The Essential Galileo

by Galilei, Galileo, Finocchiaro, Maurice A.

  SAGR. Oh, what a scholar! I am at his command; he does not want to be duped by Aristotle, but wants to lead him by the nose and make him speak as he himself commands! See how important it is to know how to seize an opportunity. One should not deal with Hercules when he is in a rage and overtaken by fury, but rather while he is playing with Lydian maids.18 Oh, the unbelievable cowardice of slavish minds! To make oneself spontaneously a slave, to accept decrees as inviolable, to be obliged to call oneself persuaded and convinced by arguments so effective and clearly conclusive that its proponents cannot decide even whether they are written for that purpose and are meant to prove that conclusion! But let us mention the greatest folly: that they themselves are still uncertain whether the same author holds the affirmative or negative side. Is not this like regarding a wooden statue as their oracle, resorting to it for answers, fearing it, revering it, and worshipping it?

  SIMP. But, if one abandons Aristotle, who will be the guide in philosophy? Name some author.

  SALV. One needs a guide in an unknown and uncivilized country, but in a flat and open region only the blind need a guide; whoever is blind would do well to stay home, whereas anyone who has eyes in his head and in his mind should use them as a guide. Not that I am thereby saying that one should not listen to Aristotle; on the contrary, I applaud his being examined and diligently studied and only blame submitting to him in such a way that one blindly subscribes to all his assertions and accepts them as unquestionable dictates, without searching for other reasons for them. This abuse carries with it another extreme impropriety, namely, that no one makes an effort any longer to try to understand the strength of his demonstrations. Is there anything more shameful in a public discussion [139] dealing with demonstrable conclusions than to see someone slyly appear with a textual passage (often written for some different purpose) and use it to shut the mouth of an opponent? If you want to persist in this manner of studying, lay down the name of philosophers and call yourselves either historians or memory experts, for it is not right that those who never philosophize should usurp the honorable title of philosopher.

  However, we should get back to shore in order not to enter an infinite ocean from which we could not get out all day. So, Simplicio, come freely with reasons and demonstrations (yours or Aristotle’s) and not with textual passages or mere authorities because our discussions are about the sensible world and not about a world on paper. In yesterday’s discussions the earth was drawn out of darkness and brought to light in the open heavens, and we showed that to want to number it among those bodies called heavenly is not so doomed and prostrate a proposition as to be left devoid of any vital energy; and so today we should examine how much probability there is in holding it fixed and completely motionless (referring to the globe as a whole) and how much likelihood there is in making it move with any motion (and if so what type this is). I am undecided about this question, while Simplicio together with Aristotle is firmly on the side of immobility; because of this, he will present step-by-step the motives for their opinion, I will present the answers and arguments for the contrary side, and Sagredo will say what goes on in his mind and to which side he feels drawn.

  SAGR. I am happy with this arrangement, but on the condition that I am free to introduce whatever simple common sense may suggest to me.

  SALV. Indeed, I beg you to do exactly that; for I think the various authors have left out few of the easier and (so to speak) cruder considerations, so that only some of the more subtle and esoteric ones may be wanting and lacking; but to investigate these, what subtlety can be more appropriate than that of Sagredo’s intellect, which is most acute and penetrating?

  SAGR. I may be all that Salviati says, but please, let us not start on another sort of ceremonial digression because right now I am a philosopher and have come to school and not to city hall.

  [§8.3 Day II: Diurnal Rotation, Simplicity, and Probability]19

  SALV. So let the beginning of our reflections be the following consideration: whatever motion is attributed to the earth, [140] it must remain completely imperceptible and seem nonexisting for us living there and sharing that motion, as long as we look only at terrestrial things; on the other hand, it is equally necessary that the same motion appear to us to be common to all other visible bodies and objects which are separated from the earth and so lack that motion.20 Thus, the true method of investigating whether any motion may be attributed to the earth and, if so, what kind it may be is to consider and observe whether in the bodies separated from the earth one sees any appearance of motion belonging equally to all; for if a motion were seen, for example, only in the moon and had nothing to do with Venus or Jupiter or other stars, it could not belong to the earth in any way but only to the moon. Now, there is a motion which is very general and most important of all: it is the motion by which the sun, moon, other planets, and fixed stars (in short, the whole universe except only the earth) appear to us to move together from east to west in a period of twenty-four hours. In regard to this first phenomenon, this motion may belong either to the earth only or to the rest of the universe without the earth, for the same appearances would be seen in the one situation as in the other. Aristotle and Ptolemy grasped this consideration, and so when they try to prove the earth to be motionless, they argue only against this diurnal motion; but Aristotle mentions something or other against another motion attributed to it by an ancient author, of which we shall speak in due course.21

  SAGR. I understand very well the necessity of which you speak, but I have a difficulty which I do not know how to remove. Copernicus attributed to the earth another motion besides the diurnal one; so by the rule just stated, as regards the appearances, that other motion should remain imperceptible when we look at the earth but be visible in the whole rest of the universe; thus it seems one can necessarily conclude either that he clearly erred in attributing to the earth a motion which does not appear to be general in the heavens, or that if it is general then Ptolemy was equally wrong in not refuting it as he did the other.

  [141] SALV. Your difficulty is a very reasonable one; when we treat of the other motion you will see how much Copernicus’ intellect surpassed Ptolemy’s in cleverness and profundity, insofar as the former saw what the latter did not, namely, the wonderful accuracy with which this motion is reflected in all other heavenly bodies.22 However, for now let us put off this aspect and return to the first point; in regard to this, I shall begin with the more general considerations and propose the reasons that seem to favor the earth’s mobility, and then I shall listen to Simplicio for the opposite ones.

  Firstly, let us consider the immense size of the stellar sphere in comparison to that of the terrestrial globe, which can fit inside the former many millions of times, and let us also think of the speed required for it to make one entire rotation in twenty-four hours; given these considerations, I cannot persuade myself that anyone can be found who would think it is more reasonable and credible for the celestial sphere to undergo rotation and the terrestrial globe to stand still.

  SAGR. Let us assume that all phenomena which may be naturally dependent on these motions are such that the same consequences follow, without a difference, from one supposition as well as the other one; if this were so, my initial and general impression would be that whoever thought it more reasonable to make the whole universe move to keep the earth motionless was more unreasonable than someone who went up to the top of your cathedral to look at the city and its surroundings and demanded that they turn around him so that he would not have to bother turning his head. To overcome this absurdity and revise my impression, thus rendering this supposition more credible than the other one, the advantages deriving from it rather than the other would have to be great and many. But Aristotle, Ptolemy, and Simplicio must think that there are advantages in it; now, if these exist we should be told what they are, or else let it be admitted that there are not or cannot be any.

  SALV. 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.

  SIMP. This doctrine is correct, sound, and entirely Peripatetic.

  SALV. 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.23

  SAGR. Please, let us not break the thread, and let us proceed with the discussion we began.

  SALV. 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?24

  SIMP. 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?

  SALV. 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.

  SIMP. 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] SALV. 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.

  SAGR. 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?

  SIMP. Let us call them contrary.

  SAGR. 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.

  SALV. 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.25 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.26 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;27 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,28 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.