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The '''light year''' (symbol: '''ly''') is the distance that the [[light]] travels in [[vacuum]] in one [[year]]. Although one might think that one [[year]] is defined  as the time for the [[Earth]] to complete one revolution of its [[orbit]] of the Sun, there are still several definitions depending on the frame of reference. According to the recommendations of the [[International Astronomical Union]] (IAU), the [[Julian year]], which has a length of 365.25 days or 31,557,600 seconds, shall be used.
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The '''light year''' (symbol: '''ly''') is the distance that [[light]] travels in a [[vacuum]] in one [[year]]. Although one might think that one [[year]] is defined  as the time for the [[Earth]] to complete one revolution of its [[orbit]] of the Sun, there are still several definitions depending on the frame of reference. According to the recommendations of the [[International Astronomical Union]] (IAU), the [[Julian year]], which has a length of 365.25 days or 31,557,600 seconds, shall be used.


The light year is a unit of distance larger than an [[astronomical unit]] (AU).  It is commonly used to measure interstellar distances, where other units of measure are too small.  It is rarely used in interplanetary distances, because it is too large to be useful.
The light year is a unit of distance larger than an [[astronomical unit]] (AU).  It is commonly used to measure interstellar distances, where other units of measure are too small.  It is rarely used in interplanetary distances, because it is too large to be useful.


Similar units are the [[light second]], [[light minute]], [[light hour]], [[light day]], [[light week]], [[light month]]. As opposed to the light year, whose value more or less depends on the definition of [[year]], the values of all these units are defined exactly.  
Also used are the [[light second]], [[light minute]], [[light hour]], [[light day]], [[light week]] and [[light month]]. As opposed to the light year, whose value more or less depends on the definition of [[year]], the values of all these units are defined exactly.  




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* [[velocity]] of [[light]] in [[vacuum]]: 299,792,458 m/s <ref>Review of Particle Physics
* [[velocity]] of [[light]] in [[vacuum]]: 299,792,458 m/s <ref>Review of Particle Physics
  [http://stacks.iop.org/JPhysG/33/1 Particle Data Group: W.-M. Yao ''et al.'', J. Phys. G 33, 1 (2006).]</ref> ([[meter]]s/[[second]])
  [http://stacks.iop.org/JPhysG/33/1 Particle Data Group: W.-M. Yao ''et al.'', J. Phys. G 33, 1 (2006).]</ref> ([[meter]]s/[[second]])
* one [[julian year]] = 365.25 days = 31,557,600 s
* one [[Julian year]] = 365.25 days = 31,557,600 s
* 1ly = 9,460,730,472,580,800 m
* 1 ly = 9,460,730,472,580,800 m




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* 1 [[parsec]] = 3.2616 ly
* 1 [[parsec]] = 3.2616 ly
* 1 [[astronomical unit| AU]] = 15.8×10<sup>-6</sup> ly
* 1 [[astronomical unit| AU]] = 0.0000158 ly = 15.8×10<sup>-6</sup> ly
* 1 ly = 0.30660 [[parsec]]s
* 1 ly = 63,241 [[astronomical unit| AU]]
* 1 ly = 5,878,625,373,183.607 [[mi]]<ref>1 mi = 1609.344 m</ref>  = 5.878625 * 10<sup>12</sup> [[mi]]
* 1 ly = 9,460,730,472,580.800 [[km]]    = 9.4607304 * 10<sup>12</sup> [[km]]
* 1 ly = 9,460,730,472,580.800 [[km]]    = 9.4607304 * 10<sup>12</sup> [[km]]
* 1 ly = 5,878,625,373,183.607 [[mi]]<ref>1 mi = 1609.344 m</ref> = 5.878625 * 10<sup>15</sup> [[mi]]
* 1 ly = 10,346,380,656,803,149.606 [[yd]]<ref> 1 yd = 0.9144 m</ref> = 10.346381 * 10<sup>15</sup> [[yd]]
* 1 ly = 31,039,141,970,409,448.818 [[ft]]<ref> 1 ft = 0.3048 m</ref> = 31.039142 * 10<sup>15</sup> [[ft]]
* 1 ly = 31,039,141,970,409,448.818 [[ft]]<ref> 1 ft = 0.3048 m</ref> = 31.039142 * 10<sup>15</sup> [[ft]]
* 1 ly = 10,346,380,656,803,149.606 [[yd]]<ref> 1 yd = 0.9144 m</ref> = 10.346381 * 10<sup>15</sup> [[yd]]




===Distances in Light-Years===
===Distances in light-years (or other light-time units)===
 
* the average distance between [[earth]] and [[moon]] is about 1.3 [[light seconds]]
* the average distance between [[earth]] and [[sun]] is about 499 [[light seconds]] or 8.3 [[light minutes]]
* the diameter of our [[solar system]] is about 150 [[light hours]]
* the [[Oort cloud]] is approximately 2 light years in diameter.
* the nearest star, [[Proxima Centauri]], has a distance of about 4.2 light years
* the diameter of our [[galaxy]] is about 100,000 light years
* the distance to a galaxy called [[Andromeda nebular]] is about 2.4 to 2.7 million light years


* The maximum distance between the [[Earth]] and the [[Moon]] is about 1.3 [[light second]]s.
* The maximum distance between the [[Earth]] and the [[Sun]] is about 499 [[light second]]s or 8.3 [[light minute]]s.
* The diameter of our [[Solar System]] is about 150 [[light hour]]s or about 6 [[light day]]s.
* The hypothetical [[Oort cloud]] is approximately 2 light years in diameter.
* The nearest star, [[Proxima Centauri]], is a distance of about 4.2 light years from the [[Sun]].
* The diameter of our [[Milky Way]] Galaxy is about 100,000 light years.
* The distance to the naked-eye [[Andromeda Galaxy]] is about 2.4 to 2.7 million light years.


===Notes===
===Notes===
 
{{reflist}}[[Category:Suggestion Bot Tag]]
<div class="references-2column">
<references />
</div>
 
 
===External links===
 
* [http://pdg.lbl.gov Particle Data Group & WWW edition of ''Review of Particle Physics'']
* [http://stacks.iop.org/JPhysG/33/1 2006 ''Review of Particle Physics'' in printed layout]
 
[[Category:CZ Live|Light year]]
[[Category:Astronomy Workgroup|Light year]]

Latest revision as of 06:00, 12 September 2024

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The light year (symbol: ly) is the distance that light travels in a vacuum in one year. Although one might think that one year is defined as the time for the Earth to complete one revolution of its orbit of the Sun, there are still several definitions depending on the frame of reference. According to the recommendations of the International Astronomical Union (IAU), the Julian year, which has a length of 365.25 days or 31,557,600 seconds, shall be used.

The light year is a unit of distance larger than an astronomical unit (AU). It is commonly used to measure interstellar distances, where other units of measure are too small. It is rarely used in interplanetary distances, because it is too large to be useful.

Also used are the light second, light minute, light hour, light day, light week and light month. As opposed to the light year, whose value more or less depends on the definition of year, the values of all these units are defined exactly.


Numerical value


Comparison to other Units

  • 1 parsec = 3.2616 ly
  • 1 AU = 0.0000158 ly = 15.8×10-6 ly
  • 1 ly = 0.30660 parsecs
  • 1 ly = 63,241 AU
  • 1 ly = 5,878,625,373,183.607 mi[2] = 5.878625 * 1012 mi
  • 1 ly = 9,460,730,472,580.800 km = 9.4607304 * 1012 km
  • 1 ly = 10,346,380,656,803,149.606 yd[3] = 10.346381 * 1015 yd
  • 1 ly = 31,039,141,970,409,448.818 ft[4] = 31.039142 * 1015 ft


Distances in light-years (or other light-time units)

Notes

  1. ↑ Review of Particle Physics Particle Data Group: W.-M. Yao et al., J. Phys. G 33, 1 (2006).
  2. ↑ 1 mi = 1609.344 m
  3. ↑ 1 yd = 0.9144 m
  4. ↑ 1 ft = 0.3048 m
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