That was a daring theory, which led its followers into many false paths. There might be a central fire around which the Earth revolves, and of which the Sun is only a reflection. Reflection was a Hindu notion too, and the Pythagoreans may have heard an echo of it. In addition they had the motions of the Moon for analogy-the Earth might revolve around something which our side never saw. Then there might be another body like the Earth--a dark planet. These suggestions sound preposterous, but, for the first time in the history of the world, the Pythagoreans and the atomists between them had reached the conclusion that the Earth moves, and the mistakes due to a great number of erroneous additions cannot deprive them of that glory.
The wonder is that they were not all put to death for what they said, but the idea was cradled in Italy, and by the time it came to Athens that city was ready to receive it kindly, due to the influence of the times in general, and of one man in particular, the philosopher--Plato. Modest, charming and always open to new views, Plato accepted the hypothesis as something which should at least be taken into consideration. "The Earth our nurse goes to and fro on its axis which stretches right through the universe," he wrote, putting the words into the mouth of a Pythagorean with due credit. Sixty years before, even Plato would have been put to death for mentioning a theory like that, and neither his great influence nor the love of the Athenian people could have saved him. But time's had changed since the days of Anaxagoras, and Plato himself had done much to alter them. His beloved master had been condemned to death on a false charge of corrupting the youth and dabbling with impious science; and Plato had used all his artistic gift to clear the name of Socrates and at the same time to open wide the path for scientific investigation. How well he succeeded is shown by the reception given to this new planetary theory.
The doctrines for which Plato fought were accepted, partly because his authority was so great, partly because his prose was so magically persuasive, very little (it is to be feared) because they were true. Many of his own ideas, such as the direction in which the planets moved, were never again seriously doubted, but the things he allowed to fight for themselves died a rapid death. In his old age, he repented that he had not laid more stress upon the place of the Earth among the planets. That is one of the few things which he had good reason to repent; for ironically it was Plato's greatest pupil, Aristotle, who went back to the old theory and claimed that the Earth was the center of the universe.
In Plato's academy, some time after his death, Aristarchus of Samos identified the central fire with the Sun, and said that the Earth went round it with a circular orbit. He further argued that, as the stars appear (in spite of their diurnal motion) to retain fixed places in the heavens, they must be at immeasurably great distances from the Earth. "The distances," he said, "bear the same relation to the Earth's orbit, as the radius of a sphere bears to its center." No one had ever come nearer to saying that the whole solar system was a mere point in the immensity of the heavens. But alas for the truth! By that time Aristotle had effectively criticized the theories of his old teacher, and neither Aristarchus nor anyone else could hold weight against so heavy an authority.
The scientific mind of Aristotle in all other realms is unquestioned. His treatise on logic has served as a clearing house for befuddled ideas ever since, and in biology he was superb. But Alexander the Great could send his old schoolteacher rare fishes from the Tigris and the Indus and the Arabian Sea, so Aristotle had genuine data with which to work; not even Alexander could send him a working model of the solar system. He had to guess, and he guessed wrong.
We can imagine the lines of argument as the peripatetic followers of Aristotle walked up and down through the groves of the Lyceum. "It is obvious that the Earth does not revolve or we should feel it. Only an impractical philosopher would say such a thing." And Aristotle would be mildly reproving when his master was too severely criticized by anyone but himself. "We Platonists used to say that the Earth moved; but," he would add seriously, "I think that we were wrong." Then the voices would go on and on now that Aristotle had conceded the point. "Something must stand still, and certainly the heavens don't. This idea that the Earth is moving is perfectly ridiculous. Why if we were revolving we'd fall off when we got to the other side." On and on with all the other palpable, obvious, plain and clearly apparent reasons, by which those who do not know, and are not able to prove their point, try to convince their listeners that they are wise, sticking to fact, and that what they say is axiomatically true.