218 THE POPULAR SCIENCE MONTHLY 



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 veloci- 

 ties 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 -150,000,000 years to travel from the center to the circum- 

 ference 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. 



Comets 



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 advo- 

 cated 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 en- 

 counter 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. Inter- 

 preted, 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 com- 

 ing 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 gen- 

 eral, 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 Strom- 



