A New Model of the Solar System

NICOLAUS COPERNICUS, On the Revolutions of the Heavenly Spheres (1542)

There was nothing revolutionary about Polish cleric and astronomer Nicolaus Copernicus’s (1473–1543) motives for developing a heliocentric (sun-centered) model of the universe. Frustrated by the cumbersome calculations and inaccuracies of the Ptolemaic model, Copernicus sought an alternative that overcame these limitations and, in his view, offered a better reflection of the perfection of God’s creation. Moreover, he retained many elements of earlier systems. For example, Copernicus still imagined the stars embedded in crystalline spheres. For all the conservatism of his approach, however, Copernicus was well aware of the stir his work might produce. Fearful of the response of Church authorities, Copernicus waited to publish his findings until 1542, more than a decade after his work was complete and shortly before his death. As you read this excerpt, consider its mix of traditional and revolutionary ideas. In what ways does Copernicus seem like a medieval scholar? In what ways does he seem closer to what we think of as a scientist?

That the Universe Is Spherical

First we must remark that the universe is globe-shaped, either because that is the most perfect shape of all, needing no joint, an integral whole; or because that it is the most capacious of shapes, which is most fitting because it is to contain and preserve all things; or because the most finished parts of the universe, I mean the Sun, Moon, and stars, are observed to have that shape, or because everything tends to take on this shape, which is evident in drops of water and other liquid bodies, when they take on their natural shape. There should therefore be no doubt that this shape is assigned to the heavenly bodies.

That the Earth Is Also Spherical

The Earth is also globe-shaped, because every part of it tends towards its center. Although it is not immediately apparent that it is a perfect sphere, because the mountains project so far and the valleys are so deep, they produce very little variation in the complete roundness of the Earth. That is evident from the fact that as one moves northward from any point that pole [the North Pole] of the diurnal [daily] rotation rises little by little, while the other pole on the contrary sinks to the same extent, and several stars round the North Pole seem not to set, while some in the South no longer rise. . . .

Whether the Earth Has a Circular Motion, and Concerning the Location of the Earth

As it has now been shown that the Earth also has the shape of a globe, I believe we must consider whether its motion too follows its shape, and what place it holds in the universe, without which it is impossible to find a reliable explanation of celestial phenomena. Among the authorities it is generally agreed that the Earth is at rest in the middle of the universe, and they regard it as inconceivable and even ridiculous to hold the opposite opinion. However, if we consider it more closely the question will be seen to be still unsettled, and so decidedly not to be despised. For every apparent change in respect of position is due to motion of the object observed, or of the observer, or indeed to an unequal change of both. (Between objects which move equally in the same direction no motion is perceived, I mean between that which is observed and the observer.) Now the Earth is the point from which the rotation of the heavens is observed, and brought into our view. If therefore some motion is imputed to the Earth, the same motion will appear in all that is external to the Earth, but in the opposite direction, as if it were passing by. The first example of this is the diurnal rotation. This seems to whirl round the whole universe, except the Earth and the things on it. But if you grant that the heaven has no part in this motion, but that the Earth revolves from west to east, as far as the apparent rising and setting of the Sun, Moon, and stars is concerned, if you consider the point seriously, you will find that this is the way of it. And as the heaven is that which contains and cloaks all things, where everything has its place, it is not at once apparent why motion is attributed to that which is contained rather than to the container, to that which is located rather than that which locates it. . . . If this assumption is made there follows another and no lesser problem about the position of the Earth, although almost everyone admits and believes the Earth to be the center of the universe. For if one argues that the Earth does not occupy the center or middle of the universe, not claiming that its distance is great enough to be comparable with the sphere of the fixed stars, but that it is appreciable and significant compared with the orbits of the Sun and other stars,1 and believing that on this account their motion seems to be variable, as if they were regular with respect to [i.e., revolved around] some center other than the center of the Earth, he would perhaps be able to put forward a not unreasonable account of the apparently variable motion. For the fact that the wandering stars are observed to be sometimes nearer to the Earth and sometimes further away from it necessarily shows that the center of the Earth is not the center of their orbits. It is also undecided whether the Earth veers toward them and away from them or they towards and away from the Earth. It would also not be surprising if in addition to this daily revolution another motion should be supposed for the Earth. Indeed that the Earth revolves, wanders with several motions, and is one of the stars [i.e., planets] is said to have been the opinion of Philolaus the Pythagorean,2 no mean mathematician. . . .

Refutation of the Arguments Quoted, and Their Insufficiency"3

From this and similar arguments, then, they say that the Earth is at rest in the middle of the universe, and that such is undoubtedly the state of affairs. Yet if anyone should hold the opinion that the Earth revolves, he will surely assert that its motion is natural, not violent. What is natural produces contrary effects to what is violent. For objects to which force or impulse is applied must necessarily be destroyed and cannot long subsist; but objects which exist naturally are in their proper state, and continue in their perfect form. There is therefore no need for Ptolemy to fear the scattering of the Earth and of all terrestrial objects in a revolution brought about through the workings of nature, which is far different from artifice, or what can be achieved by human abilities. Further, why is not the same question raised even more strongly about the universe, the motion of which must be much swifter in proportion as the heaven is greater than the Earth? Or has heaven become so immense, because it is drawn outwards from the middle by a motion of ineffable strength [i.e., centrifugal force], that it would collapse if it were not at rest? Certainly if this reasoning were to be accepted, the magnitude of the heaven will rise to infinity. For in proportion it is thrown higher by the impulse of the motion, so the motion will be swifter, on account of the continual increase in the circumference which it must traverse in the space of twenty-four hours; and on the other hand as the motion increased, so would the immensity of the heaven. So the velocity would increase the magnitude, and the magnitude the velocity, to infinity. But according to that axiom in physics, that what is infinite cannot be traversed, nor moved by any means, the heaven will necessarily be at rest. But they say that outside the heaven there is no body, no place, no empty space, in fact nothing whatsoever, and therefore there is nothing to which the heaven can go out. In that case it is remarkable indeed if something can be restrained by nothing. But if the heaven is infinite, and finite only in its hollow interior, perhaps it will be more clearly proved that there is nothing outside the heaven, since every single thing will be within, whatever amount of space it occupies, but the heaven will remain immovable. For the strongest argument by which they try to establish that the universe is finite, is its motion. Therefore let us leave the question whether the universe is finite or infinite for the natural philosophers4 to argue. What we do know for certain is that the Earth is limited by its poles and bounded by a globular surface. . . .

Surely Aristotle’s division of simple motion into three types, away from the middle, towards the middle, and round the middle, will be regarded merely as an intellectual division; just as we distinguish between a line, a point, and a surface, although one cannot exist without the other, and none of them without a body. A further point is that immobility is considered a more noble and divine state than that of change and instability, which is for that reason more appropriate to the Earth than to the universe. I also add that it would seem rather absurd to ascribe motion to that which contains and locates, and not rather to that which is contained and located, that is the Earth. Lastly, since it is evident that the wandering stars are sometimes nearer, sometimes further from the Earth, this will also be an example of motion of a single body which is both round the middle, by which they mean the center, away from the middle, and towards it. Motion round the midpoint must therefore be accepted more generally, and as satisfactory, provided that each motion is motion about its own midpoint. You will see then that from all these arguments the mobility of the Earth is more probable than its immobility, especially in the daily revolution, as that is particularly fitting for the Earth.

From Nicolaus Copernicus, De Revolutionibus Orbium Celestium, trans. A. M. Duncan (Newton Abbot, Devonshire: David and Charles, 1976), pp. 36, 37, 40–41, 43–44, 45–46.