# Why parsecs that particular distance? **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2002, 5:10pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895 "2002-09-27T17:10:47Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![ageless6](https://avatars.discourse-cdn.com/v4/letter/a/9de053/32.png) [@ageless6](https://boards.straightdope.com/u/ageless6)\ **Post date:** [September 27, 2002, 5:10pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/1 "2002-09-27T17:10:47Z")
I never can remember the distance of a parsec, so I look it up once more and think, oh yeah, 3.26 light years. And then I think once more,“But why are they that weird distance?” Why not 3.5 light years or 4.0 or some easier to work with figure? If it has something to do with parallax, I don’t understand that damned concept either. Won’t someone help this po’ brain cell challenged, cross-eyed, toofless, SF lover to unnerstan? ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [September 27, 2002, 5:17pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/2 "2002-09-27T17:17:13Z")
Lokk at something close with one eye and then the other. Notice how the view changes? for close up stars, we can figure the distance by looking at the star in June and December and doing the triangulation thing. A Parsec is the distance an object needs to be to have one second (1/3600 of a degree) of angle difference with a base leg of twice the Earth/Sun distance (2 AU) I think. Brian ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [September 27, 2002, 5:20pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/3 "2002-09-27T17:20:27Z")
The word is derived from “parallax of one second.” It is equal to the distance having a heliocentric parallax of one second, or to 206.265 times the radius of the earth’s orbit, which is 3.26 light-years (19.2 trillion miles). ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 27, 2002, 5:30pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/4 "2002-09-27T17:30:33Z")
[Here](http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html) is a diagram that describes parallax. Arcsecond may seem like an arbitrary unit as well, but it’s the standard unit of angular measurement in astronomy. The smallest things you can resolve from a ground-based telescope is about one arcsecond. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [September 27, 2002, 5:38pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/5 "2002-09-27T17:38:37Z")
Is that distance measured along the arc (i.e., of a circle, centered on the sun, with a radius 206.265 AUs), or in a straight line? Um, and frankly, why 206.265 AUs, vs. some other distance from the sun? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 27, 2002, 5:46pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/6 "2002-09-27T17:46:19Z")
AU is a measure of distance, not angle. One AU is the radius of the earth’s orbit. I’m not sure what you mean by “along the arc.” If the parallax of a star is one arcsecond, it means the distance to the star is 1/arctan(1 arcsec) = 206,265 AU. It’s just geometry. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [September 27, 2002, 6:09pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/7 "2002-09-27T18:09:08Z")
Okay, let’s try this a different way. Since the distance from Earth to Sun is effectively 93 million miles, the distance from one point in Earth’s orbit to the point exactly opposite that point is 186 million miles, or 2 A.U. Because a star will shift position slightly thanks to the fact that the earth is in two different places 186 million miles apart, that shift is its parallax. Now, given that the ancient Babylonians operated with a hexagesimal system of numbers, we have a circle of 360 degrees, each divided into 60 minutes of arc which are in turn divided into 60 seconds of arc each. This means that one second of arc is equal to 1/129,600 of a circle, if my arithmetic’s accurate. When the parallax of a given object is one second of arc, it’s at one parsec of distance, which is equivalent to 3.26 light years, or 206,265 AU. Note that a parsec is an _inverse_ relationship – an object two parsecs away has a parallax of 0.5 arc seconds, one four parsecs away, 0.25 arcsec, and so on. ---
**Author:** ![ageless6](https://avatars.discourse-cdn.com/v4/letter/a/9de053/32.png) [@ageless6](https://boards.straightdope.com/u/ageless6)\ **Post date:** [September 27, 2002, 7:36pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/8 "2002-09-27T19:36:30Z")
Give that Poly a cwacker! You’re bringing me perilously close to understanding the whole parsec, parallax concept. I’m going to keep shoe-horning it into selected leftover brain cells until it solidifies. Keep on carpin’! ---
**Author:** ![RadioWave](https://avatars.discourse-cdn.com/v4/letter/r/91b2a8/32.png) [@RadioWave](https://boards.straightdope.com/u/RadioWave)\ **Post date:** [September 27, 2002, 7:53pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/9 "2002-09-27T19:53:31Z")
Lets see if I’ve got this straight. If Earth and its evil opposite Earth’ were at opposite points in their (averaged) orbit, they would be 186 million miles apart. A line drawn between Earth, a person standing one parsec away, and Earth’ would have an angle of one second? Is that correct? So how did the Millenium Falcon make it all the way to Kessel in less than 12 parsecs. 😉 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 27, 2002, 8:20pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/10 "2002-09-27T20:20:07Z")
Quoth **toadspittle** : > [@](#): > > Is that distance measured along the arc (i.e., of a circle, centered on the sun, with a radius 206.265 AUs), or in a straight line? I think I see what you’re asking here, and the answer is, it doesn’t matter. Basically, you’re asking whether we’re interested in the angle or the tangent of the angle. For small angles, though, they’re approximately the same, and one arcsecond is easily a small enough angle that you don’t care. It makes much more difference whether you’re talking about “distance from Earth to the star” or “distance from the Sun to the star”, and even that distinction is insignificant. Yes, **RadioWave** , you’ve got it. And the usual explanation for your other question is that Han Solo was an idiot and trying to impress Luke and Ben with technobabble. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [September 27, 2002, 8:26pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/11 "2002-09-27T20:26:47Z")
> [@](#): > > So how did the Millenium Falcon make it all the way to Kessel in less than 12 parsecs. Though I know that you did it for a gag, there are three possible answers: 1. Han Solo and Chewbacca were able, through good piloting and the customizing of the Millenium Falcon’s drive, to utilize a more direct chord across hyperspace that was shorter than the normal 12-parsec distance. (One assumes FTL drive and hyperspace for purposes of discussing short-time interstellar travel in questions like this.) 2. George Lucas has no idea what a parsec is. 3. George Lucas knows quite well what a parsec is, but wanted to depict Han Solo as the sort of braggart who has no idea what a parsec is. 4. This reiteration of the answer to this often-asked question was probably not necessary – but it was fun! 🙂 ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 27, 2002, 8:52pm UTC](https://boards.straightdope.com/t/why-parsecs-that-particular-distance/129895/12 "2002-09-27T20:52:11Z")
Actually, **RadioWave** got it slightly wrong. A person one parsec away will see the Earth and Sun as being one arcsecond apart. While the diameter of the Earth’s orbit (2 AU) is used as a baseline for measuring parallax, the angle measured is divided in half when calculating parallax. Astronomical literature sometimes just gives the parallax instead of the distance. For instance, the [RECONS list](http://joy.chara.gsu.edu/RECONS/TOP100.htm) of the nearest stars (probably the most accurate and up-to-date such list on the net) has two numbers listed for parallax. The first number is the parallax and the second number is the measurement error. To get the distance in parsecs, divide the parallax into one. To get the distance in light years, divide the parallax into 3.26.