But all these must be regarded as his minor
labours, for next to the discovery of Neptune the fame of Adams
mainly rests on his researches upon certain movements of the moon,
and upon the November meteors.
The periodic time of the moon is the interval required for one
circuit of its orbit. This interval is known with accuracy at the
present day, and by means of the ancient eclipses the period of the
moon's revolution two thousand years ago can be also ascertained. It
had been discovered by Halley that the period which the moon requires
to accomplish each of its revolutions around the earth has been
steadily, though no doubt slowly, diminishing. The change thus
produced is not appreciable when only small intervals of time are
considered, but it becomes appreciable when we have to deal with
intervals of thousands of years. The actual effect which is produced
by the lunar acceleration, for so this phenomenon is called, may be
thus estimated. If we suppose that the moon had, throughout the
ages, revolved around the earth in precisely the same periodic time
which it has at present, and if from this assumption we calculate
back to find where the moon must have been about two thousand years
ago, we obtain a position which the ancient eclipses show to be
different from that in which the moon was actually situated.
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