# light year

##### Non-SI unit
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

### Definition

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:

$1 \mspace{4mu} \text{ly} = (299 \mspace{4mu} 792 \mspace{4mu} 458 \mspace{4mu} \text{m} \mspace{4mu} \text{s}^{-1}) \mspace{4mu} (31 \mspace{4mu} 557 \mspace{4mu} 600 \mspace{4mu} \text{s})\\ \\ 1 \mspace{4mu} \text{ly} = 9 \mspace{4mu} 460 \mspace{4mu} 730 \mspace{4mu} 472 \mspace{4mu} 580 \mspace{4mu} 800 \mspace{4mu} \text{m}$

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.