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The quest for longitude: the men who mapped the surface of the earth

For sailors and seamen in the 18th century, accurate navigation could mean the difference between life and death. In 1714, British authorities tried to tackle this problem by offering a vast reward for any man who could develop an accurate method of mapping longitude at sea. But, asks Rebekah Higgitt, was the right man given the credit for finding a solution?

An oil painting by John Cleveley The Elder depicting the Royal Naval Dockyard
Published: July 16, 2014 at 10:23 pm
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This article was first published in the July 2014 issue of BBC History Magazine


The longitude conundrum

Accurate navigation, on land or at sea, relies on knowing current position as well as destination. Although determining location relative to visible landmarks is fairly straightforward, defining a position out at sea is not.

As long ago as the third century BC, the Greek geographer Eratosthenes proposed a system using longitude and latitude – intersecting perpendicular lines overlaid on a map – as co-ordinates defining global position. Each is measured in degrees, equivalent to the angles on a circle representing the surface of the globe.

Lines of latitude measure positions north and south, and run parallel to the equator, while lines of longitude run pole to pole and define positions east and west.

The equator is a natural reference for latitude; by measuring the angle of the sun at its highest point from the horizon, you can calculate your distance from the equator. But establishing east–west position is trickier.

The Earth’s rotation means that local time changes in direct relation to degrees of longitude – hence, if you know the time at both your current position and at a known reference point, you can calculate your distance west or east of that point. Yet there’s no natural reference line for longitude. Historically, various locations have been used as a zero point or prime meridian; today, of course, we use Greenwich.

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Every degree of longitude (1/360th of the Earth’s circumference) is equivalent to four minutes’ difference in local time – a difference of an hour indicates 15 degrees east or west. So if it’s noon at Greenwich and 11am at your location, you are 15 degrees west of London – Iceland, for example.

The problem is how to determine the time at the reference location while at sea – and so in 1714 the British offered a reward of £20,000 to whoever solved that conundrum.

At the turn of the 18th century, Britain was a major player on the world’s seas. The expanding Royal Navy had been fighting French, Spanish and Dutch fleets, while global trade – particularly with India – was booming. Yet one major navigational problem still vexed mariners, extending voyage times and increasing risks: the inability to accurately determine longitude. The British government was persuaded to tackle the conundrum with the Longitude Act of 1714.

What was the Longitude Act?

The act saw the government offer a £20,000 reward for anyone making a significant contribution to finding a solution – an astonishingly large sum of money, echoing the huge rewards that had previously been offered by the Spanish and Dutch.

In the words of the act – one of the last parliamentary bills signed by Queen Anne – a method for precisely solving the longitude problem was desirable “for the safety and quickness of voyages, the preservation of ships and the lives of men”, and would “be of particular advantage to the trade of Great Britain”.

Yet it’s easy to overplay Britain’s anxiety to solve the longitude problem. Some historians have gone as far as to describe the 1714 bill as an act of desperation in response to an incident off the Isles of Scilly in 1707 in which four Royal Navy ships under the command of Sir Cloudesley Shovell were wrecked, with the loss of nearly 2,000 men. But this disaster was not about longitude – latitude error, fog and poor charts, instruments and observations all played a role – and the seven-year gap between the tragedy and the bill shows that the act was not a panicked political reaction.

Rather, the government was spurred into action by the lobbying of mathematician William Whiston, who had been expelled from Cambridge University, due to his unorthodox religious beliefs, and was looking for new opportunities. Regarding himself as a potential beneficiary of any longitude reward, he promised schemes that would “have sav’d all Sir Cloudesley Shovel’s Fleet”.

How close was a solution in 1714?

Maritime powers had been attempting to solve the longitude puzzle since at least the 16th century. Indeed, advised by Isaac Newton, then president of the Royal Society, the authors of the 1714 Longitude Act declared that: “Several methods have already been discovered, true in theory, though very difficult in practice.” In other words, the solution was tantalisingly close, but remained just out of reach.

The key lay in being able to establish the time in a known reference location in order to compare it with local time at the current position, calculated from observations of the sun and stars: time difference is equivalent to difference in longitude. One way would be to take a clock to sea – one that could keep the reference time reliably during a sea voyage. Another was to determine the reference time by observing heavenly bodies and checking their measurements against predictive astronomical tables.

By 1714, producing an accurate portable clock was not an outlandish idea, though many doubted it was possible. Meanwhile, improved tables and telescopes were allowing land surveyors to determine longitude by observing the movements of Jupiter’s moons. These heavenly bodies were, however, too small to be observed with existing instruments on an unsteady deck. (Galileo Galilei had earlier tried to address this problem with his design for a helmet-mounted telescope through which Jupiter’s satellites could be observed.)

So attention turned to the motions of Earth’s own moon: an easier observation to make at sea but, it turned out, a fiendishly difficult one to predict. Newton tried but failed, later recalling that “his head never ached but with his studies of the moon”.

So the key challenges for the two most promising methods – timekeepers and ‘lunar distances’ – were to improve tables of the moon’s motions and develop instruments not compromised by motion and changing conditions at sea.

How did the Longitude Act help to overcome these problems?

One of the remarkable aspects of the story is that three solutions identified as promising in 1714 were all ready for official sea trials at the same time. Tested by the Board of Longitude on a 1763–64 voyage to Barbados were the provincial carpenter John Harrison’s H4 watch, Christopher Irwin’s counterweighted ‘marine chair’ designed to steady the observer and aid measurement of Jupiter’s moons, and tables of our moon’s motion calculated by the astronomer Tobias Mayer.

The Longitude Act specified that the large reward would be payable only if longitude could be kept, or found, to within half a degree (about 30 miles) on a voyage to the West Indies. The board sent the astronomer Nevil Maskelyne to establish the longitude of Barbados, the figure against which each method would be tested, and to carry out observations for the two astronomical methods en route.

The trial resulted in the dismissal of Irwin’s chair as “a mere bauble” – despite his having earlier received £600 from the Board of Longitude – but success for the other two methods. Harrison’s watch performed brilliantly, keeping longitude on this six-week voyage to within 10 miles, while Mayer’s tables underpinned lunar-distance observations that could be accurate to half a degree. He had achieved what Newton could not.

As far as the Commissioners of Longitude were concerned, two promising and, most importantly, complementary methods required further development. Hence they invested heavily in both.
While Harrison, fairly, claimed that he had fulfilled the terms of the original Longitude Act and should immediately receive the top reward, so had the lunar-distance method. The commissioners, then, found themselves having to make decisions about how to spend money wisely. Harrison’s watch, even if it could be depended on to perform equally well on subsequent or longer voyages, was only one object. They needed many more watches, and for them to be produced by other makers, more quickly and more cheaply. For the lunar-distance method to be useful, they needed to support regular publication of pre-computed predictive tables to ease the heavy burden of calculation.
This was achieved from 1767, when Maskelyne, by then astronomer royal, organised the calculation and regular publication of the Nautical Almanac on behalf of the board. Together with better observing instruments, such as the octant developed by John Hadley, a fellow of the Royal Society, these tables supported the complementary use of both the lunar-distance and timekeeping methods.

John Harrison traditionally takes the plaudits for cracking the longitude conundrum. Is this fair?

John Harrison was, so the story goes, the ‘lone genius’ who defied the establishment with his prototype of the marine chronometer. Over three decades Harrison produced four innovative timekeepers, two of which – now known as H1 and H4 – were tested at sea and shown to track longitude with great accuracy.

Despite wrangling with the Board of Longitude over whether or not he had fulfilled the requirements of the Longitude Act, between 1737 and 1772 Harrison received rewards of over £20,000 for his sea watch, H4. With his portable timekeeper, he had solved the problem for the nation. This is, more or less, the account given by the science journalist Dava Sobel in her bestselling book Longitude (1995) – but it hides more than it reveals.
There is another version of the story – one that is richer and more revealing of the Georgian world. Though Harrison got the lion’s share of attention and money up to the 1760s, the archives of the Board of Longitude reveal not just one genius in communication with the state but a whole economy of scientific, maritime and artisanal workers.

So the result was a collaborative effort, not just individual brilliance?

John Harrison was, undoubtedly, an extraordinarily talented innovator – but he could not work alone. It is no coincidence that he moved to London in order to develop his marine timekeepers. Georgian London was a centre for this kind of activity, with a thriving instrument trade and places of scientific exchange as diverse as the Royal Society, workshops and coffee houses.

The capital gave Harrison access to skills and materials, pieceworkers and assistants that his Lincolnshire home could not offer. Neither his vastly complex and experimental H3 marine timekeeper, which never went to sea, nor his successful H4 could have been made without the move – or the ongoing communication with, and financial support of, the Commissioners of Longitude.

To understand how the story played out, we must bear in mind that the Longitude Act offered financial rewards, not simply a prize. True, a top reward of £20,000 was on offer for a single successful trial voyage, but other sums might be offered for partial success, promising ideas or expenses incurred. As the terms of the act were read with increasing flexibility, it became more a grant-giving mechanism than a competition.

Over the years, lots of people were in contact with the Commissioners of Longitude. Many received money in recognition of their efforts, or to encourage them to improve, test or share their work.

The story is not an insular one, but depends on London’s position within a global network. The desire of trading organisations such as the East India Company to turn profits from tricky maritime routes, and of the Royal Navy to protect and extend such interests, was crucial. So, too, was the availability of ships and people to test and develop new ideas.

So Harrison’s wasn’t the final solution?

Neither Harrison nor timekeepers solved the problem alone. Not only did Harrison, naturally, make use of the work of predecessors and contemporaries, but it took several others to develop a marine timekeeper that could be put into widespread use. Several clockmakers, including John Arnold and Thomas Earnshaw in London, and Pierre Le Roy and Ferdinand Berthoud in Paris, were rewarded for this work.

The Commissioners of Longitude were also right to see the timekeeping and lunar-distance methods as complementary. The latter could be rolled out more cheaply and quickly than the former – and it could be used intermittently to check the performance of a chronometer, preventing error accumulating over the course of a voyage. In the longer term, the answer was to have multiple chronometers, tested and corrected against the stars at land-based observatories. Even in today’s world of satellite navigation, a combination of tables and sextant is the ultimate fallback system.

How many proposals did the Board of Longitude reject?

Lots. The aforementioned Whiston and fellow mathematician Humphrey Ditton proposed a signal-based solution using the flash and bang of rockets, set off from known locations, as a positioning system. The idea worked in theory, but with one big snag: how could rocket platforms be moored in deep waters?

Other unsuccessful proposals were based on patterns of the Earth’s magnetic field. The commissioners quickly concluded that this approach was impracticable, because the patterns were neither well mapped nor static. Even into the 19th century, magnetic schemes and instruments were submitted for consideration – but were quickly dismissed.

What was the public view of the quest?

It certainly attracted plenty of cynicism. The apparent delay between the government’s announcement of the reward and the arrival of a “practicable and useful” solution – which, from the point of view of most mariners, was not until the early 19th century – made longitude a subject ripe for satire.

It was easy to accuse of hubris those natural philosophers who claimed that a solution was practically at hand, and the ‘projectors’ who put forward their schemes of being confidence tricksters. Jonathan Swift, who satirised experimental philosophers in Gulliver’s Travels, was not alone in linking the discovery of longitude with other ‘impossibilities’ such as perpetual motion and universal medicine.

Surely, it was thought, anyone talking about longitude was misguided, mad or bad, on an impossible quest that would land projector or backer in the madhouse or poorhouse. Thus William Hogarth’s depiction of Bedlam, in the final scene of A Rake’s Progress, has a “longitude lunatic” scribbling solutions on the wall, while Thomas Gainsborough’s brother, John, became known as Scheming Jack because of his longitude schemes. One visitor wrote: “I could scarcely detect whether his deranged imagination or his wonderful ingenuity was most to be admired.”

How crucial to Britain’s rise as a maritime power was this work?

The fact that British individuals and institutions were central to solving the longitude problem did not give the nation a significant advantage in the following century – tables, sextants and chronometers were readily traded overseas. Rather, the environment that had encouraged interest in the development of successful longitude methods put Britain in a better position to support and exploit them.

While the ability to find longitude undoubtedly helped make trade routes more predictable, and hence more profitable, it had limited impact on navigation in military contexts. More significantly, the new tables and instruments were put to use in survey and charting. Knowing a ship’s longitude was of little use without accurate information about the position of coastlines and rocks. Driven by the interests of trade and empire, in the 19th century the British Admiralty vastly expanded their charts’ global coverage.

These naval surveyors used Maskelyne’s Nautical Almanac, which was based on astronomical observations made at the Royal Observatory, Greenwich. It was this that ensured that the reference for longitude on their charts – the zero point, or prime meridian – was placed at Greenwich, and that chronometers were set to Greenwich Mean Time.

The widespread use of these charts led to the Greenwich meridian becoming the International Prime Meridian, from which the world measures time and longitude: the longest-lasting legacy of the extraordinary 18th-century quest for longitude.


Rebekah Higgitt is a historian of science based at the University of Kent.


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