Astronomical Units (AU) and Light-Years (ly) are both measurements of distance, generally used to show large amounts of distance between points. Light-years are better suited for calculating the vast distances between stars and galaxies, while AUs are better suited for calculating distance inside the (relatively) smaller space of a solar system.

One light-year is equal to about 6 trillion miles (9.5 trillion kilometers) and is the distance it takes light to travel in one year. Light, of course, travels at the speed of light -- 186,282 miles per second (299,792,458 meters per second) -- and so would travel about that distance in a perfect vacuum in one year. Because of the enormous distances involved on the galactic scale, light-years are well suited for measurement, although the astronomical society tends to prefer the Parsec, which is about 31 trillion km, to make calculations easier.

One astronomical unit is about 93 million miles (150 million kilometers) and is the approximate distance of the Earth to the Sun. This is a general unit of distance, as the Earth is closer to or farther from the Sun depending on season, but given a plus/minus of about 0.02 AU it is accurate. This unit, being much smaller than a light-year or a parsec, is very useful in calculating and graphing distance within a solar system or other body on the solar scale. Instead of working with fractions of light-years, AUs provide an easier base scale.

One light-year is about 63,281 AUs.