Long journey in time for a local hero and a £2m clock that has changed the world

A tick-tock pendulum clock. Oak, with a few embellishments, its only oddity being a hand-written table of numbers encased behind glass at the front.

Those with a very expert eye would quickly get a fix on the importance of it, though, their pulses quickening as they draw near and recognise the words "James Harrison" inscribed across the face. They're looking at a timepiece that helped to change navigation, saved lives, added greatly to Britannia's rule of the waves and international reputation, and enabled many technological advances that came after it.

It belongs to Leeds Museums and Galleries and is thought to worth at least 2m, left to the city in a bequest by a local man called William Wyrill Sissons in 1973. The documents that went with the bequest have been lost, so it's not known if he or his family realised the importance of the clock, or how many times it changed hands in 250 years.

For a number of years, it was displayed at Abbey House Museum, in a mock-up period shop. With no clock specialist on the staff, it took a long time for anyone to realise its importance. It has taken even more years for something to be done about putting it on public display, although it can be seen by appointment.

This chronographic Titan is Precision Clock Number Two, made by John and James Harrison around 1727. James is thought to have made the case and perhaps the movement, but the brains of the operation and architect of the piece's true genius was older brother John.

They made only three clocks like this, and the most shocking thing about them was that not only were they crafted so precisely that they gave the correct time to within one second each month, but the movement was made almost entirely of wood – a special self-lubricating African wood called lignum vitae. This clock was the most accurate in the world when it was built, and is still in good working order.

The Harrison brothers were born at Foulby on the Nostell Priory Estate near Wakefield. A few years later the family moved to Barrow-on-Humber, and John and James became fine joiners like their father Henry. John, born in 1693, was no ordinary craftsman, though – he became a self-taught scientist, mathematician and engineer with a particular passion for making clocks.

At the turn of the 18th century, the beginning of the great seafaring age, stories of lives, cargos and ships lost were rife due to miscalculations of where a ship was in relation to land.

They could calculate latitude using the sextant and astrolabe to make astronomical measurements, and also used a compass and "dead reckoning" to determine speed and direction. Co-ordinating longitude and latitude was necessary to pinpoint exact position but once out of sight of land, it was impossible to measure longitude, a measurement taken on land as the angular distance east or west from a standard meridian such as Greenwich.

In 1707, after a disaster in which four cargo ships sailing from Gibraltar to England crashed into the Scilly Isles with the loss of 2,000 lives, the government set up the Longitude Board, and offered a prize of up to 20,000 (worth millions today) to anyone who could come up with a way of measuring a ship's position at sea. John Harrison became obsessed with the notion of cracking the problem by making a high-precision clock that would keep time despite changes in weather and the motion of the waves.

He formulated a calculation for longitude by using his highly accurate timepiece set to time at the home port and comparing it to local time. To measure longitude with this clock it had to be 50 times more accurate than any made previously.

Harrison set about systematically eliminating all the hindrances to creating such a timepiece, including the need for lubrication (oil became sticky in heat). The clock would need to be maintenance-free but be able to keep precise time over long and rough voyages.

The clock John Harrison came up with (the one owned by Leeds Museums and Galleries) kept time to within less than a minute a week. This would have been technologically advanced had he been a grand scientist in London, surrounded by the latest instruments and greatest brains. But Harrison was, essentially, a Yorkshire joiner who worked out problems to do with varying heat and humidity affecting timekeeping accuracy by keeping clocks in different rooms inside his cottage, then building up the fire in one while leaving the other cold.

The Harrison Precision Pendulum Clock No 2, the one with which he made many of his most important technological breakthroughs and forerunner to the marine chronometer that would win him the Longitude Prize, was sold (buyer and price unknown) to finance further versions of the clock. Harrison presented his findings to the Longitude Board, but (though impressed) they were not entirely satisfied. Harrison moved to London and spent the rest of his life trying to win the prize.

He worked and reworked the problems, taking each new clock back to basics and refining every element. Eventually, in 1767 with his H4 marine chronometer, Harrison presented the Board with the solution. But still they weren't completely sure. In 1772, Captain James Cook tested the clock on his second and third great voyages, reporting on his return that he had been won over by "our never-failing friend".

Any further quibbles were quashed by King George III, who was a great fan of technology and timepieces and had Harrison's clocks tested in his own observatory. At 80 years old, John Harrison was finally awarded the Longitude Prize and he died three years later.

In terms of technology we take for granted today, we owe not only

shipping safety but the precision of quartz watches, clocks clocks and SatNav (which uses readings from three incredibly precise clocks in space) to John Harrison.

Looking again, and with a more informed eye, at the lovely long-cased clock with mind-boggling secrets, it's difficult not to genuflect to it and to the vision of the genius who invented it.

"We want to create an important display around it for Leeds City

Museum, hopefully opening some time in 2011," says conservator Ian Fraser. Leeds Museums and Galleries are fundraising to mount this

special John Harrison exhibit.

"It is close to its original condition, virtually untouched apart from some maintenance. It's amazing that after 280 years there is little appreciable wear and tear. It may look ordinary, but we cannot overestimate its contribution to navigation."

No wonder this object has been listed by the British Museum as one of the treasures in A History of the World In 100 Objects. This clock changed time.

TV PROGRAMME CLOCKS ON

A BBC documentary will be aired next Monday night which tells the story of John Harrison and his quest to solve the problem of longitude. Adam Hart-Davis investigates and recreates some of Harrison's most pivotal discoveries and also travels to the Brocklesby Farm Estate in Lincolnshire, where a John Harrison clock commissioned for a stable block is still keeping perfect time. Hart-Davis sums up Harrison, with a note of understatement, as a man "with big ideas still making waves".

n The Clock That Changed the World can be seen on BBC1 in East Yorkshire and Lincolnshire on Monday, May 17 at 7.30pm. Viewers in other parts of the region can watch this programme by tuning into Freesat channel 967 or Sky channel 977.