Page:Popular Science Monthly Volume 68.djvu/330

 326 in the original, and of comparing the opinions of learned critics, will readily be directed to them by the special literature.

Contenting ourselves with these brief literary indications, we may pass on to the more intricate questions relating to the predecessors of Aristarchus, and the influence of Pythagorean views upon later thinkers. As has been previously remarked, Heraclides Ponticus acknowledged that the heliocentric theory provided an adequate explanation of celestial phenomena, and even approached so nearly to modern ideas as to maintain the revolution of Mercury and Venus about the sun. This we know from the testimony of numerous authors, chiefly Roman, amongst whom Heraclides was held in high regard. Of interest is the passage in Simplicius ('Commentary on Aristotle's de Cælo,' Karsten's edition, p. 232), which shows Heraclides' correct apprehension of the causes determining the difference in length between the sidereal and ordinary day of twenty-four hours. We are informed, however, by Plutarch (Plac. Philos., III., 13) and later writers (e. g., Simplicius, Hippolytus, Proclus, Chalcidius, and especially Vitruvius and Terentius Varro) that although Heraclides of Pontus and Ecphantus the Pythagorean believed the earth to turn upon its axis from west to east, they distinctly denied to it a movement of translation through space. It is clear also from Aristotle that it was no unheard-of thing to explain the apparent diurnal motion of the heavens upon the hypothesis of the earth's rotation. Plato appears to have accepted this idea as the starting-point of his system, complicated as it was with superadded mechanism. But the great Athenian appears to have elaborated his cosmical theory more as a speculative abstraction than as an orderly induction from observed facts, and it was easy to explain the discrepancy of the latter as due to false appearances.

It has been claimed on the authority of Theophrastus, as reported by Plutarch and Aristotle, that Plato repented in his old age at having placed the earth at the center or 'altar' of the universe, this being deemed too sacred a position for it to occupy (Plutarch, Plat. Quæst., VIII. 1; Aristotle, de Cælo, II. 13, 3). But this is very far from indicating that the heliocentric theory ever fully shaped itself in his mind, although one sees that it required merely a combination of his views and those of the Pythagorean school to arrive at a cosmical