Greenwich's 'wandering' meridian now 335 feet to the east

GPS has revealed that marker which shows the point of zero longitude, set in 1884, is in wrong place

(Image credit: PaulDC/Flickr)

Scientists have explained why the Greenwich Meridian appears to have drifted substantially east (or to the right, if you stand in the park and face north). Now the big question is: should the marker, popular with tourists, be moved? says the discrepancy has been known about for some time. Tourists who visit Greenwich Park and pose for photographs with one foot on either side of the Prime Meridian often notice their GPS devices don't agree with the brass marker.

Now scientists from the US Naval Observatory and the National Geospatial-Intelligence Agency have worked out why the Prime Meridian appears to be some 335 feet (102 metres) east of where it is supposed to be.

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Longitude is a system of measuring the surface of the planet in order to facilitate navigation. The globe is banded with imaginary longitude lines – and these are numbered. In the 19th century it was decided that the zero point would be in Greenwich.

The 1884 International Meridian Conference of 25 nations in Washington decided there should be a Prime Meridian "to be employed as a common zero of longitude and standard of time-reckoning throughout the globe" and it should pass through the centre of the Airy Transit Circle at the Royal Observatory.

The circle was a telescopic device for measuring star positions. It was pointed straight up at a reference star overhead in order to mark the meridian – and that's where the discrepancy crept in, The Independent says.

The device was very slightly off the perpendicular because of local distortions to gravity caused by the shape of the earth's crust.

Since 1984, GPS systems using satellites have been able to measure the earth with greater accuracy than Victorian scientists could muster – and that's why the meridian appears to have "wandered".

Luckily, the real line of zero longitude does still pass through Greenwich Park – so sticklers for accuracy can use their GPS-enabled mobile phones to find just the right spot for a selfie.

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