meridian of longitude: see longitude.

longitude, angular distance on the earth's surface measured along any latitude line such as the equator east or west of the prime meridian. A meridian of longitude is an imaginary line on the earth's surface from pole to pole; two opposite meridians form a great circle dividing the earth into two hemispheres. By international agreement, the meridian passing through the original site of the Royal Greenwich Observatory at Greenwich, England, is designated the prime meridian, and all points along it are at 0° longitude. All other points on the earth have longitudes ranging from 0° to 180°E or from 0° to 180°W. Except where it is changed to account for populated areas, the international date line lies along the 180° meridian. Meridians of longitude and parallels of latitude together form a grid by which any position on the earth's surface can be specified. The term longitude is also used in various celestial coordinate systems (see ecliptic coordinate system).

Coordinate system by which the position or location of any place on the Earth's surface can be determined and described. Latitude is a measurement of location north or south of the Equator. Lines of latitude are known as parallels, or parallels of latitude. Longitude is a measurement of location east or west of the prime meridian, which passes through Greenwich, Eng. The combination of meridians of longitude and parallels of latitude establishes a grid by which exact positions can be determined: for example, a point described as 40° N, 30° W is located 40° of arc north of the Equator and 30° of arc west of the Greenwich meridian.

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