I can well believe, most holy father, that certain people, when they hear of my attributing motion to the earth in these books of mine, will at once declare that such an opinion ought to be rejected. Now, my own theories do not please me so much as not to consider what others may judge of them. Accordingly, when I began to reflect upon what those persons who accept the stability of the earth, as confirmed by the opinion of many centuries, would say when I claimed that the earth moves, I hesitated for a long time as to whether I should publish that which I have written to demonstrate its motion, or whether it would not be better to follow the example of the Pythagoreans, who used to hand down the secrets of philosophy to their relatives and friends only in oral form. As I well considered all this, I was almost impelled to put the finished work wholly aside, through the scorn I had reason to anticipate on account of the newness and apparent contrariness of my theory to reason.
My friends, however, dissuaded me from such a course and admonished me that I ought to publish my book, which had lain concealed in my possession not only nine years, but already into four times the ninth year. Not a few other distinguished and very learned men asked me to do the same thing, and told me that I ought not, on account of my anxiety, to delay any longer in consecrating my work to the general service of mathematicians.
But your holiness will perhaps not so much wonder that I have dared to bring the results of my night labors to the light of day, after having taken so much care in elaborating them, but is waiting instead to hear how it entered my mind to imagine that the earth moved, contrary to the accepted opinion of mathematicians-nay, almost contrary to ordinary human understanding. Therefore I will not conceal from your holiness that what moved me to consider another way of reckoning the motions of the heavenly bodies was nothing else than the fact that the mathematicians do not agree with one another in their investigations. In the first place, they are so uncertain about the motions of the sun and moon that they cannot find out the length of a full year. In the second place, they apply neither the same laws of cause and effect, in determining the motions of the sun and moon and of the five planets, nor the same proofs. Some employ only concentric circles, others use eccentric and epicyclic ones, with which, however, they do not fully attain the desired end. They could not even discover nor compute the main thing-namely, the form of the universe and the symmetry of its parts. It was with them as if some should, from different places, take hands, feet, head, and other parts of the body, which, although very beautiful, were not drawn in their proper relations, and, without making them in any way correspond, should construct a monster instead of a human being.
Accordingly, when I had long reflected, on this uncertainty of mathematical tradition, I took the trouble to read again the books of all the philosophers I could get hold of, to see if some one of them had not once believed that there were other motions of the heavenly bodies. First I found in Cicero that niceties had believed in the motion of the earth. Afterwards I found in Plutarch, likewise, that some others had held the same opinion. This induced me also to begin to consider the movability of the earth, and, although the theory appeared contrary to reason, I did so because I knew that others before me had been allowed to assume rotary movements at will, in order to explain the phenomena of these celestial bodies. I was of the opinion that I, too, might be permitted to see whether, by presupposing motion in the earth, more reliable conclusions than hitherto reached could not be discovered for the rotary motions of the spheres. And thus, acting on the hypothesis of the motion which, in the following book, I ascribe to the earth, and by long and continued observations, I have finally discovered that if the motion of the other planets be carried over to the relation of the earth and this is made the basis for the rotation of every star, not only will the phenomena of the planets be explained thereby, but also the laws and the size of the stars; all their spheres and the heavens themselves will appear so harmoniously connected that nothing could be changed in any part of them without confusion in the remaining parts and in the whole universe.
THAT THE UNIVERSE IS SPHERICAL
First we must remark that the universe is spherical in form, partly because this form being a perfect whole requiring no joints, is the most complete of all, partly because it makes the most capacious form, which is best suited to contain and preserve everything; or again because all the constituent parts of the universe, that is the sun, moon, and the planets appear in this form; or because everything strives to attain this form, as appears in the case of drops of water and other fluid bodies if they attempt to define themselves. So no one will doubt that this form belongs to the heavenly bodies.
THAT THE EARTH IS ALSO SPHERICAL
That the earth is also spherical is therefore beyond question, because it presses from all sides upon its center. Although by reason of the elevations of the mountains and the depressions of the valleys a perfect circle cannot be understood, yet this does not affect the general spherical nature of the earth. This appears in the following manner. To those who journey towards the North the north pole of the daily revolution of the heavenly sphere seems gradually to rise, while the opposite seems to sink. Most of the stars in the region of the Bear seem not to set, while some of the southern stars seem not to rise at all. So Italy does not see Canopes which is visible to the Egyptians. And Italy sees the outermost star of the Stream, which our region of a colder zone does not know. On the other hand to those who go towards the South the others seem to rise and those to sink where are high in our region. Moreover, the inclination of the Poles to the diameter of the earth bears always the same relation, which could happen only in the case of a sphere. So it is evident that the earth is included between the two poles, and is therefore spherical in form. Let us add that the inhabitants of the East do not observe the eclipse of the sun or of the moon which occurs in the evening, and the inhabitants of the West those which occur in the morning, while those who dwell between see those later and these earlier. That the water also has the same form can be observed from ships, in that the land which cannot be seen from the deck, is visible from the mast-tree. And conversely if a light be placed at the mast-head it seems to those who remain on the shores gradually to sink and at last still sinking to disappear. It is clear that the water also according to its nature continually presses like the earth downward, and does not rise above its banks higher than its convexity permits. So the land extends above the ocean as much as the land happens to be higher.
WHETHER THE EARTH HAS A CIRCULAR MOTION, AND
CONCERNING THE LOCATION OF THE EARTH
As it has been already shown that the earth has the form of a sphere, we must consider whether a movement also coincides with this form, and what place the earth holds in the universe. Without this there will be no secure results to be obtained in regard to the heavenly phenomena. The great majority of authors of course agree that the earth stands still in the center of the universe, and consider it inconceivable and ridiculous to suppose the opposite. But if the matter is carefully weighed it will be seen that the question is not yet settled and therefore by no means to be regarded lightly. Every change of place which is observed is due, namely, to a movement of the observed object or of the observer, or to movements of both, naturally in different directions, for if the observed object and the observe move in the same manner and in the same direction no movement will be seen. Now it is from the earth that the revolution of the heavens is observed and it is produced for our eyes. Therefore if the earth undergoes no movement this movement must take place in everything outside of the earth, but in the opposite direction than if everything on the earth moved, and of this kind is the daily revolution. So this appears to affect the whole universe, that is, everything outside the earth with the single exception of the earth itself. If, however, one should admit that this movement was not peculiar to the heavens, but that the earth revolved from west to east, and if this was carefully considered in regard to the apparent rising and setting of the sun, the moon and the stars, it would be discovered that this was the real situation. Since the sky, which contains and shelters all things, is the common seat of all things, it is not easy to understand why motion should not be ascribed rather to the thing contained than to the containing, to the located rather than to the location. From this supposition follows another question of no less importance, concerning the place of the earth, although it has been accepted and believed by almost all, that the earth occupies the middle of the universe. But if one should suppose that the earth is not at the center of the universe, that, however, the distance between the two is not great enough to be measured on the orbits of the fixed stars, but would be noticeable and perceptible on the orbit of the sun or of the planets: and if one was further of the opinion that the movements of the planets appeared to be irregular as if they were governed by a center other than the earth, then such an one could perhaps have given the true reasons for the apparently irregular movement. For since the planets appear now nearer and now farther from the earth, this shows necessarily that the center of their revolutions is not the center of the earth: although it does not settle whether the earth increases and decreases the distance from them or they their distance from the earth.
REFUTATION OF THE ARGUMENT OF THE ANCIENTS THAT THE
EARTH REMAINS STILL IN THE MIDDLE OF THE
UNIVERSE, AS IF IT WERE ITS CENTER
From this and similar reasons it is supposed that the earth rests at the center of the universe and that there is no doubt of the fact. But if one believed that the earth revolved, he would certainly be of the opinion that this movement was natural and not arbitrary. For whatever is in accord with nature produces results which are the opposite of those produced by force. Things upon which force or an outside power has acted, must be injured and cannot long endure: what happens by nature, however, preserves itself well and exists in the best condition. So Ptolemy feared without good reason that the earth and all earthly objects subject to the revolution would be destroyed by the act of nature, since this latter is opposed to artificial acts, or to what is produced by the human spirit. But why did not he fear the same, and in a much higher degree, of the universe, whose motion must be as much more rapid as the heavens are greater than the earth? Or has the heaven become so immense because it has been driven outward from the center by the inconceivable power of the revolution; while if it stood still, on the contrary, it would collapse and fall together? But surely if this is the case the extent of the heavens would increase infinitely. For the more it is driven higher by the outward force of the movement, so much the more rapid will be movement become, because of the ever increasing circle which must be traversed in 24 hours; and conversely if the movement grows the immensity of the heavens grows. So the velocity would increase the size and the size would increase the velocity unendingly. According to the physical law that the endless cannot wear away nor in any way move, the heavens must necessarily stand still.
But it is said that beyond the sky no body, no place, no vacant
space, in fact nothing at all exists; then it is strange that some thing
should be enclosed by nothing. But if the heaven is endless and is bounded
only by the inner hollow, perhaps this establishes all the more clearly
the fat that there is nothing outside the heavens, because everything is
within it, but the heaven must then remain unmoved. The highest proof on
which one supports the finite character of the universe is its movement.
But whether the universe is endless or limited we will leave to the physiologues;
this remains sure for us that the earth enclosed between the poles, is bounded
by a spherical surface. Why therefore should we not take the position of
ascribing to a movement conformable to its nature and corresponding to its
form, rather than suppose that the whole universe whose limits are not and
cannot be known moves? and why will we not recognize that the appearance
of a daily revolution belongs to the heavens, but the actuality to the earth;
and that the relation is similar to that of which one says: "We run
out of the harbor, the lands and cities retreat from us." Because
if a ship sails along quietly, everything outside of it appears to those
on board as if it moved with the motion of the boat, and the boatman thinks
that the boat with all on board is standing still, this same thing may hold
without doubt of the motion of the earth, and it may seem as if the whole
universe revolved. What shall we say, however, of the clouds and other
things floating, falling or raising in the air-except that not only does
the earth move with the watery elements belonging with it, but also a large
part of the atmosphere, and whatever else is in any way connected with the
earth; whether it is because the air immediately touching the earth has
the same nature as the earth, or that the motion has become imparted to
the atmosphere. A like astonishment must be felt if that highest region
of the air be supposed to follow the heavenly motion, as shown by those
suddenly appearing stars which the Greeks call comets or bearded stars,
which belong to that region and which rise and set like other stars. We
may suppose that part of the atmosphere, because of its great distance from
the earth, has become free from the earthly motion. So the atmosphere which
lies close to the earth and all things floating in it would appear to remain
still, unless driven here and there by the wind or some other outside force,
which chance may bring into play; for how is the wind in the air different
from the current in the sea? We must admit that the motion of things rising
and falling in the air is in relation to the universe a double one, being
always made up of a rectilinear and a circular movement. Since that which
seeks of its own weight to fall is essentially earthy, so there is no doubt
that these follow the same natural law as their whole; and it results from
the same principle that those things which pertain to fire are forcibly
driven on high. Earthly fire is nourished with earthly stuff, and it is
said that the flame is only burning smoke. But the peculiarity of the fire
consists in this that it expands whatever it seizes upon, and it carries
this out so consistently that it can in no way and by no machinery be prevented
from breaking its bonds and completing its work. The expanding motion,
however, is directed from the center outward; therefore is any earthly material
is ignited it moves upward. So to each single body belongs a single motion,
and this is evinced preferably in a circular direction as long as the singe
body remains in its natural place and its entirety. In this position the
movement is the circular movement which as far as the body itself is concerned
is as if it did not occur. The rectilinear motion, however, seizes upon
those bodies which have wandered or have been driven from their natural
position or have been in any way disturbed. Nothing is so much opposed
to the order and form of the world as the displacement of one of its parts.
Rectilinear motion takes place only when objects are not properly related,
and are not complete according to their nature because they have separated
from their whole and have lost their unity. Moreover, objects which have
been driven outward or away, leaving out of consideration the circular motion,
do not obey a single, simple and regular motion, since they cannot be controlled
simply by their lightness or by the force of their weight, and if in falling
they have at first a slow movement the rapidity of the motion increases
as they fall, while in the case of earthly fire which is forced upwards-and
we have no means of knowing any other kind of fire-we will see that its
motion is slow as if its earthly origin thereby showed itself. The circular
motion, on the other hand, is always regular, because it is not subject
to an intermittent cause. Those other objects, however, would cease to
be either light or heavy in respect to their natural movement if they reached
their own place, and thus they would fit into that movement. Therefore
if the circular movement is to be ascribed to the universe as a whole and
the rectilinear to the parts, we might say that the revolution is to the
straight line as the natural state is to sickness. That Aristotle divided
motion into three sorts, that from the center out, that inward toward the
center, and that around about the center, appears to be merely a logical
convenience, just as we distinguish point, line and surface, although one
cannot exist without the others, and none of them are found apart from bodies.
This fact is also to be considered, that the condition of immovability
is held to be nobler and more divine than that of change and inconstancy,
which latter therefore should be ascribed rather to the earth than to the
universe, and I would add also that it seems inconsistent to attribute motion
to the containing and locating element rather than to the contained and
located object, which the earth is. Finally since the planets plainly are
at one time nearer and at another time farther from the earth, it would
follow, on the theory that the universe revolves, that the movement of the
one and same body which is known to take place about a center, that is the
center of the earth, must also be directed toward the center from without
and from the center outward. The movement about the center must therefore
be made more general, and it suffices if that single movement be about is
own center. So it appears from all these considerations that the movement
of the earth is more probable than its fixity, especially in regard to the
daily revolution, which is most peculiar to the earth.