# Talk:Parsec

## Star Wars

Please, anybody, before you mention Han Solo and the Kessel Run here, read the three sections that already throw the question all over the room:

--Thnidu (talk) 02:25, 18 February 2015 (UTC)

## Precision

I was trying to correct the precision of this calculation, but I think it is only a (close) approximation anyway, using the fact that tan(θ)~=θ for small θ. So, I have trimmed the number of significant digits.
Ray Spalding 17:49, 19 Mar 2004 (UTC)

The computation offered here is not immediately clear. I think the use of the approximation tan(θ) ~= θ for small θ is acceptable here, but we should either mention this as a whole, or leave out any form of computation. I prefer the latter, that is, to just state that a parsec is approximately ... km / miles / etc . without any calculations.
Maurice Termeer 08:21, 27 Mar 2006 (UTC)

## Parsec

The derivation of Parsec is wordy and requires people to look up "subtend" to figure out what ur saying. How about this instead: Derivation: create a right triangle with one leg being from the Earth to the Sun and the other leg, the parsec, being from the Sun to a point in space. Imagine being on that point as it moves away causing the angle between the Sun and Earth to decrease. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second.

I feel this is so much cleaner I'm going to put it in. jim foit — Preceding unsigned comment added by 74.77.148.238 (talk) 14:16, 31 January 2017 (UTC)

For the non-astronomers among us, how many miles is a parsec? From a crossword puzzle clue, I'm guessing 19 billion miles. Is that right?

One lightyear = ~5.88 × 1012 miles

One parsec    = ~3.26 lightyears
= ~(3.26 × 5.88) × 1012 miles
= ~19.2 × 1012 miles


— Preceding unsigned comment added by 65.35.122.133 (talkcontribs) 16:07, 25 November 2004

So if you're talking British billion, yes that's right. If you're talking American billion, then no. It would be 19.2 trillion miles in American trillions. -- Derek Ross | Talk 16:21, 25 November 2004 (UTC)
Make that 'Traditional British billion' rather than 'modern British billion'. The Government and BBC would call the above number 19.2 trillion miles - see long scale for further information. Ian Cairns 19:23, 26 May 2005 (UTC)

Wouldn't it make more sense to state its approximate distance in normal units as 30.9 Pm instead of "trillions of kilometres", when names like trillion have different meanings to different people? Then again, why do we need this nonsense unit when expressing all spacial distances in metres with proper prefixes, such as kilo, mega, giga, tera, peta, exa, zetta, yotta can be used?

Whenever someone uses words like parsec and light year, it is always necessary to convert it normal SI units before it is understood. It seems like those who claim to be scientists, rather than acting in an enlightened manner, work in confusion. 68.105.199.216 (talk) 17:05, 7 October 2012 (UTC)

Debates like this are why I never use the words "billion" or "trillion" (or their higher-order relatives) in wiki articles. They are too ambiguous in meaning. Rhialto (talk) 13:15, 8 October 2012 (UTC)

## When to call a billion a trillion

The article now contains the phrase:

and is approximately 19,131,554,073,600 (19 trillion) miles

Someone changed "trillion" to "billion", but I have changed it back. Ray Spalding 06:42, 26 September 2005 (UTC)

What I call a billion you call a trillion. It was probably not vandalism, just a product of the fact that there are two meanings of each of these words thanks to some bunch of clowns in the 17th century. We are probably stuck with this wittled-down trillion in English, though, so "trillion" let it stay. JIMp talk·cont 01:21, 30 May 2008 (UTC)
After reading this discussion on the ways to read 109, I located the article of long and short scales. I personally prefer the short scale (million=106, billion=109, trillion=1012) because of the logical word association of 'bi' meaning 2, 'tri' meaning 3, etc. Using billiard, and trilliard as terminology doesn't make much sense. With no disrespect intended, it's like assigning two different values for a single variable (x=1 and x=2 simultaneously). It's an anomaly; there should only be one use of the 'bi' prefix.
If we do away with the names, and just stick with the long terminology (thousand-million), we could say it is similar in respects to the Roman Numeral system, in which IV means 4, or 1 less than 5, and VI means 6, or 1 more than 5 (except we're working with exponents). Although the long number system appears to be designed for larger numbers, to me it just seems like a messy way to do things. On the other hand, if we use the short number system, it's very smooth and straight forward until we get to the really larger numbers. But by the time we reach long strings of numbers, we would simply drop the word descriptions.
My preference is for the most scientific numbering system (which is why I also wish that the world would do away with the Imperial units and the United States customary units). Having spent my entire life to this point in the U.S., it is natural that I would prefer the short scale over the long scale. Because of my ancestry, I have ties to continental Europe, which mainly uses the long scale. This means that I'd have to either learn both, or drop the terminology altogether and just use the mathematical values for numbers 109 and larger (I doubt I will ever use 9 digits past decimal in all practical use).
Christopher, Salem, OR (talk) 00:44, 11 May 2010 (UTC)
I have been born and raised on the short system, and only recently came to know the long system. Your arguments for the short system, imo, are weak. In the short system where does the 'bi' meaning 2, 'tri' meaning 3, etc come from? A billion in the short system is 1,000,000,000 or 1000^3. Trillion is 1,000,000,000,000 or 1000^4. I don't see a 3 or a 4. In the long system the 'bi' and 'tri' actually DO have meaning, as a billion is 1,000,000,000,000 or 1,000,000^2. Trillion is 1,000,000,000,000,000,000 or 1,000,000^3. In the short system if I asked you for the number of zeros in the number one octillion you would have to do 3*(8+1) = 36. In the long system it would be 6*8 = 48. The long system makes more sense in this manner.
Next you say that using the word thousand-million to mean the number 1,000,000,000 is like the roman numeral system. This just doesn't connect. Both systems use the terms hundred-million, ten-million, etc. What is the difference with using thousand-million? How is that messier? Or anything like the roman numeral system. Beside this is where the milliard and billiard terms come into play as a milliard = thousand-million and a billiard = thousand-billion. They only seem weird because you haven't used them your whole life.
Finally, this is the very reason why the scientific world uses the scientific notation, and wikipedia being international should do the same. It completely does away with these arguments. However, if I had to pick, I would definitely say that the long system is by far the more logical system for the reasons I pointed out above. 24.52.246.245 (talk) 19:17, 1 June 2014 (UTC)
The long system only seems more logical to you because you were born and raised with it. "Thousand million" seems antiquated, fumbling, and awkward to me, logically, because such language constructions tend to be found in times long past to deal with numbers that were bigger than the everyday ones. Ditto "million million". And that seeming (to me) is because I was born and raised on the short system. The argument is really not about logic, and there's no overcoming what is already second nature. It's simply a misfortune that these two varieties of nomenclature exist instead of one. Logically, either one would do. But because there are two, neither of them do. Logically, I support the idea that everyone who uses one system (whichever system) should give it up and use the other. And logically, I know it will never happen. And pragmatically we must face the fact that numbers in excess of 10^12 are not only commonly encountered in everyday life today, but are growing in application.
We could talk about what to do, but the long and the short of it is, only an explicit list of digits or scientific notation are commonly enough understood to make a quantity unmistakable throughout an international community. English WP is in a kind of forefront of international operation among an unusually sized community, but this kind of internationalism has been growing for some time and continues to do so. Likewise, under-awareness of it fosters only an "either/or" attitude based on older systemic incompatibilities. By all means, we should be writing out the digits here, or providing scientific notational shortcuts, and we should deprecate both the long and the short systems above millions. As time goes on, their mutual incompatibility is what is going to do them in, because the more localized contexts where they survive will become subsumed by the need for everyone (not just WP) to deal in larger contexts just as easily. Perhaps then it will be time for another logical naming system to arise. Hmm: "kilo", "mega", "giga", "tera" ...? Could play a part. Evensteven (talk) 21:44, 1 June 2014 (UTC)

Scientific notation of numbers in English the following are the equivalent notations :- Million is 1000x1000 =1,000,000 Milljard 1,000 x 1,000,000 or 1,000,000,000 in American financial terms a Billion A Mathematical Billion is a million millions (from bi-million) 1,000,000,000,000 in American financial terms this is a Trillion Billiard is 1,000 million millions 1,000,000,000,000,000 A Mathematical Trillion is thus tri-million 1,000,000 x 1,000,000 x 1,000,000 or 1,000,000,000,000,000,000 and such

Americans have routine problems working out distances in mathematics, or even scientific measurements - getting simply scientific and mathematics right (which incidentally they misspell as a singular discipline of Math, rather then disciplines of Maths). Their use of miles, when international scientists use Km for distances, and inability to deal with standard international mathematics has caused engineers routinely to cause problems missing targets in space, or having probes crash into Mars before their parachutes deploy or landing thrusters fire, and such - often blamed on a "Martian gremlin", much to the annoyance of their colleagues in the European Space Agency, and NASA leaders. This problem of miscalculating assets was found to be partially responsible for the worldwide financial crash of 2008/9 when American investment in NINJA (No Income, No Jobs or Assets) properties - which were sold on on worldwide markets - the debts and valuations in real terms were a fraction of a percentage of their overinflated real value and thus resulted in a massive right down of losses worldwide, as banks found they were under-capitalised with their loan-books overvalued, and debts exceeding their assets as a result. In the Republic of Ireland, Government bought bonds of banks, and became majority shareholder of many banks. Whole loan-books of buildings were created, which on the basis of the National Asset Management Agency Act became administered by civil servants and bank staff acquired. While the Bill was drafted I (Damon Matthew Wise), noting previous abandoned and unfinished buildings which were stripped down and unoccupied, thus unusable due to vandalism and squatters, proposes, and had included in the act provisions to have housing to be made available to local authourities and housing associations, and commercial and retail premises to Charities, voluntary and community groups and community/cottage industries on 5 or 10 year leases, rent free - the basis of this was to prevent complete loss of value - occupied buildings made available to sitting tenants who take care of them and make productive use will always increase in value, while unoccupied buildings lose value rapidly due to damage, decay and destruction and are most have no usable value within 6 to 18 months. — Preceding unsigned comment added by 109.77.233.95 (talk) 19:17, 31 January 2013 (UTC)

Would it not be better to avoid using ambiguous terms like "billion" and "trillion"? I recently tried to reword "trillion" in the above citation as "million million" - which is an unambiguous way of saying of 1012 - only to be reverted. The problem, as already stated, is that "trillion" can mean either 1012 (short scale) or 1018 (long scale), just as "billion" can mean either 109 (short) or 1012 (long). -- Glenn L (talk) 18:07, 1 April 2013 (UTC)
...except that the "long scale" is antiquated and deprecated in virtually every English-speaking country in the world (e.g. U.S., U.K., Commonwealth, etc.). A reader won't think that "trillion" refers to 1018 any more than they would think a British "penny" refers to 1/240th of a pound, instead of the modern 1/100th.
On the other hand, the term "million million" is confusing, because it's rarely encountered in common English language speech. Have you ever heard anyone state that the U.S. goverment's debt is 16 "million million" dollars? Of course not; that would confuse people. Using "trillion" (and assuming it to be understood as "short scale" trillion) confuses perhaps 0.01% of the readers. Using "million million" confuses the other 99.99% of readers. Shouldn't we use the terminology that causes the least amount of reader confusion? — Preceding unsigned comment added by Hatster301 (talkcontribs) 08:37, 2 April 2013 (UTC)

## Discrepancy

This figure doesn't agree well with other figures in the article. A few lines from the top, it gives that 1 parsec = 3.08568×1016m
That equates to approximately 19,173,500,000,000 miles. This might seem a minor point, but if it's worth stating 12 significant figures in the article, then we've got to make sure that at least the 4th one agrees with the true value.Richard B 12:28, 8 October 2005 (UTC)

I also have a major issue with the precision given here.
• Info box says: "19.1735×1012" miles
• Main text says: "19,176,075,967,324.937" miles.
1. What is the point of giving a value to such precision if all other values in the article only have a precision of 4 or 5 decimal places?
2. The two values don't even agree with each other. If we round the precise - but not very accurate - number to the same precision as the other number, we get: 19.1761×1012, but this value does not agree with the other, which is 19.1735×1012.
Something is SERIOUSLY wrong with the accuracy here. Why bother giving it to 17 decimal places if it disagrees at the fourth place? -- B.D.Mills  (T, C) 06:23, 30 April 2008 (UTC)

That's precision, not accuracy. --Thnidu (talk) 02:11, 18 February 2015 (UTC)

## Picture not to scale

I would recommend adding some kind of notice that the picture [1] depicting the method of how to calculate a Parsec is not to scale. I think it would be great if it could be added directly into the picture, but a note in the text would be fine, too.
--capnez 18:17, 4 Mar 2005 (UTC) 13:05, 4 Mar 2005 (UTC) EDIT: Updated my signature, I finally got an account!

Sure, it's to scale. Just for an extremely nearby star. :-) --Doradus 10:52, 14 June 2006 (UTC)
No, it's not to scale. The angle as it appears in the diagram is far greater than one arcsecond! :-) 24.6.66.193 05:33, 5 August 2007 (UTC)

## Discrepancy in Parsec Definition

The illustration is not correctly drawn as it shows an astronomical object one parsec distant from the Sun at the subtended angle. To the best of my knowledge, the nearest astronomical object that approximates the distance of one parsec from the Sun is Proxima Centauri at 1.2 parsecs distant. In fact the parsec is not a distance between two astronomical objects, the Sun and one other but is the distance from the Son to a point in space at which one astronomical unit subtends an angle of one arc second.

So the two things in the illustration that need to be rectified are: 1. The 'near star' needs to be moved away from the focus of the angle along one of the lines of parallax. 2. The description MUST include the words 'astronomical unit' OR 'radius of Earth's orbit'

e.g. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. OR A parsec is the distance from the Sun to a point in space at which an astronomical unit subtends an angle of one arcsecond.

Brian Jones, bdcjones@optusnet.com.au — Preceding unsigned comment added by Briandcjones (talkcontribs) 10:19, 16 July 2018 (UTC)

There is a discrepancy on this page in the definition of a parsec. Near the top, it states: "The parsec is defined to be the distance from the Earth of a star that has a parallax of 1 arcsecond." But on the diagram towards the bottom, the parsec is labeled along the side of the triangle (which is the distance between the Sun and "D"), implying a parsec measures the distance between the Sun and another object, not between the Earth and that object.

Something doesn't add up. These two distances are quite different. What is the exact definition?

Globe199 13.00, 26 May 2005 (CDT)

Well, distance from the Solar System is the point. An object 1 parsec from the Sun is 206,265 AU away. Its distance from the Earth is 206,265 ± 1 AU; I wouldn't call that "quite different".
—wwoods 18:56, 26 May 2005 (UTC)
OK, so you're saying it doesn't really matter, since the distance is almost the same; we're talking about distance on a galactic scale, so ± 1 AU is negligible. Sounds good. On the other hand, at what point is parallax useless? The page on Deneb says the distance is between 1600 and 3200 light years because "...determination of distances at this range is very difficult because stars with such distances have negligible parallax."
Globe199 15.57, 26 May 2005 (CDT)
"On the other hand, at what point is parallax useless?"
All depends on how accurately you are able to measure the parallax. The ESA's Hipparcos mission was the most accurate that parallaxes had been measured. It measured the parallaxes of stars to a precision of a few milli-arcseconds. This means that it could measure distances reasonably accurately to a few hundred parsecs. The ESA's Gaia_probe scheduled for launch in 2011 aims to measure the parallaxes of bright stars to a few micro-arcseconds, and fainter stars to a slightly lower accuracy. With this, you could measure distances reasonably accurately to a few tens or even hundreds of thousands of parsecs. You could even measure parallaxes of some stars in some other galaxies.Richard B 11:34, 9 October 2005 (UTC)

..Earth's orbit as a baseline. The parsec follows naturally from this method, since the distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds). But isn't the earth's orbit 2AU? So isn't this a factor of 2 out? Mat-C 04:36, 28 October 2005 (UTC)

That's exactly what I thought, but then I noticed that our diagram calls the "parallax angle" half the apparent motion of the nearby star. That makes it all work out. Seems a weird definition, but who can explain astronomers... --Doradus 10:54, 14 June 2006 (UTC)

I would like to know if it's the distance between our sun and the object, or us and the object. I know it seems neglible but I'd like a more accurate definition. As far as i know a parsec is the distance of an object with a heliocentric, stellar parralax of one second of arc....The distance of the object from what???

## Hopeless

→Completely agree, the first paragraph makes very little sense. I think the author(s) are assuming knowledge I don't have.

## 4,000,000,000,000,000 kilometers?

4,000,000,000,000,000 kilometers doesn't fit here, does it?

3.08567758 E+16 m = 4,000,000,000,000,000 kilometers is not correct. First of all there are two too many zeros. Secondly, if 1 km = 1000 m then the result would start with a 3, not a 4.

3.08567758 E+16 m = 30856775800000000 m = 30856775800000 km

Or am I missing something here???

Midavalo 8 March 2006

## poorly explained

this article is good however the first paragraph which is crucial for the understanding the rest of the page is not very well explained. Actually it is pretty confusing. Fine a parsec is 3.26 ly but how that diagram determines that and what near star are you refering to? Krnchris 21:09, 15 May 2006 (UTC)

Any star. The imaginary star in the diagram is exactly 1 parsec away. -- Xerxes 00:24, 16 May 2006 (UTC)

## Error in International Units calculation

I have ammended the number of metres in a parsec as the previous value was incorrect. The figure now matches that derived later in this page. Mrfunkyostrich 16:10, 11 August 2006 (UTC)

Is a parsec/secpar 299792458*3600*24*365.25*3.26156378 metres???

## Han Solo

So how are we going to fit in Han's comment about doing "the Kessel Run in less than 12 parsecs" ? -- Beardo 01:04, 23 September 2006 (UTC)

I have heard this rationalized. If the objective of the Kessel Run is to complete a task in the minimum distance, then it makes perfect sense. (But I think Lucas screwed up.) Srain 02:12, 24 September 2006 (UTC)
Indeed, according to the (obviously rationalized) explanation of the Kessel Run, the objective is to smuggle spice from the mining colony Kessel through a nearby cluster of black holes and quantum singularities. Being such a difficult area to navigate, if you CAN do it you get away scott free, which leads to a competition between those that perform the "Kessel Run" to compare how efficiently they got through, i.e. how close they got to a black hole. Hence, 12 parsecs is a goood score. Still stupid though.

Shouldn't this reference be removed? After all, the technology behind hyperspace travel in Star Wars is unknown. If one thinks of warping space for faster-than-light travel, then a distance might be totally reasonable. To say it is "incorrect" seems a bit much, and without bringing up this question, it doesn't seem to have much use in the wiki entry.

The reason why Han Solo was boasting about that is because the Kessel Run has a greater distance than 12 parsecs, so when he was escaping from Imperials, he got so close to the one the Maw's blackholes that he actually warped distance.--209.244.43.103 (talk) 00:56, 3 October 2008 (UTC)

I think the mention is notable enough to belong in the article; and the article is not allowed to claim it's "incorrect" because the apologists, as above, may be right. But it could be "claimed to be incorrect" or "controversial". Tempshill (talk) 20:11, 18 November 2008 (UTC)
Does anyone watch the bonus features? They state in the bonus features that Han said that to brag about the capabilities of the Millennium Falcon's hyperdrive. They do in fact use warp technology, and the Millennium Falcon has a very good warp drive. They also say that the look on Ben's face is testament to the insanity of the statement. But it was not a mess up, it was the fact that he had a bitchen warp drive. John Holmes II (talk) 02:24, 23 December 2009 (UTC)

here how it works(this is the officail reasoning, you can check) the kessel run goes threw a black hole infested area. if you get too close to a black hole your screwed. he went so fast, he went past whats supposed to be the point of no return but was going so fast the ship just kept going. this shaved somedistance off. (end offical stuff) smuggelers and such knew what he ment but most people who dont realize what the kessel run is just think hes nuts, which would explain ben's look. 69.115.204.217 (talk) 02:29, 21 July 2010 (UTC)

I can't say what Lucas and his staff were thinking but see Length contraction and Time dilation. However, it's science fiction and they get to take some liberties. No one is complaining when he says the ship goes .5 past light speed. That would be faster that light travel(1.5*c). Who is to say what the effects of this are in that fictional world.

12.106.237.2 (talk) 14:24, 27 August 2010 (UTC)

Can someone remove the 'lol lucas messed it up lol?' It wasn't a mistake nor was it "intended as a joke." The Kessel Run is a competition between smugglers to plot a route that takes them near the black holes resulting in a shorter route, without falling out of minimum escape velocity and getting sucked in. It's not a typo so much as stupid people looking up the dictionary definition of parsec and assuming contextual error. 97.188.112.58 (talk) 19:43, 12 October 2010 (UTC)RLJ

These rationalizations don't fit the context though. The context of that line in the movie is Han Solo bragging about his, and I quote, "fast ship." Obi-Wan says he and Luke are looking for passage on a fast ship, and Han replies "Fast ship? You've never heard of the Millennium Falcon? It's the ship that made the Kessel Run in less than twelve parsecs." The context is clear: he's giving "12 parsecs" as an example of how fast his ship is. If he were talking about skipping through the space distorted by a black hole to cut down on the distance traveled, then he would be bragging about his own cleverness, not the speed of the ship. In addition, the "black hole shortcut" explanation is only given [i]later[/i], not in the movie but in the (non-canon) Expanded Universe, presumably for the sake of "fixing" what was, at the time of the movie, just a plain old error. Errors happen sometimes in science fiction, and it's OK to admit it. -- Anonymous — Preceding unsigned comment added by 207.111.162.65 (talk) 20:44, 16 January 2013 (UTC)
The explanation I've heard is that Solo was just throwing out technobabble to impress a couple of hicks from the sticks. He knew that a parsec was a unit of distance, but he didn't think that Luke and Ben knew that. That's why Ben winced skeptically after Han made the statement. He probably made up "the Kessel run" too. WaxTadpole (talk) 16:18, 31 May 2013 (UTC)
I have in my possession a copy of the book in which Han Solo's statement is rationalized, however I can't recall the name off the top of my head. If we were to source this, maybe we could add a "In Popular Culture"-type segment referencing the whole thing and mentioning the book's explanation? ʟʌɰȿøͷ ʈʜϵ ȷϵƌɨ 12:19, 17 April 2014 (UTC)

The "Star Wars" reference should be either deleted or moved to a "trivia" section at the bottom. I'd favour deletion. — Preceding unsigned comment added by 2A00:23C4:BF87:9B00:B49E:FC74:994C:81A9 (talk) 21:01, 19 June 2018 (UTC)

## Picture Worth 1000 Words

Stellar parallax motion

I've made an alternate version of the diagram which has "1 Parsec" "1 AU" and "1 arc second" depicted. I would like to replace the diagram, but appreciate some feedback first. Srain 02:31, 24 September 2006 (UTC)

Definition: A Parsec: is the distance that a distance of one astronomical unit subtends an angle of one arcsecond.

Fault in the illustration

The closest star to our Sun, Proxima Centauri is 4.2 light-years or 1.295 parsecs distant from the Sun. There is NO 'Near star' exactly 1 parsec from the Sun, so that any illustration that shows a Near star exactly 1 parsec distant from the Sun is incorrectly drawn.

Fault in the description of the illustration

The description in the illustration states: "A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond". This dexcription has 2 faults:

1. As shown above in 'Fault in the illustration', there is NO astronomical object exactly 1 parsec distant from the Sun, so the Sun is not 1 parsec from the Near star.

2. The distance that subtends the 1 arcsecond angle is not mentioned in the description

So both the 'Parsec' illustration 330px-Stellarparallax_parsec1.svg and its description have faults.

Corrections

1. The illustration The Earth and the Near star lie on the hypotenuse of the parsec triangle. The Near star is at the acute angle of the parsec triangle, and the distance from the Near star to the Earth, and the Sun exceeds 1 parsec. An imaginary astronomical unit lies 1 parsec from the Near star.

2. The description A parsec is the distance from a near star at which an astronomical unit (AU) subtends a parallax angle of one arc-second

I apologise that attempts to upload my modified png image were unsuccessful. However, the relative positions of the Earth, the Sun, the Near star and the imaginary astronomical unit are improved. If the creator of the original file Stellarparallax_parsec2.svg or anyone prepared to view my modifications and consider making the appropriate changes in the current image wishes to contact me at the email address in the signature line below, I will be happy to email my modified image without delay and without obligation of any kind.

jonesbdc@gmail.com Brian Jones 22 July 2018 — Preceding unsigned comment added by Briandcjones (talkcontribs) 07:29, 22 July 2018 (UTC)

I apologise that the quality of the redrawn illustration is marginal. However, the relative positions of the Earth, the Sun, the Near star and the imaginary astronomical unit are improved. The final drawing might be further improved if parallel marks are shown on the two lines from the earth and the sun to the 1 parsec line.

briandcjones jonesbdc@gmail.com Brian Jones 22 July 2018 — Preceding unsigned comment added by Briandcjones (talkcontribs) 07:41, 22 July 2018 (UTC)

A parsec is the distance from a near star at which an astronomical unit (AU) subtends a parallax angle of one arc-second

I have been unsuccessful in many attempts to upload an improved image. This is yet another attmpt/

briandcjones jonesbdc@gmail.com Brian Jones 22 July 2018 — Preceding unsigned comment added by Briandcjones (talkcontribs) 08:05, 22 July 2018 (UTC)

## Have I missed something?

I don't, given the rest of the article or picture, get this!

"There is no star whose parallax is more than 1 arcsecond."

When the parsec seems to be defined as 1 parsec per arcsecond parrallelax Frankenstien 04:21, 29 December 2006 (UTC)

No, parsecs are the reciprocal of parallax. 10 parsecs means 1/10 arcsecond parallax. No star is less than 3 parsecs away, so no star has more than 1/3 arcsecond parallax. --Doradus 15:10, 31 December 2006 (UTC)

Ahhh! Thank you, yes I would have more parsecs the closer I got! Frankenstien 11:37, 6 January 2007 (UTC)

## Usage Missing Info

As a non-astronomer, I am still confused about how parsecs are used. First, it might be useful to explain that it is the reciprocal. Second, why would an astronomer use parsecs instead of light years, when the latter would seem to be more universal?

How so? The speed of light is a universal constant, but the length of earth's year doesn't have any universal meaning. So both units are somewhat arbitrary.

Both are parochial units. The light-year depends on the length of the Earth's year (actually, a precisely and arbitarily defined value that is very close to an Earth year). The parsec definition depends ultimately on Earth's orbital radius (actually a precisely (to the extent that the gravtational constant is known) and arbitarily defined value that is very close to Earth's orbital radius). Both are parochial and locally (on a galactic scale) defined units. Rhialto 06:26, 22 April 2007 (UTC)

## Proposed WikiProject

Right now the content related to the various articles relating to measurement seems to be rather indifferently handled. This is not good, because at least 45 or so are of a great deal of importance to Wikipedia, and are even regarded as Vital articles. On that basis, I am proposing a new project at Wikipedia:WikiProject Council/Proposals#Measurement to work with these articles, and the others that relate to the concepts of measurement. Any and all input in the proposed project, including indications of willingness to contribute to its work, would be greatly appreciated. Thank you for your attention. John Carter 21:00, 2 May 2007 (UTC)

## h-1 Mpc

I've seen units of h-1 Mpc a lot in googling some cosmology I was curious about; the Mpc is obviously megaparsecs, but what's the h-1?

It could be Planck's constant. Rhialto 17:42, 21 May 2007 (UTC)
I should have mentioned it was being used to measure distances. If it was planck's constant, the units won't match up. --Starwed 19:10, 21 May 2007 (UTC)
h is the dimensionless magnitude of Hubble's constant divided by 100; that is, H0 = 100h km/s/Mpc. Converting redshift to distance, in particular, involves Hubble's constant, which is not precisely known. If H0 is 100 km/s/Mpc (as was once thought roughly correct), then the /h factor is unity and can be ignored; as the value estimated for H0 decreases (it is now thought to be about 71 km/s/Mpc), the distance expressed with a /h factor increases. Ray Spalding 06:29, 22 May 2007 (UTC)
Done. Ray Spalding 11:01, 23 May 2007 (UTC)

## Parsec to Light-Year Conversion

Many people will visit this article looking for a conversion factor between parsecs and other common units. For some reason the conversion between parsecs and light-years was present on the discussion page but not in the article itself! I have therefore added the information to the article as light-years are also a very common way of specifying astronomical distances.

Actually, that conversion (and many many others) was in the infobox on the top right corner as soon as you load the page. Rhialto 12:16, 9 June 2007 (UTC)

## Meaningfulness

I understand parsec is a useful unit for measurement when we observe objects from earth. Does the distance definition become arbitrary if the observer is not on earth or in earth orbit? --Voidvector 20:04, 29 September 2007 (UTC)

I think yes because the distance between the earth and the sun is part of the definition of the parsec. Egriffin 09:39, 18 October 2007 (UTC)

hi hello —Preceding unsigned comment added by 59.163.32.25 (talk) 14:17, 18 October 2007 (UTC)

No, because the relevance to the earth is only in the definition of the AU. Keep that, and you can carry the parsec anywhere. You can't use parallax over a half-orbit from another planet, but the parsec just as Earthbound as the light-year, no more, no less. --Thnidu (talk) 02:15, 18 February 2015 (UTC)

## Particle horizon's radius is 14 gpc???

okay so if the particle horizon's radius is 14 gpc, and each parsec is about 3.262 lightyears, that would make the radius of the observable universe around 45.7 lightyears. however, if the universe is around 13.7 billion years old, and is traveling a bit under the speed of light since, then our visible range of the universe couldn’t possibly extend out that far. so either cosmology is wrong, or the 14 gpc is wrong. —Preceding unsigned comment added by 70.228.68.29 (talk) 23:45, 25 November 2007 (UTC)

I think it is far more likely that your maths is wrong. You dropped the "giga". Rhialto (talk) 06:32, 26 November 2007 (UTC)
I don't see any "giga" in the question. What the heck is "gpc", anyway? Gene Nygaard (talk) 18:52, 26 November 2007 (UTC)
A "gpc" is a gigaparsec. Rhialto (talk) 20:35, 26 November 2007 (UTC)
We don't live in a special relativistic universe. It's not true that light travels a distance ct in a time t. That's only an approximation valid in some special cases. -- BenRG (talk) 16:57, 26 November 2007 (UTC)
Unless you are a scientist with a degree or three in astrophysics and several relevant articles in peer-reviewed journals, I think I'll take your opinion with a pinch of salt. If you are, cites please. Rhialto (talk) 18:48, 26 November 2007 (UTC)
Sorry for the late reply. I'm not an astrophysicist, but I've studied general relativity, which is necessary and sufficient for understanding this stuff. The best source of information about cosmology on the web is probably Ned Wright's site. "Expanding Confusion" by Tamara Davis and Charles Lineweaver is also worth reading, as is this Scientific American article by the same authors. This is a subject on which unfortunately a lot of seemingly reliable sources are flat wrong. As mentioned in the articles I linked, even a lot of professional astronomers get it wrong. I think the problem is that almost all introductions to special relativity spend most of their time on coordinate artifacts like time dilation, length contraction, relativity of simultaneity, and the speed c. They usually fail to clearly explain that (a) all of these concepts are meaningless except in the context of a family of inertial coordinate systems, and (b) there are no inertial coordinate systems in cosmology, or indeed coordinate systems of any kind anywhere in the real world. The result is a large number of people who think that the coordinate artifacts are general truths about nature, and use them to draw incorrect conclusions which they're confident enough to publish without checking against primary sources. For me the hardest part of learning general relativity was unlearning special relativity as it was taught to me and replacing it with coordinate-independent concepts (for example, that light travels along null geodesics and that elapsed proper time is the length of the worldline). -- BenRG (talk) 16:50, 29 January 2008 (UTC)

He means "Gpc". JIMp talk·cont 01:30, 30 May 2008 (UTC)

## Formula for parsec value

I have re-calculated the parsec value, using 149,597,870,691 m as the value for the Astronomical unit (sourced from wp article). I used the following as a formula in Excel:

That gives the number of au in one parsec, which is identical to 11 significant figures with the 360x60x60/2pi value shown in the diagram. Multiplying that number by the length of an au in m gives the pc length in m - 30,856,775,813,299,000 m. I suspect Excel will not happily calculate to more than 15 sig figs, so the last 3 zeroes can probably be considered spurious accuracy, as tools to calculate that many sig figs are not commonly available.

Rhialto (talk) 14:05, 27 November 2007 (UTC)

The 1976 IAU definition for the au is only good to +/- 30m, less than 11 significant figures, and its definition is a bit unhelpful: "the distance from the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (a Gaussian year)." Using au*radian / arcsec rather than au/ATAN(arcsec) results in a proportional difference over 25 times smaller than the uncertainty of the definition of the au. Using the latter equation anyway and plugging in the +/- 30m precision to the interval arithmetic capacity of Frink, the precision of the definition of the parsec is 30,856,775,813,299,047 +/-6,187,944m, or between 30,856,775,807,111,103 and 30,856,775,819,486,992m, or more comprehensibly, 1 parsec = 3.08567758133(62)E16 m Using the alleged 2009 value in the Astronomical unit article with a supposed uncertainty of +/- 3m gives a value of 30,856,775,815,774,225 +/- 1,237,588m or 3.08567758158(12)E16 m

Enon (talk) 03:18, 25 February 2011 (UTC)


## Non-standard display format?

Why are the SI units in the top right corner displayed in the form 30.857×10^12 km 30.857×10^15 m ? Scientific notation dictates they should be 3.0*10^13/16 respectively. --72.235.245.54 (talk) 08:54, 7 January 2008 (UTC)

It looks like the equally standard engineering notation to me, whereby the exponent is given the the smallest multiple of three which would make the main number greater or equal to one. If you want it changed, it'd be best to take it up on the talk page for Template:Unit_of_length instead of here, since that template affects many pages. Rhialto (talk) 10:57, 7 January 2008 (UTC)
Engineering notation as Rhialto points out. I almost switched {{Unit of length}} over to standard scientific notation when I overhauled it ... but I saw the light. Engineering notation nicely fits in with SI prefixes. 1015 metres ... that's a petametre ... easy. JIMp talk·cont 01:29, 30 May 2008 (UTC)

## "Misuse" section removed

I removed the following section because I consider it fandom irrelevant for a scientific article in an encyclopedia. This is not the Wokieepedia article about Parsec, you know. -- wr 87.139.81.19 (talk) 11:45, 17 April 2008 (UTC)

As a unit of astronomical distance, the parsec is mentioned in many science fiction stories. One use that stands out, however, is the remark by pilot Han Solo in Star Wars: A New Hope, that his spaceship is fast because it "made the Kessel Run in less than 12 parsecs." Later novels in this fictional universe have retconned Solo's remark to mean that the ship reached the planet Kessel in the shortest distance despite having to pass through a cluster of black holes.

It's odd to have an "earth-based" unit in a sci-fi world that doesn't even involve earth. --Voidvector (talk) 07:50, 26 August 2008 (UTC)
Well, no odder than their speaking English. Maybe "parsec" was a translator's error. -- BenRG (talk) 09:40, 26 August 2008 (UTC)
The section should be restored. It's a notable use of the term. Tempshill (talk) 20:11, 18 November 2008 (UTC)
Should not a policy be made about crapping up scientific articles with science fiction fantasy? I like Star Wars and such, but a parsec has an actual use in reality, and it isn't in starships going to such places that are parsecs away. I think the whole Science fiction section should be removed. 96.237.148.44 (talk) 04:52, 3 June 2009 (UTC)
I think it would be appropriate to include an "In Popular Culture" section which describes the use of the parsec in significant scifi works. Minetruly (talk) 01:54, 11 January 2010 (UTC)

## Distant star

I guess that we are assuming that the 'distant star' in the picture is a fixed point? —Preceding unsigned comment added by 83.101.44.217 (talk) 08:43, 28 February 2009 (UTC)

## Andromeda Galaxy

The Andromeda galaxy was previously described as being the most distant object from Earth visible to the naked eye. Bode's Galaxy (Messier 81) and the Triangulum galaxy (Messier 33) are both further away and visible to the naked eye, albeit under significantly better viewing conditions. Only the clause "the most distant object visible to the naked eye" was removed, as the rest of the sentence still makes sense. Epsilon Knight (talk) 20:08, 26 April 2009 (UTC)

## Science fiction

Hm. Should we maybe remove this section? None of it is notable at all, and if we're gonna start listing every reference to parsecs in sci fi movies, series and novels, this could be one long list. -- Nils (talk) 13:35, 11 May 2009 (UTC)

This discussion happened about a year ago too, and the decision then was to remove it all. Rhialto (talk) 13:40, 11 May 2009 (UTC)
I think it would be appropriate to include an "In Popular Culture" section which describes the use of the parsec in significant scifi works. Minetruly (talk) 01:56, 11 January 2010 (UTC)

## Parallax of the Sun

Not that it's incredibly important, but shouldn't this be 180 degrees? --Hasoan (talk) 01:34, 20 June 2009 (UTC)

No, as the article says in the History section, "The parallax of a star is half of the angular distance a star appears to move [...]" (emphasis on half added). This is because measuring the difference in angle of a star at 6-month intervals gives the angle at the apex of an isosceles triangle (with a base of 2 AU). That triangle is cut in half to get a right triangle (with base 1 AU), so that the tangent trig function can be applied. The sun, of course, is the limit case in which the isosceles triangle, with "apex" of 180 degrees, collapses into a straight line. But the parallax is still half that, or 90 degrees. Radius (talk) 03:05, 25 June 2009 (UTC)

## Thousand million?

The section "Megaparsecs and gigaparsecs" uses the term "thousand million." Shouldn't that be "billion?" I'd change it, but I don't know if there's some reason specific to the field of astronomy it's phrased that way. —Preceding unsigned comment added by Minetruly (talkcontribs) 01:36, 11 January 2010 (UTC)

There is a difference between US billion (109) and old-style English/European billion (1012 - Europeans call 109 a "milliard"). See billion. Due to that ambiguity, the term is best avoided. --Stephan Schulz (talk) 15:17, 9 February 2011 (UTC)

## Proxima Centauri

Proxima Centauri article states that Proxima Centauri is 4.2 light-years away, not 1.29 parsecs. —Preceding unsigned comment added by 85.223.209.2 (talk) 08:13, 26 August 2010 (UTC)

A parsec is 3.26 light-years. 1.29 × 3.26 = 4.2, so both values are the same. --The High Fin Sperm Whale 17:36, 25 October 2010 (UTC)

## Dots in image

I'm wondering what do the black dots in that curved line in the image mean... can anyone clarify this? --Waldir talk 19:29, 20 January 2011 (UTC)

They represent stars in the far background. --Stephan Schulz (talk) 22:58, 8 February 2011 (UTC)
I'm not sure why I didn't realize this before. Now that you've told me that, it seems obvious when looking at the image. I wonder if others would have the same doubts or if it was just a one-off case of inattention. Anyway, thanks for the clarification :) --Waldir talk 01:26, 9 February 2011 (UTC)
No problem. You're welcome. --Stephan Schulz (talk) 15:18, 9 February 2011 (UTC)

## Confusing graph

I see there is some confusion about the diagram. To be honest, what I understood when I saw that diagram is that the form of calculation always reaches the distance of 1 parsec for any star or object, and that the parsec is not a fixed unit.

Reading the article, it's quite the opposite. But the main problem is that I had to reach this discussion page ("poorly explained" section) to understand the "1 parsec" caption in the diagram. I really don't think the diagram is self-explanatory.

Wouldn't it be better if the diagram showed the distance in a different manner, for example 'x parsecs' or 'n parsecs'? —Preceding unsigned comment added by 186.215.156.157 (talk) 19:38, 22 February 2011 (UTC)

## Error in history

Quote from the article: "One of the oldest methods for astronomers to calculate the distance to a star was to record the difference in angle between two measurements of the position of the star in the sky. The first measurement was taken from the Earth on one side of the Sun, and the second was taken half a year later when the Earth was on the opposite side of the Sun."

This is plain WRONG! Because astronomy is as old as mankind, and in ancient times (before Kopernikus and Galilei), everybody believed in the GEOCENTRIC world, = the universe rotates around EARTH, not the sun! The article is based on a HELIOCENTRIC world-view, = the planets (of our solar system) rotate around the SUN, not the Earth! 93.219.151.5 (talk) 20:20, 12 May 2011 (UTC)

So I guess, "one of the oldest methods" should read, "one of the oldest accurate methods". How did geocentrists (attempt to) measure the distance to stars? Rwflammang (talk) 09:35, 13 May 2011 (UTC)
It's actually kind of correct. Until scientists discovered that the Earth revolves around the Sun, astronomy didn't really exist as a separate branch from astrology, and astrology wasn't concerned with measuring planetary or stellar distances. Rhialto (talk) 09:22, 15 May 2011 (UTC)
Ancient astronomers did indeed attempt to measure the distance to the sun and planets, and I believe, the stars as well. They were not very good at it. Read the Somnium Scipionis for some details. Rwflammang (talk) 18:57, 17 May 2011 (UTC)

## Right triangle?

"The parsec is equal to the length of the adjacent side of an imaginary right triangle in space." -- I'm probably misunderstanding this completely, but the diagrams seem to imply that this only true when the object being measured is at ("on"?) the orbital pole? Ojw (talk) 22:22, 17 September 2011 (UTC)

In order to get a correct parsec measurement and calculation, the first of two readings must be taken when the line between Sol and the target star is perpendicular to a line from the earth to Sol. If this isn't true, then you don't have a full 1 AU1 at the base of a right triangle. The second reading must be taken when the earth is 1/2 of an orbit from the first reading.

The one exception to this rule is if the target star is exactly at one of the poles of the earth ecliptic plane, then the first measurement can be take at any time.

1. One AU is the average distance between earth and Sol. Although the earth has a perigee and apogee in its orbit, the sum earth-Sol distances of the first and second readings work out to be the same regardless of when in earth's orbit the readings are taken. For example, if the first reading is at apogee, the second at perigee, the distances will be different, but together average 1 AU.

If my statements are correct, I think it should be included in the article. I'm not an astronomer, so perhaps one can confirm it.(DavidAmis (talk) 22:17, 27 April 2018 (UTC))

## Horizon scale

The article states that the present particle horizon in the Universe is 46.5 billion light-years, but the reference from Scientific American (in the same line) states 16 billion. Which one is correct? Should the value in the article be changed to that in the reference? joselotl (talk) 10 January 2012 —Preceding undated comment added 11:37, 10 January 2012 (UTC).

The linked article includes both the cosmic event horizon (16 billion ly) and the particle horizon (46 billion ly): "Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years." -- JHunterJ (talk) 12:26, 10 January 2012 (UTC)

## Distance from earth to the Andromeda Galaxy

There are two references in this article to the distance from earth to the Andromeda Galaxy: one for 2.61 Million Lightyears and one for 2.51 Million Lightyears. I don't know which one is correct but clearly both can't be... Pgcohen (talk) 00:32, 18 July 2012 (UTC)

Given the uncertainty in measuring astronomical distances, they probably can be— that is, both be within the limits of precision of the measurement. --Thnidu (talk) 02:19, 18 February 2015 (UTC)
Could be, but that would argue for changing the article to reflect the precision of the measurement by giving the measure only to the number of significant digits known, or by giving a range. Evensteven (talk) 00:39, 19 February 2015 (UTC)

## Re-work for 2012 of the A.U.

The 2012 IAU conference finally agreed to scrap various earlier definitions and re-define the Astronomical Unit as a simple integral multiple of the meter. One parsec is therefore exactly 149,597,870.7 km / atan(1"). This is the exact value of one parsec. Of course since the inverse tangent of one arcsecond is a transcendental number, in decimal notation, the value has to be truncated at some point. So, for example, to the nearest micron, that come out to 30,856,775,815,155,429,770,405 microns. No one would ever need the value of a parsec to the nearest micron. This is just an example. It doesn't matter that you can do a "small angle" approximation here. That's just another method for truncating the precision.68.9.201.92 (talk) 20:54, 17 October 2012 (UTC)

I know we're in danger here of dancing on the head of a pin, but since we're talking about "exact" (always dangerous): the IAU page referenced below defines a parsec as "the distance at which one Astronomical Unit subtends an angle of one arcsecond". In other words, in an equilateral triangle with a base measuring 1au and having an apex with an included angle of 1 arcsecond, the perpendicular from the base to the apex measures one parsec. So the formula should be 0.5au / tan(0.5"). Windows 7's calculator makes this to be 30,856,775,814,853,233.543822509736376 m, which diverges from your value after the 10th significant figure (Libre Office spreadsheet makes it 30,856,775,814,853,200 so it obviously gives up after 15 digits but that's enough to support the point). Also, your calculation should have used tan, not atan. The aforementioned small angle approximation does mean they give close results at 1 arcsecond, of course --Gobbag (talk) 16:24, 31 August 2013 (UTC)

I'm not sure I'd agree. Picture triangle ABC, where C is the distant object, A and B (i.e. the earth and the sun) are exactly one AU apart, and the angle at vertex C is 1 arcsecond, then you'd get slightly different results depending on which angle (A or B) is a right angle, which line (AC or BC) is used to define the parsec, or, as you've described, if ABC is an isosceles triangle, and the line that defines the parsec is drawn from C "down the middle" of the triangle to the midpoint of line AB. Each of those three measurements would technically fit the discription of "the distance at which one Astronomical Unit subtends an angle of one arcsecond," so that definition isn't 100% explicit. Is there a source that makes the definition more unambiguous than that? -Hatster301 (talk) 07:45, 1 September 2013 (UTC)

## In Popular Culture

In Star Wars, The Millenium Falcon (fictional spaceship) is described as "the ship that made the Kessel Run in less than twelve parsecs". This should be added to the article.— Preceding unsigned comment added by 75.150.107.217 (talkcontribs) 17:06, 7 January 2013‎ (UTC)

Why? See also Wikipedia:"In popular culture" content. -- JHunterJ (talk) 17:40, 7 January 2013 (UTC)
I recommend a link under "See Also" pointing to the article on the Millennium Falcon - perhaps directly to the section titled "Depiction." So that when someone comes here looking for the parsec article who was motivated by the line in Star Wars, which is quite conceivably culturally significant in that many people first heard of a parsec thanks to the very widely viewed film, they can at least see an acknowledgement of the connection. Agreeable? Medleystudios72 (talk) 18:12, 4 April 2013 (UTC)
I certainly think we should have _something_ (and presently it has been removed altogether). The apparent misuse in Star Wars is notable, widely remarked upon - as Medleystudios72 says, there is every reason to believe it is the first time many people heard the word. It is not simply part of the usual mess of "in popular culture" trivia. Pinkbeast (talk) 00:21, 28 June 2018 (UTC)
This article is not about Star Wars or it's alleged goofs. There is absolutely no factual evidence that the term "parsec" was misused in Star Wars, only the belief of some people that it was. Even if it was misused and can be proven (and only a public confession from George Lucas can do that), it still doesn't belong in the article. By that logic, we would have to rewrite the article about Outer space and include mention of every film and TV show that had spaceships making sound in space. It simply isn't relevant to the article. And suggesting that it's relevant because Star Wars is where most people first heard the word "parsec" is an unbelievable claim and totally unprovable. 146.90.60.100 (talk) 18:44, 28 June 2018 (UTC)
The question is not if it was misused, a meaningless question. The question is whether it was apparently misused, which it was, a fact which is well-established. I imagine any popular science book of about the right vintage mentions it, part of why it is notable.
We would not, of course, have to rewrite outer space because in general incidences of ships making sound in space in fiction are not notable. Pinkbeast (talk) 20:49, 28 June 2018 (UTC)
It's still irrelevant. Again, this is not an article about Star Wars. The article should concentrate on what a parsec is, not what it isn't, and it certainly should not be used to force the opinion that George Lucas misused it in his script. There is no conclusive evidence that he did, and it's inappropriate to include such a detail in this article even if there was. Save the debate for the fan forums, this is an encyclopedia. 146.90.60.100 (talk) 03:24, 29 June 2018 (UTC)
You seem once again to have missed the important distinction between "misused" and "apparently misused". Pinkbeast (talk) 06:18, 29 June 2018 (UTC)
And you seem to be missing the point that Wikipedia is not a fan forum or an IMDB "goofs" section. As is, you are trying to use this article to trash Star Wars. Take it elsewhere. 146.90.60.100 (talk) 14:16, 29 June 2018 (UTC)
That's absurd. I've been very specific about this mention being notable when others are not; generally, I trim "in popular culture" stuff, not extend it. And "trash Star Wars"? A piece of space opera with no pretensions to scientific rigour misuses "parsec" - so what? It's hardly the most trenchant criticism one could make. Pinkbeast (talk) 02:34, 2 July 2018 (UTC)

## Astrometry: "Megaparsecs and gigaparsecs"???

Quoting from the article: "Astronomers typically measure the distances between neighbouring galaxies and galaxy clusters in megaparsecs." Perhaps a minor quibble, but it seems to me that the notion of actually "measuring distances" (as opposed to expressing them) on the order of megaparsecs and larger is, as of 2013, pretty darn meaningless given that the usage of parsecs within astronomy is intimately (and historically) tied to directly-observed parallax. The latest best such measurements was from Hipparcos, and was good only down to 1 milliarcsecond, and therefore itself was barely adequate for merely kiloparsec range(!). Should this not be noted in this section of the article? DWIII (talk) 06:26, 20 April 2013 (UTC)

Feel free to correct it! --Regards, Necessary Evil (talk) 02:02, 21 April 2013 (UTC)

## Oh for heaven's sake

A parsec is 30.857×10^12 km. It's as simple as that.

https://www.iau.org/public/measuring/ — Preceding unsigned comment added by 71.212.78.121 (talkcontribs) 08:21, 26 June 2013‎ (UTC)

No, it isn't. Jimp 13:32, 21 February 2015 (UTC)

## Design Error?

I quote: "ESA's Gaia satellite, ... , is intended to measure ... , producing errors of 10% in measurements as far as the Galactic Center". I guess the intention is not to on purpose produce errors of 10%, but rather a maximum of 10%. — Preceding unsigned comment added by Hilmer B (talkcontribs) 11:21, 16 July 2013 (UTC)

## Comment on Broomwick's input.

It is true that it is difficult to measure distances with parallax at ranges greater than 1,000 pc (High precision parallax collecting satellite, Hipparcos). However the astronomical distance measurement is a ladder, where e.g. parallax measurements determinate the distances to standard candles like Cepheids. Cepheid variable stars in turn gives the distance to their parent galaxy, and so on. The frequency of a certain type of Cepheid, compared to the observed luminosity, gives a calculation from parsecs to megaparsecs.

The real popularity of the light-year is not because of "advancement of astrophysics and cosmology into worlds far from the Solar System". It is so much easier to explain the light-year to a layman, than the parsec. Since readers of astronomy textbooks & scientific literature, plus members of established institutions are familiar with the parsec, it is used by them. Popular science medias use the light-year instead. --Regards, Necessary Evil (talk) 21:25, 6 August 2013 (UTC)

## Why are there two tables in the 'Equivalencies' section?

I wonder why there are two tables in the 'Equivalencies' section. I'll prefer the smaller {{unit of length.. rather than the big cumbersome one. It's ok to have more decimals once, but if the conversions have been done by a Wikipedian it's OR - and should be excluded. --Regards, Necessary Evil (talk) 09:40, 8 January 2014 (UTC)

See Talk:Light-year#Why are there two tables in the 'Equivalencies' section?. Jimp 11:44, 8 January 2014 (UTC)

## SI value

I removed the following line:

Unlike lightyear (ly) and astronomical unit (au), parsec does not have an SI standardized exact definition in metres.

And in return, received the following comment on my talk page:

This is regarding my change that you have undone on Feb 20th with comment "Undid revision 596300596; actually, it does: http://www.iau.org/science/publications/proceedings_rules/units/"

The content of this IAU site is either inaccurate or more likely obsolete. As an example - Astronomical Unit has an exact definition in metres (149,597,870,700m as of 2012) but the site shows old incorrect approximate value. It has the same inaccurate data for Parsec. So, my comment is accurate that lightyear and AU have exact definitions in metres. That is not (yet) true for Parsec. Even PI (3.14...) Parsec is approximately (but not exactly) equal to 180x60x60=648000 au, because Tan(1sec) is approximately (but not exactly) equal to 1". This makes AU and lightyear different from Parsec in terms of exact vs. approximate value in SI metres. Let me know if you still disagree or need more information - Wikipedia pages on AU and lightyear have accurate data and substantiating evidence and I can provide more if needed.

Please revert your-Feb 20th-undone and add my comment back into the page. Thanks.

The IAU website is about as official a definition for parsec as it gets. If their definition of the parsec does not match others, it means the others are obsolete (or incorrect), not the IAU. The fact that the numbers in the tale are approximations should be reasonably obvious from the level of rounding, but the parsec definition in the body text is far more verbose.

An appropriate unit of length for studies of structure of the Galaxy is the parsec (pc), which is defined in terms of the astronomical unit of length (au).

Since the parsec is defined relative to the au, and the au does have a precise definition in metres (even though the au definition isn't precisely shown on that page), it follows that the pc can be precisely defined in terms of metres (albeit probably not as a rational number, as the overall definition involves arc-seconds).

Rhialto (talk) 06:49, 24 February 2014 (UTC)

## 1.3 parsecs is less than a parsec??

The line

" the parsec is shorter than the distance from our solar system to the nearest star, Proxima Centauri, which is 1.3 parsecs from the Sun"

is either incorrect or needs to be explained. I'm not an astronomer but as far as I know 1.3 parsecs is not less that 1 parsec (am I missing something?). — Preceding unsigned comment added by SomeHandyGuy (talkcontribs) 05:31, 15 September 2014 (UTC)

Yes. It is the parsec (1 parsec) that is the shorter distance, as the article says. Evensteven (talk) 06:20, 15 September 2014 (UTC)
The nearest star is Proxima Centauri. The distance from our solar system to Proxima Centauri is 1.3 pc. 1 pc < 1.3 pc. So the parsec is shorter than the distance from our solar system to the nearest star. Perhaps the sentence could be rephrased to make it clearer. Jimp 00:44, 19 September 2014 (UTC)

## New definition

Given that the parsec apparently has been redefined[2] (see footnote 4), the article would be in need of a bit of a rewrite. Jimp 05:46, 6 January 2016 (UTC)

+1 for this suggestion. The fist paragraph, "A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond." is no more correct. — MFH:Talk 15:47, 18 September 2017 (UTC)
Actually there was no redefinition of the parsec, at least not relative to the AU, so the current text is wrong. The formula 648000/pi AU is exactly the distance at which one AU subtends one arcsecond (on the other hand the AU WAS redefined in 2012). Please don't get hung up on the difference between θ (in radians) and tanθ, which is less than 1 part in 1011 for θ = 1 arcsec. No distances in parsecs will be known to anything like 1 part in 1011 for the forseeable future. In fact no measurement of anything has ever been done to 1 part in 1011. 82.29.136.2 (talk) 19:39, 12 May 2018 (UTC)
That difference does exist and hence the statement you have put in now, that these amounts are "exactly" the same, is incorrect. Pinkbeast (talk) 19:58, 12 May 2018 (UTC)
Ho hum. No working astronomer or IAU member (I'm both) would consider the footnote in the reference given above as a redefinition of the parsec. If the IAU wanted to re-define one of our primary units of distance we would certainly make it the subject of a major resolution (as we did for the 2012 redefinition of the AU), not a footnote attached to something else altogether. If you check the two references provided (thanks, Google Books), which the IAU obviously considers equivalent since they are both cited as the source of the quoted definition, you will find that Cox (2000) defines the parsec as 206 264.806 AU, which differs from both the "old" and "new" definition given by Wikipedia by 2 parts in 108, that is 2000 times more than the alleged "redefinition". Scientists (as opposed to mathematicians) do not care about unmeasurably small distinctions like this. 82.29.136.2 (talk) 20:34, 12 May 2018 (UTC)
Let me have a proper look at the references tomorrow; I'll self-revert if it seems appropriate after doing so. I take your point that the unit would not have been redefined so casually.
I've reviewed your remarks above and some of the references and agree with your point. I've reverted myself, removing the word "exactly" which seems to be incorrect, and edited further down the page to remove any implication this was an explicit redefinition. I hope this is better. Pinkbeast (talk) 23:47, 12 May 2018 (UTC)

## Assessment comment

The comment(s) below were originally left at Talk:Parsec/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

 The third paragraph of gigaparsec discussion contains the following: "one fourteenth of the distance to the horizon of the observable universe." Shouldn't this be "one fourth" instead of one fourteenth??? 98.196.183.179 18:51, 10 October 2007 (UTC)

Last edited at 18:51, 10 October 2007 (UTC). Substituted at 02:19, 30 April 2016 (UTC)

## Intentional or Unintentional Humor?

This article states: "though the light-year remains prominent in popular science texts and everyday usage." One wonders how often light year comes up in "everyday usage" for most people. — Preceding unsigned comment added by 192.158.48.160 (talk) 12:10, 4 November 2016 (UTC)

## More humor

An astronomer friend of mine suggested that the metre was a narrowly parochial measure of length and that the femtoparsec would be more universal. FYI, it's almost exactly a hundred feet...

Franciscus montmartinensis (talk) 18:30, 28 November 2016 (UTC)

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## Divided by Pi

Is it wrong to put the equasion solution in there so that we can see as simple number or do we have to divide by Pi in our heads? -Inowen (talk) 08:44, 13 March 2018 (UTC)

Inowen: I guess you are referring to the formula 648000/π, which is the formal definition of the parsec. Since π is irrational, a "solution" to the equation will only be an approximation. This has no room in the lede, but can be useful mainly for unit conversions and calculations. Therefore the approximate value is given in the article no less than three times:
• In the infobox as 2.06265×105 au
• In the section "Calculating the value of a parsec", with a bit more precision, as ${\displaystyle \mathrm {SD} \approx {\frac {\mathrm {ES} }{1}}={\frac {1\,{\mbox{au}}}{{\frac {1}{60\times 60}}\times {\frac {\pi }{180}}}}={\frac {648\,000}{\pi }}\,{\mbox{au}}\approx 206\,264.81{\mbox{ au}}.}$
• A bit lower in the same section, with no less than 15 significant digits, as 1 parsec ≈ 206264.806247096 astronomical units
There is therefore no reason to make any calculation in the head. You only need to read a bit closer... Regards! --T*U (talk) 10:30, 13 March 2018 (UTC)

## Star Wars redux

The following comment was left on my talk page and I'm moving the discussion here:

Hi there, I just wanted to circle back on an edit of mine you reverted (if this is the wrong place to do it let me know). If your argument is that no Star Wars stuff should be included on the parsec page then shouldn't you delete the existing Star Wars comment? As it stands now, the current Star Wars content is incorrect. Therefore, it should be changed if you are opting to not delete it. Or it should be deleted to be consistant with your reasoning.
Regardless, I would argue that it should be included. Most folks know what a parsec is because of Star Wars. An entire movie's plot (not just any movie but the space opera Star Wars of which its exceptionally huge impact is well-known and documented) centered around shortening the parsecs required to make a particular journey in space. The impact on most people's understanding of what a parsec is and even it's existence is clear so I think it merits inclusion. What do you think? Thanks for your time. - unsigned comment added by 2600:1700:65c0:6630:7de8:b422:d89e:3cf1
My argument is not that no Star Wars stuff should be included on the parsec page. My argument is that the mention in Star Wars is extremely well known and so probably merits inclusion, but an attempt to retcon Lucas's error decades later does not merit inclusion.
The current Star Wars content is not incorrect. The sentence in the article is completely true.
Inasmuch as what you say in the second paragraph is true (I think in fact most people do not know what a parsec is at all), it is true of of the mention in Star Wars, which is why I tend to feel it does merit inclusion. It is not true of the retcon in Solo.
I'm not quite sure which film you think has "exceptionally huge impact" and "centred around shortening the parsecs required to make a particular journey in space". Star Wars doesn't centre around that; Solo, yet another release in a tired franchise, doesn't have "exceptionally huge impact". Pinkbeast (talk) 16:40, 30 May 2018 (UTC)

## Star Wars redux (response)

Our chief concern should be clarity and making a determination on the accuracy of a statement made in a film especially when the matter is convoluted by changing context would detract from the article as the shifting context would make a statement on it subjective (subjective in the sense that there is the new issue of whether the new film should be taken in to account) in addition to (and perhaps more importantly) very likely confusing visitors to the page about whether the critique of the use of the term takes Han Solo: A Star Wars Story into consideration. I've edited the article so that the mention of Star Wars is as objective as possible and that will hopefully satisfy everyone who's been involved in this dispute. I'm not going to make any attempt to rectify the article if someone takes it upon themselves to continue this issue as I've spent enough time on this already (this is obviously very trivial) but hopefully someone cares enough about Wikipedia's clarity and objectivity that they will fix any emotionally driven edits. — Preceding unsigned comment added by 2600:1700:81E1:7C40:5877:3A23:6DDF:FB5 (talk) 07:43, 1 June 2018 (UTC)

This is largely incomprehensible; I don't think "subjective" and "objective" mean what you think they do. "Apparently misused as a unit of time" is a succinct and accurate description of the use in Star Wars, and as such I have restored it. Pinkbeast (talk) 23:36, 1 June 2018 (UTC)

## How do we get Wikipedia to stop using Parsecs?

It's not metric and it's not based on a true physical constant. It's defined in terms of the earth's orbit, not a true physical constant. It's defined in terms of arcsecond angles (1/3600 of a DEGREE), which is not metric. Every time I see it I have to go look up what it is. Why can't we use "light years" like sane people? — Preceding unsigned comment added by 128.92.10.210 (talk) 13:22, 9 April 2019 (UTC)

We do that in the talk pages for the Manual of Style. This talk page is to suggest improvements to this article. Pinkbeast (talk) 12:31, 11 April 2019 (UTC)