Longitude is the measurement east or west of the prime meridian. Longitude is measured by imaginary lines that run around the Earth vertically (up and down) and meet at the North and South Poles. These lines are known as meridians. Each meridian measures one arcdegree of longitude. The distance around the Earth measures 360 degrees.

The meridian that runs through Greenwich, England, is internationally accepted as the line of 0 degrees longitude, or prime meridian. The antimeridian is halfway around the world, at 180 degrees. It is the basis for the International Date Line.

Half of the world, the Eastern Hemisphere, is measured in degrees east of the prime meridian. The other half, the Western Hemisphere, in degrees west of the prime meridian.

Degrees of longitude are divided into 60 minutes. Each minute of longitude can be further divided into 60 seconds. For example, the longitude of Paris, France, is 2° 29' E (2 degrees, 29 minutes east). The longitude for Brasilia, Brazil, is 47° 55' W (47 degrees, 55 minutes west).

A degree of longitude is about 111 kilometers (69 miles) at its widest. The widest areas of longitude are near the Equator, where the Earth bulges out. Because of the Earth's curvature, the actual distance of a degrees, minutes, and seconds of longitude depends on its distance from the Equator. The greater the distance, the shorter the length between meridians. All meridians meet at the North and South Poles.

Longitude is related to latitude, the measurement of distance north or south of the Equator. Lines of latitude are called parallels. Maps are often marked with parallels and meridians, creating a grid. The point in the grid where parallels and meridians intersect is called a coordinate. Coordinates can be used to locate any point on Earth.

Knowing the exact coordinates of a site (degrees, minutes, and seconds of longitude and latitude) is valuable for military, engineering, and rescue operations. Coordinates can give military leaders the location of weapons or enemy troops. Coordinates help engineers plan the best spot for a building, bridge, well, or other structure. Coordinates help airplane pilots land planes or drop aid packages in specific locations.

longitude
All lines of longitude are measured from the prime meridian.

Into the Great Wide Open
It was not until the 18th century that people were able to correctly determine their longitude, even though they had been able to figure out latitude for some time. Not being able to reckon longitude was dangerous for sailors. Without an exact location, they could easily run out of food or water on a long expedition into uncharted territory.

antimeridian
Noun

imaginary line of 180 degrees longitude, completing the circle of longitude of the prime meridian.

arc
Noun

part of the outline of a circle.

coordinates
Noun

a set of numbers giving the precise location of a point, often its latitude and longitude.

curvature
Noun

shape of a bent or curved object or form.

engineering
Noun

the art and science of building, maintaining, moving, and demolishing structures.

engineering
Noun

the art and science of building, maintaining, moving, and demolishing structures.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

International Date Line
Noun

line of longitude at roughly 180 degrees. East of this line is one day earlier than west.

Noun

distance north or south of the Equator, measured in degrees.

Noun

distance east or west of the prime meridian, measured in degrees.

meridian
Noun

line of longitude, dividing the Earth by north-south.

military
Noun

armed forces.

parallel
Noun

line of latitude, dividing the Earth by east-west.

Noun

imaginary line around the Earth running north-south, 0 degrees longitude.

rescue operation
Noun

planning and delivering aid to people in an emergency situation.

vertical
Noun

up-down direction, or at a right angle to Earth and the horizon.