The Essential Galileo (35 page)

S
ALV.
    Despite my having thought about it for a long time, I have been unable to find any difference, and so my finding seems to be that there cannot be any difference; hence I feel it is useless to continue searching for one. Let me explain. Motion exists as motion and acts as motion [142] in relation to things that lack it, but in regard to things that share it equally, it has no effect and behaves as if it did not exist. Thus, for example, the goods loaded on a ship move insofar as they leave Venice, go by Corfu, Crete, and Cyprus, and arrive in Aleppo, and insofar as these places (Venice, Corfu, Crete, etc.) stay still and do not move with the ship; but for the bales, boxes, and packages loaded and stowed on the ship, the motion from Venice to Syria is as nothing and in no way alters their relationship among themselves or to the ship itself; this is so because this motion is common to all and shared equally by all; on the other hand, if in this cargo a bale is displaced from a box by a mere inch, this alone is for it a greater motion (in relation to the box) than the journey of two thousand miles made by them together.

S
IMP.
    This doctrine is correct, sound, and entirely Peripatetic.

S
ALV.
    I think it is even more ancient. Moreover, I suspect that, when Aristotle took it from some good school, he did not entirely grasp it, and that therefore he wrote it in altered form and so was the source of confusion with the help of those who want to support all his statements. I also suspect that, when he wrote that everything which moves, moves upon something unmoved, he engaged in an equivocation on the assertion that everything which moves, moves in relation to something unmoved; the latter proposition suffers no difficulties, the former many.
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SAGR. Please, let us not break the thread, and let us proceed with the discussion we began.

S
ALV.
    It is clear, then, that motion common to many movable things is idle and null in regard to their relationship among themselves (because nothing changes among them), and that it acts only in regard to the relationship between those movable things and others which lack that motion (for this is the relationship which changes). We have also divided the universe into two parts, for which it is necessary that one of them is mobile and the other immobile; in regard to whatever may depend on this motion, to make the earth alone move is equivalent to making the rest of the universe move because the action for this motion lies only in the relationship between the heavenly bodies and the earth, and this is the only relationship that changes. Again, let us assume that, in order to bring about the same effect in the finest detail, one can either have the earth alone moving with the whole rest of the universe stopped or have the earth alone still with the whole universe [143] moving by the same motion; if this assumption holds, who will believe that nature has chosen to let an immense number of very large bodies move at immeasurable speed to bring about what could be accomplished with the moderate motion of a single body around its own center? Indeed, who will believe this, given that by common consent, nature does not do by means of many things what can be done by means of a few?
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S
IMP.
    I do not understand very well how this very great motion is null for the sun, the moon, the other planets, and the innumerable array of fixed stars. How can you say it is nothing for the sun to pass from one meridian to another, rise above this horizon, set below that one, and bring day and night in turn; and also for the moon, other planets, and fixed stars to go through similar variations?

S
ALV.
    All these variations you mention are nothing except in relation to the earth. To see that this is true, imagine that the earth is taken away: there will no longer be in the world any rising or setting of the sun or moon, any horizons or meridians, any days or nights; nor would their motion ever produce any changes among the moon, the sun, or any other stars whatever (be they fixed or wandering). In other words, to say that all these changes relate to the earth means that the sun appears first in China, then in Persia, and afterwards in Egypt, Greece, France, Spain, America, etc., and that the moon and the other heavenly bodies do the same. This phenomenon occurs in exactly the same way if, without involving such a large part of the universe, the terrestrial globe is made to turn on itself.

However, let us double the difficulty with another very great one. That is, if this great motion is attributed to the heavens, it is necessary to make it contrary to the particular motion of all the planetary orbs; each of these unquestionably has its own characteristic motion from west to east, at a very leisurely and moderate speed; but then one has to let this very rapid diurnal motion carry them off violently in the contrary direction, namely, from east to west. On the other hand, by making the earth turn on itself, the contrariety of motions is removed, and motions from west to east alone accommodate all appearances and satisfy them all completely.

S
IMP.
    As for the contrariety of the motions, it matters little because Aristotle demonstrates that circular motions are not contrary to each other, and that theirs cannot be called true contrariety.

[144] S
ALV.
    Does Aristotle demonstrate this, or does he merely assert it because it fits his purpose? If, as he himself states, contrary motions are those that reciprocally destroy each other, I do not see how two moving bodies that collide along a circular line would damage each other any less than if they were colliding along a straight line.

S
AGR.
    Please, stop for a moment. Tell me, Simplicio, when two knights meet jousting in an open field, or when two whole fleets or armadas clash at sea breaking up and sinking each other, would you call such encounters contrary to one another?

S
IMP.
    Let us call them contrary.

S
AGR.
    How is it then that there is no contrariety for circular motions? For these occur on the surface of the land or the ocean, which (as you know) is spherical, and so they are circular after all. Do you know, Simplicio, which circular motions are not contrary to one another? They are those of two circles tangent to each other and such that the turning of one naturally makes the other one move in a different direction; but, if one is inside the other, it is impossible that their motions in different directions should not contrast with each other.

S
ALV.
    In any case, whether the motions are contrary or not, these are verbal disputes. I know that in fact it is much simpler and more natural to explain everything by means of a single motion rather than by introducing two of them. If you do not want to call them contrary, call them opposite. Moreover, I am not saying that this introduction of opposite motions is impossible; nor am I claiming to be giving a necessary demonstration, but only inferring a greater probability.

The unlikelihood is tripled by upsetting in a very disproportionate manner the ordered pattern we unquestionably see existing among those heavenly bodies whose revolution is not in doubt but most certain. The pattern is that when an orbit is larger, the revolution is completed in a longer period of time; and when smaller, in a shorter period.
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Thus Saturn, which traces a greater circle than any other planet, completes it in thirty years; Mars in two; the moon goes through its much smaller orbit in just a month; and, in regard to the Medicean Stars, we see no less sensibly that the one nearest Jupiter completes its revolution in a very short time (namely, about forty-two hours), the next one in three and one-half days, the third one in seven days, and the most remote one in sixteen. This very harmonious pattern is not changed in the least [145] as long as the motion of twenty-four hours is attributed to the terrestrial globe (rotating on itself). However, if one wants to keep the earth immobile, it is necessary first to go from the very short period of the moon to others correspondingly longer; that is, to that of Mars lasting two years, from there to the larger orbit of Jupiter requiring twelve years, and from this to the bigger one of Saturn with a period of thirty years; but then it is necessary to go to an incomparably greater orb and have an entire revolution completed in twenty-four hours.
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This is the least disorder that would follow; for someone may first want to go from Saturn to the stellar sphere and make it larger than the orbit of Saturn in a proportion appropriate to its very slow motion with a period of many thousands of years;
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but then one would have to make a much more disproportionate jump in going from the stellar sphere to an even larger one, and make the latter revolve in twenty-four hours. On the other hand, once we give motion to the earth, the order of the periods is very strictly followed, and from the very sluggish orb of Saturn we go to the fixed stars, which completely lack such motions.

The earth's rotation also enables one to escape a fourth difficulty, which must necessarily be admitted if the stellar sphere is made to move. The difficulty is the immense disparity among the motions of the stars: some would move at very great speed in very large circles, while others would move very slowly in very small circles, depending on whether they are respectively further away from or closer to the poles. This is problematic because we see those heavenly bodies whose motion is not in doubt all moving in great circles, as well as because it does not seem to be good planning that bodies which are supposed to move in circles be placed at immense distances from the center and then be made to move in very small circles.

Aside from the fact that the pattern of the magnitude of the circles and the consequent speed of the motions of these stars would be very different from the pattern of circles and motions of the others, each of these same stars would be changing its circle and speed,
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and this is a fifth disadvantage. For there are stars which two thousand years ago were positioned on the celestial equator, and consequently described great circles with their motion; but in our time they are located away from it by several degrees, and so one must attribute to them a slower motion and make them move in smaller circles; and it may even happen that the time will come when some star which in the past always moved will become motionless by being joined to the pole, and then again (after resting [146] for some time) it will get back in motion. On the other hand, as previously stated, all the other stars that are unquestionably in motion describe the greatest circle of their orb and keep themselves constantly in it.

The unlikelihood is increased by a sixth disadvantage. Anyone with sound common sense will be unable to conceive the degree of solidity of that very large sphere in whose thickness would be embedded so many stars so firmly that they do not change their relative positions in the least, and yet they are made to revolve together with such great disparity. On the other hand, it is more reasonable to believe that the heavens are fluid,
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so that each star wanders about in space by itself; if this belief is true, what law would regulate their motions and to what end, while making sure that (when observed from the earth) they would appear as if they were produced by a single sphere? It seems to me that an easier and more manageable way of accomplishing this would be to make them motionless, rather than making them wandering, just as it is easier to keep track of the many stones cemented into the pavement of a marketplace than of the bands of children running over them.

Finally, there is a seventh objection: if we attribute the diurnal turning to the highest heaven,
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it must have so much force and power as to carry with it innumerably many fixed stars (all very huge bodies and much larger than the earth) and also all the planetary orbs, even though both the latter and the former by nature move in the contrary direction;
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moreover, it is necessary to admit that even the element fire and most of the air would be carried along as well, and that only the tiny terrestrial globe would be stubborn and recalcitrant vis-à-vis so much power; this seems to me to be a very problematic thing, and I would be unable to explain how the earth (as a body suspended and balanced on its center, indifferent to motion and to rest, and placed in and surrounded by a fluid environment) would not yield and be carried along the rotation. However, we do not find such obstacles in giving motion to the earth; it is an insignificant and very small body compared to the universe, and thus unable to do any violence to it.

S
AGR.
    I feel some of these concepts whirling in my mind, and indeed I am very confused after the discussions we have just had; if I want to be able to concentrate on what remains to be said, I must try to put some order in my ideas and draw some useful lesson (if possible). Proceeding by questioning will perhaps help me to explain myself better. So I first ask Simplicio whether he believes that [147] different natural motions may belong to the same simple body, or else only one is appropriate as its own natural motion.

S
IMP.
    For a simple movable body, only one, and no more, can be the motion that naturally belongs to it; all other motions can belong to it only by accident or by participation.
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For example, for someone walking on a ship, his own motion is that of walking, and by participation he has the motion bringing him to port; for he will never arrive there unless the ship's motion takes him there.

S
AGR.
    Tell me a second thing, in regard to the motion that by participation is transferred to some moving body while the latter moves on its own with some motion different from the shared one; must this transferred motion belong to some subject by itself, or can it exist in nature without other support?

S
IMP.
    Aristotle answers all these questions. He says that, just as to a given moving thing there corresponds one particular motion, to a given motion there corresponds one particular moving thing; consequently, no motion can exist or be imagined without it inhering in its subject.
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SAGR. Thirdly, I should like you to tell me whether you believe that the moon and the other planets and heavenly bodies have their own proper motions and what these motions are.