stery that puzzled
Halley and his associates all their lives was finally attained.

BRADLEY AND THE ABERRATION OF LIGHT

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 and Kepler.

Bradley's studies led him to discover also the libratory motion of the
earth's axis. "As this appearance of 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 sam