THE MOON. 



it should make one revolution upon its axis in the time it takes so 

 to revolve. For let us suppose that, in any one position, it has 

 the centre round which it revolves north of it, the hemisphere 

 turned toward the centre is turned toward the north. After it 

 makes a quarter of a revolution, the centre is to the west of it, 

 and the hemisphere which was previously turned to the north 

 must now be turned to the west. After it has made another 

 quarter of a revolution the centre will be south of it, and it must 

 be now to the south. In the same manner, after another quarter 

 of a revolution, it must be turned to the east. As the same 

 hemisphere is successively turned to all the points of the compass 

 in one revolution, it is evident that the globe itself must make a 

 revolution on its axis in that time. 



It appears, then, that the rotation of the moon upon its axis 

 being equal to that of its revolution in its orbit, is 27 days, 7 

 hours, and 44 minutes. The intervals of light and darkness to the 

 inhabitants of the moon, if there were any, would then be alto- 

 gether different from those provided in the planets ; there would 

 be about 13 days of continued light alternately with 13 days of 

 continued darkness ; the analogy, then, which prevails among the 

 planets with regard to days and nights, and which forms a main 

 argument in favour of the conclusion that they are inhabited 

 globes like the earth, does not hold good in the case of the moon. 



6. While the moon revolves round the earth, its illuminated 



Fig. 2. 



hemisphere is always presented to the sun; it therefore takes 

 various positions in reference to the earth. The effects of this 

 are exhibited in the annexed fig. 2. Let E s represent the direction 

 36 



