Wolfram Alpha:

Astronomical Unit
-----------------

1 au is roughly the distance from the Earth to the
Sun. However, the distance varies as Earth orbits the
Sun.  The distance changes from a maximum (aphelion)
to a minimum (perihelion) and back again once a year.
It was originally defined as the average of the aphelion
and perihelion, but is now defined as exactly
149,597,870,700 metres (93 million miles).

Parsec
------

A parsec is a unit of length used to measure large
distances to objects outside the Solar System. One
parsec is the distance at which one astronomical
unit subtends an angle of one arcsecond.  A parsec
is equal to about 3.26 light-years.

Minute = 1/60 degrees

Arc minute 1/(60*360) = 1/21,600 degrees

Arc second 1/(60*360*60) = 1/1,296,000 degrees

+       ^
/|       |
/θ|       |
/  |       |
/   |       |
/    |       |
/     |       | = 1 parcec
/      |       |
/     .  .      |
E .     S    .  v
.  .

<- 1au ->