The Physics
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Opus in profectus

Miscellaneous Units

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Discussion

standard values for the Earth

Earth properties, because we live here and using it for comparisons make sense.

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 characteristic value
Sun
()
radius 6.957 × 108 m
irradiance 1 361 W/m2
luminosity 3.828 × 1026 W
temperature 5 772 K
mass parameter 1.327 124 4 × 1020
m3/s2
mass1 1.988 41 × 1030 kg
Earth
()
equatorial radius* 6.378 1 × 106 m
polar radius 6.356 8 × 106 m
mass parameter 3.986 004 × 1014
m3/s2
mass1 5.972 12 × 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) 9.806 65 m/s2
standard atmo­sphere4 (atm) 101 325 Pa
Mars
()
sol5 88 775.244 s
tropical year5 668.592 1 sol
Jupiter
()
equatorial radius* 7.1492 × 107 m
polar radius 6.6854 × 107 m
mass parameter 1.266 865 3 × 1017
m3/s2
mass1 1.898 12 × 1027 kg

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