Longitude

Longitude is the angular distance east or west of the prime meridian at Greenwich, 0 to 180 degrees, measured along a parallel of latitude. Unlike latitude, a minute of longitude is not a fixed distance: it equals one nautical mile only at the equator and shrinks as the cosine of the latitude toward the poles. Longitude is tied to time, 15 degrees per hour, which is why an accurate chronometer was the historic key to finding it at sea. Positions are written degrees-minutes-decimal with the suffix E or W.