- Hey, smart people, Joe here.

If someone walked up to you today and said, "Hey, the sun isn't the center of the solar system," you would think that person is weird.

Like, if they sat down next to you, you'd probably get up and move.

Because the sun being the center of the solar system, the Earth orbiting the sun, that's like elementary school stuff.

Well, today I am that weird person.

The sun isn't the center of the solar system, and it won't be until 2027.

Before you turn off this video thinking I've completely lost my mind, I need to explain two things.

One, I am right, and you'll see why.

Two, when we look at the crazy history of people figuring out what is actually in the center of the solar system, well, there's a really important lesson about where new ideas come from in science and what it takes for a new idea to actually become the truth.

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I'm not sure if I've ever actually said this out loud in a video before, but one of the fundamental principles that I believe in as a science communicator is that you never fully understand an idea until you understand where it comes from.

So let's get into it.

Pretend you're a big shot astronomer in 1400s Europe.

This is what the universe looks like as far as you know.

Earth at the center, and everything revolving around us.

Now, if that were true, if we actually lived in that geocentric universe, what would you see when you step outside to look at the sky?

You would see pretty much exactly what you do see, the sun, the moon, the stars moving across the sky, and the Earth under your feet steady and not moving.

Today, we understand that the stars and planets moving across the sky is a kind of apparent motion created by a moving Earth careening through space.

But here's a hot take.

You can debate in the comments.

If we did actually live in a geocentric universe, you probably wouldn't even notice.

It's very difficult to forget everything we know today about how the universe works.

But back in a time when our only tools for looking at the sky were the two eyes on the front of our face, this makes a whole lot more sense than this.

So why did it take thousands of years to realize that was wrong?

Well, blame this guy.

Aristotle was an ancient Greek philosopher in the mid 300s BC, and here's the universe according to him.

Earth, round, not flat, because Aristotle had seen ships coming over the horizon and Earth's round shadow on the moon, and the ancients weren't dumb, okay?

There's seven bright sky thingies, the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn.

And since they wander across the sky, they're called wanderers, or planetes, moving on invisible spheres around Earth like a Russian doll with the stars out here, like a sparkly candy shell.

We don't feel the Earth move.

So in Aristotle's universe, the Earth didn't move like at all.

Only the heavens moved in unchanging perfect circles, and that part will be important in a minute.

Now, to them, this represented the entire universe.

There was no such thing as galaxies or the big bang.

This was it.

And it was the basis for most of the world's astronomy for the next 2000 years or so.

But it didn't take long for people to notice that Aristotle's model didn't match up with what was actually happening in the sky.

Because ancient people were not stupid, they were really good at observing and calculating things.

They weren't distracted on social media all the time, like all those kids today and me.

Now, astronomers had noticed that planets like Mars sometimes seemed to stop and move backwards in the sky, or in retrograde motion, and then forwards again.

Aristotle's model couldn't explain this.

A new model was needed.

The problem is Aristotle was a BFD, a big famous dude.

We're talking like the original influencer.

You can't just throw his idea away.

We're talking about Aristotle, man.

What if we just tweak the idea instead?

You know, make a few teensy changes.

Well, that's what Claudius Ptolemy did.

Bold move with the silent P, but he was a bold guy.

So we're in the second century AD now.

Alexandria, Egypt, crossroads of all the world's great ideas about nature and the universe.

To explain retrograde motion, Ptolemy said, as they orbit the Earth, the planets are spinning on their own circles called epicycles.

What are the planets spinning around?

Doesn't matter.

Don't think about that, okay?

People had also figured out that the time between the spring and fall equinox was about a week longer than from the fall to spring equinox.

If only there were a simple and elegant way to explain that.

Hmm, nope.

Ptolemy said, let's just nudge Earth a little bit so it's not totally in the center, and everything rotates around this invisible magic point instead.

Now, Earth is still mostly in the middle unmoving because that's how Aristotle said it should be.

But all this circles on circles business is really getting quite strange.

Ptolemy's math was actually pretty impressive.

In terms of calculating the positions of planets, this was an accurate enough model.

It was just super wrong.

In fact, when it comes to complex elegant theories based on totally wrong assumptions, this might take the cake.

Ptolemy's astronomy book ended up being one of the most important science texts in history, and it basically defined how people in Europe and the Islamic world interpreted the sky for the next thousand years.

That's partly because there were some pretty powerful people out there with a strong interest in calculating astronomical events, the Catholic church.

Now, today, Christian holidays like Easter happen whenever your kittens-as-K-pop bands calendar says that they do.

But in the Middle Ages, the church had to literally tell everyone when Easter was supposed to happen.

Quick sidebar.

Let's talk about Easter.

(energetic music) The church had declared that Easter would fall on the Sunday after the first full moon after the spring equinox, and back in the year 325 at the Council of Nicaea, the church had declared that the spring equinox was always on March 21st.

Problem was they were using a Roman calendar where the year was 11 minutes too long.

So after like a thousand years of that, the actual equinox, like the astronomical event was happening way earlier than the church said it should.

Plus, the Bible claimed that the Earth didn't move, just like Aristotle said.

And whether you're a peasant or a planet, you didn't question the church.

Luckily, the church employed some of the world's best astronomers and mathematicians.

Like this guy from Poland, Mikolaj Kopernik, better known by his Latin name Copernicus.

Around 1500, Copernicus, was studying the cutting-edge astronomy of his time.

So he was likely exposed to the works of Islamic, Persian, and Indian astronomers who were making more and more precise measurements of the sky.

And they had noticed that Ptolemy's geocentric model of the universe, it wasn't matching the math.

Over the next few decades, Copernicus calculated and recalculated and re-recalculated the heavens, and the result was a new heliocentric cosmology with the sun at the center, the Earth moving around it rotating on its axis and all the other planets doing the same.

Contrary to what you might have heard, the church was totally on board with Copernicus, at least for a while.

And heliocentrism hit the best-seller list in 1543 with this book, which has a long Latin name that I'm not gonna attempt out loud.

Nobody is quite sure who the first person was to place the sun at the center of the solar system.

I mean, there are writings as far back as the 3rd century BC.

A guy named Aristarchus had the Earth going around the sun spinning on its axis.

But none of those early attempts to challenge Aristotle's universe caught on until Copernicus.

It could have been his position in the church.

Maybe history just aligned and the world was ready.

Whatever it was, Copernicus's idea changed the universe, literally.

What's notable about these early models is they really ignore why any of this is happening, right?

I mean, all these models, even Copernicus's, really, their point was giving people a way to calculate the positions of things in the sky.

They probably weren't really meant to show the universe as it actually is.

But after Copernicus, that was about to change.

Now, that we had the sun and Earth where they should be, a guy named Kepler came along and figured out that the reason planets sometimes move faster and slower through the sky is because orbits are ellipses not perfect circles.

An Italian fellow named Galileo started playing with these and noticed other planets have their own moons, and Venus has phases just like our moon does.

That only works if everything's orbiting the sun.

And this guy Newton, maybe you've heard of him, he said these things aren't held in place by magical crystal spheres.

It's something called gravity.

After almost 2000 years being stuck on Aristotle, by 1700, the universe had been totally redefined a few times.

And this brings us to why heliocentrism isn't totally accurate.

Because if you keep going down Newton's road, you realize that any two things with mass exert gravity on each other.

The apple pulls on the Earth a minuscule amount, just like the Earth pulls on the apple.

This means the Earth and the moon are actually both orbiting a point in between the two of them.

Jupiter and the sun are orbiting a point way out here.

And when you factor in the mass and position and pull of everything in the solar system, the gravitational center of the solar system actually looks like this, a point moving through space.

Right now, at this moment, the center of everything is outside the sun, and it won't be inside the sun again until 2027.

You know what they say?

- You are technically correct, the best kind of correct.

- So why do we care?

Does this affect your day-to-day life?

Honestly, probably not.

But the biggest thing that science gives us isn't an ability to see things how they are.

It's the ability to make predictions and figure out what will happen in the future and to make really precise predictions like putting space telescopes in orbit or shooting probes to Saturn or whatever.

This is a better truth than this.

We haven't had our last scientific revolution.

We've continued gazing deeper into the heavens with greater and greater precision.

And today we know that our solar system orbits the center of everything in the Milky Way.

And the Milky Way and the rest of our local group of galaxies orbit a barycenter in the Virgo Supercluster.

And that super cluster is tugged on by everything in a bigger supercluster.

All of these are bigger, more precise, more accurate truths.

Perhaps, they're just waiting for us to build something that will make them revolutionary too.

Stay curious.

And before you do anything else, head over to PBS Terra and watch "Overview."

That's right.

I make show besides this one, and you are gonna love it.

We take a whole new perspective on stories, and I mean that literally.

It's the overview perspective.

We've covered giant cranes, kelp forests, lasers.

Seriously, click up there.

It's awesome.

See ya.

Hey, we were just talking about "Top Gun."

How are you guys doing?

Oh, that has nothing to do with today's video.

Okay.

Well, today I am that weird person.