NARRATOR：

Listen to part of a discussion in a history of science class. The class is discussing the heliocentric theory.

MALE STUDENT：

What I find really difficult to understand is why the heliocentric theory… Um, why it wasn’t, like, believed by everybody right away.

MALE PROFESSOR：

Well, one thing that’s hard to do is to sort of see things from the perspective of someone who’s hearing that theory for the first time.

I mean today we tend to assume that the moment the heliocentric theory was laid out, the idea that the Sun, not the Earth, was the center of the solar system, that, you know, you’d have to be in denial not to accept it.

But it really wasn’t that easy.

MALE STUDENT：

But the idea that the Earth wasn’t the center of the universe… that had been tossed around for, like centuries, right?

I mean, lots of people had had the idea.

MALE PROFESSOR：

Yes, that’s true, going all the way back to the ancient Greeks.

But in Europe when Galileo championed it in the seventeenth century, due in part to his discoveries using a telescope, there still was some major resistance to it.

MALE STUDENT：

But I still don’t understand why. I mean, isn’t it obvious?

MALE PROFESSOR：

Well…, despite Galileo’s ingenious arguments in support of the heliocentric theory, there were still a lot of reasons why people of that period couldn’t buy into it.

Remember, we’re talking about 400 years ago. So, uh, let’s think about a few of those reasons, OK?

So, first of all, they could work out that if the Earth was going around the Sun, then it had to be traveling at many thousands of kilometers per hour.

And that was just beyond anything anyone could understand. You know, they could understand riding a horse or walking.

Maybe they could get up to 30, 32 kilometers per hour, but tens of thousands of kilometers per hour?

That was just crazy. So, to many people, whatever’s going on, it couldn’t be that.

FEMALE STUDENT：

Hmm. So people didn’t believe the heliocentric theory… because it was so hard to believe?

MALE PROFESSOR：

Exactly. But there were more scientific kinds of reactions, as well.

Cause, look, if you’ve ever been on a carousel, or you’re on a ride at an amusement park, and you’re on something that is going round and round and round.

Two things, alright? One, you know you’re moving, there’s no doubt.

And the other thing is, you know, that unless you hold on tight, you’re gonna go flying out because of centrifugal force, right?

FEMALE STUDENT：

So, if I understand you, for the average person 400 years ago, there was no evidence that we’re moving at high speed, right?

Since everything was securely on the ground, and no one was flying off into space?

MALE PROFESSOR：

Yes. And in particular. And this was one specific difficulty for people in the period, even if they thought that there was some sorta force that maybe kept you and me and buildings and things on the surface of the Earth.

Their theory about the nature of the atmosphere was that nothing was holding it down.

So, if, if you can understand that way of thinking, then clearly, if the Earth, was moving at great speed, we should’ve lost all our atmosphere a long time ago.

You know, it would be, like, trailing away behind us.

And so I wanna try a little thought experiment. Because, I, I think that what we’ll find is that some of us have ideas about motion that actually fit with antiheliocentrism.

MALE STUDENT：

Antiheliocentrism! No way. This is the twenty-first century.

MALE PROFESSOR：

Well, then, let’s see. So, picture the following: you’re at the equator moving at sixteen-hundred kilometers per hour, OK?

And you drop something, small and light, like a matchstick, for example. Where’s it gonna land?

MALE STUDENT：

That’s easy. It’ll be long gone. The matchstick is so light, it’ll fly right out of my hand and end up way behind me somewhere.

MALE PROFESSOR：

Ahh…actually, that matchstick you dropped? It’ll land right at your feet.

MALE STUDENT：

What?!

MALE PROFESSOR：

Well, let’s think about it. You forgot to consider that the Earth’s rotating at sixteen-hundred kilometers per hour at the equator…

and you, me, the air, and that matchstick, we’re all moving together at the same speed, even though it doesn’t seem or look or feel like we’re moving.

So class, clearly, even today, we actually have some inclination to think that if the Earth were moving around at a great speed, we oughta see signs of it.

Perhaps now you’re less inclined to dismiss those who once found heliocentrism so hard to believe.

OK, let’s move on…