<s class="aside-in"><em>mentioned in 5 topics, 2 evergreens</em></s> #### <s class="topic-title">[[lightyears]]</s> > [!wikipedia] [lightyears](https://en.wikipedia.org/wiki/Light-year) > > A light-year, alternatively spelled lightyear, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km), or 5.88 trillion miles (5.88×1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the time-measurement 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 non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit subtends an angle of one second of arc. > ##### ^dataviews > [!dataview]+ Related unlinked notes > > - [[A light year is not a year when you are travelling near the speed of light]] > - [[Light could circle the earth eight times in a single second]] > [!dataview]- Other unlinked mentions > > - [[Black holes]] > - [[black hole mergers]] > - [[expanding universe]] > - [[universe]]