We have already seen that the
tidal influence of the sun has the effect of moderating the
rotation of the planets. Now, this very rapid rotation of a
liquid mass, with a thin crust, would (together with the
instability occasioned by its cooling) cause it to bulge at the
equator. The bulge would increase until the earth became a
pear-shaped body. The small end of the pear would draw further
and further away from the rest--as a drop of water does on the
mouth of a tap--and at last the whole mass (some 5,000,000,000
cubic miles of matter) was broken off, and began to pursue an
independent orbit round the earth.
There are astronomers who think that other cosmic bodies, besides
our moon, may have been formed in this way. Possibly it is true
of some of the double stars, but we will not return to that
question. The further story of the moon, as it is known to
astronomers, may be given in a few words. The rotational movement
of the earth is becoming gradually slower on account of tidal
influence; our day, in fact, becomes an hour longer every few
million years. It can be shown that this had the effect of
increasing the speed, and therefore enlarging the orbit, of the
moon, as it revolved round the earth.
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