Light-year Light-year The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres or 5.88 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec
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