STATE OF THE MOON. 



515 



motions occupy almost exactly the same space of time. So we generally 

 see the same space of the moon, though there is a slight variation at times. 

 This movement or swaying of the central point is called the moon's " libra- 

 tion," and is an optical effect, due to the inequalities in the motion of the 

 moon in its orbit, and to the inclination of its equator and orbit to the 

 ecliptic. 



We append a map of the moon, on which the mountains, seas, and 

 craters can be perceived, according to the list. The hill ranges extend for 

 hundreds of miles, and the elevation reaches 30,000 feet, and even more in 

 places. The so-called craters do not resemble volcanoes when viewed closely, 

 but take the form of basins or valleys surrounded by lofty hills. One great 



Fig. 566. The Apennines and walled plain Archimedes 



plain called Copernicus is more than fifty miles across. Respecting the 

 appearance of the moon let us quote Mr. Lockycr. 



" Fancy a world without water, and therefore without ice, cloud, rain, 

 snow ; without rivers or streams, and therefore without vegetation to support 

 animal life ; a world without twilight or any gradations between the fiercest 

 sunshine and the blackest night ; a world also without sound, for as sound 

 is carried by the air, the highest mountain on the airless moon might be 

 riven by an earthquake inaudibly." 



PHASES OF THE MOON. 

 We have said that the moon revolves around the earth in the same 



