the other concept is called "longitude" (long, or the greek symbol "lambda", formula_2), sometimes referred to as "meridian". the 0° longitude line (or zero meridian) goes through the royal observatory in greenwich. greenwich is a part of london. then lines are drawn in a similar way; the opposite (or "antipodal") meridian of greenwich is considered both 180°w(est), and 180°e(ast).