Meridian Building (London)

 landmark, place with historical importance, interesting place, geodetic point

This is the building that formerly housed the Airy Transit Circle, the instrument that defined the Prime Meridian.

The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — it is the meridian at which longitude is defined to be 0 degrees. The prime meridian, and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, separate the Eastern and Western Hemispheres. The Prime Meridian and the 180th meridian form a great circle which encircles the Earth like a longitudinal equator.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator, 0°), the prime meridian is arbitrary, and multiple meridians have been used through history as the prime meridians of various mapmaking systems (including four different Greenwich meridians). The Greenwich Meridian established by Sir George Airy in 1851 was agreed upon as the international standard in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. France abstained when the vote was taken, and French maps continued to use the Paris Meridian for several decades.

Heading south from the North Pole, the Prime Meridian passes through the following countries:

United Kingdom (The most northerly land on the meridian is the shore (53°45′34″N) southeast of the Sand-le-Mere caravan park east of Kingston upon Hull, England.)
France
Spain
Algeria
Mali
Burkina Faso
Togo
Ghana
Antarctica to the South Pole
The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.[1]. WGS84 uses the zero meridian as defined by the Bureau International de l'Heure,[2] which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.[3] The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of vertical deflection, is 5.31″. The WGS84 datum is an average of the various continental drifts. As a result, the astronomical meridian between the vertical crosshairs of Airy's transit telescope drifts toward the east as it is carried by the European portion of the Eurasian tectonic plate, closer to the geodetic meridian, by about one centimetre per year.

Curiously, whether by accident or design, the location of the WGS84 0° meridian is marked in Greenwich by the presence of a waste basket on the path leading more or less due east from the observatory containing the transit telescope.

The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the line marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply continued to use it.

Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the Earth's rotation. Leap seconds are inserted periodically to keep UTC in sync with the earth.

The Greenwich Meridian is now marked at night by a laser beam emitted from the observatory.

One degree of longitude is equal to (111.320 + 0.373sin²φ)cosφ km, where φ is latitude.