Page:The story of the comets.djvu/224

170 a free comet approach, the attraction of the Sun is to the comet like the flame to the moth; the comet flutters for a moment about the Sun, and passes on its way. But not unscathed; like the moth, the comet has been singed; the fierce light of the Sun has beaten upon it, and spread out its particles and scattered them along its path.

"This idea that comets originate outside the solar system rests upon the supposed character of their orbits. The great majority of these strange bodies appear to travel in parabolas, open curves leading from infinite space to and around the Sun

(W. B. Gibbs, del.)

and thence back into the region of the fixed stars. Sir Isaac Newton first showed the possibility of comets moving in such paths, and the prestige of his name and the ease and facility with which parabolic orbits can be calculated led to the adoption of this curve as representing the motions of these bodies. Under the Law of Gravitation a body may travel about the Sun in any one of the three conic sections, or curves, known as the ellipse, the parabola, and the hyperbola. That is, if there were in the universe but two bodies, the Sun and a comet, then would the comet describe about the Sun one of these three mathematical curves, the exact character and size of the curve depending solely upon the speed of the