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Longitude], represented by a series of vertical lines on a map - or vertical rings on a globe - is the angular distance of a point on the Earth either east or west of the Prime Meridian ( 0° ) which runs through Greenwich, England. There are 360 lines of longitude, 180 east and 180 west of the Prime Meridian. E180° and W180° are the same. Lines of longitude are often refered to as "meridians".

Every circle formed by lines of longitude is a great circle - compare this to Latitude where only the equitorial line of latitude is a great circle.

Why don't lines of Lat/Lon form a square angle on a map? Because the meridians converge at the north and south poles. If you divide the earths' circumference at the Equator and through any meridian by 360, the distance on the earths' surface for each degree of longitude and longitude at the equator is approx 69 miles or 111 kilometres. As you move either north or south of the equator the meridians get closer together until they converge at the poles. At N45° & S45° one degree of longitude is approximately 49 miles or 78 kilometres. One degree of latitude is still 69 miles. Another way to explain it is that all the meridians are of a constant circumference whereas the parallels reduce in circumference as you move north or south.

See also coordinates.