Longitude

Picture an orange with lines from top to bottom.

The lines go from one end to the other, like the segments. Lines of longitude look like that on the Earth.

Longitude measures how far east or west a place is. NOAA's National Ocean Service describes lines of longitude as imaginary lines that run north to south, from pole to pole, but measure distance east or west.

Lines of longitude are also called meridians. The starting line, at 0 degrees, is the prime meridian. It runs through Greenwich, in London, England. Everything east of it is measured in degrees east, and everything west of it in degrees west.

Longitude measures distance east or west of the prime meridian. NOAA's National Ocean Service notes the same twist as for latitude, in reverse: meridians run north to south, from pole to pole, yet measure position east or west. Like latitude, longitude is measured in degrees, minutes and seconds.

The distance around Earth is 360 degrees, so longitude runs up to 180 degrees in each direction from the prime meridian, the line of 0 degrees through Greenwich, England. National Geographic Education explains that the Eastern Hemisphere is measured in degrees east of the prime meridian and the Western Hemisphere in degrees west. Its example: Brasília, Brazil, lies at 47° 55′ W.

Longitude lines are far apart in the middle of the Earth.

They get closer and closer near the top and bottom. At the poles, they all meet.

Lines of latitude stay the same distance apart, but lines of longitude do not. Meridians are farthest apart at the Equator and get closer together toward the poles, where they all meet.

NOAA gives the numbers. At the Equator, one degree of longitude covers about 111 kilometers (69 miles). By 60 degrees north or south, that has shrunk to about 56 kilometers (35 miles). At the poles, it is zero.

Unlike parallels, meridians are not evenly spaced. They are farthest apart at the Equator, where Earth bulges out, and converge until they meet at the North and South Poles. So the ground distance covered by a degree of longitude depends on latitude.

NOAA's figures show how fast it changes: about 111 km (69 miles) per degree at the Equator, the same as a degree of latitude; about 56 km (35 miles) at 60° N or S; and zero at the poles. A grid square bounded by one degree of latitude and one of longitude is therefore nearly square near the Equator but a thin wedge near the poles, which is why a flat map that draws meridians as parallel lines must stretch high latitudes.

Someone had to choose where to start counting.

In 1884, people from many countries met. They chose a line through Greenwich, in London.

The Equator is a natural starting line for latitude, halfway between the poles. But there is no natural starting line for longitude, so people had to choose one.

In 1884, representatives met at the International Meridian Conference in Washington, D.C. They chose the line through a telescope at the Royal Observatory in Greenwich, England. Many sea charts and time zones already used Greenwich, so keeping it made sense. Royal Museums Greenwich says the cross-hairs of that telescope, the Airy Transit Circle, defined zero degrees longitude for the world.

NOAA points out that, unlike the Equator, the prime meridian is arbitrary. At the 1884 International Meridian Conference in Washington, D.C., delegates chose the meridian running through the telescope at the Royal Observatory, Greenwich; many nautical charts and time zones already used it. Royal Museums Greenwich identifies the instrument as the Airy Transit Circle, designed in 1850 by George Biddell Airy, the seventh Astronomer Royal; the cross-hairs in its eyepiece defined 0° for the world.

The story has a modern twist. Nineteenth-century scientists did not allow for local variations in gravity or Earth's slightly squashed shape. NOAA says that a GPS receiver at Greenwich reads 0° only about 102 meters (334 feet) east of the prime meridian markers. In the 1980s the International Reference Meridian was set as the precise 0°, and unlike the Greenwich line it moves as Earth's surface shifts.

Go east from Greenwich and the numbers grow.

Go west and they grow too. Halfway around the world, east and west meet at 180.

Longitude starts at 0 degrees at the prime meridian and counts up to 180 degrees going east, and up to 180 degrees going west. Halfway around the world, at 180 degrees, east and west meet. This line is called the antimeridian.

The prime meridian and the antimeridian together split Earth into the Eastern and Western Hemispheres. The International Date Line is based on the antimeridian, but it zigzags around borders, because local laws decide where each day begins.

Counting from the prime meridian, longitude runs to 180° E and 180° W, which are the same line: the antimeridian, on the opposite side of Earth. The prime meridian divides Earth into Eastern and Western Hemispheres, with the antimeridian completing the circle.

NOAA notes that the antimeridian is the basis for the International Date Line, but actual date and time-zone boundaries depend on local laws, so the date line zigzags around borders near 180°. National Geographic Education adds that coordinates are valuable for engineering and rescue work, helping engineers choose sites for bridges or wells and pilots land planes or drop aid packages in exact locations.

Earth turns all the way around each day.

So noon comes at different times in different places. Sailors used clocks to work out how far east or west they were.

Earth spins 360 degrees in 24 hours, which is 15 degrees every hour. That means longitude and time are linked. NOAA explains how sailors used this: they compared local noon, when the Sun was highest, with a clock set to Greenwich time. Each hour of difference meant 15 degrees of longitude.

The hard part was building a clock that kept good time on a rocking, salty ship. Accurate marine clocks, called chronometers, were not invented until the 18th century.

Because Earth rotates 360 degrees in 24 hours, or 15 degrees an hour, the time difference between two places reveals their difference in longitude. NOAA works an example: if the Sun shows local noon and a clock set to Greenwich Mean Time reads 2 p.m., the ship is 30° west of Greenwich.

The method needed a clock that stayed accurate at sea, and the clocks of the time failed in swaying ships, changing temperatures and salty air. Starting in the 16th century, European governments offered huge rewards for solving "the longitude problem"; Royal Museums Greenwich describes Britain's 1714 Longitude Act, which offered money for finding a ship's longitude. Marine chronometers in the 18th century finally made it possible. Today precise time still underpins longitude: NOAA notes that every GPS satellite carries atomic clocks.

Every place has a longitude.

London is very close to 0. Tokyo is far to the east. New York is far to the west.

Here are some longitudes from Natural Earth, a free public map dataset. London sits almost on the prime meridian, at about 0.1 degrees west. Tokyo, Japan, is at about 140 degrees east. New York City is at about 74 degrees west, and Honolulu, Hawaii, at about 158 degrees west.

Put latitude and longitude together and you have coordinates, a pair of numbers that can find any spot on Earth, from a city to a single house.

Natural Earth's city points place central London at 0.12° W, almost on the prime meridian; Tokyo at 139.75° E; New York City at 73.98° W; and Honolulu at 157.86° W, closer to the antimeridian than to Greenwich. National Geographic Education's examples use degrees and minutes: Brasília at 47° 55′ W.

Neither number alone is an address. A longitude picks out a meridian from pole to pole; a latitude picks out a parallel around the globe. Where they cross is a single point, and that pair of coordinates means the same place on every map, globe and GPS receiver.