Q&A: General Astronomy and Space Science

Q:
What are light years and why are they used in astronomy?

A:
In astronomy, distances are measured in units of light years, where one light year is the distance that light travels in a year—10 trillion kilometers. For historical reasons having to do with measuring distances to nearby stars, professional astronomers use the unit of parsecs, with one parsec being equal to 3.26 light years.

For a complete discussion on light years and cosmic distance please see http://chandra.harvard.edu/photo/cosmic_distance.html