48 THB MOON. [Lesson XI. 



nated, the opposite half remaining dark : then, as 

 the face of the Moon visible on the Earth, is that 

 part of her body turned towards us, we shall, ac- 

 cording to the different positions of the Moon, 

 with regard to the Sun and Earth, perceive dif- 

 ferent degrees of illumination. Hence the Moon 

 appears sometimes increasing sometimes waning; 

 sometimes horned, then half round; sometimes 

 gibbous, then full and round. At the time of 

 new Moon, the Moon is between the Earth and 

 Sun ; and at the time of full Moon, the Earth is 

 between the Moon and Sun. 



This may be familiarly illustrated by means of 

 an ivory ball, which, being held before a candle in 

 various positions, will present a greater or less 

 portion of its illumined hemisphere to the view of 

 the observer. The Ear^h presents the same 

 phases to a spectator on the Moon, as she does to 

 us, only in a contrary order, the one being full 

 when the other changes, &c. and in a greater de- 

 gree, the Earth giving thirteen times as much 

 light as the Moon. 



The face of the Moon, as you may have often 

 observed, appears to have shades of different 

 colours : when viewed through a telescope these 

 shades are found to be the diversifications of 

 hills and valleys; the same are also shewn by the 

 border of the Moon appearing indented or jag- 

 ged, especially about the edge of the illumined 

 part when the Moon is either horned or gibbous. 

 Dr. Herschel has measured the heights of several 

 of the lunar prominences with greater precision 



than 



