The Prime Meridian Line
Defined by George Biddell Airy's transit circle (Airy 1801 to 1892) · Transit circle 1850; adopted internationally 1884
You are standing at longitude zero. The metal strip that runs across the courtyard marks the Prime Meridian of the world, the line from which every other longitude on earth is measured, east or west. Visitors love to plant one foot on each side, one in the eastern hemisphere and one in the western, and the line continues up the wall of the Meridian Building, where the numbers of the great cities of the world are set out with their distance from Greenwich in degrees.
The line was fixed not by a surveyor but by a telescope. Inside the building, look through the window at the Airy Transit Circle, a large instrument installed by the Astronomer Royal George Airy in eighteen fifty. It could only move north and south, so each star crossed its crosshairs once a night, and the exact moment of that crossing gave both the time and the position of the star. The centre of that telescope's eyepiece defined the meridian. Notice the small brass line on the ground and, after dark, the green laser that projects the line northwards across London.
In eighteen eighty four delegates from twenty five nations met in Washington and chose Greenwich as the world's Prime Meridian, largely because most shipping charts already used it. That decision also gave the world its time zones, all counted from Greenwich Mean Time. There is one modern twist. Satellite positioning shows that the true zero longitude of the global reference system lies about a hundred metres to the east of this line, because the old system depended on the local direction of gravity rather than the centre of the earth. Airy's line is still the historic one, and it is the one everyone photographs.
Did You Know? If you open a satellite map app while standing on the line, it will tell you that you are still a few seconds of longitude west of zero.
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