This was known long before the time of Laplace, but it did not
directly convey any explanation of the lunar acceleration. It no
doubt amounted to the assertion that the moon's periodic time was
slightly augmented by the disturbance, but it did not give any
grounds for suspecting that there was a continuous change in
progress. It was, however, apparent that the periodic time was
connected with the solar disturbance, so that, if there were any
alteration in the amount of the sun's disturbing effect, there must
be a corresponding alteration in the moon's periodic time. Laplace,
therefore, perceived that, if he could discover any continuous change
in the ability of the sun for disturbing the moon, he would then have
accounted for a continuous change in the moon's periodic time, and
that thus an explanation of the long-vexed question of the lunar
acceleration might be forthcoming.
The capability of the sun for disturbing the earth-moon system is
obviously connected with the distance of the earth from the sun. If
the earth moved in an orbit which underwent no change whatever, then
the efficiency of the sun as a disturbing agent would not undergo any
change of the kind which was sought for. But if there were any
alteration in the shape or size of the earth's orbit, then that might
involve such changes in the distance between the earth and the sun as
would possibly afford the desired agent for producing the observed
lunar effect.
Pages:
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402