Talk:Parsec
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- 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 |
[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)
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)
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)
[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)
[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)
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- 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)
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- "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)
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..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)
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- 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?
![[1]](http://en.wikipedia.org/wiki/Image:Parsec.png){kind=link}