The Essential Galileo (36 page)

S
IMP.
    They have them, and the motions are those whereby they run through the zodiac: the moon in one month, the sun in one year, Mars in two years, and the stellar sphere in so many thousands; these are their own proper and natural motions.

S
AGR.
    But in regard to the motion whereby I see the fixed stars and all the planets proceed together from east to west and return to the east in twenty-four hours, in which way does it belong to them?

S
IMP.
    They have it by participation.

S
AGR.
    Therefore, this motion does not reside in them; now, since it does not reside in them, and since there must be some subject in which it resides, it is necessary that it should be the proper and natural motion of some other sphere.

S
IMP.
    In this regard, astronomers and philosophers have found a very high sphere without stars to which the diurnal rotation naturally belongs; it is called the Prime Mobile and carries along with it all the lower spheres, thus transferring its motion to them and sharing it with them.

S
AGR.
    However, suppose everything fits and agrees with perfect harmony without the introduction of unknown and very huge spheres, [148] without additional shared motions and transfers, by giving each sphere only its own simple motion, without mingling contrary motions, but having them all go in the same direction (as they must when they all depend on a single principle); then, why reject this proposal and accept those very strange and problematic complications?

S
IMP.
    The point is to find an easy and handy way of accomplishing this.

S
AGR.
    The way is promptly found, I think. Let the earth be the Prime Mobile, that is, let it rotate on itself every twenty-four hours in the same direction as all the other planets or stars; then they will all appear to rise and set in the usual way and exhibit all the other phenomena without that terrestrial motion being transferred to any of them.

S
IMP.
    The important point is to be able to move the earth without a thousand inconveniences.

S
ALV.
    All the inconveniences will be removed as you propose them. The things said so far are only the initial and more general reasons why it seems not to be entirely improbable that the diurnal turning belongs to the earth rather than to the rest of the universe; I do not advance them as inviolable laws but as likely reasons. Now, I understand very well that a single contrary experience or conclusive demonstration suffices to shoot down these and a hundred thousand other probable arguments; thus, one must not stop here, but proceed and hear what Simplicio has to say and what better probabilities and stronger reasons he advances against them.

S
IMP.
    I will first say something in general about all these considerations taken together, and then I will come to particulars. It seems to me that in general you base yourself on the greater simplicity and facility of producing the same effects; you do this when you judge that, in regard to the fact of causing them, it is the same to move the earth alone as to move the rest of the universe without the earth, but in regard to the manner of operation, the former is much easier than the latter. To this I answer that it seems the same to me too as long as I consider my own strength, which is not only finite but very puny; but from the standpoint of the power of the Mover, which is infinite, it is no harder to move the universe than the earth or a straw. Now, if the power is infinite, why should He not exercise a greater [149] rather than a smaller part of it? Thus it seems to me that your account in general is not cogent.

S
ALV.
    If I had ever said that the universe does not move due to insufficient power in the Mover, I would have made a mistake and your correction would be appropriate; for I admit that to an infinite power it is the same to move one hundred thousand things as to move one. What I said does not regard the Mover but only the bodies moved; that is, not only their resistance, which is undoubtedly less for the earth than for the universe, but also the other particulars mentioned above. Moreover, I want to respond to your saying that an infinite power is such that it is better to exercise a greater than a smaller part of it: a part of the infinite is never greater than another, if both are finite; nor can one say that one hundred thousand is a greater part of an infinite number than two, even though the former is fifty thousand times greater than the latter; if to move the universe one needs a finite power (although very great in comparison to what would suffice to move the earth alone), one would not thereby be using a greater part of the infinite, nor would the unused part be less than infinite; thus it makes no difference to use a little more or a little less power to bring about a particular effect. It should also be mentioned that the action of such a power does not aim at the diurnal motion alone, but that there are in the world many other motions known to us, and there may be many others unknown to us. So, from the standpoint of the things moved, there is no doubt that the shorter and quicker mode of operation is to move the earth rather than the universe; let us also keep in mind the many other conveniences and benefits it brings about; and let us remember the very true Aristotelian principle saying that it is useless to do with more means what can be done with fewer;
³⁴
all these considerations render it more probable that the diurnal motion belongs only to the earth, rather than to the universe except the earth.

S
IMP.
    In mentioning this principle you left out a clause that is all important, especially in the present context; it is the phrase “equally well.” Therefore, one must examine whether everything can be accommodated equally well with each of the two assumptions.

S
ALV.
    Whether both positions satisfy equally well is something that will be understood from the particular examinations of the phenomena which must be accommodated; so far we have discussed, and we are now discussing, hypothetically, namely, by supposing that in regard to accommodating the phenomena, both [150] positions are equally satisfactory. Moreover, in regard to the phrase you say I have left out, I suspect that instead you have superfluously added it; for “equally well” is a relationship, which necessarily requires at least two terms, since a thing cannot have a relation with itself (for example, one cannot say that rest is equally good as rest); furthermore, when one says “it is useless to do with more means what can be done with fewer means,” one understands that what is to be done must be the same thing and not two different things; now, since the same thing cannot be said to be equally well done as itself, adding the phrase “equally well” is superfluous and exemplifies a relation with only one term.

[§8.4 Day II: The Case against Terrestrial Rotation, and the
Value of Critical Reasoning]
³⁵

S
AGR.
    If we do not want the same thing happening as yesterday, let us please return to the subject; and let Simplicio begin to produce the difficulties which seem to him to contradict this new arrangement of the world.

S
IMP.
    The arrangement is not new but very old. That this is true may be seen from the fact that Aristotle refuted it. His refutations are the following:
³⁶
“First, if the earth were in motion (either around itself while located at the center, or in a circle while placed outside the center), this motion would have to be a violent one because it is not its own natural motion; if it were natural, it would also belong to every one of its particles, whereas each of them moves in a straight line toward the center. Being thus violent and preternatural, it could not be everlasting. But the world order is everlasting. Therefore, etc.

“Secondly, except for the Prime Mobile, all the other bodies moving with circular motion seem to fall behind and to move with more than one motion. Because of this, it would be necessary for the earth to move with two motions. If this were so, there would necessarily have to be variations in the fixed stars. But this is not seen; instead, the same stars always rise at the same places and always set at the same places, without any variations.

“Thirdly, the motion of the parts and of the whole is naturally toward the center of the universe; therefore, the whole stands still therein.” He also asks whether the motion of the parts is to go naturally to the center of the universe or to the center of the earth; he concludes that their proper instinct is to go to the center of the universe, and that their accidental instinct is to go to the center of the earth. We discussed this question at length yesterday.

[151] Fourthly, he confirms the same conclusion with an argument based on our experience with heavy bodies. As these fall down from on high, they move perpendicularly to the earth's surface. Similarly, projectiles thrown perpendicularly upwards come back down perpendicularly by the same lines, even when they are thrown to an immense height. These experiences provide a necessarily conclusive argument that their motion is toward the center of the earth, which awaits and receives them without moving at all.

Lastly, he mentions that astronomers have produced other reasons to confirm the same conclusions, namely, that the earth is at the center of the universe and motionless. He gives only one of these; that is, all phenomena seen in regard to the motions of stars correspond to the position of the earth at the center, and there could not be such a correspondence unless it were true.

There are other arguments produced by Ptolemy and other astronomers.
³⁷
I can bring them up now, if you so desire; or I can do it after you tell me what occurs to you in response to these Aristotelian ones.

S
ALV.
    The arguments produced in this matter are of two kinds: some regard terrestrial phenomena and have no relation to the stars; others are taken from the appearances and observations of heavenly bodies. Aristotle's arguments are mostly taken from things near us, and he leaves the others to astronomers; thus, it is appropriate, if you agree, to examine the ones taken from terrestrial experience first, and then we will come to the other kind. Moreover, Ptolemy, Tycho, and other astronomers and philosophers produced other such arguments besides accepting, confirming, and strengthening those of Aristotle; hence, these can all be considered together in order not to have to repeat twice the same or similar replies. So, Simplicio, whether you wish to relate them, or whether you want me to release you from this burden, I am here to please you.

S
IMP.
    It will be better for you to present them since you have studied them more, and so you will be able to present them more readily and in greater number.

S
ALV.
    As the strongest reason, everyone produces the one from heavy bodies, which when falling down from on high move in a straight line perpendicular to the earth's surface. This is regarded as an unanswerable argument that the earth is motionless. For, if it were in a state of diurnal rotation and a rock were dropped from the top of a tower, then during the [152] time taken by the rock in its fall, the tower (being carried by the earth's turning) would advance many hundreds of cubits toward the east and the rock should hit the ground that distance away from the tower's base. They confirm this effect with another experiment. That is, they drop a lead ball from the top of the mast of a ship which is standing still, and they note that the spot where it hits is near the foot of the mast; but if one drops the same ball from the same place when the ship is moving forward, it will strike at a spot as far away from the first as the ship has moved forward during the time the lead was falling. This happens only because the natural motion of the ball in free fall is in a straight line toward the center of the earth.

This argument is strengthened with the experiment of a projectile thrown upward to a very great height, such as a ball shot by a cannon aimed perpendicular to the horizon. The time required for it to go up and down is such that at our latitude we, together with the cannon, would be carried by the earth many miles toward the east; thus the ball could never fall back near the gun, but rather would fall as far to the west as the earth would have moved forward.

Moreover, they add a third and very effective experiment, which is the following: if one shoots a cannon aimed at a great elevation toward the east, and then another with the same charge and the same elevation toward the west, the westward shot would range much farther than the eastward one. For, since the ball goes westward and the cannon (carried by the earth) goes eastward, the ball would strike the ground at a distance from the cannon equal to the sum of the two journeys (the westward one made by itself and the eastward one of the cannon carried by the earth); by contrast, from the journey made by the ball shot toward the east, one would have to subtract the one made by the cannon while following it; for example, given that the ball's journey in itself is five miles and that at that particular latitude the earth moves forward three during the ball's flight, in the westward shot the ball would strike the ground eight miles from the cannon (namely, its own westward five plus the cannon's eastward three), whereas the eastward shot would range two miles (which is the difference between the five of the shot and the three of the cannon's motion in the same direction). However, experience shows that the ranges are equal. Therefore, [153] the cannon is motionless, and consequently so is the earth.

No less than this, shooting toward the south or toward the north also confirms the earth's stability. For one would never hit the mark aimed at, but instead the shots would always be off toward the west, due to the eastward motion of the target (carried by the earth) while the ball is in midair.