The Individual Motive Forces

The mechanism of planetary motion seemed therefore to be definitely established, but a fresh difficulty immediately arose. In fact, the planets being subjected to the raptus of the solar whirlwind—and to it alone—ought to describe concentric orbits about the central body, and their motion in these orbits ought to be perfectly uniform. The ‘force’ emanating from the Sun (or from the Earth), whether it be considered after the manner of light or of magnetic force, is a purely motive one; it produces forward movement; it does not attract, nor does it repel. Consequently, the planets in their motion, should have no reason to approach the Sun (which does not attract them, as the Earth does the Moon); nor should they have reason to move away from it—in Kepler's universe the circular motion of celestial bodies does not develop centrifugal force ¹ ; so, they would revolve eternally at the same distance from their motive source, which acting in a uniform and constant manner—the supreme rule in celestial mechanics was that the revolution of the Sun, and hence its species, takes place in a uniform and constant manner ² —would confer on them likewise a uniform and constant motion. Now, we know that this is not the case. Therefore, in order to explain their true motions, it becomes necessary, in addition to having the common solar motive force, to endow each planet with its own individual motive force whose action accounts for the eccentricity of the orbits, and, at the same time, the exact non-uniformity of the motions round the Sun.