Ancient Skies

High above our heads is a glittering panorama

so vast and mysterious that we have spent all of human history

trying to unlock its secrets.

Why do the heavens move?

And where do we fit among the planets and the stars?

Looking for answers to these questions,

we embark on a 2,000-year journey of discovery

that would reshape our earth...

And replace ancient stories with a new way of thinking.

We invent incredible devices to map the heavens...

And a few risk everything to work out

our true place in the universe.

This is the story of humankind's obsession with the skies

and the ways our ancestors made sense of the universe.

This is the story of how we moved from a world

ruled by supernatural beings...

To a cosmos revealed by scientific astronomy.

This is the story of seeing other worlds

and finding our true place among them.

This is the story of our ancient skies.

From the beginning of time,

we have created stories to explain

how the universe came to be and how it works.

We have learned to watch the skies,

and to Mark the passage of time.

When the sun sets behind

that landmark again...

Boom! You know you've been waiting one year.

And ultimately look into the future.

But as different cultures began to share

their observations and ideas,

our assumptions about the nature of the cosmos

began to unravel.

For millennia, people across the world

assumed the earth was flat.

Confined within a universe of limited size and shape.

The native American Navajo tribe

modeled the universe on their traditional homes...

Where the floor was the earth and the roof was the sky.

In ancient Babylon, the world's oldest map

reveals a flat earth surrounded by water.

While these stories may seem strange,

they had a natural logic.

It does make sense that many of our ancestors

believed that the earth was flat

and that's because the earth is so huge.

It's really a matter of perspective.

But a flat earth didn't mean an infinite earth.

Because the sky was a dome, then the limit of the world

could be found where earth and sky met.

Anyone brave enough to try and reach that limit...

Had to enter the unknown.

In ancient Greek epic, the hero Odysseus

had to venture to the very limit of the world

to find the land of the dead.

The closer he came, the more he encountered

terrifying dangers and deadly monsters.

We entered the straits

in great fear of mind.

Knowing that on the one hand was Scylla,

and on the other, the whirlpool, Charybdis.

We could see the water whirling round and round,

and it made a deafening sound as it broke against the rocks.

The men were at their wit's end for fear.

Scylla pounced down upon us and snatched up my 6 best men.

But even as this story was being written down,

there was evidence for those that looked

that the world might not be flat after all.

We spotted clues in the skies.

In a lunar eclipse, the earth

comes between the moon and the sun,

so, instead of the moon being illuminated,

the earth's shadow blocks some of this light

and you can see the curvature in this shadow.

We also noticed hints much closer to home.

If you've ever been out on the ocean

and looked out in the horizon and saw a ship sailing away,

the ship doesn't just fall below the horizon all at once.

It's actually the hull that sinks below the horizon first

and then the mast.

From at least the 10th century B.C.,

sailors were making incredible voyages

over thousands of miles,

and they found no limit to their world.

The ancient Greek historian Herodotus even told a story

about Phoenician sailors who circumnavigated Africa.

But there was one part of that account he didn't believe.

As their fleet rounded Africa heading west,

something unusual happened.

The sun now appeared to their right.

While it's clear now the sailors must have crossed the equator,

for Herodotus, this made no sense.

But this was just the sort of observation that could be used

by other ancient Greeks who were starting to think

in a whole new way.

In early classical Greek culture

around 500-600 BCE, we begin to see the origin

of what we now know as Greek science and philosophy.

So, what the Greeks begin then to do

is leave out the gods and goddesses

and begin to focus on the material.

We don't know if there was ever a Eureka moment.

But by the fifth century B.C.,

this massive eruption of Greek science and philosophy

resulted in a reshaping of our earth so dramatic

that even today, some find it difficult to accept.

Our flat earth broke free from the skies that contained it,

reforming into a perfect, geometric sphere.

Our ideas about the universe would continue to change...

But a round earth was no longer in doubt.

The cosmos remained complex and mysterious.

But when it came to the earth, human ingenuity allowed us

to move from questions of shape

to those of scale.

The question isn't "is the earth round?"

The question really is "how big is it?"

A mathematician in Alexandria in Egypt

took on the task of finding out.

His name was Eratosthenes.

Eratosthenes was the head of

the library of Alexandria,

and most famous for having calculated

the circumference of the earth.

The library at Alexandria

had a remarkable ambition:

To collect all the world's knowledge under one roof.

When Eratosthenes arrived in Alexandria around 240 B.C.,

the resources available to him were unique in human history.

But the solution to the problem he faced

was actually rather simple.

So, how do you calculate

the diameter of the earth when all you've got

are the tools at hand and the ground around you?

Eratosthenes heard a story that there is a well in Syene, Egypt,

and on a certain day of the year,

if you look down into that well,

you can see all the way down to the bottom.

The sun is shining straight down that shaft.

Well, he lived in Alexandria, which was due north of Syene,

and he knew that on that same day,

a stick stuck in the ground still cast a shadow,

and he realized that using simple geometry,

he could use this information to calculate

the circumference of the earth.

Eratosthenes knew that if he put a stick straight into the ground

at the same time that somebody is looking down a well in Syene,

this stick will cast a shadow.

So, if you measure the angle from the top of the stick

to the tip of the shadow, that angle is going to be

the same as the angle from the center of the earth

to Alexandria and Syene.

Eratosthenes worked out that this angle

was about 7.2 degrees.

Using this figure and the distance

between Alexandria and Syene,

he could calculate the size of the earth using geometry.

Eratosthenes knew that Syene

was 5,000 stadia south of Alexandria,

and that's an ancient unit of measurement.

When he did the experiment and did the math,

he calculated that the circumference of the earth

was a little over 250,000 stadia all the way around.

That's about 29,000 miles.

Eratosthenes' figure was just 16% more than

the actual circumference of the earth,

which is around 25,000 miles,

making his incredible calculation

one of the greatest feats of ancient science.

For the first time in history, we knew how big the world was,

but most of its surface remained a complete mystery.

Over the following centuries, conquest, commerce,

- and the growth of the Roman empire - and the growth of the Roman empire

Connected countries as far apart as India and Italy.

In Alexandria, a scholar named Claudius Ptolemy

set out to map this new world.

Unfortunately, he didn't have much to work with.

Map making seems to have been

pretty rudimentary in antiquity and relatively local.

Lots of regional maps.

We have very few maps that survive.

Ptolemy worked out a way to unpeel and project

our spherical earth onto a flat map of the world.

His methods and data were so good,

they were used for over 1,000 years.

Even Christopher Columbus

planned his voyages to the Americas

based on Ptolemy's work.

But mapping the earth was dwarfed by

Ptolemy's crowning achievement:

An epic project, using math and geometry

to chart our vast and constantly moving heavens.

And the starting point of this new map

was one of the most fundamental ideas in ancient astronomy.

Every year, we send hundreds of satellites into space.

They orbit our earth at up to 17,500 miles per hour.

Without them, modern life would grind to a halt.

But this space-age technology effectively operates

on an ancient idea.

Satellites behave as if the planet they orbit

is the center of the universe.