Parsec Parsec The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to astronomical units. One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun. The parsec unit was probably first suggested in
Unit distance graph graph so that the edges are exactly the unit distance pairs, may greatly differ from each other: the 2"n"-vertex crown graph may be embedded in four dimensions so that all its edges have unit length, but requires at least "n" − 2 dimensions to be embedded so that the edges are the only unit-distance pairs . It is NP-hard, and more specifically complete for the existential theory of the reals, to test whether a given graph is a unit distance graph, or is a strict unit distance graph . It is also NP-complete to determine whether a unit distance graph