The Earth considered as a Planet. 79 



the nights, which is twice in every annual 

 revolution of the earth, and are called the 

 autumnal and vernal equinoxes. 



The earth's annual motion causes an 

 apparent daily declination of the sun, or 

 in other words, he appears at different dis- 

 tances from the equator every diurnal 

 turn of the earth on its axis. Thus, about 

 the 22d of December, when the earth is 

 in Cancer, the sun will be over the tropic 

 of Capricorn ; and consequently, by the 

 earth's rotation on its axis, the inhabitants 

 of every part of this circle will success- 

 ively have the sun in their zenith, or, in 

 other words, he will be vertical to them 

 that day at noon. About the 20th of 

 March the earth is at Libra, and the sun 

 will then appear in Aries ; a central solar 

 ray will terminate upon the surface of the 

 earth in the equator ; and therefore the 

 sun appears to be carried round in the 

 celestial equator, and is successively ver- 

 tical to those who live under that circle. 

 About the 21st of June, when the earth 

 is in Capricorn, a central solar ray ter- 

 minates on the surface of the earth, in 

 the northern tropic, and for that day the 

 sun appears to be carried round in the tro- 



