he will compute the number of sand grains that would fill the sun's seeming orbit, assumes as a matter of course that the earth is globular. Strabo, writing about the beginning of our era, criticises his great predecessor, Eratosthenes, for wasting argument upon such a subject. He takes it for granted that the earth is round, and that no other theory as to its shape can for a moment be considered.
That was the first great step, but there remained a harder one. Though the earth is a ball, it does not follow that this ball is not the centre of the universe. Indeed the contrary assumption is quite the natural and seemingly obvious one. To all ordinary observation our earth seems a vast immovable mass, and the sun and moon and stars appear as minor satellites circling about it. So far as we know, there was never an Oriental astronomer who questioned that such was really the fact. Yet the early Pythagoreans, possibly Pythagoras himself, were led to the strange conclusion that our seemingly stable earth is really in motion. The full argument that led them to this conclusion cannot now be followed. It appears that it was in part, perhaps largely, metaphysical rather than inductive. The Pythagoreans had the idea that ten is a sacred number, somewhat as the Babylonians before them had ascribed sacredness to the number seven. But since, counting the stars as a single series, there were only nine series of sidereal bodies, including the earth and the sun, they made up the deficit in true metaphysical fashion by inventing a "counter-earth," which was supposed to revolve in such a way as never to be visible from the inhabitable side of our globe. There was, however, an additional reason for assuming the existence of the counter-earth,—this time not a metaphysical reason. It seemed to the astronomer of the period that the shadows that produce eclipses could not be cast always by our earth, and the counter-earth could of course do service as a substitute. But these assumptions as to the counter-earth were coupled with another assumption which was perhaps their necessary counterpart,—the assumption, namely, that earth and counter-earth alike revolve about a common centre, this centre being a great mass of fire, which, like the counter-earth, is never visible from the habitable side of our globe. It came to be conceived by the Pythagorean Philolaus that the sun is merely a mirror reflecting light from this great central fire. The Anaxagorean theory that the moon shines merely by light reflected by the sun gave support through analogy to this idea that the sun itself is, as indeed it seems, merely a higher-polished disc like a burning-glass.
These obviously were steps away from the paths of tradition, but they were not all steps in the right direction. But while the Pythagoreans were pursuing this ignis fatuus, Anaxagoras, a truer scientist than they, had started in quite another direction. He had studied a famous meteoric stone that fell at Ægespotomi, and had been led to the amazing inference that space must abound with such fragments; that all these consist of matter which sometime had been thrown off by the whirling earth; that the sun and moon were themselves only larger fragments of the same mass and have the same origin. The moon, he said, has cooled until it has become a habitable earth; the sun is still a mass of molten stone and iron; a mass of iron "larger than the Peloponnesus." You smile. Yet few men in any generation have had so vitalizing a prevision. The sun a mass of molten iron! We are going far from that Egyptian conception of the sun-god that floated about the river of the world in a boat. We are getting on.
But so long as it is possible to speak gravely of the sun as "larger than the Peloponnesus," we shall not get away from the idea that this body after all is a mere satellite of the earth; or at the very most we shall conceive with the Pythagoreans that earth and sun alike revolve about some greater central mass. The notion that our earth may be subordinate to the sun in position will scarcely suggest itself; or if suggested, will gain scant credence until it is clearly conceived to be subordinate in size as well. And this conception will surely never come until some method is devised of measuring heavenly bodies. Could such a method be found? The Greek mind did not despair of solving even so inscrutable a problem. We are told that Philolaus taught that the sun is larger than the earth, but we have no clue as to how this
