Longitude

Longitude, represented by a series of vertical lines on a map - or vertical rings on a globe - is the angular distance of a point on the Earth either east or west of the Prime Meridian ( 0° ) which runs through Greenwich, England. There are 360 lines of longitude, 180 east and 180 west of the Prime Meridian. E180° and W180° are the same. Lines of longitude are often refered to as "meridians".

Every circle formed by lines of longitude is a Great Circle - compare this to latitude where only the equitorial line of latitude is a Great Circle.