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 find a unit distance graph "G" in which some pair of vertices are at distance "A" in all unit distance representations of "G" . This result implies a finite version of the Beckman–Quarles theorem: for any two points "p" and "q" at distance "A", there exists a finite rigid unit distance graph containing "p" and "q" such that any transformation of the plane that preserves the unit distances in this graph preserves the distance between "p" and "q" . The full Beckman–Quarles theorem states that any transformation of the Euclidean plane (or a higher-dimensional space) that preserves unit distances must