When working with astronomical numbers and mathematics, when is it appropriate to use astronomical unit, light year, and parsec?
Use astronomical units for bodies within our solar system, Use light years or parsecs for stars and other more distant objects.
The astronomical unit, or AU, is based on the distance from the Earth to the Sun. This is useful for bodies within the solar system. Pluto is between 30 and 50 AU away.
A light year is the distance it takes light one year to travel. It takes light from the Sun about 5.5 hours to reach Pluto when it is 40 AU a way.
When it comes to starts and other bodies outside of the solar system the AU is just too small. The light year makes more sense. The nearest star is over 4 light years away. A parsec is 3.26 light years so either are good for such distances.