The mirrors left on the Moon allow Peter to make a very accurate measurement.
Using a telescope with a built-in laser, Peter can precisely measure the distance from the Earth to the Moon.
We orient the telescope so that it is facing the Moon.
The laser light, coming out of the telescope, then goes directly up to the Moon.
It can be reflected by the reflector and it then comes right back through the tube again,
makes its way through the optics and we sense it inside the building.
OK, there it is. Look at that.
There's a really big crater, you can see the shadow in there.
Timing how long it takes the laser beam to go out and bounce back, Peter can precisely calculate the distance to the Moon.
But trying to hit that tiny mirror, so far away, requires very careful alignment and a bit of perseverance.
We may send out a thousand million billion photons,
whereas coming back... coming back into the telescope might be ten. Ten! Or five, or none!
So it is still a very hard experiment because everything has to work just right.
How accurately can you make that measurement, off Neil and Buzz's reflector and back again?
One to three centimetres. Over a quarter of a million miles.
Over a quarter of a million miles out, quarter of a million miles back.