The ‘Prime Meridian’ that’s been running through the Royal Observatory at Greenwich, UK, since 1884 is now located 335 feet (102 meters) east of its historic spot. Dr Ken Seidelmann from the University of Virginia and his colleagues investigated the cause of this apparent discrepancy.

In 1884, the International Meridian Conference recommended that Earth’s prime meridian “to be employed as a common zero of longitude and standard of time-reckoning throughout the globe” pass through the “center of the transit instrument at the Observatory of Greenwich.”

This instrument – named the Airy Transit Circle for its designer, British Astronomer Royal Sir George Biddell Airy – is a nineteenth-century telescopic device for measuring star positions, and could be used for determining local time.

Today, tourists visiting its meridian line must walk east approximately 335 feet before their satellite-navigation receivers indicate zero longitude.

Why? Because newer technologies – primarily the superb accuracy of GPS, which uses satellites to precisely measure grid coordinates at any point on the Earth’s surface – replaced the traditional telescopic observations used to measure the Earth’s rotation.

“With the advancements in technology, the change in the prime meridian was inevitable. Perhaps a new marker should be installed in the Greenwich Park for the new prime meridian,” said Dr Seidelmann, who is a co-author of the paper published in the Journal of Geodesy.

Dr Seidelmann and co-authors concluded that a slight deflection in the natural direction of gravity at Greenwich is responsible for the offset, along with the maintenance of continuity of astronomical time.

According to the team, the 335-foot offset can be attributed to the difference between two conventional methods of determining coordinates: astronomical versus geodetic, which refers to a set of reference points used to locate places on the Earth.

Their difference is known as ‘deflection of the vertical,’ and high-resolution global gravitational models confirm that the east-west component of this deflection is of the proper sign and magnitude at Greenwich to account for the entire shift.

Because our planet is not perfectly round, and because different locations on Earth have different terrain features affecting gravitational pull, traditional ways to measure longitude have built-in variations, or errors, based on the specific location where measurements are taken.

The observations were based on a vertical determined from a basin of mercury and were dependent on local conditions.

“However, GPS measures vertical from space in a straight line directly through the center of the Earth, effectively removing the gravitational effects of mountains and other terrain,” Dr Seidelmann explained.

“Meridian is dependent on the direction of the vertical, which is gravity- and observational-method-dependent. The distance and direction of the 335-foot offset is confirmed by gravitational models.”

For supporting evidence, the scientists also analyzed the differences in the coordinates of many former timekeeping observatories to affirm that the apparent longitude shift at Greenwich is a localized effect due to the direction of gravity at Greenwich, and not a global shift in the world’s longitude system.

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Stephen Malys et al. 2015. Why the Greenwich meridian moved. Journal of Geodesy, published online August 01, 2015; doi: 10.1007/s00190-015-0844-y