e year 433 B.C., but the length of the hour
varied with the time of the year, since the Greeks divided the day into
twelve equal parts. Dials were common at Rome in the time of Plautus,
224 B.C.; but there was a difficulty in using them, since they failed at
night and in cloudy weather, and could not be relied on. Hence the
introduction of water-clocks instead.

Aristarchus is said to have combated (280 B.C.) the geocentric theory so
generally received by philosophers, and to have promulgated the
hypothesis "that the fixed stars and the sun are immovable; that the
earth is carried round the sun in the circumference of a circle of
which the sun is the centre; and that the sphere of the fixed stars,
having the same centre as the sun, is of such magnitude that the orbit
of the earth is to the distance of the fixed stars as the centre of the
sphere of the fixed stars is to its surface." Aristarchus also,
according to Plutarch, explained the apparent annual motion of the sun
in the ecliptic by supposing the orbit of the earth to be inclined to
its axis. There is no evidence that this great astronomer supported his
heliocentric theory with any geometrical proof, although Plutarch
maintains that he demonstrated it. This theory gave great offence,
especially to the Stoics; and Cleanthes, the head of the school at that
time, maintained that the author of such an impious doctrine should be
punished. Aristarchus left a treatise "On the Magnitudes and Distances
of the Sun and Moon;" and his methods to measure the apparent diameters
of the sun and moon are considered theoretically sound by modern
astronomers, but practically inexact owing to defective instruments. He
estimated the diameter of the sun at the seven hundred and twentieth
part of the circumference of the circle which it describes in its
diurnal revolution, which is not far from the truth; but in this
treatise he does not allude to his heliocentric theory.

Archimedes of Syracuse, born 287 B.C., is stated to have measured the
distance of the sun, moon, and planets, and he constructed an orrery in
which he exhibited their motions. But it was not in the Grecian colony
of Syracuse, but of Alexandria, that the greatest light was shed on
astronomical science. Here Aristarchus resided, and also Eratosthenes,
who lived between the years 276 and 196 B.C. The latter was a native of
Athens, but was invited by Ptolemy Euergetes to Alexandria, and placed
at the head of the library.