The line of 0° longitude that passes through the Old Royal Observatory at Greenwich, London. It was adopted in 1884 as the prime meridian but has been superseded for all scientific and navigational purposes by the zero meridian of the International Terrestrial Reference Frame (ITRF). The ITRF zero meridian is approximately 5 arc-seconds east of the Greenwich meridian, and as a result GPS receivers do not display zero longitude when situated on the Greenwich meridian.