It is well known by all that are acquainted with the Art of Navigation, That nothing is so much wanted and desired at Sea, as the Discovery of the Longitude, for the Safety and Quickness of Voyage, the Preservation of ships, and the Lives of Men.

The British Longitude Act (1714)*

As a sort of vague follow-up to my review of three travel books about lines of latitude, here are two books about longitude—specifically, about the historical problem of finding one’s longitude at sea.

Missing, you may notice, is the “obvious” book about the longitude problem, which almost everyone has heard of—Dava Sobel’s Longitude (1995). I’ve omitted it deliberately, because I find it difficult to say anything polite about it—Sobel frames her narrative as the struggle of an oppressed working-class hero against a self-serving intellectual elite. That styling brought her book great popular success, but at the expense of … well … at the expense of the facts.

I offer these books, instead, as a sort of antidote to Sobel’s version. Derek Howse’s Greenwich Time And The Longitude (1997) is a reissue of his Greenwich Time And The Discovery Of The Longitude (1980). It was updated and republished by the National Maritime Museum as an “Official Millennium Edition”, presumably in anticipation of millennial interest in the topic of the Greenwich meridian and the International Date Line. And Richard Dunn and Rebekah Higgitt’s Finding Longitude (2014) is a companion volume to the Ships, Clocks & Stars exhibition at the National Maritime Museum, held to mark the tercentenary of the British Longitude Act (a quote from which opens this post). In fact, the book was published in the USA under the title Ships, Clocks & Stars. Since both books come from the National Maritime Museum at Greenwich, they’re copiously illustrated with photographs, charts and documents from the museum’s collection.

Both tell the story of how, in the eighteenth century, the problem of how to find longitude at sea was solved. Finding Longitude is the better illustrated; Howse’s book gives a more extensive treatment, and moves on in its second half to the related topics of Greenwich time, timekeeping in general, and how the prime meridian came to be defined by the Greenwich Observatory.

By the time the story opens, mariners were able to find their latitude fairly easily, if they had a sight of the sun or the stars. But they could not work out their longitude except by dead reckoning. What was needed to determine longitude was a way of comparing the local time (determined from the sun and stars) with the time at some remote reference point with known longitude—either the ship’s home port or some  standard location like Greenwich or Paris.  From the difference between local time and reference time, and the knowledge that one hour’s difference in time equates to fifteen degrees of longitude, it would then be straightforward to work out how far one’s ship was to the east or west of the reference point.

But how do you work out the time at some reference location you can’t see? By the start of the eighteenth century, there were two techniques that looked like they might just, possibly, provide a solution.

The first potential solution was to use the moon as a clock. It moves quite briskly across the background stars, covering a distance equal to its own diameter in about an hour. So if you could measure the angular distance between the moon and a reference star (or, in daytime, between the moon and the sun) you’d have an absolute measure of time, which you could compare to the local time of day and deduce your longitude. There were several problems with that. At the start of the eighteenth century, astronomers didn’t know the position of the stars or the orbit of the moon accurately enough to make this “lunar distance method” work. And even if they had known the position of the stars and moon well enough to work out exactly where they should be at a given time and date, they didn’t have portable instruments that could make the necessary angular measurements accurately enough from the heaving deck of a ship at sea.

The second potential solution was to carry reference time with you—setting an extremely accurate clock before you left home, and checking this timekeeper against the local time of day whenever you needed to know longitude. The problem with that was that the clocks of the day were simply not accurate enough, even on land, let alone when exposed to the temperature variation, dampness and hectic motion of shipboard life.

That was the background to the British government’s Longitude Act of 1714, which established a prize of £20,000 of public money, to be awarded to anyone who could come up with a sufficiently accurate solution to the problem, which was “Practicable and Useful” (two words that would be the cause of immense ill-feeling in years to come). This was a positively jaw-dropping sum of money—estimates vary, but it was certainly the equivalent of one or two million pounds in today’s currency, if not more. It was also, as Dunn and Higgitt point out, an early example of scientists managing to drive government policy. The august members of the Royal Society were pretty sure that the longitude problem could be solved, if only enough people could be persuaded to think about it. And £20,000 certainly proved to be persuasive. The members of the Board of Longitude (who administered the prize money) received all sorts of more-or-less crazy submissions, many of which were, after the fashion of the time, lampooned in cartoons and scurrilous verse. I feel particularly sorry for William Whiston and Humphry Ditton, whose proposal came to the attention of the satirical Scriblerus Club. Both Finding Longitude and Greenwich Time reproduce the poem that resulted, which was structured around the irresistible rhymes of Whiston / pissed on and Ditton / shit on.

The membership of the board was made up of politicians, senior officers of the Royal Navy, and scientists. Being of a mathematical bent, the scientists had a keen interest in the lunar distance method of finding longitude, which was gradually becoming more practical with the publication of John Flamsteed’s catalogue of accurate star positions (1725), the independent invention of the double-reflection quadrant (a precursor to the marine sextant) by John Hadley and Thomas Godfrey (1731), and the publication of Tobias Mayer’s improved tables for the motion of the moon (1752).

In the midst of all this, in 1736, there appeared John Harrison, a self-taught clock-maker, who offered for the Board’s consideration a prototype marine timekeeper that addressed many of the problems that had, up to that point, bedevilled clocks at sea. The Board ordered a sea-trial, and Harrison’s clock performed well. Over the next 24 years, Harrison went on to refine his ideas through a further three timekeepers before submitting his final version, a large watch designated H-4 by historians, for a further sea-trial in 1761.

Sobel’s version of this story has unfortunately become the default narrative. Harrison slaves away for decades on his timekeepers, while the scientists on the Board of Longitude (particularly the Astronomer Royal, Nevil Maskelyne) rush to perfect the lunar distance method, in direct competition with Harrison for the coveted £20,000 prize. To this end, Maskelyne dishonestly exploits his position to misrepresent the performance of Harrison’s H-4, and to persuade the Board and government to keep changing the rules so that Harrison is denied the prize he deserves, despite H-4 performing to all the required criteria in its sea-trial.

But Howse, Dunn & Higgitt are careful to tell the other side of the story. While Harrison was developing his timekeepers, the Board repeatedly provided him with interim awards—£3250 over 24 years, the equivalent of £135/year. If we check typical annual incomes for the period concerned, we find that the Board was providing Harrison with an income that placed him in the top few percent of earners in England at the time. Harrison of course had to buy tools and materials from this allowance, but if influential members of the Board had really wished to blight Harrison’s chance of winning the prize, why would they have afforded him such a good income over so many years? And there’s no evidence that Maskelyne ever made any application for prize money himself, despite his key role in the development of the lunar distance method into a practical (if time-consuming) solution to the longitude problem. Tobias Mayer’s widow received a payment of £3000 in recognition of her late husband’s work on improved lunar tables, and Leonhard Euler received £300 for his work on the mathematics of the moon’s orbit, but Maskelyne seems to have regarded his work on the problem as being no more than his duty as Astronomer Royal.

Towards the end of Sobel’s narrative, Harrison is denied the longitude prize unless he hands over his four timekeepers to Maskelyne, publicly reveals all the secrets of their design, and undertakes to build two more. At this point, Sobel portrays Harrison as a broken man being cheated and bullied into submission. But the problem for Harrison was always with those two words “Practicable and Useful” in the original Longitude Act. Harrison seems to have been naive, imagining he was entering a competition to produce one timekeeper that could perform to the required standard in one test. Whereas he was actually being asked to provide a solution that could be rolled out and used by hundreds of ships. Until the workings of Harrison’s timekeeper were generally known, and there was a demonstrated potential for other devices to reproduce H-4’s success, the government was unlikely to disburse a large quantity of public funds.

But in exchange for his devices, an explanation of their workings, and an undertaking to make copies, Harrison was offered half the prize money—£10,000, or something like a million pounds in modern money. Does that seem unreasonable?

In total, after King George III had intervened to order a further payment, Harrison received £23,065 of public funds for his work on marine timekeepers. The total disbursed for the successful development of the lunar method of finding longitude was just the £3,300 that went to Mayer and Euler. That hardly supports the idea that there was an institutional bias against the timekeeper method. Indeed, Maskelyne and his colleagues had always understood something that Harrison seems never to have grasped—timekeepers and lunar distances were not competitors, but complementary techniques. If the timekeeper stopped, there was no way to recover your longitude unless you could do a lunar distance observation. But conversely, if the moon was invisible (because of cloud or proximity to the sun), then a reliable timekeeper would fill the gap.

And finally, when ships were able to set off to sea equipped with both marine timekeepers and lunar distance tables, Dunn and Higgitt offer a killer statistic—despite all that effort and grief, there had been no reduction in shipping losses by the end of the eighteenth century. What eventually changed the game was not the trick of finding longitude at sea (canny mariners had managed to work around that problem for centuries), but the nineteenth century practice of using these navigational techniques to prepare more accurate sea charts. Only once the charts had hazards correctly marked could knowing your longitude protect you from danger.

Of the two books, Finding Longitude deals with the Harrison episode in most detail, and has many beautiful illustrations. Howse provides more practical detail, but fewer and smaller illustrations, and less coverage of the Harrison/Maskelyne conflict.

And if you would like to understand why I have such an antipathy to Sobel’s Latitude, I can recommend Davida Charney’s critique in her article “Lone Geniuses In Popular Science” (210KB pdf).

* Actually officially named An Act for providing a Publick Reward for such Person or Persons as shall discover the Longitude at Sea. But life’s too short.