FORM, MOTIONS, LATITUDE, AND LONGITUDE 209 



ago chosen as the meridian from which distances east and west are 

 to be reckoned (Fig. 204). This meridian is the meridian of 0, 



North Pole 



South Pole 



Fig. 204. Diagram showing the position of the axis of the earth, the poles, 

 the equator, the meridian of Greenwich, and the meridian of 180. 



and is sometimes called the prime meridian. Distance east or 

 west of this meridian is known as longitude. Places east of longi- 

 tude are in east longitude, and those west of it are in west longi- 

 tude. East and west longitude respectively are regarded as ex- 

 tending 180 from the meridian of 0; that is, half-way around the 

 earth. 



The position of a place on the earth's surface may be fixed ex- 

 actly by means of meridians and parallels. If a place is in longi- 

 tude 30 E., its distance east of the meridian is known. If at 

 the same time it is in latitude 30 N., it must be on the parallel of 

 30 N. where it is crossed by the meridian of 30 E. 



Every meridian extends to each pole. It might at first seem, 

 therefore, that each pole has all longitude. But longitude is dis- 

 tance east or west of the meridian 0, and at the North Pole the 

 only direction is south, while at the South Pole the only direction 

 is north. The poles therefore cannot be said to have longitude, 

 since they are neither east nor west of the meridian of 0. 



Longitude and time. There is a definite relation between 



