Department of History
University of California, Irvine
Instructor: Dr. Barbara J. Becker
Week 8. Cosmological Questions.
The System of the World (1796)
by Pierre-Simon de Laplace (1749-1827)
|Let us cast our attention on the arrangement of the solar system and
its relation to the stars. The immense globe of the sun, focus of
the principal planetary motions, turns on itself in twenty-five and a half
days; its surface is covered with an ocean of luminous material whose lively
effervescence forms varying spots, often quite numerous and sometimes larger
than the earth.
Above this ocean rises a vast atmosphere; it is here that the planets with their satellites move in nearly circular orbits which are inclined very little to the solar equator. Countless comets, after approaching the sun, recede to distances which prove that the sun's domain extends far beyond the known boundaries of the system of planets. Not only does this star act on these bodies and force them to move around it, it spreads on them its light and heat. Its benevolent action permits animals and plants to spring up and cover the surface of the earth, and analogy leads us to believe that it produces similar effects on the planets; because it is not natural to suppose that the material whose fecundity we see develop in so many ways would be sterile on so large a planet as Jupiter, which, like the earth, has its days, nights and years, and where changes implying very active forces have been observed. Man, made for the temperature which he enjoys on earth, apparently could not live on other planets; but ought there not be an infinite variety of configurations suitable to the varied temperatures of celestial bodies? If the mere difference of composition and climate gives such variety in earthly objects, how much more must those of the various planets and their satellites differ? The most active imagination cannot conceive them, but their existence is most likely.
Although the geometrical elements of the planetary system are physically independent of each other, there are, nevertheless, certain relationships among them which can clarify their origin. On close consideration, it is astonishing to find all the planets moving about the sun from west to east and almost in the same plane; all of the satellites move about their planets in the same sense and nearly in the same plane as the planet; finally, the sun, the planets and their satellites, whose rotary motion we can observe, turn on themselves in the direction and nearly in the plane of their orbital motion.
Such an extraordinary phenomenon can hardly have haphazard causes; it suggests that a general cause has established all of the motions. To obtain an estimate of the probability of such a cause, we remark that the planetary system as it is known today, comprises seven [planets] and fourteen satellites; we have observed the rotation of the sun, of five planets, of the moon, the ring of Saturn and one of his moons. These form an ensemble of thirty motions directed in the same sense.
An equally remarkable phenomenon of the solar system is that the orbits of the planets are nearly circular, while those of the comets are highly elongated; the orbits of the system offer no intermediate nuances. Again we are compelled to recognize the effect of a regular cause; happenstance alone could not possibly give an almost circular form to the orbits of all the planets. Whatever arranged these orbits also made them nearly circular. Moreover, the same cause must explain the great elongation of cometary orbits, and the fact that comets move in all directions as though they had been thrown at random.
Thus to trace back to the cause of the original motions of the planetary system, we have the following five facts: the motions of the planets in the same direction, and in almost the same plane; the motions of the satellites in the same direction as that of the planets; the rotary motions of these various bodies, and of the sun, in the same directions as their trajectories about the sun and in approximately the same planes; the nearly circular orbits of the planets and satellites; finally, the great elongation of the orbits of the comets, although their orientations have been left to chance.
M. [Georges Louis Leclerc, Comte de] Buffon [1707-1788] is the only one I know who has attempted, since the discovery of the true system of the world, to trace the origin of the solar system. He supposes that a comet, by falling into the sun, expelled a torrent of material which reunited into globes of various sizes and at various distances; these globes, after cooling and becoming opaque and solid, are the planets and their satellites.
This hypothesis fits the first of the five phenomena listed previously, because it is clear that all the bodies thus formed will move near to a plane passing through both the center of the sun and the torrent of material which produced them; the other four phenomena appear inexplicable to me by this hypothesis. In fact, the absolute movement of the molecules of a planet ought then to be in the direction of the motion of its center of gravity; but it does not at all follow that the motion of the rotation of the planets should be in the same direction as that of the orbital motion..... [The same can be said of the satellites.]
The circularity of the planetary orbits is not only very difficult to explain with this hypothesis, the facts seem to contradict it. If a body moving in an orbit about the sun comes close to the surface of this star, it will return unfailingly at each revolution; from this it follows that if the planets had originally been detached from the sun they would touch it again at each return and their orbits would be far from circular. It is true that a mass of matter driven from the sun cannot be exactly compared to a globe which touches its surface, for the impulse which the particles of this mass receive from one another and the reciprocal attractions which they exert among themselves, could, in changing the direction of their movements, remove their perihelions from the sun; but their orbits would be always very elongated, or at least they would not be nearly circular except by the most extraordinary chance. Finally, there is no evident reason in the hypothesis of Buffon why the eighty comets observed thus far should have such elongated orbits. This hypothesis is far from fitting the facts. Let us see whether it is possible to deduce their true cause.
Whatever the sun's nature, it must have encompassed all of the planets; and considering the enormous distances separating these bodies, it must have been a fluid of an immense extent. In order to have given the planets almost circular motions in the same direction, this fluid must have surrounded the sun like an atmosphere. The consideration of the planetary motions thus leads us to think that, by virtue of an excessive heat, the solar atmosphere originally extended beyond the orbits of all the planets and that it progressively shrank to its present limits. This might have occurred through causes similar to those which made the famous star of 1572 suddenly shine so brightly for several months in the constellation of Cassiopeia.
The great eccentricity of cometary orbits leads to the same result; it suggests the disappearance of a large number of comets with less-eccentric orbits, as though they had been destroyed by passing through an atmosphere. If this were the case, the only comets existing today would be those which were outside the atmosphere during its enormous extension; and, as we can only observe comets which come quite close to the sun, the observable comets would not be those with very eccentric orbits. And, at the same time, we can see that the orientation of their orbits would be scattered at random, because the solar atmosphere could not have influenced them.
But how did the extended atmosphere of the sun produce the rotations of the planets? As these bodies would have fallen into the sun if they had penetrated the atmosphere, we may conjecture that they were formed at the successive limits of the atmosphere, in zones of material abandoned as it contracted toward the surface of the sun. We may further conjecture that the satellites were formed in a similar fashion from the atmospheres of the planets....
The five phenomena of the solar system, mentioned earlier, flow naturally from these hypotheses, to which the rings of Saturn add a further degree of likelihood.
Whatever may be the fate of this theory, which I present with the diffidence appropriate to what is not the result of observation or calculation, it is certain that the arrangement of the solar system is such as to assure its greatest stability--supposing no trouble from external forces. To that end, the motions of the planets and the satellites are almost circular and are directed in the same sense and nearly in a common plane. This system will merely oscillate by a small amount about its average state. It seems that nature disposed everything in the sky to assure the stability of the planetary system, by a design similar to what it follows so wonderfully on earth for the preservation of humans and for the perpetuity of species.
Now let us cast our eyes beyond the solar system. Numberless suns are scattered through the immensity of space at such great distances form us that the entire orbit of the earth, observed from their positions, would be imperceptible. Several stars display remarkable periodic variations in their color and brightness....
Other stars have appeared quite suddenly and then have disappeared after several months. (One example of this was the star observed by Tycho Brahe in 1572, in the constellation Cassiopeia. It quickly surpassed the brightest stars, even Jupiter, and it was seen in the daytime. Its light then decreased and it vanished sixteen months after its discovery. Its color varied widely: it was first dazzling white, then reddish yellow and finally a leaden white like Saturn.) What amazing changes must have occurred on these great bodies to be observed from our distance. Think, how they must surpass what we see on the surface of the sun, and what convincing proof they give that nature is not everywhere and always the same. All such stars which again become invisible remained in precisely the same position during their apparition; thus, there exist large bodies in space, perhaps as numerous as the stars.
A luminous star of the density of the earth, and having a diameter two hundred and fifty times that of the sun would not, by virtue of its gravitational attraction, let any of its light escape to us. Thus, it is possible that the largest luminous bodies of the universe are, for that very reason, invisible.
It appears that the stars, far from being scattered at equal distances, are collected in various groups containing billions of stars. Our sun and the brightest stars are probably part of one such group, which seems to surround the sky and form the Milky Way. The large number of stars seen in the field of a powerful telescope aimed at the Milky Way gives proof of its immense depth, which is more than a thousand times the distance to Sirius.
An observer receding from the Milky Way would ultimately see a white and continuous light because irradiation, which persists even in the best telescopes, would cover and hide the gaps between the stars; it is thus likely that the starless nebulae are groups of stars seen from afar, and that they would give the appearance of the Milky Way if only we were to draw near them.
The mutual distances of the stars forming each group are at least one hundred thousand times greater than the distance from the sun to the earth; from the multitude of stars in the Milky Way, one can judge the prodigious extent of these groups. If one now reflects on the immense intervals separating them, the imagination is stunned by the grandeur of the universe and can hardly conceive it to be limited.
From these considerations, based on telescopic observations, it follows that the nebulae which appear sharply outlined and whose centers can be located with precision are, from our standpoint, the most fixed of the celestial objects; they are the objects to which the positions of the stars might be related.
...Seen in its entirety, astronomy is the handsomest monument to the human spirit, the most noble sign of man's intelligence. Seduced by the illusions of his sense and by self-esteem, man has long regarded himself as the center of celestial motion, and his vain pride has been punished by fears inspired by the stars. Centuries of work have finally pulled down the veil which had hidden from him the system of the world. Man is now seen to reside on a small planet, almost unseen in the vast extent of the solar system which, itself, is only a point in the immensity of space. The sublime consequences of this discovery are sufficient to give solace, despite the small space assigned to him in the universe.
Let us carefully conserve and continue to augment the body of this profound knowledge--the delight of thinking beings. It has rendered important service to navigation and geography, but its greatest benefit is to have dissipated the fears caused by celestial phenomena and to have destroyed the errors born out of ignorance of our true relationship with nature--errors that are all the more distressing because our social order rests solely on these relationships. TRUTH, JUSTICE: These are her immutable laws. How far removed they are from our own dangerous maxim that it is sometimes necessary to mislead, deceive and subjugate man to ensure his welfare. Cruel experience inevitably proves that these sacred laws may not be violated with impunity.