Prime Meridian

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Prime Meridian is a meridian at which longitude is defined 0 degrees. Together with the 180th meridian that forms International Date Line, it divides Earth into Eastern and Western Hemispheres. Prime Meridian runs through the London part Greenwich and therefore it is also known as Greenwich Meridian or International Meridian.

In the past mapmakers used at least 14 different prime meridians, including four different Greenwich Meridians. Greenwich Observatory was set up by King Charles II in 1675 to study means of fixing longitude, and became the acknowledged world authority on the subject. The exact Greenwich Meridian was established in 1851 by Sir George Airy. However, it was agreed to use this prime meridian just in October 1884 on the International Meridian Conference held in Washington D.C. at the behest of the American president Chester A. Arthur. There were 41 delegates from 25 nations there. However, the decision was not unanimous. The French, for political reasons, said that they would only accept Greenwich as the prime meridian if Great Britain adopted the metric system. Over the following years the various governments adopted Greenwich officially as prime meridian, with only France going its own way. France adopted universal time just in 1978.

Leading from the North to the South, Prime Meridian passes through United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and Antarctica.

Prime Meridian passes right through the Royal Observatory in Greenwich. Prime Meridian is marked by red line on the building and it continuous on the pavement. It is therefore possible to stand astride the line with a foot in each hemisphere what is a favourite tourist attraction. At night, there is a laser beam used to mark the line.

Greenwich Meridian is also the base of Greenwich Mean Time (GMT). It is mean solar time, with midday defined as the time at which the sun crosses the Greenwich Meridian, 0 degrees longitude.
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