Talk:Light-year

Duration of travel at light speed
Does it take a year to travel a light year from the perspective of the traveler or from the perspective of the traveler's twin left behind? I think the article would benefit from clarification. — Preceding unsigned comment added by 72.161.2.130 (talk) 04:10, 27 March 2016 (UTC)


 * It's not possible to ride on a photon, so we don't know what the universe would look like to a "light traveller". Light travels at the same speed relative to all real observers, so the unit distance of one light-year is the same in all real reference frames, though actual distances (in miles or any other unit) will vary with the reference frame of the observer.  The distances to various astronomical objects are measured in the reference frame of our own solar system.    D b f i r s   07:11, 27 March 2016 (UTC)


 * That's a question of special relativity which isn't the topic of the article, but, to answer it anyway, it would be the twin left behind. From the perspective of the photon, the Universe is flat and it travels no distance in no time.  We see it moving at light speed though.  The traveller riding on the photon (which is impossible, of course, as mentioned above ... but anyway) will look frozen, the hands on his watch have stopped and he's flat from our perspective.  Jimp 00:47, 7 March 2017 (UTC)

Wrong info Rajbeer Chauhan (talk) 03:22, 3 June 2022 (UTC)

Meter or Metre
I notice that a change from kilometer to kilometre was reverted. Looking across the entire article, with that one exception, the article is written using British English. That suggests that the article is primarily British English, and should be kept as such, as per WP:ART1VAR. I have reverted the edit and added the British English template to this talk page in case this gets queried in future. Rhialto (talk) 18:57, 5 March 2017 (UTC)
 * Thanks, . I was confused by the use of "trillion", having overlooked that Brits and Americans now agree on the the definition of that word. Progress does happen. Favonian (talk) 19:04, 5 March 2017 (UTC)
 * Call that "progress", ay? It's true, though, that most of us English speakers have adopted the new illogical definitions of "billion", "trillion", etc.  We'd have done better going the other way if you ask me (but it's hard to compete against US English) ... anyway, enough of the rant.  MOS:NUMERAL prescribes the American version (per increasing common usage in English globally), so, no, it doesn't mean anything in terms of consistency of style.  Jimp 00:32, 7 March 2017 (UTC)
 * I haven't adopted the illogical definitions! I still use the long scale billions and trillions that were the only sort taught when I was at school.  (I do use short scale in Wikipedia, with an explanatory link for the many readers of English in countries where the long scale is still common.)  Not all of us in the UK accepted the Prime Ministerial Executive Order of "President" Harold Wilson in December 1974.  I gather that not all executive orders are accepted in other countries?     D b f i r s   09:03, 7 March 2017 (UTC)
 * This number thing seems to be a distraction from the main point. Because it is specified elsewhere in the MoS, the number system isn't an indicator of national variety of English. The spelling, however, is. Rhialto (talk) 10:06, 7 March 2017 (UTC)
 * Agreed. Jimp and I were making off-subject comments about the loss of the traditional British billion (though it is still common in many other countries), and we were not suggesting that this usage had any influence on the ENGVAR issue. Favonian also acknowledges the MoS ( and calls it "progress" ).    D b f i r s   12:34, 7 March 2017 (UTC)
 * I don't understand why either "long scale" or "short scale" violates any law of "logic". They each have their own consistent system which specifies clearly, univocally, what is meant. The scale one uses is an arbitrary choice as far as logic is concerned.   TomS TDotO (talk) 14:14, 7 March 2017 (UTC)
 * No, the short scale doesn't violate any "law of logic". For the long scale, bi means two, tri means three millions multiplied together and so on for "n"-illion.  In short scale, the formula is (n+1) thousands multiplied together which doesn't directly translate into "-illions".  I think that's the logic that Jimp had in mind.    D b f i r s   16:49, 7 March 2017 (UTC)
 * The thing with the long scale words "milliard", "billiard", "trilliard" etc. is that the sound a lot like "million", "billion" and "trillion" especially in nonrhotic or r-dropping speech, so could be confused in speech. I'd prefer going with a new system entirely to adopting the long scale. Maybe just do away with "million" and other "-illion" words, and just invent new words like "bousand" for 1,000,000, "trousand" for 1,000,000,000 etc. 99.101.56.68 (talk) 01:13, 9 March 2017 (UTC)
 * If we didn't already have words with conflicting meanings, that would be a good idea, but I prefer the two other precise and unambiguous systems used in science and engineering (Metric prefixes and standard form). I prefer to avoid the words billion and trillion in any context where precision is important, and to link to the appropriate scale whenever the words are used informally in Wikipedia.  (I make an exception for economics where billion seems to have an agreed usage.)  The words "milliard", "billiard", "trilliard" are rare and seldom used.    D b f i r s   09:02, 9 March 2017 (UTC)

Why the reluctance to provide sources for kly, Mly, etc.?
A light year is not an SI unit, and I know of no international standard that supports ly as a unit symbol for it. If the symbols kly, Mly, etc are used then I'm sure someone can find a source. If they are not used they should not be promulgated by Wikipedia. Dondervogel 2 (talk) 20:40, 9 June 2018 (UTC)


 * I provided the requested sources. I simply searched for " kly", " Mly", and " Gly". — Joe Kress (talk)
 * Thank you. I think you have improved the article. I was not questioning the use - just pointing out it's not self-evident, and therefore needed a citation to support it. Dondervogel 2 (talk) 22:17, 10 June 2018 (UTC)

What English should this article use?
I've removed the British English header as part of this discussion. IPs are people too 🇺🇸🦅 21:12, 18 September 2023 (UTC)
 * Slightly odd to have that template on a talk page. I don't think there is any *should* for the article, but generally once an article is written in one particular dominant variant of English, that should be continued unless there is an overriding reason why a different variant would suit the topic better.  See MOS:ENGVAR for more details.  A very quick scan of the article finds a number of British spellings and I didn't come across any specific American ones, but that it would take longer to come to a firm conclusion.  Lithopsian (talk) 14:43, 19 September 2023 (UTC)
 * "British" spellings aren't necessarily British. They could be Canadian. IPs are people too 🇺🇸🦅 20:30, 19 September 2023 (UTC)
 * The British English banner was presumably there by consensus. I see no evidence that consensus has changed so I put it back. Dondervogel 2 (talk) 16:24, 19 September 2023 (UTC)
 * Ah, right. Perhaps I was thinking of Use British English.  Lithopsian (talk) 16:47, 19 September 2023 (UTC)

Is someone tampering with this article?
I'm reading this on the day you'll see posted below and I see this exact wordage because I immediately copied it:

"As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year (280.25 days, as opposed to the 280.2425-day Gregorian year or the 280.24219-day Tropical year that both approximate)"

I researched the source for the numbers I'd seen that I know are wrong just for the heck of it (or in case I'm going mad), then log back in, and then I see the correct(ed) year measure change to what it is supposed to be! (Do I need to even say "365.25?!") Anyone with authority want to put a lock or semi-protected gate on here. Coming to WP and seeing stupid stuff in scientific articles is insane! Thanks StyxinConn47 (talk) 21:42, 5 February 2024 (UTC)