A series of four marine timekeepers which eventually solved the ‘problem’ of longitude – revolutionising sea travel by allowing mariners to accurately locate their position – are housed at the Royal Observatory in Greenwich.

The three clocks and one watch were designed by John Harrison, a working class joiner from Lincolnshire who made it his life’s mission to design a device which would keep accurate time at sea.

Calculating longitude – an east-west position on the earth – is relatively simple. Because time moves forward one hour for every 15 degrees one travels in an eastward direction (or back an hour for every 15 degrees one travels westward), it’s possible to calculate a position by simply knowing the local time at two different places on earth.

The problem was that the pendulum clocks of the 1600s were affected by changes in temperature and humidity  and couldn’t keep accurate time on board a ship meaning that while a mariner might know the local time, he could not get an accurate measurement of time elsewhere to compare it to.

While mariners could use the ‘lunar distance method’ to measure longitude – this involved measuring the motion of the moon relative to the stars – it relied on clear skies and was not very accurate. So in 1714 the British Government announced it would award a prize of £20,000 to anyone who could come up with a solution to longitude which was accurate to within half a degree (or two minutes).

Clockmaker John Harrison was among those who took up the challenge (his somewhat tragic story is told in great detail in Dava Sobel’s terrific 1995 book Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time). His first effort, known simply as H1, was constructed between 1730 to 1735 and, using a counterbalanced spring mechanism making it independent of gravity, was successfully tested on a voyage to Lisbon.

But Harrison wasn’t satisfied and began work on H2 in 1737 before, after realising its shortcomings three years later, starting work on another clock, H3, in 1740. He worked on this for 19 years but it failed to meet the accuracy requirements of the Board of Longitude which was charged with looking overseeing the awarding of the £20,000.

In 1753, Harrison asked a London watchmaker John Jefferys to create a watch to his designs, initially for his own personal use. But he soon discovered that with a few improvements, H4 – which looks like a large pocketwatch (see picture), could be the answer he was looking for.

In 1761 and 1764, Harrison’s son William took the watch on two voyages to the West Indies, yet, despite the fact that its accuracy was well within the requirements of the Board of Longitude, the board initially refused to pay up. After much wrangling Harrison was finally paid £10,000 but told that to obtain the other half of the money, he would have to create at least two more copies of H4.

Harrison went on to make one copy – H5 – while watchmaker Larcum Kendall, made another, K1, at the direction of the Longitude Board. When Harrison suggested that Kendall’s K1 could be considered the second of the two copies he was required to make, the board rejected the idea. Harrison then appealed directly to the king, George III, and finally to parliament before he was eventually awarded a further £8,750 in 1773.

Kendall’s watch, meanwhile, was taken by Captain James Cook on a three year voyage as far afield as Antarctica. It was a test which proved beyond all doubt the accuracy of the timekeepers. A year after Cook’s return in 1775, John Harrison died in his house in Red Lion Square on 24th March, 1776. It is not known whether he knew of the success of the timekeeper taken on Cook’s voyage.

Harrison’s four timekeepers are now housed at the Royal Observatory where the intriguing story of their creation is told.

WHERE: Royal Observatory, Blackheath Avenue, Greenwich (nearest DLR stations are that of Greenwich and Cutty Sark and it can also be reached by river – stop at Greenwich Pier); WHEN: 10am to 5pm (last admission 4.30pm) daily; COST: £7 adult/£5 concessions/free for children 15 and under (annual passes available – £10 an adult/£7.50 concessions) ; WEBSITE: www.nmm.ac.uk/places/royal-observatory/

PICTURE: Courtesy of the Royal Observatory, Greenwich