meridianer

Meridianer are imaginary half-circles that run from the North Pole to the South Pole, forming lines of longitude in the geographic coordinate system. Each meridian represents a specific longitude, and locations on Earth are described by a pair of coordinates: a longitude (meridian) and a latitude (parallel). Longitudes are measured in degrees east or west of a reference line, the prime meridian.

The prime meridian is designated 0 degrees longitude and passes through the Royal Observatory in Greenwich, London. Its adoption as the international standard was formalized at the International Meridian Conference in 1884. The line opposite it, near 180 degrees longitude, is close to the International Date Line, although it is adjusted in places to accommodate political and geographic boundaries. Time zones are roughly aligned with meridians, with each roughly 15-degree step corresponding to a one-hour difference from Greenwich Mean Time.

Meridians converge toward the poles and thus, on the surface of the Earth, are not parallel in a strict sense, but on a globe they form a network of interlocking half-circles. They are essential for navigation, mapping, and global positioning, providing a framework to specify absolute locations and to measure east–west distances. Historically, several different prime meridians were used before Greenwich was established as the international standard.

Other uses of the term meridian exist in different fields, such as traditional medicine, where meridians refer to energy channels; those are conceptually distinct from geographic meridians.