Prime meridian

A prime meridian is a special line on Earth that helps us measure longitude. Longitude is like invisible lines that go from the North Pole to the South Pole. The prime meridian is the line where longitude is set to 0°. From this line, we can measure distances east and west all around the world. This line helps us give every place on Earth a unique address so we can find it easily on maps.

Earth has had many different starting points for measuring longitude in the past. But today, there is an agreed-upon international standard called the IERS Reference Meridian. This line is very close to the Greenwich Meridian, which was used before. Unlike the equator, which is based on Earth’s shape, the prime meridian is chosen for convenience.

For Earth and the Moon, we measure longitude from 0° to 180° east and west. For other objects in our Solar System like planets and moons, we measure from 0° all the way around to 360°. This system helps scientists and mapmakers describe exactly where things are in space.

History

See also: History of longitude

Long ago, people needed a way to know where they were on Earth. The ancient Greeks, like Eratosthenes and Hipparchus, started to figure this out. They used lines called meridians to measure distances east and west from a starting point.

Later, a man named Ptolemy made maps using a special line called a prime meridian. He picked a group of islands in the Atlantic as his starting point. Over time, different places chose their own starting lines. In the 1700s, many countries used lines near their capitals. Finally, in 1884, leaders from many countries agreed to use a line that runs through Greenwich in the United Kingdom as the world's prime meridian. This line is still used today.

International prime meridian

Since 1984, the world has used the IERS Reference Meridian as the main line of zero longitude. Before that, from 1884 to 1984, the Greenwich Meridian was used. These two lines are very close to each other.

In October 1884, leaders from many countries met in Washington, D.C., and chose the Greenwich Meridian as the world’s zero point for longitude and time. This line was set at the Royal Observatory in Greenwich.

Today, scientists use new tools like satellite measurements to define the IERS Reference Meridian. This line goes through the Earth’s center. It is also used by the Global Positioning System to help us know exactly where we are on the planet.

Prime meridian on other celestial bodies

See also: Longitude (planets)

"Prime meridian (planets)" redirects here; not to be confused with Central meridian (planets).

Just like on Earth, planets and moons have a line where longitude starts at 0°. These lines are picked in different ways. Sometimes they go through a special spot like a crater, or they are set using magnetic fields.

Here are some examples:

• The Sun has two systems. One starts from a point seen from Earth in 1853, when an astronomer began watching sunspots. The other comes from a place in England.
• Mercury's line is near a crater and was picked because it gets the most sunlight there.
• Venus's line goes through a crater's peak, chosen randomly.
• The Moon's line runs through the middle of the side we see from Earth.
• Mars's line goes through a crater and is tied to a spacecraft called Viking 1.
• Ceres's line runs through a crater close to its equator.
• On Vesta, the line is near a crater for easy reference.
• Jupiter and its large moons each have their own ways of setting these lines, often using craters or the way they face Jupiter.
• Titan, Saturn's largest moon, has its line through the middle of the face that always looks toward Saturn.
• Neptune's large moon Triton has its line set in a specific year.
• Pluto and its moon Charon each have lines that face each other because they always show the same side to one another.

List of historic prime meridians on Earth