While the Ionian school, which Anaxagoras and A•chelaus had transported from Miletus to Athens, was enlightening that renowned seat of philosophy, the science of astronomy was making rapid advances in Italy under the direction of Pythagoras. This illustri ous philosopher, after having been instructed by Thales, travelled in search of knowledge into Phoenicia, Chal dea, and India. From these countries he went to Egypt, and, after a stay of several years, he returned to Samos loaded with intellectual spoils, and instructed • in all the mysteries of the East. The inhabitants of Samos re jected the instruction which Pythagoras had previously offered them, and thus compelled him to seek in Italy for more grateful and willing disciples. To the secret association which he established at Crotona, Pythagoras taught all the doctrines of the Ionian school, improved and perfected by the wisdom which he had acquired. He demonstrated, from the different altitudes of stars at different places, that the earth was round; and he allowed that there might be antipodes in the opposite part of the globe. He considered the planet Venus as the morning and evening star, and he conceived the uni verse to be composed of twelve different spheres; the sphere of the stars, of Saturn, of Jupiter, of Mars, of Mercury, of Venus, of the Sun, and of the Moon ; and these were again succeeded by the spheres of fire, air, water, and the terrestrial globe. The plurality of worlds suggested by Anaxagoras, was likewise admit ted by Pythagoras, who went so far as to calculate the size of the lunar animals. The harmony of the spheres, taught by Pythagoras, is a stronger proof of the bril liancy of his imagination, than of the soundness of his judgment. He supposed that the solid spheres, in which the planets were fixed, ought, during their revo lutions, to emit sounds pieportional to their mutual dis tances, but that this celestial concert, though loud and grand, was not audible to the feeble organs of human beings. The most distinguishing tenet of the Pythago rean school was the diurnal revolution of the earth, and its annual motion round the sun, which they supposed to occupy the centre of the planetary system. This opi nion, so opposite to vulgar prejudice, seems to have been adopted, rather as a probable hypothesis which explained the phenomena, than as an incontestible truth, founded on observation. The fate of Anaxagoras warned Pythagoras of the danger to which he would be exposed by teaching it in public, and hence he was obliged to communicate it to his disciples under the seal of mys tery, while he flattered the prejudices of his country men, by maintaining in public the immobility of the earth. The motion of the planets round the sun, seems to have suggested to Pythagoras the opinion, that the comets were not fleeting meteors, but bodies resem bling the planets, and circulating round the central lu minary.
The true system of the universe which Pythagoras had taught in private, was publicly maintained by his disciple Philolaus, who flourished about the year 450 A. C. The people were instantly in arms against an opinion so hostile to their prejudices, and Philolaus was obliged to fly from Italy for that protection which the laws of his country denied him. Philolaus, according to Plutarch, believed that the sun was a mass of glass, which reflected all the light that was diffused through the universe ; and he is said to have made the great year consist of 59 years, in which there were 21 inter calary months; a proof that he made the solar year only 365 days, and the revolution of the moon 27 days 13 hours.
About this time flourished a number of astronomers, who, though not distinguished by any remarkable dis covery, contributed in some measure to the advance ment of the science. Seleucus of Erithrea maintained, that the earth turned round like the circumference of a wheel ; and Nicetas of Syracuse taught that the stars were at rest, and that their apparent motion arose from the diurnal rotation of the earth. Cleostratus of Cnidos proposed a lunisolar period of eight years, consisting of 3651 each, or 2922 days. An intercalary lunar month was added at the end of the third, the fifth, and the eighth years ; but, as the basis of the period was not exact, it produced an error of thirty-six hours at the end of the eighth year. Several other attempts were made, at this time, to reform the calendar ; but though they gradually approximated to the truth, it was reserved for the celebrated Meton, assisted by his friend Euctemon, to accomplish this desireable object. He proposed, for this purpose, a cycle of nineteen solar years, or 235 lu nations, which was equal to nineteen lunar years and seven intercalary months. These intercalations took place on the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth years of the cycle; and, instead of making the lunar months consist alternately of twenty-nine and thirty days, as had been done before, he proposed that 110 months should contain twenty nine days, while the remaining 125 contained thirty days. By this contrivance, the motion of the sun and moon were very nearly reconciled ; and the Metonic cycle began to be used on the 16th of July A. C. 433. It would be a fruitless and unprofitable task to detail all the astronomical opinions which were hazarded by the Greek philosophers. An inordinate desire to explain and generalise, without facts and observations, led them to the most absurd and extravagant notions; though, in a few cases, they have displayed the most wonderful in genuity, and sagaciously anticipated the discoveries of modern times. Parmenides, for example, explained the stability of the earth, by saying, that no reason could be assigned why it should fall to one side rather than ano ther; and thus anticipated the aujficiens mum of Archi medes and Leibnitz : a principle of reasoning which has not yet been banished from science. Democritus of Abdera advanced the very same explanation of the whiteness of the milky way that Dr Herschel has dis covered by his powerful telescopes, which separate that immense nebula into its component stars; and he main. tained, though from an erroneous theory, that the num ber of the planets was not known, and that more would be discovered in succeeding ages. This prediction has been accomplished after an interval of more than 2000 years.
The progress of astronomy was considerably accele rated by the establishment of the Platonic school. Though its illustrious founder was chiefly celebrated for his knowledge in mathematics and philosophy, he entertained a correct idea of the science of astronomy, and saw the steps that must be taken to bring it to per fection. It would appear from some passages in his Timms, and in Diogenes Laertius, that he supposed the planets to have been projected in right lines, and to have afterwards moved in circular paths by the action of gravity. Plato believed that the earth remained at rest in the centre of the universe; but, in the decline of life, he is said to have abandoned this opinion, and to have asserted, that the centre of the system was occu pied by some body more worthy of its place.