Solving the Longitude Problem

How time can tell you where you are at sea
25 February 2020

Interview with 

Josh Nall, University of Cambridge


Ocean and an island


Time doesn’t just change over the course of a day. It changes as you go east or west. And as countries like the UK, traditionally a nation of seafarers, took to the waves, while sailors could work out their latitude - how far north or south they were - from the positions of stars or the sun, knowing how far they’d gone around the world - their longitude - was impossible. Because rough seas meant that a traditional pendulum couldn’t keep a clock ticking. This was called the Longitude Problem. Josh Nall from the Department of History and Philosophy of Science at the University of Cambridge joined Chris Smith and Adam Murphy to explain how it was ultimately solved...

Josh - Well, particularly in the British context, Britain is an expanding Imperial nation, it's active in the Atlantic slave trade, it has colonial holdings in places like India and that means long range seafaring; and knowing your position in East and West becomes extremely important when you begin to approach land so that you don't, for example, run aground and lose ships. And this was becoming by the turn of the 18th century a very serious problem for the British, in terms of losing ships going out to India and coming back.

Chris - And it just comes down to the simple fact that a clock would not tick and keep time reasonably when you're rocking all over the place on rough seas.

Josh - Absolutely. I mean at that point, not only could a clock not work, but when people like Isaac Newton were asked about the potential to solve this problem, he stated very clearly that it was effectively impossible to make a clock that would work at sea.

Chris - Why are we solely married to a clock as the solution? Indeed, is that the only way to solve this problem?

Josh - Well, many possible solutions were offered, but a clock seemed the best. For the simple reason that very intuitively we understand that as we move East or West, there's a time difference. So if you have a one hour time difference between two places on earth, we know that that's a 15 degree change in longitude. So if you know your local time, which you can find from the altitude of the sun, and you know the time at a reference location, which the British have always taken to be Greenwich observatory, then the difference in time gives you a very easy and quick read of your difference in longitude. So of all of the possible schemes proposed, it was always seen that timekeeping at sea would be the best solution.

Chris - And what, did someone throw down the gauntlet then? And say, "This is a major problem. It's costing us ships. It's costing us lives. It's costing us money. We need to solve it?"

Josh - Absolutely. So following quite a serious Naval disaster in 1707, where the fleet of the wonderfully named Sir Cloudesley Shovell ran aground on the Scilly Isles, and over 1600 British sailors were drowned. Parliament was pushed to act, and in 1714 they passed an Act of Parliament offering very significant financial rewards for anyone who could propose a workable solution to finding longitude at sea.

Chris - The one person who is often hailed as the hero of the day is John Harrison who was a watchmaker. He made timepieces didn't he? Was he the only player? Presumably, if there was a big financial incentive, didn't lots of people enter the race?

Josh - A very large number, to the point that there were a great many satires at the time of the number of absurd proposals that were being put forward! But certainly Harrison has become known, probably better than any other, because he is the person who first begins to develop clocks that can be effectively used at sea. But he's certainly not the only that the board of longitude is supporting and funding through the 18th century. At the same time that Harrison is working, there is an equivalently well funded project to try and develop an astronomical solution, which is effectively to use the moon as a clock because you can actually at any point on the earth if you measure the angle between the moon and certain given stars with the right kinds of astronomical data to hand, you can calculate time at Greenwich. And so that solution which involved using an instrument called a sextant and using astronomical tables that the Royal observatory had issued was also very well funded and was in a certain sense set up as something of a rival solution to Harrison. But in actual fact, the reality is by the end of the century, both techniques, taking working clocks but also taking sextants and astronomical tables, they tended to be used in conjunction.

Adam - Why would you want to use the system of measuring the moon and complex angles and sextants as opposed to just having a really good clock onboard?

Josh - There are two really good reasons. The first is that clocks are always going to be dangerously unreliable. So if your clock breaks at sea, you have no way of recovering the time that it was carrying, because it was carrying Greenwich time. And so you've then lost your one navigational technique. The second reason is cost. Chronometers were exceedingly expensive valuable pieces. Whereas taking astronomical tables, taking a sextant was a much more affordable, and therefore a much more general and repeatable technique.

Chris - So John Harrison invents a clock that solves the problem - A, how did he do that? And B, what did he win for doing it?

Josh - It's really a lifelong journey of clock making and clock improving that takes him to this eventual clock, which is generally now referred to as H4. And that's because it's the fourth major clock that he develops. And he starts working on these clocks in the early 1730s, and it's not until the mid 1760s that these are trialled at sea and demonstrated to be keeping time reliably at sea. And at that stage he has gone through many, many iterations and improvements, improving things like temperature compensation. You would think that motion at sea would be a really big problem, but if you think about it, pocket watches already existed. One of the really big problems alongside motion was temperature change, which really wreaks havoc with a clockwork mechanism because all of your metal parts expand and contract, and that just means your rate of timekeeping varies unreliably.

Chris - And what did, did he, when did he get the full price? Did it just go to him or any of these other solutions sort of co-awarded?

Josh - The answer is yes and no. Harrison got a lot of money from the board of longitude. They kept giving him money over the years to keep funding his work and his development, but at the end of his life, they didn't then decide to give him the full 20,000 pound award. Very controversially. Harrison was extremely upset. They were concerned principally about reproducibility. Their worry was that he had developed a clock, a single clock that worked, but they were extremely worried that they wouldn't then be able to produce more. So they offered him 10,000 pounds of the 20,000 prize if he would reveal the workings of his clock, which he did very reluctantly. And then they said they would give the other 10,000 if then the clock could be proved to be generally useful and usable. And this is the point that Harrison gets very upset and ultimately appeals to George III because he thinks he should just get the 20,000 pounds straight off.

Chris - And did they manage to reproduce it easily and give him the other 10,000 or did he go away an unhappy man?

Josh - They did. Ultimately parliament awarded him nearly all of the rest of the 10,000. They offered him 8,750 pounds. So no one in the end was happy because he got almost 20,000 uh, but not quite


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