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This article is within the scope of WikiProject Measurement, a collaborative effort to improve the coverage of Measurement on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
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. Dbfirs 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)
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★pIronIV 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★pIronIV 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★pIronIV 21:27, 4 January 2016 (UTC)
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 22.214.171.124 (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. Dbfirs 07:11, 27 March 2016 (UTC)