Gravity or curved space doesn't always
make things fall in.
Sometimes objects are in orbit...like the
Moon around the Earth. 
The Moon is like a giant marble.
Okay, it's actually a ball of rock out in
space.
Why doesn't it fall to Earth? 
Child: Because it orbits.
That's right it orbits the Earth!Absolutely. 
But why is it orbiting?
Now, Einstein's concept: that matter 
curves space around it
also determines how objects move around
massive objects, like the Moon around the
Earth or the Earth around the Sun.
Okay let's see what that means all right?
This is space. Now here's a planet...
here's a planet moving through space.
What kind of a path does it take across
there? Is it straight or is it curved? 
Presenter: You want to try?
Child: Straight
Okay go ahead.  Now let's take a star
like the Sun and let's put it
into space over there.
Wow! What happens to the fabric of space?
Woman: Dips?
Presenter: It dips, you bet!
Now, let's move a planet through this
area of space.
Go ahead. What happens to it now?
Presenter: Is it still going in a straight line?
Child: It goes around goes around.
Absolutely. Okay.
Now, can you make it escape from
 the star, and keep on going?
Child: Yeah
Presenter: Let's let's make it escape-
 like that. Try again?
How fast did we need to try and push
this planet to make it get out of there?
Faster, or slower, than to put it into
orbit?
Woman: Faster
Presenter: Faster,right. The
Earth orbits the Sun because space is
curved around the Sun. Now, why does the
Moon orbit the Earth? 
Because space is curved around Earth, too.
How fast objects move through space due to
gravity, and how much space is curved at
any point depends on two things:
How much mass the central object has, and
your distance from the center 
of that massive object.
Close to the stars, space is 
curved more than it is out here,
where space is curved less. Right? Now if
you're far away out here, in this region
of space and you're in orbit,
are you going to move faster or slower
than when you're in closer?
(Both Visitors) Slower. 
Yeah, exactly right. 
Let's make the star here the Sun.
Let have this marble represent Mars.
And this marble represents Earth
Okay? Now, which planet is actually
farther away from the Sun?
Woman: Mars
Presenter: Mars, right. Exactly.
Which one is going to travel
faster through space?
Boy: Earth
Presenter: Very good! How come?
Woman: It's closer.
Presenter: Were is space curved more?
Boy: Closer to the central object.
Presenter: Exactly: where the Earth is.
So let's try this. Do you see?
Let's try that one more time.
Did you see how that was? 
Do you want to try?
Now how long does it really take the
Earth to go around the Sun once? 
How long is our year?
(Both Visitors):  365 Days.
That's it.
Now Mars only travels about one and a
half times  as far around the Sun
than the Earth does, but Mars has a
year that's almost twice as long.
that's almost twice as long now why do
you suppose that is? 
Boy: Because it's far far away.
Right, Mars is farther away and it has a
little longer distance to go than Earth-
- but space is curved less out where
Mars is and you saw that Mars traveled
more slowly than the Earth traveled.
So when you're farther away from the center
of mass do you orbit faster or slower?
Boy: Slower 
Presenter: Slower, absolutely
Notice that if we have a large planet
orbiting a small star, the star
noticeably moves too. All massive objects
exert the force of gravity
 on their surroundings.
This is one way NASA scientists and
others detect planets around other stars,
detecting the wobble of the star 
caused by orbiting planets.
