When trying to navigate your way across an open ocean, it is essential to know your longitude to determine your position. Not knowing their position lead to the deaths of many early sailors, who could rely only on instinct to find their destinations. The longitude problem, of how a ship could know how far west or east it had sailed from its home port, was the most complex for centuries.

Galileo Galilei, Jean Dominique Cassini, Sir Isaac Newton, and Edmond Halley all tried to solve it, but none of them managed it. They all thought the answer lay in the "clockwork" of the heavens - in mapping the stars. Meanwhile, less significant minds devised schemes that depended on the yelps of wounded dogs, or the cannon blasts of signal ships strategically anchored somehow on the open ocean.

John Harrison believed there was a mechanical answer and after 40 years of work proved in 1764 that a clock could be used to locate a ship's position at sea with extraordinary accuracy. John Harrison was a working class joiner who developed an obsession for creating high-precision clocks. Between 1730 and 1759, he produced a series of timekeepers, H1, H2 and H3. These were all large clocks that had special balance mchanisms, which compensated for the motion of the sea. They were accurate, but not accurate enough.

Harrison radically re-thought his design and produced H4, a timekeeper that resembled a large pocket watch. In six weeks, it was out by just five seconds; accuracy three times better than that required to win the £20,000 prize.

A copy of H4 made by another watchmaker accompanied Captain James Cook on his second voyage to the Pacific, where it was hailed as having an amazing degree of accuracy.

A version of the H4 clock was still in use by navies and merchant shipping throughout the world up to the 1940s.