<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> <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> <P> The notion of longitude was developed by the Greek Eratosthenes (c. 276 BC--c. 195 BC) in Alexandria, and Hipparchus (c. 190 BC--c. 120 BC) in Rhodes, and applied to a large number of cities by the geographer Strabo (64 / 63 BC--c. 24 AD). But it was Ptolemy (c . AD 90--c . AD 168) who first used a consistent meridian for a world map in his Geographia . </P>

What is another name for the prime meridian