The prime meridian is the line of 0 longitude, the starting point for measuring distance both east and west around the Earth.

The prime meridian is arbitrary, meaning it could be chosen to be anywhere. Any line of longitude (a meridian) can serve as the 0 longitude line. However, there is an international agreement that the meridian that runs through Greenwich, England, is considered the official prime meridian.

Governments did not always agree that the Greenwich meridian was the prime meridian, making navigation over long distances very difficult. Different countries published maps and charts with longitude based on the meridian passing through their capital city. France would publish maps with 0 longitude running through Paris. Cartographers in China would publish maps with 0 longitude running through Beijing. Even different parts of the same country published materials based on local meridians.

Finally, at an international convention called by U.S. President Chester Arthur in 1884, representatives from 25 countries agreed to pick a single, standard meridian. They chose the meridian passing through the Royal Observatory in Greenwich, England. The Greenwich Meridian became the international standard for the prime meridian.

UTC

The prime meridian also sets Coordinated Universal Time (UTC). UTC never changes for daylight savings or anything else. Just as the prime meridian is the standard for longitude, UTC is the standard for time. All countries and regions measure their time zones according to UTC.

There are 24 time zones in the world. If an event happens at 11:00 a.m. in Houston, Texas, it would be reported at 12 p.m. in Orlando, Florida; 4:00 p.m. in Morocco; 9:00 p.m. in Kolkata, India; and 6:00 a.m. in Honolulu, Hawaii. The event happened at 4:00 p.m. UTC.

The prime meridian also helps establish the International Date Line. The Earth's longitude measures 360, so the halfway point from the prime meridian is the 180 longitude line. The meridian at 180 longitude is commonly known as the International Date Line. As you pass the International Date Line, you either add a day (going west) or subtract a day (going east.)

Hemispheres

The prime meridian and the International Date Line create a circle that divides the Earth into the eastern and western hemispheres. This is similar to the way the Equator serves as the 0 latitude line and divides the Earth into the northern and southern hemispheres.

The eastern hemisphere is east of the prime meridian and west of the International Date Line. Most of Earths landmasses, including all of Asia and Australia, and most of Africa, are part of the eastern hemisphere.

The western hemisphere is west of the prime meridian and east of the International Date Line. The Americas, the western part of the British Isles (including Ireland and Wales), and the northwestern part of Africa are landmasses in the western hemisphere.

prime meridian
The Royal Observatory, which identified the prime meridian in 1884, is no longer a working scientific facility. It is maintained as a tourist attraction.

Laser Meridian
Today, the prime meridian is marked by a laser beam that shoots out northward from the Royal Observatory in Greenwich, England.

Planetary Prime Meridians
The Earth is not the only planet with a prime meridian. Scientists use craters or other geographic features to mark prime meridians on other planets and celestial bodies. The prime meridian of Mars runs through a crater named Airy-0. The prime meridian of the Earth's moon runs near a crater named Bruce.

Airy-0
Noun

crater on Mars which marks the prime meridian on that planet.

arbitrary
Adjective

determined by choice, not by standards or rules.

Bruce
Noun

crater on the moon which marks its prime meridian

Noun

city where a region's government is located.

cartographer
Noun

person who makes maps.

Noun

type of map with information useful to ocean or air navigators.

Chester Arthur
Noun

(1829-1886) 21st president of the United States (1881-1885).

convention
Noun

formal meeting, usually with representatives from different regions or parties.

Coordinated Universal Time (UTC)
Noun

time standard based on atomic time that is coordinated with rotational timethe cycles of day and nightby the introduction of leap seconds at certain intervals. For practical purposes, sometimes referred to as Greenwich Time.

daylight savings
Noun

practice of moving one hour forward in the spring (spring forward) and one hour backward in the fall (fall back) to gain an extra hour of daylight.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Eastern Hemisphere
Noun

area of the Earth east of the prime meridian and west of the International Date Line.

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.

landmass
Noun

large area of land.

laser
Noun

(acronym for light amplification by stimulated emission of radiation) an instrument that emits a thin beam of light that does not fade over long distances.

Noun

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

Noun

symbolic representation of selected characteristics of a place, usually drawn on a flat surface.

meridian
Noun

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

Moon
Noun

Earth's only natural satellite.

Noun

art and science of determining an object's position, course, and distance traveled.

Noun

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

Royal Observatory
Noun

museum and planetarium in Greenwich, England, that marks the prime meridian and determines Universal Time.

time zone
Noun

one of Earth's 24 divisions distinct by one hour, roughly 15 degrees of longitude.

Western Hemisphere
Noun

area of the Earth west of the prime meridian and east of the International Date Line.