- Earth properties, because we live here and using it for comparisons make sense
- Jupiter properties, because so many exoplanets are similar in size to Jupiter
- Solar properties, because what other star do we know as well
- galactic year (cosmic year): 225 million years, but I've seen 250 million as well
- Mars properties, because Mars has been occupied by space probes for so long we need to know what time it is there
- sol (Martian day)
- mean tropical year (Martian year)
- 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
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 = c∆t||⇐||
Numbers in, answer out.
|∆s = c∆t
∆s = (3.00 × 108 m/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)
∆s = 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 (the 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 13.8 billion light years.
|Sun (☉)||radius||6.957 × 108||m|
|luminosity||3.828 × 1026||W|
|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|
|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|
|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|
- 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 pitches — between the wickets
- 1 chain, 1/80 mile, 22 yards (20.12 m)
- banana equivalent dose
- "Bananas produce antimatter, releasing one positron—the antimatter equivalent of an electron—about every 75 minutes. This occurs because bananas contain a small amount of potassium-40, a naturally occurring isotope of potassium. As potassium-40 decays, it occasionally spits out a positron in the process."