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Sasha Stolt
The line in Greenwich represents the historic Prime Meridian of the World - Longitude 0º. Every place on Earth was measured in terms of its distance east or west from this line. The line itself divided the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres. If you stand with one foot on one side and the other on the left, you are perfectly in the middle of east and west, according to the prime meridian line.
Between 1984 and 1988 an entirely new set of coordinate systems were adopted based on satellite data and other measurements and required a prime meridian that defined a plane passing through the centre of the Earth.
The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0 longitude by international agreement. The IRM passes 102.5 metres to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian.
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