Greenwich Royal Observatory: How the Prime Meridian line is actually 100 metres away from where it was believed to be

The line actually lies more than 100 metres to the east, cutting across a nondescript footpath in Greenwich Park near a litter bin

Chris Green
Thursday 13 August 2015 02:17 BST
The location of the ‘Prime Meridian of the World’ at the Royal Observatory in Greenwich was agreed at a conference in America in 1884
The location of the ‘Prime Meridian of the World’ at the Royal Observatory in Greenwich was agreed at a conference in America in 1884

Every year, hundreds of thousands of tourists from all over the world descend on the Royal Observatory in Greenwich to pose for a photograph astride the Prime Meridian, the famous line which divides the eastern and western hemispheres of the earth.

There is just one problem: according to modern GPS systems, the line actually lies more than 100 metres to the east, cutting across a nondescript footpath in Greenwich Park near a litter bin. Now scientists have explained why – and it all comes down to advances in technology.

According to a newly published paper on the discrepancy, which has existed for many years, tourists who visit the observatory at Greenwich often discover that they “must walk east approximately 102 metres before their satellite navigation receivers indicate zero longitude”.

The reason, the researchers at the US Naval Observatory and the National Geospatial-Intelligence Agency concluded, is simply that modern devices rely on satellite GPS technology introduced in 1984, which is better at calculating longitude than the one previously relied upon by earth-bound astronomers.

The location of the original Greenwich Meridian was agreed upon at a meeting in Washington in 1884, after a vote involving 25 nations. Creating a universally recognised position of 0 degrees longitude allowed accurate global navigation, standardised maps and the creation of time zones.

The line, which is used to define Greenwich Mean Time, was calculated by pointing a telescope straight up at a so-called “clock star”. However the scientists’ telescope was very slightly off the perpendicular because of the way local terrain and the shape of the Earth cause local distortions to gravity. But the measurements taken from space by satellites, which have been used by GPS systems since 1984, are not affected by such conditions, resulting in a more accurate line being drawn.

“With the advancements in technology, the change in the prime meridian was inevitable,” said Ken Seidelmann, an astronomer at the University of Virginia and co-author of the study, published in the Journal of Geodesy. “Perhaps a new marker should be installed in the Greenwich Park for the new prime meridian.”

A tourist stands astride at the 'Prime Meridian of the World' (Getty)

Dr Marek Kukula, the Royal Observatory’s public astronomer, said staff were frequently being questioned by puzzled tourists “helpfully pointing out that our meridian is in the wrong place”. He welcomed the idea of a permanent marker for what he described as the “GPS Meridian” in Greenwich Park.

“We’re forever telling this story, making the point that as we refine our measurements and get better technology, of course these things change, because we want to have the best possible data,” he said. “I think having a marker in the park would be brilliant, to update the story of the Greenwich meridian line into the 21st century. At the moment the nearest thing there is to a marker is a litter bin.”

There are already two other meridian lines at Greenwich which were used before the 1884 conference. The Halley meridian was defined in 1721 by the English astronomer Edmond Halley, while the Bradley meridian of 1750 is still used as the standard definition of zero longitude in modern Ordnance Survey maps, which began in 1801 and have not moved onto the new system.

Although the idea that the Greenwich meridian is now effectively obsolete may come as a surprise to some, visitors to the observatory should not forget the importance of the original line, Dr Kukula said. “The reason that all these meridian lines go through Greenwich is because for hundreds of years, the observatory has been churning out the best available navigational data, using the best technology of the time,” he added.

However, a visit to the GPS meridian is certainly cheaper than posing for a photo by the old one. An adult day ticket giving access to the Meridian Courtyard at the Royal Observatory currently costs £9.50, whereas entry to the park is free.

Fortunately, there is no need for the entire population of the world to adjust their watches. The researchers said that while the location of the meridian may have moved under the new system, the plane used to measure Universal Time (the modern equivalent of Greenwich Mean Time) has remained “essentially unchanged”, so there are no knock-on effects.

The current meridian, which is used by most in-car sat navs and services such as Google Maps, may have to be updated again as technology moves on. “Is that going to be the final one – who knows what refinements may take place in the future?” Dr Kukula said. “But the great thing about meridian lines is that it doesn’t really matter – as long as everybody is using the same one.”

Greenwich Meridian and the bowl of mercury

The Greenwich meridian does not just mark the man-made divide between east and west, it was the place where the clocks were set in the form of Greenwich Mean Time, now known as Universal Time.

The Greenwich Shepherd Gate Clock at the Royal Observatory (Getty)

The fact that the meridian is 102m (334ft) east of the telescope used to determine the meridian is a feature of the way the Greenwich meridian was calculated with the help of a basin of mercury to ascertain the precise vertical from the telescope to the “clock stars” overhead. This introduces errors because it assumes the Earth is perfectly round, which it is not, and that the gravitational pull of the terrain is uniformly the same as in other parts of the world, which is not so. The telescope measures local time by marking the exact moment a clock star passes overhead, which means that you must point the telescope absolutely perpendicular to the Earth.

It is assumed the surface of the liquid mercury is exactly parallel to the surface of the Earth. Unfortunately, if the Earth is not exactly round and the local gravitational effects are not uniform, then this can give a wonky reading.

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