﻿Character of Cometary Orbits. 49 



I now wish to point out a possible method by which comets 

 of very long period may have become permanently attached to 

 the solar system. 



The known elliptic comets are supposed to have had their 

 orbits changed into ellipses by perturbations produced by the 

 planets; and there are several reasons for believing this to be the 

 true account. Thus the planes of their orbits have a decided 

 tendency to make a small angle with the plane of the ecliptic, 

 and a large percentage of them have a direct motion. Both 

 these facts agree well with the theory that they have had their 

 orbits changed by the perturbing influence of the planets. 



It may be well here to remark that this tendency of the planes 

 of the orbits of periodic comets to have a small inclination to the 

 plane of the ecliptic does not arise from any disturbing action of 

 the planets upon their orbits after they have become periodic, 

 but arises from the fact that a non-periodic comet is more liable 

 to become changed into a periodic comet the smaller its inclina- 

 tion to the ecliptic upon its first approach to the sun. The 

 disturbing influence of the planets upon the planes of the orbits 

 of periodic comets will on an average act as much to increase 

 as to diminish their inclinations to the ecliptic. But when 

 we come to consider the parabolic comets, we find that the comets 

 having a retrograde motion are more numerous than those which 

 have a direct motion, and also that the planes of their orbits ex- 

 hibit no tendency to make a smaller inclination with the plane 

 of the ecliptic than with any other plane. Out of 1 74 parabolic 

 comets, 101 or 58 per cent, have a retrograde motion, and 102 

 or 59 per cent, have the planes of their orbits inclined at an 

 angle greater than 45° to the ecliptic. These facts are opposed 

 to the theory that these comets have had their orbits changed by 

 the attractions of the planets ; and we are thus induced to seek 

 for some other cause. 



May not an adequate cause be found in the resisting medium 

 which is supposed to surround the sun ? We have seen that 

 many comets must approach the sun for the first time moving in 

 elliptic orbits. The aphelion distances of some of these orbits 

 will be less than the distance of the nearest stars, and may be so 

 small that, if they be diminished to any considerable extent, the 

 comets will become permanently attached to the sun. Now the 

 effect of a resisting medium upon a very excentric orbit in 

 changing the length of the major axis will be much greater in 

 the course of one revolution than it is in the case of an orbit of 

 moderate excentricity such as Encke's. We may form a rough 

 notion as to what effect the resisting medium would have in di- 

 minishing the semiaxis of the orbit of a comet of long period by 

 the following approximate calculations. 



Phil. Mag. S. 4. Vol. 41. No. 270. Jan. 1871. E 



