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
Expansion of the universe an observer it appears that space is expanding and all but the nearest galaxies are receding into the distance. During the inflationary epoch about 10 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 10 (an expansion of distance by a factor of at least 10 in each of the three dimensions), equivalent to expanding an object 1 nanometer (10 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 10 m or 62 trillion miles) long. A much