Terminology & Information Sources


Metric vs Imperial
In the past, I have tried to quote distances in both metric (kilometers, meters, cms) and Imperial (miles, feet,  inches) units.  The metric scale is used pretty much everywhere outside the US, and is used by scientists in the US.  In the future, therefore, I plan to give distances only in metric units, unless there is some specific reason not to do so.  A km is approximately 0.625 of a mile, and a mile is approximately 1.6 km.  To convert km to miles, take half the km value, then add one quarter of the result; for example, 200 km is 100 + 25 = 125 miles, and 74 km is 37 + 9.25 = 46.25 miles. 

Representing Numbers
I use scientific notation extensively, where the superscript represents the number of decimal places moved to the right, if positive, or the number of decimal places moved to the left, if negative.  Some examples: 2.125 × 108 is 212,500,000.  2.125 × 10-8, on the other hand, is 0.00000002125.  This becomes very important when trying to express really large or really small numbers. 

I use the short scale numbering system that increases from a million by factors of 1,000.  Thus, one billion is 1,000 million or 109, and one trillion is 1,000 billion or 1012.  Beyond those two, I always use the exponent numbering system described above.  The US has always used the short scale, and the UK officially moved to it in 1974.  The long scale is still used outside of scientific circles in most of Europe, however. 

Light-Years & Parsecs
Astronomical distance is expressed in either Light-Years, or Parsecs.  1 light-year is about 9,460,000,000,000 km (or 5,878,630,000,000 miles).  Light-years are used generally only for intragalactic measurements.  On larger intergalactic scales, astronomers generally use the parsec, megaparsec or gigaparsec.  The word parsec comes from the "parallax of one arc second", and is the distance from the Earth to a star or other astronomical object that subtends a parallax angle of one arc second when viewed from Earth.  1 parsec is approximately 3.26156 light-years or 19.174 × 1012 miles or 30.857 × 1012 kilometers.  It first came into use in 1913, and is abbreviated to "pc".  A Kiloparsec (Kpc) is 1,000 parsecs, Megaparsec (Mpc) is a million parsecs and a Gigaparsec (Gpc) is one billion parsecs.

Hubble Constant
The Universe is approximately 13.75 billion years old.  However, due to the "stretching" of space, the radius of the observable universe is approximately 14 Gpc (46.5 billion light-years or nearly 2.7 × 1023 miles!), although no one knows how far the universe extends beyond that.  Note that all distances quoted here assume a value of ~71 km/sec/Mpc for the Hubble Constant, although the latest WMAP figure is 70.5 ±1.3 km/sec/Mpc. 

I quote most temperatures in degrees Kelvin (K).  Zero Kelvin (0K), also call Absolute Zero, is -273.15 Celsius (°C) and -459.67 Fahrenheit (°F).  A change of 1K equals a change of 1°C or 1.8°F.  Thus 0°C = 32°F = 273.15K, or 500°C = 932°F = 773.15K.  Note the degrees symbol (°) is not generally used for Kelvins. 

Expressing Particle Masses
Particle masses are generally quoted in electron volts (eV).  MeV is 1 million electron volts, GeV is a billion electron volts and TeV is a trillion electron volts.  An electron volt is defined as the amount of kinetic energy gained by an electron when it accelerates through an electrostatic potential difference of one volt.  As a unit of energy, to use it to define mass, it should be divided by c2; that is eV/c2  (where c is the speed of light).  This is just a derivation of E=Mc2, but usually the c2 is left off, and taken as understood.  The kinetic energy of a flying mosquito (not its mass; just the kinetic energy of its flight) is approximately 1TeV.  The Large Hadron Collider at CERN is planned for an energy level of 14TeV, 7TeV for each of the two beams, but the LHC concentrates the energy into a pair of protons, which are a lot smaller than a mosquito!. 
For anyone not familiar with scientific terminology, I have defined some of the terminology and concepts that I use.