.

The studies of Hipparchus led him to observe the stars chiefly with
reference to the meridian rather than with reference to their rising,
as had hitherto been the custom. In making these studies of the relative
position of the stars, Hipparchus was led to compare his observations
with those of the Babylonians, which, it was said, Alexander had caused
to be transmitted to Greece. He made use also of the observations
of Aristarchus and others of his Greek precursors. The result of his
comparisons proved that the sphere of the fixed stars had apparently
shifted its position in reference to the plane of the sun's orbit--that
is to say, the plane of the ecliptic no longer seemed to cut the sphere
of the fixed stars at precisely the point where the two coincided in
former centuries. The plane of the ecliptic must therefore be conceived
as slowly revolving in such a way as gradually to circumnavigate the
heavens. This important phenomenon is described as the precession of the
equinoxes.

It is much in question whether this phenomenon was not known to the
ancient Egyptian astronomers; but in any event, Hipparchus is to be
credited with demonstrating the fact and making it known to the
Western world. A further service was rendered theoretical astronomy by
Hipparchus through his invention of the planosphere, an instrument for
the representation of the mechanism of the heavens. His computations
of the properties of the spheres led him also to what was virtually a
discovery of the method of trigonometry, giving him, therefore, a high
position in the field of mathematics. All in all, then, Hipparchus is a
most heroic figure. He may well be considered the greatest star-gazer of
antiquity, though he cannot, without injustice to his great precursors,
be allowed the title which is sometimes given him of "father of
systematic astronomy."

CTESIBIUS AND HERO: MAGICIANS OF ALEXANDRIA

Just about the time when Hipparchus was working out at Rhodes his
puzzles of celestial mechanics, there was a man in Alexandria who was
exercising a strangely inventive genius over mechanical problems of
another sort; a man who, following the example set by Archimedes a
century before, was studying the problems of matter and putting his
studies to practical application through the invention of weird devices.
The man's name was Ctesibius. We know scarcely more of him than that he
lived in Alexandria, probably in the first half of the second century
B