SBCT. IX. ROTATION OF JUPITER'S SATELLITES. 65 



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of the western edge of her disc is visible, which is in- 

 visible at her setting, and the contrary takes place with 

 regard to her eastern edge. There are also librations 

 arising from the relative positions of the earth and 

 moon in their respective orbits ; but as they are only 

 optical appearances, one hemisphere will be eternally 

 concealed from the earth. For the same reason, the 

 earth, which must be so splendid an object to one lunar 

 hemisphere, will be forever veiled from the other. On 

 account of these circumstances, the remoter hemi- 

 sphere of the moon has its day a fortnight long, and a 

 night of the same duration, not even enlightened by a 

 moon, while the favored side is illuminated by the re- 

 flection of the earth during its long night. A planet 

 exhibiting a surface thirteen times larger than that of 

 the moon, with all the varieties of clouds, land, and 

 water coming successively into view, must be a splen- 

 did object to a lunar traveler in a journey to his an- 

 tipodes. The great height of the lunar mountains prob- 

 ably has a considerable influence on the phenomena of 

 her motion, the more so as her compression 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 per- 

 manent meridian, to which the different spots on her 

 surface hare been referred, and their positions are de- 

 termined 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 exceeded each other in brilliancy, and, by 

 comparing the maxima and minima of then' illumination 

 with their positions relatively 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. 



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