So while everyone did regular Italian stuff... - The astronomers looked at the sky and studied what the Sun and Moon and stars and planets were up to.

- Good, 'cause I have a question about the Sun, Mr. Galileo the astronomer.

It looks like the Sun is moving around the Earth, - Aha!

But it isn't.

It's the other way around.

First of all, it looks like the Sun is moving around the Earth because the Earth is also rotating as it goes around the Sun.

I'll demonstrate.

Cosmo, would you play the Sun?

[whimsical music] - Rotate with me, Mindy, and you'll see the Sun over there rise up and set down.

- Oh!

When I spin, it looks like Sunspot moves, just like it looks when the Sun moves across the sky!

Cool!

I think I get it, Mr. Galileo.

- Eccellente.

That's Italian for "excellent."

- Okay, but still, how do you figure that the Sun and the planets don't go around the Earth?

- I'll explain.

One night, just like this one, I was looking at the planets in the sky-- which people can do today, too!

All you need is a telescope, or a pair of binoculars.

[otherworldly music] Anyways, I was looking at Jupiter, and I saw four little moons.

Up until that time in history, Earthies thought that all the planets and the Sun orbited the Earth.

Together: Woo-hoo!

We're orbiting the Earth!

- But I, Galileo, realized something wasn't right, because I watched Jupiter's moons every night.

That's when I realized something: something important.

- What?

- Each night, I could see that the moons were in different positions.

They were orbiting around Jupiter!

all: Ooh... - So my hypothesis was that maybe all the planets and the Earth could be orbiting around the Sun, which turned out to be true!

Boo-yah!

Science.

- [chirps] [all cheering] - Molto buono , Cosmo.

That's Italian for "really good, Cosmo."

This is known as the heliocentric theory.

"Helio-" means "sun" in Greek.

The Sun is the center of everything in the solar system.

- Heliocentric.

I love that word!

[light rock music] ♪ Heliocentric ♪ all: ♪ Heliocentric ♪ - ♪ I love the way it sounds ♪ ♪ Have you heard helio- is a word ♪ ♪ For the Sun we spin around ♪ ♪ Heliocentric ♪ all: ♪ Heliocentric ♪ - ♪ Just saying it is fun ♪ ♪ It has a lovely ring ♪ ♪ It means every little thing ♪ ♪ Orbits all around the Sun ♪ all: ♪ Yeah ♪