On a layman level I'm pretty familiar with the principles of modern physics but there's something about relativity which I'm afraid I do not understand properly. Relativity tells us that non-accelerating motion is relative to the observer. Saying that I am travelling at 100 Km/h relative to you who are sitting still motionless is indistinguishable from saying that it's me who is motionless and you who are travelling at 100 Km/h relative to me. Unless we define the reference frame, one description is physically equivalent and (as I have read frequently and this is the crucial word) indistinguishable from the other. This is pretty straightforward when referring to linear motion, but I'm not sure I get it properly when it comes to gravitational orbital motion (for the time being I'm not going to complicate things by arguing that 'gravitational orbital motion' is 'linear', geodesically speaking). Let's say, Wikipedia says that the ISS is orbiting the Earth at 27,000 Km/h. Of course this is relative to the Earth's close environment (we can not even say 'relative to the Earth's surface' since the Earth is also spinning). An astronaut out on a spacewalk at the ISS feels like floating motionless, due to the absence of aerodynamic drag and acceleration. But can the astronaut really say that he is motionless and it's the Earth (not meaning some particular point in the Earth's surface) which is rotating around him at 27,000 Km/h? Is this description really indistinguishable? By extension the same goes to the fact that both the Earth and the ISS are travelling much faster than that in their orbit around the Sun, not to mention the movement of the Solar system around the Milky Way and the Milky Way's relative to other galaxies. My point is that even if the physical behavior is equivalent, I don't see the two descriptions as indistinguishable. We have a physics theory explaining why the ISS and its astronaut orbit the Earth, why the Earth orbits the Sun, why the Sun orbits the Milky Way and so forth. But if the astronaut is the one considered to be motionless and everything else moving relative to him, we need a physics theory to explain why a hugely massive Earth orbits a tiny astronaut, why a hugely massive Sun orbits a tiny Earth and so forth. Obviously we do not have such a theory. General relativity tells us that both descriptions are equivalent in terms of physical behavior, but surely not indistinguishable from each other. By observing the facts we can deduce that it's the ISS and its astronaut which are orbiting the Earth and not the other way around, can't we? For one description we have a theoretical explanation but for the other one we don't. So the two descriptions are equivalent but surely distinguishable which would contradict the standard interpretations about 'the relativity of motion' which I usually find in popular science books. Or am I missing something here?