Page:Popular Science Monthly Volume 87.djvu/222

218 a very great variety of speeds, yet, on the average, that group of stars seems to be approaching ns at the rate of 19 kilometers per second. In a similar manner it has been found that the stars near the opposite point of the sky, while moving individually with a great variety of velocities of approach and recession, are, on the average, receding from the solar system with a speed of 19 kilometers per second. No one questions the explanation of these facts: the solar system is traveling toward the Hercules-Lyra region with a speed of 19 kilometers per second. If, now, the speed of 19 kilometers per second be maintained, and the longer radii of our stellar system be 30,000 light years, we should require a period of 450,000,000 years to travel from the center to the circumference of our system. The youth of the solar system was probably spent in a very different part of the stellar system from where it now is.

Are the comets bona fide members of the solar system as the planets are, or are they transient visitors from the greater stellar system? Immanuel Kant in 1755 advocated the view that the comets are genuine members of the solar system. From 40 to 70 years later Laplace advocated the other view, that the comets belong to the great stellar system, and that a few of them happen, in the course of their travels, to encounter the solar system. The latter view prevailed from Laplace's time almost up to to-day. If the comets are of our solar system they should move in elliptic orbits; that is, they should return again and again to the vicinity of the Sun.

If the Sun were at rest with reference to the stellar system and the comets should start with exceedingly small velocities from a very great distance, say 20 or more light years away, they would travel around our Sun in curves which we could not distinguish from parabolas. Interpreted, this means that they would eventually go back to approximately the same distant region of space from which they started and never again return to the solar system. If the comets should start toward us, from interstellar space, with appreciable velocities, they would move around the Sun in hyperbolic orbits, curves whose branches, one coming in toward the Sun and one going out from the Sun, diverge widely; such comets would go away to a region of space totally different from that which they had occupied before their solar visits and never return either to us or to their original habitation. Since the Sun is not at rest in the stellar system, but is traveling 19 kilometers per second toward the Lyra-Hercules constellations, it can be shown that the forms of the orbits of comets coming from interstellar space, whether they start from rest or with the average speed of the stars, would, in general, be strongly hyperbolic. The observed facts are that of the more than 400 cometary orbits determined, only 8 or 9 have been suspected to be hyperbolic. Further, the recent researches of Fabry and