Talk:Parsec

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Additional information: This article is an assessed orphan; it has been assessed by Work via WikiProjects, but should be rated by subject experts in a WikiProject. See the FAQ for more info. The following comments have been left for this page: 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) (edit)

It is therefore 360×60×60/2π AU = 206,264.806 247 AU = 3.085 677 581 31×10¹⁶ m = ~3.26 light-years.

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(a)~=a for small a. 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(a) ~= a for small a 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 10:21, 27 Mar 2006 (CEST)

Contents 1 Parsec 2 When to call a billion a trillion 3 Discrepancy 4 Picture not to scale 5 Discrepancy in Parsec Definition 6 Hopeless 7 4,000,000,000,000,000 kilometers? 8 poorly explained 9 Error in International Units calculation 10 Han Solo 11 Picture Worth 1000 Words 12 Have I missed something? 13 Usage Missing Info 14 Proposed WikiProject 15 h-1 Mpc 16 Parsec to Light-Year Conversion 17 Meaningfulness 18 Particle horizon's radius is 14 gpc??? 19 Formula for parsec value 20 Non-standard display format? 21 "Misuse" section removed

[edit] Parsec

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

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

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)

[edit] 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)

[edit] 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)

[edit] 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.  :-) --P3d0 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)

[edit] Discrepancy in Parsec Definition

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... --P3d0 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???

[edit] Hopeless

Unfortunately, this article is so very ugly and confusing that it really calls for a complete rewrite. leschatz 23:00, 29 January 2006 (UTC) I agree CPS

[edit] 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

[edit] 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)

[edit] 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???

[edit] 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.

[edit] Picture Worth 1000 Words

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)

[edit] 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. --P3d0 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)

[edit] 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)

[edit] 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)

[edit] 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)

Would it make sense to add that into this article? --Starwed 19:16, 22 May 2007 (UTC)

Done. Ray Spalding 11:01, 23 May 2007 (UTC)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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:

1/ATAN(RADIANS(1/60/60))

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)

[edit] 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)

[edit] "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.