserve. The speculation is intended to answer the
question, What brought the moon into that position, close to the surface
of the earth? We will only say that there is the gravest reason to
believe that the moon was, at some very early period, fractured off from
the earth when the earth was in a soft or plastic condition.

At the beginning of the history we found the earth and the moon close
together. We found that the rate of rotation of the earth was only a few
hours, instead of twenty-four hours. We found that the moon completed
its journey round the primitive earth in exactly the same time as the
primitive earth rotated on its axis, so that the two bodies were then
constantly face to face. Such a state of things formed what a
mathematician would describe as a case of unstable dynamical
equilibrium. It could not last. It may be compared to the case of a
needle balanced on its point; the needle must fall to one side or the
other. In the same way, the moon could not continue to preserve this
position. There were two courses open: the moon must either have fallen
back on the earth, and been reabsorbed into the mass of the earth, or it
must have commenced its outward journey. Which of these courses was the
moon to adopt? We have no means, perhaps, of knowing exactly what it was
which determined the moon to one course rather than to another, but as
to the course which was actually taken there can be no doubt. The fact
that the moon exists shows that it did not return to the earth, but
commenced its outward journey. As the moon recedes from the earth it
must, in conformity with Kepler's laws, require a longer time to
complete its revolution. It has thus happened that, from the original
period of only a few hours, the duration has increased until it has
reached the pres