It's an essential question

that humans have been wondering about for centuries.

But the journey toward an answer

has been a strange one.

If you wanted to know the answer to this question

in third century B.C.E. Greece,

you might look up at the night sky

and trust what you see.

That's what Aristotle,

THE guy to ask back then, did.

He thought that since we're on Earth, looking up,

it must be the center, right?

For him, the sphere of the world

was made up of four elements:

Earth,

water,

air,

and fire.

These elements shifted around a nested set

of solid crystalline spheres.

Each of the wandering stars, the planets,

had their own crystal sphere.

The rest of the universe and all of its stars

were on the last crystal sphere.

If you watch the sky change over time,

you could see that this idea worked fine

at explaining the motion you saw.

For centuries, this was central to how Europe

and the Islamic world saw the universe.

But in 1543, a guy named Copernicus

proposed a different model.

He believed that the sun

was at the center of the universe.

This radically new idea

was hard for a lot of people to accept.

After all, Aristotle's ideas made sense

with what they could see,

and they were pretty flattering to humans.

But a series of subsequent discoveries

made the sun-centric model hard to ignore.

First, Johannes Kepler pointed out

that orbits aren't perfect circles or spheres.

Then, Galileo's telescope caught

Jupiter's moons orbiting around Jupiter,

totally ignoring Earth.

And then, Newton proposed the theory of universal gravitation,

demonstrating that all objects are pulling on each other.

Eventually, we had to let go of the idea

that we were at the center of the universe.

Shortly after Copernicus, in the 1580s,

an Italian friar, Giordano Bruno,

suggested the stars were suns

that likely had their own planets

and that the universe was infinite.

This idea didn't go over well.

Bruno was burned at the stake for his radical suggestion.

Centuries later, the philosopher Rene Descartes

proposed that the universe was a series of whirlpools,

which he called vortices,

and that each star was at the center of a whirlpool.

In time, we realized there were far more stars

than Aristotle ever dreamed.

As astronomers like William Herschel

got more and more advanced telescopes,

it became clear that our sun is actually

one of many stars inside the Milky Way.

And those smudges we see in the night sky?

They're other galaxies,

just as vast as our Milky Way home.

Maybe we're farther from the center than we ever realized.

In the 1920s, astronomers studying the nebuli

wanted to figure out how they were moving.

Based on the Doppler Effect,

they expected to see blue shift

for objects moving toward us,

and red shift for ones moving away.

But all they saw was a red shift.

Everything was moving away from us, fast.

This observation is one of the pieces of evidence

for what we now call the Big Bang Theory.

According to this theory,

all matter in the universe

was once a singular, infinitely dense particle.

In a sense, our piece of the universe

was once at the center.

But this theory eliminates the whole idea of a center

since there can't be a center to an infinite universe.

The Big Bang wasn't just an explosion in space;

it was an explosion of space.

What each new discovery proves

is that while our observations are limited,

our ability to speculate and dream

of what's out there isn't.

What we think we know today can change tomorrow.

As with many of the thinkers we just met,

sometimes our wildest guesses

lead to wonderful and humbling answers

and propel us toward even more perplexing questions.