Wikipedia talk:Manual of Style/Dates and numbers/Archive 163

Archive 160

Archive 161

Archive 162

Archive 163

Conversion of light years and parsecs

Hi, all. We're trying to harmonize the units used in astronomy articles by building consensus for Wikipedia:WikiProject Astronomy/Manual of Style. It currently seems to be general practice to convert interplanetary distances and smaller to km, but not convert interstellar distances measured in light-years (and to show conversion to parsecs). Given that these units are not used outside astronomy and astrophysics, this contradicts the part of MOS:CONVERSIONS which advises converting "units of measure that are ... obscure outside of a particular specialty or geography". Would it be OK to add an official exception noting interstellar and larger distances should be given in light-years and parsecs and not converted to SI units? -- Beland (talk) 07:11, 23 July 2024 (UTC)

Can you give an example (point to a page) where that conversion to km doesn't happen? Gawaon (talk) 07:15, 23 July 2024 (UTC)

I see that in all the infoboxes of the first extrasolar objects I could think of, namely Alpha Centauri, Betelgeuse, Wolf 359, and Andromeda Galaxy. -- Beland (talk) 07:28, 23 July 2024 (UTC)

Thanks. As someone who's not a editor or expert in the field, but an occasional reader, I'm not in favour. I find 0.024 AU (3,600,000 km) much more helpful than a naked distance in AU, and I find 7.86 light-years (7.44×10¹³ km) more relatable than 7.86 light-years (2.41 parsecs). Light-year and parsec essentially just mean "unimaginably long" to me, but I know what a kilometre is and 10¹³ allows me get a better understanding of the dimensions involved. Gawaon (talk) 09:12, 23 July 2024 (UTC)

Well you're lucky then, because there's no way I'd relate to 10¹³ km in any remotely sensible manner. I can at least think of a light year as about a quarter of the distance to the nearest star to the Sun. Praemonitus (talk) 18:21, 23 July 2024 (UTC)

You quite have a perspective there, because "10¹³ km" is essentially meaningless to me. It is very hard to think about that number in terms of human scale alone. You will not encounter that number on a daily basis except perhaps when discussing countries' GDP or debts.

7.86 light-years (2.41 parsecs) is much more sensible on my perspective. That means light reaches that star in just under 8 years time, and that it makes a parallax angle of 1/2.41ths of an arcsecond every six months in the sky. It is more than just being "unimaginably long." SkyFlubbler (talk) 11:41, 28 July 2024 (UTC)

Would it be better to say that distances given in light-years should only be converted to parsecs and vice versa? Looking at Proxima Centauri and Antares reminds me that distances between binary stars may be appropriately given in smaller units such as AU and billions or trillions of kilometres, which would technically be contrary to an unqualified "interstellar". NebY (talk) 08:47, 23 July 2024 (UTC)

Good point; I mean distances between stellar systems, rather than between stars in the same system. We could say "between stellar systems" or "distances typically measured in light years by reliable sources" to clarify. -- Beland (talk) 16:29, 23 July 2024 (UTC)

No, that would not be OK. That would be like saying it's OK not to convert knots to m/s because nautical people are familiar with knots. What unites all of us is the SI system, so a conversion to SI is needed. Always. Dondervogel 2 (talk) 12:16, 23 July 2024 (UTC)

Would you be in favor of converting from light-years to km only, and dropping parsecs, as in Gawaon's examples above? Parsec says light-year is more common in popular science and general media, which aligns with my experience. I have always found parsecs redundant to light-years and confusingly different. -- Beland (talk) 16:34, 23 July 2024 (UTC)

I would support making a choice between light-years and parsecs (I don't care which, but both would be overkill). Once that choice is made, convert to SI and we're done - in the sense that everyone can then comprehend the distance. Dondervogel 2 (talk) 17:21, 23 July 2024 (UTC)

I agree, that sound reasonable. Among the two, light-year seems to be better known, as Beland noted. Gawaon (talk) 17:26, 23 July 2024 (UTC)

I agree with Dondervogel 2 as well. Avi8tor (talk) 19:56, 23 July 2024 (UTC)

I see little reason not to. Most of the units that aforementioned policy in MOS:CONVERSIONS refer to are still "human-scale", or at least close to it. When one furlong is thrown out, for example, its conversion to 220 yards/0.125 mi/~201 m is easy to visualize, as such lengths are encountered frequently in life. People generally have first-hand experience with distances that long.

One light-year is nearly 1e13 km. I can't speak for everyone, but personally that figure is almost completely meaningless to me; 1e13 km simply is not a comprehensible figure. Even 1 AU (~1.5e11 m) is difficult to comprehend. This is why internet demonstrations of the "true scale" of the Solar System and interstellar space often go viral: people just do not intuitively grasp these distances and scales very well, and no conversion will change that. As Praemonitus mentioned in Wikipedia talk:WikiProject Astronomy/Manual of Style#Conversion of light years and parsecs into km, this issue only gets worse for intergalactic distances on the order of megaparsecs. Ultimately, I fail to see how useful such conversions really would be. ArkHyena (talk) 17:24, 23 July 2024 (UTC)

Thanks for writing this comment, that's precisely my opinion and you saved me the work. Tercer (talk) 17:29, 23 July 2024 (UTC)

I was not aware of this attempt to write a specific MOS for astronomy; I imagine you'd get more informed input at WT:AST than here. Astronomical distances are measured in au within a star/planetary system (including the Solar System), and in parsecs for everything larger (extending to kiloparsecs for galaxies, megaparsecs for galaxy groups etc.). Converting either of those to km would not be useful to readers - the numbers are incomprehensibly vast, which is one of the reasons why astronomers don't use km in the first place. Light years are used only in popular science accounts and press releases, where they do have some utility, but are essentially never the original astronomical measurement. I think it's fine to provide a conversion of pc to ly, but pc should be the primary unit. Converting to km is generally pointless, unless there's some unusual situation. Linking the au or pc unit on first appearance would be more useful to readers. Modest Genius ^talk 17:48, 23 July 2024 (UTC)

PS. There's some relevant discussion in the archives at Wikipedia_talk:WikiProject_Astronomy/Archive_12#Should_we_decide_on_a_default_unit_to_use_across_WP? and Wikipedia_talk:WikiProject_Astronomy/Archive_11#Units_to_be_used_for_distances_and_sizes_in_infoboxes Modest Genius ^talk 17:54, 23 July 2024 (UTC)

The previous discussions linked above seemed to concern whether light-years or parsecs should be used for large distances, and the general consensus seemed to be that astronomers use parsecs, readers are much more likely to understand light-years, and we should just use {{convert}} to display both. That has since at least mostly happened, but no one suggested converting to kilometers, and at that time the language about "obscure outside of a particular specialty" was not in MOS:CONVERSIONS. Hence the current question about resolving the conflict.

Per WP:JARGON, Wikipedia articles are written for the broadest possible audience, and we are advised to "write one level down" if necessary to make technical articles understandable to non-specialists. So if we need to pick two units and one of them is km, then parsecs may have to get the boot because it's mostly only specialists who use them. Fortunately, astronomers should be able to convert from light-years to parsecs easily, unlike the general public, so we don't need to sacrifice level of technical detail.

In other unit-related discussions, we've decided to use {{convert}} to display Wikipedia house style to readers and in some cases hide the units used by sources. Using a parameter like disp=out can do that while preserving the original units for verification against the cited source, and to avoid losing precision if someone comes by later and adds a second conversion. -- Beland (talk) 18:52, 23 July 2024 (UTC)

I disagree, as noted above. 10¹³ km means something to me, even if (admittedly) the dimensions involved are hard to grasp, while throwing "parsecs" or "light-years" around is essentially meaningless. (I agree one of them should be used too, but not exclusively.) Gawaon (talk) 18:16, 23 July 2024 (UTC)

I wonder then how many lay readers are familiar with distances expressed in exponential notation? Because that isn't everyday usage. If you saw 1×10¹³ in a lay readers article, it would be more likely to be written as "10 million million kilometers". I look at the public facing NASA article The Galaxy Next Door and it gives distances and dimensions in light years, so NASA is expecting the public to be familiar with that distance scale. The distance to the Andromeda Galaxy is something like "25 million million million kilometers", surely a cumbersome statement. Praemonitus (talk) 22:01, 23 July 2024 (UTC)

I must agree with this, I—anecdotally—would not expect most laypeople to understand what 1e13 km really means. To my knowledge, most educational platforms do not convert lyr or pc to km/mi (some examples: [1] [2] [3] [4]). It is clear that science communication as a whole deems conversions of lyr into km/mi as unhelpful and unneeded for most purposes. ArkHyena (talk) 22:20, 23 July 2024 (UTC)

I checked my state's curriculum standards, and exponential notation is required to be learned in eighth grade. So I would guess this has been taught to anyone who has completed their primary school education in a country with a decent education system. That doesn't include all our readers, and some people will have failed math or completely forgotten this concept. On radio broadcasts for popular consumption, I have definitely heard constructions like "billion billion" or "6 with 20 zeroes after it" in lieu of exponential notation, and also light-years.

Based on editors' personal reports here, it seems there are some people who think in light years and some who think in large numbers of kilometers. Why not have both to maximize accessibility and intuitive understanding? Anyone who knows what a parsec is almost certainly has a firm grasp on what a light-year is. -- Beland (talk) 22:38, 23 July 2024 (UTC)

The issue is that there are situations where this may introduce clutter with arguably marginal benefit, especially in infoboxes. I suppose introduction of conversion in main article text is fine, so long as km values are given in exponential notation to limit clutter. A potential compromise would be to add a note upon first mention of a lyr/pc in an article that provides km values for one lyr/pc; this is broadly similar (though not perfectly analogous) to how hurricane articles handle major hurricane status (e.g. at Hurricane Beryl). ArkHyena (talk) 23:27, 23 July 2024 (UTC)

That's an interesting idea...you're thinking light years converted to parsec in infoboxes, light-years converted to kilometers in article prose (and maybe only at first mention)? -- Beland (talk) 04:47, 24 July 2024 (UTC)

I'm opposed to the latter. If we're expecting students to know what a kilometre is and to grok out exponential notation, then it's reasonable to expect that they will also understand a light year. I see no need to provide a conversion to km in most cases. A link should suffice. Praemonitus (talk) 14:47, 24 July 2024 (UTC)

What do you mean with "understand" here? I know what a light-year is, in the abstract sense: It's the distance light travels in a year. And I, and guess most people, also know that light travels "very fast". But how much is "very fast" times one year? I don't have the foggiest idea, to be honest. Never having travelled at the speed of light, it is very hard to fathom for me, and as such essentially meaningless. Exponential notation, on the other hand, is not very hard to get, if you know how to do basic addition and multiplication. If I read 10¹³ km, I know I have to take one kilometre, multiply it with 1000, and again, and again, and again, and then finally with 10. Still abstract, admittedly, but now I have a much better sense of the dimension involved compared to "unimaginably fast times one year". Which is why I'm in favour of using exponential notation in addition to light-years or parsecs (I don't care which of them is chosen). Gawaon (talk) 15:09, 24 July 2024 (UTC)

I would perhaps prefer 10 Pm to 10¹³ km, but other than this detail I strongly agree with Gawaon. I can accept 10¹³ km if that is the consensus. I cannot accept omitting the conversion to SI - to do so would suggest that astronomy is beyond metrology, when it clearly is not. Notice the link to petametre takes the reader directly to an equation stating that a petametre is about 0.1 light-year. Dondervogel 2 (talk) 15:20, 24 July 2024 (UTC)

Oh come on. It suggests nothing of the sort (about astronomy being beyond metrology). Metrology is a whole discipline; SI is just a system of units, roughly as arbitrary as any other.

That said, I don't have a hardened objection to including an SI conversion, at least in infoboxes, though I wouldn't be thrilled to see them repeated over and over again in the running text. --Trovatore (talk) 16:18, 24 July 2024 (UTC)

OK, so I was exaggerating, for effect. A more measured remark would have been that even the IAU defines its units in terms of the SI, so why mark it unnecessarily hard on the reader by omitting that conversion on Wikipedia? I accept the SI is arbitrary, but it is THE single arbitrary system that we all (including Americans) learn at school, is used in day to day scientific work and is defined by international standards (BIPM).

Providing a conversion to SI in info boxes seems a good compromise. Dondervogel 2 (talk) 18:47, 24 July 2024 (UTC)

I would argue for the inverse, actually. Providing SI conversions in maintext whilst omitting them in infoboxes seems to be the best practice besides not having any. Infoboxes, especially those of astronomy, are already crowded with numbers; it would not help readability to shove yet more in them. ArkHyena (talk) 18:55, 24 July 2024 (UTC)

Right now light-years and parsecs are typically presented side-by-side in infoboxes. If one of them is replaced with a different unit, that wouldn't result in any additional clutter. Gawaon (talk) 19:04, 24 July 2024 (UTC)

Per SevenSpheres: The astronomical literature almost exclusively uses parsecs, because distance in parsecs can be derived directly from parallax, which is used to measure stellar distances. So it makes sense to use both units. I don't think I've seen interstellar distances expressed in kilometers much if at all before today ... On the topic of deriving distance from parallax, {{Starbox astrometry}} can do this automatically and is used this way in most star articles. So even if parsecs aren't used in the text, it doesn't make sense to remove them from the infobox

If we are to implement SI conversions, they are better-suited for the maintext (either as first mention or throughout), since that is presumably where most readers read. Astronomy infoboxes typically hold information about more obscure/technical (even if still very much relevant) properties for their respective objects. ArkHyena (talk) 19:32, 24 July 2024 (UTC)

"So I would guess this has been taught to anyone who has completed their primary school education in a country with a decent education system." - And most people don't work in science-related fields, and so never use scientific notation and thus have forgotten it. "10^13 km" and "1 light year" both translate to "very, very large", but the latter can at least allow comparison of distances--"it's only a few light years to the nearest star, but a million light years to the nearest galaxy."--whereas 10^13 km and 10^19 km are both equally meaningless as "very large", because they don't think in subtracting exponents. The prefixes are even worse, as anything beyond giga or tera are completely unfamiliar to most (and even those just mean "big" to many). - Parejkoj (talk) 06:50, 25 July 2024 (UTC)

No one is suggesting we stop using light-years (or parsecs if you prefer those - take your pick). We just prefer to include a conversion to SI, because we are all taught SI units. It's called the International System of Units for a good reason. Dondervogel 2 (talk) 07:08, 25 July 2024 (UTC)

I disagree. In this field of astronomy the governing body would be the International Astronomical Union, and the SI would only be focused on standardizing units relevant to common everyday measurements (and perhaps those in technology). And parsec is specifically defined in the notes of Resolution B2 in 2015. SkyFlubbler (talk) 12:02, 28 July 2024 (UTC)

Not everybody is an astronomer though, and our articles shouldn't be accessible to specialists only. Gawaon (talk) 12:58, 28 July 2024 (UTC)

But this is still a topic of astronomy, so of course we have to use units used by astronomers. By your logic we should not use radians as an angle measurement despite countless mathematical areas using it because "not everybody is a mathematician."

If they are seeking astronomy topics here in Wikipedia, the article for parsec is as simple as a mouse click. SkyFlubbler (talk) 23:31, 30 July 2024 (UTC)

In general, we don't use radians for angles, such as when discussing bicycle frames or sports or flowers). We use the much better-known degrees, unless there is a specific reason that radians make calculations or geometric expressions easier (which is why they exist). Radians are also much better-known than parsecs. I checked the Massachusetts curriculum standards; radians are part of the secondary school math requirements. Parsecs are not a requirement, and I would expect them to be first taught in undergrad astronomy classes (which obviously hardly anyone takes, though I did) or picked up as an extracurricular interest.

Certainly linking unfamiliar units like parsecs helps a lot of readers make more sense of them, but not everyone using Wikipedia has a mouse. Sometimes articles are printed out. Sometimes they are read out loud by a text-to-speech system - quite common for blind and visually impaired people, and also among people like me who use TTS to read articles while doing something else like yard work or doing the dishes. All of us screenreader users potentially have to listen to conversions for every single field in an infobox, which can get a bit tedious and make things harder to follow, especially if we don't know the definition of one of the units. -- Beland (talk) 02:55, 31 July 2024 (UTC)

What do you mean by "your logic"? I'm in favour of using parsecs, but against using only parsecs (or only non-SI units). Gawaon (talk) 05:58, 31 July 2024 (UTC)

This astronomy MOS was announced at WT:AST and considerable discussion has already happened there, and I already posted there a link to this discussion. I started this discussion because I wasn't comfortable creating an exception to MOS:CONVERSIONS without consulting the wider editor community beyond the WikiProject Astronomy. -- Beland (talk) 18:31, 23 July 2024 (UTC)

• Light-years are not an obscure or specialized unit; along with parsecs, they are the standard units used to measure interstellar and larger distances. While it's true that they're only used in one field, astronomy is the only field that deals with such large distances! This is different from, say, furlongs, which are on the same scale as kilometers.
• My thought is that interstellar distances expressed in kilometers are meaninglessly large numbers, while such distances expressed in light-years are more accessible to the general reader. Surprisingly though, there are comments saying the opposite! I find it hard to believe this is representative of the average reader though; surely most people who understand scientific notation also understand what a light-year is? That if a star is 100 light-years away, its light takes 100 years to reach us and so we see it as it was 100 years ago?
• In terms of common usage, sources aimed at the general public (like the NASA pages linked above) almost exclusively use light-years, presumably because, again, this is the most accessible unit to the general public. The astronomical literature almost exclusively uses parsecs, because distance in parsecs can be derived directly from parallax, which is used to measure stellar distances. So it makes sense to use both units. I don't think I've seen interstellar distances expressed in kilometers much if at all before today.
  • (On the topic of deriving distance from parallax, {{Starbox astrometry}} can do this automatically and is used this way in most star articles. So even if parsecs aren't used in the text, it doesn't make sense to remove them from the infobox.)
• A proposed change of this kind that would affect so many articles should be more widely advertised; I suspect everyone who's commented here watches either this page and/or WikiProject Astronomy where this was mentioned.

SevenSpheres (talk) 00:21, 24 July 2024 (UTC)

If we decide to suppress parsecs from reader view, {{Starbox astrometry}} can simply be changed to convert to light-years or whatnot. It's actually a lot easier to do that than change 1,000 articles that are not using a template feature like that, which I am sadly already doing for a lot of problems. -- Beland (talk) 04:51, 24 July 2024 (UTC)

There's no way a light-year is a standard unit. It is not defined by international standards bodies, and according to the IAU, light-years are "mostly confined to popular publications and similar media". And I suspect there are more readers who think the light-year is a unit of time than ones who would be confused by use of the metre (or kilometre) as a unit of distance. If there is a standard unit in astronomy, it is the parsec, not the light-year. Dondervogel 2 (talk) 11:21, 24 July 2024 (UTC)

I don't there exists anyone who thinks light-year is a unit of time but understands what a parsec is. Ironically enough, the creators of Star Wars thought parsec was a unit of time. Tercer (talk) 12:32, 24 July 2024 (UTC)

@SevenSpheres: FYI 1 furlong is 201.168 m, so not quite the "same scale". Martin of Sheffield (talk) 14:14, 24 July 2024 (UTC)

Corrected. SevenSpheres (talk) 15:10, 24 July 2024 (UTC)

Where would you like to see this discussion "more widely advertised"? -- Beland (talk) 15:57, 24 July 2024 (UTC)

WP:RFC? SevenSpheres (talk) 16:16, 24 July 2024 (UTC)

If we're doing that, we should summarize the discussion into some clear options. How about this for a question with neutral background material:

----

How should distances between stellar systems and longer be presented in infoboxes? (These are often calculated automatically from different units presented in astronomy sources, such as parallax.)

Examples from Local Interstellar Cloud, Andromeda Galaxy, Proxima Centauri, and Betelgeuse.

A.) Light-years and kilometers only in infoboxes:

• 30 ly (2.8×10¹⁴ km)
• 2.50 Mly; 2.36×10¹⁹ km
• 4.2465 ± 0.0003 ly
  4.0175×10¹³ ± 2.8382×10⁹ km
• 408–548+90
  −49 ly (3.86×10¹⁵–5.18+0.85
  −0.46×10¹⁵ km)

B.) Light-years and parsecs only in infoboxes

• 30 ly (9.2 pc)
• 2.50 Mly; 765 kpc
• 4.2465 ± 0.0003 ly
  1.3020 ± 9.1980×10⁻⁵ pc
• 408–548+90
  −49 ly (125-168.1+27.5
  −14.9 pc)

C.) Light-years and meters with larger prefixes in infoboxes

• 30 ly (280 Pm)
• 2.50 Mly; 23.6 Zm
• 4.2465 ± 0.0003 ly
  40.1750 ± 0.0028 Pm
• 408–548+90
  −49 ly
  3.86–5.18+0.85
  −0.46 Em

D.) Light-years and parsecs only in infoboxes (like B), with conversion to kilometers on first mention in prose

E.) Something else

If conversion to kilometers in infoboxes is not required, this would be added as an explicit exception to MOS:CONVERSIONS.

Previous discussions identified that parsecs are used by professional astronomers and light-years are used in popular news and educational media. Editors disagreed on whether light-years or kilometers with exponential notation were easier to read and understand intuitively for most readers.

----

-- Beland (talk) 18:10, 24 July 2024 (UTC)

(Edited to link units.) -- Beland (talk) 18:16, 24 July 2024 (UTC)

How about "Parsecs and kilometers only in infoboxes" as further option, since that combination has been mentioned as well? Also, if you suggest 5 or 6 different options, it's quite likely that none of them will gain an absolute majority, resulting in an unclear outcome.

An alternative question might be something along the lines of "Since it's cumbersome to present more than two alternative units, which two should preferably to used for interstellar distances?", with the options being:

A. Light-year

B. Parsec

C. Kilometre with exponential factor

D. Metre with SI prefix

And every editor asked to pick their two favourites. Gawaon (talk) 18:54, 24 July 2024 (UTC)

OK, maybe we should ask the "pick two" separately for prose and infoboxes, since there seems to be a stronger leaning toward different practices, and it would be nice to get a clear result for both if we're bothering everyone to consider the question. -- Beland (talk) 22:02, 24 July 2024 (UTC)

Okay, why not, though personally I see no good reason to treat them differently. Gawaon (talk) 06:11, 25 July 2024 (UTC)

Some editors have expressed different preferences for prose vs. infobox...at this point I've thought about all this too much and don't know how I feel about anything. Revised draft RFC posted in subsection below; everyone feel free to tweak or critique. -- Beland (talk) 07:32, 25 July 2024 (UTC)

If you're referring to my comment, I haven't expressed different preferences for prose vs. infobox, only a stronger preference to retain the status quo in infoboxes. SevenSpheres (talk) 14:55, 25 July 2024 (UTC)

I was thinking of ArkHyena's proposal to convert to km on first mention in prose but not infoboxes. -- Beland (talk) 19:48, 25 July 2024 (UTC)

IAU definitions are about as standard as you're going to get in astronomy. Per the IAU style manual, the "unit known as the light-year is appropriate to popular expositions on astronomy and is sometimes used in scientific papers as an indicator of distance".[5] "The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU."[6] The parsec does have a standard IAU definition, although it is a much less well known unit in the public space. Praemonitus (talk) 14:54, 24 July 2024 (UTC)

Looking again at MOS:CONVERSIONS, I think In some topic areas [...] it can be excessive to provide a conversion for every quantity applies here, though unlike light-years the examples given are at a scale where metric units are typically used. As I've said, I don't think For units of measure that are [...] obscure outside of a particular specialty or geography applies; not part of the SI or US customary systems may apply but that's not the part that was mentioned. SevenSpheres (talk) 18:22, 24 July 2024 (UTC)

Are you arguing for leaving some quantities in infoboxes unconverted from the preferred unit, or are you thinking of omitting conversions only in prose? -- Beland (talk) 18:39, 24 July 2024 (UTC)

The opposite, as per e.g. [7] I would more strongly prefer to retain the status quo in infoboxes than in prose. SevenSpheres (talk) 19:03, 24 July 2024 (UTC)

OK, that looks like a vote for light-years and parsecs in infoboxes with kilometers in prose but not to excess. I consider the status quo to be "SI conversions are required in infoboxes" because of MOS:CONVERSIONS, but in practice for the ones I've seen, the status quo is light-years and parsecs. -- Beland (talk) 22:01, 24 July 2024 (UTC)

All I will say is that this discussion seems to connect with the one I began about prefixes. Looking at this thread it seems I might not be alone here. Avenues2009 (talk) 23:18, 29 July 2024 (UTC)

Sure; feel free to share your preferences in the RFC thread. -- Beland (talk) 01:19, 30 July 2024 (UTC)

Draft RFC question

Version 1

How should distances between stellar systems and longer be presented in astronomy articles? (These are often calculated automatically from different units presented in astronomy sources, such as parallax.) Previous discussions identified that parsecs are used by professional astronomers and light-years are used in popular news and educational media; those two units are currently the most commonly used and converted to each other. Editors disagreed on whether light-years or kilometers with scientific notation were easier to read and understand intuitively for most readers. If conversion to SI units (like kilometers) is not required in certain contexts, this would be added as an explicit exception to MOS:CONVERSIONS.

More than two units per quantity is cumbersome, so we're asking folks to pick their TOP TWO from:

• Light-year (ly)
• Parsec (pc)
• Kilometer in scientific notation (km)
• Meter with metric prefix (?m)

Infobox examples using quantities from Local Interstellar Cloud, Andromeda Galaxy, Proxima Centauri, and Betelgeuse:

• ly and km:
  • 30 ly (2.8×10¹⁴ km)
  • 2.50 Mly; 2.36×10¹⁹ km
  • 4.2465 ± 0.0003 ly
    4.0175×10¹³ ± 2.8382×10⁹ km
  • 408–548+90
    −49 ly (3.86×10¹⁵–5.18+0.85
    −0.46×10¹⁵ km)
• ly and pc:
  • 30 ly (9.2 pc)
  • 2.50 Mly; 765 kpc
  • 4.2465 ± 0.0003 ly
    1.3020 ± 9.1980×10⁻⁵ pc
  • 408–548+90
    −49 ly (125-168.1+27.5
    −14.9 pc)
• ly and ?m:
  • 30 ly (280 Pm)
  • 2.50 Mly; 23.6 Zm
  • 4.2465 ± 0.0003 ly
    40.1750 ± 0.0028 Pm
  • 408–548+90
    −49 ly
    3.86–5.18+0.85
    −0.46 Em

Please let us know if you prefer the SAME or DIFFERENT for INFOBOXES vs. PROSE (or if you prefer some other solution). We assume that for prose the "unless this would be excessive given the context" rule from MOS:CONVERSIONS will still apply.

Version 2

How should distances between star systems and galaxies be presented? These are measured in light years (ly) in popular news and educational media; professional astronomers use parsecs (pc). Articles currently use a variety of units (some only ly and ly converted to km) but most commonly use ly converted to pc in infoboxes (often automatically from technical data). If conversion to SI units (like kilometers) is not required in certain contexts, this would be added as an explicit exception to MOS:CONVERSIONS. The maximum distance in the observable universe is under 100 billion light-years, and interplanetary distances (inside a star system) are a fraction of a light-year and are measured in astronomical units (AU or au).

Two formats are needed: an expanded format for use in article prose (especially on first mention), and a compact format for infoboxes, tables, and prose where the expanded format would be excessively long. The "unless this would be excessive given the context" rule from MOS:CONVERSIONS will still apply when the units are used several times in prose.

The units nominated in previous discussion are:

• Light-year (ly)
• Parsec (pc)
• Kilometer (km)
• Meter (m)

Units can be written in words or symbols, but if symbols are used, MOS:NUMNOTES says the number part must be written in numbers (e.g. 12 million km, not twelve million km). First use of light-year/ly and parsec/pc must be linked to their articles, per MOS:UNITS.

The notations that can express very large quantities are:

• Words (million, billion, trillion, and so on in names of large numbers)
• Metric prefixes, namely: Gly, Mly, kly, Gpc, Mpc, kpc, Ym, Zm, Em, Pm, Tm
• Scientific notation, often used with km (which already have a metric prefix), e.g. 8.8×10²⁰ km
• Long numerals, e.g.: 93,000,000 ly (88,000,000,000,000,000,000 km)

Formatting details are delegated to {{convert}} and {{val}}, which may be helpful in expressing your preferences below.

The chosen formats need to accommodate simple cases and complex expressions of precision like:

• 30 ly (2.8×10¹⁴ km)
• 2.50 Mly (765 kpc)
• 4.2465 ± 0.0003 ly
  4.0175×10¹³ ± 2.8382×10⁹ km
• 408–548+90
  −49 ly (125-168.1+27.5
  −14.9 pc)

Please specify which order the units should appear in, if you have a preference.

(sample votes shown)

Compact format (specify preferred units, notation, and order)

• ly only, with metric prefixes. "34.5 Mly ± .3Mly". Infoboxes get overwhelmingly numbery if they have conversions. - User 1
• ly converted to pc, both in exponential notation. "34.5×10⁹ ly (1.06×10¹⁰ pc)". Astronomers need parsecs for convenience. - User 2

Expanded format (specify preferred units, notation, and order)

• "Light year" converted to km, in exponential notation. "34.5 million light years (3.26×10²³ km) " - User 1
• "Light years" converted to "kilometers" then "parsecs", in words, but no higher than trillion. "34.5 million light years (326 billion trillion kilometers; 10.6 billion parsecs) plus or minus .3 million light years." - User 2

Version 3

What units should be used for distances between star systems and galaxies? These are measured in light years (ly) in popular news and educational media; professional astronomers use parsecs (pc). Articles currently use a variety of units (some only ly and some ly converted to km) but most commonly use ly converted to pc in infoboxes (often automatically from technical data). If conversion to SI units (like kilometers) is not required in certain contexts, this would be added as an explicit exception to MOS:CONVERSIONS. The maximum distance in the observable universe is under 100 billion light-years, and interplanetary distances (inside a star system) are a fraction of a light-year and are measured in astronomical units (AU or au).

Two formats are needed: an expanded format for use in article prose (especially on first mention), and a compact format for infoboxes, tables, and prose where the expanded format would be excessively long. The "unless this would be excessive given the context" rule from MOS:CONVERSIONS will still apply when the units are used several times in prose. First use of light-year/ly, parsec/pc, and rare meter prefixes (e.g. zettameter) must be linked to their articles, per MOS:UNITS. The chosen formats need to accommodate simple cases and complex expressions of precision (examples below).

The choices nominated for inclusion are:

• Light-year (ly) with SI prefixes (kly, Mly, Gly) and large number words (million, billion) in prose
  • 34.6 ± 0.3 million light-years (ly) [first mention in prose]
  • 34.6 ± 0.3 Mly [compact]
  • 408–548+90
    −49 ly [compact]
• Parsec (pc) with SI prefixes (kpc, Mpc, Gpc)
  • 765 ± 2 kiloparsecs (kpc) [first mention in prose]
  • 765 ± 2 kpc [compact]
  • 125-168.1+27.5
    −14.9 pc [compact]
• Kilometer (km) with scientific notation
  • 3.27×10¹⁴ km [compact, secondary in prose]
  • 3.273×10¹⁴ ± 2.8×10¹² km [compact, secondary prose]
  • 68.1+7.5
    −4.1×10¹⁴ km [compact, secondary in prose]
  • 3.27×10¹⁴ kilometres [primary expanded]
• Meter with SI prefixes (Ym, Zm, Em, Pm, Tm)
  • 68.1 zettameters (Zm) [first mention in prose]
  • 68.1 Zm [compact]
  • 68.1+7.5
    −4.1 Zm [compact]

You can of course advocate for as many or few options as you find appropriate, or assert multiple options are equally good, but previous discussion has assumed at most two units would be used because many editors find three to be excessive. Please specify your preferred order; "primary" units come first and other units are converted to (typically in parentheses in prose, sometimes on a new line or after semicolon in infoboxes).

(sample votes shown)

Your preferred units

Please note your preferred units for both compact-in-infobox and expanded-in-prose if they are different.

• Light-years only. Conversions make science overwhelmingly numbery. - User 1
• Light years converted to parsecs in infoboxes for astronomers, kilometers in prose for general audience. - User 2

Discussion

• For clarity I would suggest using the same format for all examples (always put the second unit in brackets or maybe always use a semicolon between them, instead of semicolons, brackets, and line breaks mixed). Otherwise it looks fine to me. Gawaon (talk) 08:07, 25 July 2024 (UTC)
• I'm afraid it's hard to decipher your illustrative possibilities; I think it's better to say explicitly that you are displaying the same data using the three combinations (ly, km), (ly, pc), and (ly, ?m). Also, it might be worth noting that (ly, pc) is already a de facto standard. Tercer (talk) 08:35, 25 July 2024 (UTC)

  Tweaked to add headers for clarity. -- Beland (talk) 09:27, 25 July 2024 (UTC)

  And added note about the status quo in articles, per your suggestion. -- Beland (talk) 09:28, 25 July 2024 (UTC)

In articles, I see semicolons, parentheses, and new lines in infoboxes. -- Beland (talk) 09:24, 25 July 2024 (UTC)

Okay. Gawaon (talk) 10:28, 25 July 2024 (UTC)

I have two comments

• I object to using megalight-years in the examples. While light-years are commonly used, megalight-years are not (and the unit in any case should be light-megayear, not megalight-year). Saying "2.5 million light-years" is fine. Saying "2.5 megalight-years" is not.
• I'm not sure the question is well posed. Surely we are all assuming the primary unit is either light-year or parsec, and the question should then be "what are converting it to?"

Dondervogel 2 (talk) 09:59, 25 July 2024 (UTC)

That's not true. Mly is commonly used. "light-megayear" is non-existent. Tercer (talk) 10:31, 25 July 2024 (UTC)

I meant that in a relative sense. In other words, use of the megalight-year is less common than that of the light-year. I accept that light-megayear is rarely used, but if one uses light-second, light-minute, light-day and light-year, the obvious next steps are light-century, light-kiloyear and light-megayear. That was my point.

That said, my main objection to use of megalight-year, gigalight-year is the absence of an authoritative/standard definition of these units. The IAU does not define them, so who does? Perhaps the same question applies to the megaparsec, but that seems somehow less controversial. I'm not sure why. Dondervogel 2 (talk) 16:44, 25 July 2024 (UTC)

I'd say if Mly is used in existing pages, it's okay to use it in an example too. Gawaon (talk) 17:36, 25 July 2024 (UTC)

I mean, why would the IAU have to define them when mega- and giga- prefixes are already well-defined? This doesn't seem to be a very good justification to not use them; AFAIK, no organization "officially defines" what a kiloton or megaton is, but articles about nuclear tests and volcanic eruptions use them all the time in TNT-equivalent units simply because kt and Mt are useful and widely used. ArkHyena (talk) 17:40, 25 July 2024 (UTC)

I'll pay you 10€ if you can find a single example of light-megayear used in a peer-reviewed paper. I see your point about the logical progression of the units, but the fact of the matter is that this is not how astronomers use it. Tercer (talk) 17:54, 25 July 2024 (UTC)

Hmmm ... that's an interesting challenge. I can't guarantee to find light-megayear, but I would be surprised if a search for light-kiloyear, light-megayear and light-gigayear doesn't come up with something. Watch this space ... Dondervogel 2 (talk) 19:20, 25 July 2024 (UTC)

... I found this mention of a light-gigayear. See also [8] Project Astronomy Archive 17, where it appears the same question arose 9 years ago, and with the same outcome. Dondervogel 2 (talk) 19:37, 25 July 2024 (UTC)

Light-gigayear is fair enough, but I did specify a peer-reviewed paper. Tercer (talk) 20:08, 25 July 2024 (UTC)

You did indeed, so I am not claiming my 10 EUR just yet. I'll keep watching out for an example :P Dondervogel 2 (talk) 20:13, 25 July 2024 (UTC)

It just dawned on me that the reason I can't find light-megayear in peer-reviewed papers is that peer-reviewed papers use the parsec (and megaparsec), not the light-year. The light-year is used in popular literature only, where it does not need a rigorous definition. Perhaps that explains why Mpc seems less controversial (to me) than Mly. Dondervogel 2 (talk) 20:23, 25 July 2024 (UTC)

While parsecs are clearly the unit of choice, plenty of peer-reviewed astronomy papers do use light-years: [9] [10] [11]. Tercer (talk) 20:44, 25 July 2024 (UTC)

Sometimes I really do wish astronomers and science communicators would use just one unit instead of two... ArkHyena (talk) 20:50, 25 July 2024 (UTC)

Yes, I see those articles use the light-year (though in one case only in the title). Do you know one that uses the megalight-year? Dondervogel 2 (talk) 20:56, 25 July 2024 (UTC)

It looks like Morrison and Sartori 1965 might use the light-kiloyear. I can't be sure because it's behind a paywall. Does anyone have access to Phys Rev Lett? Dondervogel 2 (talk) 21:19, 25 July 2024 (UTC)

I found it and yes, they talk about "R ~ 10 light kiloyears". Gawaon (talk) 22:07, 25 July 2024 (UTC)

I have checked, and indeed, that's a light kiloyear. Not a light-megayear, but I think it's good enough. Please email me your IBAN through Special:EmailUser/Tercer so that I can pay you what I owe. Tercer (talk) 08:33, 26 July 2024 (UTC)

Light-year#Definitions has citations to textbooks for kly, Mly, and Gly, kilolight-year, megalight-year, and gigalight-year. English isn't always a logical or consistent language, but we use it as it is. There doesn't seem to be any disagreement about what these units mean; they adopt the SI prefixes straightforwardly. Mly, Gly, kpc, and Mly are currently used in a lot of articles, but I also see constructions like "70,000,000 ly" in infoboxes and "70 million light years" in prose. These units are a lot more compact and a lot cleaner when there are ±, and they have commonly-used prefixes people are familiar with from computer hardware. Beland (talk) 17:56, 25 July 2024 (UTC)

We can ask this question two ways: 1. pre-discuss possibilities and offer up the most popular (among astro-enthusiast editors) and logically coherent choices in an effort to make the question easy to answer and consensus easy to discern, or 2. open voting to arbitrary combinations and have faith that The People will Do the Right Thing. I started with approach 1, but Gawaon requested approach 2. If we're sticking with approach 2, I'm inclined to let the wisdom of the crowd decide which unit they want as primary. If 70% of people vote to make kilometers primary, then that's surprising but useful information to make the encyclopedia more readable to the general public, and no one can complain the question was biased and we need to do another RFC. We've been talking about two units of measure as the optimal number, but the more articles I look at, the more I wonder if 1 or 3 wouldn't be better, depending on whether we want to make things clear and easy to understand (which some articles already do) or give everyone immediate handy access to a number in the units they are thinking or calculating in.

I was originally thinking of this only as a question for infoboxes, but the more articles I look at, the more I realize that quantities are presented very differently in prose. For example, "72 million light-years" is much more reader-friendly than either "7.2 × 10⁶ ly" or "7.2 Mly", and the friendly version is often used in prose. Perhaps we should frame the question as asking people to define "compact" and "expanded" formats. In prose, we often use an expanded format on first mention of an unfamiliar-to-everyday-life unit (like light-years), and then compact formatting for later uses to avoid excessive length. We also use compact formats for tables, not just infoboxes. MOS:NUMNOTES has some things to say about this already, but doesn't make all the choices we have questions about.

When we write $2M in prose, we write two million dollars, not two megadollars, and I agree 2Mly should be written as two million light years, megatons notwithstanding. I'll add some prose examples showing the default interpretation of choosing certain units. Beland (talk) 18:23, 25 July 2024 (UTC)

I didn't mean to ask about "primary" and "secondary", only about the preferred combinations of two units (considering that regularly using more would be impractical). I think the only truly "odd" result from the astronomers' viewpoint would be if kilometres and prefixed metres were chosen, with both light-years and parsecs discarded. But I think that's a very unlikely outcome, hence I believe we can trust the "wisdom of the crowd" to find a reasonable combination. Gawaon (talk) 19:08, 25 July 2024 (UTC)

(Also I didn't mean to "request" anything, it was just a suggestion to make the multiple logical possibilities easier to handle.) Gawaon (talk) 19:12, 25 July 2024 (UTC)

Sure, it was a good suggestion! BTW, I've drafted a second version of the RFC question making the variables more explicit and connecting to some existing rules so hopefully we don't have to re-debate those. Too much? -- Beland (talk) 19:37, 25 July 2024 (UTC)

@Beland: I question the assertion that there is no dispute about the meaning of megalight-year. According to this unit converter, a megalight-year is equal to 999315.53730322 light-years. Is that the conversion you would expect? Dondervogel 2 (talk) 19:25, 25 July 2024 (UTC)

Seems like a floating point error. SevenSpheres (talk) 19:35, 25 July 2024 (UTC)

I doubt it's a floating point error. More likely a consequence of the ambiguity between a megaannum (precisely 365.25 million days, following the IAU convention, using Julian years) and one million years (approximately 365.2422 million days). I don't think the arithmetic works out to explain that weird conversion, but my fundamental point is that such units are undefined, and therefore ambiguous. Dondervogel 2 (talk) 19:52, 25 July 2024 (UTC)

This website is clearly suffering from rounding errors. It converts 9999999999999 mm to 9999.999999999001 Mm. It's pretty clearly a not-so-carefully semi-automatically created SEO honeypot. -- Beland (talk) 19:56, 25 July 2024 (UTC)

Somebody used "365.5" when entering one unit into the database, and "365.25" for the other: 999315.53730322 / 365.25 = 365.5×10⁻⁶. Indefatigable (talk) 19:58, 25 July 2024 (UTC)

Nice catch! Tercer (talk) 20:11, 25 July 2024 (UTC)

...hmm, it does actually say that in the text though... but it also says One megalight-year [...] is one million light-years. That website is probably not the most reliable source in any case. SevenSpheres (talk) 19:37, 25 July 2024 (UTC)

Shitty websites making mistakes prove nothing. If you want to demonstrate that there is a dispute about the definition of megalight-year you need to find reliable sources saying so. Tercer (talk) 19:50, 25 July 2024 (UTC)

Uh-uh. The unit remains ambiguous until it is defined. And there is never a justification for using this particular ambiguous unit when we can easily write "one million light-years", with no ambiguity. Dondervogel 2 (talk) 19:54, 25 July 2024 (UTC)

I fail to see how "megalight-year" would remotely be ambiguous, unless the metric prefix "mega-" itself is ambiguous. ArkHyena (talk) 20:01, 25 July 2024 (UTC)

But the prefix mega- (as in megabyte) IS ambiguous. I suspect you realised that before adding the "metric" qualifier. For the definition we rely on a handful of sources from the light-year article. Is that really enough? And I repeat we can always write "one million light years" (or "one billion light years" for giga), so why confuse our readers with the prefix? Dondervogel 2 (talk) 20:10, 25 July 2024 (UTC)

That particular case is does seem to be exceptional due to its usage within compsci, but honestly I doubt people who are aware of that oddity would apply it to other units. "Mega-", regarding units, seems to be universally understood to mean 10⁶ unless specified otherwise, regardless if the unit it is applied to is SI or not. A megatonne of TNT equivalent is one million tonnes of TNT equivalent. A megaelectronvolt is one million electronvolts. And so on.

The one possible point of confusion is that "mega-" in colloquial usage does not strictly refer to the metric prefix, e.g. "megadonor", but TMK people generally recognize that, when affixed to a unit of measurement, "mega-" indeed means 10⁶. ArkHyena (talk) 20:22, 25 July 2024 (UTC)

Can we agree to write "6.7 million light years" instead of "6.7 mega-light-years" and "9.8 megaparsecs" (which seems standard) and 6.7 Mly and 9.7 Mpc for short? I don't think anyone is going to be confused into thinking that the "M" uses the 1000x scale instead of 1024x for light years but not parsecs. Certainly not people who know enough to need the numbers for precise calculations, who seem to be using Mly in technical papers with zero confusion. 1000x is what everyone in America is taught in school when we learn the metric system. Almost anyone who knows about the 1024x scale should know it's only used for computers. Our main audience is the general public to whom we're giving these numbers just to get a general sense of things (at which point 1000x vs. 1024x doesn't matter). -- Beland (talk) 22:19, 25 July 2024 (UTC)

Reliable sources do define Mly as exactly one million light-years, as cited at light-year. Definitions of units don't need to be blessed by a government or professional body to have a clear meaning, any more than "straight up" needs an English equivalent of the French Academy to legally define which vector I mean by that. This web site is unambiguously making an error. -- Beland (talk) 20:01, 25 July 2024 (UTC)

• Since some votes will differ in options for infobox vs. prose (or some contributors may want to add notes specific to one or the other), it may be best to set up two separate surveys (e.g. survey for infoboxes/survey for prose) once the RfC is pushed out for organizational reasons. ArkHyena (talk) 10:56, 25 July 2024 (UTC)

  Excellent suggestion; implemented in Version 2 of the draft above. -- Beland (talk) 19:38, 25 July 2024 (UTC)

  My preference (and I don't think I am alone here) would be

  • primary unit: parsec or light-year
  • converted unit: any SI unit

  I don't see this preference represented in the choices offered. Dondervogel 2 (talk) 19:59, 25 July 2024 (UTC)

  That seems like a perfectly valid not-vote. We didn't say you couldn't pick multiple items off the menu as equally good in whatever slot. I assume "any SI unit" means you don't care if we use m or km, and don't care if we use exponential notation or metric prefixes or long numerals or words. -- Beland (talk) 20:06, 25 July 2024 (UTC)

  Yes, that is a correct interpretation. Dondervogel 2 (talk) 20:17, 25 July 2024 (UTC)

  While I see the ideas behind Version 2, I think it's too complicated. We can't expect everyone to express separate preferences for compact and expanded format, and nobody should have to think about which prefixes for light-years they prefer. I'd keep it simpler, more in line with version 1: You have four possible units (ly, pc, km/scientific, m with prefix) – pick the two you prefer. Maybe Preferably ask for first and second unit too. My choices, similar to Dondervogel 2: ly or pc as first, km/scientific as second. Gawaon (talk) 20:08, 25 July 2024 (UTC)

  I'd also argue for this. More complex options would probably prolong discussion and make things messier than they need to be. This discussion here is already quite protracted. ArkHyena (talk) 20:13, 25 July 2024 (UTC)

  OK, so to clarify:

  • We're limiting this RFC to the compact format intended for infoboxes and tables, and after we get a clear answer on that discuss what to do about prose?
  • Are we pre-selecting a notation for extremely long distances for ly and pc? According to parsec, Gpc, Mpc, and kpc are standard. Despite the one-editor objection above, Gly, Mly, and kly seem to be used and understood, and seem a lot more compact than 93,000,000 ly (especially if there's an error margin) and more comprehensible and less cluttery than exponential notation.

  -- Beland (talk) 22:10, 25 July 2024 (UTC)

  We can probably survey for infoboxes and prose simultaneously, especially since I'd figure someone would bring up prose conventions at some point if it is not included in the first place—we'd just have to partition the survey as aforementioned to hopefully keep things smooth. And yes, Gpc, Mpc, kpc; and Gly, Mly, and kly seem to be well-established standards in relevant articles. ArkHyena (talk) 22:35, 25 July 2024 (UTC)

  Should we then assume the expanded form (prose first or infrequent mention) would use unit names instead of symbols?

  Should we also assume we're not using scientific notation for the expanded form and we'll use the words for multipliers greater than "thousand"? For example, "34.5 million light years (326 billion trillion kilometers)" if those units win the not-vote? ("Sextillion" isn't even in all dictionaries listed by Names of large numbers; when English Wikipedia uses it, it tends to be accompanied by exponential notation or other -illion words to explain what it means.) -- Beland (talk) 02:49, 26 July 2024 (UTC)

  No, of course not, with distances like this, scientific notation will have to be used for kilometres in any case. Stuff like "billion trillion" is impractical to write and confusing to read. (And probably also confusing to write – indeed I think it should have been "billion billion" or "million trillion" (10¹⁸).) Gawaon (talk) 05:35, 26 July 2024 (UTC)

  Also, it's common to abbreviate converted units in brackets also in prose, and I would assume that for our survey too. So the above example could be written as 34.5 megalight-years (3.26×10²⁰ km) – that's how {{convert}} does it. Though actually "million light-years" is probably better than "megalight-years" in prose. While I agree with that, I don't think such details should be part of this RfC, that would just be a needless overcomplication. Gawaon (talk) 05:58, 26 July 2024 (UTC)

  OK, I've integrated all your suggestions into Version 3, above, which does feel a lot less overwhelming and easier to answer. I've tried to leave no ambiguity in the examples and use only formats with the strongest support, in case they are later taken as prescriptive (which they kind of will be if no one complains about them), and so no one can complain "if I'd known that's how we'd be writing this, I'd have voted differently". -- Beland (talk) 16:13, 26 July 2024 (UTC)

  Yeah, that version looks fine to me. Gawaon (talk) 16:18, 26 July 2024 (UTC)

  Yes, I agree it's reasonable to survey for infoboxes and prose together. People who have different preferences of units for one vs. the other should simply express that as part of their vote – I for one don't. But keep it one survey, with the option to express separate preferences for both in case they differ – nobody should be forced to vote twice, and certainly not in different sections. Stuff like Gpc and Mly sounds reasonable too, so we can used it in examples, but it's not what the RfC is about. Gawaon (talk) 05:34, 26 July 2024 (UTC)

• Version 3 reads good to me! Gawaon (talk) 16:12, 26 July 2024 (UTC)

  Agree here, version 3 looks good to go. ArkHyena (talk) 21:36, 26 July 2024 (UTC)

• I just noticed that astronomical unit is absent. Is this a deliberate omission? Dondervogel 2 (talk) 16:23, 26 July 2024 (UTC)

  Yes. Based on comments by Modest Genius and general practice I see observed in articles and the popular media, AU are used for distances inside star systems, but not the much larger distances between star systems. One light-year is about 63,240 AU, so even for relatively short interstellar distances, the numerical quantities are getting into the range where we'd normally start considering applying metric prefixes or exponential notation. 1 AU is the size of the Earth's orbit, so it makes sense to use these units for easy, intuitive comparisons of planetary orbits, and not for measurements on vastly different scales. I think it's unlikely that even if offered the choice, the wisdom of the crowd will favor AU, but that said, if anyone has strong feelings and wants to nominate AU to be included, I'm happy to add them. -- Beland (talk) 17:02, 26 July 2024 (UTC)

  That's very reasonable. No need to add them on my behalf. I just wanted to check it was not an oversight. I also approve of the Version 3 wording. Thanks to those who have invested their time in the multiple iterations. Dondervogel 2 (talk) 20:27, 26 July 2024 (UTC)

  Woo! If no one has any further suggestions or objections, I'll start the RFC with the version 3 text in the next 24-48 hours. -- Beland (talk) 20:33, 26 July 2024 (UTC)

Primary units and flipping

External videos
"It's called flipping, and it almost oughta be illegal"

May I suggest that the units shown first should always be those quoted in the source document. The reason is that this is less likely to be distorted by rounding errors, and is likely to be more accurate than any alternative, converted figure. If "flipping" is allowed, then the user is left in doubt as to which of the two (or more) figures is more accurate, and in many cases also as to the precision of each figure. This means that some information is lost. Ehrenkater (talk) 15:17, 30 July 2024 (UTC)

Your rounding-errors argument makes no sense, since Convert will be working from the source's unit whether the display is flipped or not. The "user left in doubt" argument has at least some merit, but I believe it's completely outweighed by the fact that we'd be presenting values from different s sources with different unit orders, which will appear random to the reader. Johnuniq may have useful insight on this. EEng 16:50, 30 July 2024 (UTC)

I prefer using the unit used in the source as the first in the Convert template, but usually flip to put the metric output first. Care must be taken to minimize rounding errors whether the outputs are flipped or not. Donald Albury 17:01, 30 July 2024 (UTC)

{{Convert}} is quite good with keeping the approximate precision of the source, and I agree that it's better to use flip as needed to get a consistent output. Consistency requirements within the same article suggest that the same unit should always come first (if possible), otherwise readers could get confused. Gawaon (talk) 17:06, 30 July 2024 (UTC)

Yes, and editors might use one source rather than another so that their preferred system of units comes first (see this talk-page's archives, too often). NebY (talk) 17:35, 30 July 2024 (UTC)

This is addressed in the Wikipedia:Manual of Style/Dates and numbers/Units of measurement/Unit choice and order. Shortcut MOS:UNIT. Basically this states that with the exception of the USA and the UK, the primary unit will be SI. Note that metric and SI are not the same thing. Sources can be cherry picked, the official government or company reference is a better choice. Magazines or newspapers generally round articles to their preferred unit. Avi8tor (talk) 19:44, 30 July 2024 (UTC)

Oh, I know MOS:UNIT says that and that sources can be cherry-picked - that's exactly my point and I've got the t-shirt. However, an official government or company release will not always be a better source, per WP:PRIMARY. NebY (talk) 19:53, 30 July 2024 (UTC)

Is there an example where distortion from rounding errors has occurred? Johnuniq (talk) 23:57, 30 July 2024 (UTC)

I commonly see in Wikipedia a distance of about 100 yards (91 m) when it's an estimation not a measurement, far to accurate in metres for "about or approximately." I have a copy somewhere of an article on a roman building excavation from 3 different newspaper sources all with different dimensions in feet only. Finding it on my computer may take a while, it's been a few years. Another example is engine power in motor vehicles, which since ~1980 in the EU has required it be stated in kilowatts. This hasn't stopped people from using PS, CV or HP because it's from their source. The best source is the owners manual, most available online. Avi8tor (talk) 04:59, 31 July 2024 (UTC)

Thanks but I meant an example of an article where someone had used {{convert|...|order=flip}} and where the result was distorted. It sounded as if the OP might have encountered the issue. Johnuniq (talk) 05:27, 31 July 2024 (UTC)

Flip was not involved. What I was thinking of was this, where {{Convert}} had a round number (700 km2) as input, and sigfig set to 1, which caused the output to round to 300 sqmi. This caused the sqmi output to be larger than the output from a conversion of 703 km2 to 271 sqmi. This was a list ranked by size, and it would have ended up with an entry showing as smaller than another entry when nmeasured in km2, and larger than that entry when measured in sqmi, if I had not changed the sigfig. Pay attention to the sigfig setting. Donald Albury 16:48, 31 July 2024 (UTC)

Interesting case, but I'd tend to leave out that parameter and let {{Convert}} do the right thing. Most of the time it seems to do a good job (700 km² (270 sq mi) looks reasonable too). Gawaon (talk) 17:03, 31 July 2024 (UTC)

'sigfig' has its uses. As with everything else in editing Wikipedia, one should inspect the output of any use of the convert template. Donald Albury 18:56, 31 July 2024 (UTC)

I suspect that you have something like this: a reference of 6 miles and an article that uses SI first. And that you "flip" by hand to get 10 km to make the SI appear first like {{convert|10|km/h|mph|0}} to display as 10 kilometres per hour (6 mph).
The answer (assuming I have constructed the right strawman) is to not flip by hand but to tell {{convert}} to swap the display order. Hence, you do {{convert|6|miles|km|0|order=flip}} to display as 10 kilometres (6 mi). This always uses the reference value in the wiki mark-up, displays SI first and does not do a double conversion.  Stepho  talk  07:56, 31 July 2024 (UTC)

If the reader sees 10 kilometres (6 mi) he or she will most likely assume that the accurate distance is somewhere between 9.5 km and 10.5 km (or possibly the 10 km could be only one sig. fig., there is no way of telling). However in reality the length is probably somewhere between 5.5 miles and 6.5 miles (or about 8.9 km to 10.5 km) so some information has been lost in the flipping. Ehrenkater (talk) 07:25, 2 August 2024 (UTC)

Few readers will care whether the exact length is 9.2 or 9.6 km and those that do will do better by either looking it up in the provided reference (where they'll find the original unit) or looking it up in some kind of primary reference collection that presents all distances in a uniform way and according to a uniform standard of reliability. Face it, a tertiary source like Wikipedia that gathers information out of all kinds of (hopefully reliable) sources is not the place to go if you're interesting in absolute precision. Gawaon (talk) 08:37, 2 August 2024 (UTC)