Page:Popular Astronomy - Airy - 1881.djvu/242

228 velocity with which the planet is set going. It is quite conceivable that if a planet were started with very great velocity, it might go away and never come back. The idea which Newton suggested was, that the motion of a comet was of that kind. And, upon pursuing the investigation, he found that a body subject to the attraction of a central body (as the sun) might, if the force varied inversely as the square of the distance, describe the curve called the parabola, (see page 131); but no other law of force would account for the description of such a curve. The form of the parabola is represented in Figure 58, C being the sun;

and this curve it is evident, possesses two of the peculiarities which most markedly distinguish the motions of comets; it comes very near to the sun at one part, and it goes off to an indefinitely great distance at other parts.

Now, when Newton had found out that the same laws of gravitation which were established from the consideration of elliptic motion would account for motion in a parabola, he began to try whether the parabola would not represent the motion of a cornet. It was found, that by taking a parabola of certain dimensions, and in a certain position, the motions of the comet which had then been observed most accurately could be represented with the utmost precision. Since that time, the same investigation has been repeated for hundreds of comets, and it has been found in every instance that the comet's movements could be exactly represented by supposing it to move in a parabola of proper dimensions and in the proper position, the sun being always situated at a certain point