(Hint: A Parsec measures distance, not time.)

When Han Solo bragged about the Millennium Falcon being able to complete the “Kessel Run in less than 12 parsecs,” Obi-Wan and Luke had a right to be a little skeptical.

Parsecs are actually units of distance, not time—and they exist in real life, not just in the Star Wars universe. (Sorry, Han.)

In the way that miles or kilometers are used to measure distances on Earth, parsecs are used to measure distances between objects like stars in our galaxy and between galaxies. Because those distances are so much larger than what we would need to measure on Earth, or within our Universe, parsecs make it easier for astronomers to discuss how far it is between stars or between galaxies, instead of having to describe distances in huge numbers. In fact, one parsec is equal to about 19 trillion miles, or 30 trillion kilometers.

Thanks to Star Wars, you may be more familiar with measuring distances by light years. That is an actual unit of measurement, too: A light year equals a little under 6 trillion miles, or 9.5 trillion kilometers. Professional astronomers, however, tend to use the parsec as the preferred unit of measurement.

Astronomers calculate parsecs using the Sun and the Earth as reference points, and measuring something called a parallax angle. Here’s how it works: Hold your thumb at arm’s length. Blink your eyes, and you’ll see your thumb dart back and forth, relative to objects in the background. That darting is the “parallax” of your thumb. Increase your arm length, and the you’ll see less of that darting. So, the smaller the parallax angle, the larger the distance.

So, the Millennium Falcon completing the Kessel Run is impressive in that 12 parsecs is a huge distance, but it doesn’t really say anything about how long it took him. Would it be possible for Han to make the distance shorter? The theory of special relativity means that if he travels close to the speed of light, the distance he travels gets shorter. It also means that hitting even a tiny meteoroid would tear apart the Millennium Falcon—but never tell Han the odds.