He had indeed proved that the motion
round the focus of the ellipse in which the sun lies is not of this
description. One of his most important discoveries even related to
the fact that at some parts of its orbit a planet swings around the
sun with greater angular velocity than at others. But it so happens
that in elliptic tracks which differ but little from circles, as is
the case with all the more important planetary orbits, the motion
round the empty focus of the ellipse is very nearly uniform. It
seemed natural to assume, that this was exactly the case, in which
event each of the two foci of the ellipse would have had a special
significance in relation to the movement of the planet. The youthful
Halley, however, demonstrated that so far as the empty focus was
concerned, the movement of the planet around it, though so nearly
uniform, was still not exactly so, and at the age of nineteen, he
published a treatise on the subject which at once placed him in the
foremost rank amongst theoretical astronomers.
But Halley had no intention of being merely an astronomer with his
pen. He longed to engage in the practical work of observing. He saw
that the progress of exact astronomy must depend largely on the
determination of the positions of the stars with all attainable
accuracy.
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