Meridian (geography)

Lore & Background

In geography and geodesy, a meridian is the line connecting all points sharing the same longitude, measured as an angle east or west of a prime meridian (currently the IERS Reference Meridian). It is a coordinate line for longitude. A point’s position along a given meridian is defined by its latitude, measured north or south of the Equator. On projections like Mercator or Gall-Peters, each meridian is perpendicular to every circle of latitude. If Earth is modeled as a sphere, a meridian forms a great semicircle; if modeled as a spheroid or ellipsoid, it is half of a north-south great ellipse. The length of a meridian is twice the length of an Earth quadrant—on the WGS 84 ellipsoid, this equals a specific modern value. The term originates from Latin *meridies* (“midday”), because the subsolar point crosses a given meridian at solar noon. This same root appears in a.m. and p.m. The first prime meridian was established by Eratosthenes in 200 BCE but suffered from inadequate latitude measurement. By the early modern era, multiple prime meridians were used, with Paris and Greenwich being the most commonly acknowledged. The 1884 International Meridian Conference, attended by twenty-six countries, resolved to adopt a single prime meridian passing through Greenwich (0°), with two longitude directions up to 180° (east positive, west negative), a universal day beginning at mean midnight of that meridian, and no interference with local time. The Greenwich meridian’s position has shifted slightly due to observatory relocations; the modern geodetic prime meridian (WGS84) lies 102.478 meters east of the older astronomic marker at Greenwich. The antimeridian at 180° forms the opposite half of the circle, near the International Date Line. The magnetic meridian connects the magnetic poles; a compass needle aligns with it, though not steadily due to longitudinal geodesic effects. The angle between the true and magnetic meridians is magnetic declination.

Reader's Guide

The term "meridian" originates from the Latin word for "midday," as the Sun crosses a given meridian at solar noon. In geography and geodesy, a meridian is the line connecting points of equal longitude, measured east or west of a prime meridian. On a spherical model of Earth, a meridian forms a great semicircle; on an ellipsoidal model like WGS 84, it is half of a great ellipse. Its length is twice that of an Earth quadrant. The first prime meridian was established by Eratosthenes around 200 BCE, but early measurements suffered from a lack of latitude data. By the early modern era, many countries used their own preferred meridians, leading to inconsistencies in coordinates and local time. This became problematic for coordinating railway services, as each town effectively had its own time zone. The growing international economy and the need for standardized navigation and trade prompted the International Meridian Conference of 1884 in Washington, D.C. Twenty-six nations voted to adopt a single prime meridian passing through Greenwich, England, with longitude measured up to 180° east and west. The antimeridian, at 180° longitude, lies opposite Greenwich near the International Date Line. The prime meridian’s position has shifted slightly over time; the adoption of WGS84 moved the geodetic prime meridian 102.478 meters east of its previous astronomic position at Greenwich. The magnetic meridian connects the magnetic poles, and the angle between it and the true meridian is magnetic declination, essential for compass navigation.

Did You Know?

• The term meridian comes from Latin meridies, meaning 'midday'.
• The first prime meridian was set by Eratosthenes in 200 BCE.