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
Pace (unit) it was standardized as the distance of two steps (') or five Roman feet ('), about . There were 1000 paces in the Roman mile, which was named after that distance as the ' or '. The Byzantine pace (, "bḗma") was an adaption of the Roman step, a distance of 2½ Greek feet. The double pace (, "bḗma diploûn"), meanwhile, was similar to the Roman unit, comprising 5 Greek feet. The Welsh pace () was reckoned as 3 Welsh feet of 9 inches and thus may be seen as similar to the English yard: 3 paces made up a