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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 [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.

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 [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 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.