On February 10, 1773, Pierre Simon Laplace, a twenty-three years old scientist, read before the Academy of Sciences in Paris a paper in which, on the basis of the Newtonian theory of gravitation, he announced the invariability of planetary mean motions. This was the first step in the establishment of the stability of the solar system, says the Encyclopaedia Britannica (14th ed.). A mathematical genius, Laplace showed in a mathematical analysis that the planets must proceed on their paths to the end of time and that, accordingly, they have been on their present orbits since the very beginning. In a series of papers Laplace and Lagrange, another mathematical genius whose ideas went in the same direction, vied in a complete substantiation of this thesis of invariability of the planetary mean motions. No planet could ever have joined the family of the planets; no planet has ever changed its orbit. It was a work of stability in the cosmos carried through to the very eve of the French Revolution. In 1796, in a note to the Exposition du systeme du monde, Laplace offered his idea of the origin of the solar system. it was a large nebula, it rotated, and because of the gravitation of the mass to its center, a sun formed itself in the middle, and condensed. The outer parts of the nebula broke into rings, and the rings rolled themselves into globesthe planets. He insisted that there could be no accident in the fact that the sun, all known planets, all known satellites, roll in the same direction, counterclockwise. And, being a master of the theory of probabilities, he concluded that there are four billion chances against one that this plan is not the result of chance. Even the best known historical events have not been authenticated at the same ratio of four billion against one. By today we known that Laplace was wrong: with the discovery of the first retrograde satelliteand today more than ten retrograde satellites are known. The rotation of Venus is also retrograde, as is that of Uranus, discovered in 1781. The four billion against one odds became zero against one: there may still be a common plan in the arrangement, but this plan was no more evident.
Nevertheless, the estimate of the twenty-three year old Laplace that the planetary orbits are eternal became the pricipal statute of faith, or the supreme dogma of the astronomers of the nineteenth and twentieth centuries. On it is based the astronomy of today.
According to Laplace, gravitation, in order to keep this system together,
must propagate with a velocity that, compared to the velocity of light,
is at least fifty million times greater. And since light propagates with
the velocity of 300,000 kilometers in a second (186,000 miles), the velocity
with which gravitation must propagate in order that the solar system should
not fall apart must be infinite, or instantaneous. This last postulate
of Laplace was sometimes silently dropped out of his theory; and the permanency
of the celestial orbits remained, and served as alpha and omega of all