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[[File:lighterstill.jpg]][[File:Sun_to_Earth.jpg|right|frame|<center>[[Light]] takes about 8 minutes,19 seconds to reach [[Earth]].</center>]]

[[File:lighterstill.jpg]][[File:Sun_to_Earth.jpg|right|frame|<center>[[Light]] takes about 8 minutes,19 seconds to reach [[Earth]].</center>]]

A '''light-year''' or [[light]] year (symbol: ly) is a unit of length, equal to just under 10 trillion (i.e. 1013) kilometres. As defined by the [http://en.wikipedia.org/wiki/International_Astronomical_Union International Astronomical Union] (IAU), a light-year is the distance that light travels in a [[vacuum]] in one Julian year.

A '''light-year''' or [[light]] year (symbol: ly) is a unit of length, equal to just under 10 trillion (i.e. 1013) kilometres. As defined by the [https://en.wikipedia.org/wiki/International_Astronomical_Union International Astronomical Union] (IAU), a light-year is the distance that light travels in a [[vacuum]] in one Julian year.

The [[light]]-year is often used to [[measure]] distances to [[stars]] and other [[distances]] on a [[Galaxy|galactic]] scale, especially in non-specialist and [http://en.wikipedia.org/wiki/Popular_science popular science] publications. The preferred unit in [http://en.wikipedia.org/wiki/Astrometry astrometry] is the [http://en.wikipedia.org/wiki/Parsec parsec], because it can be more easily derived from, and compared with, [[observation]]al [[data]]. The parsec is defined as the distance at which an object will appear to move one [http://en.wikipedia.org/wiki/Arcsecond arcsecond] of [http://en.wikipedia.org/wiki/Parallax parallax] when the observer moves one [http://en.wikipedia.org/wiki/Astronomical_unit astronomical unit] perpendicular to the line of sight to the [[observer]], and is [[equal]] to approximately 3.26 light-years.

The [[light]]-year is often used to [[measure]] distances to [[stars]] and other [[distances]] on a [[Galaxy|galactic]] scale, especially in non-specialist and [https://en.wikipedia.org/wiki/Popular_science popular science] publications. The preferred unit in [https://en.wikipedia.org/wiki/Astrometry astrometry] is the [https://en.wikipedia.org/wiki/Parsec parsec], because it can be more easily derived from, and compared with, [[observation]]al [[data]]. The parsec is defined as the distance at which an object will appear to move one [https://en.wikipedia.org/wiki/Arcsecond arcsecond] of [https://en.wikipedia.org/wiki/Parallax parallax] when the observer moves one [https://en.wikipedia.org/wiki/Astronomical_unit astronomical unit] perpendicular to the line of sight to the [[observer]], and is [[equal]] to approximately 3.26 light-years.

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.

* about 0.306601 parsecs

* about 0.306601 parsecs

The figures above are based on a [http://en.wikipedia.org/wiki/Julian_year_(astronomy) Julian year] (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[4] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) [http://en.wikipedia.org/wiki/Astronomical_constant System of Astronomical Constants], used since 1984. The DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.[http://en.wikipedia.org/wiki/Light_year]

The figures above are based on a [https://en.wikipedia.org/wiki/Julian_year_(astronomy) Julian year] (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[4] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) [https://en.wikipedia.org/wiki/Astronomical_constant System of Astronomical Constants], used since 1984. The DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.[https://en.wikipedia.org/wiki/Light_year]

[[Category: Astronomy]]

[[Category: Astronomy]]