East Grinstead

The Greenwich Meridian passes through East Grinstead. You can see a marker in stone on a wall at the Chequer Mead Theatre and Arts centre.

The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England — 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, separates the eastern and western hemispheres.

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. The Greenwich Meridian 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:

• The United Kingdom (note London is in both the Western Hemisphere and Eastern Hemisphere due to the proximity of the Prime Meridian)
• France
• Spain
• Algeria
• Mali
• Burkina Faso
• Togo
• Ghana

and finally joins all other longitude lines in Antarctica at the South Pole.

The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres to the east of the line marked at Greenwich.[1] 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 through Airy’s transit instrument 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.

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 be discrepant from the observed time on the meridian by up to about one second, because of changes in the earth’s rotation. Leap seconds are inserted periodically to keep UTC in sync with the earth. One second theoretically corresponds to a variation in longitude of roughly 300 metres either way on the ground at Greenwich.

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