On the Revolutions of the Heavenly Spheres

[...the space] it occupies in relation to their length becomes incomparable to the senses, in the manner demonstrated in Optics Copernicus refers here to the science of perspective and light, likely drawing on Euclid or Ptolemy to explain why distant objects appear to lose their measurable size. By this argument, it is clear enough that the heavens are immense compared to the Earth, and present an appearance of infinite magnitude. In the estimation of the senses, the Earth is to the heavens as a point is to a body, and as the finite is to the infinite in magnitude.

Yet nothing else seems to have been demonstrated. For it does not follow that the Earth must necessarily rest in the middle of the universe. Indeed, we should wonder even more if such a vastness of the world should revolve in the space of twenty-four hours, rather than its smallest part, which is the Earth.

For as for the claim that the center is immobile and that things closest to the center move the least, this does not prove that the Earth rests in the middle of the world. It is no different than if you were to say that the sky turns while the poles remain still, and those things closest to the poles move the least. In the same way, the Little Bear original: "Cynosura" — the constellation Ursa Minor, which stays near the celestial pole is seen to move much more slowly than the Eagle original: "Aquila" or the Little Dog original: "Canicula" — usually referring to the bright star Sirius, because being near the pole, it describes a smaller circle. Since all these things belong to a single sphere, whose motion ends at its axis, the sphere does not allow for an equal motion among all its parts; nevertheless, the revolution of the whole brings them all back in the same amount of time, though not across an equal distance.

The reasoning of the argument rests on this: as if the Earth were a part of the celestial sphere and shared its nature and motion, so that, being near the center, it would move very little. Therefore, as a body and not the center itself, the Earth would also move, covering similar—though smaller—arcs of a celestial circle in the same amount of time [as the stars].

How false this is, is clearer than light! For it would mean that it would always be noon in one place and always midnight in another, so that neither the daily rising nor setting could occur, since the motion of the whole and the part would be one and inseparable.

However, the nature of things that are distinct is far different: those things enclosed in a smaller circuit revolve more quickly than those which travel a larger circle. Thus, Saturn, the highest of the wandering stars original: "errantium sydus" — the planets were known as 'wandering stars' as they moved against the fixed backdrop of the constellations, completes its revolution in the thirtieth year; and the Moon, which is undoubtedly nearest the Earth, completes its circuit in a month. Finally, the Earth itself will be thought to circle in the space of a day and a night.

Therefore, the same doubt regarding the daily revolution arises again. But its location is also still being sought, and is even less certain from the things said above. For that demonstration proves nothing else than that the size of the heavens is indefinite compared to the Earth; but how far this immensity extends is not at all clear.