Chapter 10

The Planets the Earth and Moon

by Rene Descartes

Similarly, there are several things to note concerning the planets.

1. They all tend toward the center of the system that contains them.

They can never arrive at those centers since the sun and fixed stars already occupy the center.

But, in order to make you understand distinctly in what places the planets should stop, look for example at the one marked ~ [Saturn], which I suppose to follow the course of the matter of the heaven toward the circle K

If Saturn had a bit more force to continue its motion in a straight line than the aethereal air surrounding it, then, instead of always following that circle `K`, it would go toward circle `Y`, making it farther from center S.

The aethereal air that surround it at orbit Y would move faster and even are a bit smaller (or at least are not larger) than those at `K`. They would give it still more force to pass beyond toward F allowing it to the circumference of that system without stopping.

It would easily pass into another heaven. Instead of being a planet, it would become a comet.

No star can stop anywhere in all that vast space between the circle `K` and the circumference of the heaven FGGF, through which the comets take their course.

In addition, the planets of necessity cannot have more force to continue their motion in a straight line than have the parts of the aethereal air at `K`, when those planets move with the same agitation along with these parts; and all bodies that have more are comets.

Saturn has less force than the aethereal air surrounding it.

Those parts that follow it and that are placed a bit lower than it can divert it with the result that, instead of following circle `K`, it descends toward Jupiter.

Saturn can be exactly as strong as the aethereal air that will then surround it. This is because this aethereal air is more agitated than those at `K`. It will also agitate the planet more (being smaller). It will not be able to resist the planet as much.

In this case, the planet will:

• remain perfectly balanced in the middle of them
• from there take its course in the same direction as they about the sun, without being at one time or another more or less distant from the sun, except insofar as they can also be more or less distant from it.

But if Saturn being at Jupiter still has less force to continue its motion in a straight line than has the matter of the heaven found there, it will again be pushed lower by the matter, towards Mars, and so on. Until finally, it is surrounded by a matter that has neither more nor less force than it.

Thus there can be diverse planets, some more and others less distant from the sun, such as:

• Saturn
• Jupiter
• Mars
• Earth
• Venus
• Mercury[46].

Of these, the lowest and least massive can reach to the sun’s surface.

But the highest never pass beyond circle `K` which, although very large in comparison with each planet in particular, is nevertheless so extremely small in comparison with the whole of heaven `FGGF` that it can be considered as its center.

How can

The parts of the heaven beyond circle `K` are incomparably smaller than the planets.

, do not cease to have more force than they to continue their motion in a straight line, consider that this force does not depend solely on the quantity of the matter that is in each body, but also on the extent of its surface.

When 2 bodies move equally fast, if one contains twice as much matter as the other, it also has twice as much agitation.

• But it does not follow that it has twice as much force to continue to move in a straight line.
• Rather, it will have exactly twice as much if, in addition, its surface is exactly twice as extended, because it will always meet twice as many other bodies resisting it, and it will have much less force to continue if its surface is extended much more than twice.[47]

The parts of the heaven are all round.

All shapes, they have the one that includes the most matter within the least surface. The planets are made up of small parts having very irregular shapes.

• They have large surfaces in proportion to the quantity of their matter.

Thus, the planets:

• can have a greater ratio of surface to volume than most of those parts of the heaven
• nevertheless also have a smaller surface than some of the smallest parts that are closest to the centers.

Among 2 balls, the smaller ball always has more surface per matter than the large ball. [48]

If we push a large ball composed of many tree branches confusedly joined and piled on top of one another (as one must imagine are the parts of matter of which the planets are composed), even if it be pushed by a force entirely proportional to its size, it will not be able to continue its motion as far as would another ball, very much smaller and composed of the same wood, but wholly massive.

By contrast, we could make another ball of the same wood and wholly massive, but so extremely small that it would have much less force to continue its motion than had the first.

This first ball can have more or less force to continue its motion according as the branches composing it are more or less large and compressed.

This is how:

• diverse planets can be suspended within circle K at diverse distances from the sun
• it is not simply those that outwardly appear the largest, but those that are the most solid and the most massive in their interior, that are the most distant.

Boats following the course of a river never move as fast as the water that bears them, nor indeed the larger among them as fast as the smaller, so too, even though the planets follow the course of the matter of the heaven without resistance and move with the same agitation as it,

that is not to say thereby that the planets ever move entirely as fast as the matter.

The inequality of their motion must bear some relation to the inequality between the size of their mass and the smallness of the parts of the heaven that surround them.

This is because the larger a body:

• the easier it is for it to communicate a part of its motion to other bodies
• the more difficult it is for the others to communicate to it something of their own motion.

Many small bodies all working together to act on a larger one may have as much force as it. But they can never make it move as fast as they in all directions. This is because if they agree in some of their motions which they communicate to it, at the same time they most certainly differ in others which they cannot communicate to it.

1. The matter of the heaven must make the planets turn not only around the sun, but also about their own center, except when there is some cause that hinders them

Consequently, the matter must compose around the planets small heavens that move in the same direction as the greater heaven.

1. If two planets, unequal in size, but at the same orbit, , and the planets are such that the one is exactly as much more massive as the other is larger, then the smaller of the two, having a faster motion than that of the larger, will have to link itself to the small heaven around that larger planet and turn continually about it.

The parts of the heaven that are at A move faster than the planet marked T, which they push toward Z, they must be diverted by it and constrained to take their course toward B.

I say toward B rather than toward D; for, having inclination to continue their motion in a straight line, they must go toward the outside of the circle ACZN they are describing, rather than toward the center S.

Now, passing from A to B, they force the planet T to turn with them about its center.

In turn, this planet in so turning gives them occasion to take their course from B to C, then to D and to A, and thus to form about the planet a particular heaven, with which it must thereafter continue to move from the direction one calls the “occident” toward that which one calls the “orient,” not only about the sun but also about its own center.

Moreover, knowing that the planet marked ~ [Moon] is disposed to take its course along the circle NACZ (just as is the planet marked T) and that it must move faster because it is smaller, it is easy to understand that, wherever it might have been in the heavens at the beginning, it shortly had to tend toward the exterior surface of the small heaven ABCD, and that, once having joined that heaven, it must thereafter always follow its course about T along with the parts of the second element that are at that surface.

Since we suppose that it would have exactly as much force as the matter of that heaven to turn along circle NACZ, if the other planet were not there, then we must imagine that it has a bit more force to turn along circle ABCD, because it is smaller and consequently always moves as far away as possible from the center T.

In the same way, a stone being moved in a sling always tends to move away from the center of the circle it is describing. This planet, however, being at A, will not thereby act to move off toward L, in as much as it would then enter a place in the heaven of which the matter had the force to push it back toward circle NACZ.

By the same token, being at `C`, it will not act to descend toward `K`, in as much as it would there be surrounded by a matter that would give it the force to ascend again toward that same circle `NACZ`. Nor will it go from B toward Z – much less from `D` toward `N` – in as much as it could not go as easily nor as fast as it could toward C and toward A.[49] Thus, it must remain as if attached to the surface of the small heaven ABCD and turn continually with it about T. That is what impedes its forming another small heaven about it, which would make it turn again about its own center.

I shall not add here how one can find a greater number of planets joined together and taking their course about one another, such as those that the new astronomers have observed about Jupiter and Saturn.[50]

I only mention the two planets above only to represent to you (by the planet marked T) the earth we inhabit and (by that marked ¢ [Moon]) the moon that turns about it.

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