359 CHAPTER I. PRELUDE TO THE INDUCTIVE EPOCH OF THE doctrine of Copernicus, that the sun is the true centre of the celestial motions, depends primarily upon the consideration that such a supposition explains very simply and completely all the obvious appearances of the heavens. In order to see that it does this, nothing more is requisite than a distinct conception of the nature of relative motion, and a knowledge of the principal astronomical phenomena. There was, therefore, no reason why such a doctrine might not be discovered, that is, suggested as a theory plausible at first sight, long before the time of Copernicus; or rather, it was inevitable that this guess, among others, should be propounded as a solution of the appearances of the heavens. We are not, therefore, to be surprised if we find, in the earliest times of astronomy, and at various succeeding periods, such a system spoken of by astronomers, and maintained by some as true, though rejected by the majority, and by the principal writers. When we look back at such a difference of opinion, having in our minds, as we unavoidably have, the clear and irresistible considerations by which the Copernican doctrine is established for us, it is difficult for us not to attribute superior sagacity and candour to those who held that side of the question, and to imagine those who clung to the Ptolemaic hypothesis to have been blind and prejudiced;—incapable of seeing the beauty of simplicity and symmetry, or indisposed to resign established errors, and to accept novel and comprehensive truths. Yet in judging thus, we are probably ourselves influenced by the prejudices of the knowledge and received opinions of our own times. For is it, in reality, clear that, before the time of Copernicus, the heliocentric theory (that which places the centre of the celestial motions in the sun,) had a claim to assent so decidedly superior to the geocentric theory, which places the earth in the centre? What is the basis of the heliocentric theory?—That the relative motions are the same, on that and on the other supposition. So far, therefore, the two hypotheses exactly on the same footing. But, it is urged, on the heliocentric side we have the advantage of simplicity:true; but we have, on the other, the testimony of our senses; that is, the geocentric doctrine is the obvious and spontaneous interpretation of the appearances. Both these arguments, simplicity on the one side, and obviousness on the other, are vague, and we may venture to say, both indecisive. We cannot establish any strong preponderance of probability in favour of the former doctrine, without are going much further into the arguments of the question. Nor, when we speak of the superior simplicity of the Copernican theory, must we forget, that though this theory has undoubtedly, in this respect, a great advantage over the Ptolemaic, yet that the Copernican system itself is very complex, when it undertakes to account, as the Ptolemaic did, for the inequalities of the motions of the sun, moon, and planets; and that, in the hands of Copernicus, it retained a large share of the eccentrics and epicyles of its predecessor, and, in some parts, with increased machinery. The heliocentric theory, without these appendages, would not approach the Ptolemaic, in the accurate explanation of facts; and as those who placed the sun in the centre had never, till the time of Copernicus, shown how the inequalities were to be explained on that supposition, we may assert that after the promulgation of the theory of eccentrics and epicycles on the geocentric hypothesis, there was no published heliocentric theory which could bear a comparison with it. It is true, that all the contrivances of epicycles, and the like, by which the geocentric hypothesis was made to represent the phenomena, were susceptible of an easy adaptation to a heliocentric method, when a good mathematician had once proposed to himself the problem; and this was precisely what Copernicus undertook and executed. But, till the appearance of his work, the heliocentric system had never come before the world except as a hasty and imperfect hypothesis; which bore a favourable comparison with the phenomena, so long as their general features only were known; but which had been completely thrown into the shade by the labour and intelligence bestowed upon the Hipparchian or Ptolemaic theories by a long series of great astronomers of all countries. But, though the astronomers who, before Copernicus, held the heliocentric opinion, cannot, on any good grounds, be considered as much more enlightened than their opponents, it is curious to trace the early and repeated manifestations of this view of the universe. Its distinct assertion among the Greeks is an evidence of the clearness of their thoughts, and the vigour of their minds; and it is a proof of the feebleness and servility of intellect in the stationary period, that, till the period of Copernicus, no one was found to try the fortune of this hypothesis, modified according to the improved astronomical knowledge of the time. The most ancient of the Greek philosophers to whom the ancients ascribe the heliocentric doctrine, is Pythagoras; but Diogenes Laertius makes Philolaus, one of the followers of Pythagoras, the first author of this doctrine. We learn from Archimedes, that it was held by his contemporary, Aristarchus. "Aristarchus of Samos," says he', "makes this supposition, that the fixed stars and the sun remain at 2 rest, and that the earth revolves round the sun in a circle." Plutarch asserts that this, which was only a hypothesis in the hands of Aristarchus, was proved by Seleucus; but we may venture to say that, at that time, no such proof was possible. Aristotle had recognised the existence of this doctrine by arguing against it. "All things," says he, "tend to the centre of the earth, and rest there, and therefore the whole mass of the earth cannot rest except there." Ptolemy had in like manner argued against the diurnal motion of the earth: such a revolution would, he urged, disperse into surrounding space all the loose parts of the earth. Yet he allowed that such a supposition would facilitate the explanation of some phenomena. Cicero appears to make Mercury and Venus revolve about the sun, as does Martianus Capella at a later period; and Seneca says*, it is a worthy subject of contemplation, whether the earth be at rest or in motion: but at this period, as we may see from Seneca himself, that habit of intellect which was requisite for the solution of such a question, had been succeeded by indistinct views, and rhetorical forms of speech. If there were any good mathematicians and good observers at this period, they were employed in cultivating and verifying the Hipparchian theory. Next to the Greeks, the Indians appear to have 2 Quest. Plat. Delamb. A. A. vi. 3 Copernic. i. 7. Quest. Nat. vii. 2. |