Longitude is the east-west
geographic coordinate measurement most commonly utilized in cartography and global navigation.The
Greek letter λ (lambda), describes the location of a place on
Earth east or
west of a north-south line called the
Prime Meridian. Longitude is given as an
angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Unlike
latitude, which has the
equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference
meridian had to be chosen. While
British cartographers had long used the
Greenwich meridian in
London, other references were used elsewhere, including:
El Hierro,
Rome,
Copenhagen,
Jerusalem,
Saint Petersburg,
Pisa,
Paris,
Philadelphia and
Washington. In
1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude. Each degree of longitude is further sub-divided into 60
minutes, each of which divided into 60 seconds. A longitude is thus specified in
sexagesimal notation as 23° 27′ 30" E. For higher precision, the seconds are specified with a decimal fraction. An alternative representation uses degrees and minutes, where parts of a minute are expressed in decimal notation with a fraction, thus: 23° 27.500′ E. Degrees may also be expressed as a decimal fraction: 23.45833° E. For calculations, the angular measure usually must be converted to
radians, so longitude may also be expressed in this manner as a signed fraction of π (
pi), or an unsigned fraction of 2π.
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