Reading on: The universe according to Copernicus
Nicholas Copernicus – translation quoted in Danielson, Dennis R., Editor The Book of the Cosmos, Perseus Publishing, Cambridge, MA, 2000 [abridged — 1900 words]
THE UNIVERSE IS SPHERICAL
The first thing for us to realize is that the universe is spherical. This is so either because, of all forms, the sphere is the most perfect, requiring no joins, and being an integrated whole; or because it is the most capacious of all forms, and so best fitted to enclose and preserve all things—or also because the most perfected parts of the universe such as the sun, the moon, and the stars display this shape; or because all things strive to be bounded thus, as we observe in drops of water and other liquids when they seek to be bounded within themselves. There can be no doubt, then, about the rightness of ascribing this shape to the heavenly bodies.
THE EARTH TOO IS SPHERICAL
The earth also has the shape of a globe, because all of its parts tend towards its center. We do not immediately perceive it as a perfect sphere because the mountains are so high and the valleys so deep, and yet these hardly affect the overall sphericity of the earth. This is clear from the fact that if one travels northward, the pole of the diurnal rotation gradually rises, while the opposite pole sinks accordingly, and more stars in the northern sky seem never to set, while some in the south seem never to rise… Conversely, as one travels southward, such stars rise higher, while those which appear high to us sink lower…
Seafarers know that the waters too conform to this shape, for land that is not visible from the ship is observed from the top of the mast. And if a bright light is placed at the top of the mast, then to those remaining on the shore it appears gradually to sink as the ship moves farther off from land. Finally, the light as it were sets and disappears…
THE MOTION OF THE HEAVENLY BODIES IS UNIFORM, PERPETUAL, AND CIRCULAR, OR MADE UP OF CIRCULAR MOTIONS
…the sun metes out our year and the moon our month, these being the other most common measures of time. In this way too each of the five planets completes its circuit. Yet there are differences among their various motions. First, these do not turn about the same axis as the primary motion but take a slantwise course through the zodiac. Secondly, they are not observed moving uniformly in their orbits. For we see that the sun and moon in their courses sometimes move slowly and sometimes more quickly.
As for the other five planets, as we observe, sometimes they even come to a stop and retrace their steps. And while the sun always keeps strictly to its own pathway, these others wander in various ways, sometimes towards the south, sometimes towards the north—which is why they are called planets [from Greek planetes, “wanderer”]. Moreover, sometimes they are nearer the earth and said to be “in perigee”; at other times they are farther off and said to be “in apogee.”
We must admit, nonetheless, that their motions are circular, or made up of several circles, because these nonuniformities conform to a consistent law and to the fact that the planets return to where they began, which could not be the case unless the motions were circular, for only a circle can replicate what has already taken place… For example, by a motion made up of circles the sun causes for us a repetition of unequal days and nights and of the four seasons. In this cycle we discern several motions, since no simple heavenly body can move irregularly in a single sphere. For such irregularity would have to result either from an inconstancy in the force of movement, whether arising internally or externally, or from some irregularity in the revolving body. But either alternative is abhorrent to reason. We must not ascribe any such indignity to things framed and governed optimally.
We must conclude, then, that their uniform motions appear to us as irregular either because they take place around different axes, or else because the earth is not at the center of their circles of revolution…
Admittedly, virtually everyone has been taught, and believes, that the earth is the center of the universe. However, anyone who denies that the earth occupies the center or midpoint may still assert that its distance from the center is negligible by comparison with that of the sphere of the fixed stars, yet noticeable and noteworthy relative to the spheres of the sun and other planets. He may consider that this is why their motions appear nonuniform, and that they are regular relative to some center other than that of the earth. In this way, perhaps, he can offer a not-so-inept explanation for the appearance of irregular motion. For the fact that we observe the planets sometimes nearer the earth and sometimes farther away is logical proof that the center of the earth is not the center of their orbits…
And yet the question of earth’s location remains uncertain… Even though it is not in the center of the universe, its distance from the center is nevertheless inconsiderable when compared to the distance of the sphere of the fixed stars…
[If the heaven of fixed stars moved about the earth would not their] swiftness of motion be that much greater in proportion as the heavens are greater than the earth? Or are the heavens so immense precisely because the ineffable force of their motion impels them away from the center? Would they otherwise collapse if they did stand still? If this reasoning were sound, then surely the magnitude of the heavens must expand to infinity. For the higher they are impelled by the force of their motion, the faster their motion will be on account of the continuously expanding circumference which has to make its revolution every twenty-four hours. In turn, as the motion increased, so would the immensity of the heavens—speed thus increasing size, and size increasing speed, ad infinitum. Yet according to that axiom of physics, nothing that is infinite can be traversed nor moved by any means, and so the heavens are necessarily at rest…
Why, then, do we still hesitate to accept the earth’s movement in keeping with the nature of its form instead of attributing motion to the whole universe, whose bounds are unknown and unknowable. As regards the daily rotation, why not grant that in the heavens is the appearance but in the earth is the reality? For when a ship glides along smoothly, its passengers see its motion reflected by everything outside of the ship and, by contrast, suppose themselves and everything else on board to be motionless. No wonder, then, that the movement of the earth makes us think the whole universe is turning round.
CAN THE EARTH BE SAID TO MOVE IN MORE WAYS THAN ONE? WHERE IS THE CENTER OF THE UNIVERSE?
Since therefore nothing precludes the earth’s movement, I propose we now consider whether it may be thought to move in more than one way: can it be regarded as one of the planets?
For earth is not the center of all the revolutions. This claim is demonstrated by the apparently nonuniform motion of the planets and by their variable distances from the earth, which cannot be conceived as implying circles concentric to the earth. Therefore, there being numerous centers, it is worth asking whether the center of the universe, or some other, is the center of earthly gravity. In my view gravity is nothing but a certain natural desire which by divine providence the Creator of all has infused into the parts, whereby they draw themselves into a unity and an integrity in the form of a globe. The same desire may be credibly predicated also of the sun, the moon, and the other luminous planets; by its efficacy they persist in the rounded shape in which we behold them, although they pursue their own various orbits.
Therefore, if the earth too moves in other ways—about a center, for example—then this must similarly be reflected in many external things. Among them, it would seem, is the annual revolution. For if, granting immobility to the sun, we exchange earthly movement for solar movement, then the risings and settings of the constellations and the fixed stars which accompany morning and evening will appear just as they do. Furthermore, the stations as well as both the backward and forward motions of the planets will be seen not as their own motions but as earthly motion transmuted into apparent planetary motions. Finally, it will be accepted that the sun occupies the center of the universe.
We learn all these things by discerning the order whereby the planets follow one another and by the harmony of the entire universe-if only we examine these matters (as they say) with both eyes open.
THE ORDER OF THE HEAVENLY SPHERES
We should not be ashamed to admit that this whole domain encircled by the moon, with the center of the earth, traverses this great orbit amidst the other planets in an annual revolution around the sun, and that near the sun is the center of the universe; and moreover that, since the sun stands still, whatever motion the sun appears to have is instead actually attributable to the motion of the earth. Furthermore, although the distance between the earth and the sun is quite noticeable relative to the size of the other planetary orbits, it is imperceptible as compared with the sphere of the fixed stars—so great indeed is the size of the universe. I think it is a lot easier to accept this than to drive our minds to distraction multiplying spheres almost ad infinitum, as has been the compulsion of those who would detain earth in the center of the universe. Instead, it is better to follow the wisdom of nature, which just as it strongly avoids producing anything superfluous or useless, so it often prefers to endow a single thing with multiple effects.
This whole matter is difficult, almost paradoxical, and certainly contrary to many people’s way of thinking. In what follows, however, God helping me, I shall make these things clearer than sunlight, at least to those not ignorant of the art of astronomy. And so, with the first principle firmly established (for nobody can propose one more fitting than that the magnitude of a planet’s orbit is proportionate to its period of revolution), the order of the spheres is as follows, beginning with the highest:
First and highest of all is the sphere of the fixed stars, containing itself and all things, and therefore immovable, the very location of the universe, that to which the motion and position of all the other heavenly bodies is referred....
This is followed by the first of the planets, Saturn, which completes his circuit in thirty years. Then comes Jupiter, moving in a revolution with a twelve-year period. Next, the circuit of Mars is two years. Fourth comes the annual revolution in which, as mentioned earlier, the earth is carried along, with the moon as it were in an epicycle. Venus, in fifth place, circles round in nine months. And then in sixth place Mercury completes his course in the space of eighty days.
And behold, in the midst of all resides the sun. For who, in this most beautiful temple, would set this lamp in another or a better place, whence to illuminate all things at once?