# Whats a parsec? **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 1999, 4:03am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450 "1999-08-11T04:03:00Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![falcon2](https://avatars.discourse-cdn.com/v4/letter/f/dbc845/32.png) [@falcon2](https://boards.straightdope.com/u/falcon2)\ **Post date:** [August 11, 1999, 4:03am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/1 "1999-08-11T04:03:00Z")
I knew it was 3.3 c-yr, and looking it up its says some object having a parallax second. What the heck does that mean? Theres nothing out there a parsec away, the closest star is over 4c-yr away. ---
**Author:** ![Jothar](https://avatars.discourse-cdn.com/v4/letter/j/ee7513/32.png) [@Jothar](https://boards.straightdope.com/u/Jothar)\ **Post date:** [August 11, 1999, 4:12am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/2 "1999-08-11T04:12:00Z")
In Star Wars it was a time measure- “I made the Kessel run in under 12 parsecs” ---
**Author:** ![Undead\_Dude](https://avatars.discourse-cdn.com/v4/letter/u/e19adc/32.png) [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)\ **Post date:** [August 11, 1999, 4:23am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/3 "1999-08-11T04:23:00Z")
I believe that means that an object at that distance from the Earth would appear to change it’s position in the sky by one second (i.e. 1/3600th of a degree) when measured six months apart (i.e. maximal parallax). ---
**Author:** ![falcon2](https://avatars.discourse-cdn.com/v4/letter/f/dbc845/32.png) [@falcon2](https://boards.straightdope.com/u/falcon2)\ **Post date:** [August 11, 1999, 4:24am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/4 "1999-08-11T04:24:00Z")
Uh yeah. That annoyed me too. I seemed to remember the line “Dont take a light-year to (do somehting)” in more than on b sci-fi. Warning! Lame dialog meter is on the red. ---
**Author:** ![Alan\_Q](https://avatars.discourse-cdn.com/v4/letter/a/a698b9/32.png) [@Alan\_Q](https://boards.straightdope.com/u/Alan_Q)\ **Post date:** [August 11, 1999, 4:24am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/5 "1999-08-11T04:24:00Z")
A parsec is the distance from earth where something would show a parallax (apparent shift in space) of 1 second of arc from opposite sides of Earth’s orbit. It’s a pretty arbitrary distance, because as you say, there’s nothing precisely there–but it is a constant, which makes life easier for the astronomers dealing with those distances. –Alan Q ---
**Author:** ![Undead\_Dude](https://avatars.discourse-cdn.com/v4/letter/u/e19adc/32.png) [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)\ **Post date:** [August 11, 1999, 4:24am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/6 "1999-08-11T04:24:00Z")
Jothar: That quote always bugged me. Chalk it up to poor technobabble. A parsec is definitely a unit of distance. 🙂 ---
**Author:** ![matt\_mcl](https://avatars.discourse-cdn.com/v4/letter/m/7ab992/32.png) [@matt\_mcl](https://boards.straightdope.com/u/matt_mcl)\ **Post date:** [August 11, 1999, 4:44am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/7 "1999-08-11T04:44:00Z")
The official line is that it was intentional that Han Solo screw that up so we would know it was idle braggadocio. Right. 🙂 ---
**Author:** ![lvick](https://avatars.discourse-cdn.com/v4/letter/l/d07c76/32.png) [@lvick](https://boards.straightdope.com/u/lvick)\ **Post date:** [August 11, 1999, 5:13am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/8 "1999-08-11T05:13:00Z")
I knew it was boasting by the look on Alec Guiness’s face, one of the brief actual acting moments of the film, not that there’s anything wrong with that, Larry ---
**Author:** ![bernard](https://avatars.discourse-cdn.com/v4/letter/b/df705f/32.png) [@bernard](https://boards.straightdope.com/u/bernard)\ **Post date:** [August 11, 1999, 12:28pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/9 "1999-08-11T12:28:00Z")
Re: Han Solo’s Kessel Run Well, this will make me look like a techie-geek but what the heck. In Star Wars, the course you plot in hyperspace is of far more importance than your speed. Han Solo plotted a course that ran from Kessel to Corellia in only 12 parsecs which apparently was very dangerous because it sent him near a black hole. Needless to say, since he didn’t have to take a long looping course instead he did the Kessel Run in record time. ---
**Author:** ![Akatsukami](https://avatars.discourse-cdn.com/v4/letter/a/ea666f/32.png) [@Akatsukami](https://boards.straightdope.com/u/Akatsukami)\ **Post date:** [August 11, 1999, 1:39pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/10 "1999-08-11T13:39:00Z")
Alan Q writes: > [@](#): > > It’s a pretty arbitrary distance, because as you say, there’s nothing precisely there–but it is a constant, which makes life easier for the astronomers dealing with those distances. ## Easier still, of course, is that a parsec _can be measured directly_ (measure the parallactic shift at various points in Earth’s orbit, find the line of sight thereby, and invert to get the distance), unlike a light-year, which has to be guesstimated. This is why the parsec is now the preferred unit of distance for all serious astronomy, whilst the light-year is relegated to popularizations and SF. “Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.” ---
**Author:** ![torq](https://avatars.discourse-cdn.com/v4/letter/t/dfb087/32.png) [@torq](https://boards.straightdope.com/u/torq)\ **Post date:** [August 11, 1999, 6:43pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/11 "1999-08-11T18:43:00Z")
> [@](#): > > Easier still, of course, is that a parsec can be measured directly (measure the parallactic shift at various points in Earth’s orbit, find the line of sight thereby, and invert to get the distance), unlike a light-year, which has to be guesstimated. Parsecs can be directly measured for relatively near objects, but distant objects don’t show any measureable parallax shift. After all, you measure parallax with respect to the “fixed stars”, meaning those that are so far away that they don’t seem to change positions over such a piddly distance as 2 AU. Also, it’s not clear to me that the definition of parsec includes a specification of where in the Earth’s orbit it should be measured (remember that it’s not a perfect circle), which makes it a somewhat inexact quantity… and I’d guess that we know the speed of light to a higher precision than we know the length of any arbitrary axis of Earth’s orbit anyway. My copy of the CRC Handbook of Chemistry and Physics includes conversion factors in the definition of “parsec” and says that 3.262 ly = 1 pc. Granted, this may not be an exact equality for all the reasons listed above, but I hardly think it’s fair to call it a “guesstimate”. > [@](#): > > This is why the parsec is now the preferred unit of distance for all serious astronomy, whilst the light-year is relegated to popularizations and SF. Astronomers (and astronomy journals) are, I suppose, free to prefer any unit they want, but the logic seems flawed to me. ---
**Author:** ![Nickrz](https://avatars.discourse-cdn.com/v4/letter/n/bbce88/32.png) [@Nickrz](https://boards.straightdope.com/u/Nickrz)\ **Post date:** [August 11, 1999, 7:00pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/12 "1999-08-11T19:00:00Z")
When all else fails, look it up. This from Merriam Webster: -Main Entry: par·sec Pronunciation: 'pär-"sek Function: noun Etymology: parallax + second Date: 1913 : a unit of measure for interstellar space equal to the distance to an object having a parallax of one second or to 3.26 light-years. That requires this - Main Entry: par·al·lax Pronunciation: 'par-&-"laks Function: noun Etymology: Middle French parallaxe, from Greek parallaxis, from parallassein to change, from para- + allassein to change, from allos other Date: 1580 : the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object; especially : the angular difference in direction of a celestial body as measured from two points on the earth’s orbit. I take the “two points on the earth’s orbit” to mean “maximum separation” or six months, but these definitions by themselves lend no credence to the assumption this is correct. ---
**Author:** ![EnigmaOne](https://avatars.discourse-cdn.com/v4/letter/e/d2c977/32.png) [@EnigmaOne](https://boards.straightdope.com/u/EnigmaOne)\ **Post date:** [August 11, 1999, 9:22pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/13 "1999-08-11T21:22:00Z")
{{{No C&P}}} The parsec, a unit of length commonly used by astronomers (along with AU-astronomical unit), is equal to 3.26 light years. The parsec is defined as the distance at which 1AU perpendicluar to the observer’s line of sight subtends an angle of 1 arc second. How to put this into meaningful geometric terms? Maybe not so meaningful, but plausible (given some license): Place two light sources 93 Million miles (1AU) apart in space. Put the ol’ Enterprise in reverse and back away from one of the lights, in a straight line until the the distance measured between the lights is 1 arc second. Now, ask the helmsman to check the trip odometer and give you the distance you just covered. If he says something like “ **19173103.2 Million miles** ,” which happens to be **1 parsec** , ask him when the Enterprise is due for an oil change. ## If, however, he says something that differs significantly from that figure, have him chart a course to the nearest starship dealership saying, “the odometer is on the fritz,” and hope your warranty is still good. – **Kalél** (The Original **EnigmaOne** ) _Common ¢ for all ages._ ---
**Author:** ![Akatsukami](https://avatars.discourse-cdn.com/v4/letter/a/ea666f/32.png) [@Akatsukami](https://boards.straightdope.com/u/Akatsukami)\ **Post date:** [August 11, 1999, 9:44pm UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/14 "1999-08-11T21:44:00Z")
torq opines: > [@](#): > > I’d guess that we know the speed of light to a higher precision than we know the length of any arbitrary axis of Earth’s orbit anyway. ## About three orders of magnitude, IIRC. But, of course, we have _no idea_ how long it takes a photon to get from Barnard’s Arrow to here. OK, we can measure the parallactic shift, and _calculate_ how long it would take. We can’t measure it directly, though; we _can_ measure the parallactic shift. After all, we know the value of “pi” to far more places than we need to calculate the universe’s circumstance to subnuclear dimensions, given its radius; we just don’t know its radius (actually, it probably doesn’t _have_ a radius, but I think that that’s the “The Shape of the Universe” thread). “Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.” ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [August 12, 1999, 1:30am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/15 "1999-08-12T01:30:00Z")
> [@](#): > > But, of course, we have no idea how long it takes a photon to get from Barnard’s > Arrow to here. OK, we can measure the parallactic shift, and calculate how long it would take. We can’t measure it directly, though; we can measure the parallactic shift. Unless, through some inconcievable advance in technology, we could bounce a radar signal off a star and be able to detect it’s return years later. ---
**Author:** ![Troy\_McClure\_SF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troy_mcclure_sf/32/4150_2.png) [@Troy\_McClure\_SF](https://boards.straightdope.com/u/Troy_McClure_SF)\ **Post date:** [August 12, 1999, 5:30am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/16 "1999-08-12T05:30:00Z")
Bernard: I like the way this man thinks. * * * – JMcC from SFCA [http://members.tripod.com/~weirdstuff/index.html](http://members.tripod.com/~weirdstuff/index.html) ---
**Author:** ![E1skeptic](https://avatars.discourse-cdn.com/v4/letter/e/3da27b/32.png) [@E1skeptic](https://boards.straightdope.com/u/E1skeptic)\ **Post date:** [August 12, 1999, 8:01am UTC](https://boards.straightdope.com/t/whats-a-parsec/2450/17 "1999-08-12T08:01:00Z")
According to “The Facts On File Dictionary of Astronomy”, a **parsec** (short for “parallax second”. Symbol: pc) is …_the distance at which the **semimajor** axis of the earth’s orbit substends an angle of one arc second_.