Oftentimes in my attempts to write blog posts I tend to fall back on the science-fiction and fantasy writers’ stories that drew me to study science in the first place. Today is no different. But rather than draw you in with the excitement inherent in a Game of Thrones anecdote only to then hit you with the “SURPRISE, it’s really about science” pattern I’m oh so fond of, today I’ll stick to a scientific peek at the world of science fiction. At the risk of posting another listicle (there’s only two here!) I’ve brought it upon myself to define some terms that you may or may not have overheard in your no doubt voracious consumption of science-fiction over the course of your life. I also may or may not continue this post later with more sci-fi concepts that interest me, in the hope that they will also interest you. Today, the terms I’ve picked are both ways that we measure distance in space. Put any science terms you want defined in the comments!

Beware: comma splices abound.

Parsec triangle formation

Parsec – This one became famous when Han Solo (of Star Wars fame. If I’ve already lost you, please go read one of our other articles on the weather or something) claimed to have “made a Kessel Run in less than 12 parsecs.” But after you learn what a parsec is, you’ll be able to report that Han’s claim isn’t quite based in fact. A parsec, although it sounds like a form of the commonly used term “second” for 1/60^th of a minute, is actually a unit of length. The “second” it’s referring to is an arcsecond. When a circle is split up into units, the first division is degrees. You probably know that there are 360 degrees in a circle. Furthermore, each degree is divided into 60 minutes, and each minute is divided into 60 seconds. Don’t ask me why they chose to divide it up this way. Just kidding, like the complete nerd I am, I looked it up, and apparently it comes from the ancient Babylonians obsession with a base-60 system, and the fact that “minute” used to just mean “really small”.

Back to parsec. A parsec is a portmanteau (did I use that right? I’m a scientist, not a linguist) of the phrase “parallax of an arcsecond.” It’s a way to measure how far away a star is from wherever you are, and it’s based on the astronomical unit (AU, or the average distance from the Earth to the Sun). So think about our distance to the sun, and making a triangle with that distance as the base. If you extended a leg of that triangle all the way out to until the degree measure of that top angle was 1 arcsecond (1/3600 of a degree, that’s really small), the length of that leg would be a parsec. If you’re an American homebody like me, you can just think of it as being 19174000000000 miles. Space scientists, however, need units that are much bigger than the ones we use to get around on Earth, so they use things like “parsec.”

Light year – A light year is more common in the lexicon of today’s society, so it might be that you already know what a light year is. If so, skip the next four sentences. A light year, similar to parsec, sounds like a unit of time, but is more commonly used as a unit of distance. Light travels really fast, and in the classical sense is generally accepted to be the fastest speed anything in our universe travels. So a light year is the distance that light can travel in one year (thereby determining the limit of how far anything could travel in one year). If a planet was 15 light years away, then your spaceship would take at least 15 years to get there travelling at the speed of light.

Our galaxy measured in light years.

Pretty simple. You know what’s related, and, in my opinion, cooler? A light mile. Proposed by the science fiction writer Isaac Asimov in his essay “The Ultimate Split of the Second,” a light mile, in the same vein as a light year and a parsec, sounds like a unit of distance but is actually a unit of time. As a light year defines the distance that a parcel of light travels in one year, a light mile defines the amount of time it would take a light parcel to travel one mile. Incidentally – no light ray pun intended there – this is about 0.0000054 seconds. Take this a step further, and instead of using a mile as the unit of distance, we use a Fermi (otherwise known as a femtometer at 0.00000000000001 meters). A light Fermi is then on the order of a hundredth of a thousandth of a millionth of a trillionth of a second. Forgive me, I didn’t want to write out all those zeros. Trust me that it’s really small. This might seem trivial, but it can be useful in describing events that take place over very short time intervals. For example, the half-life of the radioactive isotope hydrogen-7 would be about 7.5 light Fermis. This rolls off the tongue a little more nicely than 23 x 10^-24 seconds.