Newton then made it plain that the
rise and fall of the water was simply a consequence of the attractive
power which the moon exerted upon the oceans lying upon our globe. He
showed also that to a certain extent the sun produces tides, and he
was able to explain how it was that when the sun and the moon both
conspire, the joint result was to produce especially high tides,
which we call "spring tides"; whereas if the solar tide was low,
while the lunar tide was high, then we had the phenomenon of "neap"
tides.
But perhaps the most signal of Newton's applications of the law of
gravitation was connected with certain irregularities in the
movements of the moon. In its orbit round the earth our satellite
is, of course, mainly guided by the great attraction of our globe. If
there were no other body in the universe, then the centre of the moon
must necessarily perform an ellipse, and the centre of the earth
would lie in the focus of that ellipse. Nature, however, does not
allow the movements to possess the simplicity which this arrangement
would imply, for the sun is present as a source of disturbance. The
sun attracts the moon, and the sun attracts the earth, but in
different degrees, and the consequence is that the moon's movement
with regard to the earth is seriously affected by the influence of
the sun.
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