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Meridian (Geography)

Pre-Greenwich
The first prime meridian was set by Eratosthenes in 276 BCE. This prime meridian was used to provide measurement of the earth, but had many issues because of the lack of latitude measurement. Many years later around the 19th century there was still concerns of the prime meridian. The idea of having one prime meridian came from William Parker Snow, becuase he realized the confusion of having multiple prime meridian locations. Many of theses geographical locations were traced back to the ancient Greeks, and others were created by several nations. Multiple locations for the geographical meridian meant that there was inconsistency, because each country had there own guidelines for where the prime meridian was located.

Geographic
Toward the ending of the 19th century there were two main locations that were acknowledge as the geographic location of the meridian, France and Britain. These two locations often conflicted and a settlement was reached only after there was a International Meridian Conference held, in which Greenwich was recognized as the 0° location.

The meridian through Greenwich (inside Greenwich Park), England, called the Prime Meridian, was set at zero degrees of longitude, while other meridians were defined by the angle at the center of the earth between where it and the prime meridian cross the equator. As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich, the antimeridian, forms the other half of a circle with the one through Greenwich, and is at 180° longitude  near the International Date Line (with land mass and island deviations for boundary reasons). The meridians from West of Greenwich (0°) to the antimeridian (180°) define the Western Hemisphere and the meridians from East of Greenwich (0°) to the antimeridian (180°) define the Eastern Hemisphere. Most maps show the lines of longitude.

The position of the prime meridian has changed a few times throughout history, mainly due to the transit observatory being built next door to the previous one (to maintain the service to shipping). Such changes had no significant practical effect. Historically, the average error in the determination of longitude was much larger than the change in position. The adoption of WGS84 ("World Geodetic System 84") as the positioning system has moved the geodetic prime meridian 102.478 metres east of its last astronomic position (measured at Greenwich). The position of the current geodetic prime meridian is not identified at all by any kind of sign or marking (as the older astronomic position was) in Greenwich, but can be located using a GPS receiver.

Measurement of Earth Rotation
Many of these instruments rely on the ability to measure the longitude and latitude of the earth. These instruments also were typically effected by local gravity, which paired well with existing technologies such as the magnetic meridian.

Tools of Measurement


 * Visual and Photographic Zenith Tubes
 * Circumzenithals
 * Danjon Astrolabes
 * Transit Telescopes - Airy Transit Circle

Magnetic Meridian
The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the horizontal component of magnetic force lines along the surface of the earth. Therefore, a compass needle will be parallel to the magnetic meridian. However, a compass needle will not be steady in the magnetic meridian, because of the longitude from east to west being complete geodesic. The angle between the magnetic and the true meridian is the magnetic declination, which is relevant for navigating with a compass.

Geomagnetism
Muilticulturalism