Page:On the connexion of the physical sciences (1834).djvu/26

14 nature of the conic sections in which the celestial bodies move, depends upon the velocity with which they were first propelled in space: had that velocity been such as to make the planets move in orbits of unstable equilibrium, their mutual attractions might have changed them into parabolas, or even hyperbolas, so that the earth and planets might, ages ago, have been sweeping far from our sun through the abyss of space: but as the orbits differ very little from circles, the momentum of the planets, when projected, must have been exactly sufficient to ensure the permanency and stability of the system. Besides, the mass of the sun is vastly greater than that of any planet; and as their inequalities bear the same ratio to their elliptical motions as their masses do to that of the sun, their mutual disturbances only increase or diminish the eccentricities of their orbits by very minute quantities; consequently, the magnitude of the sun’s mass is the principal cause of the stability of the system. There is not in the physical world a more splendid example of the adaptation of means to the accomplishment of the end, than is exhibited in the nice adjustment of these forces, at once the cause of the variety and of the order of Nature.

The mean distance of a planet from the sun is equal to half the major axis of its orbit: if, therefore, the planet described a circle round the sun at