<P> The position of the Greenwich Meridian has been defined by the location of the Airy Transit Circle ever since the first observation was taken with it by Sir George Airy in 1851 . Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal, Edmond Halley in 1721 . It was set up in the extreme north - west corner of the Observatory between Flamsteed House and the Western Summer House . This spot, now subsumed into Flamsteed House, is roughly 43 metres to the west of the Airy Transit Circle, a distance equivalent to roughly 0.15 seconds of time . It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the Paris meridian abstaining) as the Prime Meridian of the world at the 1884 International Meridian Conference . </P> <P> All of these Greenwich meridians were located via an astronomic observation from the surface of the Earth, oriented via a plumb line along the direction of gravity at the surface . This astronomic Greenwich meridian was disseminated around the world, first via the lunar distance method, then by chronometers carried on ships, then via telegraph lines carried by submarine communications cables, then via radio time signals . One remote longitude ultimately based on the Greenwich meridian using these methods was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States . </P> <P> Satellites changed the reference from the surface of the Earth to its centre of mass around which all satellites orbit regardless of surface irregularities . The requirement that satellite - based geodetic reference systems be centred on the centre of mass of the earth caused the modern prime meridian to be 5.3" east of the astronomic Greenwich prime meridian through the Airy Transit Circle . At the latitude of Greenwich, this amounts to 102 metres . This was officially accepted by the Bureau International de l'Heure (BIH) in 1984 via its BTS84 (BIH Terrestrial System) that later became WGS84 (World Geodetic System 1984) and the various ITRFs (International Terrestrial Reference Systems). </P> <P> Due to the movement of Earth's tectonic plates, the line of 0 ° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres; that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map . With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning . As a result, the International Reference Meridian was established and is commonly used to denote Earth's prime meridian (0 ° longitude) by the International Earth Rotation and Reference Systems Service, which defines and maintains the link between longitude and time . Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres per year relative to this Earth - centred 0 ° longitude . Circa 1999 the international reference meridian (IRM) passed 5.31 arcseconds east of Airy's meridian or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich, London . It is also the reference meridian of the Global Positioning System operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). A current convention on the Earth uses the opposite of the IRM as the basis for the International Date Line . </P>

They are measured from the prime meridian in a