ledge of geometry to perceive that
correct ideas on this point could be readily obtained by measuring a
degree on her surface. Probably there were early attempts to accomplish
this object, the results of which have been lost. But Eratosthenes
executed one between Syene and Alexandria, in Egypt, Syene being
supposed to be exactly under the tropic of Cancer. The two places are,
however, not on the same meridian, and the distance between them was
estimated, not measured. Two centuries later, Posidonius made another
attempt between Alexandria and Rhodes; the bright star Canopus just
grazed the horizon at the latter place, at Alexandria it rose 7 1/2
degrees. In this instance, also, since the direction lay across the sea,
the distance was estimated, not measured. Finally, as we have already
related, the Khalif Al-Mamun made two sets of measures, one on the shore
of the Red Sea, the other near Cufa, in Mesopotamia. The general result
of these various observations gave for the earth's diameter between
seven and eight thousand miles.

This approximate determination of the size of the earth tended to
depose her from her dominating position, and gave rise to very serious
theological results. In this the ancient investigations of Aristarchus
of Samos, one of the Alexandrian school, 280 B.C., powerfully aided.
In his treatise on the magnitudes and distances of the sun and moon, he
explains the ingenious though imperfect method to which he had resorted
for the solution of that problem. Many ages previously a speculation had
been brought from India to Europe by Pythagoras. It presented the sun
as the centre of the system. Around him the planets revolved in circular
orbits, their order of position being Mercury, Venus, Earth, Mars,
Jupiter, Saturn, each of them being supposed to rotate on its axis as it
revolved round the sun. According to Cicero, Nicetas suggested that,
if it were admitted that the earth revolves on her axis, the difficulty
presented by the inconceivable velocity of the heavens would be avoided.

There is reason to believe that the works of Aristarchus, in the
Alexandrian Library, were burnt at the time of the fire of Caesar. The
only treatise of his that has come down to us is that above mentioned,
on the size and distance of the sun and moon.

Aristarchus adopted the Pythagorean system as representing the actual
facts. This was the result of a recognition of the sun's amazing
distance, and therefore of his enormo