One of the most fundamental pieces of information an astronomer wants to get  
about an object in the universe is always, "How far away is it?"  Alas, distance 
is one of the hardest things in astronomy to measure accurately - everything is so 
far away that calibrations are very difficult and systematic errors abound.  For 
nearby objects, we can use a geometric method called parallax; at larger 
distances, we can try to use Cepheid variables and other kinds of "standard 
candles" that are readily recognizable and have predictable energy outputs.

One unit of distance often used by astronomers is the average distance between 
Earth and the Sun.  This distance is called an "astronomical unit," or AU, and is 
equal to about 149,600,000 km - usually, this is rounded off to 150 million km.

Astronomers also often measure distances using the speed of light
in vacuum, 3.0 * 10^5 km/sec.  The distance that light travels in one second is
called a "light-second"; the distance light travels in one minute is called a
"light-minute"; and so on up.  The distance light travels in one year is usually
most useful, and that's called a "light-year" (ly).

   1 ly = (3.0 * 10^5 km/sec) * (3.16 * 10^7 sec/yr) = 9.5 * 10^12 km

A light-year is thus just under ten trillion km, or about six trillion miles.
Notice that a parsec - defined by a "parallax arcsecond" - is about 3.3 light-
years.  Astronomers tend to use parsecs more often than light-years; generally, 
though, astronomical distances can be conveniently expressed using either unit of 
length.