Meridian (geography)

A meridian is a coordinate line used to measure longitude on Earth. Longitude is the angle measured in degrees east or west from a starting point. This starting point is known as the prime meridian.

While latitude measures how far north or south a point is from the Equator, meridians define the east-west position. If we assume the Earth is a perfect sphere, a meridian is a great semicircle. However, Earth is actually a spheroid or an ellipsoid. In this model, a meridian is half of a north-south great ellipse.

Meridians function as part of a global grid system. On certain maps, such as the Mercator or Gall-Peters projections, every meridian is perpendicular to all circles of latitude. This means they cross at right angles. The prime meridian is set at 0° longitude. It divides the planet into two halves. The area to the east is the Eastern Hemisphere. The area to the west is the Western Hemisphere. On the opposite side of the Earth, the antimeridian sits at 180° longitude. This line is located near the International Date Line.

The term meridian has interesting roots in history. It comes from the Latin word *meridies*, which means "midday." This name refers to the subsolar point. This is the point where the Sun passes through a given meridian at solar noon. Solar noon occurs midway between sunrise and sunset. This same Latin root gives us the terms a.m. and p.m. These terms help us tell time using a 12-hour clock. They stand for *ante meridiem* and *post meridiem*.

Finding a single, universal meridian was a major historical challenge. The first prime meridian was set by Eratosthenes in 200 BCE. However, early measurements faced problems because latitude was hard to measure. By the early modern era, different countries used their own preferred meridians. This lack of consistency made it difficult to record local times and coordinates. It even caused trouble for railway services. Each town often had its own time zone, making coordination very hard. Two locations, Paris and Greenwich, were most commonly used as reference points.

To solve these problems, nations met for the International Meridian Conference in 1884. This meeting took place in Washington, D.C. Twenty-six countries attended the conference to vote on a single standard. They wanted to support a growing international economy. A standard meridian would make worldwide travel and international trading much easier. The conference decided there would be only one prime meridian. It would pass through Greenwich, England, at 0°. They also established a universal day that begins at mean midnight. They decided that east would be represented by plus values and west by minus values up to 180°.

There are different types of meridians used in science and navigation. A true meridian is a chord that goes from pole to pole through an observer. In contrast, a magnetic meridian connects the magnetic north and south poles. A compass needle will point parallel to the magnetic meridian. The difference between the true meridian and the magnetic meridian is called magnetic declination. Navigators must understand this angle to calibrate their compasses correctly. Henry D. Thoreau used these distinctions to account for variations in his own compass.

Meridians are also essential for tracking celestial objects. A meridian passage occurs when a star or the Sun reaches its highest point. This happens when the object crosses the observer's meridian. Navigators used the Sun's altitude during this passage to calculate their latitude. They used a specific formula involving the Sun's declination and its noon altitude. Today, we use the World Geodetic System 84, or WGS84, for positioning. This system moved the geodetic prime meridian 102.478 meters east of the old astronomic position at Greenwich. You can now locate this precise point using a GPS receiver.