Meridian (geography)

Lore & Background

The term meridian comes from the Latin *meridies*, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between sunrise and sunset on that meridian. The same Latin stem gives rise to a.m. (*ante meridiem*) and p.m. (*post meridiem*). In geography and geodesy, a meridian is the locus connecting points of equal longitude, measured in degrees east or west of a prime meridian. It is a coordinate line for longitudes, also called a line of longitude. A point’s position along a meridian is given by its latitude, measured north or south of the Equator. On Mercator or Gall-Peters projections, each meridian is perpendicular to all circles of latitude. Assuming a spherical Earth, a meridian is a great semicircle; on a spheroidal or ellipsoid model, it is half of a north-south great ellipse. The length of a meridian is twice the length of an Earth quadrant, equal to that value on the WGS 84 ellipsoid. The first prime meridian was set by Eratosthenes in 200 BCE, but had problems due to lack of latitude measurement. By the early modern era, multiple locations were used, with Paris and Greenwich most often acknowledged. The International Meridian Conference in 1884, with twenty-six countries present, established a single prime meridian through Greenwich, England, set at zero degrees longitude. Other meridians were defined by the angle at Earth’s center between them and the prime meridian at the equator. The antimeridian at 180° longitude, opposite Greenwich, forms the other half of the circle. Meridians from Greenwich west to the antimeridian define the Western Hemisphere; east to the antimeridian define the Eastern Hemisphere. The prime meridian’s position has shifted slightly; the adoption of WGS84 moved the geodetic prime meridian 102.478 meters east of its last astronomic position at Greenwich. The magnetic meridian is an imaginary line connecting the magnetic poles; a compass needle aligns with it, but the angle between the magnetic and true meridian is magnetic declination.

Reader's Guide

Meridians are the north-south lines connecting points of equal longitude, measured as an angle east or west of a prime meridian. On a spherical model of Earth, each meridian forms a great semicircle; on an ellipsoid model, it is half of a great ellipse. The length of a meridian equals twice the length of an Earth quadrant. The term derives from the Latin for "midday," as the Sun crosses a given meridian at solar noon. Historically, the first prime meridian was established by Eratosthenes around 200 BCE, but early systems lacked accurate latitude measurement. By the early modern era, multiple prime meridians were used, with Paris and Greenwich being the most common. This caused inconsistencies in coordinates and local times, complicating railway coordination. The 1884 International Meridian Conference, attended by twenty-six countries, established a single prime meridian passing through Greenwich, with longitude directions extending 180° east and west, and a universal day beginning at mean midnight. The Greenwich meridian defines the Western and Eastern Hemispheres, with the antimeridian at 180° longitude near the International Date Line. The geodetic prime meridian under WGS84 is now 102.478 meters east of the older astronomic position at Greenwich, though no physical marker indicates this current location. The magnetic meridian connects Earth's magnetic poles, and a compass aligns with it. The angle between the magnetic and true meridian is magnetic declination, essential for navigation.

Did You Know?

• The term meridian comes from Latin meridies, meaning 'midday'.