Miscellaneous Units

Discussion

astronomical bodies

light year

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
Δt
 
 
Δs =  Δt  
 
Δs =  (3.0 × 108 m/s)(365.25 × 24 × 3600 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
Δt
 
 
Δs =  Δt  
 
Δs =  (299,792,458 m/s)(365.25 × 24 × 3600 s)  
 
Δs =  9,460,730,472,580,800 m  
 

Some distances in light years are provided below.

parsec

parallax second

human constructions