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Line between the poles with the same longitude

In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle (in degrees or other units) east or west of a given prime meridian (currently, the IERS Reference Meridian). In other words, it is a line of longitude. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall Peters projection, each meridian is perpendicular to all circles of latitude. A meridian is half of a great circle on Earth's surface. The length of a meridian on a modern ellipsoid model of Earth (WGS 84) has been estimated as .

The first prime meridian was set by Eratosthenes in 200 BCE. This prime meridian was used to provide measurement of the earth, but had many problems because of the lack of latitude measurement. Likewise, the Sun crosses the celestial meridian at the same time. The same Latin stem gives rise to the terms a. m. (ante meridiem) and p. m. (post meridiem) used to disambiguate hours of the day when utilizing the 12 hour clock.

Because of a growing international economy, there was a demand for a set international prime meridian to make it easier for worldwide traveling which would, in turn, enhance international trading across countries. As a result, a Conference was held in 1884, in Washington, D. C. Twenty six countries were present at the International Meridian Conference to vote on an international prime meridian. Ultimately the outcome was as follows: there would be only one prime meridian, the prime meridian was to cross and pass at Greenwich (which was the 0°), there would be two longitude direction up to 180° (east being plus and west being minus), there will be a universal day, and the day begins at the mean midnight of the initial meridian.

Toward the ending of the 12th century there were two main locations that were acknowledged as the geographic location of the meridian, France and Britain. These two locations often conflicted and a settlement was reached only after there was an 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 Greenwich (0°) west to the antimeridian (180°) define the Western Hemisphere and the meridians from Greenwich (0°) east to the antimeridian (180°) define the Eastern Hemisphere. Unreliable source: certain= 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 World Geodetic System 84" (WGS84) 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 at Greenwich (as the older astronomic position was), but can be located using a GPS receiver.

The true meridian is the chord that goes from one pole to the other, passing through the observer, and is contrasted with the magnetic meridian, which goes through the magnetic poles and the observer. The true meridian can be found by careful astronomical observations, and the magnetic meridian is simply parallel to the compass needle. The arithmetic difference between the true and magnetic meridian is called the magnetic declination, which is important for the calibration of compasses. Henry D. Thoreau classified this true meridian versus the magnetic meridian in order to have a more qualitative, intuitive, and abstract function. He used the true meridian since his compass varied by a few degrees. There were some variations. When he noted the sight line for the True Meridian from his family's house to the depot, he could check the declination of his compass before and after surveying throughout the day. He noted this variation down.

A standard meridian is a meridian used for determining standard time. For instance, the 30th meridian east (UTC+02:00) is the standard meridian for Eastern European Time. Since the adoption of time zones – as opposed to local mean time or solar time – in the late 19th century and early 20th century, most countries have adopted the standard time of one of the 24 meridians closest to their geographical position, as decided by the International Meridian Conference in 1884. Although, a few time zones are offset by an additional 30 or 45 minutes, such as in the Chatham Islands, South Australia and Nepal.

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