<P> A prime meridian is a meridian ( a line of longitude ) in a geographical coordinate system at which longitude is defined to be 0 ° . Together , a prime meridian and its antimeridian ( the 180th meridian in a 360 ° - system ) form a great circle . This great circle divides the sphere , e.g. , the Earth , into two hemispheres . If one uses directions of East and West from a defined prime meridian , then they can be called Eastern Hemisphere and Western Hemisphere . </P>
where is the meridian that is opposite the prime meridian located
antimeridian