Location of the Prime Meridian
Prime Meridian in Greenwich
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 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.
The meridian was agreed upon 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:
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 about 100 metres to the east of the line marked at Greenwich. The shift was due firstly to errors in determining its position from other observatories, secondly to recalculations of the position of the Earth's centre, and thirdly to the basing of calculations on where the meridian crosses the Equator, not where it passes through Greenwich. The Royal Observatory's lack of modern instruments prevented accurate measurements there in any case. The meridian is not fixed on the ground as the continents are moving because of continental drift, and the WGS84 datum is an average of the various continental movements - the datum stays where it is and the continents slide around underneath it.
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 carried on using 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. This theoretically corresponds to a variation in longitude of roughly 300 metres either way on the ground at Greenwich.
Other reference meridians
The meridian through Greenwich was selected as the Prime Meridian because over two-thirds of all ships and tonnage already used it as the reference meridian on their maps in 1884. Other reference meridians used previously include:
- WGS84 longitude zero - about 100 metres east of the traditional Prime Meridian
- WGS84 longitudes will eventually supplant traditional ones. The offset at other locations can be as much as 30" east or west.
