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 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