us to the fact that we are dealing with a scientific hypothesis. It was a great thing to see that the phenomena could best be "saved" by a central luminary, and that the earth must therefore be a revolving sphere like the other planets.[1] Indeed, we are tempted to say that the identification of the central fire with the sun was a detail in comparison. It is probable, at any rate, that this theory started the train of thought which made it possible for Aristarchos of Samos to reach the heliocentric hypothesis,[2] and it was certainly Aristotle's successful reassertion of the geocentric theory which made it necessary for Copernicus to discover the truth afresh. We have his own word for it that he started from what he had read about the Pythagoreans.[3]
In the form in which it was now stated, however, the theory raised almost as many difficulties as it solved, and it did not maintain itself for long. It is clear from Aristotle that its critics raised the objection that it failed to "save the phenomena" inasmuch as the assumed revolution of the earth would produce parallaxes too great to be negligible and that the Pythagoreans gave some reason for the belief that they were negligible. Aristotle has no clear account of the arguments on either side, but it may be pointed out that the earth was probably supposed to be far smaller than it is, and there is no reason why its orbit should have been thought to have an appreciably greater diameter than we now know the earth itself to have.[4]
- ↑ Aristotle expresses this by saying that the Pythagoreans held τὴν . . . γῆν ἓν τῶν ἄστρων οὐσαν κύκλῳ φερομένην περὶ τὸ μέσον νύκτα τε καὶ ἡμέραν ποιεῖν (De caelo, B, 13. 293 a 23).
- ↑ I do not discuss here the claims of Herakleides to be the real author of the heliocentric hypothesis.
- ↑ In a letter to Pope Paul III., Copernicus quotes Plut. Plac. iii. 13, 2–3 (R. P. 83 a) and adds Inde igitur occasionem nactus, coepi et ego de terrae mobilitate cogitare.
- ↑ Cf. Ar. De caelo, B, 13. 293 b 25 ἐπεὶ γὰρ οὐκ ἔστιν ἡ γῆ κέντρον, ἀλλ' ἀπέχει τὸ ἡμισφαίριον αὐτης ὅλον, οὐθὲν κωλύειν οἴονται τὰ φαινόμενα συμβαίνειν ὁμοίως μὴ κατοικοῦσιν ἡμῖν ἐπὶ τοῦ κέντρου, ὥσπερ κἂν εἰ ἐπὶ τοῦ μέσου ἧν ἡ γῆ· οὐθὲν γὰρ οὐδὲ νῦν ποιεῖν ἐπίδηλον τὴν ἡμισεῖαν ἀπέχοντας ἡμᾶς διάμετρον. (Of course the words τὸ ἡμισφαίριον αὐτης ὅλον refer to Aristotle's own theory of celestial spheres; he really means the radius of its orbit.)
