Prime meridian (Greenwich)

The prime meridian was therefore long symbolised by a brass strip in the courtyard, now replaced by stainless steel, and since 16 December 1999, it has been marked by a powerful green laser shining north across the London night sky.

In the past, this offset has been attributed to the establishment of reference meridians for space-based location systems such as WGS-84 (which the GPS relies on) or to the fact that errors gradually crept into the International Time Bureau timekeeping process.

The actual reason for the discrepancy is that the difference between precise GNSS coordinates and astronomically determined coordinates everywhere remains a localized gravity effect due to vertical deflection; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.

[6] In the 19th century, astronomers and geodesists were concerned with questions of longitude and time, because they were responsible for determining them scientifically and used them continually in their studies.

The International Geodetic Association, which had covered Europe with a network of fundamental longitudes, took an interest in the question of an internationally-accepted prime meridian at its seventh general conference in Rome in 1883.

It seemed natural that it should contribute to the achievement of significant progress in navigation, cartography and geography, as well as in the service of major communications institutions, railways and telegraphs.

[8] From a scientific point of view, to be a candidate for the status of international prime meridian, the proponent needed to satisfy three important criteria.

The conference concluded that Greenwich Observatory best corresponded to the geographical, nautical, astronomical and cartographic conditions that guided the choice of an international prime meridian, and recommended the governments should adopt it as the world standard.

Laser projected from the Royal Observatory , marking the prime meridian of Greenwich
A group of people waiting in a line curving to the left on a cobblestone surface. Behind it is an ornate brick building with a red ball on top. The people at the end of the line, closest to the camera, are taking pictures of other people near a shiny metal monument on the right, under a tree. A line in the cobblestone connects them
Tourists queuing to take pictures on the brass line marking the prime meridian at the Royal Observatory, Greenwich
The line down the middle of this 1911 map is the prime meridian, shown passing through Greenwich . Saxavord and Balta at the top of the map are in the Shetland Islands , the most northerly parts of Scotland and the United Kingdom. Shetland lies 1° W of the prime meridian.
A GPS receiver at the marking strip of the Greenwich Meridian in front of the Royal Observatory . The indicated longitude is not exactly zero because the geodetic zero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using the IERS Reference Meridian ) is 102 metres east of this strip. [ 1 ]
Greenwich meridian and Earth