Longitude clock comes alive

H4 is telling the time once more

By BBC News Online's Julianna Kettlewell

One of the most famous clocks ever built is running again this week - the first time it has done so in over a decade.

H4, the marine chronometer that the 18th Century engineer John Harrison constructed to solve the "longitude problem", has been wound as part of Science Week in the UK.

When Harrison said watches were the solution everyone thought he had finally gone mad

Jonathan Betts

The "pocket watch", which has a rapid little beat, ticking five times a second, has been set going alongside Harrison's other revolutionary timepieces - H1, H2 and H3 - in a special exhibition at the Royal Observatory and National Maritime Museum in Greenwich, London.

Jonathan Betts, curator of horology at the observatory, said the simultaneous running of all four chronometers would not be repeated in the foreseeable future.

"It is a huge privilege; it is one I won't take for granted," he told BBC News Online. "I think it is incredibly exciting. It is a true honour."

'Useless' technology

The longitude problem - how a ship could know how far west or east it had sailed from its home port - was perhaps the most vexing of the 18th Century.

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.

H4 being cleaned before being wound up

But 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.

"When Harrison said watches were the solution everyone thought he had finally gone mad, because everyone knew that watches were useless," said Jonathan Betts.

But not H4. The watch did phenomenally well in tests and the Board of Longitude (the first science funding body in the UK) set up by the King and Parliament to find a solution was forced to hand over a £20,000 prize.

"He had a huge battle on his hands convincing people that he had solved the problem," said Betts. "They thought H4 was a fluke, or even that John Harrison himself was a fluke."

Choppy seas

Longitude is the angular distance east or west from a standard meridian, such as Greenwich, to the meridian of any place. On global maps, longitude lines loop from the North Pole to the South in great circles that converge at the ends of the Earth.

When trying to navigate your way across an open ocean, the ability to calculate longitude is vital. An ignorance of longitude killed many early sailors, who could rely only on instinct to find their destinations.

John Harrison had to fight the establishment

Early sailors were well aware of the principle behind calculating longitude. They knew that for every 15 degrees travelled eastward, the local time moves forward one hour.

Likewise, they knew it moves back one hour for every 15 degrees travelled west. So, they understood that if they had the local time at two points on Earth, they could use the difference between them to calculate longitude.

The problem was this. Although they could measure the local time, wherever they were, by observing the sun, sailors also needed to know the time at a reference point such as Greenwich. And they only had pendulum clocks, which were rendered hopelessly inaccurate by the motion of the sea.

A lone genius

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 mechanisms, 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. It was a revelation. In six weeks, it was out by just five seconds; an accuracy three times better than that required to win the £20,000 prize.

But Harrison was an old man of 79 when, in 1772, he finally received the recognition, and prize money, he deserved, after King George III intervened on his behalf. In the meantime Captain Cook had embarked on his second voyage of discovery with a copy of H4, which he referred to as "our faithful guide through all the vicissitudes of climates".

Even today H4's legacy remains very much alive. "What H4 was doing is still current today because in GPS, for example, accurate time standards are required for navigation," said Jonathan Betts. "You'd be surprised how clocks rule our modern lives."

The Harrison clocks are running for the duration of National Science Week, 8-17 March, 2002.