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By using a high-resolution gravitational model, a research team consisting of geodesists from the National Geospatial-Intelligence Agency (NGA), astronomers from the US Naval Observatory and the University of Virginia, and an engineer from Analytical Graphics Inc. (AGI), conclude that a slight deflection in the natural direction of gravity at Greenwich is responsible for the 102-meter offset, along with the way astronomical time was maintained over the 20th century.
In 1884, an international delegation convened in Washington D.C. to recommend that zero longitude pass through the Airy Transit Circle at the Royal Observatory in Greenwich, England. This telescope was used to accurately measure the rotation of the Earth, with its location established partly by the direction of local gravity. Over the twentieth century, space-age technologies replaced telescopic observations and changed the conventions for determining latitude and longitude. These conventions differ by small variations in local gravity known as “deflection of the vertical”.
“Astronomical time and longitude are inextricably linked, but astronomical measurements of time are affected by direction of gravity as well as physical location,” says John Seago, AGI’s contributor to the research. “The Greenwich meridian marker considers the location, but not the ‘up’ direction — or zenith — established by local gravity. Greenwich Mean Time, or Universal Time, was originally measured against the direction of gravity at Greenwich.”
Contrary to popular belief, GPS has nothing to do with the shift. In 1984 — before GPS was operational — the International Bureau of Time (BIH) at the Paris Observatory defined a new world-reference system to measure Earth rotation, to take advantage of highly accurate space-based technologies. The new reference system was later adopted by GPS, and it maintained Greenwich Time according to the same direction established by the Airy Transit. But because Earth-rotation was no longer measured in a way that included the effects of local gravity, zero longitude could not pass through Airy’s instrument.
“Forcing zero in the BIH system to coincide with the Airy Transit Circle could have caused a historical disruption in the Greenwich Time it was designed to measure, and it would have also caused a small shift in world-wide longitudes,” Seago says. “Yet it would be fitting to still refer to the zero-longitude line indicated by GPS as the ‘Greenwich’ meridian, because that line was established by both the location and the direction of gravity at the Greenwich observatory. Zero longitude is the origin for Greenwich Mean Time, and the GPS line is where Universal Time continues to be maintained relative to the center of the Earth.”
The paper is available in the Journal of Geodesy (DOI: 10.1007/s00190-015-0844-y) though this link: http://link.springer.com/article/10.1007/s00190-015-0844-y. Access is free.