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 unit of time. Parsecs measure extremely long distances to astronomical objects outside of our solar system. One parsec is equal to about 31 trillion kilometers, and most of the stars visible to the unaided eye in the night sky are within 500 parsecs of our sun.
Also equivalent to 3.26 light-years, a parsec is a nearly unimaginable distance packaged in palatable terms. While it might not seem like a lot, it might help to put it in perspective: it would take photons of light three years to travel one parsec's distance.
In our solar system, kilometers or astronomical units tend to get the job done in measuring distance in space. Yet, for the distance between stars outside of our solar system, a larger unit is necessary. For the vast gulfs between galaxies, astronomers use kiloparsecs, megaparsecs and even gigaparsecs (feel small yet?).
A parsec is defined as: "the distance corresponding to a parallax of one arcsecond." While that might read like absolute gibberish, there is a rational explanation. A parallax is when a foreground object moves compared to a more distant background. By measuring the changing angles from the observer to the star and the background universe, you can calculate the distance.
Learn more about parsecs below.