THE SOLAR REGION. 353 



which the reflected light of a satellite revolving round 

 a Centauri would have to traverse in order to reach our 

 telescopic vision. But is it absolutely necessary that we 

 should assume the existence of satellites around the fixed 

 stars ? For when we cast a glance at the subordinate par- 

 ticular systems within our large planetary system, we find 

 that, notwithstanding the analogies which may present them- 

 selves in planets-attended by many satellites, there are others, 

 such as Mercury, Venus, and Mars, which have no attendant 

 moons. If we disregard that which is merely possible, and 

 limit ourselves to the consideration of that which is actually 



the extreme known boundary of the planetary system. If 

 we assume the distance of Neptune from the Sun to be 30-04 

 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. 261), 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 2,484 millions 

 of geographical miles, and that of Uranus, according to 

 Hansen, about 1,586 millions; the distance of Sirius 

 amounts, according to Galle, assuming the parallax computed 

 by Henderson, to 896,800 radii of the Earth's orbit, or 

 74,188.000 millions of geographical miles, a distance which 

 gives fourteen years for the passage of light.] The aphelion 

 of the Comet of 1680 is forty-four times the distance of 

 Uranus, and therefore twenty-eight times that of Neptune 

 from the Sun. According to these assumptions the Sun's 

 distance from the star a Centauri is nearly 270 times that of 

 this Comet in its aphelion, which we regard as the minimum 

 of the very bold estimates of the radius of the solar system 

 (see p. 277). The estimate of such numerical relations has, 

 at all events, this merit, notwithstanding other defects, that 

 the assumption of a very high standard of measurement of 

 space leads to results which may be expressed in smaller 

 numbers. 



