260 SPECIAL RESULTS IN THE URA.NOLOGICAL 



mical bodies revolve round one which is self-luminous; 

 or, to speak more precisely, round a common centre of 

 gravity, which is sometimes included within, and sometimes 

 falls without, the central body. " The great ellipse which 

 the Earth describes round the Sun is reflected in a small but 

 otherwise entirely similar ellipse, in which the centre of the 

 Sun moves round the common centre of gravity of the Earth 

 and Sun." Whether the planetary bodies, among which the 

 interior and exterior comets must also be included, are not 

 also partially capable of originating light of their own, be- 

 sides that which they receive from the, central body, is a 

 question which in these general indications needs not to be 

 further touched upon. 



"We have hitherto no direct evidence of the existence of 

 dark planetary bodies revolving round other fixed stars. 

 Should such exist, as was surmised long before Lambert by 

 Kepler, the faintness of reflected light must probably for 

 ever forbid their being seen by the inhabitants of the 

 Earth. If the nearest fixed star, a Centauri, is distant from 

 the Sun, 226,000 semi-diameters of the Earth's orbit, or 

 7523 semi- diameters of Neptune's orbit, and if the solar 

 distance of the aphelion of a comet of very wide elongation, 

 that of 1680 (to which, although on very insecure grounds, 

 a period of 8800 years has been attributed), is equal to 28 

 distances of Neptune, the distance of o Centauri will still be 

 270 times more than the extent of our solar domain taken 

 to the aphelion of that most distant comet. We see the 

 reflected light of Neptune at 30 times the distance of the 

 Earth from the Sun : if in more powerful telescopes to be 

 hereafter constructed there should be discovered three 

 more planets at distances successively increasing, so that the 



