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 more symmetrical than the C-G coefficient. Since the state formula_5 is unchanged by rotation, one also says that the contraction of the product of three rotational states with a 3-"j" symbol is invariant under rotations. The Wigner 3-"j" symbol is zero unless all these conditions are satisfied: A 3-"j" symbol is invariant under an even permutation of its columns: An odd permutation of the columns gives a phase factor: Changing the sign of the formula_11 quantum numbers (time-reversal) also gives a phase: The 3-"j" symbols also have so-called Regge symmetries, which are not due to permutations or time-reversal. These symmetries