[PLATE: CAMBRIDGE OBSERVATORY.]
It has been pointed out that, in consequence of the solar
disturbance, the orbit of the moon must be some what enlarged. As it
now appears that the solar disturbance is on the whole declining, it
follows that the orbit of the moon, which has to be adjusted
relatively to the average value of the solar disturbance, must also
be gradually declining. In other words, the moon must be approaching
nearer to the earth in consequence of the alterations in the
eccentricity of the earth's orbit produced by the attraction of the
other planets. It is true that the change in the moon's position
thus arising is an extremely small one, and the consequent effect in
accelerating the moon's motion is but very slight. It is in fact
almost imperceptible, except when great periods of time are
involved. Laplace undertook a calculation on this subject. He knew
what the efficiency of the planets in altering the dimensions of the
earth's orbit amounted to; from this he was able to determine the
changes that would be propagated into the motion of the moon. Thus
he ascertained, or at all events thought he had ascertained, that the
acceleration of the moon's motion, as it had been inferred from the
observations of the ancient eclipses which have been handed down to
us, could be completely accounted for as a consequence of planetary
perturbation.
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