longitude

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is the angular distance, measured in degrees, minutes, and seconds, between the prime meridian and the meridian passing through the point. All lines of longitude, or meridians, are great circles that converge at the poles and are used with latitude to locate a point on the globe. The prime meridian, defined as 0°, runs through the Royal Observatory in Greenwich, England.

Historically, longitude determination was challenging at sea. Early navigation used celestial observations and lunar distances; accurate longitude measurement eventually depended on precise timekeeping. The development of accurate marine chronometers in the 18th century, notably by John Harrison, made practical determination of longitude feasible. In 1884, the International Meridian Conference selected the Greenwich meridian as the international standard, establishing 0° longitude and the system of east and west longitude up to 180°. The 180th meridian forms the opposite side of the globe, near the International Date Line, where days are changed.

Today, longitude coordinates are used with latitude in geographic information systems, maps, and navigation. Timekeeping underpins longitude: each 15° of longitude corresponds to one hour of time, so time zones are broadly aligned with meridians, though political boundaries produce deviations. At the poles, longitude lines converge and the concept of a single longitude becomes undefined.