“Meridian is dependent on the direction of the vertical, which is gravity- and observational-method-dependent. “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. The observations were based on a vertical determined from a basin of mercury and were dependent on local conditions. 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.īecause 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. 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.ĭr 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.Īccording 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. “With the advancements in technology, the change in the prime meridian was inevitable. 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. Today, tourists visiting its meridian line must walk east approximately 335 feet before their satellite-navigation receivers indicate zero longitude. 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. 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.”

Image credit: Google Maps / Infoterra Ltd. The Airy meridian marked as the ‘Prime Meridian of the World’ (dotted line), and the modern reference meridian indicating zero longitude using GPS (solid line).

The Change by C.M. Meridian