If the error margin in measuring a star's position on a single photographic plate is $\pm 0.005$ arcseconds, what challenge arises when measuring a tiny proper motion displacement over two decades?
Answer
The measurement becomes statistically precarious
When the expected angular displacement over a long period is minute—only a few thousandths of an arcsecond—it can easily be lost or statistically overwhelmed by the inherent error margin associated with measuring the positions on individual plates.
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