What is the opposite direction anyways? Opposite to what? The original message? What if the two transceivers are rotating around the sun on opposite sides? What

What is the opposite direction anyways? Opposite to what? The original message? What if the two transceivers are rotating around the sun on opposite sides? What is one direction at one point in time is now another direction at another point. Is there a difference between bouncing a signal off of a mirror back to yourself and sending a signal which then gets a response from the receiving party? Is my point coming across? Because like, are we talking about direction relative to a receiver or direction relative to some fixed body in space?