Relative to our sun, it's possible to calculate the speed at which the Earth is moving through space in it's orbit. So it's possible for an astronaut to position himself in a fixed position just slightly outside the Earths orbital path and observe the Earth passing by at the velocity the mathmatics say it should be traveling. This speed obviously changes depending on what point Earth is in it's eliptical orbit around the sun since it speeds up and slows down during each pass around the sun. Our sun being part of the milkyway is "swirling" around with all the other stars in our galaxy. So relative to the black hole that is theorized to be in the center of our galaxy, it's possible for an astronaut to position himself at a fixed point relative to the center of our galaxy where he could observe the sun passing by as it orbits this black hole. Since the universe is expanding and galaxies are moving away from each other, it should also be possible for an astronaut to position himself at a fixed point in space where he could observe the milkyway galaxy passing by. However, in my two examples above, this "fixed" point is always relative to another existing object. So I'm questioning the possiblity to measure TRUE zero velocity. If an astronaut is floating in a "fixed" position in space, is there any mathmatics that can prove he is indeed staying in one spot? What would be the determining factor of true stillness in space if there is no known point that can be used as a reference?