1873.] The Planet Mars in 1873. 179 



is illuminated renders the study of her surface exceedingly 

 difficult. A similar remark applies to Mercury ; and it need 

 hardly be said that the proximity of Mercury to the sun 

 presents a yet more serious difficulty to the telescopist, in 

 the fact, that Mercury is never far removed from the sun a's 

 respects apparent position. 



On the other hand, when we pass from Mars to the other 

 superior planets, we find that we must necessarily study the 

 surface even of Jupiter and Saturn under conditions very 

 much less favourable than those which exist in the case of 

 Mars. When at his nearest,, that is, when he is in opposition 

 near the perihelion of his orbit, Mars is but about 35 million 

 miles from the earth ; and even when he is in opposition 

 near aphelion his distance does not exceed 61 million miles. 

 But Jupiter is never less than 360 millions of miles from 

 the earth, and Saturn never less than 732 millions of miles. 

 So that taking the case of Jupiter at his nearest as compared 

 with Mars in opposition near perihelion, we see that in the 

 first place Jupiter is more than ten times as far away, and the 

 apparent dimensions of equal parts of his surface are therefore 

 reduced more than a hundred times as much, and also Jupiter 

 is very much less brilliantly illuminated by the sun. For the 

 least distance of Jupiter from the sun is 452,692,000 miles, 

 the least distance of Mars 126,318,000 miles, the former 

 distance exceeding the latter about 3* times ; and as 

 illumination varies as the square of the distance, it follows 

 that equal surfaces of Jupiter and Mars (both in perihelion) 

 receive from the sun quantities of light in the proportion 

 of about 13 to 1. Now this consideration is important in 

 comparing the circumstances under which we study Jupiter 

 and Mars. For, although in the case of Venus we have 

 spoken of a degree of brightness which interferes prejudicially 

 with observation, yet in the case of Mars and Jupiter we 

 are not troubled with an excess of light, insomuch that the 

 smaller quantity received from (equal surfaces of) Jupiter 

 introduces a difficulty. When the highest magnifying 

 powers are used, on the best observing nights, there is a 

 want of luminosity in the disc of Jupiter which renders the 

 study of his surface more difficult than it would otherwise be.* 



* The intrinsic brightness of Jupiter is not reduced to the same extent as 

 the quantity of light received by equal portions of his surface (compared with 

 that of Mars). Whether this be owing to the greater reflective power of his 

 surface (that is, of the surface which forms his visible disc), or to some in- 

 herent luminosity possessed by the planet, is not as yet determined. Adhuc 

 sub judice lis est. But that the peculiarity is very noteworthy will appear from 

 the considerations discussed farther on. In fact, Zollner estimates the reflective 

 power of the surface of Jupiter at more than twice that of the surface of 

 Mars, or greater in the ratio of 624 to 267. Prof. Bond (the elder;, of America, 

 estimated the reflective power of the surface of Jupiter still higher. 



