BOOK I – CHAPTER XII
WHETHER THE SUN MOVES THE PLANETS
The solution to this problem cannot be achieved by Geometrical demonstration. Nevertheless, from reasons adduced in our Philolaus book 4, chapter 2, that which corresponds to reality seems more apt and congruent, and what becomes much more probable is that the planets and the rest of the heavenly bodies are moved by their own form rather than by an outside force; for since they have a form through which they exist and proceed, they should also be directed by that form to the end for which they came into existence, indeed, we see that they were born towards motion, therefore they have motion from their form. Furthermore, I offer another reason, taken from the necessity of motion of planets, which is more compelling, since with a planet having been positioned and being movable, it is put into motion, towards which motion its own nature renders the planet, as a necessary action, and as an action proceeding from its foundation, also because the planet is such. Thus, the origin of a necessary action is in the agent, just as combustion is in fire and illustration is in light, so motion is in being movable. On this account, since the planets are unavoidably moved, the effort is not by an external mover which moves them, but by some unspecified capacity; internal form suffices as much towards the constitution of the nature of the planets as towards the attained end; nevertheless, even though they are self-moved, the effort is not that of living organisms, in the same manner as it illustrates with light, as it consumes with fire, as it soaks with water, the labor is not by living organisms, rather, the capacity of moving, emanating from its form, is sufficient.
The most ingenious Kepler wanted the planets to be at rest [of their own accord] and on account of their natural inertia, but that they should be seized by solar light, as though by an agent of motive power, and thereby led round [about the Sun], as he described in chapter XXXIII of his Commentariade MotibusStellaeMartis, and in other terms in Book IV of the Epitome AstronomiaeCopernicanae, part iii, question 5: ‘The Sun, in order to carry the planets around, makes use, as though they were arms, of the power of its body emitted in straight lines throughout the whole extent of the world’; and in defense of this proposition he puts forward the following reasons:
The first of these reasons is that the planet is subject to a retardation during one part of its revolution, where it is, in fact, slower; but in another part it is faster. Now, it is slower where it is farther from the Sun, and faster where it is nearer the Sun. However these retardations and accelerations must not be ascribed to the motive power of the planet, for in that case, it would tire and weaken with time.
The second adds [the consideration drawn] from the motion of planets compared among themselves: they complete their orbit more slowly when they are more distant from the Sun, and more quickly when closer.
The third rests on the dignity and aptitude of this body which is the source of life and heat, and from which all vegetative life originates; in such a manner heat, as well as light, may be regarded to some extent as the agents of its action.
The fourth is drawn from the rotation of the Sun on its axis, which takes place in the same direction as all the others.
He would, furthermore, endow the Sun with a soul which subdues and enflames this great mass of matter, producing therein diverse changes subject to mutations, whence it appears that there is not a continuous, perpetual and uniform power in the body of the Sun; he proves [the animation of the Sun] from the very existence of light which he claims is related to the soul. As for the power by which the Sun seizes the planets, it is corporeal, not animal or mental; for which reason, namely, it is an immaterial kind of body, the virtue turns together with the Sun in the manner of a very rapid whirlpool, covering the whole extent of the path which it fills with the same velocity as the Sun turns about its center in its own small space.
He also puts forward the analogy of the magnet which attracts iron; he compares its action to the magnetic virtue of the Sun in the following way: the magnet, according to the position of its poles, attracts on one side only, and repels on the other side only, but the Sun possesses throughout its parts the active and energetic faculty to attract, repel, and retain the planet.
He maintains that one pole of the planets is friendly to the Sun, the other hostile; consequently, the Sun attracts them by one part and repels them by the other; and if there were no rotation, it would have attracted and united them to itself; but because the Sun turns on its axis, its species therefore turns with it, together with the motive power that moves the planets.
In reply to these premises, I say concerning the first that it does not prove that the planets are moved by the Sun, although it states a truth, namely, that the planets move more quickly nearer perihelion, and more slowly farther away. Indeed, the Sun does not cause this velocity and state of motion, but they depend entirely on the system of planetary motions and the manner in which it has been ordered, as we shall soon show.
To the second, I reply that it is true that the superior planets complete their revolutions more slowly due to the distance covered, as well as their form; it would seem [that in order to provide an explanation] that the two causes must be combined. However, it is not the distance from the Sun, nor the attenuation of the motive power that cause this retardation: for the greater distance of the planet from the Sun only makes its revolution more extended, and for that reason it needs a longer period to cover the orbit.
As for the third, I say that it merely proves that the rays from the Sun, as well as its motive power, act on the planet through light and heat, not by causing it to move, but by producing changes and, if such be its nature, generations and corruptions of the kind produced on Earth.
The fourth, in my opinion, proves nothing, unless it be that it is fitting for all bodies in the world to move, either locally, or by orbiting. I willingly grant this.
However, I should like to ask Kepler why he would endow the Sun with a motive soul and a corporeal life, whereas the other planets are stupid and so inert that they have within themselves a resistance to motion? Is it because changes such as we see on the Sun are never produced in the planets? For, if sunspots and the other things that show changes, mutations, and activity in parts of the Sun persuade that it is endowed with the semblance of life, is not our Earth, the planet we inhabit, perhaps subject to even more frequent and still graver changes? Our planet emits flames, vapors, waters; it causes trees to fall from great heights, sends clouds into the air. This proves the activity and changes in generation and corruption in various parts of the Earth: what, then, prevents it from being animated by a motive soul, and consequently moving of its own accord?
Then, how could the following propositions also be true: that which asserts that the Sun moves the Earth which, ]ike the other planets, possesses a certain inertia and resistance to motion; and this other, on page 175 of the Commentaria de MotibusStellaeMartis, which states that the motion of the chief, monthly Moon [derives] entirely from the Earth as its source; and further on, To be sure, such is the strength of the Earth’s immaterial species, and doubtless, too, the rarity of the lunar body is equally great, and the rli~like of this body [for motion] is equally feeble. If the Sun moves the Earth by means of its immaterial species, and if the Earth is inert with respect to motion like the other planets, how then is it able to impress a monthly motion on the Moon? This is absurd and impossible. Perhaps Kepler means that the Earth, impelled [in the first place] by the Sun, moves the Moon in a subordinate manner–but it is not difficult to close off this means of flight. If the planets are seized and carried on their paths by the light of the Sun in the guise of an instrument of the motive power, then the earth in the grip of this motive power would not be able to lay hold of the Moon and carry it along except by a repercussion [an effect] of the solar light, in so far as it (the Earth) reflects light from the Sun to the Moon. In that case, the Earth would cease to do so at the time of Full Moon, and the power of the Earth would increase from full Moon up to New Moon, at which time the Earth would move the Moon very rapidly. But nothing of the kind is revealed by the phenomena.
As for the power by which the Sun seizes or grips the planets, and which, being corporeal, functions in the manner of hands, it is emitted in straight lines throughout the whole extent of the world, and like the species of the Sun, it turns with the body of the Sun; now, seeing that it is corporeal, it becomes weaker and attenuated at a greater distance or interval, and the ratio of its decrease [in strength] is the same as in the case of light, namely, the duplicate proportion, but inversely, of the distances [that is, 1 /d 2 ]. Kepler does not deny this, yet he claims the motive power decreases only in direct proportion to the distance [that is, 1 /d]; furthermore, he says that this attenuation of the motive power produces a weakening of the power only in longitude, because local motion impressed by the Sun on the planets (which motion similarly animates the corporeal parts of the Sun itself) occurs only in longitude, not in latitude; consequently, he offsets the inadequacy of this analogy by increasing the [quantity] matter in the slower planets.
But Kepler’s reply carries little weight. For if he considers the amount of this motive power with respect to area, he must necessarily decrease it according to the duplicate proportion of the distance; on the other hand, if he regards it only as lines, he contradicts the previous proposition which states that this motive power is corporeal: for if it were such, it would not be able to rest only in lines. Furthermore, this power acts by contact of the Sun’s species which emanates from the Sun together with the motive power; now, this species touches the body of the planet in the same manner that one surface [contacts] another surface; consequently, the power touches it in the same manner, seeing that it emanates from the sun in the same fashion. It follows that the power must decrease in the duplicate proportion of the distance, as does the species. However, if we were to accept this argument, Saturn would make but one revolution while Jupiter, in the same period, would make three revolutions and part of the next, Mars 39, Venus 173, and Mercury nearly 557; as for the Earth, it would make 90. This is clearly contrary to the phenomena. In order to show agreement among the ratios between revolutions, he appeals in vain to the quantity of matter by compensating [balancing] the distance of one by the moles of the other. For example, Jupiter at its mean distance from the Sun is three and one-eighth times further away from the Sun than is Mars; for this reason, if the proportion between the periods of revolution were as the lines of distance, Mars would complete only three and one-eighth revolutions while Jupiter completed its revolution; consequently, Kepler had to increase the corporeal moles of Jupiter, or else decrease that of Mars, so that the moles of Mars being less, the species would show less resistance to move it [Mars], and therefore this planet would be accelerated to the degree where Mars would complete nearly seven revolutions. However, observations show that Mars is less than one-thousandth the size of Jupiter; where, then, is the proportion between motions? For the motion of Mars should have exceeded that of Jupiter beyond all measure; therefore, no compensation between the matter and moles [of the planet] and the distances is permissible. Moreover, Kepler cannot avoid objections by saying: all the fibers of the globe taken together are moved by all the circles of the motive power taken together in simple proportion, in the same manner as the individual lines, or the fibers of two isolated planetary bodies would be moved by the individual circles of the motive power in the simple proportion of the distances, as if corporeal power could reside in lines. When diminution founders on these reefs, the astronomy of fibers is included in the shipwreck.
Comparison with the magnet, as put forward [by Kepler], does not agree very well with the Sun’s virtues as postulated by him: for a magnet attracts with one of its poles, and repels, as it were, with the other, but that results from the nature of its parts, which cannot be changed. Now the Sun has no distinct parts, as are found in the planets, though Kepler says so without offering proof. A magnet attracts those parts of iron which are cognate to it, and does not make them turn [round about it], and attracts them by one part, while it repels with the other. When the planet’s fibers are in equilibrium, then the Sun either attracts the planet when at aphelion, or repels it at perihelion. On the other hand, a magnet never repels, but only tends to cause cognate parts to unite with it, and to set them in accordance with the direction of its poles. If the fibers, when at the apsides, attract and repel uniformly, ought not the Sun to propel the planet by the one, and attract it by the other, because the power and the body which is moved are in equilibrium? The planet ought, on this account, to remain stationary; nevertheless it continues to move. Finally I ask, how can he prove that the Sun would draw all the planets to itself, if it did not turn on its axis, and why, seeing that it does turn, it does not attract them, but makes them turn round about itself. I fear Kepler’s devices are merely fantasies engendered by his extremely lively, and most ingenious, mind which imagines the cause of things where the real cause is hidden.
Thus at last, the question resolved, I say that the Sun is moved by its own form around its axis, by which form it was ignited and made light, indeed I say that no kind of motion presses upon the remaining planets, which [kind of motion] carries them, indeed that the individual planets are driven round by individual forms with which they were provided, as we indicated sufficiently in the Philolaus, book 4, chapter 2; and now, in the following books, we will show more adequately and completely than Kepler the cause of the real acceleration and retardation of the planets, first in some, and then other parts of their orbit.