<P> A prime meridian is a meridian (a line of longitude) in a geographic 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., Earth, into two hemispheres . If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere . </P> <P> A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation--and various conventions have been used or advocated in different regions and throughout history . The most widely used modern meridian is the IERS Reference Meridian . It is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884 . </P>

The starting line to number the meridian is the