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
Pearson symbol more commonly used hexagonal setting has 3 translation equivalent points per unit cell. The Pearson symbol refers to the hexagonal setting in its letter code ("hR") but the following figure gives the number of translation equivalent points in the primitive rhombohedral setting. Examples: "hR"1 and "hR"2 are used to designate the Hg and Bi structure, respectively. The Pearson symbol should only be used to designate simple structures (elements, some binary compound) where the number of atoms per unit cell equals, ideally, the number of translation equivalent points. Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as