compute the actual distance of the sun in terms not merely of the
moon's distance but of the earth's radius. And now there was the
experiment of Eratosthenes to give the length of that radius in
precise terms. In other words, Hipparchus might have measured the
distance of the sun in stadia. But if he had made the
attempt--and, indeed, it is more than likely that he did so--the
elements of error in his measurements would still have kept him
wide of the true figures.

The chief studies of Hipparchus were directed, as we have seen,
towards the sun and the moon, but a phenomenon that occurred in
the year 134 B.C. led him for a time to give more particular
attention to the fixed stars. The phenomenon in question was the
sudden outburst of a new star; a phenomenon which has been
repeated now and again, but which is sufficiently rare and
sufficiently mysterious to have excited the unusual attention of
astronomers in all generations. Modern science offers an
explanation of the phenomenon, as we shall see in due course. We
do not know that Hipparchus attempted to explain it, but he was
led to make a chart of the heavens, probably with the idea of
guiding future observers in the observation of new stars. Here
again Hipparchus was not altogether an innovator, since a chart
showing the brightest stars had been made by Eratosthenes; but
the new charts were much elaborated.

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.C. His antecedents, the
place and exact time of his birth and death, are quite unknown.
Neither are we quite certain as to the precise range of his
studies or the exact number of his discoveries. It appears that
he had a pupil named Hero, whose personality, unfortunately, is
scarcely less obscure than that of his master, but who wrote a
book through which the record of the master's inventions was
preserved to posterity. Hero, indeed, wrote several books, though
only one of them has been preserved. The ones that are lost bear
the following suggestive titles: On the Construction of Slings;
On the Construction of Missiles; On the Automaton; On the Method
of Lifting Heavy Bodies; On the Dioptric or Spying-tube. The work
that remains is called Pneumatics, and so interesting a work it
is as to make us doubly regret the loss of its companion volumes.
Had these other books been preserved we should doubtless have a
clearer insight than is now possible into some at least of the
mechanical problems that exercised the minds of the ancient
philosophers. The book that remains is chiefly concerned, as its
name implies, with the study of gases, or, rather, with the study
of a single gas, this being, of course, the air. But it tells us
also of certain studies in the dynamics of water that are most
interesting, and for the historian of science most important.

Unfortunately, the pupil of Ctesibius, whatever his ingenuity,
was a man with a deficient sense of the ethics of science. He
tells us in his preface that the object of his book is to record
some ingenious discoveries of others, together with additional
discoveries of his own, but nowhere in the book itself does he
give us the, slightest clew as to where the line is drawn between
the old and the new. Once, in discussing the weight of water, he
mentions the law of Archimedes regarding a floating body, but
this is the only case in which a scientific principle is traced


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