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
3-j symbol and formula_21 integers. Similar relations exist for the spin-weighted spherical harmonics if formula_22: For formula_25 a non-zero 3-"j" symbol has where formula_27 and formula_28 is a Wigner function. Generally a better approximation obeying the Regge symmetry is given by where formula_30. The following quantity acts as a metric tensor in angular momentum theory and is also known as a "Wigner 1-jm symbol", It can be used to perform time-reversal on angular momenta. Wigner 3-j symbols are related to Racah -coefficients by a simple phase: 3-j symbol In quantum mechanics, the Wigner 3-j symbols, also called 3"-jm" symbols, are an alternative