Parsec can refer to a couple of different concepts depending on the context: 1. **Astronomy**: In astronomy, a parsec (pc) is a unit of distance used to measure astronomical distances. It is defined as the distance at which one astronomical unit (the average distance from the Earth to the Sun) subtends an angle of one arcsecond. One parsec is equivalent to approximately 3.26 light-years, or about 30.9 trillion kilometers (19.
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The parsec is a distance unit defined by the exact conventionThe astronomical unit is exactly ; one arcsecond is exactly radians. The IAU resolution explicitly records the parsec convention. A megaparsec is one million parsecs. [ref]. [ref].
The familiar description "the distance giving one arcsecond of stellar parallax" uses the small-angle approximation. Exact right triangle geometry instead gives , slightly shorter than the conventional parsec. The relative difference is approximately . This is negligible for the examples here, but distinguishing an exact unit convention from an approximate geometric interpretation prevents a misleading equality.