JESSICA WILSON / NASA / SCIENCE PHOTO LIBRARY JESSICA WILSON / NASA / SCIENCE PHOTO LIBRARY
Outline of world map, showing the continents, oceans and prime meridian (in red). The prime meridian (also known as the Greenwich meridian) exactly marks the position of 0 degrees longitude. It is from this that the longitude of all points on the surface of the Earth are measured. This was agreed by an international convention in 1884. The meridian passes through the original site of the Greenwich observatory.
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