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Greenwich Mean Time
London

The Royal Observatory in Greenwich is the home of Greenwich Mean Time (GMT) and the Prime Meridian of the world.

The Greenwich Meridian was chosen as the Prime Meridian of the World in 1884. There were two main reasons for the choice. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.

What is a meridian? A meridian is a north-south line selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the sky. The line in Greenwich represents the Prime Meridian of the World - Longitude 0º. Every place on Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres.