What is Longitude?

Longitude is a geographic coordinate that defines the east-west position of a point on the Earth’s surface. In simpler terms, it’s the lines that go north to south, and all lines cross at the north and south poles. Longitude in pair with latitude defines one unique position given as coordinates.

Greenwich, England

Greenwich is known as 0 degrees, or the prime meridian, and is ground zero for measuring. Therefore, going east, it counts positive from 0 to 180 degrees and going west it counts negative from 0 to -180 degrees. We measure each degree of longitude by 60 minutes and then subdivide that into 60 seconds.

Longitude Map

On a flat map this type of calculations actually works out well. However the world is not flat and the measurements will differ at the poles since longitude lines cross one another. A degree of longitude is widest at the equator and equals 69.172 miles. At 40° north or south, the distance between a degree of longitude is 53 miles. Eventually the measurements will converge to 0 miles at both the poles.

Longitude is a Problem

The idea of longitude and latitude has actually be around since 3rd century BC, when Eratosthenes proposed these two to map the world. Hipparchus was the first to actually use them to label places on Earth. He understood that knowing the time between two locations and celestial knowledge would give you longitude.

Earth's Rotation and Orbit

Visual of what’s happening when the Earth rotates and orbits

Because you need to know time between two places, longitude has always been hard to calculate. Today we have computers and satellites to do the work for us but back then it was a problem to calculate correctly, especially at sea. There was a challenge in 1714 to come up with a solution and many great thinkers of our past tried to solve this problem.


To get an exact location of where you are in the world you need three things, altitude, latitude and longitude. On land, with enough time, celestial studying and knowing your altitude, it was possible. But at sea, it was a different story. Constant movement and the celestial sky changing every night, meant nobody could accurately know what time it was at a given place at sea. Navigators needed something that could keep time in rough seas and could handle constant movement.

John Harrison

John Harrison

It took the greats minds of the 18th century to measure longitude and to calculate it while at sea. For nearly 50 years, engineers, scientists and thinkers spent time finding the right device that could withstand rough seas to take time properly.

In the 1760’s, the marine chronometer was invented by John Harrison. The purpose was to accurately keep time on a ship of a known fixed location, such as Greenwich time. Knowing Greenwich’s time allows a navigator to use the time difference between the ship’s position and the Greenwich Meridian. As the Earth rotates at a regular rate, the time difference between the chronometer and the ship’s local time is used to calculate the longitude of the ship from the Greenwich Meridian using spherical trigonometry with the help of the moon. Then an accurate location is plotted using celestial navigation knowledge and calculations.

Longitude Changed the World

Plotting the world with longitude and latitude was a big achievement. It opened the door for many explorers to properly map the world and to understand it. Countries and people would not be where they are today if countries of the past were not able to understand and map the world like they did. Today GPS and satellites map the world for us; they are essential for daily life and travel.

Map of the World Philipp Eckebrecht




images by: wikimedia, royalmuseumgreenwich, eckebrecht

By | 2017-05-31T09:14:35+00:00 May 31st, 2017|