Thanks to Brilliant for helping support this episode.
Hey Crazies.
Ancient humans really wanted Earth to be the center of the universe.
To be fair, when we look into the sky it certainly seems like the universe is revolving around us.
So it's only natural we came up with wacky models, all just to explain our weird observations.
Of course in modern times, we have a simpler explanation.
Earth revolves around the Sun.
Or does it?
Before you blow the lid off your brain, I'm not saying that modern astrophysics is wrong.
I'm just saying there's a nuance here you may not have considered.
But before we get into that, we need some history.
To the timeline!
There's evidence we were tracking celestial objects as early as 40,000 years ago.
Cave paintings showing phases of the moon were commonplace,
although people that far back didn't know the Earth was a sphere.
We wouldn't figure that out until we started traveling longer distances.
You know, venturing into the ocean and making world maps and stuff like that.
But that didn't stop us from tracking celestial objects in the meantime.
Many historical monuments were built expressly for that purpose.
My point is tracking objects in the sky is a time honored tradition among humans;
mostly just for timekeeping, though, we had to know when to do stuff.
Explaining why celestial objects move the way they do.
Well, that was a bit more niche.
Niche? Niche? Eh, whatever.
Back in ancient Greece, like 2400 years ago, they loved geocentric models.
Geocentric meaning “Earth-centered.” That's the literal translation.
To give you an example: Imagine a model with Earth at the center of the universe.
And because the ancient Greeks were absolutely obsessed with geometry,
Earth was surrounded by eight concentric spheres:
seven for the sky wanderers and one for the stars.
The sky wanderers? Yep, and I'm sure you've heard of all of them.
From inside to outside:
the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
Aren’t most of those planets?
Yeah, and this is actually where the word “planet” comes from.
Planetos means “wanderer” in ancient Greek.
Okay okay. So why are they called wanderers?
Because they wander against the background stars.
Let's say one night we observe Mars to be here in the sky.
We can record its position in relation to all the surrounding stars.
But then the next night we check again and notice its position is changed.
It was here last night, but now it's here tonight.
Which seems a little sus, so we keep checking to see what happens.
On night number three, it's here.
Night four, here.
Night five, here. And so on. Mars wanders.
The same goes for the other six planets.
Yes, they called both the Moon and the Sun planets back then.
So stop being grumpy about Pluto being reclassified.
The Moon and the Sun were reclassified too, and we don't think less of them.
Anyway, each of the seven wanderers were on their own rotating sphere.
It creates a mechanism that shows how they can wander.
But it didn't take us very long to figure out this was wrong.
All you have to do is watch Mars for a little while longer and you'll notice something strange.
At very regular intervals, it backtracks a little before continuing on its way.
We call this retrograde motion
and the concentric sphere model doesn't explain it.
There's no mechanism for it.
I suppose they could have said a god steps in and does it, but that wouldn't really be the ancient Greek style.
While they did believe in powerful gods, those gods were also messy,
imperfect, and never said to be the creators of the universe.
Would you want a being like that messing up the perfection of nature?
Which finally brings us to Ptolemy. Back to the timeline!
It took about 530 years, but a guy named Ptolemaeus (or Ptolemy for short) proposed a fix.
Rather than spheres, Ptolemy imagined planets revolved around the Earth on circular orbits...
although he didn't call them orbits and a single orbit still didn't explain retrograde motion,
a type of motion that five of the seven wanderers exhibited.
So he didn't stop at a single orbit for those five.
For each of the planets, Ptolemy imagined one large circle around Earth called the deferent.
A point on this deferent revolved around the Earth, but that point wasn't the planet.
The planet itself was on a second circle called an epicycle, centered on that point.
Each planet orbited along its epicycle, while the epicycle orbited along its deferent.
Tuned properly, he could explain retrograde motion.
Sometimes the planets wandered backward.
It also explained why their brightness changed.
Sometimes they were closer to Earth, sometimes there were farther away.
So, success? Not so fast.
Watching a planet like Mars very closely, you'll notice another minor detail.
It's not always going the same speed.
For Ptolemy's model to work, the planets had to change orbital speed throughout the year.
Unacceptable!
So what did he do? More epicycles!
Actually, no, that's a historical myth.
The way the story's usually told is that Ptolemy added epicycle upon epicycle until it worked.
Would that have worked? Absolutely.
It's just Fourier series. It'll work on any shape.
But that wouldn't be discovered until 1807 CE.
Ptolemy didn't do this. His orbits never had more than one epicycle.
It's a myth.
But he was definitely in a bit of a pickle.
He needed a solution, and he opted for one a little simpler than extra epicycles.
Ptolemy zoomed in on Earth and asked himself:
What if the Earth isn't perfectly at the center?
What if it's off just a tiny amount?
It's still center-ish, just not perfectly centered.
Of course, this didn't go over well at first.
But this tiny fix actually worked,
at least to the precision they were capable of measuring at the time.
In fact, this model was so good at predicting the motion of celestial objects that it was essentially the word of the gods for 1500 years.
Oh, the arrogance.
But predictability alone does not make a model correct.
Sun-centered or heliocentric models are far superior.
They're not only simpler, they're more accurate, too.
And they predicted later observations like stellar parallax.
That's when a nearby star wobbles slightly against the background stars.
That wouldn't be possible if Earth didn't move through space.
Wait, didn't you start this whole discussion by saying the Earth could be at the center?
Did I?
Earth revolves around the Sun.
Or does it?
Oh, I guess I did. Where were you going with that?
Reference frames.
Those ancient Earth-centered models were a mistake.
But the mistake wasn't that Earth was at the center.
It was that they equated the center with importance.
And we still kind of make this mistake sometimes.
These days we say the Sun is at the center of the solar system.
But what we should really be saying is that the Sun is in control of the solar system.
Newtonian physics tells us that mass causes gravity
and that gravity is what controls a planetary system.
The Sun contains 99.8% of the mass of the solar system,
which means it's always in control. That can't be disputed.
But that doesn't mean the Sun has to be at the center.
It just means putting the Sun at the center results in the simplest picture.
A fundamental truth of the universe is that all objects observe themselves to be stationary.
Motion is measured from a reference, and the easiest reference is oneself.
Putting Earth at the center of your measurements is a perfectly valid point of view.
It's the point of view of all humans standing on the Earth.
You can center your coordinates wherever you want,
as long as you obey the laws of physics.
Do you want Earth to be the center of the solar system?
That's fine, as long as the Sun is still in control.
What would that look like? Complicated.
If we stick with tradition and put the Sun at the center, the model is simple.
All the planets, including Earth, orbit the sun in elliptical paths
just as Johannes Kepler imagined in 1604 CE.
Retrograde motion is caused by the motion of each planet relative to a moving Earth.
To give you some perspective, I've stabilized the sky in this clip.
Our planet Earth is a giant rock tumbling through space.
If we shift our coordinates, though, the paths of the planets aren't so simple anymore.
This isn't particularly beautiful, but it's physically accurate.
And it's what we see from Earth.
From this point of view, retrograde motion happens because of a complicated path relative to a stationary Earth.
This is the point of view we actually have where the sky moves around us.
All we did was change the coordinates and coordinates are just a tool.
Which coordinates you'll want to use depends on the question you're trying to answer.
If you like science questions, you should check out today's sponsor Brilliant.
Brilliant is a tool for learning STEM interactively.
You can play around with concepts in real time.
Working with science yourself is honestly the best way to learn new skills in math, science, and computer science.
If you like this video, you'll probably like their course on gravitational physics.
There's a whole section full of lessons and quizzes on orbital mechanics.
You can learn about how the planets orbit in ellipses and about the math behind those patterns.
If that isn't what you're into, Brilliant has thousands of lessons with new content added monthly.
It's college-level content for everyone.
If you want to get started with a free trial,
go to Brilliant dot org slash Science Asylum or click the link in the description below.
The first 200 of you will get 20% off Brilliant’s annual premium subscription.
It’ll also let Brilliant know you heard about them from me, which helps out the channel.
So who's the center of your universe?
Let us know in the comments.
Thanks for liking and sharing this video.
A special thanks goes out to all my Patreon patrons and YouTube members for all their generous support.
Don't forget to subscribe if you'd like to keep up with us.
And until next time, remember: It's okay to be a little crazy.
I've been informed by many commenters that phone chargers aren't transformers anymore.
They used to be, you know, back in the ancient days of the nineties and the early 2000s.
But apparently they switched over to something called a switched-mode power supply.
Must have happened when the cords upgraded to USB.
Anyway, thanks for the correction and thanks for watching.