The Essential Galileo (41 page)

S
ALV.
    Having accommodated Venus, you should think about Mercury; as you know, the latter stays always near the sun and recedes from it much less than Venus does.

S
IMP.
    There is no doubt that, since it imitates Venus, a very appropriate place for it will be a smaller circle inside that of Venus and also around the sun; a very conclusive argument or reason for this, especially for its vicinity to the sun, is the vividness of its shining, which is greater than that of Venus and the other planets. So, on this basis we can draw its circle, marking it with the letters
BG.

S
ALV.
    Where, then, shall we place Mars?

S
IMP.
    Because Mars reaches opposition to the sun, it is necessary that its circle enclose the earth. But I see that it must necessarily enclose the sun as well; for when this planet reaches conjunction with the sun it would appear horned (like Venus and the moon) if it were not beyond the sun but rather in between; however, it always appears round. Therefore, its circle must enclose both the earth and the sun.

Moreover, I remember your having said that when it is in opposition to the sun it appears sixty times larger than when it is near conjunction; so, I think these appearances will agree very well with a circle around the center of the sun and enclosing the earth, which I am now drawing and marking
DI.
Here, at the point
D
, Mars is closest to the earth and in opposition to the sun; but, when it is at the point
I
, it is in conjunction with the sun and farthest from the earth.

Finally, the same appearances are observed in regard to Jupiter and Saturn, although with much less variation for Jupiter than for Mars, and still less with Saturn than with Jupiter; so, I think I understand that these two planets will also be very adequately accommodated by means of two circles also around the sun. The first one is for Jupiter and is marked
EL;
the other larger one is for Saturn and is labeled
FM.

S
ALV.
    So far you have conducted yourself splendidly. Now, as you can see, the variation in distance for the three superior planets is measured by an amount twice the distance between the earth and the sun; hence, [353] the variation is greater for Mars than for Jupiter since Mars's circle
DI
is smaller than Jupiter's circle
EL;
similarly, because
EL
is smaller than Saturn's circle
FM
, the variation is even less for Saturn than for Jupiter; this corresponds exactly to observation. What remains for you now is to think about the place to assign to the moon.

S
IMP.
    Let us use the same argument, which seems to me to be very conclusive. Because we see the moon reach both conjunction and opposition with the sun, it is necessary to say that its circle encloses the earth; but we must not say that it encloses the sun because then near conjunction it would not appear horned but always round and full of light; furthermore, it could never produce, as it often does, an eclipse of the sun by coming between it and us. Therefore, it is necessary to assign to it a circle around the earth, such as this marked
NP;
thus, when positioned at
P
, from the earth
A
it appears in conjunction with the sun and so can eclipse it sometimes; and when located at
N
it is seen in opposition to the sun, and in this configuration it can come into the earth's shadow and eclipse itself.

S
ALV.
    What shall we do now with the fixed stars, Simplicio? Do we want to spread them in the immense space of the universe, at different distances from any determinate point? Or do we want to place them on a surface extending spherically around its center, such that each of them is equidistant from the same center?

S
IMP.
    I would rather follow an intermediate path. I would assign them an orb constructed around a determinate center and contained between two spherical surfaces, namely, a very high concave one and another convex one below it; and I would place the countless multitude of stars between them, but at different heights. This could be called the sphere of the universe, and it would contain inside it the orbs of the planets we have already drawn.

S
ALV.
    So far, then, Simplicio, the heavenly bodies have been arranged just as in the Copernican system, and you have done this yourself. Moreover, you have assigned individual motions to all except the sun, the earth, and the stellar sphere; to Mercury and Venus you have attributed a circular motion around the sun, without enclosing the earth; you make the three superior planets (Mars, Jupiter, and Saturn) move around the same sun, encompassing the earth inside their circles; then the moon can move in no other [354] way but around the earth, without enclosing the sun; and in regard to these motions you again agree with Copernicus.

Three things now remain to be assigned to the sun, earth, and stellar sphere: that is, rest, which appears to belong to the earth; the annual motion along the zodiac, which appears to belong to the sun; and the diurnal motion, which appears to belong to the stellar sphere and to be shared by all the rest of the universe except the earth. Since it is true that all the orbs of the planets (namely, Mercury, Venus, Mars, Jupiter, and Saturn) move around the sun as their center, it seems much more reasonable that rest belongs to the sun than to the earth, inasmuch as it is more reasonable that the center of moving spheres rather than any other point away from this center is motionless; therefore, leaving the state of rest for the sun, it is very appropriate to attribute the annual motion to the earth, which is located in the middle of moving parts; that is, between Venus and Mars, the first of which completes its revolution in nine months, and the second in two years. If this is so, then it follows as a necessary consequence that the diurnal motion also belongs to the earth; for if the sun were standing still and the earth did not rotate upon itself but only had the annual motion around the sun, then the cycle of night and day would be exactly one year long; that is, we would have six months of daylight and six months of night, as we have stated other times. So you see how appropriately the extremely rapid motion of twenty-four hours is taken away from the universe, and how the fixed stars (which are so many suns) enjoy perpetual rest like our sun. Notice also how elegant this first sketch is for the purpose of explaining why such significant phenomena appear in the heavenly bodies.

S
AGR.
    I see it very well. However, just as from this simplicity you infer a high probability for the truth of this system, others, on the other hand, might perhaps draw contrary conclusions; because such an arrangement is the very ancient one of the Pythagoreans and agrees so well with the observations, one might wonder (not without reason) how it could have had so few followers in the course of thousands of years, how it could have been rejected by Aristotle himself, and how even after Copernicus it could continue to suffer the same fate.

S
ALV.
    Sagredo, if you had ever happened to hear (as I have very many times) what kinds of stupidities suffice to make the common people stubbornly unwilling [355] to listen to (let alone accept) these novelties, I think you would wonder much less about the fact that there have been so few followers of this view. However, in my opinion, we should pay little attention to such brains; to confirm the earth's immobility and to remain unmoved in this belief, they regard as a very conclusive proof the fact that they cannot eat in Constantinople in the morning and have supper in Japan in the evening; and they are certain that the earth, being very heavy, cannot go up above the sun only to come back crashing down. We need not take into account these people, whose number is infinite, nor keep track of their stupidities; we need not try to gain the support of men whose definition contains only the genus but lacks the difference, in order to have them as companions in very subtle and delicate discussions. Moreover, what gain would you think you could ever make with all the demonstrations in the world when dealing with brains so dull that they are incapable of recognizing their extreme follies?

My wonderment, Sagredo, is much different from yours. You are surprised that there are so few followers of the Pythagorean opinion, whereas I am amazed at how there could ever have been anyone who accepted and followed it; nor can I ever sufficiently admire the eminence of mind of those who have accepted and regarded it as true, and who with the liveliness of their intellect have done violence to their own senses, so much so that they have been able to prefer what their theorizing told them over what their sensory experiences showed them very clearly to the contrary. We have already seen that the reasons against the earth's diurnal rotation, which have been examined, appear to be very good; the fact that they have been regarded as most conclusive by the Ptolemaics, Aristotelians, and all their followers is a very good argument for their effectiveness. However, the observations that clearly contradict its annual motion appear to be even more powerful, so much so that (I repeat it) there is no end to my admiration of how in Aristarchus and Copernicus their reason could have done so much violence to their senses as to become, in opposition to the latter, mistress of their belief.

S
AGR.
    Are we, then, also going to hear other powerful objections against this annual motion?

S
ALV.
    We are. These are so clearly based on our sense experience that, if a higher and better sense than the common and natural ones had not joined with [356] reason, I suspect that I too would have been much more recalcitrant against the Copernican system than I have been since a lamp clearer than usual has shed light on my path.

S
AGR.
    Now then, Salviati, let us join the fray, for any word uttered for any other purpose seems to me to be wasted.

S
ALV.
    I am ready to serve you.
⁵⁴
I have already explained to you the structure of the Copernican system. [357] Against its truth the first extremely fierce assault comes from Mars itself: if it were true that its distance from the earth varies such that the farthest minus the closest distance [358] equals twice the distance from the earth to the sun, it would be necessary that when it is closest to us its disk should appear more than sixty times larger than when it is farthest; [359] however, this variation of apparent size is not perceived; instead, at opposition to the sun, when it is close to the earth, it appears barely four or five times larger than near [360] conjunction, when it is hidden behind the sun's rays. Another and greater difficulty is due to Venus: if (as Copernicus claims) it should turn around the sun and be sometimes beyond the sun and sometimes in between, and if it should recede from us [361] and approach us by a difference equal to the diameter of the circle it describes, then when it is positioned between us and the sun and is closest to us its disk would appear almost forty times larger [362] than when it positioned beyond the sun and is near its other conjunction; however, the difference is almost imperceptible. To this we should add another difficulty: it seems reasonable that the body of Venus is inherently dark and shines only because of the sun's illumination, like the moon; if this is so, then when positioned between us and the sun it should appear sickle shaped, as the moon does when it is likewise near the sun; but this phenomenon is not observed in Venus. Thus, Copernicus declared that either it is inherently luminous, or its substance is such as to be capable of absorbing sunlight and transmitting it through its interior, so that it appears to be always shining; this is how Copernicus excused Venus for not changing its apparent shape;
⁵⁵
as regards the small variation in its apparent size, he said nothing. About Mars he said much less than was necessary; I believe the reason is that he was unable to account to his own satisfaction for a phenomenon so incompatible with his position; and yet, persuaded by many other confirmations, he stuck to it and regarded it as true. Furthermore, there is a feature that alters the order in such a way as to render it unlikely and false: all planets together with the earth move around the sun, which is at the center of their revolutions; only the moon perturbs this order, by performing its proper motion around the earth; and then it, the earth, and the whole elemental sphere all together move around the sun in one year.

These are the difficulties that make me marvel at Aristarchus and Copernicus; they must have known about those difficulties but were unable to solve them; and yet, because of other wonderful confirmations, they trusted what reason told them so much that they confidently asserted that the [363] structure of the universe can have no other configuration but the one constructed by them. There are then other very serious and very beautiful difficulties which are not easily solved by mediocre intellects, but which were understood and explained by Copernicus; we will discuss them below, after answering other objections which seem to undermine this position. Now, coming to the clarifications and solutions of the three very serious objections advanced above, I say that the first two not only do not contradict the Copernican system, but favor it considerably and absolutely; for both Mars and Venus do vary in apparent size in accordance with the required proportions, and Venus does appear sickle shaped when between us and the sun and, in general, changes in apparent shape exactly like the moon.

S
AGR.
    But how could this be hidden from Copernicus and revealed to you?

S
ALV.
    These things can be understood only with the sense of vision, which nature has not granted us in such a perfect state as to be able to discern such differences; indeed the very instrument for seeing contains impediments within itself. However, in our time God saw fit to allow the human mind to make a marvelous invention, which renders our vision more perfect by increasing its power by four, six, ten, twenty, thirty, and forty times; as a result, countless objects that were invisible to us because of their distance or extremely small size are now rendered highly visible by means of the telescope.