Astronomical distances are so large that using meters is cumbersome. For really large distances the light year is the best unit. A light year is the distance that light would travel in one year in a vacuum. Since the speed of light is fast, and a year is long, the light year is a pretty good unit for astronomy. One light year is about ten trillion meters as the following calculation shows.
|Δs =||(3.0 × 108 m/s)
|Δs =||9.46 × 1015 m|
Since both the speed of light and the year have exact defined values in the International System of Units, the light year can be stated with an unnecessarily large number of significant digits.
|Δs =||(299,792,458 m/s)
|Δs =||9,460,730,472,580,800 m|
Some distances in light years are provided below.
- The distance to Proxima Centauri (the star nearest the sun) is 4.3 light years.
- The diameter of the Milky Way (a collection of stars that includes the sun and all the stars visible to the naked eye) is about 100,000 light years.
- The distance to Andromeda (the nearest galaxy outside the Milky Way) is about 2 million light years.
- The radius of the universe (the observable part of it) is 13.7 billion light years.