The Prime Meridian, also known as the Greenwich Meridian, passes through longitude 0° 0' 0'' and on its journey from pole to pole, it passes through England, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and Antarctica, dividing the earth into east and west, just as the Equator splits it into north and south. The meridian’s position is marked in hundreds of places, but the best place to see this all important imaginary line is in Greenwich Park in London. The marker is located at the Royal Observatory, a former observatory and now a museum, that played a major role in the history of astronomy and navigation. Its path is determined by the location of a historic telescope, the Airy Transit Circle, which is housed at the observatory’s premises.

The Royal Observatory was established in 1675, and since then the British astronomers have used it as a basis for astronomical measurement. In 1851, the astronomer Sir George Biddell Airy built an instrument called the transit circle for timing the passage of stars across the local meridian, and in doing so established the location of the Prime Meridian. Three years later, in 1884, the Prime Meridian was adopted, by an international decree, as the official zero-degree longitude. To mark the line, a brass strip was laid down across the courtyard, which was later upgraded to stainless steel. In December 1999, an additional marker was installed – a powerful green laser that shines north across the London night sky.

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Such is the fame of the Meridian that each year, hundreds of thousands of visitors from all around the world make their way to the Observatory and have their picture taken as they stand astride the Line. However, if you hold a GPS receiver (any smartphone will do) over the stainless steel strip you will discover that the GPS will give a reading that is offset by about 100 meters. You don’t have to be at Greenwich to verify this. Just open Google Maps and look up Greenwich. You will notice that the Prime Meridian does not pass through the Royal Observatory. The apparent anomaly arises from the assumed shape of the Earth and the way longitudes were determined and maps were historically constructed.

A GPS receiver held over the Meridian Line at Greenwich does not read a longitude of 0° 0' 0''. Photo credit

When constructing maps it was necessary to choose an ellipsoid that represent the shape of the Earth, based on which the maps would be made. When Greenwich was an active observatory, it was the practice to pick an ellipsoid, called a datum, whose surface closely matched local mean sea level. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres worldwide. The advent of satellite technology enabled ellipsoids to be defined for the first time with their centre coincident with the Earth’s centre of mass. This new datum is called WGS84 (World Geodetic System, version 84) and is used in all modern cartography, and navigation, including by GPS satellites. The adoption of the WGS84 datum caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is 102.5 metres east of the Greenwich astronomical meridian represented by the stainless steel strip.

The Prime Meridian and many other places continue to shift coordinates even as you read this. This is for another cause – continental drift. The WGS84 datum is defined such that it remains stationary with respect to the average motion of the Earth’s crustal plates. As a consequence, all individual locations are in motion relative to them. In UK, WGS84 latitudes and longitudes are changing at about 2.5 cm per year in a north-easterly direction.

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Laser projected from the observatory marking the Prime Meridian line. Photo credit

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Sources: Wikipedia / www.thegreenwichmeridian.org / Royal Museums Greenwich