The Physics
Opus in profectus

Miscellaneous Units

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Standard values for the Earth

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

Standard values for our Sun

The sun is my favorite star. It's right next door.

Quasi-standard values for some other astronomical objects

The universe has a lot of stuff in it. How can we make measurements of astronomical objects more meaningful?

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

Numbers in, answer out.

s = c∆t
s = (3.00 × 108 m/s)(365.25 × 24 × 60 × 60 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 × 60 × 60 s)
s = 9,460,730,472,580,800 m

Some distances in light years are provided below.


The parsec is a unit used by astronomers to describe distances from the Earth to objects outside our solar system. It a combination of the geometric words parallax and second.

Parallax is the apparent shift in the position of one object (usually one that is nearby) relative to others (usually those that are behind it) when an observer changes location (usually laterally, which is a fancy way to say sideways).

Everyone with two working eyes is familiar with this effect. Make a fist. Stick out one thumb like you are getting ready to hitchhike across America but, instead of holding your arm at your side, stick your arm out horizontally in front of you with your thumb pointing up. Look at that thumb with one eye closed. Now switch eyes. Your thumb appears to have moved relative to the stuff behind it. That's parallax.


By analogy…


The complete range of angles covering one complete rotation were traditionally divided into 360 parts called degrees (symbol °). This number 360 was probably chosen because it has lots of factors to make arithmetic easier. Also the year has around 360 days to it, so it was probably considered lucky. The degree was originally divided into small, or minute, parts called minutes (symbol ′). The minutes were then divided a second time into even smaller parts called seconds (symbol ″). 60 of these smaller units fit into the next bigger unit.

1 rotation =  360 degrees
1 degree =  60 minutes
1 minute =  60 seconds

Or equivalently…

degree =  1360 rotation    
minute =  160 degree 121,600 rotation  
second =  160 minute 13,600 degree 11,296,000 rotation



Solar and planetary units Sources: When not indicated IAU, otherwise 1NIST, 2IAU, 3BIPM, 4BIPM, 5NASA. The values from NASA don't have any offical unit status, yet. * If equatorial vs. polar radius is not explicitly specified, then equatorial radius is assumed.
object characteristic value
radius 695,700,000 m
irradiance 1,361 W/m2
luminosity 3.828 × 1026 W
temperature 5,772 K
mass parameter 1.3271244 × 1020
mass1 1.98841 × 1030 kg
equatorial radius* 6,378,100 m
polar radius 6,356,800 m
mass parameter 3.986004 × 1014
mass1 5.97217 × 1024 kg
astronomical unit (au)2 149,597,870,700 m
parsec (pc)2 648,000π au
day (d) 86,400 s
Julian year (y)2 365.25 day
light year (ly)2 9,460,730,472,580,800
standard gravity (g)3 9.80665 m/s2
standard atmo­sphere (atm)4 101,325 Pa
sol5 88,775.244 s
tropical year5 668.5921 sol
equatorial radius* 71,492,000 m
polar radius 66,854,000 m
mass parameter 1.2668653 × 1017
mass1 1.89812 × 1027 kg

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