Longitude and Latitude
The text and graphics below are from Larry Braile's Website
Positive and Negative Longitude and Latitude
The surface of the globe is divided into lines of longitude and latitude defining 360° of angle (and therefore location) in an east-west direction (lines of longitude) and 180° of angle in a north-south direction (-90 to +90, with zero at the equator). Using these definitions of longitude and latitude, any position on the Earth can be delineated by a unique pair of coordinates. The lines of longitude extend from pole to pole and are called "meridians". The meridian at 0° (through Greenwich, England, Spain and western Africa) is the Prime Meridian. The meridians are labeled from 0° to 180° (positive) for locations to the east of the Prime Meridian and 0° to -180° ( negative) for locations to the west of the Prime Meridian. This system divides the globe into eastern and western hemispheres. In this system, -180°=180° (the meridian approximately between Alaska and eastern Russia), and marks (approximately) the International Date Line.