> In astronomy, which of the following units of measurement is the longest?Considering gigametre, light-year, parsec and lunar distance, the answer is: parsec.Contrary to what Han Solo says in Star Wars, a parsec is a distance, not a unitof time. Parsecs measure extremely long distances to astronomical objectsoutside of our solar system. One parsec is equal to about 31 trillionkilometers, and most of the stars visible to the unaided eye in the night skyare within 500 parsecs of our sun.Photo credit: Pinterest.Also equivalent to 3.26 light-years, a parsec is anearly unimaginable distance packaged in palatable terms. While it might notseem like a lot, it might help to put it in perspective: it would take photonsof light three years to travel one parsec's distance. In our solar system, kilometers or astronomical units tend to get the job donein measuring distance in space. Yet, for the distance between stars outside ofour solar system, a larger unit is necessary. For the vast gulfs betweengalaxies, astronomers use kiloparsecs, megaparsecs and even gigaparsecs (feelsmall yet?).A parsec is defined as: "the distance corresponding to a parallax of onearcsecond." While that might read like absolute gibberish, there is a rationalexplanation. A parallax is when a foreground object moves compared to a moredistant background. By measuring the changing angles from the observer to thestar and the background universe, you can calculate the distance. Learn more about parsecs below.