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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 petameters (ten quadrillion 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 =  s
t

Numbers in, answer out.

s = c∆t
s = (3.00 × 108 m/s)(365.25 × 24 × 3600 s)
s = 9.46 × 1015 m
Δs ≈ 10 petameters

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.

s = c∆t
s = (299,792,458 m/s)(365.25 × 24 × 3600 s)
s = 9,460,730,472,580,800 m

Some distances in light years are provided below.

parsec

parallax second

Solar and planetary units Sources: IAU with contributions from 1NIST, 2IAU, 3BIPM, 4BIPM, 5NASA * If equatorial vs. polar radius is not explicitly specified, then equatorial radius is assumed.
object property value
Sun () radius 6.957 × 108 m
irradiance 1361 W/m2
luminosity 3.828 × 1026 W
temperature 5772 K
mass parameter 1.3271244 × 1020 m3/s2
mass1 1.98841 × 1030 kg
Earth () equatorial radius* 6.3781 × 106 m
polar radius 6.3568 × 106 m
mass parameter 3.986004 × 1014 m3/s2
mass1 5.97212 × 1024 kg
astronomical unit2 (au) 149,597,870,700 m
parsec2 (pc) 648,000π au
day (d) 86,400 s
julian year2 (y) 365.25 day
light year2 (ly) 9,460,730,472,580,800 m
standard gravity3 (g) 9.80665 m/s2
standard atmosphere4 (atm) 101,325 Pa
Mars () sol5 88,775.244 s
tropical year5 668.5921 sol
Jupiter () equatorial radius* 7.1492 × 107 m
polar radius 6.6854 × 107 m
mass parameter 1.2668653 × 1017 m3/s2
mass1 1.89812 × 1027 kg

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