What is a Parsec?

In Star Wars*, Han Solo assures Luke and Obi-Wan that the Millennium Falcon "made the Kessel run in less than twelve parsecs."

I've decided to blog about this because that sentence implies that a parsec is a unit of time (for example, it'd make sense if you changed the sentence to "made the school run in less than twelve minutes."), which means that there's a misconception here: a parsec is actually a unit of distance (so, in essence, Han could be boasting that his Chelsea tractor "made the school run in less than twelve miles," which doesn't make a lot of sense, unless he's talking about offroading a more direct route through a field, of course).

Important edit: In 2018, nine years after this post was first published - NINE YEARS! - the movie Solo: A Star Wars Story was released. I won't provide any spoilers here, except to say that the previous sentence was remarkably prescient.


So what's a parsec?
A parsec is a unit of distance**, so you'd use it to describe how far away something is, rather than how long it took to get there.

Why the confusion?

The second half of the word 'sec' is also commonly used as an abbreviation for the unit of time, the second (as in, 'I'll be back in a sec,'). I'd guess that this is where confusion may be introduced.


How far is a parsec?
One parsec (shortened to 'pc') is almost the same distance as 31 trillion kilometres*** or 19 trillion miles. Or, if you want it in more space-y terms, it's about 3.26 light-years****, or a bit more than 200,000 AU.


What does 'parsec' mean?

The word 'parsec' is a contraction of 'parallax arcsecond'. It comes from the use of parallax***** to determine how far away from the sun you'd have to be for the Sun and Earth to appear to be one arcsecond apart in the sky.


So Han Solo, epitome of man-ness and worthy role-model for all growing boys (of all ages), was wrong?
Not necessarily. Like all good works of sci-fi with glaring scientific errors, fans are duty-bound to make their hero's apparent mistake fit in with what we know to be right, and show that it is, in fact, us who have been mistaken all along. Here's my effort:

Han was plainly talking about a route which is usually all wibbly-wobbly, possibly for saftey reasons (asteroid fields******, empire blockades, black holes etc), but which Han took a less round-about approach to, being all heroic and daring and all. This would mean that, while most ships took the safe option and clocked up an extra 12pc (at least) on their parsecometer, Han managed to shave off some of that extra distance and get his per-parsec expenses claim down below 12pc. If this was the case (and surely it must be), then the distance reduction, with corresponding danger-level increase, would be worth boasting about in the Mos Eisley Cantina.




Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.




* Episode IV: A New Hope, if you're interested. Which was the first one (Episode I was the 4th. Simple.)
** Miles, kilometers, feet, inches, millimetres and Astronomical Units are other examples of distance units.
*** That's 31,000,000,000,000 km, or 19,000,000,000,000 miles. Ish.
**** Another unit of distance that's commonly misconceived to be a unit of time. If you want to know more about light-years, get in touch and suggest I write a post about them.
***** Next time you're outside, stand still, look at a lamppost and close one eye. Without moving, swap the eye you're using to see. The lamppost has apparently 'moved' with respect to its background. This is a basic idea of the effect of parallax. Beware of wallies who spell it 'parallex'. That's actually an alternative pop/rock band from the US.
****** An arcsecond is a unit of angle measurement (like degrees) that astronomers use because instead of degrees because they're too big. One arcsecond is the same as about 0.0003 degrees. Click here to find out more about it!
****** That's an idea- might do a misconceptions post about those too at some point, as well.

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