CHAPTER IX. SCIENTIFIC KNOWLEDGE AMONG THE ROMANS.
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." [Footnote: Lewis, p. 190.] This speculation, resting on the authority of Archimedes, was ridiculed by him; but if it were advanced, it shows a great advance in astronomical science, and considering the age, was one of the boldest speculations of antiquity. Aristarchus also, according to Plutarch, [Footnote: Plut., Plac. Phil., ii. 24.] 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. [Footnote: Quaest. Plat., viii. 1.] This theory gave great offense, 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 has 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 sound by modern astronomers, [Footnote: Lewis, p. 193.] but 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, 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. He was a native of Athens, but was invited by Ptolemy Euergetes to Alexandria, and placed at the head of the library. His great achievement was the determination of the circumference of the earth. This was done by measuring on the ground the distance between Syene, a city exactly under the tropic, and Alexandria situated on the same meridian. The distance was found to be five thousand stadia. The meridional distance of the sun from the zenith of Alexandria, he estimated to be 7 degrees 12', or a fiftieth part of the circumference of the meridian. Hence the circumference of the earth was fixed at two hundred and fifty thousand stadia, not far from the truth. The circumference being known, the diameter of the earth was easily determined. The moderns have added nothing to this method. He also calculated the diameter of the sun to be twenty-seven times greater than of the earth, and the distance of the sun from the earth to be eight hundred and four million stadia, and that of the moon seven hundred and eighty thousand stadia - a very close approximation to the truth.
[Greatness of Hipparchus.]