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 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