longitudes

Longitudes, or meridians, are imaginary lines that run from the North Pole to the South Pole on the Earth's surface. They form a coordinate system used with latitudes to specify a location. Each longitude is measured in degrees east or west of the Prime Meridian, a reference line that passes through the Royal Observatory in Greenwich, England. Longitudes range from 0° at the Prime Meridian to 180° east or 180° west, with the 180th meridian forming the approximate International Date Line opposite Greenwich. All meridians converge at the poles, so their distance apart varies with latitude.

The Prime Meridian was adopted as the international standard in 1884 at the International Meridian Conference. Greenwich was chosen because of its historical role in navigation and mapmaking. The International Date Line is not a perfectly straight line; it detours to accommodate political boundaries and practical travel considerations.

Longitudes are typically expressed in degrees, minutes, and seconds, or in decimal degrees. They may be labeled with E or W to indicate the direction from the Prime Meridian, or shown with positive and negative signs in some systems.

Historically, determining longitude at sea was a major navigational challenge. The invention of accurate marine chronometers and the development of celestial navigation during the 17th and 18th centuries enabled sailors to compute longitude from time differences with Greenwich mean time. In modern practice, longitude is defined within geodetic reference systems such as WGS84, and is determined precisely by satellite navigation systems like GPS.

Applications of longitude include navigation, timekeeping and time zones, cartography, surveying, and geospatial data analysis.