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.
the starting line to number the meridian is the
0 ° 0 °