John Harrison and the Problem of Longitude

Portrait of John Harrison by Thomas King, 1767

1693-1776

The beginning of the 18^th century was a time of great economic expansion as sailing ships crossed the seas to trade with far-off ports and colonies.  But sailing was a dangerous business because there was no way to accurately determine location.  A ship’s captain needed to know the ship’s latitude, its north-south position, and longitude, its east-west position, to safety navigate the oceans of the world.  Determining latitude was not difficult; the ship’s navigator simply measured the angle of the sun or the star Polaris.  Measuring longitude accurately, however, was virtually impossible.  As a result, early 18^th century captains charted their position using their speed and bearing from a previous ‘fix’.  This ‘dead reckoning’ approach meant that the longer a ship went without sighting a known landmark, the more uncertain it was of its position.  Crossing large bodies of water or sailing in inclement weather were extremely dangerous.  The toll in shipwrecks, lost cargo, and lost lives was enormous.

Lost on the Rocks of Scilly, October 22, 1707
National Maritime Museum, UK

On October 22, 1707, four ships of the British fleet ran aground and sank off the Isles of Scilly.  More than 1,400 sailors drowned.  Investigation into the disaster determined that the fleet did not accurately calculate its position.  The public outcry was immediate.  Surely there had to be some way to navigate other than by dead reckoning.  The British Parliament passed the Longitude Act of 1714 which empowered a Board of Longitude to offer a £20,000 reward to anyone who could accurately measure longitude at sea to within 30 nautical miles.  Most scientists of the day, including Sir Isaac Newton, believed that the solution lay in observation of the moon and stars. Therefore, the Board of Longitude was made up of astronomers.

However, there was another solution to the longitude problem.  If you knew the time at your position (which you can get from the sun) and you knew the time in a reference location, such as London, then you could calculate your longitude using the difference.   For example, if it’s noon at your location and six o’clock PM in London (Longitude 0), you are one quarter around the earth west of London at Longitude 90.  Unfortunately, 1714 clocks were not accurate enough to keep the reference time, particularly on a pitching and rolling ship.  In addition, a marine clock would need to stand up to the wet and salty conditions on board a ship. Such a clock, it was felt, simply couldn’t be built.

H1, the First Sea Clock
Image: National Maritime Museum, UK

John Harrison was an innovative English clockmaker who thought he could build a marine clock accurate enough to solve the longitude problem (and win the prize!)  Earlier in his career, he had invented the grid-iron pendulum, which kept accurate time as the temperature changed, and the grasshopper escapement, which regulated his clocks without a pendulum.  In 1730, Harrison approached Edmond Halley, the Astronomer Royal, with plans for a marine clock.  Halley referred him to George Graham, a renowned clockmaker, who was so impressed that he footed the bill for its development.  Five years later, the clock later named ‘H1’ was ready for testing.  On a voyage from England to Lisbon, Portugal and back, Harrison and H1 successfully tracked the ship’s position and predicted landfall far better than the ship’s navigator.  The Board of Longitude did not award Harrison the prize because the voyage was not transatlantic, as the prize demanded.  But the Board granted Harrison £500 for development of a second marine clock, H2.  Another five years passed while Harrison made this smaller, more rugged timepiece.  Unfortunately, H2 was never sea-tested because England went to war with Spain.  He moved on to H3, spending the next 17 years on its development.  Then the oddest thing happened.  

Harrison's Masterpiece, H4
Image: National Maritime Museum, UK

In London, Harrison discovered pocket watches that kept time almost as well has his complex, bulky sea clocks.  He abandoned H3 and the track he had taken for almost 30 year to begin work on H4, essentially an over-sized pocket watch.  It took 6 years to build and incorporated all the technical innovations Harrison had made over his lifetime.  H4 is his masterpiece.  In 1761, H4 went on a transatlantic voyage under the care of Harrison’s son.  When it arrived in Jamaica, it had lost only 5 seconds, the equivalent of one nautical mile.  Regardless of this spectacular success, the Board of Longitude refused to award Harrison the £20,000 prize, stating that the presumed accuracy was just dumb luck.  The Board’s real issue was that a second method for determining longitude, the Lunar Distances Method, was gaining credence and the Board members felt compelled to save the award for their fellow astronomers rather than give it to some clockmaker.  A second voyage confirmed the accuracy of H4 and again the Board refused to award the prize.  The irate Harrison took the matter to King George III who tested H4 himself, found it to be highly accurate, and told Parliament to settle the matter once and for all.  In 1773, when Harrison was 80 years old, Parliament awarded him £8,750 for his achievements, but he never received the official prize.  He lived for only three more years.

In the decades that followed, the price of the clocks dropped to the point that the marine chronometer would become the navigational method of choice for most seamen.  Ships would not leave harbor without two on board, set to Greenwich Mean Time. 

Harrison’s marine clocks, H1, H2, H3 and H4, are on display at the National Maritime Museum in Greenwich, England.   H3 was recently dismantled for cleaning and research.


Conserving the H3 Timekeeper part 1 from Royal Observatory Greenwich on Vimeo.

There is a terrific TV mini-series about Harrison and his pursuit of the longitude solution called Longitude (2000) starring Michael Gambon and Jeremy Irons.  The trailer and the entire show can be found on Youtube.

Question of the Day: There is an odd-looking device on top of the Royal Observatory, Greenwich.  What was it used for?

Royal Observatory, Greenwich