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From Wikipedia Article on Parsec:

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System. A parsec is obtained by the use of parallax and trigonometry, and is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, i.e. 1/3600th of a degree.

From Wikipedia Article on Astronomical Unit:

The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun and equal to about 150 million kilometres (93 million miles). However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 1.495978707×1011 m. The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

So, as Star Wars happens in a galaxy far far away, why are they using the average distance between Earth and the Sun as a distance of measure and how do they know it?

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    How do they know what a year is? How do they know what an hour is, or a day? How are they (mostly) speaking English? This is the sort of question that's pretty commonly answered by, "You have to suspend disbelief here." Or you can assume that whatever translates their alien languages into English also handles time/distance/etc terms. – Jeff Jan 11 at 17:55
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    @Jeff If only every SF/F writer was like Tolkien and wrote hefty appendices detailing the original languages, calendrical systems, etc. used by their characters before the "translation" of the story. – Rand al'Thor♦ Jan 11 at 17:57
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    Pretty sure you could argue they don't know what a parsec is. – Misha R Jan 11 at 18:40
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    @Valorum Yeah... I dunno, sounds just a little post hoc :) I kinda feel like, if that was the intent, there are far better ways to write that line than to expect the viewer to piece this together. – Misha R Jan 11 at 19:31
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    How has no answer yet mentioned that the weird part in the definition of "parsec" is not the "parallax" part! The weird part is the notion of dividing the unit circle (τ radians) into 360 little parts, and then subdividing it further into 1296000 teeny tiny little parts called "seconds." Blame the Babylonians. Who lived on... wait for it... Earth. – Quuxplusone Jan 12 at 4:10
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They also use hours to measure time, which are just as Earth-centric. But remember, they don’t actually speak English in a galaxy far far away. Every word that is spoken on screen has been translated from the original language, so presumably the units have been translated into our units as well.

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  • Hours aren't directly determined by our local astronomy; the hour is an artificial length of time into which some cultures of humans decided to divide the natural length of time we call a day. That said, I agree with your answer. – Rand al'Thor♦ Jan 11 at 17:56
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    @Shade A light year isn’t a universal constant because a year isn’t a universal constant. It’s just as dependent on the exact details of the Earth‘s orbit around the sun as the AU and thus the parsec. – Mike Scott Jan 11 at 18:08
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    @Shade Here's one possible, purely speculative explanation: in Galactic Basic, there's a unit commonly used by pilots, travelers, and space enthusiasts when talking about intragalactic astronomical distances, but it's not used very often by anyone else. That means the word for that unit carries a connotation of being a little bit technical. Parsec fills a similar role in English - everyone says "light year" but you're more likely to hear "parsec" from astronomers and, well, scifi fans - so one might argue the translator of the script chose to use "parsec" to keep that impression. – David Z Jan 12 at 10:09
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    @chepner Yes, but the people of the galaxy far far away have no reason to use that distance as a unit of measurement — it has no meaning to them. – Mike Scott Jan 12 at 17:10
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    @chepner: I guess the concern here is how a light year could be a universal constant while a parsec is not (in reference to Shade's comment). Both are based on Earth orbital data, both have exact definitions that approximate said orbital data, both could have been "translated" from the "real" units used in the Star Wars universe, and neither parsecs nor light years were explicitly defined this way until long after the original movie was released. (A parsec is exactly 96,939,420,213,600,000 ÷ π meters by 2015 IAU definition.) – MichaelS Jan 14 at 2:41
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Parsecs, along with most other measurements of space and time in Star Wars most likely derive from Coruscant, the cradle of human civilisation and earliest explorers of the Galaxy.

The orbital period of Coruscant is 128-155 million miles, a fast elliptical orbit of roughly 1.3 - 1.6AU (as compared to Earth's 93 million miles - 1AU).

CORUSCANT

Coruscant orbits relatively far from its small sun, varying from 207 million to 251 million km (128 million to 155 million miles)

Star Wars: Complete Locations (2016)

enter image description here
Star Wars: Absolutely Everything You Need to Know, Updated and Expanded

That being the case, and given that we know that Coruscant's rotational period is 24 hours (giving us the concept of seconds of arc), one PARallex SECond of arc would be a simple matter of calculation and would be similar to an Earth parsec, albeit not identical, probably about 30% larger.

Obviously this presents a canon discontinuity, but not a large one, and easily explained by a writing error in the Complete Locations book. If we assume they meant 155 million kilometers (instead of miles), this distances would be only a few percent difference from the distance from the Earth to the Sun and would give us a parallax calculation that was almost identical.

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    Equal orbital period is not sufficient for a distance of 1AU; your assumption of 1AU holds only if the mass of Coruscant's sun is also equal to the mass of our own Sun. – desertnaut Jan 11 at 18:21
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    @desertnaut - Ah, well my new line of thinking is that it's all messed up. A SW parsec seems to be the same as an Earth parsec, but the AU of Coruscant is wrong to do that measurement. Even though pretty much all measurements in Star Wars come from Coruscant. Either they've measured something else or the writers have just fouled up – Valorum Jan 11 at 19:00
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    @desertnaut - If Coruscant orbits its star at 1.4AU, their parsec should be 40% larger. But it's not – Valorum Jan 11 at 19:05
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    1) how do we know it's not (it's not that we have known waypoints to compare their distance)? 2) if it has nothing to do with the Coruscant's distance from its sun (as you seem to imply here), why including that part in your answer? – desertnaut Jan 11 at 19:08
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    @desertnaut - It's worthy of mention because all distances and times in Star Wars derive from Coruscanti measurements. – Valorum Jan 11 at 19:19
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Legends answer*: Parsecs aren't parallax-seconds, but are (somehow) the same length as real-world parsecs

In the Legends continuity, Coruscant's year (and by extension a standard year) is 368 24-hour days long, not 365. Star Wars: The Essential Atlas (2009) mentions this, as well as confirming that Star Wars parsecs are 3.26 light-years long, the same as Earth parsecs.

I noticed that this seemed like a discrepancy, since a light-year with a longer year would be a different length than real-world light years (even assuming Coruscant otherwise had the same orbit as Earth for the purposes of parallax), so I emailed Daniel Wallace, the author of The Essential Atlas, in order to ask which values were being used. I asked whether A) in-universe light-years were longer than ours, B) Coruscant years were the same length as ours (and just had shorter days), or C) in-universe light-years and parsecs were the same length as ours and just arbitrarily defined.

Here is an excerpt from his reply, from 2009 (emphasis added):

[...] I think our approach to units of timekeeping is to try to make them as equivalent to "real world" units of timekeeping as possible. This is both for the sake of our sanity and for the sake of making the universe something readers can relate to. So, while it's true that West End Games introduced a ten-month calendar, I personally will never use it, and Lucasfilm basically uses a 12-month calendar anyway.

[...]

Therefore, I'd be happiest if it turned out that Coruscant had a 24-hour rotation cycle and a 365-day orbital period, and an AU that is 149,597,870,691 meters. Turns out it doesn't, but we're still going to use real world references to avoid the need to make conversion calculations every time we provide a specific bit of data. If you think about it, the meter is one ten-millionth of the distance from Earth's equator to the North Pole, so technically the meter, kilometer, etc. don't have a basis in the GFFA but we keep using them anyway.

You really hit on the problem when you reference the Decoded episode below. The writers of Decoded (and 99% of all novels, comics, and spinoff products) are going to use "real" figures because that's what they know -- therefore the sane solution is to make all in-universe figures from the GFFA the same as ours.

From an in-universe perspective, I guess this makes it closest to your third solution, namely that parsecs are either arbitrary or based on some other planet's measurements. This is weird, I don't deny it -- but I think it does less damage to the SW universe than the alternative.

Thanks,

Dan

So, it looks like for the purposes of the authors of the Expanded Universe, the answer was closest to C): parsecs are the same as ours but not based on similar measurements. Now that an in-universe year is canonically 365 days, hopefully this is less of a headache for the authors.

*based on correspondence probably not authorized by Lucasfilm

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    Worthy of an upvote (for original research), even if the answer is unsatisfying. – Valorum Jan 14 at 22:00

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