Prime meridian

A prime meridian is a meridian, i.e., a line of longitude, at which longitude is defined to be 0°. A prime meridian and its opposite in a 360°-system, the 180th meridian (at 180° longitude), 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.


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