The mutual attraction of matter, so far as things terrestrial are concerned, had occurred to the inquiring intellect of Francis Bacon; but it was left for Newton to propound it as the great principle that governs the physical universe.
Now let us see how all this bears on the truth of the Copernican system. Newton proved--and I may add that the improved methods of mathematics which have been adopted since his day make the proofs more simple and easy--that if any body moves in an ellipse, or indeed, in one of the other conic sections, the law of force, tending to the focus, is that of the inverse square of the distance.[26] Conversely, he proved that a body under the action of a central force, varying in intensity as the inverse square of the distance, will move in a conic section.
Then if the Moon moved in an ellipse, as it was easy to perceive that she did, and if her motion corresponded precisely with what it would be on the theory of universal gravitation; if also, as seemed evident, the planets revolved in ellipses, then the inference that the law of gravitation, as stated by Newton, was true became irresistible; not susceptible, as before stated, of direct and absolute proof, but established conclusively by a sound and legitimate induction.
What I have just stated shows that Kepler’s first law corresponds with Newton’s discovery; but the same is true of the two other laws. It would of course be out of place here to go minutely into all the evidence which can be gathered in support of the doctrine of universal gravitation. I may briefly state that all of Kepler’s laws are simply explicable by that hypothesis, and that the evidence derives additional confirmation from the following curious fact: observation shows that Kepler’s laws, though approximately true, are not strictly and accurately so; if the planets were mere particles revolving round the Sun, they would then be quite rigidly true, but the planets have a certain mass (though very small compared to the Sun) and so do in some measure attract the Sun as well as being attracted by him, and they, moreover, exercise a disturbing influence on each other. These perturbations, however, have been calculated, and the result is that they agree with what ought reasonably to be expected, supposing the theory of universal gravitation to be true. This confirmatory proof has been acquired, I need not add, since the time of Newton by the labours of astronomers, Laplace and others, who have succeeded him, and who have had the advantage of that more manageable method of mathematical calculation to which I have just alluded.