1883. Prowe's Life of Copernicus. 331
It is extremely difficult for us, in our superabundant wealth of conviction, to realise the paucity of cogent evidence as to the truth of the heliocentric system at the disposal of Copernicus. Proof of the rotation of the earth he had absolutely none to offer; all he could do was to point out the reasonableness and probability of the supposition. One of his arguments indeed, though of a highly abstract kind, had almost the force of a demonstration ; but its cogency was not likely to be perceived in the confusion of thought respecting the metaphysical foundations of mechanics then prevailing. Its drift may be conveyed as follows : — Motion is an attribute of matter. Space may be its scene, but cannot be its subject. But the 'celestial sphere' signifies nothing more than the depths of space surrounding us on every side. Its movement is then only apparent — an optical transference of the real movement of the earth.[1]
The reasons alleged by him in favour of the earth's revolution round the sun were of a more special and technical character. They rested mainly on the simplification which the adoption of such an hypothesis introduced into the complicated theories of the several planets. The backward loopings of the paths pursued by them in the heavens were then at once seen to have no substantial existence — to be, in fact, mere perspective effects of the combination of their real movements with the no less real movement of the earth. And the circumstance that the so-called ' stations and retrogradations ' of all the planets could be abolished, as it were, at one stroke, by simply making the point from which they were viewed revolve in their midst in a period of one year, was in itself a powerful argument in favour of that assumption.
It would, however, be a mistake to suppose that the system of the world was wrought up at Frauenburg into anything approaching its present state of perfection. By retaining the ancient postulate of ' uniform circular motion,' and the epicycles thus rendered necessary for the ' saving of appearances,' Copernicus gave to his scheme somewhat of a hybrid character. The irregularities of Mercury still required a combination of seven circles to explain them ; each of the other planets was dexterously conveyed by means of five associated orbs ; the moon made a shift to get along with four ; the earth itself was modestly content with three. ' Thus,' Copernicus exclaimed, in a rare outburst of exultation, ' thirty-four circles
- ↑ De Revolutionibus, lib. i. cap. 5, 8.
