Talk:Light-year/Archive 1

1 year equals 365.25 days
Article said light year was defined using the time the Earth takes to orbit the sun. It isn't -- its defined in terms of the Julian year (365.25 days of 86400 SI seconds each). The reason for this is that the time it takes for the Earth to orbit the sun can be known only to limited accuracy, and varies over time. Thus if the light year was defined based on that, it would not be a stable nor accurate measurement. (Although arguably the amount of inaccuracy and stability would be insignificant over the distances the light year is used to measure.)


 * In german Wikipedia the light year is said to be based on the tropical year. Is that definitely wrong? Could anyone please point out a reference where the internationally accepted definition (if any exists) of a light year is shown?--SiriusB 13:50, 20 May 2005 (UTC)


 * According to an IAU style guide recommendation (§5.15), "Although there are several different kinds of year (as there are several kinds of day), it is best to regard a year as a julian year of 365.25 days (31.5576 Ms) unless otherwise specified."


 * Yes, it is definitely wrong. The International Astronomical Union defines a light year as the distance light travels in one Julian year (365.25 days exactly). Here's the URL to confirm it: http://www.iau.org/public_press/themes/measuring/ (see the 4th sentence of the 4th paragraph)Mtiffany71 (talk) 07:54, 7 May 2008 (UTC)

The text above mentions 365.25 but the calculation below uses 365. Which one is it?
 * Which calculation?


 * A light-year, as defined by the International Astronomical Union (IAU) is the distance light travels in one Julian year. Double check me if you like: http://www.iau.org/public_press/themes/measuring/, therefore the table with 'other light years' is not only a source of possible confusion, it is plainly wrong. Deleting accordingly.Mtiffany71 (talk) 07:54, 7 May 2008 (UTC)

Spelling: lightyear?
Would there be any objection to using the compound form lightyear? It actually gets about 7 times as many yahoo! returns. Pizza Puzzle


 * At least I don't object. But is it necessary? User:Wshun

Nothing is necessary. Pizza Puzzle


 * please could someone check with the OED. My dictionary gives "light year" in two words. -- Tarquin 21:34, 22 Sep 2003 (UTC)

Google gives a ratio of 5:1 in favour of lightyear. Dictionary.com also gives preferences to a combined form. Pizza Puzzle


 * Regardless of what's more popular among the general public, practicing physicists and astronomers always use "light year" or "light-year", never "lightyear". So the page should be moved back. -- BenRG 21:49, 22 Sep 2003 (UTC)

I agree with BenRG. Most of books I read use "light year" not lightyear. A number of Google hits hardly matter because articles on the wikipedia should sound authentic, not popular, common or ordinary. -- Taku 21:55, 22 Sep 2003 (UTC)


 * I moved the page back. -- BenRG 22:15, 22 Sep 2003 (UTC) (Incidentally, regarding the 5:1 ratio on Google, notice that all of the top 10 hits for "light year" point to astronomy-related sites, while none of the top 10 hits for "lightyear" point to anything remotely astronomy-related. I went as far as the eighth page of hits without finding any astronomy sites. Buzz Lightyear appears to be mostly responsible for the large total hit count.)

So how about light-year? Pizza Puzzle

hyphen
Why the modern hatred of the standard (traditionally standard, anyway) use of hyphens? Anyone who doubts their utility should consider the difference in meaning between a headline that says


 * New Age-Discrimination Rules Released

and one that says


 * New-Age Discrimination Rules Released.

I found "light-year" redirecting me to "light year"; I have interchanged the two pages. -- Mike Hardy


 * But since you're not a logged in user, you've done a cut & paste job -- so we've left the article history behind. I've seen your name on many talk pages -- have you any plans on creating a login for yourself here? I'm restoring the old way round -- my dictionary (Collins) has "light year" and doesn't mention a hyphenated form -- Tarquin 00:02 Dec 17, 2002 (UTC)


 * It should be "light year". Hyphens have a grammatical function when they connect nouns in English in phrases such as the new-age example above and are not part of the English-language spelling itself. It's still "new age" without hyphen. One could write: "A light-year-long distance", but "a distance of one light year". Due to the talk-page discussion under "light year", I cannot move this quite-well-written page to "light year", so I'll leave it as it is. - Hankwang 13:24, 24 Mar 2004 (UTC)

Yottametres
How much would a ly be in Ym or similar SI prefixes?--Sonjaaa 21:02, Nov 9, 2004 (UTC)

Distance to center of Galaxy
In the last few years (not sure when exactly) the center of the galaxy has been more closely determined to be around 26,000 LY, (7.9±0.2 kPc) instead of 28,000. This info is also in the Milky Way galaxy page. 130.253.146.97 14:52, 17 March 2006 (UTC)

Move to light-year
This page should be moved to light-year, which is the proper form for a compound word like this. A light-year is not a year that has some property of lightness; it has a special, separate meaning. This is the spelling used in the Oxford English Dictionary and in Webster's. -- Centrx 00:53, 11 June 2006 (UTC)


 * Reluctant agreement. Although the unhyphenated spelling is somewhat more common in the literature, it seems to me that it is an abbreviated form of "the product of the speed of light by one year". So as in foot-pound, it should be hyphenated. -- Xerxes 02:16, 11 June 2006 (UTC)

Where it was
ANY PROBLEM with clarifying that any object viewed from Earth is seen NOT where it is right now, but where it WAS (depending on the distance it's light has to travel) ? -- PFS 12 June 2006 (UTC)


 * Such a statement is meaningless, since it assumes a simultaneity that does not exist in special relativity. -- Xerxes 20:37, 12 June 2006 (UTC)


 * No it doesn't. Special relativity simply says that simultaneity is relative, not that it doesn't exist.  In a sub-luminal observer's reference frame, no event can be simultaneous with its observation.  --Doradus 15:47, 14 June 2006 (UTC)


 * The expression "right now" implies simultaneity. Certainly you can say something about the time-like separation of the emission and absorption events, but that's not the same as what the orginal poster said. -- Xerxes 00:49, 15 June 2006 (UTC)

Another question: we say e.g. "the distance to the Andromeda galaxy is 2.5 million light years". Does this mean the light has taken 2.5 million light years to reach us, or the distance now as estimated by us is 2.5 million light years. They are not the same thing since the Andromeda galaxy is approaching us and will have moved an appreciable distance in 2.5 million years. I assume since we have estimated the distance using e.g. delta Cepheids it is the second definition? Aarghdvaark (talk) 11:49, 3 March 2009 (UTC)


 * OTOH we use stars' positions and luminosities in the Andromeda galaxy from 2.5 million light years ago to make the estimate. So perhaps it is both the light has taken 2.5 million light years to reach us and the distance now as estimated by us is 2.5 million light years to where the Andromeda galaxy was 2.5 million years ago? Aarghdvaark (talk) 11:55, 3 March 2009 (UTC)

small point - commas vs NBSP
I was wondering if it would be best to use commas rather than &amp;nbsp;s. Commas might make it more clear that its one number, and are easier to read on the wikiside. Also, copy pasting numbers with spaces might multiply the pieces together (say in mathematica), while commas would alert the user (ie error). Fresheneesz 03:56, 13 June 2006 (UTC)


 * Unfortunately commas are confusing to international readers, fubar cut-and-pastes and just look ugly. I much prefer &amp;nbsp;s even though they make the soruce a bit hard to read. Actually, perhaps it would be better to change the whole thing to scientific notation. -- Xerxes 16:52, 13 June 2006 (UTC)


 * They may be confusing to non-native speakers of English, but in every variety of English, commas are standard separators for thousands. In non-English editions of wikipedia, I would agree to remove the commas. But not for the English version. Rhialto 00:05, 4 October 2006 (UTC)

I figure 1 light-year as being about 63000 AU, not 90000 as suggested in the conversions section.


 * You wouldn't have to use either, if the numbers here were expressed with a precision appropriate for any measurement ever made in light years, and you used either the SI prefixes or scientific or engineering notation. Gene Nygaard 22:53, 8 October 2006 (UTC)


 * And then you run into the issue of accessibility for people not familiar with scientific notation. Rhialto 04:20, 24 October 2006 (UTC)

There is a considerable numerical error: "A light-year is equal to about 9.461 Tm." The correct value is 1000 times larger and reads "9.461 Pm". Maybe someone was confused by commas and dots. By the way - international standard for the decimal sign is a comma in every language of the world according to ISO. Correspondingly, adjacent groups of ciphers should be separated by a single blank character. Achim10 01:47, 7 February 2007 (UTC)


 * I'm not really interested in that specific ISO standard regarding number format. English wikipedia has its own manual of style, which overrides any external opinion on that specific point. And wikipedia's manual of style requires commas as a thousands separator and a point as a decimal separator.


 * Regarding that Pm/Tm error, I rather think it is being caused by the base value being quoted in km and not m, and people trying to work it out from that figure without othering to check the unit initially being quoted.


 * Rhialto 03:45, 7 February 2007 (UTC)

The box of equivalent units at the top right of the page is wrong
I'm a new user and can't figure out how to change it. Specifically the exponent for statue miles (5.559x10^15) should be 12, not 15, and the value 5.559 should be made consistent with the value in the body of the article (5.878). (By the way, how *do* you edit the box?) (JWBlair) —Preceding unsigned comment added by JWBlair (talk • contribs) 07:41, 8 August 2006 (UTC)

Recalculate the figures
Unless anyone has some objections, I would like to recalculate the distances based of a year of exactly 365.25 days (each of exactly 24 hours). That's the value used by the IAU, and as the only widely-recognised organisation that both has a standard definition for the unit and uses it with any real frequency, it seems reasonable to use them as the ultimate authority for choosing which definition of a year should be used. Rhialto 04:29, 24 October 2006 (UTC)

Kilolightyear (kly)?
Is this a real unit? It seem a bit like a kiloweek or a kilopoundfoot - perfectly clear what it means, but, as far as I am aware, no one actually uses it as a unit. The same goes for megalightyear, etc. Astronomers would use the parsec or kiloparsec anyway. -- ALoan (Talk) 12:18, 1 December 2006 (UTC)

Meaningless precision
14 significant figures is much too much precision for light-year, so I changed it, and removed the discussion of the exact figure. Even the number of figures I left in is on the high side.Saros136 12:58, 8 February 2007 (UTC)

I'm reverting. Long figures can't be calculated that precisely, it is true. But since the metre is defined in terms of a precise relatiosnhip with the ligh-year, it is indeed a precise value, even though it cannot be measured in any practical sense. the astronomical unit can't be measured so precisely (it is only known to about 15 decimal places), and the parsec is defined ina precise way relative to the AU, so those two will never be a precise value. But the km and mile have known and exact values. Rhialto 13:37, 8 February 2007 (UTC)


 * Yes, the light-year does have an exact value. But the exactness is meaningless, physically, so why include it? Round values are easier to read and comprehend. Saros136 06:42, 10 February 2007 (UTC)


 * They are easier to include, true, and I don't have much issue with including them, IN ADDITION to the precise values (note that I did exactly that for the astronomical unit). Wikipedia is first and foremost an encyclopaedia, and to remove relevant information simply to dumb down the content seems rather counter-productive. Someone somewhere probably does need that information, it is correct information, and the only logical place for that information is in this article. There is nothing wrong with giving information to the highest level of precision known, within the limits of rounding errors from conversion factors and uncertainty from fundamental physical constants. Woudl you also argue that the exactness for 'e' is also meaningless? For most people, it probably is, but for those who need the exactness, we should make it available. Rhialto 08:38, 10 February 2007 (UTC)


 * Thinking about it more, anyone who would need the full precision would almost certainly be using metres, or at least be readily able to convert metres to another unit, seeing as how all the other units noted include some level of rounding error. Since such high levels of precision are therefore not really needed for the other conversion factors, I've decided to round those others (ie not metres, for which the full precision should be retained, but a rounded approximation included) to 4 significant figures, in line with muy usual approach to rounding in metrology issues. Rhialto 12:39, 10 February 2007 (UTC)

I still don't understand the lack of precision in the first listing for miles. This is illogical for two reasons The first is that Rhialto stated that this is inherently inprecise. Why is this? This conflicts with this quote from the miles page "the statute mile of 5,280 feet (1,609.344 m exactly)" The second problem is that the more precise mile value is listed below in the included table. Why one time use a less precise value and below use a more precise value? Please enlighten me. 221.218.190.172 15:53, 20 August 2007 (UTC) Steve

unreferenced
I tagged the article as unreferenced because it completely lacks in any published sources. Those are surely easy to find, but nevertheless must be included. Kncyu38 05:30, 2 December 2006 (UTC)


 * The IAU style guide 


 * It directly cites the definition provided by the IAU, which is as authoritative a definition as you could hope for on the topic. Rhialto 12:52, 2 December 2006 (UTC)


 * Agree on authority of IAU, but I just visited their page and couldn't find where it states that "The Andromeda Galaxy is 2.5 megalight-years away", for example. And the Miscellaneous facts section features notes that lead nowhere and none of the facts is so far backed up by any reliable source. For example, the section states that


 * In the Disney movie Toy Story one character was named Buzz Lightyear. Buzz referring to Buzz Aldrin - one of the first men on the moon, and Lightyear referring to astronomical distance.


 * But without giving a source for that, it's just speculation, and that's putting aside my personal opinion that this is particularly useless trivia not worthy of inclusion. Kncyu38 13:10, 2 December 2006 (UTC)


 * Well, a lot of the trivia section needs citing, I agree. But the core of the article with the important information is sound. Putting that needs references on the header makes it look like the article is fundamentally unsound. I think it would be better to place < > tags on each specific item that needs citing, rather than label the entire article as being unreferenced, purely on account of the trivia section. Rhialto 20:35, 2 December 2006 (UTC)


 * I know an article looks bad being tagged as a whole, but in my opinion that is the very point of a tag like unreferenced: It isn't there for the reader (who can easily spot the lack of sources), but rather for the editors, as an incentive to improve the article. The problem I have is not only with the trivia section, but also with my first example above: How are bits like "The Andromeda Galaxy is 2.5 megalight-years away" being backed up by the IAU? Kncyu38 21:24, 2 December 2006 (UTC)


 * It might be more efficient to remove the trivia section entirely than to find cites for all the tangential information. That removes the need for the cite tags just as effectively. Another approach would be to place that big cite header over the sections which contain the uncited data, instead of over the entire article. Rhialto 02:47, 3 December 2006 (UTC)

iau definition

 * To be precise: The IAU paper doesn’t state that a light-year is “the distance light travels in vacuum in one Julian year”, just that “it is best to regard a year as a julian year of 365.25 days (31.5576 Ms) unless otherwise specified.” I think the IAU doesn’t make an authoritative definite statement about which year to use for the light-year here. It would be good to express this (slight) uncertainty more appropriately in the first sentence, or to write just “in one year” and discuss the details below. Moreover, though I didn’t find a suitable reference, the tropical year seems to have been used in the past, possibly by the SI before 1978. Finally, though it is desirable to clear up this issue, it is difficult to find an example where the precision would matter. Maybe the aphelion of (87269) 2000 OO67 is an example where the light-year as a unit could make sense and the precision is high enough. --80.129.113.90 16:19, 28 April 2007 (UTC)

I haven't seen any reason to consider (87269) 2000 OO67 to have any relevance to the light-year definition. The common definition is the distance light travels in one year in an ideal vacuum. The speed of light is precisely defined, and the only question is which definition of the year is used.

From :


 * "The IAU has used the julian century of 36 525 days in the fundamental formulae for precession, but the more appropriate basic unit for such purposes and for expressing very long periods is the year. The recognised symbol for a year is the letter a, rather than yr, which is often used in papers in English; the corresponding symbols for a century (ha and cy) should not be used. Although there are several different kinds of year (as there are several kinds of day), it is best to regard a year as a julian year of 365.25 days (31.5576 Ms) unless otherwise specified."

Obviously, that 31.5576 megaseconds in a julian year should be taken as a rounded value; the precise number of seconds can be easily calculated, given the standard definition of a day.

To be sure, the IAU site doesn't specifically say a ly *must* be based on a julian year, but that is the only year they recomend for use in this definition. I think when they say "it is best to regard a year as a julian year" can be taken as a recomendation, which is the wording used in the article at present. Note that the article doesn't directly state that this (julian year-based ly) is the only value; it even gives alternative values as shown by the google and yahoo calculators. The values given in the header are those based on the recomended ("best to regard") value given by the IAU, which by earlier discussion was considered the best authority to use for a primary definition on this unit.

Rhialto 20:19, 28 April 2007 (UTC)


 * Let me put it as follows: Mr Wilkinson, the author of the IAU “Recommendations concerning Units”, can not be held responsible for the narrowed Wikipedia interpretation expressed by the most prominent statement of the article, “A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one Julian year”, which is the wording currently used in the article. Maybe there is no need to bother about this. Now I will try to explain you why (87269) 2000 OO67 has some relevance to the light-year definition: Because all candidates for the length of a year (Julian, Gregorian, tropical, sidereal) are within a 0.005 percent margin, it only makes sense to specify the year used if the distance one wishes to express in light-years is known to at least five digits. Therefore, if such distances don’t exist, it doesn’t make sense at all. And this is of some relevance to the definition, I daresay. --80.129.113.90 21:21, 28 April 2007 (UTC)


 * I reworded the opening sentence to remove that absolute certainty previously expressed. It no longer defines the julian year as the only acceptable year in that opener.


 * You are of course right that such distances can't be measured to that level of precision with existing technology, but that doesn't change the fact that the unit can be defined with such precision. Consider that we can't measure the speed of light with perfect precision either, but that doesn't prevent us from defining it with perfect precision. Ditto for almost every other base SI unit. In any case, presently, no authoritative body has used that astronomical body in any light-year definition, so to use it as such would constitute original research. Rhialto 22:06, 28 April 2007 (UTC)


 * Thank you. You are right, the example should certainly not be made part of the definition. It was intended to illustrate what I wrote, and giving examples complies with WP:NOR, I think. Still, I am in doubt myself whether it is fit for inclusion in the article. --80.129.116.39 06:47, 29 April 2007 (UTC)

Light-year table
Please provide the Google and Yahoo references for the days in a year. They are search engines, not references. Google gives this value when "light-year" is entered in the search field as part of its calculator function. Except for myriad "answers" by anonymous individuals, I don't see a Yahoo value. 365.242199 days is obviously the 1900 value for the tropical year rounded to six digits. The 2000 value is 365.242190 days rounded to six digits. 365.2422 days is simply the tropical year rounded to four digits and is widely used as such in the literature. 365.2411 days is an obvious error, and without an authoritative reference, will be removed. Such an obvious error justifies its removal in any case. — Joe Kress 21:54, 5 June 2007 (UTC)


 * Personally, I'd be tempted to drop the Yahoo and Google lines in that table. They aren't definitions so much as measurements. Formal documenst have said things along the lines of a light-year is the distance light travels in one Julian year or one Gregorian year, or one floopydoopy year. No one has offically defined a ly by the measurement that those search engines use; they aren't authoritative sources, and that listing turns the article into one about those websites' calculators, rather than about the ly itself. Rhialto 23:27, 5 June 2007 (UTC)

Light-year and relativity
When describing what a light-year is, wouldn't it be prudent to state that light takes a year to travel this distance based on an observer's watch on Earth. In actuality, if a spaceship were to leave Earth at the speed of light and head to a star 10 light years away, that trip would be instantaneous for the person on the spaceship but it would appear to take 10 years for a stationary observer on Earth.

Something that truly travels at light speed should, by the nature of the theory of relativity, be able to reach any other point in the universe instantly. When thought of in this fashion, it is easier to understand why going faster than light violates causality (because that would mean reaching point B before ever having left point A). —Preceding unsigned comment added by Aphexcoil (talk • contribs) 05:27, 3 November 2007 (UTC)

Light speed is relative not constant. A researcher in Massachusetts has even slowed it to a stop at temperatures that approach absolute zero. read: www.smithsonianmag.com/science-nature/phenom-200801.html —Preceding unsigned comment added by 74.70.206.210 (talk) 02:37, 5 January 2008 (UTC)

Trailing zeroes
I have left the trailing zeroes that have been re-added for now. I would like to invite a discussion as to why they have been added though. In giving the conversion in km (and miles), the values are exact values, not rounded. In the case of the years, it makes no sense to talk of a year "1900.5", so there is no need for "1900.0", since there is nothing that the trailing zero could meaningfully distinguish a non-trailing zero value from. Rhialto (talk) 06:56, 23 April 2008 (UTC)


 * The reason for the trailing zeros in the conversion is that it is the product of a time in seconds and speed in metres per second, so the two trailing zeros indicate that the original conversion was metres, not kilometres. However, I understand your reasoning that it is still an exact value. But both of the mean tropical years are conventions used in astronomy that have a specific meaning. In astronomy 1900.0 means the instant in time 1900 January 0 12:00:00 (noon), where 1900 January 0 = 1899 December 31. Thus both the 1900.0 and 2000.0 mean tropical years refer to instantaneous mean tropical years, which are calculated from a polynomial in time, where there is a significant difference between the length of the mean tropical year at the beginning and the end of a single calendar year. Specifically, in 1895, the mean tropical year was defined in Newcomb's Tables of the Sun as 365.24219879 – 0.00000614T days, where T is the time in Julian centuries since 1900 January 0 12:00:00 (its length is smaller for later times and is larger for earlier times). Only at the instant 1900.0 can the linear change be zero. 1900.0 is already linked in the article to aide the reader in understanding it (it redirects to epoch (astronomy)). — Joe Kress (talk) 08:55, 28 April 2008 (UTC)


 * Based on that explanation, I will leave the years as they are, but change the distance value. When I originally calculated those values, I had noted them as "exactly" those values in plain text. It seems more sensible to me to indicate that a value is exact in plain English rather than use a shorthand notation, unless the context doesn't allow the space for such a longhand notation. In this case of the distance, there is sufficient space. Rhialto (talk) 09:08, 28 April 2008 (UTC)


 * The statute miles conversion is not exact. I suspect you are using a conversion utility or calculator that is limited to double precision, 64 bits or about 16 decimal digits. When using a quad precision calculator, 128 bits or about 32 digits, like that in Microsoft Windows (Start - Programs - Accessories - Calculator), division of the exact value 9,460,730,472,580,800 metres by the exact conversion 1609.344 metres/statute mile (based on 25.4 mm/inch, actually the international mile) the quotient is 5,878,625,373,183.607,730,851,825,340,014,3 statute miles, which shows no signs of terminating. — Joe Kress (talk) 02:41, 1 May 2008 (UTC)

We should not use word "billion"
[ Sorry for my bad English :-) ]

IMHO we should not use termin "billion" and we should correct phrases like this: "One gigalight-year, abbreviation "Gly", is one billion light-years — one of the largest distance measures used". There are two diferent billions exists: one is 109 and one is 1012! Please see Long and short scales. I think we can use word "milliard" for 109 and nothing like this for higher powers. At higher power words "billion" and "trillion" have double meanings. Other words after them have well-defined meanings but this words based on "billion" and "trillion": billiard likes something after billion but we have two billions, quadrillion sounds like something after trillion but we have two trillions. 88.84.193.7 (talk) 22:06, 25 April 2008 (UTC)


 * The problem with "milliard" and related words is that, in the English-speaking world, very few people use them this side of WW2. The wiki needs to be readily understandable to the majority of modern English speakers. The 10^12 billion was formerly used mainly in the UK, but for all official documents, even the UK government uses a 10^9 billion these days. Rhialto (talk) 22:25, 25 April 2008 (UTC)


 * I had a quick look over the Long and short scales article. Every English speaking country, without exception, officially uses the short scale, as used in this article. The one exception is Canada, which notes that the French-speaking portion uses long scale. This article's usage is in line with standard usage in every English-speaking country, and so there is no grounds at all to change the scale usage. Rhialto (talk) 22:30, 25 April 2008 (UTC)


 * I'm afraid it is so. Hold-outs like me clinging desperately to the logical and original definitions of the words are getting fewer and further between. We'll just have to roll our eyes, shake our heads and adapt. J IM ptalk·cont 07:44, 28 August 2009 (UTC)

Who defines the light year?
Are there any bodies or organizations other than the International Astronomical Union which are recognized as being responsible for defining the light year? Yes? Then please cite them when providing their definition. No? Then the IAU definition is the one that can be used, because, in case anyone has forgotten, you can't just make stuff up, and the IAU definition is the only definition for which anyone has provided a citation. And the IAU definition defines a light year as being the distance light travels in a vacuum in one Julian year, which is 365.25 days, where a day is 86400 seconds in duration... see http://www.iau.org/public_press/themes/measuring/ fourth sentence of the fourth paragraph.

Mtiffany71 (talk) 19:56, 7 May 2008 (UTC)


 * According to the IAU, "Although there are several different kinds of year (as there are several kinds of day), it is best to regard a year as a julian year of 365.25 days (31.5576 Ms) unless otherwise specified." That is far from making an authoritative definition of the unit. It is weasel-wordy to the point that no objective reading could really consider it a definition; it really is a mere recommendation. And their wording clearly implies at least acknowledgement that there are other year definitions that can and are used as a basis for measuring a light-year.


 * However, I'm happy to leave it as being "defined", since it fits in with my personal biases. But I am aware enough to recognise that leaving it that way is far from objective. Rhialto (talk) 20:17, 7 May 2008 (UTC)


 * I dispute on its face your assertion that the IAU is merely 'recommending' that people use the Julian year and that other definitions of year are perfectly acceptable to substitute based on nothing more than mere whim or ignorance. The fact that the IAU does not use the sentence construction "A light year is defined as..." does not mean that the sentence construction that is used is not a definition. If you want to call it weasel-wordy, that's fine by me, since it is still a definition because any objective reading would make it obvious that the "year" in "light year" refers to the Julian year (because that's what the definition explicitly says) unless of course "light year" is explicitly modified by another type of year, which could perhaps be a "tropical light year" or "sidereal light year" or an "anomalistic light year" or even (why not) a "Chinese luni-solar light year." But since I've never encountered any of those particular variations of "light year" in any of the popular (amateur) literature, then I guess it's safe to assume that the year which is being referred to is the Julian year. But hey, if anyone can find an article or other lit that uses a "tropical light year," "sidereal light year," or any other variation, please cite it.


 * And of course, I'm happy that you're happy to leave it as it, even if you have to resort to saying that you're only doing so because it fits with your own biases. But wouldn't it just be more logical and defensible to leave the definition as it is because it is the only definition provided so far which explicitly cites its source? I mean, if there are other institutions which use another type of year in their definition of a light year, then cite them, cite the source, and we can have as many definitions as there are cited, verifiable sources. Do we really need to be reminded that one of the requirements for inclusion in an article is verifiability?


 * If you can find an alternative definition, provide it and cite the source.
 * Mtiffany71 (talk) 10:48, 8 May 2008 (UTC)

Of course, it doesn't actually matter if you use the Julian year, the Gregorian year, the sidereal year, the tropical year, the Besselian year or the mean Chinese lunar-solar year! Measurements of distances to stars (other than the Sun) are simply not that accurate: you're lucky to get a distance at ±10% in the Hipparcos catalogue, the most accurate set of parallax measurements available at present. Not to mention that the metre is simply undefined as a unit of proper length at astronomical time scales. The debate is pointless. Physchim62 (talk) 14:34, 10 May 2009 (UTC)

Distances in light-years
In the chart in the section "Distances in ligh-years" there was a single use of kiloparsec for a distance so I changed it to light-years. The edit was reverted though with the explanation that it was "unused terminology". From the sounds of it though the intent of the section is to list out distances in light-years. A similar listing of distances in parsecs already exists in the parsec article. --GrandDrake (talk) 18:46, 4 May 2009 (UTC)

In the table it says "The Andromeda Galaxy is approximately 2.5 megalight-years away". I have sometimes read in Astronomy books that this means the light has taken 2.5 million years to reach us, but this gives a different distance (as it is moving towards us) from the reading that the Andromeda Galaxy is currently estimated to be 2.5 million light years away. Which is the correct interpretation? Aarghdvaark (talk) 10:37, 15 July 2009 (UTC)


 * To my knowledge, we are yet to be able to measure these huge distances at such an accuracy that it would frankly matter. Andromeda is estimated to approach us with a speed of ~300 km/s, it is still only 1/1000 of the speed of light. So in 2.5 million years the galaxy would travel only 1/1000 of the distance. Afaik, there's no distance-estimates given with such an accuracy, error margin alone being some percentages. --J. Sketter (talk) 05:18, 12 August 2009 (UTC)

Gravity has effect at infinite distance
The phrase "Maximum extent of the Sun's gravitational pull" should be corrected since gravity has no known maximum distance effect. —Preceding unsigned comment added by 66.81.112.220 (talk) 14:49, 11 October 2009 (UTC)


 * Changed "gravitational pull" to "gravitational dominance". — Joe Kress (talk) 18:36, 11 October 2009 (UTC)

Opening sentence of article is too cryptic for the non-scientific reader
Wikipedia is a public use encyclopedia, saying that a light-year is ten to the thirteenth power kilometers is obscure language considering the audience. Its OK to use scientific language in Wikipedia articles but that should be in parallel to language and descriptions understandable to the average reader where possible. Simply put-- how many kilometers (and miles) is a light-year in everyday language?

Sean7phil (talk) 02:27, 4 December 2009 (UTC)


 * I've rephrased it to say "...just under 100 trillion (i.e. 1013) kilometres" - is that better/? Olaf Davis (talk) 10:05, 4 December 2009 (UTC) Also moving this section to the bottom of the talk page.Olaf Davis (talk) 10:06, 4 December 2009 (UTC)


 * An anonymous editor has already corrected this to 10 trillion kilometres. — Joe Kress (talk) 18:59, 4 December 2009 (UTC)

Thanks Olaf! Thats a good alternative. Thanks also for your kind consideration.

Sean7phil (talk) 19:07, 5 December 2009 (UTC)

History section
I reverted the beginning of a History section in the article. The first paragraph perpetuated the misconception that a light-year is a unit of time. It is not. It is purely a unit of distance, and has nothing to do with how long it would take to travel the distance (all practical means of travel are tiny fractions of the speed of light). It is not like saying, "The nearest supermarket is 25 minutes from here." The second paragraph is unsourced speculation that the first measurement of 61 Cygni's distance from Earth was the impetus for using the light-year as a unit for measuring astronomical distances. The third paragraph is impermissible original research because it is a synthesis of primary sources. The edit summary acknowledges that "sources are scanty", but that does not justify adding content that is not supported by reliable sources. A History section would have to be based on reliable sources that give the history of the light-year as such. There probably are such sources. Perhaps other encyclopedias or science dictionaries. Perhaps the Oxford English Dictionary.—Finell 07:16, 11 December 2009 (UTC)


 * 1. Good luck defining light-year without 'how long it takes to travel the distance.' It's right there in the header: 'One Julian year.' That could have been written better, but I felt it was a start. 2. The last sentence of para 2 was unreferenced, but the literature of the time is full of discussions of light-speed - in place of huge, unwieldy trilions-of-miles numbers - before the term was coined. But I can't talk about that, that's not blind copying, that's decades of reading talking. 3. There is no substantial 'history of the light-year as such' in print, thus a history would have to be 'synthesized' - like most sections in Wikipedia - from multiple sources. By editors doing original research, if you understand policy right. 4. Most 'authoritative' sources quote the range of 1885-1890 for the coining of the term. That means noone knows for sure. They're all wrong; the term appeared no later than 1868. But that fact will never get to WP because: no research allowed. Unlike -real- encyclopedias.Twang (talk) 18:34, 13 December 2009 (UTC)


 * I disagree with the complete removal of the history section. Instead it should be reworded and cited to meet the objections. A source for the second paragraph is Light-years by ASTROLab of Mont-Mégantic National Park of Quebec, Canada, which states that Bessel was the first to use a light-year as a unit of measurement in 1838. But this is not literal because his 1838 article in Astronomische Nachrichten, "Bestimmung der Entfernung des 61sten Sterns des Schwans" (Determine the distance of the 61st star of the Swan) only uses an equivalent phrase (col. 93), "die Zeit, welche das Licht gebraucht, um diese Entfernung zu durchlaufen" (the time, which the light needs, to traverse this distance). The third paragraph is a reasonable summary of a history of the term light-year which actually appears in the cited reference, [Sten F. Odenwald, Back To The Astronomy Cafe. Westview Press, 2003, p.220]. It begins with an 1844 book, A system of natural philosophy by J. L. Comstock, which also contains an equivalent phrase, "a distance so immense, that light cannot pass through it in less than three years". This identical quote appears in the 1830 edition of this book, making it even earlier than Bessel's equivalent phrase. Oldenwald also notes that William Herschel may have used it. Indeed an article by him in the 1802 edition of the Philosophical Transactions of the Royal Society of London contains the equivalent phrase "the rays of light which convey its image to the eye, must have been ... almost two millions of years on their way". The Oxford English Dictionary lists quotations from 1888 to 1973. The earliest quote for 1888 is from the Athenæum 27 Oct 558/2. The next quote is from 1890, C. A. Young, Elements of Astronomy xii. §433. See also Science Forums. — Joe Kress (talk) 02:55, 19 December 2009 (UTC)

"Farthest known astronomical objects per year of record"
I do not believe this section, "Farthest known astronomical objects per year of record" is appropriate for this article, it should be in some other article. Say a List of most distant astronomical object record holders or something (at WT:AST, it there was a new discussion titled Timeline of farthest known astronomical objects, which would also work).

70.29.210.242 (talk) 10:52, 17 February 2010 (UTC)


 * You are right. I have started the suggested page and moved the information there. It would be great if I could use your table to complete the List of most distant astronomical object record holders --Micru (talk) 12:30, 17 February 2010 (UTC)

Text under the picture: "Outer shell is one light year from Sun and left yellow line is Comet 1910 A1's orbit"
All lines (both curvy and straight) on the picture are yellow. Whitch one is the "left yellow line"? 84.50.182.154 (talk) 09:43, 23 August 2010 (UTC)
 * Perhaps the writer thought a line is by definition straight ^^ ..even if it's in reality an ellipse. This version of the image is a bit unclear in this size, true. --J. Sketter (talk) 20:04, 27 August 2010 (UTC)

Nomination for Good Article Status: Light-Year article
With respects to the previous discussion by Rhialto 'Trailing zeroes' ...

As noted, this is a very important topic and should be improved. Further discussion and a reasonable timeline for resolution should be considered. Therefore, an election should be held to establish new standards and points of referral for detailed information and concise material.

You may participate in organizing an article improvement campaign by registering at the main Wikipedia article improvement page

As a start point, it is suggested that all scientific notation; unless needed, should be rounded up (during the winter solstice) and rounded down (during the summer solstice) that a more accurate standard system may be observed. This simple rule of operation should be standard across all Wikipedia and its sister sites. (i.e.- the Parsec Shift Project)
 * (Note)-

In observation of Rhialto's conceptualization, a series of scientific notation data tables may be implemented and referenced to summarize all precision data needed to make the article(s) more accurate. This will add to the flexibility of editors to gather detailed information and help reader groups learn the difference between standard notation and scientific notation.

Statements concerning measuring large distances and other modes of astronomical standards should be respectively subtitled "The Wikipedia Standard System (or WiS)"

In improving this article and implementing a more detailed standard for referencing, the entire encyclopedia may benefit from its newest features.

Please respond to this post if improvement is needed or if available for election contribution. Provisions for Good Article status are disclosed here on the main Wikipedia site.

Habatchii (talk) 04:45, 28 February 2011 (UTC)

Illustration


I removed the illustration which conveys a wrong impression.

The aphelion of comet 1910 is less than 3,000 UA. One light year is over 63,000 UA, which is over 20 times more. 109.9.80.45 (talk) 09:46, 5 February 2012 (UTC)


 * I've added the illustration here so we can see what we're talking about (first picture on the right). According to the Comet 1910 A1 article, yes, the aphelion is 2974 AU. So either the article is wrong or this picture is wrong.  Note: there are more of these pictures (which may be worth adding, if it turns out that they're okay afterall). J IM ptalk·cont 00:54, 6 February 2012 (UTC)

Conversion overkill
Currently the first sentence reads like this.

"A light-year ... is ... just under 10 trillion kilometres (10 metres, 10 petametres or about 6 trillion miles)."

Recently I switched it to the following.

"A light-year ... is ... just under 10 trillion kilometres (1016 metres or about 6 trillion miles)."

But I was reverted with the following comment.

"approximate value in meters should not be in scientific notation but in engineering notation to match 10 trillion km and 10 Pm immediately before and after, averting endless 'corrections' because 10 is missing"

I disagree about the idea that engineering notion is better than scientific notation (though 1016 isn't strictly scientific notation either, 1 is the standard scientfic notation). I don't believe that engineering notation is all that familiar outside of circles which use it (i.e. engineers, etc.) This is not an engineering article. So the points it scores for matching "10 trillion kilometres" it looses for not fitting the context. As for it's matching 10 petametres ... hadn't I got rid of those? If the purpose is to avert endless "corrections", I don't see the need. A hidden note to editors would do the trick. In fact "10" could even encourage mislead "corrections" by people (unfamiliar with engineering notation) inserting the missing decimal point.

However, I'm wondering why we even need metres at all. This is the first sentence of the article and we have three conversions to metric. Isn't this overkill? As I've just mentioned, I'd got rid of the petametre conversion. This issue is raised couple of sections above. The question raised there was whether the conversion adds anything to a reader's understanding of the light-year. I don't believe it does, petametres aren't exactly in common use. So is there any use in including petametres? Well, it gives the reader an idea of the nearest SI unit and this is good but does it belong in the introduction? No, this is useful information but it's more about the petametre than the light-year. Petametres are better saved for the Numerical value section (and they already do appear there). Then why not get rid of the metre conversion whilst we're at it? Is a conversion from kilometres to metres of any use at all? Multiplication and division by a thousand is pretty straight-forward arithmetic. It's arguably helpful for readers who might not get what we mean by "trillion" but the short scale is used all over WP (for better or for worse) and those of us who still call a million million "a billion" mostly know that this usage is dying out. Piping the link to 1000000000000 (number) should suffice. I suggest we get rid of both the metre and petametre conversions from the intro and leave them for the Numerical value section. This would give us the more concise

"A light-year ... is ... just under 10 trillion kilometres (or about 6 trillion miles)."

J IM ptalk·cont 06:24, 18 May 2012 (UTC)


 * The several parts of the lead sentence were accreted over several years. It began without any length, then 10 trillion km was added. Possibly to avoid problems with long and short scales, this was clarified as 10 trillion (i.e. 10$1 m$) km. Then the power was moved after as 10$1 m$ m. Finally, Woodstone added 10 Pm and another editor added 6 trillion miles. Since that time, several editors have "corrected" 10$1 m$ to 10$13$ and were reverted. Those were the edits I was trying to avoid via 10 m. I have no objection to removing all except 10 trillion km (along with 6 trillion miles). — Joe Kress (talk) 07:38, 18 May 2012 (UTC)


 * I'm glad we can agree. J IM ptalk·cont 02:14, 21 May 2012 (UTC)

Numerical Value
This section was originally created to have a layman-readable set of values in units that a layman is likely to be familiar with.

Extreme amounts of significant figures reduce the human-readable aspect of this section (I'd make an exception for for precise values). Additionally, some of these units, most notably nautical miles, have no real familiarity to a layman, making them listcruft. And some are excessive duplication (metres, kilometres, and petametres? really?). The section has become a case of too much information.

Finally, tables should not be used to present this information, especially tables that use blank cells as an integral part of the formatting. This makes the article almost imparsable for readers that are used by the visually-impaired.

Rhialto (talk) 13:49, 18 May 2012 (UTC)


 * These are fair points.
 * We could get rid of some of these units and round others off.
 * The light-second could go. Yes there are 31,557,600 seconds in a Julian year.  Anyone with a calculator (or enough patience) can figure that out.  Then multiply by the speed of light.
 * The metre should stay and should be exact.
 * The kilometre could go. I'd thought it might be worth including to help clarify that we mean a million million when we write "trillion" but it's probably unnecessary.
 * The petametre could go. As mentioned already on this page it's not a familiar unit so doesn't help understanding the topic at hand.  Its inclusion has some marginal use in that it gives us the closest SI unit but, of course, we can find that elsewhere.  Note that 1 petametre is linked from See also.
 * The statute mile should stay. A good proportion of English speakers use this unit.  How about we round it off to the nearest million miles?
 * The nautical mile could go. It doesn't really make sense to use this unit for extraterrestrial distances.  I'd added it because apparently NASA uses nautical miles (though probably not for distances comparable to the light-year).
 * The astronomical unit, I think, should stay. Whilst not an every-day unit, it is used for outer space distances.  If we round if off to the nearest one AU we'll have five significant figures which seems a reasonable degree of precision.
 * The parsec should stay. The parsec is the main alternative to the light-year, the unit actually used in astronomy.  It deserves a mention.  It's probably reasonable to round it off to five or six significant figures to match the AU precision.
 * If a table shouldn't be used here, should a table be used anywhere? The table is much clearer than the bullet points we used to have (for those who can see).  There must be a way to cater for the visually impared whilst not making the article less clear for those who can see.  It would be nice is somehow we could solve this.  In the meantime, how about something like this as a sort of compromise?


 * 1 light-year = 9,460,730,472,580,800 metres (exactly)
 * ≈ 5,878,625 milllion miles
 * ≈ 63,241 astronomical units
 * ≈ 0.306601 parsecs
 * J IM ptalk·cont 00:55, 21 May 2012 (UTC)

That looks reasonable to me (but without the blockquote tags, which have a specific purpose and are interpreted in a special way by readers). Rhialto (talk) 06:34, 21 May 2012 (UTC)


 * Yes, no, the blockquotes were just intended as a quote on this talk page. J IM ptalk·cont 15:36, 21 May 2012 (UTC)


 * I agree with the precision of astronomical units and parsecs but for a different reason than 1 AU. They should have as many significant digits as needed to distinguish their Julian year values from their Gregorian year values. This ratio differs from unity by 2 so five significant digits should be enough. Unfortunately the five significant digit parsec value, 0.30660, cannot distinguished the two, so another digit, 0.306601, is needed. BOMBINI recently tried to change the parsec value to 0.306595, its Gregorian year value, which is still cited in many sources. — Joe Kress (talk) 17:58, 22 May 2012 (UTC)

Meters vs. km
The approximate values in km and miles are mentioned in the lede. In the section on numerical value, it's appropriate to use the SI units in which the speed of light and the light year are defined, i.e. meters and seconds, as also used in the sources. And I don't think very many people have an easier time visualizing 9460730472580.8 km than 9460730472580800 m. Both numbers are far beyond everyday experiences. --Stephan Schulz (talk) 13:00, 23 May 2012 (UTC)

Request for protection
I have declined a request for protection. Please resolve this by attempting local discussion first. If you want to see both imperial and metric measurements to avoid any claim of geographic bias, have a look at using the  template. --Fæ (talk) 13:08, 23 May 2012 (UTC)

Typo in speed of light?
In this section: "Numerical value ...

1 light-year = 9460730472580800 metres (exactly)

≈ 5 878 625 trillion miles"

perhaps the "5 878 625" was to be "5.878,625". -Jonathan King


 * Actually, it should be 5.878626 trillion miles (I don't think you can use a coma for grouping digits after the decimal point). Corrected. Thanks! --Stephan Schulz (talk) 21:29, 21 October 2012 (UTC)


 * The link from "trillion" leads to an ambiguity. Would anyone object to using "million million" instead, since this means the same world-wide, even in translation?    D b f i r s   20:55, 24 October 2012 (UTC)
 * Fine with me. I'd go with 1e12 personally, but scientific notation is not the best for all readers... ;-). --Stephan Schulz (talk) 21:02, 24 October 2012 (UTC)


 * That a light-year is "about 6 trillion miles" is mentioned in the intro. The link is to 1000000000000 (number) (not to Trillion), it's there to clarify what we mean by "trillion" in this article and I think it does the job.  "5  878  625 million miles" (as opposed to "5.878  625 trillion miles") was a deliberate attempt to further clarify the ambiguous word.  As for "5.878  625 million million", I'm sorry but I disagree.  Firstly, people rarely use this (e.g. when defining numbers like "billion" or "trillion").  Secondly, I don't see how it's better than "5  878  625 million miles"; would you write "63.241 thousand"? By "1e12" you mean "$16$ miles", right? No, it should be as simple as possible at this point in as general-interest an artlce as this; besides scientific notation appears in the infobox anyway. J IM ptalk·cont 23:58, 24 October 2012 (UTC)


 * But the link is a redirect to an article, Orders of magnitude (numbers) that gives two different definitions of the trillion. I suppose most readers know that Wikipedia uses short scale, but it could be very confusing to those (including myself) who use the long scale (Long and short scales).    D b f i r s   07:16, 25 October 2012 (UTC)


 * I concur with the idea of writing "xyz millions", as it absolutely avoids any confusion surrounding the definition of the word "trillions". Rhialto (talk) 10:00, 25 October 2012 (UTC)


 * We hold outs who stick to the original definition of trillion (except perhaps when editing Wikipedia) mostly know we're a dying breed. Yes, the link goes to Orders of magnitude (numbers) but, specifically, it goes to the section 1012 of the article; that section gives two names for the number, one the one used in the article, the other the older name billion, but I think it's clear enough that we mean a million million. J IM ptalk·cont 23:40, 25 October 2012 (UTC)


 * It's not just us "British oldies" who might be confused, but most of Europe uses long-scale (though only a small proportion will read Wikipedia in English). I tried the link in a different browser and it did go to the correct place, and my confusion was only because I was half asleep and couldn't remember what American trillions were.  I always taught that real scientists use powers of ten, not ambiguous words like billion, trillion, squillion and zillion, but I suppose the usage is beneficial for non-scientific non-Hispanic Americans, so I don't object to retaining it.    D b f i r s   07:27, 26 October 2012 (UTC)

Three months later and it's happened again. Does "5 878  625 million miles" actually work to clear up ambiguity or does it just create confusion? Should we stick with this? Should we go for scientific notation? Should we try "5 878  625  373  184 miles"? Should we just write "5.878 625 trillion miles"? J IM ptalk·cont 23:46, 21 January 2013 (UTC)


 * I definitely think any usage of "billion" or "trillion" should be avoided, since it genuinely does create ambiguity for those familiar with the full meaning of those words. It's all very well writing for the layman, but the article must still remain unambiguous for the expert. I'm okay with any of the following:


 * ≈ 5,878,625 million miles
 * ≈ 5,878,625,000,000 miles


 * Adding more significant numbers (as opposed to just writing out the zeroes) creates "needless precision" and detracts from the readability of the information. I'd actually be in favour of reducing it to four significant figures, as that is the highest level of precision in which numbers are routinely read aloud as a contiguous number, and not just a string of digits. Rhialto (talk) 07:56, 22 January 2013 (UTC)


 * Needless precision, yes, but the zeros would probably be just as likely to be "corrected". Leaving it as is might be best after all. J IM ptalk·cont 16:04, 22 January 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) 09:36, 8 January 2014 (UTC)


 * Multiplication isn't OR if the Wikipedian knows how to do arithmetic. Anyhow, though, the only place we really need a high level of precision is in the metre conversion (the reader can work anything else out from that since it's exact).  Lots of significant figures elsewhere is simply unuseful clutter.  As for the big cumbersome table, I've made a few suggestions below to make it less cumbersome but my conclusion, as you'll read, is that the best solution would be to ditch it in favour of  what we had before. Jimp 11:31, 8 January 2014 (UTC)

New tables
Previously we had this.

1 light-year = 9 460  730  472  580  800 metres (exactly)


 * ≈ 5.878 625 trillion miles
 * ≈ 63 241.077 astronomical units
 * ≈ 0.306 601 parsecs

Now we have this.

I'm sure AshleyJames had the best intentions but to me it doesn't really seem to be an improvement. Firstly, here are a few minor points.
 * There is no need to link the units twice.
 * The numerals should not be separated from the number words & unit names.
 * The number words should not be capitalised.
 * It should be mentioned that the metre (and kilometre) conversions are exact.
 * Numbers should conform with the gaps formatting used elsewhere in the article.
 * "Tenth of a parsec" was a mistake.
 * The decimal point is in the wrong place for AU.
 * The term "short scale" is a WP invention so needs a link or explanation.

Fixing these we'd get something like this.

Also, regarding the units:
 * Do we need kilometres and metres?
 * Do we need feet?
 * Perhaps nautical miles would be more useful.

So, is this new and improved table any better than the old version? I don't think it seems any clearer. It seems harder to digest. Perhaps the number of significant figures is overwhelming me. We could even that score by adding the extra sig figs into the old version.

This is kind of cluttered though. I don't think that so many significant figures are necessary. Anyone who wants an exact conversion to miles, astronomical units, parsecs, etc. can take the metre figure and go and work it out for themselves. So, let's go the other way.

verses

No, I'm afraid that no matter which way I look at it I cannot see any advantage in the table version.
 * The old version is less cluttered without the headers, columns and lines.
 * With the old version we don't need links to Scientific notation or Short and long scales. Note that comparing the 9  460  730  472  580  800-metre figure here to the 9.5 trillion kilometres mentioned in the intro made it clear which scale we're using.
 * Including scientific notation is a bit of an overkill especially when there's engineering notation in the infobox to the right. Anyone familiar with the concept could figure the scientific notation for these few numbers out in their heads from the old version.  Those unfamiliar with scientific notation, on the other hand, would just be bewildered by this column in the table.
 * It's not immediately clear that these are actually meant to be the value of one light-year in other units. The word "conversions" is a little vague.  That we're converting one light-year into various other units isn't clear.  We could be converting other units into light-years or even converting any given measurement to or from light-years.  Perhaps something like "Equivalence of one light-year in other units" would be clearer than "Light-year Unit Conversions".  It is pretty wordy, though, unlike the short and sweet "1 light-year =" of the old version.

I'm proposing we revert this change. This also applies to the changes made at Astronomical unit & Parsec. In fact, I'm not going to wait around for further discussion before I do so. These tables contain errors: I've mentioned the erroneous "tenth" and the misplaced decimal point on this page; Astronomical unit has similar problems ("0.00000484813681 Millionth of a parsecs" and "0.00000015812507 Millionth of a light-years", which besides being ungrammatical are plain wrong). It's time to revert. Anyone who supports using tables feel free to comment but if we're going to go down that path, let's get it right first. Jimp 12:04, 8 January 2014 (UTC)


 * I second a reversion. In addition to Jimp's objections, the excess level of precision creates a "number salad" effect, in which it is no longer a human-readable (as in, it can be read out loud and be parsed by an ordinary listener). The table also duplicates the functions of the existing table, and finally, my original objection for using a table --- it breaks machine readers for blind-person access --- still stands.

I'd like to note that the original listing had four significant figures (not necessarily four decimal places), and that the reversion shoudl also apply to the parsec and the astronomical unit articles. Rhialto (talk) 18:33, 8 January 2014 (UTC)
 * Jimp's replacement of the table with a human-readable statement is a great improvement. Johnuniq (talk) 22:47, 8 January 2014 (UTC)


 * I concur with the reversion. BTW does anyone know how to edit the {{unit of length…? --Regards, Necessary Evil (talk) 23:04, 8 January 2014 (UTC)


 * I've got a bit of an idea of the workings of {{tl|unit of length}} but I wonder whether we shouldn't just get rid of it in favour of {{tl|infobox unit}}. Jimp 08:17, 9 January 2014 (UTC)

It seems alot happened after I created those tables, it was a small project I included which actually was left a bit unfinished I apologize for that. Jimp has taken a personal approach towards the issue I can quote several instances of sarcasm where it was unnecessary. What was quite obvious, where the existing {{tl|unit of length}} template which had been transcluded previously to my editing already include feet; perhaps I will also remove them from the {{tl|unit of length}} as we should replace it with nautical miles as I don't use the imperial system it makes no difference to me, also quite obviously is again the {{tl|unit of length}} includes both metre and kilometer units.

I did not know about the "machine readers for blind-people" issue if it is one, alot of other pages must need some immediate revisions then also... Aside from misplaced decimals, the only issue with the table was the non-inclusion of the scientific notation not being a multiple of the power to 10, eg. 123*10{{sup|5}} I will be recreating tables for these, I don't mind input or alternative options such as a {{tl|infobox unit}} template, however I am free to conduct editing at whim and as a scientific article the editing supports reasoning and clear understanding if formatted more clearly without disregarding any of Wikipedia's policies. You may challenge me further if you please, provide all reasoning in the discussion and ensure you are correct when dismissing facts, such as tenths and millionths. I am able to hold editing until a discussion is undertaken within reason. Ashley James  08:21, 11 January 2014 (UTC)
 * The need for both {{tl|unit of length}} and the table in question are agreeably a mistake; refer to previous comment of previous transclusion.
 * Capitalization was in fact necessary, as the short scale begins a sentance, that must start with a capitalization of the first letter.
 * I agree the choice of separating the units from the short scale was a bad decision, using that edit you are then correct in non-capitalization of any letters and it's more readable formatting, this is correct English grammar.
 * "Tenth of a parsec" is actually correct, stating it is a mistake is a mistake itself. In fact, is is approximately equal to 3 tenth's of a parsec, this is unchallengeable notation; refer to metric prefix in correcting yourself. The mistake is made within the Units column, where it states .3 tenths. What you are also stating µg is incorrect being 1 millionth of gravity.
 * The methodology of 9460730472580800 metres (exactly) is unnecessary, an equal sign should follow an absolute value unless noted, no approximation sign is used so why state it is is exactly twice in the one sentence?
 * If there is an issue regarding which unit notation to use, I can easily remove the engineering notation from the {{tl|unit of length}} template, there are people out there bewildered by engineering notation so why is that any different? I don't require citations to explain this.
 * Light-year Unit Conversions is again correct, I'm sorry you cannot grasp the concept however you stated yourself We could be converting other units into light-years. To explain what you wrote to yourself, 1 light-year = 9.4607304725808 quadrillion metres therefor 9.4607304725808 quadrillion metres = 1 light-year, if you don't understand it doesn't matter which unit is converted to which, nowhere does it state 1 metre = 1 light-year it cannot be read that way if its not stated.


 * Ashley, having a third column in that table ("short value") was confusing and misleading. Normal reading of tables is to assume each column of data is a separate item, whereas in fact you intended column 2 and column 3 to be a single connected item. When I saw that "tenth of a parsec", my first thought was that this was a really awful rounding error. Your formatting caused more confusion than it solved. Rhialto (talk) 00:29, 12 January 2014 (UTC)


 * My intention was not to take a personal approach nor to be sarcastic, sorry if that what it came out like. The inclusion of feet in {{tlf|unit of length}} makes sense for smaller units such as the fathom, smoot or hectometre, but not here.  This is just one of the issues facing {{tlf|unit of length}}.  We could go in for editing this template but it seems to me that the best approach would be to replace it with the more comprehensive and versatile {{tlf|infobox unit}}.
 * It is quite usual to have important information presented in an infobox as well as in the text. {{tlf|Unit of length}} is like an infobox but, as it lacks a lot of important info, it doesn't really make the cut and, as such, is more or less a redundant piece of clutter.  {{tlf|Infobox unit}}, on the other hand, is clearly an infobox.  So, yes, the Definitions doesn't really need to double up by using {{tlf|unit of length}} and another means (be it a table or a plain mathematical statement); however, this would not apply to {{tlf|infobox unit}}.
 * "Quadrillion metres" is not a sentence (but this is a moot point).
 * I'm pleased you agree that a quantity should not be split into two columns.
 * Okay, "3 tenths of a parsec" as opposed to "0.3 parsecs". Please excuse my mistaking where you made the mistake when you wrote "0.3 tenths of parsec".  (I'm not sure to what you refer when you talk about microgravity.)
 * Noting that the conversion is exact can be helpful for clarity. It's interesting, though, that you'd bring up the issue of "=" vs "≈" when the table version you're proposing ditches them.
 * I agree that the engineering notation used in {{tlf|unit of length}} is unhelpful. Scientific notation may be somewhat unfamiliar to the average person but engineering notation is even more so.  Again, though, this is another issue facing a template that, in my opinion, should be deprecated.
 * With a header like "Light-year Unit Conversions" one might expect some sort of table of conversions like this one.

{{Pressure Units}}
 * "1 light-year =" is much clearer than "Light-year Unit Conversions" even if it were used as the header of the table (as below). (Though I would really call this great style.)
 * {| class="wikitable"

! colspan=2 | 1 light-year = ! Scientific Notation     !! Short Scale
 * ~$16$ ||&ensp;&#8199;9.460  730  472  5808 quadrillion metres (exactly)
 * ~$15$ ||&#8199;~5.878 625 trillion miles
 * ~$5.879$ ||~63.241  077 thousand astronomical units
 * ~$9.461 m$ ||&#8199;~{{gaps|3.066|01}} tenths of a parsec
 * }
 * Sure, you are free to recreate your tables and free to edit at a whim. Yes, it would be best to do so without disregarding any of Wikipedia's policies.  Of course you'd be aware of the policy regarding consensus.  Yes, feel free to edit at a whim as long as you've got consensus behind you (or sincerely believe you would have) but if you're editing against consensus, please don't take offence if you're reverted.  I'm sure consensus will be on your side if you can format these conversions more clearly than what we've currently got but as things stand I see a different picture.
 * You suggest I ensure I be "correct when dismissing facts, such as tenths and millionths". By this I assume you refer to my pointing out that one light-year is not 0.30660139378555 tenths of a parsec and that one astronomical unit is neither 0.00000484813681 millionths of a parsec nor 0.00000015812507 millionths of a light-year.  Okay, these would be "facts" if the decimal points were in the correct place.  However, whether you meant 3.0660139378555 tenths of a parsec or 0.30660139378555 parsecs, whether you meant 4.84813681 millionths of a parsec and 1.5812507 millionths of a light-year or 0.00000484813681 parsecs and 0.00000015812507 light-years, wasn't obvious.  My guess was that you'd meant 0.30660139378555 parsecs rather than 3.0660139378555 tenths of a parsec (it seems a more usual way of putting things to me).  My guess was wrong.  As for the millionths on the other page, I'd have guessed that the decimal was in the wrong spot (but I didn't mention it either way). Anyhow, I can assure you that there were errors to be corrected.  I shall do my best but it can be difficult to guess in cases like this exactly what was intended (whether, for example, the error was in the placement of the decimal point or in the words).
 * Note, as mentioned above, infoboxes such as {{tlf|infobox unit}} are not alternatives to putting information in the body of an article but are included as additional places to find information.
 * Jimp 10:41, 13 January 2014 (UTC)
 * Sure, you are free to recreate your tables and free to edit at a whim. Yes, it would be best to do so without disregarding any of Wikipedia's policies.  Of course you'd be aware of the policy regarding consensus.  Yes, feel free to edit at a whim as long as you've got consensus behind you (or sincerely believe you would have) but if you're editing against consensus, please don't take offence if you're reverted.  I'm sure consensus will be on your side if you can format these conversions more clearly than what we've currently got but as things stand I see a different picture.
 * You suggest I ensure I be "correct when dismissing facts, such as tenths and millionths". By this I assume you refer to my pointing out that one light-year is not 0.30660139378555 tenths of a parsec and that one astronomical unit is neither 0.00000484813681 millionths of a parsec nor 0.00000015812507 millionths of a light-year.  Okay, these would be "facts" if the decimal points were in the correct place.  However, whether you meant 3.0660139378555 tenths of a parsec or 0.30660139378555 parsecs, whether you meant 4.84813681 millionths of a parsec and 1.5812507 millionths of a light-year or 0.00000484813681 parsecs and 0.00000015812507 light-years, wasn't obvious.  My guess was that you'd meant 0.30660139378555 parsecs rather than 3.0660139378555 tenths of a parsec (it seems a more usual way of putting things to me).  My guess was wrong.  As for the millionths on the other page, I'd have guessed that the decimal was in the wrong spot (but I didn't mention it either way). Anyhow, I can assure you that there were errors to be corrected.  I shall do my best but it can be difficult to guess in cases like this exactly what was intended (whether, for example, the error was in the placement of the decimal point or in the words).
 * Note, as mentioned above, infoboxes such as {{tlf|infobox unit}} are not alternatives to putting information in the body of an article but are included as additional places to find information.
 * Jimp 10:41, 13 January 2014 (UTC)

This sentence is a mess
I've just deleted a hidden comment which stated "This sentence is a mess: 'The product of [...] and [...] produced a light-year of [...] found in [...] was probably derived from [...], which was updated in 2000. HUH?! I suggest splitting it in two or three sentences."

The sentence in question is the following.

"The product of Simon Newcomb's J1900.0 mean tropical year of 31 556 925.9747 ephemeris seconds and a speed of light of 299  792.5 km/s produced a light-year of $9.461 km$ (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000."

I agree. It is a mess and it would be better split up. However, I thought I'd bring it here so as to be clear what we actually mean here. I suppose it's clear enough that "rounded to the seven significant digits in the speed of light" means that c was measured to the seven sig figs, of course, this should be put in a clearer way (next to the actual 299 792.5 km/s figure). The problem is that it's not so clear what was found in the old source (I guess it's the speed of light but is it the light-year or both?). Jimp 09:09, 29 January 2014 (UTC)

Did Bessel really introduce the word light-year?
The article currently says: "Bessel ... announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a unit of distance." No reference is given to support that claim. To me it seems doubtful that the meticulous scientist Bessel would have introduced the confusing name light-year as a unit of distance. In his 1838 article he mentioned that it takes 10.3 years for light to travel from 61 Cygni to us, but he did not use the word light-year. Ceinturion (talk) 12:25, 29 March 2014 (UTC)


 * I have removed Bessel's connection with the light-year unit from the article. While the source (National Geographic) did say "10.3 light-years" in one place, it immediately qualified the use of the unit as sometimes being attributed to Bessel, but noting that that is not a well-accepted view. It was a casual mention only, and lacked any implication of scholarly backing even then. Evensteven (talk) 23:50, 29 March 2014 (UTC)
 * Ok. Using Ngram viewer I came across an early use of the light-year unit in the Monthly Notices of the Royal Astronomical Society from 1864. The article was a review of new parallax data from the Königsberg Observatory, where Bessel had worked until his death in 1846. The distance of two stars is given in light-years (61 Cygni 5.77 ± 0.017 light-years, and another star 12.01 ± 0.50 light-years). The reviewer did not pay attention to the light-year unit, which suggests that it already was a common unit for star distances before 1864. Ceinturion (talk) 21:00, 30 March 2014 (UTC)
 * That's useful to the article in itself. Would you like to put it in? Evensteven (talk) 21:13, 30 March 2014 (UTC)


 * Let's have Ngram search for German sources first. The first appearance of Lichtjahr is in 1851, in an article by Otto Ule (Was wir in den Sternen lesen, in Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben, Volume 1): Wo aber finden wir ein größeres Maß, ein Maß, das unserer Vorstellung noch Raum fur seine Vervielfachung gibt? Wir wollen uns auf eine Reise begeben und auf Reisen pflegt man nicht nach Meilen, sondern nach Stunden zu fragen. Wie, wenn wir auch am Himmel nach Stunden, oder vielleicht nach Jahren die Entfernungen messen, wenn wir uns aber nach einem etwas schnellern Läufer als den langsamen menschlichen Fuß, oder selbst der schleichenden Locomotive umfähen? So muß also wohl das Licht diesen Läufer abgeben, das Licht, das in einer Secunde 42,500 Meilen .. zurücklegt. Das Maß, mit dem wir den Himmel zu messen wagen, sei also das Lichtjahr, den das Licht in einem Erdenjahr durchlauft .. Das sei unsere himmliche Wegstunde! Another interesting explanation is given in 1855 by Adolph Wilhelm Diesterweg (in Populäre Himmelskunde und astronomische Geographie): Man hat diesen Raum (seltsamer Weise) ein Lichtjahr gennant. .. Die Natur hat uns ein absolutes Zeit-, nicht ein absolutes Maaß fur den Raum gegeben. Jenes besitzen wir in dem sich immer gleichbleibenden Sterntag. Die Franzosen glaubten in den zehnmillionsten Theile eine Meridian-Quadranten, den sie Metre nannten, ein sich immer gleich bleibendes Naturmaaß fur den Raum zu finden; aber die Unregelmäßigkeit des Erdkörpers vereitelte die Hoffnungen. Somit bleibt uns nur die Wahl unter künstlichen (conventionellen) Raummaaßen übrig. Wünschenswert aber bleibt die Wahl eines Raummaaßes fur alle Länder der Erde. Ceinturion (talk) 20:56, 31 March 2014 (UTC)

Measuring in light years
@Eric, from your edit summary "not an improvement" is clearly a subjective evaluation. Mine is that it was an improvement. So just what is it that seems different to the two of us? My point is that the measuring of such distances is done by professional astronomers, who are among the most qualified to do so, and that when they do, they generally use parsecs. It seems to me that we shouldn't be making claims about measurements being made in light years. My edit was clear as to this fact, and used acceptable equivalent language without being repetitive. Please say what is not better about that. Evensteven (talk) 16:38, 29 September 2014 (UTC)
 * I agree with the global objective of your recent edits. But though it eliminated the repeated use of "are measured", the wording of your edit to the above-mentioned paragraph was awkward--that is what I meant in my edit summary. Look at the diff, you'll see what I mean. For example, distances can't be said to be apart from each other.
 * I find the section name awkward as well, and am changing it to something I think makes more sense. See what you think. Eric talk 17:01, 29 September 2014 (UTC)
 * Thanks. I'm now away from Internet again for a time, but I'll check on it when I'm online. Evensteven (talk) 17:18, 29 September 2014 (UTC)
 * Ok. I did some rewording there that I think is in the spirit of what you wanted. Also did a little re-ordering of the intro sentences. Eric talk 19:35, 29 September 2014 (UTC)
 * Thanks, Eric. That's pretty much what I was looking for. Evensteven (talk) 13:44, 6 October 2014 (UTC)

Petametres
Someone has added the term "(about 9.5 Pm)" to the length of the light-year in kilometres.

Although this is undoubtedly correct, I wonder if I am the only person who thinks the use of petametres adds confusion rather than clarity to the article.--Oz1cz (talk) 06:48, 29 June 2011 (UTC)

Precision of values in the lead
The conversion to metric and imperial/US in the lead has gone from one to two significant figures (10 trillion kilometres has become 9.5 and 6 trillion miles has become 5.9). I wonder whether this isn't a bit of overkill considering that more precise (exact in the case of metric) conversions are given elsewhere (in the first section). It seems to me that the rough figures we had before are easier to wrap your head around. I don't see the point of this extra precision. Jimp 04:50, 5 January 2015 (UTC)
 * I tend to agree. An in-between solution might be to simply remove the just under and or about. Eric talk 05:26, 5 January 2015 (UTC)
 * I restored 10 trillion because round numbers are much better in the lead. The infobox shows five significant figures, and the second section of the article has the precisely precise value. Johnuniq (talk) 06:12, 5 January 2015 (UTC)
 * My problem is with the "just under". It's obviously impossible to precisely quantify what counts as just over, but I would think of 9.9 million or perhaps 9.8 million as just under 10, whereas 9.5 million is quite a bit under. Perhaps about would be better?  Λυδ α  cιτγ  09:24, 5 January 2015 (UTC)
 * It's a 5% difference. No matter what word is chosen, it will strike someone else in a different way, because the words are meant to be subjective. We could also try "approximately" or "roughly", but I doubt they would do better. To some people (including me), the 5% seems larger, and to others (Johnuniq) it's enough less significant that he thinks one significant digit is "much better" in the lead. Who I am to argue, even if 9.5 Tkm and 5.9 T miles seem better to me? I think that's why this lead keeps going back and forth. I suspect it's just better not to worry it so much. It's in the gray area, and whatever we do, that's where it will remain. Evensteven (talk) 22:49, 5 January 2015 (UTC)

Unit system: Astronomical units
That is in the infobox. That doesn't make sense to me. Could someone explain? As far as I know, a unit system would be imperial, metric, etc... Thanks Buckbill10 (talk) 19:37, 13 July 2015 (UTC)
 * Check the link to astronomical system of units. The astronomical units are part of a loosely-defined system used for astronomical scales in place of or in addition to SI. —Alex (Ashill &#124; talk &#124; contribs) 19:43, 13 July 2015 (UTC)
 * Oh I understand now. A light year is an astronomical unit (it is used to measure large distances in space). That is kinda confusing, considering there is a unit called exactly Astronomical Unit (AU). But nevermind that, I guess. Buckbill10 (talk) 19:55, 13 July 2015 (UTC)
 * It's a good point. I changed the text from "astronomical units" to "astronomical system of units". —Alex (Ashill &#124; talk &#124; contribs) 20:05, 13 July 2015 (UTC)
 * I don't see how that helps. In what sense is the light year part of the astronomical system of units, given that in that system the unit of length is the astronomical unit. Dondervogel 2 (talk) 22:15, 13 July 2015 (UTC)
 * Maybe "units in astronomy" or "other units"? —Quondum 22:47, 13 July 2015 (UTC)
 * The light-year is a unit used in astronomy and included in the article astronomical system of units. Note that the "astronomical system of units" is not a formally recognized thing. There is an IAU resolution which is cited in the Wikipedia article that defines an "IAU (1976) System of Astronomical Constants", but there is no cited source that links that to the generic concept of an "astronomical system of units", and, in my experience, this IAU recommendation is no more widely-used than the IAU's recommended symbol for the astronomical unit (to reference a long-standing discussion). So the wikilink to astronomical system of units seems the most relevant to me; the text in the infobox on this page certainly could be changed to "units in astronomy" or any number of other things, since there's no formal name for the collection of units used in astronomy. Or if we don't want to include the "Unit system" entry in the infobox, that's OK too, though I do think it's somewhat helpful. —Alex (Ashill &#124; talk &#124; contribs) 23:21, 13 July 2015 (UTC)
 * I think the page astronomical system of units is useful, and I think it should be linked from this article on light-year. I also think the infobox is now clear as it is. So, something good has actually come from all this commotion! Isambard Kingdom (talk) 23:38, 13 July 2015 (UTC)
 * Indeed :) Buckbill10 (talk) 00:14, 14 July 2015 (UTC)
 * For the changed text in this edit, my proposal would be to either remove that entry of the table altogether, or to change the visible text to "IAU astronomical constants", which might be what the link's target Astronomical system of units should be renamed to. —Quondum 03:21, 14 July 2015 (UTC)

The observable universe is bigger than 13 billion light years
Due to expansion of space, the Hubble Volume is about 46 Billion light years. I've corrected the paragraph. (jcl July 17 2005) you r right

The particle horizon section
...Should be removed unless a citation is added; the figure seems dubious.

Move to speed of light?
Axel removed this:
 * Under normal circumstances, no material object can travel faster than the speed that light propagates in a vacuum. Particles routinely move faster than light in some media, such as the water used as coolant in nuclear reactors(see Cherenkov radiation). However, even the general light-speed rule seems to be abrograted by cases of quantum tunneling, and several laboratory experiments have suggested that light can, in some cases, move faster than the standard 299,792,458 m/s. See Theory of relativity.

Is it incorrect, or just in the wrong place? Can it be be moved to speed of light? -- Tarquin

Yes, it belongs in speed of light, although the last sentence has to be qualified; these experiments show a high speed of light for some suitable definition of "speed of light" and in no case can you transmit information that fast. AxelBoldt 00:29 Jan 9, 2003 (UTC)

Why not just say it's 9.5 Pm?
I mean, a kilometer is already a derived unit from the meter using a metric prefix, and the trillion word just makes things more confusing than they should, imho. What's the point of having all those prefixes if we never use them? I suggest we write that one light-year is about 9.5 Pm.--Grondilu (talk) 12:40, 28 February 2014 (UTC)


 * Two reasons. First, we are able to give the metres value precisely, which isn't actually possible for the others. That gives people a base from which to perform precise conversions for other scales. Second, petametres aren't really a unit that people can relate to in the same way they can relate to metres. There are dozens of situations in which one of the various metric prefixes could be used but aren't (eg. earth's circumference = 4 myriametres; Earth-Moon distance = 380 megametres). This is no different. Rhialto (talk) 22:09, 28 February 2014 (UTC)


 * I agree with Grondilu that using the ambiguous word "trillion" is confusing because the word means something different to me (I would use "billion" for a million million). However, the note clarifies the meaning sufficiently, and the "peta" prefix is seldom used in everyday English, so I'm happy to leave the article as it is.    D b f i r s   11:17, 1 March 2014 (UTC)


 * I'm going to argue that the high prefixes Mega, Giga, Tera, Peta and so on are getting more and more known even by the layman. This is due to their relatively recent use in consumer electronics.  At some point in the future, the fact that Wikipedia does not use those prefixes despite the fact that they are available and well known, is going to be seen as an oddity and will fail under the principle of least astonishment.  I believe this time has pretty much come.--Grondilu (talk) 22:16, 3 March 2014 (UTC)


 * At this stage, those only see regular usage within the field of consumer electronics. Once they see regular usage in the wider world in relation to units of length, then yes, WP should follow suit. But WP should follow current trends, not lead them. The current trends in measuring distance (or anything really outside of consumer electronics) do not yet include widely using those prefixes. Rhialto (talk) 09:47, 4 March 2014 (UTC)
 * When I saw this discussion I reverted my own edit. The self-revert was not due to changing my mind (I still prefer Pm to trillion km) but so that it could be discussed first.  Trillion km is just plain clumsy. It can't be avoided with miles but it can (and therefore should) for km.  Dondervogel 2 (talk) 23:33, 27 December 2015 (UTC)

abbreviation for light year
The source given clear states that the abbreviation for light year is "l.y.". There is no mention of "ly". Dondervogel 2 (talk) 13:19, 4 January 2016 (UTC)
 * The source is a Wikipedia article, which we can't use for such confirmation as it is WP:CIRCULAR, and it is unsourced there, although attributed to ISO 8000. Now, if the original text were to be found, it would be useful for your purposes.   Scr ★ pIron IV 17:12, 4 January 2016 (UTC)
 * The source is ISO 800000-3:2006, which reads 'light year: (l.y.)a', where a points to a footnote that reads '“l.y.” is an abbreviation for the name light year.' Dondervogel 2 (talk) 19:40, 4 January 2016 (UTC)
 * Except the citation you have given is to the Wikipedia article, which is what makes it WP:CIRCULAR - cite chapter and verse of the original document, and not the wikipedia article, and it would be fine.  Scr ★ pIron IV 20:09, 4 January 2016 (UTC)


 * I see no mention of the wp article except here on the talk page. The article cites the ISO standard directly. Dondervogel 2 (talk) 21:13, 4 January 2016 (UTC)
 * Actually, it is a bare ref, and has been challenged - I'm not looking to change anything here. But you did ask, so I am recommending that you provide a clearer citation, and include page and section.  Scr ★ pIron IV 21:27, 4 January 2016 (UTC)
 * No problem. Dondervogel 2 (talk) 21:32, 4 January 2016 (UTC)
 * The actual ISO standard PDF seems to be behind a paywall, and I don't want to tromp over the local Indiana University library and dig around in the stacks for it. So I can't provide a page number for Annex C, along with appropriate page, paragraph, and sentence (aka book cipher).   But I did manage to find two other references that can be seen on Google Books..  Both use "l.y." with the periods.  But I'm not sure if this is "primary" enough.  Here they are:  1)  and 2)   I am surprised however that I can't find a pirated version of the actual standard PDF somewhere in Google's pile-o-Internet-bones..  Jimw338 (talk) 01:18, 7 October 2016 (UTC)

Decimal mark?
In the pre-text 9 trillion kilometres (or about 6 trillion miles). In the detail 1 light-year = 9460730472580800 metres (exactly) ≈ 9.461 petametres ≈ 9.461 trillion kilometres ≈ 5.878625 trillion miles. The number is hereby questionmarked by a factor of 1000 for kilometers or 1000000 for miles. Rune — Preceding unsigned comment added by 2.107.163.134 (talk) 22:11, 16 October 2016 (UTC)
 * The dot is a decimal mark, not a thousands separator. Therefore, 9.461 ≈ 9 and 5.878625 ≈ 6. Ceinturion (talk) 06:18, 17 October 2016 (UTC)