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THE MOST SALIENT EXCERPTS
(if you don’t want to tackle the whole)
translated by Edward Rosen
[T]he planetary theories of Ptolemy and most other astronomers, although consistent with the numerical data…present no small difficulty. For these theories were not adequate unless certain equants were also conceived….I often considered whether there could perhaps be found a more reasonable arrangement of circles, from which every apparent inequality would be derived and in which everything would move uniformly about its proper center, as the rule of absolute motion requires. After I had addressed myself to this very difficult and almost insoluble problem, the suggestion at length came to me how it could be solved with fewer and much simpler constructions than were formerly used, if some assumptions (which are called axioms) were granted me. They follow in this order.
1. There is no one center of all the celestial circles or spheres.
2. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere.
3. All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe.
4. The ratio of the earth’s distance from the sun to the hieght of the firmament is so much smaller than the ratio of the earth’s radius to its distance from the sun that the distance from the earth to the sun is imperceptible in comparison with the height of the firmament.
5. Whatever motion appears in the firmament arises not from any motion of the firmament, but from the earth’s motion. The earth…performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged.
6. What appears to us as motions of the sun arise not from its motion but from the motion of the earth and our sphere, with which we revolve about the sun like any other planet. The earth has, then, more than one motion.
7. The apparent retrograde and direct motion of the planets arises not from their motion but from the earth’s. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.
…[L]et no one suppose that I have gratuitously asserted, with the Pythagoreans, the motion of the earth; strong proof will be found in my exposition of the circles. For the principal arguments by which the natural philosophers attempt to establish the immobility of the earth rest for the most part on the appearances; it is particularly such arguments that collapse here, since I treat the earth’s immobility as due to an appearance.
On the Revolutions of the Heavenly Spheres
translated by Charles Glenn Wallace
7. Why the Ancients Thought the Earth Was at Rest at the Middle of the World as Its Center
…[T]he ancient philosophers tried to force the earth to remain at rest at the midpoint of the world and as principal cause they put forward heaviness and lightness. For earth is the heaviest element; and all things of any weight are borne towards it and strive to move towards
the very center of it.
For since the earth is a globe towards which from every direction heavy things by their own nature are borne at right angles to its surface, the heavy things would fall on one another at the center if they were not held back at the surface; since a straight line making right angles
with a plane surface where it touches a sphere leads to the center. And those things which are borne toward the center seem to follow along in order to be at rest at the center. All the more then will the earth be at rest at the center and, being the receptacle for falling bodies, will remain immobile because of its weight.
They strive similarly to prove this by reason of movement and its nature. For Aristotle says that the movement of a body which is one and simple is simple and the simple movements are the rectilineal and the circular. And of rectilineal movements, one is upward, and the other is downward. As a consequence every simple movement is either toward the center, i.e., downward, or away from the center, i.e., upward, or around the center, i.e., is circular. It is a property of earth and water only, which are heavy, to be borne downward, i.e., to seek the center: for air and fire, which possess lightness, move upward, i.e., away from the center. It seems fitting to attribute rectilineal movement to the four elements and to give the heavenly bodies a circular movement around the center. So Aristotle. Therefore, said Ptolemy of Alexandria, if the earth moved, even if only by its daily rotation, the contrary of what was said above would necessarily take place. For this movement which would traverse the total circuit of the earth in twenty-four hours would necessarily be very headlong and of an unsurpassable velocity. Now things which are suddenly and violently whirled around are seen to be utterly unfitted for reuniting, and the more unified are seen to become dispersed, unless some constant force constrains them to stick together. And a long time ago, he says, the scattered earth would have passed beyond the heavens, as is certainly ridiculous; and a fortori so would all the living creatures and all the other separate masses which could by no means remain unshaken. Moreover, freely falling bodies would not arrive at their destination, and certainly not along the perpendicular line which they assume so quickly. And we would see clouds and other things floating in the air always borne toward the west.
8. Answer to the Aforesaid Reasons and Their Inadequacy
For these and similar reasons they say that the earth remains at rest at the midpoint of the world and that there is no doubt about this. But if some one opines that the earth moves, he
will also say that the movement is natural and not violent. Now things which take place naturally produce effects contrary to those which take place violently. For things which are moved by force or vehemence necessarily get broken up and are unable to subsist for a long time. But things which are caused by nature are in a right condition and are kept in their best organization. Therefore Ptolemy had no reason to fear that the earth and all things on the earth would be scattered in a revolution caused by the efficacy of nature, which is greatly different from that of art or from that which can result from the genius of man….
…[L]et us hold as certain that the earth held together between its two poles terminates in a spherical surface. Why therefore should we hesitate any longer to grant to it the movement which accords naturally with its form, rather than put the whole world in a commotion–the world whose limits we do not and cannot know? And why not admit that the appearance of diurnal revolution belongs to the heavens but the reality belongs to the earth? And things are as when Aeneas said in Virgil: “We sail out of the harbor, the land and cities move away.” As a matter of fact, when a ship floats on over a tranquil sea, all the things outside seem to the voyagers to be moving in a movement which is the image of their own, and they think on the contrary that they themselves and all the things with them are at rest. So it can easily happen in the case of the movement of the earth that the whole world should be believed to be moving in a circle….
9. Whether Many Movements Can Be Attributed to the Earth, and Concerning the Center of the World
…[S]ince nothing hinders the mobility of the earth, I think we should now see whether more than one movement belongs to it, so that it could be regarded as one of the wandering stars. For the apparently irregular movement of the planets and their variable distances from the earth, which cannot be understood as occurring in circles which are homocentric with the earth, show that the earth is not the center of their circular movements. Therefore since there are many centers, it does not require audacity to doubt whether the center of gravity of the earth rather than some other is the center of the world….[I[f the annual revolution were changed from being solar to being terrestrial, and immobility were granted to the sun, the risings and setting of the
signs and of the fixed stars–whereby they become morning or evening stars–will appear in the same way; and it will be seen that the stoppings, retrogressions and progressions of the wandering stars are not theirs, but are a movement of the earth and that they borrow the appearances of this movement. Lastly, the sun will be regarded as occupying the mid-point of the world. The reason for the order in which all these things succeed one another and the harmony of the whole world teaches us their truth, if only–as they say–we would look at the thing with both eyes.