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
Unit distance graph graph so that the edges are exactly the unit distance pairs, may greatly differ from each other: the 2"n"-vertex crown graph may be embedded in four dimensions so that all its edges have unit length, but requires at least "n" − 2 dimensions to be embedded so that the edges are the only unit-distance pairs . It is NP-hard, and more specifically complete for the existential theory of the reals, to test whether a given graph is a unit distance graph, or is a strict unit distance graph . It is also NP-complete to determine whether a unit distance graph