. THE SOLAR REGION. 55 



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 particular systems within our large plan- 

 etary system, we find that, notwithstanding the analogies 

 which may present themselves 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 explored, we shall be vividly impressed 

 with the idea that the solar system, especially in the great 

 mutual connection revealed to us during the last ten years, 

 yields the richest image of the evident and direct relations 

 borne by many cosmical bodies to a special one. 



The more limited sphere of the planetary system affords 

 by its very limitation undoubted advantages, both as to the 

 certainty and correctness of the facts ascertained by measuring 

 and calculating astronomy, over the results of a contempla- 

 tion of the heaven of the fixed stars. Many of these results 

 are only connected with contemplative astronomy, through the 

 medium of stellar swarms and nebulous groups, as well as of 

 the insecurely-based photometric arrangement of the stars. 

 The most certain and brilliant portion of a^trognosy is the 

 determination of positions by right ascension and declination 

 a department of astronomical science that has been very 

 extensively improved and increased in our own day, in refer- 

 ence to isolated fixed stars, double stars, stellar masses, and 

 nebulae. Equally difficult, although more or less accurately 

 measurable relations likewise present themselves in the prop- 

 er motion of the stars the elements from which their paral- 

 laxes are determined telescopic star-gauging, which leads 



that of Uranus, according to Hansen, about 1586 millions. The dis- 

 tance 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 four- 

 teen 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 

 limes 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. 204). The es- 

 timate of such numerical relations has, at all events, this merit, not- 

 withstanding other defects, that the assumption of a very high standard 

 of measurement of space leads to results which may be expressed in 

 smaller numbers 



