SECT, ix.] EOTATTON OF JUPITEE's SATELLITES. 79 



night of the same duration, not even enlightened by a moon, 

 while the favoured side is illuminated by the reflection of 

 the earth during its long night. A planet exhibiting a sur- 

 face thirteen times larger than that of the moon, with all 

 the varieties of clouds, land, and water, coming successively 

 into view, must be a splendid object to a lunar traveller 

 in a journey to his antipodes. The great height of the 

 lunar mountains probably has a considerable influence on 

 the phenomena of her motion, the more so as her compres- 

 sion is small, and her mass considerable. In the curve 

 passing through the poles, and that diameter of the moon 

 which always points to the earth, nature has furnished a 

 permanent meridian, to which the different spots on her 

 surface have been referred, and their positions are deter- 

 mined with as much accuracy as those of many of the most 

 remarkable places on the surface of our globe. 



The distance and minuteness of Jupiter's satellites render 

 it extremely difficult to ascertain their rotation. It was, 

 however, accomplished by Sir William Herschel from their 

 relative brightness. He observed that they alternately ex- 

 ceed each other in brilliancy, and, by comparing the maxima 

 and minima of their illumination with their positions rela- 

 tively to the sun and to their primary, he found that, like 

 the moon, the time of their rotation is equal to the period 

 of their revolution about Jupiter. Miraldi was led to the 

 same conclusion with regard to the fourth satellite, from 

 the motion of a spot on its surface. 



