|Name||Symbol||Quantity||Expressed in terms of SI base units||Expressed in terms of other SI units|
|light year||ly||length||9 460 730 472 580 800 m||9.46… Pm|
The light year, symbol ly, is a non-SI unit of length.
The light year is not accepted for use with the SI.
One light year is defined as the distance travelled by light in vacuum during a time interval of one Julian year, 365.25 days, or 31 557 600 seconds.
One light year is equal to 9 460 730 472 580 800 m, or approximately 9.46 petametres.
The length of one light year in metres is calculated by multiplying the speed of light in vacuum, c, by the number of seconds in one Julian year:
The light year is used in the field of astronomy, primarily to express distances on interstellar and intergalactic scales.
Examples of distances of stars from Earth
|Name||Distance in light years||Distance in SI units|
|Proxima Centauri||4.24 ly||40.1 Pm|
|Barnard’s Star||5.96 ly||56.4 Pm|
|Sirius||8.58 ly||81.2 Pm|
|Tau Ceti||11.9 ly||112.5 Pm|
|Altair||16.7 ly||158 Pm|
|Vega||25 ly||237 Pm|
|Arcturus||36.7 ly||347 Pm|
|Aldebaran||65 ly||615 Pm|
|Betelgeuse||724 ly||6850 Pm|
|Rigel||860 ly||8136 Pm|
|One light year is equal to approximately 9.46 petametres.|
Examples of distances of galaxies from Earth
|Name||Distance in millions of light years||Distance in SI units|
|Andromeda Galaxy (M31)||2.54 Mly||24.0 Zm|
|Triangulum Galaxy (M33)||2.64 Mly||25.0 Zm|
|Messier 83 (NGC 5236)||14.7 Mly||139 Zm|
|Messier 63 or (M63)||29.3 Mly||277 Zm|
|Sombrero Galaxy (M104)||31.1 Mly||294 Zm|
|Cartwheel Galaxy (PGC 2248)||500 Mly||4730 Zm|
|One million light years is equal to approximately 9.46 zettametres.|
The radius of the observable universe is approximately 440 000 zettametres, or 440 yottametres.