Hello, I am having one question about whether we can realize we are moving in a system, or we think that everything around us is moving away from us. This question popped up while I was thinking about two let's say planets, orbiting each other... If we are to live on one of them, we'd probably be able to see that the other is moving around us... and doing the calculations in relative coordinates we could understand we are also moving around our CM... However, what about the motion of the CM? In what way could we "realize" that we are not just orbiting each other, but we are also moving through the whole space?