Longitude lines are usually semicircular. Longitude ( :) is a measure of the angle that a point on the earth's surface produces along a perpendicular circumference (which intersects the north and south poles) with the equator , east or west of the center of the coordinate system . The line that is found with all the points of the same longitude is called the meridian. Each equator is a semicircle but not parallel to any one. By definition, each line meets the north and south poles. Historically, the equator that passes through the Royal Observatory , Greenwich (near London, UK) is considered to be the zero-longitude or the authentic equator. For example, the longitude of Dhaka is 90 ° 22 ′ 30 . East.