great
predecessor of Samos, but he declined to accept his theories. For him
the world was central; his problem was to explain, if he could, the
irregularities of motion which sun, moon, and planets showed in
their seeming circuits about the earth. Hipparchus had the gnomon of
Eratosthenes--doubtless in a perfected form--to aid him, and he soon
proved himself a master in its use. For him, as we have said, accuracy
was everything; this was the one element that led to all his great
successes.

Perhaps his greatest feat was to demonstrate the eccentricity of the
sun's seeming orbit. We of to-day, thanks to Keppler and his followers,
know that the earth and the other planetary bodies in their circuit
about the sun describe an ellipse and not a circle. But in the day of
Hipparchus, though the ellipse was recognized as a geometrical figure
(it had been described and named along with the parabola and hyperbola
by Apollonius of Perga, the pupil of Euclid), yet it would have been the
rankest heresy to suggest an elliptical course for any heavenly body.
A metaphysical theory, as propounded perhaps by the Pythagoreans but
ardently supported by Aristotle, declared that the circle is the perfect
figure, and pronounced it inconceivable that the motions of the spheres
should be other than circular. This thought dominated the mind of
Hipparchus, and so when his careful measurements led him to the
discovery that the northward and southward journeyings of the sun did
not divide the year into four equal parts, there was nothing open to him
but to either assume that the earth does not lie precisely at the centre
of the sun's circular orbit or to find some alternative hypothesis.

In point of fact, the sun (reversing the point of view in accordance
with modern discoveries) does lie at one focus of the earth's elliptical
orbit, and therefore away from the physical centre of that orbit; in
other words, the observations of Hipparchus were absolutely accurate. He
was quite correct in finding that the sun spends more time on one side
of the equator than on the other. When, therefore, he estimated the
relative distance of the earth from the geometrical centre of the sun's
supposed circular orbit, and spoke of this as the measure of the sun's
eccentricity, he propounded a theory in which true data of observation
were curiously mingled with a positively inverted theory. That the
theory of Hipparchus was absolutely consistent with all the facts of