The Greenwich Prime Meridian Has Moved: Scientists Explain Why


The Royal Observatory in Greenwich is one of the most interesting places to visit in the world as many tourists love to stand on the very spot that divides the Eastern and Western Hemispheres of the Earth – the prime meridian.

However, map developers, navigators and even tourists have noticed that the zero longitude runs 100 meters, about 334 feet, east of their telescope as per their global positioning system (GPS) devices, leaving everyone puzzled.

Experts have acknowledged this slight deviation and in a new study, the reasons for it were explained.

Basically, the changes were discovered due to the rise of new technologies. With the remarkable accuracy of GPS and other innovations that can come up with more detailed grid information on all points of the Earth, the traditional telescope used to determine the prime meridian is now looking to have some lapses.

"With the advancements in technology, the change in the prime meridian was inevitable," says Ken Seidelmann, co-author of an article in the August issue of Journal of Geodesy and an astronomer at the University of Virginia.

The modification of the long vertical line that marks zero degrees longitude was partly due to a minimal deflection in the natural direction of gravity in Greenwich, explained Seidelmann and fellow researchers from the U.S. Naval Observatory, the National Geospatial-Intelligence Agency and a U.S. company called Analytical Graphics Inc.

The Earth does not have a flawless round shape. The surfaces are also composed of a wide variety of terrains, affecting the gravitational pull in different locations. The conventional methods of measuring longitude were indeed prone to errors, taking into particular consideration the location where the measurements were obtained.

The prime meridian was determined in 1884, during a gathering of experts at the International Meridian Conference in Washington, D.C. Representatives from 25 countries voted on establishing a recognized position for 0 degrees longitude to achieve standardized maps and time zones. The method used back then involved the use of a basin of mercury to determine a vertical line that was dependent on local conditions. More accurate measurements have been available since 1984, as GPS is able to measure longitude from space in a direct straight line to the center of the planet, without the effects of gravity and other environmental influences.

"Today, tourists visiting its meridian line must walk east approximately 102 m before their satellite-navigation receivers indicate zero longitude. This offset can be accounted for by the difference between astronomical and geodetic coordinates – deflection of the vertical – in the east-west direction at Greenwich, and the imposed condition of continuity in astronomical time," the article states.

So now, maybe it is time to institute a new indicator at the Greenwich Park for the new prime meridian, comments Seidelmann.

Photo: Ben Sutherland | Flickr

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