||Halley was succeeded
as astronomer royal by a man whose useful additions to the science were
not to be recognized or appreciated fully until brought to light by the
Prussian astronomer Bessel early in the nineteenth century. This was Dr.
James Bradley, an ecclesiastic, who ranks as one of the most eminent astronomers
of the eighteenth century. His most remarkable discovery was the explanation
of a peculiar motion of the pole-star, first observed, but not explained,
by Picard a century before. For many years a satisfactory explanation was
sought unsuccessfully by Bradley and his fellow-astronomers, but at last
he was able to demonstrate that the stary Draconis, on which he was making
his observations, described, or appeared to describe, a small ellipse.
If this observation was correct, it afforded a means of computing the aberration
of any star at all times. The explanation of the physical cause of this
aberration, as Bradley thought, and afterwards demonstrated, was the result
of the combination of the motion of light with the annual motion of the
earth. Bradley first formulated this theory in 1728, but it was not until
1748 - twenty years of continuous struggle and observation by him - that
he was prepared to communicate the results of his efforts to the Royal
Society. This remarkable paper is thought by the Frenchman, Delambre, to
entitle its author to a place in science beside such astronomers as Hipparcbus
Bradley's studies led him to discover also
the libratory motion of the earth's axis. "As this appearance of g Draconis.
indicated a diminution of the inclination of the earth's axis to the plane
of the ecliptic," he says; "and as several astronomers have supposed THAT
inclination to diminish regularly; if this phenomenon depended upon such
a cause, and amounted to 18" in nine years, the obliquity of the ecliptic
would, at that rate, alter a whole minute in thirty years; which is much
faster than any observations, before made, would allow. I had reason, therefore,
to think that some part of this motion at the least, if not the whole,
was owing to the moon's action upon the equatorial parts of the earth;
which, I conceived, might cause a libratory motion of the earth's axis.
But as I was unable to judge, from only nine years observations, whether
the axis would entirely recover the same position that it had in the year
1727, I found it necessary to continue my observations through a whole
period of the moon's nodes; at the end of which I had the satisfaction
to see, that the stars, returned into the same position again; as if there
had been no alteration at all in the inclination of the earth's axis; which
fully convinced me that I had guessed rightly as to the cause of the phenomena.
This circumstance proves likewise, that if there be a gradual diminution
of the obliquity of the ecliptic, it does not arise only from an alteration
in the position of the earth's axis, but rather from some change in the
plane of the ecliptic itself; because the stars, at the end of the period
of the moon's nodes, appeared in the same places, with respect to the equator,
as they ought to have done, if the earth's axis had retained the same inclination
to an invariable plane."