Talk:Light-year

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The page light year has been moved around a bit. For some reason, the page history is now scattered over three pages. A part of the page history is in light-year/more (history) as well as light year (history.

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[edit] 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)

[edit] 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 1005)

[edit] 1 year equals 365.25 days

JHKJHArticle 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. TJHhus 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)

[edit] 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)

[edit] 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


[edit] 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)



[edit] Yottametres

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

[edit] 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)

[edit] 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)

[edit] 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. --P3d0 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)

[edit] small point - commas vs NBSP

I was wondering if it would be best to use commas rather than  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  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)

[edit] The particle horizon section

...Should be removed unless a citation is added; the figure seems dubious.

[edit] 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)

[edit] 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)

[edit] 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

[edit] 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 [1]
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 <<citeneeded>> 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)

[edit] 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 [2]:

"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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)