Meridian (geography)

Lore & Background

In geography and geodesy, a meridian is the line connecting points of equal longitude, measured as an angle east or west of a prime meridian (currently the IERS Reference Meridian). It functions as a coordinate line for longitude. A point’s position along a given meridian is defined by its latitude, measured in degrees north or south of the Equator. On map projections such as Mercator or Gall-Peters, each meridian is perpendicular to all circles of latitude. Assuming a spherical Earth, a meridian forms a great semicircle; under a spheroidal or ellipsoidal model, it is half of a north-south great ellipse. The length of a meridian equals twice the length of an Earth quadrant, a value defined on the modern WGS 84 ellipsoid. The term derives from Latin *meridies* (“midday”), as the subsolar point crosses a given meridian at solar noon; the same root gives a.m. and p.m. The first prime meridian was established by Eratosthenes in 200 BCE, but lacked latitude measurement. By the early modern era, multiple prime meridians were used, with Paris and Greenwich most often acknowledged. The 1884 International Meridian Conference, attended by twenty-six countries, established a single prime meridian passing through Greenwich, set at 0° longitude, with east and west directions up to 180°. The antimeridian at 180° forms the other half of the circle, near the International Date Line. The geodetic prime meridian under WGS84 lies 102.478 meters east of the older astronomic marker at Greenwich. A magnetic meridian connects the magnetic poles; a compass needle aligns with it, and the angle between the magnetic and true meridian is magnetic declination.

Reader's Guide

The meridian is fundamental to global navigation, timekeeping, and mapping. The concept of the meridian also underlies the magnetic meridian (used with compasses) and the true meridian (found by astronomical observations). Standard meridians, such as the 30th meridian east for Eastern European Time, are used to determine standard time zones, though some countries use offsets of 30 or 45 minutes. Navigators historically used the Sun's altitude at local meridian passage to calculate latitude.

The term itself 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 measurements suffered from a lack of latitude data. By the early modern era, many nations used different prime meridians, notably Paris and Greenwich, causing inconsistencies in coordinates and local times. The rise of global travel and railways, where each town had its own time, created pressure for a single standard. The 1884 International Meridian Conference in Washington, D.C., with twenty-six countries, resolved that one prime meridian would pass through Greenwich (set at 0°), with longitude measured east and west up to 180°. This also established a universal day beginning at midnight. The Greenwich meridian’s exact position has shifted slightly; the modern geodetic prime meridian (WGS84) lies 102.478 meters east of the older astronomic marker at Greenwich, though no physical sign marks it. The antimeridian at 180° longitude, opposite Greenwich, lies near the International Date Line. The magnetic meridian, aligned with Earth’s magnetic poles, differs from the true meridian by an angle called magnetic declination.

Did You Know?

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