EVOLUTION OF THE STABS 219 



gren have shown that all of the suspected cases either rest upon insuffi- 

 cient observations of the comets at the time of their appearance, so that 

 the orbits are uncertain, or that the disturbing attractions of our 

 planets have converted the orbits from the elliptic to the hyperbolic 

 form after the comets have got well within our planetary system. 

 Another fact is equally important. By virtue of our rapid travels toward 

 the Lyra-Hercules region we should meet more comets coming from that 

 direction than there are comets overtaking us from the opposite direc- 

 tion. To state this point differently : of the comets which swing around 

 the Sun, a greater number should have come into our system from the 

 Lyra-Hercules region than from any other region, and especially from 

 the region of sky which we are leaving behind. The facts are other- 

 wise: we can not say that the approaches of comets favor any particular 

 direction. 



The orbits of the great majority of comets are very close to the para- 

 bolic form. The nature of comets is such that they are under observa- 

 tion for a few weeks or a few months, and only an occasional one for 

 a year or more. When but a small section of the orbit has been thus 

 observed it is difficult to decide between the parabola and a very elon- 

 gated ellipse. It happens, however, when these comets have been ac- 

 curately observed through many months, and the disturbing attractions 

 of our planets have been taken into account, that the orbits are found 

 to be very long ellipses and not parabolas; some of the ellipses are so 

 elongated that thousands, and occasionally hundreds of thousands of 

 years, are required to complete one circuit of the Sun. Let us assume 

 that a comet belonging to the solar system starts at rest, with reference 

 to the solar system, from a point midway between a Centauri and our 

 Sun, and travels around our Sun. It would be 00.000.000 years in 

 reaching us, or 120.000,000 years in completing its circuit. It is evi- 

 dent that an immense amount of cometary material must exist in the 

 outer regions of our Sun's gravitational field in order that a minute 

 part of it may visit the Sun every three months, which is about the 

 average interval of time between the coming of these bodies. 



It should lie noted that the planes of the very elongated comet orbits 

 show no preference for small angles with the plane of the solar system: 

 they intersect the solar system plane at all angles, and these comets 

 come into our system from all directions indifferently. 



We must hold, I think, that the comets are genuine members of our 

 solar system : the great majority spend most of their time in the outer 

 parts of our system, far beyond the orbits of Xeptune, but they are mov- 

 ing through space as companions to our Sun as truly as the Earth and 

 Jupiter are. 



Aside from the comets which come from great distances, there are, of 

 course, the so-called periodic comets which move in relatively short 

 ellipses, revolving around the Sun in a few years, and reappearing at 



