## Unit Conversion

A common problem in astronomy is the conversion of a quantity
measured in some units into a quantity based on other units.

There are also several `systems of measurement' in use in physics
and astronomy. The common systems are *mks*, for meter-kilogram-second,
*cgs*, for centimeter-gram-second, and *SI* or the
System Internationale (SI). SI is essentially an expanded mks system that
includes a large variety of other standard units for quantities such
as energy, force and magnetic flux.

#### Here is a list of useful conversions:

Length:

- 1inch = 2.54 cm
- 1 mile = 1.6 km
- 1 km = 1000 m
- 1 m = 100 cm = 1000 mm = 10
^{9} nm
- 1 Angstrom = 10
^{-10} m = 10^{-8} cm
- 1 AU = 1.4960x10
^{11} m = 1.4960x10^{8} km (Astronomical Unit)
1 ly = 9.4605x10^{15} m = 9.4605x10^{12} km (light year)

1 pc = 3.0857x10^{16} m = 3.0857x10^{13} km = 3.2616 ly = 206265 AU (parsec)

1 Mpc = 10^{6} pc = 3.2616x10^{6} ly (Megaparsec)

Angles:

- 1°
= 1 degree = 60' = 60 arcminutes
- 1' = 60" = 60 arcseconds
- 1 radian = 360°
/2p
= 57.3°
= 206265"

Mass:

- 1 kg = 2.20462 lb
- 1 Solar Mass = 1.9891x10
^{33} gram = 1.9891x^{30} kg

Time:

- Sidereal Day = 23
^{h} 56^{m} 04.09054^{s}
- Solar Day = 24
^{h} = 86400 s
- Sidereal Year = 3.155815x10
^{7} s
- Tropical Year = 3.155693x10
^{7} s

Energy:

- 1 Joule = 2.39x10
^{-1} calorie
- 1 Joule = 10x
^{7} ergs
- 1 Solar Luminosity = 3.826x10
^{33} ergs/s = 3.826x^{26} Joules/s = 3.826x^{26} Watts
- 1 Jansky = 10
^{-26} Joules s^{-1} m^{-2} = 10^{-26} Watts m^{-2}