The Parsec ($ exttt{pc}$) is geometrically defined based on the distance at which one AU subtends what specific angular measurement?
Answer
One arcsecond ($ exttt{1''}$), or $ exttt{1/3600th}$ of a degree.
The parsec is the distance at which one $ exttt{AU}$ subtends an angle of exactly one arcsecond ($ exttt{1''}$) when viewed from that distance, linking the unit directly to observable geometry via stellar parallax.
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