The Physics
Hypertextbook
Opus in profectus

# Miscellaneous Units

## Discussion

### astronomical bodies

• acceleration due to gravity on the surface of the Earth
• 1 g = 9.80665 m/s2 exactly by definition
• re or r = 6371 km
• earth mass
• me or m = 5.9736 × 1026 kg
• jupiter mass (for exosolar planets)
• mj or m = 1.8986 × 1029 kg
• astronomical unit [slide]
• 1 au = 149,597,870.691 kilometers (NASA)
• solar masses
• ms or m = 1.989 × 1030 kg
• galactic year (cosmic year)
• 225 million years, but I've seen 250 million as well
• crab nebula
• "The Crab is arguably the most exhaustively studied celestial object beyond the solar system, and the apparent stability of its luminosity at all wavelengths has made it an attractive reference for calibrating observing instruments. The brightness of other objects is often quoted in "millicrabs." But the startling observation of a powerful gamma-ray flare from the Crab last September belies that vaunted stability, and it challenges the accepted theory of how charged particles are accelerated inside supernova remnants. That theory has its beginnings in Enrico Fermi's 1949 attempt to explain the origin of cosmic rays." doi:10.1063/1.3563807

### 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.

s = vt  ⇐
 v = Δs Δt
 Δs = vΔt Δs = (3.0 × 108 m/s)(365.25 × 24 × 3,600 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 = vΔt Δs = (299,792,458 m/s)(365.25 × 24 × 3,600 s) Δs = 9,460,730,472,580,800 m

Some distances in light years are provided below.

• The distance to Proxima Centauri (the star nearest the sun) is 4.3 light years.
• The diameter of the Milky Way (a collection of stars that includes the sun and all the stars visible to the naked eye) is about 100,000 light years.
• The distance to Andromeda (the nearest galaxy outside the Milky Way) is about 2 million light years.
• The distance to the edge of the universe (the observable part of it) is 13.77 billion light years.

parallax second

### human constructions

• stories — typical multiunit, residential building
• 3 meters
• 10 feet (3.048 m)
• city blocks — gridded cities
• 1/20 mile (80.47 m): New York
• 1/16 mile (100.6 m): Houston, Milwaukee
• 1/8 mile (201.2 m): Chicago
• olympic swimming pools
• 50 × 25 × 2 m = 2500 m3 = 2,500,000 L
• football fields — International, American, Candian or Australian?
• 100 m
• 100 yards (91.44 m)
• cricket pitchs — between the wickets
• 1 chain, 1/80 mile, 22 yards (20.12 m)