Precision and the Prime Meridian

The Prime Meridian is the imaginary line of zero longitude, the geographic starting point for any East-West degree measurement of any place on the Earth.  It was selected by an international delegation that convened in 1884 in Washington, DC.  It’s a North-South imaginary line that run right through the Airy transit circle at the Royal Observatory in Greenwich, England.  This is also where we get the measurement of Greenwich Mean Time (GMT).

Solid line shows original Prime Meridian, while the dashed line shows the actual location updated through GPS. Credit: Imagery copyright 2014 Google Maps, Infoterra Ltd. & Bluesky

As our ability to measure position and time improves with the Global Positioning System (GPS), the precise position of the prime meridian has actually changed, moving 102 meters East of the original line, as shown in the above image.

A slight deflection in the direction of the Earth’s gravity, along with the maintenance of continuity of Astronomical time, together are responsible for the offset. “With the advancements in technology, the change in the prime meridian was inevitable,” says Ken Seidelmann, an astronomer at the University of Virginia and co-author of the study sponsored by the U.S. Naval Observatory, the National Geospatial-Intelligence Agency and the U.S. company Analytical Graphics Inc.

Determining precise coordinates on Earth is not easy, especially for reference points.  You want to get it exactly right because all other measurements follow from this.  This is difficult because the Earth is not perfectly round, and different terrain and surface features result in a difference in the gravitational pull at any one point.  Traditional measurements have corrections built in at different locations, and were based on a vertical line measured with a small basin of Mercury metal.  However, modern GPS techniques measure vertical as an imaginary line that passes directly through the centre of the Earth, effectively removing the gravitational effects of varying terrain on Earth.

The difference in these two measurements is called the ‘deflection of the vertical,’ and accounts for the difference in the Prime Meridian’s position.  The entire difference is due to the slightly different gravity on an observer at Greenwich.

Amazing how such a precise measurement and a tiny difference can result in a 102 meter shift when we scale it up to the Earth.  Precision counts in time and coordinates!



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