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Cosmos (1980): Season 1, Episode 7 - The Backbone of Night - full transcript

Carl Sagan looks at the work of ancient Greek scientists and astronomers including Thales, Anaximander, Democritus, and Pythagoras.


SAGAN:
The sky calls to us.

If we do not destroy ourselves...

...we will one day venture
to the stars.

There was a time when the stars
seemed an impenetrable mystery.

Today, we have begun
to understand them.

In our personal lives also, we
journey from ignorance to knowledge.

Our individual growth reflects
the advancement of the species.

The exploration of the cosmos is...

...a voyage of self-discovery.

When I was a child, I lived here...

...in the Bensonhurst section of
Brooklyn in the city of New York.

I knew my immediate
neighborhood intimately...

...every candy store, front stoop...

...back yard, empty lot...

...and wall for playing
Chinese handball.

It was my whole world.

But more than a few blocks away...

...north of the raucous traffic
and elevated railway on 86th Street...

...was an unknown territory
off-limits to my wanderings.

It could have been Mars
for all I knew.

Even with an early bedtime
in the winter...

...you could occasionally see
the stars.

I would look up at them
and wonder what they were.

I'd ask other kids and adults...

...and they would answer:

"They're lights in the sky, kid."

Well, I could tell they were lights
in the sky, but what were they?

There had to be some deeper answer.

I remember I was issued
my first library card.

It was some library on 85th Street.
Anyway, it was in alien territory.

And I asked the librarian
for a book on stars.

She gave me...

...a picture book with portraits
of men and women...

...with names like
Veronica Lake and Alan Ladd.

I explained that wasn't what
I wanted at all.

And for some reason,
then obscure to me, she smiled...

...and got me another book,
the right kind of book.

I was so excited to know
the answer...

...I opened the book breathlessly,
right there in the library...

...and the book said
something astonishing...

...a very big thought.

Stars, it said, were suns...

...but very far away.

The sun was a star, but close-up.

How, I wondered, could anybody
know such things for sure?

How did they figure it out?
Where did they even begin?

I was ignorant of the idea
of angular size.

I didn't know about the inverse square
law of the propagation of light.

I didn't have any chance of
calculating the distance to the stars.

But I could tell that if
the stars were suns...

...they had to be awfully far away.

Further away than 86th Street,
further away than Manhattan...

...further away, probably,
than New Jersey.

The universe had become
much grander...

...than I had ever guessed.

And then I read
another astonishing fact.

The Earth, which includes Brooklyn...

...was a planet.

It went around the sun.

There were other planets.

They also went around the sun...

...some closer to the sun,
some further from the sun.

But planets didn't shine by their
own light the way the sun does.

No, planets simply reflected
the little bit of light...

...that shines on them
from the sun back to us.

If you were a great distance
from the sun...

...you wouldn't be able to see
the Earth or the other planets at all.

Well, then, it stood
to reason, I thought...

...that those other stars ought to
have their own planets...

...and some of those planets
ought to have life.

Why not?

And that life ought to be pretty
different from life as we know it...

...life here in Brooklyn.

Ganymede. Look at this
amazing Ganymede stuff.

Wait, wait, wait.

As a child, it was
my immense good fortune...

...to have parents and a few teachers
who encouraged my curiosity.

This was my 6th-grade classroom.

I came back here one day
to remember what it was like.

I brought some of the pictures
of other worlds...

...that were radioed back
by the Voyager spacecraft...

...of Jupiter and its moons.

This is Calisto which is...

(SAGAN LAUGHS)

What is a Calisto?
I want a Calisto.

Now you got it.
What is it?

It's the outermost
big moon of Jupiter.

Who is this guy? Europa.

Another Europa.

A black-and-white picture
of a ring of Jupiter.

There you go.
That's a prize for honesty.

You didn't get a second.

Which one would you like?

Every one of us begins life
with an open mind...

...a driving curiosity,
a sense of wonder.

I thought it might be fun
if we now had some questions.

Why is the Earth round?
Why isn't it square or any other shape?

That's a good question.

That's a question I've asked myself.
The answer has to do with gravity.

The Earth has a strong gravity.

If you were to make a mountain
very high...

...higher than Everest,
the biggest mountain on Earth...

...it would be crushed
by its own weight.

Gravity pulls everything
towards the center.

So any really big bump
on the Earth is crushed.

But if you had a small object,
a tiny world...

...the gravity is very low...

...and then it can be very different
from a sphere.

I think I have here
a world that isn't a sphere.

Here.

Look at this one.

See? It's lumpy.

It's a lumpy world.

It looks like a potato.

There's a large potato
orbiting the planet Mars.

This is one of the moons of Mars.

That's a perfect example.

You can have big departures from
a sphere if your gravity is low.

Now the question in the front.

Is the sun considered part
of the Milky Way galaxy?

Sure, you're considered part
of the Milky Way galaxy.

Everything except other galaxies is
part of the Milky Way galaxy.

The sun is one star.

There is a few hundred billion stars
in the Milky Way.

Around each star, maybe,
is a whole bunch of planets.

And on one of those planets is life...

...and one of the life forms
on that planet is you.

You're a part of
the Milky Way galaxy too.

Sometimes I think, how lucky we are
to live in this time...

...the first moment in human history
when we are, in fact...

...visiting other worlds...

...and engaging in a deep
reconnaissance of the cosmos.

But if we had been born
in a much earlier age...

...no matter how great our dedication,
we couldn't have understood...

...what the stars and planets are.

We would not have known that there
were other suns and other worlds.

This is one of the great secrets
wrested from nature...

...through a million years of patient
observation and courageous thinking.

Human beings have always asked
questions about the stars.

It's as natural as breathing.

But imagine a time before science
had found out the answers.

Imagine what it was like, say...

...hundreds of thousands
of years ago...

...soon after the discovery of fire.

We were just as smart
and just as curious then...

...as we are now.

Sometimes it seems to me that...

...there were people then
who thought like this:

We are wandering hunter folk.

Fire keeps us warm.

Its light makes holes in the darkness.

It keeps hungry animals away.

In the darkness,
we can see each other and talk.

We take care of the flame.

The flame takes care of us.

The stars are not near to us.

When we climb a hill or a tree,
they are no closer.

They flicker with
a strange, cold, white...

...faraway light.

Many of them, all over the sky,
but only at night.

I wonder what they are.

One night I thought
the stars are flames.

They give a little light
at night as fire does.

Maybe the stars are campfires...

...which other wanderers
light at night.

The stars give a much
smaller light than campfires...

...so they must be very far away.

I wonder if our campfires...

...look like stars to the people
in the sky.

But why don't those campfires
and the wanderers who made them...

...fall down at our feet?

Why don't strange tribes
drop from the sky?

Those beings in the sky
must have great powers.

I don't suppose
that every hunter-gatherer...

...had such thoughts about the stars.

But we know from contemporary
hunter-gatherer communities...

...that very imaginative ideas arise.

The Kung Bushmen...

...of the Kalahari Desert
in the Republic of Botswana...

...have an explanation
of the Milky Way.

At their latitude,
it's often overhead.

They call it the "backbone of night."

They believe it holds the sky up.

They believe that if not
for the Milky Way...

...pieces of sky would come crashing
down at our feet.

So the Milky Way, in their view,
has some practical value.

The backbone of night.

Later on, metaphors about...

...campfires or backbones...

...or holes through which
the flame could be seen...

...were replaced in most human
communities by another idea.

The powerful beings in the sky
were promoted to gods.

They were given names and relatives...

...and special responsibilities...

...for the cosmic services they were
expected to perform.

There was a god
for every human concern.

Gods ran nature.

Nothing happened without the direct
intervention of some god.

If the gods were happy,
there was plenty of food...

...and humans were happy.

But if something
displeased the gods...

...and it didn't take much,
the consequences were awesome:

Droughts, floods, storms, wars...

...earthquakes, volcanic eruptions,
epidemics.

The gods had to be propitiated.

And a vast industry
of priests arose...

...to make the gods less angry.

But because the gods
were capricious...

...you couldn't be sure
what they would do.

Nature was a mystery.

It was hard to understand the world.

Our ancestors groped in darkness...

...to make sense
of their surroundings.

Powerless before nature...

...they invented rituals and myths...

...some desperate and cruel...

...others imaginative and benign.

The ancient Greeks explained...

...that diffuse band of brightness
in the night sky...

...as the milk of the goddess Hera...

...squirted from her breast
across the heavens.

We still call it the Milky Way.

In gratitude for the many gifts
of the gods...

...our ancestors created works
of surpassing beauty.

This is all that remains...

...of the ancient temple of Hera,
queen of heaven:

A single marble column standing
in a vast field of ruins...

...on the Greek island of Samos.

It was one of the wonders
of the world...

...built by people with
an extraordinary eye for clarity...

...and symmetry.

Those who thronged to that temple...

...were also the architects
of a bridge...

...from their world to ours.

We were moving once again
in our voyage of self-discovery...

...on our journey to the stars.

Here, 25 centuries ago...

...on the island of Samos
and in the other Greek colonies...

...which had grown up
in the busy Aegean Sea...

...there was a glorious awakening.

Suddenly, people believed
that everything was made of atoms...

...that human beings and other animals
had evolved from simpler forms...

...that diseases were not caused by
demons or the gods...

...that the Earth was only
a planet going around a sun...

...which was very far away.

This revolution made cosmos
out of chaos.

Here, in the sixth century B.C.,
a new idea developed...

...one of the great ideas
of the human species.

It was argued that the universe
was knowable.

Why? Because it was ordered.

Because there are regularities
in nature...

...which permitted secrets
to be uncovered.

Nature was not entirely unpredictable.

There were rules which even
she had to obey.

This ordered and admirable character
of the universe...

...was called cosmos.

And it was set
in stark contradiction...

...to the idea of chaos.

This was the first conflict
of which we know...

...between science and mysticism...

...between nature and the gods.

But why here?

Why in these remote islands and inlets
of the eastern Mediterranean?

Why not in the great cities of...

...India or Egypt, Babylon,
China, Mesoamerica?

Because they were all
at the center of old empires.

They were set in their ways,
hostile to new ideas.

But here in Ionia...

...were a multitude of newly colonized
islands and city-states.

Isolation, even if incomplete,
promotes diversity.

No single concentration of power
could enforce conformity.

Free inquiry became possible.

They were beyond the frontiers
of the empires.

The merchants and tourists
and sailors of Africa...

...Asia and Europe
met in the harbors of Ionia...

...to exchange goods
and stories and ideas.

There was a vigorous and heady
interaction...

...of many traditions, prejudices,
languages and gods.

These people were ready to experiment.

Once you are open to
questioning rituals...

...and time-honored practices...

...you find that one question
leads to another.

What do you do when you're faced
with several different gods...

...each claiming the same territory?

The Babylonian Marduk
and the Greek Zeus...

...were each considered
king of the gods...

...master of the sky.

You might decide, since they otherwise
had different attributes...

...that one of them was merely
invented by the priests.

But if one, why not both?

And so it was here
that the great idea arose:

The realization that there
might be a way...

...to know the world
without the god hypothesis.

That there be principles,
forces, laws of nature...

...through which the world might be
understood without attributing...

...the fall of every sparrow to
the direct intervention of Zeus.

This is the place
where science was born.

That's why we're here.

This Greek revolution happened
between 600 and 400 B.C.

It was accomplished by the same
practical and productive people...

...who made the society function.

Political power was in the hands
of the merchants...

...who promoted the technology
on which their prosperity depended.

The earliest pioneers
of science were...

...merchants and artisans
and their children.

The first Ionian scientist was
named Thales.

He was born over there
in the city of Miletus...

...across this narrow strait.

He had traveled in Egypt...

...and was conversant
with the knowledge of Babylon.

Like the Babylonians, he believed
that the world had once all been water.

To explain the dry land...

...the Babylonians added
that their god, Marduk...

...had placed a mat on the face
of the waters...

...and piled dirt on top of it.

Thales had a similar view...

...but he left Marduk out.

Yes, the world had once been
mostly water...

...but it was a natural process
which explained the dry land.

Thales thought it was similar to
the silting up he had observed...

...at the delta of the river Nile.

Whether Thales' conclusions
were right or wrong...

...is not nearly as important
as his approach.

The world was not made by the gods...

...but instead was the result
of material forces...

...interacting in nature.

Thales brought back from
Babylon and Egypt...

...the seeds of new sciences:

Astronomy and geometry...

...sciences which would
sprout and grow...

...in the fertile soil of Ionia.

Anaximander of Miletus, over there...

...was a friend and colleague
of Thales...

...one of the first people
that we know of...

...to have actually done
an experiment.

By examining the moving shadow
cast by a vertical stick...

...he determined accurately
the lengths of the year and seasons.

For ages, men had used sticks...

...to club and spear each other.

Anaximander used a stick
to measure time.

In 540 B.C., or thereabouts,
on this island of Samos...

...there came to power a tyrant
named Polycrates.

He seems to have started
as a caterer...

...and then went on to
international piracy.

His loot was unloaded
on this very breakwater.

(DRUM BEATS)

But he oppressed his own people,
he made war on his neighbors.

He quite rightly feared invasion.

So Polycrates surrounded his capital
city with an impressive wall...

...whose remains stand till this day.

To carry water from a distant spring
through the fortifications...

...he ordered this great tunnel built.

A kilometer long,
it pierces a mountain.

Two cuttings were dug
from either side...

...which met almost perfectly
in the middle.

The project took some 1 5 years
to complete.

It is a token of the civil engineering
of its day...

...and an indication of the
extraordinary practical capability...

...of the Ionians.

The enduring legacy of the Ionians...

...is the tools and techniques
they developed...

...which remain the basis
of modern technology.

This was the time of Theodorus...

...the master engineer of the age...

...a man who is credited with
the invention of...

...the key, the ruler,
the carpenter's square...

...the level, the lathe,
bronze casting.

Why are there no monuments
to this man?

Those who dreamt and speculated...

...and deduced about
the laws of nature...

...talked to the engineers
and the technologists.

They were often the same people.

The practical and the theoretical
were one.

(DRUM BEATS)

This new hybrid of abstract thought...

...and everyday experience
blossomed into science.

When these practical men turned
their attention to the natural world...

...they began to uncover
hidden wonders...

...and breathtaking possibilities.

Anaximander studied the profusion
of living things...

...and saw their interrelationships.

He concluded that life had originated
in water and mud...

...and then colonized the dry land.

"Human beings," he said...

"...must have evolved
from simpler forms."

This insight had to wait 24 centuries
until its truth was demonstrated...

...by Charles Darwin.

Nothing was excluded from the
investigations of the first scientists.

Even the air became the subject
of close examination...

...by a Greek from Sicily
named Empedocles.

He made an astonishing discovery...

...with a household implement
that people had used for centuries.

This is the so-called
water thief.

It's a brazen sphere with a neck
and a hole at the top...

...and a set of little holes
at the bottom.

It was used as a kitchen ladle.

You fill it by immersing it in water.

lf, after it's been in there
a little bit...

...you pull it out
with the neck uncovered...

...then the water trickles out
the little holes making a small shower.

Instead, if you pull it out
with the neck covered...

...the water is retained.

Now try to fill it...

...with the neck covered
with my thumb.

Nothing happens.

Why not?

There's something in the way.

Some material is blocking the access
of the water into the sphere.

I can't see any such material.

What could it be?

Empedocles identified it...

...as air.

What else could it be?

A thing you can't see
can exert pressure...

...can frustrate my wish to fill
this vessel with water...

...if I were dumb enough to
leave my thumb on the neck.

Empedocles had discovered...

...the invisible.

Air, he thought, must be matter...

...in a form so finely divided...

...that it couldn't be seen.

This hint, this whiff
of the existence of atoms...

...was carried much further by
a contemporary named Democritus.

Of all the ancient scientists, it is
he who speaks most clearly to us...

...across the centuries.

The few surviving fragments
of his scientific writings...

...reveal a mind of the highest
logical and intuitive powers.

He believed that a large number of
other worlds wander through space...

...that worlds are born and die...

...that some are
rich and living creatures...

...and others are dry and barren.

He was the first to understand
that the Milky Way...

...is an aggregate of the light
of innumerable faint stars.

Beyond campfires in the sky,
beyond the milk of Hera...

...beyond the backbone of night,
the mind of Democritus soared.

He saw deep connections between
the heavens and the Earth.

"Man," he said, "is a microcosm...

...a little cosmos."

Democritus came from
the Ionian town of Abdera...

...on the northern Aegean shore.

In those days, Abdera was
the butt of jokes.

lf, around the year 400 B.C...

...in the equivalent
of a restaurant like this...

...you told a story about
someone from Abdera...

...you were guaranteed a laugh.

It was, in a way...

...the Brooklyn of its time.

For Democritus, all of life was to be
enjoyed and understood.

For him, understanding and enjoyment...

...were pretty much the same thing.

He said, "A life without festivity is
a long road without an inn."

Democritus may have come from Abdera,
but he was no dummy.

Democritus understood
that the complex forms...

...changes and motions
of the material world...

...all derived from the interaction
of very simple moving parts.

He called these parts atoms.

All material objects are
collections of atoms...

...intricately assembled...

...even we.

When I cut this apple...

...the knife must be
passing through...

...empty spaces between the atoms,
Democritus argued.

If there were no such empty spaces,
no void...

...then the knife would encounter
some impenetrable atom...

...and the apple wouldn't be cut.

Let's compare the cross sections
of the two pieces.

Are the exposed areas exactly equal?

No, said Democritus,
the curvature of the apple...

...forces this slice to be slightly
shorter than the rest of the apple.

If they were equally tall,
then we'd have...

...a cylinder and not an apple.

No matter how sharp the knife...

...these two pieces have
unequal cross sections.

But why?

Because on the scale
of the very small...

...matter exhibits some
irreducible roughness...

...and this fine scale of roughness
Democritus of Abdera identified...

...with the world of the atoms.

His arguments are not those
we use today.

But they're elegant and subtle
and derived from everyday experience.

And his conclusions were
fundamentally right.

Democritus believed that nothing
happens at random...

...that everything has
a material cause.

He said, "I would rather understand
one cause...

...than be king of Persia."

He believed that poverty
in a democracy was far better...

...than wealth in a tyranny.

He believed that the prevailing
religions of his time were evil...

...and that neither souls
nor immortal gods existed.

There is no evidence that Democritus
was persecuted for his beliefs.

But then again, he came from Abdera.

However, in his time...

...the brief tradition of tolerance
for unconventional views...

...was beginning to erode.

For instance,
the prevailing belief was...

...that the moon and the sun
were gods.

Another contemporary of Democritus,
named Anaxagoras, taught...

...that the moon was a place
made of ordinary matter...

...and that the sun was a red-hot stone
far away in the sky.

For this, Anaxagoras was condemned...

...convicted and imprisoned
for impiety...

...a religious crime.

People began to be persecuted
for their ideas.

A portrait of Democritus is now...

...on the Greek 100-drachma note.

But his ideas were suppressed...

...and his influence on history
made minor.

The mystics were beginning to win.

(DRUM BEATS)

You see, Ionia was also the home...

...of another quite different
intellectual tradition.

Its founder was Pythagoras...

...who lived here on Samos
in the 6th century B.C.

According to local legend...

...this cave was once his abode.

Maybe that was once his living room.

Many centuries later...

...this small Greek Orthodox shrine
was erected on his front porch.

There's a continuity of tradition
from Pythagoras to Christianity.

Pythagoras was the first person
in the history of the world...

...to decide that the Earth
was a sphere.

Perhaps he argued by analogy
with the moon or the sun...

...maybe he noticed the curved shadow
of the Earth on the moon...

...during a lunar eclipse.

Or maybe he recognized
that when ships leave Samos...

...their masts disappear last.

Pythagoras believed that
a mathematical harmony...

...underlies all of nature.

The modern tradition
of mathematical argument...

...essential in all of science
owes much to him.

And the notion that the heavenly bodies
move to a kind of...

...music of the spheres...

...was also derived from Pythagoras.

It was he who first used
the word cosmos...

...to mean a well-ordered
and harmonious universe...

...a world amenable
to human understanding.

For this great idea,
we are indebted to Pythagoras.

But there were deep ironies
and contradictions in his thoughts.

Many of the Ionians believed...

...that the underlying harmony and
unity of the universe was accessible...

...through observation
and experiment...

...the method which dominates
science today.

However, Pythagoras had
a very different method.

He believed that the laws of nature
can be deduced by pure thought.

He and his followers were not
basically experimentalists...

...they were mathematicians...

...and they were
thoroughgoing mystics.

They were fascinated by these
five regular solids...

...bodies whose faces
are all polygons:

Triangles or squares...

...or pentagons.

There can be an infinite number
of polygons...

...but only five regular solids.

Four of the solids were associated
with earth, fire, air and water.

The cube, for example,
represented earth.

These four elements, they thought,
make up terrestrial matter.

So the fifth solid...

...they mystically associated
with the cosmos.

Perhaps it was the substance
of the heavens.

This fifth solid was called...

...the dodecahedron.

Its faces are pentagons, 1 2 of them.

Knowledge of the dodecahedron...

...was considered too dangerous
for the public.

Ordinary people were to be
kept ignorant of the dodecahedron.

In love with whole numbers,
the Pythagoreans believed...

...that all things could be
derived from them...

...certainly all other numbers.

So a crisis in doctrine occurred
when they discovered...

...that the square root of two
was irrational.

The square root of two could
not be represented as the ratio...

...of two whole numbers
no matter how big they were.

Irrational originally meant
only that...

...that you can't express a number
as a ratio.

But for the Pythagoreans,
it came to mean something else...

...something threatening...

...a hint that their world-view
might not make sense...

...the other meaning of irrational.

Instead of wanting everyone to share
and know of their discoveries...

...the Pythagoreans suppressed
the square root of two...

...and the dodecahedron.

The outside world was not to know.

The Pythagoreans had discovered...

...in the mathematical underpinnings
of nature...

...one of the two most
powerful scientific tools.

The other, of course, is experiment.

But instead of using their insight
to advance...

...the collective voyage
of human discovery...

...they made of it little more
than the hocus-pocus of a mystery cult.

Science and mathematics were to be
removed from the hands...

...of merchants and artisans.

This tendency found its most
effective advocate...

...in a follower of Pythagoras
named Plato.

He preferred the perfection
of these mathematical abstractions...

...to the imperfections
of everyday life.

He believed that ideas were far more
real than the natural world.

He advised the astronomers
not to waste their time...

...observing stars and planets.

It was better, he believed,
just to think about them.

Plato expressed hostility to
observation and experiment.

He taught contempt
for the real world...

...and disdain for the practical
application of scientific knowledge.

Plato's followers succeeded
in extinguishing the light...

...of science and experiment...

...that had been kindled
by Democritus and the other Ionians.

Plato's unease with the world
as revealed by our senses...

...was to dominate
and stifle Western philosophy.

Even as late as 1600...

...Johannes Kepler was still
struggling to interpret...

...the structure of
the cosmos in terms of...

...Pythagorean solids
and Platonic perfection.

Ironically, it was Kepler who helped
re-establish the old Ionian method...

...of testing ideas
against observations.

But why had science lost its way
in the first place?

What appeal did Pythagoras'
and Plato's teachings...

...have for their contemporaries?

They provided, I believe...

...an intellectually
respectable justification...

...for a corrupt social order.

The mercantile tradition which had
led to Ionian science...

...also led to a slave economy.

You could get richer...

...if you owned a lot of slaves.

Athens, in the time
of Plato and Aristotle...

...had a vast slave population.

All of that brave Athenian talk
about democracy...

...applied only to a privileged few.

Plato and Aristotle were comfortable
in a slave society.

They offered justifications
for oppression.

They served tyrants.

They taught the alienation
of the body from the mind...

...a natural enough idea, I suppose,
in a slave society.

They separated thought from matter.

They divorced the Earth
from the heavens.

Divisions which were to dominate
Western thinking...

...for more than 20 centuries.

The Pythagoreans had won.

In the recognition by
Pythagoras and Plato...

...that the cosmos is knowable...

...that there is a mathematical
underpinning to nature...

...they greatly advanced
the cause of science.

But in the suppression
of disquieting facts...

...the sense that science should be
kept for a small elite...

...the distaste for experiment,
the embrace of mysticism...

...the easy acceptance
of slave societies...

...their influence has
significantly set back...

...the human endeavor.

The books of the Ionian scientists
are entirely lost.

Their views were suppressed,
ridiculed and forgotten...

...by the Platonists
and by the Christians...

...who adopted much of
the philosophy of Plato.

Finally, after a long,
mystical sleep...

...in which the tools of
scientific inquiry lay moldering...

...the Ionian approach was
rediscovered.

The Western world reawakened.

Experiment and open inquiry...

...slowly became respectable
once again.

Forgotten books and fragments were
read once more.

Leonardo and Copernicus
and Columbus...

...were inspired by
the Ionian tradition.

The Pythagoreans
and their successors...

...held the peculiar notion that...

...the Earth was tainted...

...somehow nasty...

...while the heavens were
pristine and divine.

So the fundamental idea
that the Earth is a planet...

...that we're citizens
of the universe...

...was rejected and forgotten.

This idea was first argued
by Aristarchus...

...born here on Samos,
three centuries after Pythagoras.

He held that the Earth moves
around the sun.

He correctly located our place
in the solar system.

For his trouble,
he was accused of heresy.

From the size of the Earth's shadow
on the moon during a lunar eclipse...

...he deduced that the sun
had to be much, much larger...

...than the Earth,
and also very far away.

From this he may have argued
that it was absurd...

...for so large an object as the sun
to be going around...

...so small an object as the Earth.

So he put the sun rather than the Earth
at the center of the solar system.

And he had the Earth and the other
planets going around the sun.

He also had the Earth rotating
on its axis once a day.

These are ideas that we ordinarily
associate with the name Copernicus.

But Copernicus seems to have gotten
some hint of these ideas...

...by reading about Aristarchus.

In fact, in the manuscript
of Copernicus' book...

...he referred to Aristarchus,
but in the final version...

...he suppressed the citation.

Resistance to Aristarchus,
a kind of...

...geocentrism in everyday life,
is with us still.

We still talk about a sun rising...

...and the sun setting.

It's 2200 years since Aristarchus...

...and the language still pretends
that the Earth does not turn...

...that the sun is not at the center
of the solar system.

Aristarchus understood the basic
scheme of the solar system...

...but not its scale.

He knew that the planets move
in concentric orbits about the sun...

...and he probably knew their order
out to Saturn.

But he was much too modest
in his estimates...

...of how far apart the planets are.

In order to calculate the true scale
of the solar system...

...you need a telescope.

It wasn't until the 1 7th century
that astronomers were able to get...

...even a rough estimate
of the distance to the sun.

And once you knew
the distance to the sun...

...what about the stars?

How far away are they?

There is a way to measure
the distance to the stars...

...and the Ionians were
fully capable of discovering it.

Aristarchus had toyed
with the daring idea...

...that the stars were distant suns.

Now, if a star were as near
as the sun...

...it should appear as big
and as bright as the sun.

Everyone knows that the farther away
an object is, the smaller it seems.

This inverse proportionality between
apparent size and distance...

...is the basis of perspective
in art and photography.

So the further away we are
from the sun...

...the smaller and dimmer it appears.

How far from the sun would we
have to be for it to appear...

...as small and dim as a star?

Or equivalently,
how small a piece of sun...

...would be as bright as a star?

An experiment to answer this question
was performed in 17th-century Holland...

...by Christiaan Huygens and is
very much in the Ionian tradition.

Huygens drilled a number of holes
in a brass plate...

...and held the plate up to the sun.

He asked himself,
which hole seemed as bright...

...as he remembered the star Sirius
to have been the previous evening.

Well, the hole that matched was
effectively...

...1 l28,000th the apparent size
of the sun.

So Sirius, he reasoned,
must be 28,000 times...

...further away than the sun,
or about half a light-year away.

It's hard to remember
just how bright a star is...

...hours after you've looked at it,
but Huygens remembered very well.

If he had known that Sirius was
intrinsically brighter than the sun...

...he would've gotten
the answer exactly right.

Sirius is 8.8 light-years
away from us.

Between Aristarchus and Huygens...

...people had answered that question
which had so excited me...

...as a young boy growing up
in Brooklyn:

The question, "What are the stars?"

And the answer is that the stars are
mighty suns, light-years away...

...in the depths
of interstellar space.

And around those suns,
are there other planets?

And on those other worlds...

...are there beings
who wonder as we do?

Here is a light bulb...

...which is supposed to represent
a nearby star.

Next to it, and very hard to see
because of the bright light...

...is a planet.

We'll need a volunteer.
Who would like to come up, please?

Ordinarily, it's hard to
see the planet...

...because it's so close that the star
washes out the planet.

But if we're able to put something
in front of the star...

...to make an artificial eclipse,
then we might be able to see the planet.

I'm gonna stand over here.
Imagine that I'm a telescope...

...somewhere near the Earth.

And, Tab, if you'd slowly move
the disc across.

Good. A little faster would be nice.

Now you're just beginning to cover
over the star.

I really can't see the planet at all.
Keep going.

Now, right there...

...I can't see the star at all...

...and I see the planet lit
by the light of the star.

Now, that is a method for looking
for planets...

...around nearby stars.

And that method uses a spacecraft
to hold the disc...

...and scan the sky
for another telescope...

...to see if there are any planets.

Tab, you accomplished your mission
to look for planets around other stars.

Thank you for being
our interplanetary spacecraft.

So this is one way.

And there are spaceships
that will be able to do this...

...in the next 1 0 years or so.

And there's another way.

This has already been tried
from the Earth.

Imagine that there's a nearby star
that you can see.

It's bright and it has
a dark companion, a planet...

...shining only by reflected light
near it, so dim you can't see it.

But imagine that this planet
and its star...

...are going around each other.

Like that:

You can see the star,
you can't see the planet.

So now I'm gonna need two volunteers.

You two.

Just to save time
because they're in the front row.

I need one of you to turn
the star and the planet...

...and another person to pull
the star and planet along.

And what you will see...

...is that the star
you can make out...

...will be moving
in a funny, wiggly pattern...

...which will be the clue,
the evidence...

...for the existence
of the dark planet.

Okay, let's have a spin. Good.
And a pull.

And you see this funny motion...

...that the star makes
because of the planet. Thank you.

That's another way of finding out
the existence of a planet...

...that you couldn't see directly.

Well, both of these methods are
being used.

And by the time that you people are...

...as old as I am...

...we should know,
for all the nearest stars...

...if they have planets
going around them.

We might know dozens or even hundreds
of other planetary systems...

...and see if they're like our own
or very different...

...or no other planets
going around other stars at all.

That will happen in your lifetime.

It'll be the first time in the world's
history that anybody found out...

...if there are planets
around the other stars.

Now, the nearby stars, the ones
you can see with the naked eye...

...those are all
in the solar neighborhood.

That's what astronomers call it:
The neighborhood.

But it's a very tiny place
in the Milky Way galaxy.

The Milky Way is that band of light...

...that you see across the sky
on a clear night.

I can't tell if there are any more
clear nights in Brooklyn.

You must've seen the Milky Way,
a faint band of light at night.

Well, that's just 100 billion stars...

...all seen together...

...edge on, as in this picture.

If you could get out of
the Milky Way and look down on it...

...it would look like that picture.

If we did look down
on the Milky Way...

...where would the sun
and nearby stars be?

Would it be in the center where things
look important...

...or at least well-lit?

No. We would be way out here...

...in the suburbs,
in the countryside of the galaxy.

We're not in any important place.

All the stars you could see would be
in a little place like that.

And the Milky Way would be
this band of light...

...100 billion stars all together.

The fact that we live
in the outskirts of the galaxy...

...was discovered a long time ago...

...towards the end
of the First World War...

...by a man named Harlow Shapley...

...who was mapping the position
of these clusters of stars.

See, every one of these is a bunch...

...of maybe 1 0,000 stars all together.

It's called a globular cluster.

And you can see that they are
centered around the middle...

...the center of the galaxy.

People used to think that the sun was
at the center of the galaxy...

...something important about our
position. That turns out to be wrong.

We live in the outskirts...

...the globular clusters are
centered around...

...the marvelous middle
of the Milky Way galaxy.

And then it turned out
that this isn't the only galaxy.

We live in this one...

...but there are many others.

And as this picture reminds us...

...there are many different kinds
of galaxies...

...of which ours might be
just this one.

There are, in fact,
100 billion other galaxies...

...each of which contains
something like 100 billion stars.

Think of how many stars and planets
and kinds of life there may be...

...in this vast and awesome universe.

As long as there have been humans...

...we have searched
for our place in the cosmos.

Where are we? Who are we?

We find that we live
on an insignificant planet...

...of a humdrum star...

...lost in a galaxy
tucked away in some...

...forgotten corner of a universe
in which there are...

...far more galaxies than people.

We make our world significant by
the courage of our questions...

...and by the depth of our answers.

We embarked on our journey
to the stars...

...with a question first framed...

...in the childhood of our species...

...and in each generation
asked anew...

...with undiminished wonder:

"What are the stars?"

Exploration is in our nature.

We began as wanderers...

...and we are wanderers still.

We have lingered long enough...

...on the shores of the cosmic ocean.

We are ready at last...

...to set sail for the stars.