Longitude is the angular distance around Earth from a reference plane that defines a prime meridian. A meridian is synonymous with a line of longitude. Some conventions apply to angular expressions of longitude. One is that the angle is expressed counterclockwise with respect to the view from the North Pole to the South Pole. Sometimes, the angle from the zero meridian is stated as a value from 0° to 360°. The other convention is to express the longitude as a negative value from 0° to -180° west from the prime and as a positive value from 0° to +180° east of the prime. Longitude can be expressed in a variety of units (radians, grads, semicircles, arc seconds, or mils) and formats (+/- or E/W, to indicate the hemisphere; degrees: minutes and fractional minutes; or degrees: minutes: seconds and fractional seconds).
The determination of longitude on the surface of Earth is difficult because there is no physical meaning to any prime meridian. The prime meridian has been variously located in Amsterdam, Athens, Beijing, Djakarta, Berlin, Bern, Brussels, Copenhagen, the Canary Islands, Helsinki, Istanbul, Lisbon, Madrid, Moscow, Oslo, Paris, Rio de Janeiro, Rome, St. Petersburg, Stockholm, Tokyo, Washington, and other places. It was not until the 1884 Meridian Conference that Greenwich, England, was selected as a suitable common prime meridian for the 22 attending countries, a decision based on the dominance of British charts in navigation. The Greenwich prime was defined as the center of the Royal Observatory transit telescope. Since then, most geodetic datums in use place the actual zero line of longitude a few tens of meters east or west of the original transit instrument. Even a datum used within Britain, such as the Ordnance Survey of 1936, places the zero line of longitude several meters away from the original Greenwich meridian. Without reference to a specific geodetic datum, longitude values will point to different places on Earth.
Since the establishment of an implied zero meridian in the 1960s by the Bureau International de L'Heure, modern geodetic datums fix the zero line of longitude not with a single point or line but based on a network of observatories around the world with longitudes whose offset from zero is defined. Geodesists may base their zero line of longitude on the International Earth Rotation Service's International Terrestrial Reference Frame (ITRF), which is updated to a new version every few years to account for continental drift. Modern global geodetic datums, such as the World Geodetic System of 1984 (WGS-84), are occasionally reset to ITRF, slightly changing the location of the zero line of longitude.
Datums, Earth's Coordinate Grid, Equator, Latitude
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