Original subtitles

In 1852, clockmaker Edward Dent set out

to construct the largest and most accurate public clock in the world.

It took seven years to build.

A testament to a very human need.

Our modern day lives are completely driven by precise measurement.

Take Big Ben. For over 150 years it's been ringing out

the correct time to the people of London.

When built, it was an engineering marvel

accurate to an incredible one second an hour.

But times have changed.

Today we can build clocks which lose one second in 138 million years.

And now there are plans for a clock accurate to within one second

over the lifetime of the universe.

What is it that drives us to such extremes of ever greater precision?

Why do we feel the need to quantify and measure,

to impose order on the world around us?

Since our ancestors first began to count the passing of the seasons,

successive civilisations have used measurement

to help master the world around them.

It's taken us to the moon and split the atom.

And it fascinates me.

Ever since I was young, I've been obsessed with measuring things,

trying to make sense of the world around me.

But where do those measurements come from?

I mean, who decided a kilo was a kilo, and a second a second?

What we measure, how we measure it,

and how accurately we can measure it are surprisingly complex questions.

Questions which have obsessed generations of great minds,

and created a system that describes everything in our world with

just seven fundamental units of measurement.

And the quest to define those seven units with ever greater precision

has changed our world.

In this series, I want to explore why we measure.

What drives us to try and reduce the chaos

and complexity of the world to just a handful of elementary units.

In this first programme, I'm going to be looking at two of the most

fundamental measurements, namely the metre and the second.

It's likely that time and distance

were the first things people ever tried to measure.

They seem closely linked in our minds.

We even talk about length of time.

And as we'll see, time and distance

are inextricably connected by modern science.

Being able to measure time actually means spotting patterns and

that's actually a very mathematical way of looking at the world.

In fact, measuring time is an incredibly sophisticated act.

So where did it all begin?

Our ancestors would have first picked up

on the patterns of the seasons.

Marking time as the leaves turned brown, or the days got shorter,

when rivers flooded, or berries ripened.

These very practical observations would have helped them

in the daily struggle to survive.

One of the first examples of humans' attempts to measure

was discovered here in Southern France by four teenagers

and their dog called Robot.

It was 1940 and the 18-year-old Marcel Ravidat

was exploring these woods

when he came across a hole where a tree had been uprooted by a storm.

He needed some tools to make the hole bigger

so he came back four days later with his three friends,

and they uncovered the entrance to a huge system of unexplored caves.

But what they discovered inside was even more exciting.

Wow!

The boys must have been absolutely staggered to come in here

and see these images painted on the wall.

I mean, these are some of the oldest cave paintings.

Oh, look at this!

All over the wall!

Marcel and his friends had discovered

some of the earliest cave paintings ever found.

These date back 17,000 years and were painted by Cro-Magnon man.

It's beautiful!

You can really feel the energy of these animals

rushing across the walls.

This cave is a replica of the original

which is a few hundred metres from here and is now carefully preserved.

Dr Michael Rappenglueck believes that these paintings

are evidence of man's first attempt to measure time.

This one is very, very beautiful.

To him, this is a giant calendar.

The clues lie in these strange patterns of dots.

Each dot represents a week.

13 dots represent one quarter of the year.

His theory is that each seven-day phase of the moon,

what today we'd call a week,

is marked with a dot on the wall to chart the passing of time.

It was a distinctively-shaped cluster of dots

that eventually allowed him to unlock

the full meaning of the paintings.

Look up to the ceiling. You see six dots.

It reminds a little dipper,

and I think this is the star pattern of the Pleiades.

Oh, so these dots are not representing weeks any more,

these are stars up there?

Yes. These are stars, and they serve to start the year.

When our ancestors saw the stars form this same alignment in the sky,

it would mark the start of their year.

Dr Rappenglueck believes the animals have meaning too.

The stag represents autumn equinox

and it's starting a time cycle to the horse.

The horse represents spring time

and you see the horse is pregnant, highly pregnant,

so three-quarters of the year are represented on the wall.

So, it's the star calendar

followed by the calendar marking the weeks that allows them to know

when the stags are rutting, or pregnant animals...

Yes, they synchronised biological rhythms of animals

with astronomical rhythms.

It's an extraordinarily sophisticated system... Yes, it is.

..for 17,000 years ago. It is.

It's amazing!

With the aid of this basic calendar, for the first time,

our ancestors could start to predict what would happen, and when.

They could prepare to hunt when animals migrated close by

or, as agriculture developed, determine the best time to plant crops.

Measurement was making life easier.

But as communities grew, so did the need for more precise timekeeping

beyond the cycles of the moon, the stars and the seasons.

13,000 years after our ancestors painted the caves in Lascaux,

first the Mesopotamians and then the Egyptians

started to tackle the problem of dividing up the day.

And they took their inspiration from the sun.

By observing how the length of a shadow changed through the day,

they found an easy way to measure time.

And they used a device just like this.

This is a replica of an Ancient Egyptian sundial.

It's one of the first instruments ever created to measure time.

Now at midday, this stone here would have cast no shadow.

But, as the day went on, the shadow would get longer and longer,

so the Ancient Egyptians decided to divide the day up into 12 units.

You can see the lines here - we've got one, two, three...

We've got six lines for the afternoon, and six for the morning.

It's just coming up to three o'clock.

By linking time and distance, they could reliably measure

shorter periods of time.

Telling the time, by measuring the length of a shadow.

Although the sundial was a brilliant invention,

it was fundamentally flawed.

It didn't work at night.

Like the cavemen of Lascaux, who used stars to mark the seasons,

the Egyptians went one step further.

They used them to divide up the hours of darkness.

But on a cloudy night, just as on a cloudy day,

they still had no way of telling the time,

and this is where they made a conceptual leap.

This is a water clock.

It's a very simple idea.

Basically, what they did was to take a bucket

and make a hole in the bottom.

Then as night fell, they would fill the bucket with water.

Now, as the water drips out,

they can use lines marked on the side of the bucket

to tell how much time has passed through the night.

They could measure 12 hours

independently of the sun or the stars.

But why count 12 hours at all?

The answer lies in how business was done thousands of years ago.

Throughout the Middle East,

the number 12 and the number 60 were important in commerce.

They're numbers that were familiar to traders in markets just like this.

And the reason they use them is all to do with arithmetic.

As a mathematician, I love the answer

because it's about the mathematical properties of these two numbers.

They're highly divisible.

Take the number 60.

I can divide 60 beans into six groups of ten beans,

five groups of 12 beans...

..four groups of 15 beans...

..three groups of 20 beans.

Five, there.

Two groups of 30 beans...

..or one group of 60 beans.

But take 100 beans, how can I divide that?

I can divide it into two groups of 50

but divide by three and I've got to start cutting a bean!

Because the numbers 12 and 60 were so familiar to the Egyptians,

it was perhaps no great conceptual leap

for them to come up with a 12-hour night and day.

So the idea stuck.

It wasn't just the measurement of time that the Egyptians needed to tackle.

They also needed to find better ways to measure distance.

Every year the Nile would flood,

bringing great fertility to the land.

But with each flood,

the borders of the farmers' land would be washed away.

So when the waters receded, an accurate way of measuring

field size and re-establishing boundaries was critical.

They needed a reliable and uniform measure of length.

And their solution was this.

It's a cubit rod and it's the Egyptian equivalent of a ruler.

Its length was the distance of the pharaoh's cubit,

which was the length from his elbow to the tip of his middle finger.

So actually, my cubit is slightly shorter than the pharaoh's.

But this led to the Egyptians creating some of the most remarkable buildings the world has ever seen.

This is the great pyramid of Cheops,

built over 4,500 years ago for the fourth-dynasty pharaoh, Khufu.

It is said 20,000 men took 20 years to build it,

using over two million limestone blocks,

all meticulously aligned and measured with the cubit rod.

This is a miraculous building.

The length of the side is 440 cubits exactly.

Exactly? Exactly.

And the height is 280 cubits exactly.

Also it is very square.

It has perfection in every part of it.

Absolutely, and with so many people working on it,

spread over, I guess, a large area and a large amount of time,

I mean, actually having a standard unit of measurement

must have been absolutely essential.

Exactly. They had a rope which is 100 times this

that has knots in it every one cubit or every ten cubits,

which is called khet.

OK. We want to measure 440 so we need to take the corner stone

as our starting point, so if you start measuring. Yes.

The original cornerstones are no longer visible

but the foundations are still here for all to see.

I think I chose the easy job.

430.

Wow!

440 cubits, pretty much on the knot! Exactly!

What's so remarkable about the Egyptian system is that they

were one of the first to standardise length measurement.

It's said that every full moon,

the surveyors across the land would gather

and compare their wooden cubit rod against the royal master cubit.

Made of granite, this was held by the royal surveyor.

Failure to maintain an accurate cubit was punishable by death.

It was a very simple and efficient way

to standardise length measurement across the land.

And it enabled the Egyptians to measure things with phenomenal accuracy.

Mastering and standardising time and length measurement

was really key to the success of the ancient Egyptian empire.

The power of measurement is that it created order out of chaos

and allowed civilisation to flourish.

The standardisation of measurement which began here in Egypt

several millennia ago is now central to all our lives.

Nearly every country in the world has a national measurement body

whose master lengths and weights are calibrated

by one international body,

a little bit like the modern day pharaohs,

trying to bring standardisation of measurement across the globe.

But despite the obvious logic of having one international system,

it hasn't been completely embraced.

Take me, for example.

I'm going to the airport in this cab which measures its speed

in kilometres per hour and miles per hour.

When I'm up in the air, they'll be measuring their altitude in feet.

My clothes are measured in inches and my shoes are measured in...

..well, frankly I've never quite understood

what the unit of measurement for shoe size is!

Shoe sizes aside,

standardisation of measurement underpins all modern science.

Though the route to standardisation has not been an easy one.

Throughout history,

rulers had a nasty habit of ripping up measurement systems

and demanding that they be replaced by lengths based on their own body parts.

In 12th-century England

the yard was defined as the length from the tip of the King's nose

to the top of his outstretched thumb.

But as each new reign came in, so things changed.

Henry VII, he defined a yard as the length of his arm.

Elizabeth I, not to be outdone by her male predecessors,

added a few more inches.

And so the chaos continued.

Lack of standardisation was a problem on the Continent, too.

If you thought the British had it bad,

then spare a thought for the French.

On the eve of the French Revolution,

the Ancien Regime had over 250,000 different weights and measures,

including several thousand for length.

By the end of the 18th century,

people realised that something needed to be done.

Trade was impossible and open to fraud, navigation was treacherous

and building plans made by an architect in one city

couldn't be reproduced in the other

because they didn't have the same measurements.

The mess was finally sorted out by the French Academy of Sciences.

It was the last few days of the French monarchy,

and buoyed by the revolutionary spirit of the time,

a sense of egalite and rationalism,

France's best scientists decided to form

a ground-breaking and revolutionary plan of their own.

No longer would measurement be based on the human body,

or the vanity of kings and queens.

They decided that it should be based

on something permanent and unchanging. They chose the Earth.

It's really exciting to be here.

This is really one of the great scientific centres in the whole of the world.

And this is where the modern story of measurement really began.

Where a new standardised unit of length was introduced.

One that is familiar to us all today.

On the 26th March 1791, the Academy here decided to call

this new length measurement the metre.

Named after the Greek word "metron", meaning measure,

they decided it should be one ten-millionth of the distance

between the North Pole and the equator.

It was very clever.

The Academy knew that a French colloquial measure

would never be accepted by the rest of the world.

By basing the metre on the planet itself,

no one country could argue for their own measure.

They had transcended the politics of nations.

"This is a system for all people for all time",

announced the Revolutionary government.

There was one problem, though.

Nobody knew accurately what the distance between the North Pole and the equator actually was.

Getting an accurate figure would mean embarking on the most

ambitious and complex large-scale measurement project ever attempted.

Two scientists were tasked with turning the theory into reality.

They were Pierre M�chain and Jean Baptiste Delambre.

Their task was to measure the distance between two points on a meridian, or line of longitude.

Then using fairly simple mathematics,

and knowing the latitude of each point,

they could extrapolate and calculate the distance

from the Pole to the equator.

This experiment would be difficult enough under normal conditions

but France was in the middle of a revolution.

It was a dangerous time to have big ideas that were not

necessarily easy for the new order to understand.

Nevertheless, undaunted, the scientists pushed ahead.

It was here in 1793, from this bell tower in Dunkirk,

that Jean Baptiste Delambre started the northernmost part

of his epic quest to measure the Earth.

While 800 miles to the south,

Barcelona was chosen for Pierre M�chain.

Their plan was to work towards each other

and meet in Rodez in southern France.

You can imagine Delambre's excitement as he stood up here

200 years ago, ready to start his journey.

A journey that would take him seven years to complete.

And the rather splendid piece of equipment they used was this,

a repeating circle.

A device that measures angles extremely accurately

and as good today as the day it was made.

Now, obviously, Delambre wouldn't measure every distance from here to Barcelona

but what he can do is use a method called triangulation.

So, the first point of the triangle is the top of this belfry.

Then Delambre would have looked across the countryside, trying to find two high points.

And he would use this piece of equipment to line up the telescopes

on those two other points.

Then all he had to do was measure the angle between the two points

and measure the distance to the closest one.

By then moving to the next high point

and measuring the angles again,

simple geometry gave him the distances between all three.

So it's an amazing principle because just one measurement of distance

and then it's triangles all the way to Barcelona.

Delambre had a number of close scrapes along the way.

He was arrested several times, accused of being a spy.

Why else would he be scaling towers carrying strange equipment?

He tried to explain that he was measuring the size

of the Earth for the Academy of Sciences

but a drunk militiaman interrupted,

"There is no more Academy. We are all equal now. You'll come with us."

But in general, they were literally above it all.

On rooftops, towers and church spires they carried out their quest.

It was an extraordinary feat.

Seven long years later, the two men had measured the exact distance

between Dunkirk and Barcelona.

Now the metre was just a simple calculation.

The result of all M�chain and Delambre's hard labour, the prototype metre bar,

is held here at the French National Archives in Paris.

Made in 1799 of pure platinum, it's meant to represent

one ten-millionth of the distance

between the North Pole and the equator.

In fact, due to errors that M�chain made early on

in his survey, it's fractionally wrong.

The errors M�chain made were pretty much irrelevant

because for the first time,

the world had a unit of length

that was based on something they believed was permanent and unchanging - the Earth.

There it is. The metre.

A thing of beauty.

Not so much the object but the idea it represents.

This metre bar ushered in the era of metrification.

And the achievement is immense.

Even Napoleon, in a moment of humility, admitted that

"Conquests come and go, but this work will endure."

And he was right,

this lump of metal really represents a change in our thinking.

For the first time,

we had measurement based on something fundamental and universal.

The concept was brilliant,

but the metre's triumphant arrival

was not embraced with universal enthusiasm.

In fact, it took several decades before the metre was finally

accepted as a standard international unit of measurement.

It was on a spring day in 1875 that it all became official.

The historic Metre Convention was signed

and metre clones sent out around the world.

It was the beginning of our global system of precision and accuracy.

17 countries signed the convention to form the BIPM,

the Bureau International de Poids et Measures.

The custodians of international weight and measurement.

It's a role they still perform today.

Metrication was to be the basis for a new system of measurement,

the System Internationale or SI.

It even led to a new science, metrology,

the study and refinement of measurement.

The metre had united the world. At least, in theory.

Alongside the metre, seismic changes had happened in how we measured time.

For more than 3,000 years,

the sundial was the timekeeper of choice across the world.

But it was not without its problems.

And the reason is it's just not possible to fix the exact

length of an hour because the shadow cast on the dial

alters daily throughout the seasons.

The Greek astronomer Hipparchus was the first to notice

the equal length of day and night at the spring and autumn equinoxes

and that this could give us a standard for setting a fixed length of hour.

But up until the 14th century, we had no practical way of doing this.

It took the invention of the mechanical clock to change everything.

This is the Salisbury Cathedral clock.

It dates back to 1386 and it's believed to be the oldest surviving mechanical clock in the world.

For me this is an absolutely staggering achievement,

I mean, this is the 14th century,

the medieval time,

and here a blacksmith and a stonemason have created something

that is able to regulate time.

Now, it isn't driven by a pendulum,

those sort of clocks wouldn't be invented until the 17th century.

Instead it's these weights at the back

which are controlling the clock.

And as the weights fall they unwind the ropes around these barrels.

It's gravity that drives the clock,

and all you need to power it is some muscle to raise the weights.

The intriguing thing is there isn't any clock face on this clock.

It was already quite an achievement in that time

just to get that bell to bong every hour.

By the end of the 14th century many cathedrals across Europe

had built clock towers, towering up to the heavens, glorifying God,

but perhaps more importantly,

controlling the lives of us mere mortals down below.

The clocks weren't terribly accurate,

probably the best ones lost 15 minutes a day,

but they began to irrevocably change people's lives.

No longer dependent on the sun,

we were tied to the chimes of man-made clocks.

In the 15th and 16th centuries, as the mechanisms became more accurate,

the clock face itself appeared, something we now take for granted.

It then became possible to break down our day

into even smaller units.

For the first time, the hour could be divided

into minutes and seconds.

The idea came from the Greek mathematician Ptolemy

who divided a circle into 360 equal parts called degrees.

He then split each degree into 60 minutes

and each minute into 60 second minutes...

..which gave us the words we use today.

The relationship between time and length was getting closer.

We now measured the passage of time by the distance the hand

travelled around the clock face.

Mechanical clocks gave us a fixed hour.

But actually setting them to the right time was still a problem.

We still looked to the sun and set our clocks and watches to noon

when the sun was directly overhead.

But that meant that each town had its own different time.

For example, here in Salisbury,

the clocks were over seven minutes later than the clocks in London.

The reason?

Well, we're further west here, so the sun arrives overhead later.

But with the development of steam power in the early 19th century,

things had to change because it was impossible

to set busy train timetables

if every town had its own different time.

A single national time was urgently needed.

Under the unswerving leadership of Sir George Airy,

the Astronomer Royal at the Greenwich Observatory,

Greenwich time became the time for Great Britain.

The railways were the first to switch their entire timetable

to this new time.

And they did it by sending the correct time to virtually

every station in the country by the new telegraph lines

which often ran alongside the railways.

Gradually, national and international time

became essential for business and in 1884,

Greenwich time was universally adopted as the basis

for a new system of international time zones.

The reason for its enthusiastic adoption was because

the Greenwich Observatory produced the most accurate

nautical almanacs used by mariners throughout the world.

And as these almanacs were all set with Greenwich lying on zero degrees of longitude,

the prime meridian, at a stroke,

Great Britain became the centre of the world.

Time was no longer calibrated locally

by when the sun was at its highest,

it was set astronomically at Greenwich.

But while Greenwich time had gone international,

for most people, actually getting your hands on the correct time

was still a challenge.

And for businesses, this was fast becoming a problem.

And one family realised a cunning way to exploit this need.

Every week, John Henry Belville would come up the hill

here to Greenwich and set his chronometer to the correct time.

And then he'd go back down to London

to sell the right time to watchmakers and businesses.

By the 1940s, thanks to the radio and cheap clocks and watches...

..we could all run on time.

Time was money.

International trade, business and travel were all thriving.

As the world embraced Greenwich time,

our journey towards globalisation started.

While universal time was transforming our world,

the same could not be said for the metre.

17 countries had enthusiastically signed up to the historic

metre convention but, in practice, few had enforced it.

And the muddle of different measurements continued,

with standards and gauges differing from town to town,

and even factory to factory,

which was to have dire consequences, here in the United States.

When a huge fire ripped through the American city of Baltimore in 1904,

a disaster of epic proportions was unfolding.

As fire crews from the nearby cities of Washington

and New York rushed to the scene, all they could do was sit

and watch the inferno engulf the city.

None of their fire hoses would fit Baltimore's fire hydrants.

Despite being less than 200 miles apart,

all the fire crews were using different-sized equipment.

The fire raged out of control for two days, destroying 1,500 homes.

Length measurement needed to be standardised and fast.

NIST, America's measurement body,

started campaigning for better standards.

Spurred on by the NIST campaign,

American industrialists soon realised

that they could capitalise on improvements in accuracy.

Henry Ford started commissioning increasingly accurate gauges and measures.

Precise and standardised measurement

meant that mass production was possible.

At the same time, strict patterns of shift work tied their workforces to the clock.

It was the dawn of the modern age.

For the first time, millions of identical parts

could be produced at rapid speed and minimal cost.

The American boom was underway.

'And when you see inspectors checking parts for accuracy

'to dimensions measured in 10,000ths of an inch,

'you see where quantity production of quality products

'actually begins because parts must fit together perfectly.'

It would provide a profound lesson to the world.

Precise measurement had the power to change the fortunes of a nation.

But the problem with any technological breakthrough

is no-one quite knows where it will lead.

It took the paranoia of the Cold War and the resulting arms race

to trigger the next big leap in length measurement.

And it led us further than we ever thought possible.

'But history and our own conscience will judge us harshly,

'if we do not now make every effort to test our hopes by action.'

The stakes were rising but our level of accuracy

was failing to keep up with our aspirations.

Up to the 1960s,

we could measure with an accuracy of one ten-millionth of a metre.

But an error of this magnitude in the components of a rocket navigation system

would mean missing the moon by 4,000 miles.

Now the challenge was to improve the accuracy a hundredfold.

'We choose to go to the moon in this decade and do the other things,

'not because they are easy but because they are hard.

'Because that goal will serve to organise

'and measure the best of our energies and skills.'

The metre bar was no longer accurate enough.

A new and more precise way of measuring length was needed.

The answer lay in the fundamental properties of the universe.

It was the dawn of the quantum age.

Since the 1870s,

there had been a growing desire to take measurement away from earthly

constants like circumference of the globe or the length of the day...

..and to tie measurement to the fundamental and unchanging laws of nature.

Things like the speed of light or the charge on a single electron.

It was a Scottish genius, James Clerk Maxwell,

who first suggested that these universal constants

could hold the key to more precise measurement.

Considered by many to be the 19th century's most influential physicist,

Maxwell's theories would change the course of measurement history.

He said at the time,

"If then we wish to obtain standards which shall be absolutely permanent,

"we must seek them not in the dimensions or motion of our planet,

"but in the wavelength, the period of vibration

"and the absolute mass of these imperishable and unalterable

"and perfectly similar molecules."

Maxwell's idea was as revolutionary as the decision a century earlier

to take length measurement away from the human body and base it on the Earth.

Maxwell changed the direction of the science of measurement.

Maxwell, it's hard to overestimate the influence he had

on scientific thought in the 19th century.

It was a very influential idea he had and he said,

"We should be measuring length

"in terms of the wavelength of a colour of light."

But even he couldn't figure out how to really do it

to the accuracy that would be required to replace the, sort of,

metre definition.

Maxwell was never able to turn his dream of using

the wavelength of light to measure distance into reality

because the technology to achieve it simply didn't exist.

But his ideas were revolutionary.

It wasn't until decades later,

a scientist at the BIPM, the same place where the world's master metre bar is held,

would start to bring Maxwell's vision to life.

Albert Michelson began to design

and build machines called interferometers

that would actually measure the wavelength of different light sources.

So this is one of Michelson's original interferometers.

What was he using it for and how did he use it?

Well, he wanted to demonstrate that it would be possible

to measure a wavelength of light, because light travels in waves,

and then in a future time,

define the metre in terms of this wavelength of light.

Wavelengths of light are invisible to the human eye.

Michelson's genius was realising that when light is split

and then recombined, it forms a unique pattern called interference

that can be used to count wavelengths.

So by counting how many, going from light to dark, light to dark,

take a metre, divide by the number of those,

you'll get the wavelength of light,

something you can't see with your naked eye. Right.

What he had to do was build up from a wavelength of light to a metre.

And in a half a millimetre,

there are about more than 1,000 wavelengths. Extraordinary.

It was the breakthrough that was to change the destiny of the metre.

After over half a century of laborious research,

scientists were ready.

Maxwell's dream was about to become a reality.

On Friday the 14th of October 1960, delegates from across the globe,

from Russia and America, gathered here in the grounds of the BIPM.

The fate of the metre was in the balance.

At six o'clock that evening, to much applause,

the metre was redefined in terms of the number

of wavelengths of light emitted by a special krypton lamp.

Finally, the metre bar was consigned to history.

But I don't think those French Revolutionaries

who first came up with the idea of the metre would be too disappointed

because it was really realising their dream of tying

the metre to something unchanging and universal.

Distance could be measured accurately

using a universal constant, the wavelength of light.

But how could we put this new science into practice?

That would need the help of a project codenamed Laser.

It was the brainchild of Californian Theodore Maiman.

Well, this device happens to be the original laser.

The beauty of the laser, is that it is light of a precise,

fixed wavelength.

By bouncing this beam off an object,

and precisely measuring the time it takes to bounce back,

suddenly we could measure distances with incredible precision.

Within years, the laser was helping us

to measure our world in ways we never thought possible.

And there was no better illustration of this

than the Apollo 11 lunar landings.

'One small step for man, one giant leap for mankind.'

When Neil Armstrong and Buzz Aldrin

landed on the Sea of Tranquillity more than 40 years ago

on the 21st July 1969,

they left a mirror on the moon's surface.

When astronomers later fired a laser pulse at it,

Maiman's invention was also about to make history.

The beam took just 2.5 seconds to reflect back to Earth.

For the first time, scientists could calculate the distance

to the moon at any phase of its orbit

to an accuracy of three centimetres.

Lasers changed everything.

They made scientists rethink what was possible.

We could measure distance with extraordinary precision.

Distance was tied to a universal, unchanging constant

but time was not.

The second was still based on the rotation of the Earth,

which is actually rather variable.

Finding a better way of defining time

was to come from an unexpected quarter.

Just a few years before that landmark 1960 meeting in Paris,

an English scientist called Louis Essen

was working here at the UK's National Physical Laboratory.

His passion was precision timekeeping,

and he was beginning work on a new generation of clock,

the atomic clock.

We set our quartz clocks to keep time with the rotation of the Earth.

But for some of our modern problems, this is not quite accurate enough,

and now we're setting our quartz to keep time

with the vibrations of the atom.

The theory was to define time

through the vibration of individual atoms.

Across the Atlantic, the Americans, at their national laboratory,

were already pushing forward with a well-funded programme.

Back in Britain, Essen was struggling.

There was little enthusiasm for his clock project

and funding was always a problem.

His first experiment imploded, destroying much of his equipment.

But in a classic story of the underdog winning through,

Essen eventually created the world's first working atomic clock.

It was called the Caesium I.

And it was accurate to one second in 300 years.

The second was no longer based on the movement of our planet.

Time was now locked to the beating heart of a caesium atom.

A movement that was unchanging and fundamental across the universe.

In Britain, the latest incarnation of Essen's atomic clock is the CsF2.

It's one of a global network of atomic clocks that sets our time.

To most people this doesn't look like a clock at all,

so how does it actually measure time?

Well, what we're doing here is using lasers

to slow down the caesium atoms.

We form a cloud of very slowly moving caesium atoms

and we use the lasers to throw that cloud upwards through

an enclosure containing microwaves.

Then they fall back through it a second time under gravity.

When the atoms change from one energy level to another,

they emit or absorb one very precise frequency,

and we can use that frequency to keep track of time.

We simply count up the oscillations.

So it's the number of oscillations that will define the length of a second.

Those oscillations are a particular property of that caesium atom.

That's right, yes.

So, any caesium atom always has the same number

of oscillations per second.

The oscillations of these caesium atoms are the ticking of the clock.

and they give the CsF2 accuracy to one second in 138 million years.

It's a degree of precision our ancestors could never have imagined.

The genius of Maxwell, Michelson

and Essen now touch every part of our lives.

They could never have guessed their work would one day

be at the centre of everything from our banking systems to phones,

GPS and the internet.

These only exist because of the accuracy of atomic clocks

and their ability to synchronise time across the planet.

Measurement has taken us in directions we could never have dreamt possible.

But the story doesn't end there.

In one last twist, scientists looked at the metre again...

..and realised that they could now redefine length

using the new accuracy of the second.

It was 1983 and in a collaboration between different measurement labs

across the world, atomic clocks measured the speed of light

with incredible precision.

The metre could finally be defined by how far light

travels in a tiny fraction of a second.

Time and length were intimately intertwined.

We've come a long way since the days of the pharaohs,

when time was defined by the length of a shadow.

After 3,000 years, time and distance are once again linked,

joined together by one of the most fundamental

and universal constants of nature, the speed of light.

Despite all the great advances in time and length measurement,

the quest is still on.

Scientists are trying to create ever more accurate clocks.

Clocks that will only lose one second

in the lifetime of the universe.

And once they're deployed we can only begin to imagine how it's going to change our world.

Instant communication, quantum computers,

planes that can land themselves.

Science fiction will become a reality.

And that's the beauty of measurement.

Every leap in precision, from the cubit rod to the atomic clock,

has led to a technological revolution.

Through history measurement has changed every aspect of our lives...

..splitting the year into seasons and lunar cycles

allowed man to plan ahead for the first time

and gain advantage over the rest of nature.

Dividing the day still further into 24 hours

was the bedrock for civilisation.

The fixed hour controlled the working day.

And uniform national and international time

allowed the globalisation of industry.

The world would never be the same.

The story of measurement has shaped and changed our history.

And will continue to do so as we delve deeper into the atomic

fabric of the universe in search of greater precision.

Next time, I meet the biggest problem in measurement, the kilogram.

This 19th-century artefact is the world's master kilo,

and it's losing weight.

Now, a head-to-head race is on to replace it...

..as the best minds in measurement science fight it out,

there can only be one winner.