Say you have a gyroscope, rotating in the ole' proverbial Star Trek "dead of space", far enough away from any single/multiple point of mass, to where the gravitational attraction of the Universe as a whole would be expressed. Or at least at a point to where the local bodies gravitational attraction would cancel out enough for this experiment. If Mach's Principle is true, wouldn't you expect the axis of rotation of the gyroscope to precess, and if it didn't precess then Mach's Principle couldn't be true? That is because the distribution of matter in the Universe is not isotropic .. there is stucture and clumps of matter however far we look. (at least it is that way out to our gravitational horizon that we can currently see). Then looking at the practibility of doing such an experiment. Do you have any idea of where that point in space would be, and how far from Earth it would be? Have any idea of the amount of precession that would be measured? I've got more questions about Mach's Principle .. this is just a starter!