Longitude, angular distance on the earth's surface measured in degrees from a certain meridian.A meridian is an imaginary line extending from the north pole to the south pole at right angles to the equator. Usually the meridian that passes through Greenwich (a part of London, England) is used in determining longitude. This meridian is called the prime meridian.The longitude of a place reveals how far east or west it is, in degrees, of the prime meridian. The corresponding north-south measure is called latitude.
The prime meridian marks 0° longitude. Halfway around the earth, passing through the Pacific Ocean, is 180° longitude. Thus the earth is longitudinally divided into two hemispheres of 180° each—the Eastern Hemisphere, east of Greenwich, and the Western Hemisphere, west of Greenwich.
Meridians are spaced farthest apart at the equator and converge at the poles. At the equator, 1° of longitude spans about 69 miles (111 km); at 30° north and south, about 60 miles (97 km); at 60° north and south, about 35 miles (56 km); and at the poles, 0 miles. Meridians on maps, however, appear in varying shapes, depending on the map projection. (See Figures C and D.) Such distortions occur in making the earth's spherical surface appear flat.
Longitudinal degrees are divided into minutes (60 minutes in a degree) and seconds (60 seconds in a minute). The longitude of a place is usually expressed in degrees and minutes. The longitude of Washington, D.C., for example, is 77°02' west. More exact longitudes are stated in degrees, minutes, and seconds.
Longitude is also an important factor in determining time. The earth makes one complete rotation on its axis every 24 hours. Therefore, it turns 15° of longitude (1/24 of 360°) in one hour. In general, each of the world's time zones spans an area equal to 15° of longitude and its time is based on its distance from the prime meridian.
