The time was now approaching when Bradley was to make the first of
those two great discoveries by which his name has acquired a lustre
that has placed him in the very foremost rank of astronomical
discoverers. As has been often the case in the history of science,
the first of these great successes was attained while he was pursuing
a research intended for a wholly different purpose. It had long been
recognised that as the earth describes a vast orbit, nearly two
hundred million miles in diameter, in its annual journey round the
sun, the apparent places of the stars should alter, to some extent,
in correspondence with the changes in the earth's position. The
nearer the star the greater the shift in its apparent place on the
heavens, which must arise from the fact that it was seen from
different positions in the earth's orbit. It had been pointed out
that these apparent changes in the places of the stars, due to the
movement of the earth, would provide the means of measuring the
distances of the stars. As, however, these distances are enormously
great in comparison with the orbit which the earth describes around
the sun, the attempt to determine the distances of the stars by the
shift in their positions had hitherto proved ineffectual.
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