the revolutionary doctrine that our world is not the centre of the
universe. We should not have to tell of the persecution of a Bruno or
of a Galileo for teaching this doctrine in the seventeenth century of
an era which did not begin till two hundred years after the death of
Aristarchus. But, as we know, the teaching of the astronomer of Samos
did not win its way. The old conservative geocentric doctrine, seemingly
so much more in accordance with the every-day observations of
mankind, supported by the majority of astronomers with the Peripatetic
philosophers at their head, held its place. It found fresh supporters
presently among the later Alexandrians, and so fully eclipsed the
heliocentric view that we should scarcely know that view had even found
an advocate were it not for here and there such a chance record as the
phrases we have just quoted from Archimedes. Yet, as we now see, the
heliocentric doctrine, which we know to be true, had been thought out
and advocated as the correct theory of celestial mechanics by at least
one worker of the third century B.C. Such an idea, we may be sure, did
not spring into the mind of its originator except as the culmination of
a long series of observations and inferences. The precise character of
the evolution we perhaps cannot trace, but its broader outlines are open
to our observation, and we may not leave so important a topic without at
least briefly noting them.

Fully to understand the theory of Aristarchus, we must go back a century
or two and recall that as long ago as the time of that other great
native of Samos, Pythagoras, the conception had been reached that the
earth is in motion. We saw, in dealing with Pythagoras, that we could
not be sure as to precisely what he himself taught, but there is no
question that the idea of the world's motion became from an early day a
so-called Pythagorean doctrine. While all the other philosophers, so far
as we know, still believed that the world was flat, the Pythagoreans out
in Italy taught that the world is a sphere and that the apparent motions
of the heavenly bodies are really due to the actual motion of the earth
itself. They did not, however, vault to the conclusion that this true
motion of the earth takes place in the form of a circuit about the sun.
Instead of that, they conceived the central body of the universe to be a
great fire, invisible from the earth, because the inhabited side of the
terrestrial ball was turned away from