An analogy might be the horizon. There is no fixed horizon, it is just the boundary of how far your can view, given both the curvature of the earth and the qual

An analogy might be the horizon. There is no fixed horizon, it is just the boundary of how far your can view, given both the curvature of the earth and the quality of your eyes. It is relative to where you are on the earth, and by your altitude. So while it is calculable, it isn't a fixed boundary like a river, or a wall.