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Ship's chronometer from 'HMS Beagle' (made early nineteenth century), from England

In our History of the World told through things, we've now entered the long nineteenth century - the years between the French Revolution in 1789, and the beginning of the First World War in 1914 - years in which western Europe and America were transformed from agricultural societies into industrial powerhouses.

This week's programmes track some of the extraordinary changes that resulted. New technologies led for the first time to mass production of luxury goods, societies reorganised themselves politically at home, while overseas empires expanded to secure raw materials and new markets. This programme is about time, and our object is a clock - a marine chronometer made around 1800. But the point of this clock wasn't so much to tell you the time as to tell you where you were.

The pips that announce the passing of the hours on Radio 4 are the familiar sound of Greenwich Meantime, and the story of our chronometer is intimately connected to Greenwich. It explains why in one crucial sense the world divides here, and why every part of the world measures its time, and defines its position, in relation to this south-east London suburb.

"In a lot of accounts, what you find is people saying that actually the Industrial Revolution fuels exact time-keeping. But I'd argue, it's exact time-keeping that in part fuels the Industrial Revolution." (Nigel Thrift)

"So this object is really . . . considerably more important than the landing on the moon, let's say." (Steve Jones)

In 1831, this chronometer accompanied Darwin on his great voyage to South America and the Galapagos Islands, that ultimately led to his theories of evolution, and the great work on the origin of species. It typifies the advanced technology which in the nineteenth century led to a new ability to measure time, which fundamentally changed the way we live and the way we think. It's hardly an exaggeration to say that the very idea of time changed in the nineteenth century, and in consequence so did our idea of ourselves and our understanding of humanity's proper place in history.

The ticking and whirring noises just now round about me tell me that I'm in the Clocks and Watches Gallery of the British Museum. In the seventeenth and eighteenth centuries clock-making was a key European technology, and London was its cutting edge. As a maritime nation, the British were concerned with one time-keeping problem in particular - they could make clocks that kept very good time as long as they stayed perfectly still, but not when they were shaken about, and particularly not on board a rolling ship. If you wanted to sail, it was impossible to keep a precise record of passing time, and at sea, if you can't tell the time, you don't know where you are. The answer to this problem is the object I'm standing in front of now - a marine chronometer.

It's based on one invented in the mid-eighteenth century by John Harrison, the man who finally cracked the problem of accurate time-keeping at sea and, for the first time, made it possible for ships anywhere to establish their longitude.

It's relatively easy to calculate latitude - your distance north or south of the Equator - by measuring the position of the heavenly bodies. But those alone won't let you calculate longitude - your position east or west. The answer to that problem turned out to be an accurate clock - a chronometer. Before a ship set sail, its chronometer would be set to the local time in harbour - for the British this was usually Greenwich. Once at sea, you could then compare the time at Greenwich with the time on board ship, wherever you were, which you could tell by the sun. And the difference between the two times gave you your longitude.

There are 24 hours in the day so, as the earth rotates, every hour that the sun seems to move across the sky is one twenty-fourth of a complete circle of the globe - 15 degrees. So if know that you're three hours behind the time in Greenwich, you're 45 degrees west - in the middle of the Atlantic. If you're three hours ahead, then you're 45 degrees east - and so, on Greenwich latitude, you're somewhere south-west of Moscow.

What John Harrison did was to invent a clock, a chronometer, that would go on accurately telling the time set in Greenwich, despite the constant movement of the ship and, just as important, despite any fluctuations in temperature and humidity. It was a great feat of precision engineering, but Harrison's chronometers were pioneering, high-quality instruments, made in tiny numbers and affordable only by the Admiralty. Then, around 1800, two London clock-makers managed to simplify the mechanisms of his chronometer, so that virtually any ship - and certainly the whole of the Royal Navy - could carry them as routine equipment.

I'm with one of those lower-cost chronometers now. It was made in 1800 by one of the two Londoners, Thomas Earnshaw. It's brass and it's around the size of a large pocket watch, with a normal clock dial showing Roman numerals, and at the bottom where the six would be, a smaller dial for the second hand. The dial lies flat, horizontal in its wooden box. Of course I can't see its internal workings, which actually delivered the high accuracy, lower-cost time-keeping. What I can see is that the clock is suspended inside a swivelling brass ring fitted to the inside of the box, so that however much the box moves, the clock itself will remain horizontal. Here's geographer Nigel Thrift:

"The chronometer, in a sense, is the pinnacle of a long history of clock- making. Clocks have been around since 1283 in England. Everyone talks about Harrison and the fact that he was a genius. He was, but you have to understand the extraordinary innovative efforts made, literally, by hundreds and thousands of unknown clock-makers and general mechanics that actually, in the end, produced that object. These kinds of chronometers were phenomenally accurate. For example, one of the first ones was used by Captain Cook, and when Cook made final landfall in Plymouth in 1775, after circumnavigating the globe, it gave an error of less than eight miles in calculated longitude."

This particular chronometer sailed on many ships - always issued and set, as others were, at Greenwich. But this one is famous, because in 1831 it was issued to 'HMS Beagle', the ship which carried Charles Darwin round the world.

The 'Beagle' was on a mission to map the coastline of South America, and this work relied on the most accurate possible measurements of longitude and latitude. The chronometer allowed for the first time absolutely precise charting of the oceans, with all that that implied for establishing safe and rapid shipping routes. It was another great step in the Enlightenment project of mapping, and therefore controlling, the world. To allow for any discrepancies or failures, the 'Beagle' carried no less than 22 chronometers - ours was one of them.

By the middle of the nineteenth century, it was established that all British shipping would take Greenwich as its point of reference for time, and therefore for longitude. By then, virtually all the oceans of the world had been mapped by British ships on the basis of the Greenwich Meridian - and these maritime charts were widely used by the international community. As a result, both Greenwich Meantime and the Greenwich Meridian were gradually adopted by more and more countries until, in 1884, the Washington Convention formally ratified the practice as effectively universal. There was only one notable exception, the French, who defiantly stuck to their Paris Meridian for some decades more, until eventually they too fell into line. By the early twentieth century, every country fixed its time zone by reference to Greenwich Meantime. For the first moment in history, the world was working to one timetable. Global time, a concept unimaginable a hundred years before, had arrived.

But our chronometer was witness to another quite separate shift in the nineteenth century's understanding of time. Darwin's voyage on the 'Beagle', and his subsequent work on evolution, pushed human origins - and indeed the origins of life itself - into an unthinkably distant past, and geologists had already demonstrated that the earth was far older than previously believed. This new concept of deep time - going back tens of millions of years - shattered the established historical and biblical frameworks of thought. These new and constantly expanding parameters of time forced the nineteenth century to re-think from scratch the very nature and meaning of human existence. Here's Steve Jones, expert on Darwin and evolution:

"I think what deep time did was to make people realise that the earth was not unchanging. I think the biggest change really, since the Enlightenment, has been a shift in our attitude to time - the feeling that time is effectively infinite, the time that's gone, and the time that's to come.

"It's worth remembering, of course, that the summit of Everest, not long ago in the context of deep time, was at the bottom of the ocean, and some of the best fossils of whales are actually found high in the Himalayas. When you begin to think in those terms, it makes you . . . I suppose the word has to be humble, in the face of the extraordinary period we have to play with, and of the extraordinary events which that allows history - geological and biological history - to accomplish."

These were enormous and belief-shattering ideas for many people in the nineteenth century, but time was also changing in a much more day-to-day, or rather hour-to-hour, way. Thanks to clock-makers like Earnshaw, precise and reliable clocks and watches became ever more affordable. Before long, the whole of Britain, the whole of the industrialised world, was running by the clock, and the measurement of time had been severed from the natural cycle of sunrise and sunset. It was the clock that ruled every aspect of life - shops and schools, pleasure and work. As Charles Dickens wrote: "There was even railway time, observed in clocks, as if the sun itself had given in." Here's Nigel Thrift again:

"Standard time, based on the Meridian, was first applied by the First Great Western Railway in 1840, and gradually that standard time became general. By 1855, 95 per cent of towns had switched to GMT, but it's worth remembering that, until that point, certainly until the beginning of railway time, places had basically all run to local time. For example, Leeds was six minutes behind London, Bristol was ten minutes - it didn't matter. But it mattered once you start getting very fast travel, and as soon as that happens, everyone went on to one time, gradually but very certainly."

So the nineteenth century gave us global time, deep time, and a new sort of everyday uniform time, and there's one time that the whole of Britain has always been glad to move on to - tea time. Our next object is a genteel, nineteenth-century tea set . . . and we'll look at how the taste for a nice cup of tea changed the agriculture of continents, and the economy of empires.