Since astrometry is inherently relative, what determines the accuracy of any single measurement?
Answer
The accuracy and stability of the entire reference frame being used
Astrometry does not measure an object's position in an absolute sense; all angular measurements are fundamentally relative, determined by comparing the target star's position against other known background objects. Consequently, the absolute precision ceiling for any astrometric catalog is set by the quality of the fixed coordinate system used as the comparison basis. If the reference frame itself shifts, warps, or is inaccurately measured, those errors are directly transferred to every measurement tied to that frame, irrespective of how superior the observational instrument might be.
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