by tirial

In the eighteenth century, Longitude positions could not be measured. Ships were being wrecked so the British Parliament passed an Act setting a prize for a solution.

Longitude is a measure of position vital for navigation. In the eighteenth century there was no way to measure it and ships were being lost at sea. Parliament passed an Act setting a prize for anyone who could solve the problem.

The problem was solved by John Harrison, but the Board continually refused to award the prize until George III of England eventually stepped in. "Longitude" is the book and miniseries that covers his struggle, first with the scientific problem and then with an establishment that did not want to believe he had solved it.

The inability to find longitude had been a hazard to sea travellers for many years. Then,k in the reign of Queen Anne, there was a disaster: over 2,000 crew were drowned when warships lost in fog were wrecked on the coast. Parliament hastily convened and passed the Longitude Act to try to solve the problem.


The Longitude Act set three prize limits, the top level being £20,000. In 1714 this was more than it sounds as the equivalent modern earnings would be

£30 Million

How to calculate longitude from time

The Harrison equations

The calculations to work out longitude from time are very, very, basic. You just need your local time (LMT), and Greenwich Mean Time (GMT). Then you do these three steps:

  • Local time - GMT = Time difference
  • Time difference divided by 24 hours = Time factor
  • Time factor multiplied by 360 degrees = Longitude

These steps can be reduced to a single equation:

(LMT - GMT) * 15 = Longitude

It seems almost laughably simple, but the problem Harrison had to solve was that there simply weren't any clocks accurate enough to keep GMT on a sea voyage.

As a result, most of the establishment believed they would find a solution through astronomical observation, rather than a time, which no device available could measure.


Harrison vs. Maskyline

A detail of the feud and the creation of H3

Harrison's main rival for the prize was the astronomer Reverend Nevil Maskalyne. Maskalyne believed that observations of the stars were the best way to determine longitude. An establishment figure to the core Maskalyne was aided by the fact that the Board was comprised of astronomers, not engineers, who naturally supported one of their own.

A short documentary uploaded to youtube details the rivalry between the self educated Yorkshire clockmaker Harrison, and the university educated London astronomer Maskalyne.

Wooden Clocks

John Harrison was already an experience clock maker, who had made several wooden clocks known for accuracy and precision.

The Harrison Clocks

Harrison's solution to the Longitude problem was the engineer's answer: to build a more accurate clock. By knowing the time where you were and the time at a known location (e.g. Greenwich) a navigator could calculate their exact location. The problem was the inaccuracy of clocks in the period, meaning that no existing clock was accurate enough.

In total John Harrison produced four versions of his Marine Chronometer, each building on the one before.

H1 - The pendulum clock

The first of Harrison's Chronometers

Driven by pendulums, H1 was very much a traditional clock.

When installed in a ship it required a huge protective case and a gimbol (or universal joint) to help ensure it stayed steady.

Given a sea trial in 1736, it managed to predict the ship's true landfall correctly. Although praised, it did not fulfill all the requirements, but the Board granted Harrison £500 to continue experiments.

H2 - the marine chronometer

The second of Harrison's marine chronometers

Initially intended to be more compact than H1, to take into account the lack of space on ships, H2 was also designed to be more rugged. Unfortunately when finally built it was slightly larger and heavier than H1, due to the corrections Harrison had made to the design. Its trials were delayed due to the outbreak of war in 1741 and Harrison, duely granted another £500, went to work on H3.

It was while working on H3, the most complex of his clocks, that Harrison had a revelation about the problems of pendulums on ships. He abandoned the ever-more complex adjustments he was making to H3 and changed direction completely. His new solution would be H4.

H4 - The final watch

The solution to the problem

H1, H2 and H3 were pendulum powered devices in huge cases. After seeing how little room there was on onboard ship, and discovering the problems with a pendulum which moved if the ship tossed in a storm or could be thrown off by the simple yaw (side to side) motion of the ship, Harrison changed direction.

H4, shown to the left, was an elegant pocket watch produced by a watchmaker called Jeffries. It used different types of metal to remove the issue of heat expansion, and was designed to be as accurate as possible. It was used successfully in sea trials.

However despite the efficiency of Harrison's method being proved, the Board refused to award the prize. They were convinced the solution lay in the stars, and preferably in the solution developed by Maskalyne who was part of the establishment. By a quirk of circumstance, Maskalyne had been appointed to the Longitude board with responsibility for judging the award.

"These two people have been cruelly wronged...
...By God sir, I will see you righted!"
George III.

The conclusion of the Longitude tale

Royal Intervention

Reverend Maskalyne had been appointed Astronomer Royal, and as such was now a member of the Longitude Board, despite the conflict of interest this caused. His final interference in the Longitude Prize came when he changed the reporting criteria for the test of H4, ensuring that although it had in fact passed the test on the voyage, the report to the board stated that it had failed.

Furious, Harrison's son wrote to King George III who granted them an audience. It is recorded that he was less than pleased with the way Parliament and the Board of Longitude had handled the entire affair.

At the King's insistence, John Harrison got his prize. He was eighty one.

Harrison died three years later, but his Chronometers were used by historic figures such as James Cook, and rapidly replaced astral navigation as the method of determining location at sea. The original Chronometers were found in the 1920's still in the museum, in delapidated condition, by a watchmaker called Rupert Gould who restored them to running condition.

All Harrison's devices can be seen at the Royal Observatory, now part of the National Maritime Museum at Greenwich.

1767 John Harrison & Marine Timekeeper H4
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Longitude by Dava Sobel

Dava Sobel's book

Dava Sobel's book propelled John Harrison's work back into the public eye. Winner of the Whitbread prize, and an excellent read, it was converted into the miniseries starring Jeremy Irons and Micheal Gambon.

Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

Anyone alive in the eighteenth century would have known that "the longitude problem" was the thorniest scientific dilemma of the day--and had been for centuries. Lacking the abi...

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Longitude - the series

Jeremy Irons and Micheal Gambon

The series juxtaposes the story of John Harrison (Micheal Gambon) with the story of Rupert Gould (Jeremy Irons) the man who rediscovered his watches, neglected and damaged, in the basement of the observatory and restored them. Somewhat dramatised for television, not all the incidents shown actually occurred, and real-life incidents were conflated, but it is unusually accurate for a period drama.

Winner of three Bafta's, and a record breaking eight nominations, this is a gripping piece of drama.


Academy Award winner Jeremy Irons stars in this sweeping adaption of Dava Sobel's best-selling book of high seas adventure and political intrigue. Determined to stop shipping lo...

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H5, H6 and K1

Harrison's remaining watches

Chronometers H1 to H4 were created directly for the Longitude Challenge. However, there were three further watches linked to Harrison's work on the challenge and his attempts to claim the prize.

H5 was built and given to George III as proof that Harrison had solved the longitude problem. Tests on this watch were what convinced the King that Harrison was right.

H6 is a rumour. Harrison's notes indicate it was built, but if so its fate is a mystery. Many watchmakers would dearly love to find out the truth about this mysterious timepiece, which is described as a large pocket watch. H6 was also the subject of the final episode of "Only Fool's and Horses" when it turned up in Del Boy's garage and the Trotter brothers finally struck it rich. If only they hadn't invested it all in Far East Futures...

K1 was a copy of H4 made by a watchmaker named Kendall to prove to the Longitude board that others could replicate the Harrison watches and that they were not flukes. K1 was also the watch used by Captain Cook on his later voyages. Comparing the picture of K1 on the left, to the H4 image above, shows you how close a copy it was. (Sadly this image is now only available on the wayback machine - here)

More detail on the history of the Harrison Watches can be found in this free e-book available from the National Maritime Museum (of which the Royal Observatory is part).


John Harrison (1693-1776) and Lt. Cdr Rupert T. Gould R.N. (1890-1948) by J.Betts

The final sabotage

Maskalyne's last blow against the clocks

As part of Maskalyne's attempt to prevent Harrision claiming the prize money he turned up at Harrison's door in 1766. He took the three larger timepieces (H1, H2 and H3) and moved them to the observatory in an unsprung cart. As you can imagine, such handling was not good for the delicate clocks' mechanisms, and they arrived in an unfit state to run. Soon after 1770, still unworkable, they were placed in storage. Fortunately H4 and K1 did not suffer this treatment. Carefully preserved, they never became as damaged as the older machines.

Despite brief interest in restoring them in 1891, the Harrison watches (including H4, by this time unworkable) languished in obscurity until 1920.

Time Restored: The Harrison timekeepers and R.T. Gould, the man who knew (almost) everything

This is the story of Rupert T. Gould (1890-1948), the polymath and horologist. A remarkable man, Lt Cmdr Gould made important contributions in an extraordinary range of subject ...

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Rupert Gould, an amateur horologist and clockmaker, arrived at the museum and volunteered to repair them in his spare time. Already known as a polymath, the museum reluctantly let him try.  This was no small task given both the degree of damage and the complexity of the devices themselves. Working evenings and weekends, between 1920 and 1933, he managed to restore the watches to running condition.

Rupert Gould's Notes

John Harrison and His Timekeepers

Book by Gould, Rupert T.

View on Amazon

Since his initial restoration the Harrison watches have been preserved carefully to this day. Two can be seen running at the Royal Observatory, although H4 is kept stopped as it would slowly degrade if run.

The Harrison Chronometers Site

The official site for the clocks can be found at:

Author's Notes

This lens was originally on Squidoo, as a purple star and multi-award winner, but it appears to have never made the migration to Hubpages. I have recovered and rewritten what I can, but unfortunately this has caused the loss of many years of comments and discussion.

Updated: 01/22/2015, tirial
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