54 • COSMOS. 



them, self-luminous fixed stars revolve round one common 

 center of gravity, which is not filled with visible matter ; 

 while in our solar system dark cosmical bodies rotate around 

 a self-luminous body, or, to speak more definitely, around one 

 common center of gravity, which lies at different times either 

 within or without the central body. " The great ellipse 

 which the Earth describes round the Sun is reflected in a 

 small perfectly similar one, in which the central point of the 

 Sun moves round its own and the Earth's common center of 

 gravity." In general notices like the present, we need hard- 

 ly enter into any special consideration of the question as to 

 whether the planetary bodies, among which we must class 

 interior and exterior comets, may not be capable, at least in 

 part, of generating some special light of their own, in addition 

 to that which they receive from the central body. 



We have hitherto acquired no direct evidence of the exist- 

 ence of dark planetary bodies revolving round other fixed 

 stars. The faintness of the reflected light would prevent 

 their ever being visible to us, if, as Kepler conjectured (long 

 before Lambert), such bodies actually revolve round every 

 fixed star. If the nearest fixed star, a Centauri, be 226,000 

 times the Earth's distance, or 7523 times the distance of Nep- 

 tune ; if a very distant comet, that of 1680 (to which has been 

 ascribed, although on very uncertain data, a revolution of 

 8800 years), is twenty-eight times the distance of Neptune 

 from our solar system when in its aphelion, then the distance 

 of the fixed star a Centauri is still 270 times greater than 

 the distance of our solar system from the aphelion of the most 

 remote comet. The light of Neptune is reflected to us from 

 a distance thirty times greater than our distance from the 

 Sun. If, by the future construction of more powerful tele- 

 scopes, three additional planets should be recognized, each 

 situated at about 100 times the Earth's distance from the 

 other, even this would not amount to the eighth part of the 

 distance int'ervening to the aphelion of the comet referred to, 

 or to the 2200th part of the distance^ which the reflected 



* See Cosmos, vol. i., p. 109, 148, where I based my calculations on 

 the distance of Uranus, which then constituted the extreme known 

 boundary of the planetaiy system. If we assume the distance of Nep- 

 tune from the Sun to be 3004 times that of the Earth, the distance of 

 a Centauri from the Sun would still be 7523 times that of Neptune, the 

 parallax being assumed as 0' -9128 {Cosmos, vol. iii., p. 191),. yet the 

 distance of 61 Cygni is nearly two and a half, and that of Sirius (with 

 a parallax of 2"-230) four times that of a Centauri. The distance of 

 Neptune from the Sun is about 2484 millions of geographical miles, and 



