In sci-fi when an author is talking about space travellers or describing the movement of galaxies they will say something like “movement in space only means anything in relation to another object”. Examples of this would be, a space ship moving away from earth at 100 km/s, or 2 galaxies moving towards each other at one light year per century. I think it would make it easier to describe movement in space if we had three axis that we all agree on and we used 0 km/s relative to the speed of...