The Physics
Opus in profectus

Miscellaneous Units

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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.

Start with the definition of speed and solve it for distance. The traditional symbol for the speed of light is c from the Latin word for swiftness — celeritas.

s = ct  ⇐ 
c =  Δs

Numbers in, answer out.

Δs = cΔt
Δs = (3.00 × 108 m/s)(365.25 × 24 × 3600 s)
Δs = 9.46 × 1015 m

Since both the speed of light and the year have exactly defined values in the International System of Units, the light year can be stated with an unnecessarily large number of significant digits.

Δs = cΔt
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.


parallax second

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