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1787 he gave the true explanation. The
principal effect of the sun on the moon’s or-
bit is to diminish the earth’s influence, thus
lengthening the period to a new value gen-
erally taken as constant. But Laplace’s cal-
culations showed the new value to depend
upon the excentricity of the earth’s orbit,
which, according; to theory, has a period-
ical variation of enormous period, and has
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