nt; and this base-line of the earth's diameter gives us again
some extra glimpses. This diurnal or parallactic libration is
really more effective than the other two in extending our vision
into the space-facing hemisphere of the moon.

These simple matters may as well be understood, but there is
nothing in them to dwell upon. The far side of the moon is probably
but little worth seeing. Its features are likely to be more blurred
with accumulations of meteoric dust than are those of our side, but
otherwise they are likely to be of the same general character.

The thing of real interest is the fact that the moon does turn the same
face towards us; _i.e._ has ceased to rotate with respect to the earth
(if ever it did so). The stability of this state of things was shown by
Lagrange to depend on the shape of the moon. It must be slightly
egg-shape, or prolate--extended in the direction of the earth; its
earth-pointing diameter being a few hundred feet longer than its visible
diameter; a cause slight enough, but nevertheless sufficient to maintain
stability, except under the action of a distinct disturbing cause. The
prolate or lemon-like shape is caused by the gravitative pull of the
earth, balanced by the centrifugal whirl. The two forces balance each
other as regards motion, but between them they have strained the moon a
trifle out of shape. The moon has yielded as if it were perfectly
plastic; in all probability it once was so.

It may be interesting to note for a moment the correlative effect of
this aspect of the moon, if we transfer ourselves to its surface in
imagination, and look at the earth (cf. Fig. 41). The earth would be
like a gigantic moon of four times our moon's diameter, and would go
through its phases in regular order. But it would not rise or set: it
would be fixed in the sky, and subject only to a minute oscillation to
and fro once a month, by reason of the "libration" we have been speaking
of. Its aspect, as seen by markings on its surface, would rapidly
change, going through a cycle in twenty-four hours; but its permanent
features would be usually masked by lawless accumulations of cloud,
mainly aggregated in rude belts parallel to the equator. And these
cloudy patches would be the most luminous, the whitest portions; for of
course it would be their silver lining that we would then be looking
on.[24]

Next among the investigations of Lagrange and Laplace we wi