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."

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

Which calculation?

[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)

[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)
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