We're familiar with kilometers (and miles) as measures of distance here on Earth. But when it comes time to measure distances of heavenly objects, it makes more sense to use the units AU, LY, and PC. That is, Astronomical Units, Light Years, and Parsecs. That's because space is REALLY big! This video examines these units that are most commonly used to measure distances in space, and when it makes more sense to use one rather than another.