, for near a century and a half
after its foundation, an exclusive one. Delambre remarked that, had all
other materials of the kind been destroyed, the Greenwich records alone
would suffice for the restoration of astronomy. The establishment was
indeed absolutely without a rival.[2] Systematic observations of sun,
moon, stars, and planets were during the whole of the eighteenth century
made only at Greenwich. Here materials were accumulated for the secure
correction of theory, and here refinements were introduced by which the
exquisite accuracy of modern practice in astronomy was eventually
attained.

The chief promoter of these improvements was James Bradley. Few men have
possessed in an equal degree with him the power of seeing accurately,
and reasoning on what they see. He let nothing pass. The slightest
inconsistency between what appeared and what was to be expected roused
his keenest attention; and he never relaxed his mental grip of a subject
until it had yielded to his persistent inquisition. It was to these
qualities that he owed his discoveries of the aberration of light and
the nutation of the earth's axis. The first was announced in 1729. What
is meant by it is that, owing to the circumstance of light not being
instantaneously transmitted, the heavenly bodies appear shifted from
their true places by an amount depending upon the ratio which the
velocity of light bears to the speed of the earth in its orbit. Because
light travels with enormous rapidity, the shifting is very slight; and
each star returns to its original position at the end of a year.

Bradley's second great discovery was finally ascertained in 1748.
Nutation is a real "nodding" of the terrestrial axis produced by the
dragging of the moon at the terrestrial equatorial protuberance. From it
results an _apparent_ displacement of the stars, each of them describing
a little ellipse about its true or "mean" position, in a period of
nearly nineteen years.

Now, an acquaintance with the fact and the laws of each of these minute
irregularities is vital to the progress of observational astronomy; for
without it the places of the heavenly bodies could never be accurately
known or compared. So that Bradley, by their detection, at once raised
the science to a higher grade of precision. Nor was this the whole of
his work. Appointed Astronomer-Royal in 1742, he executed during the
years 1750-62 a series of observations which formed the real beginning
of exa