Solving the Longitude Problem

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Navigation on the open ocean is extremely difficult. It is a skill that takes years to master. 

Even with years of skill, an experienced mariner was still able to ground their ship on an unseen reef, underwater rocks, or a sandbar, because they didn’t know their precise location. 

The main problem, which was unsolved for centuries, was determining your longitude.

Learn more about the longitude problem, and how it was eventually solved, on this episode of Everything Everywhere Daily. 


In an earlier episode, I spoke about the navigational achievements of the  Polynesians. They managed to explore the Pacific Ocean without the use of any advanced navigational tools. Moreover, they sailed large outrigger ships that sat very high in the water. 

Not only did they have advanced knowledge of reefs and how to navigate them, but their ships, at high tide could often just sail right over them. 

European navigators weren’t so lucky. They had much larger ships that sat much lower in the water. If there was an underwater hazard such as a rock or a reef, and they were to unknowingly hit it, the results could be devastating. 

In fact, this happened far more often than most people realize. Just as an example, Sable Island which is a large sand bar off the coast of Nova Scotia is estimated to have caused at least 350 shipwrecks and possibly as many as 500. 

This was just one island. There were many other such hazards all over which ships had to avoid. 

This was one of the reasons why accurate mapping was so important. There were many voyages where a cartographer was sent along to bring back information to improve their maps. 

However, the best map in the world doesn’t mean anything if you don’t know where you are. 

This was the problem that beset sailors. 

To know where you were, you needed to know two things: your latitude and your longitude. 

Latitude, which are the imaginary horizontal lines around the Earth, was a rather easy problem to solve. Because the Earth rotates around its poles, and because there is a star in the northern hemisphere where the pole is, it is possible to determine your latitude by knowing the angle between the pole star and the horizon. 

This was done via a sextant which was a device that could measure angles. 

If you add to this the use of a compass, you could know your direction, but that was it.

There was no way, however, to determine your longitude.

Longitude lines are the imaginary vertical lines that go from north to south pole. Because the Earth is always rotating, it’s near impossible to get a fix on your longitude based on the stars. 

This problem was so important that the British Parliament passed the Longitude Act 1714 which offered a prize to anyone who could solve the problem. 

The prize was between £10,000 and £20,000 depending on the accuracy of the technique. In modern-day money, that would be between $2 to $4 million dollars. Money went a lot further back then. 

The key to determining longitude was time. If you knew the local time and the time at a fixed location, you can determine the difference in time, and hence the difference in longitude. 

The technique that everyone thought would win out was an astronomical technique. 

One of the ideas was the technique of Lunar distance. The moon moves across the stars at a rate of about 0.5 degrees per hour. If you have good tables, a good measurement, and did the calculations, you could determine your local time. 

Another technique was to use the moons of Jupiter. Because the four major moons of Jupiter orbited at a specific rate, if you knew the position of the moons, again with some very good tables, you could determine the time. 

The problem with these techniques is that they didn’t really work on the deck of a ship that was pitching and rolling on the sea. You can’t use a telescope in that sort of environment. 

Moreover, if the moon wasn’t out, or Jupiter wasn’t in the sky at the right time of the year, or if there were clouds, the techniques wouldn’t work. 

The first British Astronomer Royal in 1675 was basically commissioned to work on this problem and develop tables at the observatory in Greenwich.

These astronomical techniques did actually work on land. If you were in say Newfoundland or Kingstown, you could figure out the longitude of that spot with decent accuracy, and that helped in mapmaking.

However, it did nothing for aiding navigators on a ship. 

The solution which eventually won out was the development of an accurate clock. 

Originally, no one thought that a clock would work because early clocks required the use of a pendulum. A pendulum clock on a rocking ship was about as useless as trying to make astronomical observations on a ship with a telescope. 

An English clockmaker by the name of John Harrison knew that building a chronograph was the key to solving the problem of latitude. The trick was engineering a clock that could work at sea, yet still, remain accurate. 

Harrison spent 40 years working on five different clocks. His first clock, which was tested on a voyage in 1736, was able to determine the longitude of a ship to within 60 miles. Good but not great.

Over the years he kept constantly improving his clocks. His final version looked like a very large pocket watch. 

He has solved many problems associated with keeping time on a ship, including the motion of the ship and changes in temperature.

His final clock, which was tested in 1772 by King George III himself, was found to be accurate to within ? of one second per day. More than accurate enough to claim the prize.

However, he never did technically claim the prize. The astronomer royal was on the board overseeing the prize, and the board was very biased against any non-astronomical solution. In the end, Harrison did collect the equivalent of the prize, but never the acknowledgment of parliament. 

All the clocks in the British Navy were synchronized to the Greenwich Observatory, which is the reason why the Prime Meridian is there, and why Greenwich Mean Time, now called Coordinated Universal Time or UTC, is centered on Greenwich. 

In fact, if you visit the Greenwich observatory today, you can see the first four Harrison timepieces. All of them still work today. You can also hear the cannon which is fired at noon every day, which was used so ships could synchronize their clocks. 

The early clocks were expensive, but they proved their worth. Captain Cook used a clock very similar to Harrison’s fifth clock on his second and third voyages, and he had only glowing things to say about them.

In the 19th century, the clocks continued to get better and cheaper, and as a result, navigation became better and safer. As the clocks were installed on more ships, the number of disasters involving ships running aground plummeted, and sailing became much safer.

Eventually, as the telegraph became widespread, time signals would be sent along the wire. As the signal was almost instantaneous, measurements of longitude between points became even more precise. 

Telegraphs didn’t do much for ships, but when wireless communication was developed, it allowed ships to get time signals wirelessly, which improved the precision of their measurements. 

In 1912, a conference was held in Paris to coordinate global wireless time signals on different frequencies, allowing ships to get accurate time information almost anywhere on the globe. 

The pursuit of longitudinal measurement culminated in the development of the Global Positioning System, which allows for pinpoint measurement anywhere on Earth. The development of GPS is a topic which I went into much greater detail in a previous episode. 

If you’ve ever traveled or purchased something which was made overseas, you have been the beneficiary of the solution to the problem of longitude. A problem solved by one determined English clockmaker in the 18th century.