## 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 = 109 nm
• 1 Angstrom = 10-10 m = 10-8 cm
• 1 AU = 1.4960x1011 m = 1.4960x108 km (Astronomical Unit)
• 1 ly = 9.4605x1015 m = 9.4605x1012 km (light year)

• 1 pc = 3.0857x1016 m = 3.0857x1013 km = 3.2616 ly = 206265 AU (parsec)

• 1 Mpc = 106 pc = 3.2616x106 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 = 1.9891x1033 gram = 1.9891x30 kg

Time:

• Sidereal Day = 23h 56m 04.09054s
• Solar Day = 24h = 86400 s
• Sidereal Year = 3.155815x107 s
• Tropical Year = 3.155693x107 s

Energy:

• 1 Joule = 2.39x10-1 calorie
• 1 Joule = 10x7 ergs
• 1 Solar Luminosity = 3.826x1033 ergs/s = 3.826x26 Joules/s = 3.826x26 Watts
• 1 Jansky = 10-26 Joules s-1 m-2 = 10-26 Watts m-2