Longitude

Longitude, sometimes denoted by the Greek letter λ, 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: Ferro, 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.

Related Topics:
Greek letter λ - Earth - East - West - Prime Meridian - Angular measurement - Latitude - Equator - Meridian - British - Greenwich - London - Ferro - Rome - Copenhagen - Jerusalem - Saint Petersburg - Pisa - Paris - Philadelphia - Washington - 1884

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Each degree of longitude is further sub-divided into 60 minutes, each of which divided into 60 seconds. A longitude is thus specified as 23° 27′ 30" E. For high accuracy, the seconds are specified with a decimal fraction. An alternative representation uses degrees and minutes, where parts of a minute are expressed as a decimal fraction, thus: 23° 27.500′ E. Degrees expressed as a decimal number is also used: 23.45833° E. Sometimes, the West/East suffix is replaced by a negative sign for West. Confusingly, the convention of negative for East is also sometimes seen. The preferred convention that East is positive is consistent with the right-handed x-axis in the Cartesian coordinate system.

Related Topics:
Minutes - Cartesian coordinate system

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A specific longitude may then be combined with a specific latitude to give a precise position on the Earth's surface.

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As opposed to a degree of latitude, which always corresponds to about 111 km (69 mi), a degree of longitude corresponds to a distance from 0 to 111 km: it is 111 km times the cosine of the latitude, when the distance is laid out on a circle of constant latitude; if the shortest distance, on a great circle were used, the distance would be even a little less.

Related Topics:
Cosine - Great circle

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Longitude at a point may be determined by calculating the time difference between that at its location and Coordinated Universal Time (UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves 15 degrees per hour (360°/24 hours = 15° per hour). So if the time zone a person is in is three hours ahead of UTC then that person is near 45° longitude (3 hours × 15° per hour = 45°). The word near was used because the point might not be at the center of the time zone; also the time zones are defined politically, so their centers and boundaries often do not lie on meridians at multiples of 15°. In order to perform this calculation, however, a person needs to have a chronometer (watch) set to UTC and needs to determine local time by solar observation or astronomical observation. The details are more complex than described here: see the articles on Universal Time and on the Equation of time for more details.

Related Topics:
Coordinated Universal Time - Time zone - Meridians - Chronometer - Universal Time - Equation of time

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A line of constant longitude is a meridian, and half of a great circle.

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