436 ASTRONOMY. 



through the poles, and the days and nights are 

 equal. 



After this the earth advances to the position D, 

 at which time the inhabitants of the northern 

 hemisphere have winter, and their days are shorter 

 than their nights. 



The positions B and D are the solstitial points ; 

 and A and C the equinoctial points ; they are not 

 equidistant from each other, because the sun is not 

 in the centre, but in the focus of the ellipsis. In 

 summer, when the earth is at B, the sun is farther 

 from it than in the winter, when the earth is at D; 

 and, in fact, the diameter of the sun appears longer in 

 winter than in summer. The difference of heat is 

 not owing to the sun's being nearer to us, or more 

 remote, but to the degree of obliquity with which 

 its rays strike any part of the earth. 



The Moon is, next to the sun, the most remark- 

 able of the celestial objects. Its form is spherical, 

 like that of the earth, round which it revolves, and 

 by which it is carried round the sun. Its orbit is 

 also elliptical, having the earth in one of the foci 

 of the ellipsis. The moon always keeps the same 

 side towards us, showing only at one time a little 

 more of one side, and at another time a little more 

 of the other side. When the moon is viewed 

 through a good telescope, its surface appears 

 covered with ridges, mountains, pits, and cavities 

 of great variety. 



It was formerly supposed that the height of the 

 lunar mountains exceeded those of our earth j but 

 Herschel has re-measured them, and supposes that 

 they generally do not exceed half a mile in height. 

 He has observed some of the mountains luminous; 

 these he supposes to be volcanoes. 



Some parts of its surface also reflect less light 



