The IAU-recognized abbreviation for light-year is " ly", although other standards like ISO 80000 uses " l.y." and localized abbreviations are frequent, such as "al" in French (from année-lumière), Spanish (from año luz), Italian (from anno luce), "Lj" in German (from Lichtjahr), etc. From this, the following conversions can be derived. Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. Definitions Īs defined by the IAU, the light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light ( 299 792 458 m/s). 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 (au) subtends an angle of one second of arc. 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.
Because it includes the time-measurement word " year", the term light-year is sometimes misinterpreted as a unit of time. 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). Map showing the stars that lie within 12.5 light-years of the Sun Ī light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers ( 9.46 ×10 12 km), or 5.88 trillion miles ( 5.88 ×10 12 mi).
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