There is one explanation in the show that I just don't understand:
the theory behind the experiment with the perpendicular mirrors where they found out that the speed of light is constant (in the late 1800s I think)

It's a bit hard to explain,
so let me start with that image of the ball, thrown on a moving boat:

If you throw a ball against a wall on a moving boat, and along the movement vector of the boat, someone on the shore in the same line of the movement vector of the boat, would see the ball move faster on the way to the wall (speed of the ball plus speed of the boat), and slower on its way ricocheting back from the wall (speed of the ball minus speed of the boat).

Pretty straightforward, no problem so far.

Now, if you throw the ball sideways (perpendicular to the movement vector of the boat),
both you and the observer on the shore would see it moving at the exact same speed, both on its way to the wall and back, right?

So far so good.

However, an observer on the same boat with you
would ALWAYS see the ball moving at the exact same speed,
no matter what direction you throw it in, right?

Since you AND the observer are both moving on the same boat, in the exact same direction and at the exact same speed, the movement of the boat would not at all affect the observed speed of the ball.

So ONLY someone standing on the shore would observe a difference in the speed of the thrown ball, and ONLY if it is NOT thrown perpendicular to the movement vector of the boat.

RIGHT?

Now back to the experiment with the perpendicular mirrors:
- the light is the ball thrown at the wall
- the mirrors are the walls
(one in the line of movement of the earth, the other perpendicular)
- the earth is the moving boat

But hey, all the observers are also on that moving boat (Earth),
so the speed of the ball (light) shouldn't be affected,
no matter what direction it is thrown in.

Where is the 'independent' observer on the shore???
Only a observer who is NOT on the same boat (earth) should observe any difference, RIGHT?

So the fact that both light beams in that experiment traveled at the same speed, does not prove anything, does it???

Sorry, this is a bit long, but I just can't figure it out...
Can anybody help me?!