At present it happens to lie near
the Pole Star, but it will not always remain there. It describes a
circle around the pole of the Ecliptic, requiring about 25,000 years
for a complete circuit. In the course of its progress the pole will
gradually pass now near one star and now near another, so that many
stars will in the lapse of ages discharge the various functions which
the present Pole Star does for us. In about 12,000 years, for
instance, the pole will have come near the bright star, Vega. This
movement of the pole had been known for ages. But what Bradley
discovered was that the pole, instead of describing an uniform
movement as had been previously supposed, followed a sinuous course
now on one side and now on the other of its mean place. This he
traced to the fluctuations of the moon's orbit, which undergoes a
continuous change in a period of nineteen years. Thus the efficiency
with which the moon acts on the protuberant mass of the earth varies,
and thus the pole is caused to oscillate.
This subtle discovery, if perhaps in some ways less impressive than
Bradley's earlier achievements of the detection of the aberration of
light, is regarded by astronomers as testifying even in a higher
degree to his astonishing care and skill as an observer, and justly
entitles him to a unique place among the astronomers whose
discoveries have been effected by consummate practical skill in the
use of astronomical instruments.
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