The Prime Meridian is the meridian (line of longitude) at which the longitude is defined to be 0°. The Prime Meridian and its opposite, the Antimeridian (at +180°/-180° longitude), form a "great circle" that divides the Earth into the Eastern and Western Hemispheres. By international convention, the Prime Meridian passes through the Royal Observatory, Greenwich, in southeast London, known as the International Meridian or Greenwich Meridian.
info: http://en.wikipedia.org/wiki/Prime_Meridian
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