longitude (lŏnˈjĭtōdˌ) [key], angular distance on the earth's surface measured along any latitude line such as the equator east or west of the prime meridian. A meridian of longitude is an imaginary line on the earth's surface from pole to pole; two opposite meridians form a great circle dividing the earth into two hemispheres. By international agreement, the meridian passing through the original site of the Royal Greenwich Observatory at Greenwich, England, is designated the prime meridian, and all points along it are at 0° longitude. All other points on the earth have longitudes ranging from 0° to 180°E or from 0° to 180°W. Except where it is changed to account for populated areas, the international date line lies along the 180° meridian. Meridians of longitude and parallels of latitude together form a grid by which any position on the earth's surface can be specified. The term longitude is also used in various celestial coordinate systems (see ecliptic coordinate system).