An east-west measurement of position on the Earth. Measurements of longitude range from 0° at the Prime Meridian at Greenwich to + 180° West and - 180° East of it. A line connecting places of the same longitude is a meridian. It is measured in degrees, minutes, and seconds. Unlike latitude, longitude cannot take the poles and equator as fixed points of reference. It must rely more on time, which is why accurate long-voyage navigation had to await the invention of an accurate timepiece. See Latitude.