Comment by Gravityloss

Maybe this thought experiment makes it clear. There's a cannon firing straight up from the lunar pole. One cannot observe the explosion charge. The ball goes straight up, and then falls down and goes back to the barrel. The cannon is filmed from afar firing and you can see the whole ball travel.

Then you are shown two films, the normal and a reversed one. Would you be able to tell which one is which, and if so, how?