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

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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 miles). 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)

Ammunition calibre/length naming conventions

See discussion at WT:WikiProject Military history#Space before unit symbol. I had moved 14.5×114mm to 14.5×114 mm; another user pointed out it should be 14.5 × 114 mm; then I noticed it should be 14.5 mm × 114 mm per MOS:UNITSYMBOLS and said I don't want to go there. Can we consider revising the guidelines to allow dropping the unit symbol in such cases instead of repeating it? There are about 20 cartridge sizes with similar article title question; maybe more. Dicklyon (talk) 22:43, 8 August 2024 (UTC)

Hmm. In that article, "14.5x114mm" seems to be used as much as a proper noun as a pair of dimensions, in which case it might have any form (cf 2x4) and, like Nine Inch Nails, not require an exception here. NebY (talk) 23:05, 8 August 2024 (UTC)

Well, it's pretty clearly a descriptive name, not a proper name, and it appears various ways in sources, including with the mm twice in one source I found. So I'd say it's pretty clearly subject to editorial styling, by us and others, and it would be good to have an easy way to fit it into our style rules without using that "rarest" version. Dicklyon (talk) 00:14, 9 August 2024 (UTC)

It is a proper name, not a descriptive one. Although ammunition calibres may look like measurements and in some cases actually are, they often are not. This arises from the common practice of changing the name when the ammunition is incompatible rather than its actual size. That does not mean that it is not subject to our style rules, but do not try to use conversion templates. Hawkeye7 (discuss) 03:37, 9 August 2024 (UTC)

Having a space before the "mm" looks fine to me, since that's how metric units are officially written. Otherwise I agree we don't have to follow the intricacies of our style rules in all details, since as others have noted, it's a name, not really a measurement. So I'd leave the article name as it's now. Gawaon (talk) 06:09, 9 August 2024 (UTC)

My gut feeling is that this is the name of the ammunition, and was therefore probably OK as it was before the move (if it's a name there's no need for the space, and no need to repeat the unit symbol): '14.5×114mm' seems fine. If I'm wrong and this is a description of the dimensions, the space is needed and the unit symbol should appear twice, but I am not saying we should use '14.5 mm × 114 mm', because that implies a rectangular shape (and I kinda doubt we are talking about rectangular ammunition). If one of the dimensions is a diameter, a meaningful description would need to convey that, with a Φ symbol or similar.

My advice: keep things simple by treating it as a name. Dondervogel 2 (talk) 06:38, 9 August 2024 (UTC)

There are names and there are proper names. The wide variation in styling in sources, and the structure of it, suggest that it's a descriptive name, subject to editorial styling. Even at the milhist discussion the idea that it's a proper name didn't come up; go there if you want, since this discussion is about how our styling guidelines apply to treating it as a descriptive name. Dicklyon (talk) 15:59, 9 August 2024 (UTC)

This discussion is about whether our styling guides apply to names. MOS:UNITS applies to the names of units, but why should it apply to the names of anything else? I notice no enthusiasm at WT:WikiProject Military history#Space before unit symbol for being governed or guided by MOS:UNITS in this regard, or much concern about the matter at all. NebY (talk) 16:37, 9 August 2024 (UTC)

If we accept that it's a name, how would you style it, given that sources are all over the map on it? Dicklyon (talk) 22:38, 9 August 2024 (UTC)

I don't know that I've ever written about such ammunition and I may never, so I'm happy to leave it to the people who do have an interest in it. Indeed, if the sources are all over the map on it, it might not even be possible to find an acceptable and universally applicable norm, and there might be very little to be gained and something to be lost from seeking or imposing uniformity. NebY (talk) 22:35, 10 August 2024 (UTC)

I agree. I suppose what people mean (and I agree with that too) is that, since we're talking about a name rather than a number, MOS:UNITNAMES doesn't really apply. Though interestingly, MOS:UNITNAMES itself says: "The unspaced letter x may be used in common terms such as 4x4." That might be applicable to this case too. Gawaon (talk) 16:39, 9 August 2024 (UTC)

And notably, the linked article actually uses the spelling 4×4 – using × instead of the letter x, but without surrounding spaces. Gawaon (talk) 16:42, 9 August 2024 (UTC)

They appear to be names (which at least in the US are approved by the appropriate trade organisation - Sporting Arms and Ammunition Manufacturers' Institute) - see [12].Nigel Ish (talk) 08:57, 11 August 2024 (UTC)

Also note that while some ammunition names/designations appear to be dimensions that isn't necessarily the case - for example 357 Magnum and .38 Special both fire bullets of the same diameter, just the first has a longer cartridge and is much more powerful.Nigel Ish (talk) 09:11, 11 August 2024 (UTC)

I've infomed the firearms wikiproject.Wikipedia_talk:WikiProject_Firearms#Possibly_relevant_discussion_at_Manual_of_Style/Dates_and_numbers.Nigel Ish (talk) 09:23, 11 August 2024 (UTC)

Thanks! I hadn't noticed that one. Dicklyon (talk) 22:42, 11 August 2024 (UTC)

I've moved a couple: 12.7 × 108 mm and 14.5 × 114 mm. If anyone objects, say so or revert, and I'll start a multi-RM discussion. Dicklyon (talk) 22:42, 11 August 2024 (UTC)

Use of metric prefixes

As someone with high functioning autism I find it all nice, consistent and straightforward to use the whole range of metric prefixes (see article on those) at every opportunity (even instead of non-SI units even specialists tend to use), this to me (forgive me for any slight inaccuracies here these are to rounded figures and are off the top of my head ), the mass of an electron is 9.109 qg, the mass of a proton is 1.69 yg the electronic charge is 160 zC, the the dialect of a proton is about 1 fm, the depth of the world’s planned deepest swimming pool would be 5 dam, the height of the Eiffel Tower is 3.24 hm, Everest is 8.863 km high, the earth has a circumference of 40 Mm and a surface area of 510 Mm2, the volume of water in the Pacific Ocean is about 1 Mm3, the distance to the Moon is 384 Mm, the distance to the Sun is 150 Gm, the distance from the Sun to Saturn is 1.4 Tm, the distance to Alpha Centauri is 40 Pm, the distance to Betelgeuse is roughly 3 Em, the diameter of the Milky Way is 1 Zm with Andromeda 21 Zm away, the diameter of the Observable Universe is a Comoving 880 Ym, the mass of the Earth is 5.98 Rg and the mass of Jupiter is about 2 Qg. To me, it would be much more consistent and straightforward if all Wikipedia articles used the full range of metric prefixes to get people using them more and thus making more sense to me. Perhaps that change could be made, with people being directed to the metric prefixes page as needed to help them understand the wider range of prefixes. From the age of 12 I started to use the full range of prefixes them known to me in my schoolwork, and at virtual astronomy society I tend to butt in with the distance or mass expressed that way instead of earth masses or light years, for example. Avenues2009 (talk) 09:13, 21 July 2024 (UTC)

While I'm in favour of primarily using SI units whenever reasonable, we chiefly use the units that are most common in any given area, so general custom needs to change first before we follow suite. Hence we'll continue to use units like solar mass and astronomical unit in astronomy, nautical miles and knots in marine navigation (which fit the Earth's coordinate system better than SI units), and even feet for aviation. Also I'm pretty sure that years and centuries will remain more popular for expressing longer periods of time than mega- and gigaseconds, charming as the latter might theoretically be. Gawaon (talk) 09:44, 21 July 2024 (UTC)

Could both then be provided (including use of exotic prefixes in case of one) so not only is everybody else satisfied, so am I. Perhaps this could be done by always providing the SI with the exotic prefix in brackets (you can tell I am British here). Even if SI is already used but with a common prefix, perhaps the more exotic prefix could be provided in brackets. That would again satisfy all round. Avenues2009 (talk) 10:03, 21 July 2024 (UTC)

Template:Convert exists and can be used for such purposes. However, in less obvious cases or if you are reverted, it might be best to open a discussion on the talk page of the article in question. Gawaon (talk) 10:28, 21 July 2024 (UTC)

How does one use that to do two different conversions at the same time - eg thousands of kilometres to both miles and megametres, or astronomical units to both millions of kilometres and gigametres? Avenues2009 (talk) 11:24, 21 July 2024 (UTC)

One conversion should be enough, and units like mega- or gigametres that don't see any real world usage shouldn't be used at all. Gawaon (talk) 11:29, 21 July 2024 (UTC)

But that’s the whole point of me having started this discussion. With my high functioning autism, to me they should be used everywhere because that means everything is all straightforward and consistent. That’s why I wanted to be able to do two conversions at the same time, to bring them into use as well as what others prefer. Avenues2009 (talk) 11:57, 21 July 2024 (UTC)

Your high functioning autism doesn't mean the world will magically take the shape you would like it to take. Wikipedia is consensus-driving, and there is no consensus for the use of exotic SI combinations such as gigametres or megaseconds, even though they are theoretically valid. Gawaon (talk) 12:06, 21 July 2024 (UTC)

And it is much easier to say a measurement when using all the prefixes not just the common ones. Avenues2009 (talk) 11:58, 21 July 2024 (UTC)

@Avenues2009 Whats wrong with being autistic and not ranking it with "high functioning"? I'm autistic but I never say what functioning I might be. Anthony2106 (talk) 10:53, 15 October 2024 (UTC)

I echo the other concerns mentioned in other replies. I'll add that from the time the metric system was created about 7 Gs ago until the introduction of the International System of Units (SI) about 2 Gs ago a variety of ad hoc units were added, such as mmHg. Also, some of the original prefixes were found to be inconvenient. So SI recommends that hecto-, deka-, deci-, and centi- not be used, and only coherent units be used. However, the prefixes for multiplication or division by 10 and 100 are still used in entrenched cases, such as centimeter or decibel. Jc3s5h (talk) 13:22, 21 July 2024 (UTC)

Don’t you mean 7 Gs and 2 Gs? Avenues2009 (talk) 13:32, 21 July 2024 (UTC)

You're right, I fixed it.

It is not Wikipedia's job to try to promote metric prefixes in contexts where they are not used in the wider world, even if some individuals do use them in those circumstances. Most scientists in many fields - including, since you raise it, astronomy - don't use them - particularly the more extreme ones - because they are obscure and serve to confuse rather than enlighten.

I'd also note that even the largest metric prefixes aren't large enough for some astronomical data. I note that you fail to mention the mass of the sun, which is around 2 (non-existent-prefix)-grams. And there's plenty of of masses you might want to express that are many orders of magnitude larger than the mass of the sun. Kahastok talk 15:20, 21 July 2024 (UTC)

Perhaps a parallel example might be useful. I recently learnt a major reason why Americans keep rejecting the metric system. A major benefit of metric is the use of prefixes makes it trivial to convert from mm, to m, to km, and so-on. Except Americans are so used to the imperial (customary) system making it so hard to convert between inches, feet, yards and miles that they simple learn to not convert between units. So one of metric's major advantages is simply a non-issue to them.

The parallel here is that people use solar masses, AU, light-year, etc and just never convert them to other units. Only people writing software for things like interplanetary probes would ever do such conversions to kg, m, etc and they don't need Wikipedia for this info. So converting them to metric is not useful and the clutter it causes makes articles harder to read.  Stepho  talk  03:18, 22 July 2024 (UTC)

For me, articles would not be too crowded with such conversions thrown in. To me, they would be quite useful, as well as educational for all those who live between 5 and 12 Mm away from me in the US. Avenues2009 (talk) 06:47, 22 July 2024 (UTC)

The larger figure being included to encompass Hawaii by the way. Avenues2009 (talk) 06:50, 22 July 2024 (UTC)

Do you see what I mean there? I live in the UK, hence the distance I gave. Avenues2009 (talk) 22:30, 22 July 2024 (UTC)

I don't. Gawaon (talk) 05:54, 23 July 2024 (UTC)

Therefore such unit conversions would be quite useful and not make articles too crowded, because then you might. It would help Americans understand how much more useful the metric system would be for them. Avenues2009 (talk) 06:55, 23 July 2024 (UTC)

Nobody argues against the usefulness of commonly used metric units. However, Mm (as opposed to mm) is not one of them. Gawaon (talk) 07:05, 23 July 2024 (UTC)

Er, I'm American, and the ease of converting metric units is refreshing compared to trying to remember how many ounces are in a pound or a gallon, and I think makes metric quite attractive. We have to convert between customary units (we don't use imperial units since that system came after US independence) frequently, and to do that I either ask a smart speaker or look it up online if I can't remember the conversion factor. Generally, I think Americans find the metric system difficult to learn simply because they (outside of STEM fields and certain industries) don't use it every day and have no intuitive sense of what various quantities (like 100 km or 10 degrees C) mean. I think trying to add more prefixes into common use like gigameters or whatnot, would just mean more to memorize about the metric system, and thus make it slightly harder to learn, not easier. -- Beland (talk) 07:05, 23 July 2024 (UTC)

It doesn’t take much to memorise 24 prefixes. Avenues2009 (talk) 07:38, 23 July 2024 (UTC)

That's why I say "slightly harder" and not "a lot harder". I'm an American who does have a feel for what a kilometer is, and if you start speaking in megameters, you'll just make me do extra mental work to convert back to thousands of kilometers. If you start speaking in megaseconds, I and my European friends are probably just going to stop listening, because it's too much work to figure out what that means in years, which is the conventional and intuitive unit for long time. I'm an enthusiastic promoter of the metric system and use it whenever feasible, but anything above "tera" or below "milli" I'd have to look up.

I think a much better way to get Americans to learn the metric system (and one which already has broad consensus) is to make sure that every time a US unit is given on Wikipedia, there's a metric conversion right there. Since Wikipedia is consulted so frequently, I expect this would increase American exposure to the metric system significantly, since it rarely comes up in the news or in everyday life (unless you work in STEM or a few other narrow areas). Eventually I hope people would generate an intuition for metric units by sheer exposure.

This is currently required by MOS:CONVERSIONS, but there are hundreds of thousands of instances violating the guideline. I am actually working on adding conversions in my spare time, using moss to scan database dumps and JWB to quickly add in {{convert}}. I have tens of thousands of articles in a queue to be fixed for feet and inches alone, if you'd be interested in helping out. -- Beland (talk) 08:07, 23 July 2024 (UTC)

I don't think those conversions really do the job you think they do. I (and presumably many other Americans) simply learned to figure out which measurement to skip in any particular work, the one before the parentheses or the one inside them. They don't impinge on my consciousness anymore than a footnote number does. --User:Khajidha (talk) (contributions) 14:38, 9 September 2024 (UTC)

Sure, some people will gain more than others from exposure, and different media have different levels of effectiveness. Children, for example, who are continually exposed to the metric system might find it useful in enough contexts to never learn to gloss over it. I've noticed some American TV broadcasts giving temperatures in both Fahrenheit and Celsius, which is a bit jarring in contexts like that where Celsius is never mentioned. It's more difficult to unhear that than to have your eyes skip over a parenthetical. -- Beland (talk) 17:16, 9 September 2024 (UTC)

If you read the article on metric prefixes, there is a hint that at some stage in the future, double prefixes may return, with the restriction that the last one be quecto or quetta, but that has not happened her. That problem would be solved if they did return. Avenues2009 (talk) 06:23, 22 July 2024 (UTC)

So they can’t be used before that time if it ever happens. Avenues2009 (talk) 06:24, 22 July 2024 (UTC)

How is it more convenient to keep swapping prefixes instead of just using a single unit with scientific notation if needed? I'd much rather compare 0.04 km with 2000 km than compare 40 m with 2 Mm. Double sharp (talk) 09:17, 23 July 2024 (UTC)

Well I find it more convenient, it’s easier to say the measurement for a start if the prefixes are used. And instead of saying 40 m, you’d say 4 dam. Avenues2009 (talk) 10:03, 23 July 2024 (UTC)

Car power is expressed in 3 different units in Wikipedia articles, kW, hp and PS, so having 3 different units for astronomical distance could work. Some people do use these prefixes, look at the computer world, Gigabyte and PB are very common. When people see this it might open their mind to look into this unit and learn something, which after all is the reason for Wikipedia. Avi8tor (talk) 15:36, 23 July 2024 (UTC)

What a good idea. It would be great if that was implemented not only in astronomy but other fields as well. Avenues2009 (talk) 16:41, 23 July 2024 (UTC)

Different countries have different definitions of horsepower, including the German PS. I think the general trend is that kW is replacing horsepower in international commerce (and using kW as primary is now mandatory in the EU). It doesn't seem useful to encourage anyone to learn about horsepower or start using it when they otherwise wouldn't.

If you meant to reply to the question about light-years, this is the wrong section. -- Beland (talk) 17:02, 23 July 2024 (UTC)

Oh, when you say "3 different units for astronomical distance", you mean e.g. AU, km, and Gm or something? Having two meter-based conversions would seem to me to be adding clutter because a.) those conversions are easy for readers to do just by moving the decimal place, and b.) Gm is virtually unused, and thus not useful for conveying information about the subject of the article. The only reason to have it there would be to teach readers about obscure units in the metric system, which is not the point of Wikipedia articles except for Metric system and Metric prefix and friends. -- Beland (talk) 17:09, 23 July 2024 (UTC)

Then let’s have two metre-based conversions then, for that very purpose of teaching readers those obscure units. And let’s do it for distances beyond the solar system as well. Avenues2009 (talk) 17:13, 23 July 2024 (UTC)

Wikipedia is not a textbook. Teaching is not our primary task, and we don't have a right to decide who should be taught what. Gawaon (talk) 17:18, 23 July 2024 (UTC)

Yes, and we convert to alternative units to communicate quantities to people accustomed to this or that set of units, not to train people in the use of an unfamiliar one. NebY (talk) 17:36, 23 July 2024 (UTC)

But the more the unfamiliar ones were taught, the more they might catch on and become familiar. Avenues2009 (talk) 18:24, 23 July 2024 (UTC)

As WP:RIGHTGREATWRONGS says, Wikipedia is intentionally "behind the curve"; I think it would happily adopt such a change if it had already substantially caught on, but does not itself want to lead that type of movement. Sorry that's unsatisfying, given that I'm sure using the obscure units makes you and the original designers of the system happier than watching people use the metric system as they actually do. 8) -- Beland (talk) 19:01, 23 July 2024 (UTC)

I was thinking more AU or light years, miles and Gm or Pm for the 3 units. I've found that in general citizens of different countries use what they are fed, miles in the UK and USA and km elsewhere. The same with every other unit. The British empire used stones for weight, now only the UK used stones. It appears most younger people outside the UK have never heard of stones even though their mother tongue is English. Avi8tor (talk) 19:36, 23 July 2024 (UTC)

Currently, it's not required to convert into miles for STEM articles. In many cases we don't, and in some cases we're actively removing them. -- Beland (talk) 19:57, 23 July 2024 (UTC)

Well for astronomy you could instead convert into Mm,Gm, Tm, Pm, Em, Zm or Ym as the case may be, or for the Comoving circumference of the observable universe at least, Rm. That would make a lot more sense than miles. Avenues2009 (talk) 20:25, 23 July 2024 (UTC)

And the same with mass. It would make sense to have both kilograms, and the relevant prefix that would apply to a given value. Avenues2009 (talk) 21:34, 23 July 2024 (UTC)

And for large objects like stars, galaxies etc the second figure would be the number of Qg until such time as double prefixes are allowed again. Avenues2009 (talk) 21:40, 23 July 2024 (UTC)

Of the distance units mentioned, the units people actually use are AU/light years and kilometers. Ym might make more sense to you, but they don't make sense to hardly anyone else, and Wikipedia's goal is to communicate to as broad an audience as possible. -- Beland (talk) 22:58, 23 July 2024 (UTC)

Hence the suggestion of converting to both km and a more appropriate prefix so things make sense to me as well as to others. Avenues2009 (talk) 06:09, 24 July 2024 (UTC)

When I say people use kilometers, I mean people use kilometers, not Ym. How many people do you think actually use Ym or would understand them on sight? -- Beland (talk) 15:41, 24 July 2024 (UTC)

I’ve met one or two actually. Avenues2009 (talk) 19:20, 24 July 2024 (UTC)

And they even understand what it stands for - yottametres. Avenues2009 (talk) 19:21, 24 July 2024 (UTC)

That's not really enough information to get a handle on the overall ratio of English speakers. How many people have you met who would not understand Ym? What is your sampling bias compared to English Wikipedia readership? -- Beland (talk) 22:06, 24 July 2024 (UTC)

The sampling size is the groups I belong to, including astronomy society, and all my friends. But it really does make sense to use the prefix just below a value. It makes measurements much easier to write or to say. Avenues2009 (talk) 22:14, 24 July 2024 (UTC)

My astronomy society do not seem to understand Mm to Rm (megametres to ronnametres) so if these changes were implemented on Wikipedia that I am here asking for, we’d teach them. They’re technical and brainy enough, there are several of my fellow PhD holders among them, as well as a few teachers, so it would be very easy for them to learn and adopt any missing prefixes out of the 24 available. Avenues2009 (talk) 22:17, 24 July 2024 (UTC)

If even a specialty audience needs to be taught how to read the units, the vast majority of readers will certainly not know them on sight. We've already established it's a non-starter if readers don't already understand the units Wikipedia would be using across tens of thousands of articles. -- Beland (talk) 23:15, 24 July 2024 (UTC)

The goal is to communicate clearly with as wide a readership as possible. Do you believe that there are people who understand what a Ym is but don't understand what a km is? What then is the advantage of adding measurements in Ym (and the other obscure units you've proposed), other than satisfying your own personal sense of consistency? CodeTalker (talk) 22:12, 24 July 2024 (UTC)

It would satisfy my desire for consistency. Having the obscure unit included besides km (or kg) in a double conversion, would do that as well as be educational and informative that there are many other prefixes people can use as well. Avenues2009 (talk) 22:20, 24 July 2024 (UTC)

Although people can read the article on prefixes if they want to, they would be more likely to learn about them if they saw them in use, which such double conversions would enable. Avenues2009 (talk) 22:26, 24 July 2024 (UTC)

You're just repeating the same arguments you've already made and which have already been objected to. I could make the same objections again, but we're just going in circles, so it seems there's no point continuing this thread. -- Beland (talk) 23:18, 24 July 2024 (UTC)

The Manual of Style does ask for things to be in SI where possible and for unfamiliar units to be explained. So that is why I would like to see the full range of prefixes brought in using double conversions. Avenues2009 (talk) 07:55, 26 July 2024 (UTC)

I thought double conversions would help explain them. Anyway, that is my view. Avenues2009 (talk) 09:43, 26 July 2024 (UTC)

The MOS says that because unfamiliar units that must be used to explain the subject need to be explained. It's not asking to use unfamiliar units on purpose. Double conversions would add clutter which would make it slightly more difficult for readers to learn about the subject of articles. -- Beland (talk) 14:41, 26 July 2024 (UTC)

Ok. Avenues2009 (talk) 18:40, 26 July 2024 (UTC)

Ok, I run with that, although I still feel that if a metric prefix has been devised, one might as well use it because that is what it was devised for. Avenues2009 (talk) 08:49, 28 July 2024 (UTC)

No, some were devised merely for completeness, at a time when there was no foreseeable use for them. For example, it appears that while ronna (10^27) and quetta (10˄30) were added in anticipation of the need to express ever-larger quantities of data, ronto (10˄-27) and quecto (10˄-30) were added simply for symmetry with the other two [13] -- no one envisions anything (yet) they might actually be used for. EEng 13:55, 1 August 2024 (UTC)

Using circa template only at first occurence

Is there a reason why the c. template should only be used at the first occurence in an article? To me, this rule seems weird, and it also just looks quite inconsistent. I can remember reading the guideline a long time ago, when it wasn't like that (I checked the version history and saw this has indeed not always been the case). I'm asking out of curiosity, because I can't think of any reason for it. Thanks in advance. Maxeto0910 (talk) 21:51, 14 July 2024 (UTC)

Because it brings up a tooltip to explain what it means. It's annoying to see that at every occurrence. (Honestly I think it's a little annoying to have it at all, but the one occurrence I can live with.) --Trovatore (talk) 21:54, 14 July 2024 (UTC)

The dotted underline? I understand that by comparison as I said below, but this may truly boil down to a matter of taste. Different strokes and all. Remsense诉 21:57, 14 July 2024 (UTC)

To be honest I don't really like the tooltip interface for Wikipedia at all. The little floating question mark does not seem to be that much used these days (it reminds me of — Encarta, I think? Something of that vintage anyway). I don't think Wikipedia should be proliferating UI elements, particularly ones that show up only occasionally.

And I don't really think "c." needs explanation. Give readers some credit.

That said, I can live with the one occurrence. --Trovatore (talk) 22:11, 14 July 2024 (UTC)

It's not a Wikipedia thing, it's an HTML thing. And I imagine it's distinctly less pleasant reading for those using screen readers to hear "see dot" each time. (One can disable the tooltip, but I still don't think it should be the guideline across the encyclopedia to do so.) Remsense诉 22:13, 14 July 2024 (UTC)

Ah, do they hear "circa" when the tooltip is used? Is that true for other tooltips as well? I wasn't aware of that. --Trovatore (talk) 22:17, 14 July 2024 (UTC)

Quite! See also the MDN doc for the <abbr>...</abbr> tag. Remsense诉 22:20, 14 July 2024 (UTC)

Also, not to say it's a replacement for this discussion, but since you dislike it, you could add

abbr { text-decoration: none; } to your common.css to hide them all forever. Remsense诉 22:53, 14 July 2024 (UTC)

It seems a reasonable rule, consistent with the MOS on overlinking and saving both readers and editors from a repeatedly cluttered experience. NebY (talk) 22:18, 14 July 2024 (UTC)

I also think this shouldn't be the guideline. I imagine the reason is that it's visually obtrusive à la one of the reasons against overlinking—but I simply don't think they're comparable, especially given accessibility reasons. Remsense诉 21:56, 14 July 2024 (UTC)

I agree, and I fact I wasn't aware that we are supposed to use it just once per article (or per section, maybe?). In my experience, it's most often used in captions, where it's quite reasonable to treat each caption as fairly independent of the rest of the article. I'd suggest writing something like "the use of the {{circa}} template is preferred over just c., at least for the first occurrence in a section or caption. At later occurrences, writing c. (followed by a non-breaking space) or using the {{circa}} template is preferred over ..." Gawaon (talk) 07:03, 15 July 2024 (UTC)

Or actually, because of the accessibility issue discussed above, the best course of action (and also a very simply one) is surely to recommend always using the template. Gawaon (talk) 07:08, 15 July 2024 (UTC)

I agree, if it makes the text more accessible for people using screen readers, that seems a good thing. Also, while many or most readers may not need an explanation of what c. means, the benefit of helping those who do need an explanation seems to outweigh any harm from using the template, especially as the text decoration can be hidden if particularly disliked. Mgp28 (talk) 09:37, 15 July 2024 (UTC)

I agree it's exceptionally annoying and should be deprecated in all circumstances. -- Necrothesp (talk) 13:48, 16 July 2024 (UTC)

Just a reminder that mobile users are unable to interact with tooltips at all. For me, {{circa}} yields c. No amount of peering directly at the character seems to activate the onHover action. Folly Mox (talk) 01:36, 4 August 2024 (UTC)

Does it do anything if you tap it? --Redrose64 🌹 (talk) 07:04, 4 August 2024 (UTC)

I agree, we don't need the tooltip at all. It's no more needed that a tooltip for vs., a.m., p.m., etc., or any other abbreviation of a term that originally came from Latin but is now just a common English word. oknazevad (talk) 06:44, 6 September 2024 (UTC)

RFC for units of the longest distances

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). 01:40, 28 July 2024 (UTC)

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).

01:40, 28 July 2024 (UTC)

Your preferred units

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

• Light-years only. If there's a second unit, strong preference for kilometers. Astronomy infoboxes are already number-heavy; adding conversions makes them even more overwheming and unapproachable to the general public, which is a chronic problem in popularizing science. Light-years give nice, easy-to-understand small numbers for which we don't need exponential notation, avoiding overloading brains and having people mentally label everything from planets to galaxies as "incomprehensibly far" and more or less equally distant. It's easy to remember Alpha Centauri is about 4 ly away and the Milky Way is ~100,000 ly across and calibrate intuition from there. The speed of interstellar spacecraft (of which there are none) are sensibly measured in fractions of c, not km/s, and not parsecs per year (!?). If there is going to be a secondary unit, I strongly prefer kilometers, for consistency with all other articles and smaller measurements in infoboxes. Parsecs are too close to light-years and are only useful to specialists, who can easily do the conversion if they need to. I'm less opposed to conversion to km in prose because it feels less overwhelming, especially if it's only done sparingly, and it gives a sense of just how big a light-year is compared to everyday life. That said, it's not wrong to expect people to develop an intuition of how big a light-year is on their own or by reading light-year if they are new to the concept. -- Beland (talk) 01:44, 28 July 2024 (UTC)
• Parsecs or light years as primary, kilometres as secondary (converted) unit. I have no strong opinion regarding the use of pc or ly as primary unit, but figure that pc are to be more useful since they are preferred by specialists. I strongly favour the use of SI units with scientific notation as secondary unit, since it'll give normal (lay) readers a better sense of the dimensions involved – most people know what a kilometre is, and even those more accustomed to miles will know that they are of roughly the same scale. Using other SI prefixes such as zettametres would in theory comply even better with the SI, but few people know prefixes of such dimensions, so in practice it would be much less helpful than km with scientific notation. Gawaon (talk) 05:44, 28 July 2024 (UTC)
• Light-years first, parsecs second. We need to use light-years because that's what our readers understand, and parsecs because that's what our sources use. km is useless, both for intuitive understanding (a gigantic exponent gives no intuition other than gigantic distance), and for calculations (as relevant speeds are given in fractions of c). Em, Zm, Ym are even worse, these SI prefixes are way too obscure. Tercer (talk) 08:17, 28 July 2024 (UTC)
• Light-years only. Agree (mostly) with Beland. Lightyears give an understandable number for the general public such that distances to different stars can be compared (eg 4 light-years to Alpha Centauri vs 100,000 light-years for the width of the galaxy). km's at that scale are all are read as an equal "damn that's big!" and are totally unrelatable - the average reader simply does not think in terms of numbers that big. km clutters up the article with no payback. Parsecs are not known to the general public (professionals know how to convert and probably get their information from better sources than WP anyway).  Stepho  talk  08:47, 28 July 2024 (UTC)
• Light-years first, whatever unit is the primary unit used in the source second. We should be making it easy for our readers to understand, and not trying to shoehorn the content into some "official" style that will be more or less meaningless to many readers. I do support converting the primary unit used in the source to light-years, flipping the output as needed. - Donald Albury 17:45, 28 July 2024 (UTC)
• Light-years, because that's the unit used by Patrick Moore, one of the most prolific authors of popular astronomy books. If the source uses another unit, state that as well, using {{convert}} with |order=flip. For example, if the source says 123 parsecs, we would enter {{convert|123|pc|order=flip}} which emits 400 light-years (123 pc). --Redrose64 🌹 (talk) 18:08, 28 July 2024 (UTC)
• Parsecs, or if necessary pc+ly (pc first) using {{convert}}, for consistency with the professional literature. We need Wikipedia to be usable as a professional resource, not merely a dumbed-down only-for-the-public childrens' encyclopedia. —David Eppstein (talk) 18:34, 28 July 2024 (UTC)

  The guideline Wikipedia:Make technical articles understandable makes a useful distinction between what I would consider "dumbing down" - WP:OVERSIMPLIFY which says not to oversimplify or tell lies-to-children - and "writing one level down", which WP:ONEDOWN says is a good rule of thumb for technical subjects.

  In finding a middle ground between the overly technical and the oversimplified, §Avoid overly technical language specifically advises: "If no precision is lost, use common terms instead of technical terms. Substitute technical terms with common terms where they are completely equivalent." Whether that advice should apply in this situation is a matter of opinion, but I think it's important to remember that "making accessible" and "dumbing down" are not always the same thing. -- Beland (talk) 15:54, 29 July 2024 (UTC)

  Making accessible should also mean, making accessible for professionals to read and to edit. "The widest possible general audience" should include professionals, not just non-professionals. Gratuitously avoiding the preferred unit of professionals makes our articles less usable to and less editable by them, for no good reason. It sends the message that their participation is not welcome here, the opposite of what we want. —David Eppstein (talk) 01:49, 30 July 2024 (UTC)

  The discussion in the previous thread has several examples of peer-reviewed astronomy papers that use light-years, even though it's not the majority, and the unit is well-known in the popular press, which professional astronomers are also exposed to. Light-years should be perfectly accessible to professional astronomers and those familiar with the peer-reviewed literature, and no doubt already occasionally use the conversion factor to compare figures from different sources.

  I was also pondering whether or not it would be good to also have parsecs to attract more professionals to use Wikipedia articles for quick reference, and possibly fix things while they are here. This past week I went through and fixed the densities and surface gravities of a lot of exoplanets; a few seemed completely wrong because someone had confused g/cm³ with kg/m³ or missed undoing a logarithm or just pulled data from a contradictory source without leaving a citation. Those sorts of things I would expect a professional to occasionally spot.

  I think it would be good to get actual data about whether or not this makes a certain community feel unwelcome, rather than go on the guesses of non-astronomers about other people's emotional reactions. Does not using US units in science articles make Americans feel unwelcome? This American certainly does not, so I'm a bit skeptical of this idea. I might feel differently if someone did it intentionally to spite Americans rather than for good reasons, like reducing clutter for a global audience and the fact that STEM fields tend to use metric and Americans are an inconvenient minority on the planet in this aspect. I took some astronomy and planetary science classes while I was an undergrad at MIT, but I'm definitely not a career astronomer. Is anyone else here a professional astronomer or know someone who is and is uninvolved in this discussion so far? -- Beland (talk) 03:29, 30 July 2024 (UTC)

  (See Parejkoj's reply, below, for one answer.) -- Beland (talk) 18:34, 1 August 2024 (UTC)

• Light-years as primary unit, as the most familiar and understandable way of expressing interstellar distances. Parsecs as secondary unit, as the unit most often used by professional astronomers. Sources always use one or both of light-years and parsecs. While I prefer the status quo of using both I would also be okay with only light-years; compact tables like the list of nearest stars should use only light-years. Using large numbers of kilometers is not supported by common usage, and it seems clear that this being more understandable than light-years is a minority position. SevenSpheres (talk) 18:38, 28 July 2024 (UTC)
• Light-years as primary unit (and perhaps the only unit) as the mostly widely understood unit. I don't like the idea of using parsecs. The point of providing multiple units is to allow the reader to understand the number if they are unfamiliar with the primary unit. I can't imagine that there's anyone who knows what a parsec is but doesn't know what a light-year is. Parsecs are used only by professional astronomers, who certainly know what a light-year is and can easily convert between light-years and parsecs. Furthermore, it seems unlikely to me that a professional astronomer would be doing research using Wikipedia rather than more professional resources. Well-respected popular science magazines like Astronomy and Scientific American use light-years. Weak preference for km as a secondary unit if a secondary unit is necessary, although the huge numbers involved in the conversion to km will probably be hard to understand for a significant number of our readers. CodeTalker (talk) 19:05, 28 July 2024 (UTC)

  Well, if edited with that attitude, it is unlikely that our astronomy articles will be usable by professional astronomers. But in other areas of science, Wikipedia is usable, useful, and used by professional researchers, not so much as a source for data but as a good starting point for literature reviews and starting material for understanding topics with which they may not already be familiar. —David Eppstein (talk) 20:02, 28 July 2024 (UTC)

  I certainly know chemists who use Wikipedia to look up reference data on certain molecules sometimes, but when doing so occasionally spot errors. We generally don't convert those data to secondary units in chem infoboxes (though I do see some Fahrenheit). It seems the benefits of using a single set of units - which we're lucky enough to also be those used in industrial and academic chemistry - outweigh the convenience for Americans who might be thinking or calculating in US units. It seems a bit much to claim that using light-years and not parsecs would make Wikipedia articles unusable for astronomers; how hard is it to divide by 3.26? -- Beland (talk) 05:24, 29 July 2024 (UTC)

  @CodeTalker: The "huge numbers involved in the conversion to km" are one reason why such a conversion is helpful. An astronomer does not need reminding of that vastness, and uses parsecs for convenience, but the mindboggling vastness of space is lost when we use correspondingly vast units. The mindbogglingly large numbers resulting from a conversion to km (or mi) is one way of conveying the vastness to a lay reader. Dondervogel 2 (talk) 19:12, 1 August 2024 (UTC)

  My opinion is that "conveying vastness" should not be a goal. The goal should be writing in a way that helps the reader to understand, not to awe them with wonder. I think few readers who are not already mathematically inclined will take anything away from numbers like 10²⁰ km or 10²⁵ km, except that they're both "very big", without any understanding of what they really mean or the difference between them. CodeTalker (talk) 19:36, 1 August 2024 (UTC)

  If it is not Wikipedia's goal to convey how far away the stars are, then I too see little point in converting to km (or Zm). There we can agree.

  But in my view a good encylopaedia should strive to convey precisely that. It seems this is where we differ. Dondervogel 2 (talk) 20:10, 1 August 2024 (UTC)

  Of course our goal is to convey how far away the stars are. I don't see how you can read my response to mean that we should not. But it should be done in a way that is understandable to readers, not to deliberately use inappropriate units so that we can impress the reader with big numbers. CodeTalker (talk) 20:16, 1 August 2024 (UTC)

  Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the street to the chemist, but that's just peanuts to space. Listen... – Douglas Adams, "Fit the Second", The Hitchhiker's Guide to the Galaxy.

  The point is that we cannot conceive very large numbers, so there's no point in choosing one unit over another unless you are comparing one distance with another, in which case you can judge that this object is ten times as far away as that object, when measured using the same units. --Redrose64 🌹 (talk) 21:01, 1 August 2024 (UTC)

  Your precise words were "conveying vastness" should not be a goal. I interpreted that as meaning Wikipedia should not try to convey the vastness of space. I see no other reasonable interpretation. If not that, what did you mean instead? Dondervogel 2 (talk) 22:35, 1 August 2024 (UTC)

  Not to put words in anyone's mouth, but I think the idea is that light years give small numbers that can be readily compared to easily comprehend relative distance. Distances in light years convey the mathematical information about how far away the stars are, but do not convey a realistic sense of sheer vastness unless one already has an intuition for how vast a light-year is. As compared to kilometers, which can (for those who understand scientific notation and who stop to think for a moment) convey an intuitive sense of how far away stars are in absolute terms, for example by making it easy to see how long it would take to drive there at 100 km/h or how long it would take Voyager 2 to get there at 15 km/s. -- Beland (talk) 03:43, 2 August 2024 (UTC)

  Yep. The distances need to be presented in light-years (or parsecs), to provide relative distances and in kilometres (or metres) to convey the vastness. Very well put. Dondervogel 2 (talk) 06:37, 2 August 2024 (UTC)

  The main sticking point here seems to be over if it is within our purpose to do so (re: km conversion).

  Personally, I would say it is not. ... for example by making it easy to see how long it would take to drive there at 100 km/h or how long it would take Voyager 2 to get there at 15 km/s. Including conversions solely for this purpose feels uncomfortably close to WP:INDISCRIMINATE to me. Unit conversions are included for reasons of practicality, not for conveying "vastness" or other arbitrary properties.

  There are, of course, other arguments for include km conversions that others have laid out, but I feel that this specific line of reasoning is a weak one. ArkHyena (talk) 08:05, 2 August 2024 (UTC)

  Conveying vastness essentially means providing a comparison with a familiar unit. Thus providing distances in, say, kilometres enables readers to compare the distance to a measure they use frequently, and providing them in a large unit such as light years enables readers to more easily compare the relative distance for different stars. isaacl (talk) 13:35, 2 August 2024 (UTC)

  My stance that such a reason alone is not sufficient still stands. I fail to see how SI conversions would enable a reader to more easily compare the relative distance for different stars, because that is, in part, why we use lyr/pc in the first place. A star 50 lyr away being twice as distant as a star 25 lyr away is perfectly understandable to the reader. On the topic of vastness, we don't provide the masses of planets in kg just to convey to the reader how massive they are with impressively large numbers, we include them because

  • Kg are occasionally, if not frequently used by astronomers when dealing with the masses of celestial objects, including planetary ones
  • There are relevant attributes that rely on their masses being given in kg, such as planetary densities, planetary compositions, or the masses of a planet's internal layers

  If we are to include SI conversions for lyr/pc, we ought to do so for similarly practical reasons. ArkHyena (talk) 19:50, 2 August 2024 (UTC)

• My previous position was parsec or light-year as primary unit, converted to SI, but I now favour parsec as primary unit, converted to SI. I will explain the rationale behind the shift in a follow-up post, but my thoughts remain in a state of flux and the popularity of the light-year makes me consider the need for a 3-way conversion in some situations. I will be back. Dondervogel 2 (talk) 21:14, 28 July 2024 (UTC)

  … and my considered preference is Parsec as primary unit, converted to an SI unit. In prose, convert also to light-years, thousands of light-years, millions of light-years, billions of light-years (avoiding the abbreviations 'ly', 'kly', 'Mly', 'Gly'), as appropriate.

  My preferred SI unit would be Pm, Em, Zm or Ym, linked on first use to petametre, exametre, zettametre, yottametre. I can see the benefit of relating to the kilometre (km), so I would not object to that option, although I find the exponential notation cumbersome.

  As mentioned in my previous post, I no longer favour the use of light-year as a primary unit. I was already uneasy about “megalight-year” and similar but was unsure why. When I dug a little deeper I discovered the IAU style manual, which lists SI units and non-SI units recognised for use in astronomy. The units recognised by the IAU are the metre (symbol m), the astronomical unit (symbol au = 0.149 60 Tm) and the parsec (symbol pc = 30.857 Pm). In other words, the IAU does not recognise the light-year for use in astronomy. And if the IAU does not recognise the unit, we should not use it, right? No, not quite. Many Wikipedia readers will know a light-year is the distance light travels in a year, and this familiarity makes it relevant, hence the proposed additional conversion (in prose) to light-years, but avoiding the abbreviations ly, Mly, etc., which I find unhelpful.

  The conversion to SI provides a scale (whether the metre or kilometre) that all readers are familiar with and conveys the vastness of space in a way that parsec (or light-year) on their own do not achieve (Readers who understand the meaning of ‘light-year’ as the distance travelled by light in a year do not necessarily have a grasp of how fast light travels; only yesterday I was asked “what travels faster, light or sound?”)

  Examples

  • Sirius B is 2.670 pc (82.4 Pm; 8.71 light-years) from the Sun.
  • The Whirlpool Galaxy is 7.220 Mpc (223 Zm; 23.5 million light-years) from the Sun and 23.58 kpc (0.728 Zm; 76,900 light-years) in diameter.

  The use of 0.728 Zm in preference to 728 Em avoids an unnecessary conversion between Zm and Em in the 2nd example. The SI values could be replaced with exponential notation if that is preferred. I find it cumbersome but that is a personal preference. Dondervogel 2 (talk) 13:57, 29 July 2024 (UTC)

  Finally someone who would agree with me in the discussion on prefixes I started. Avenues2009 (talk) 21:37, 30 July 2024 (UTC)

  I can see the benefit of SI prefixes in astronomy and nuclear physics, because the prefixes obviate the need for cumbersome exponential notation. Where we differ is your statement it would be much more consistent and straightforward if all Wikipedia articles used the full range of metric prefixes, which differs from my position. In 99.9 % of our articles it is preferable to use a scale between "millionths of a millimetre" and "millions of kilometres", as appropriate. Dondervogel 2 (talk) 09:11, 31 July 2024 (UTC)

• Light-years as primary unit, parsecs as secondary unit, as lyr are indisputably the primary unit used to convey interstellar distances or larger. I don't see why distances measured by lyr/pc should be converted as such distances are well into the region where scales conveyed by kilometers are, at best, impractical and unintuitive to most. This is evidenced by numerous science communication outlets—including NASA itself—excluding SI (or US customary) conversions for interstellar distances (some examples: [14] [15] [16] [17]). It is clear that the overwhelming majority of science communication, nevermind technical introductory sources such as astronomy textbooks, deems such conversions as largely unnecessary, and I fail to see why we should be any different. ArkHyena (talk) 02:52, 31 July 2024 (UTC)

  None of the web pages you linked use parsecs, either. -- Beland (talk) 03:17, 31 July 2024 (UTC)

  Yes, but this is balanced by the fact that academic sources overwhelmingly use pc over lyr. Popular sources use lyr, academia uses pc; per this, we should use both, with the less technical lyr being our primary unit. ArkHyena (talk) 03:19, 31 July 2024 (UTC)

  Interestingly, our dab page lyr doesn't mention light-years. --Redrose64 🌹 (talk) 19:59, 31 July 2024 (UTC)

  Should be fixed now, thanks for the heads up :) ArkHyena (talk) 20:04, 31 July 2024 (UTC)

  Our article light-year also doesn't mention "lyr", a contraction which I had never come across until your post of 02:52, 31 July 2024 (UTC) - everybody else in this section, when abbreviating, has used "ly". I hope that "lyr" is not something that you made up. --Redrose64 🌹 (talk) 07:13, 1 August 2024 (UTC)

  I suppose it's a mistake. Gawaon (talk) 07:33, 1 August 2024 (UTC)

  Both "ly" and "lyr" are legitimate abbreviations of "light-year". I have seen "lyr" in reliable sources (e.g., Mutel et al 1981; Chen & Chen 2016). It should be mentioned in Light-year as a legitimate alternative to "ly". Dondervogel 2 (talk) 08:05, 1 August 2024 (UTC)

  I have added "lyr", "klyr" and "Glyr" to Light-year, citing RS. Dondervogel 2 (talk) 08:17, 1 August 2024 (UTC)

• Light years as primary, small preference for conversion to parsecs in extended prose. Professional astronomer here (there's about a half dozen that I know of that semi-regularly edit Wikipedia): parsec is really useful because the relationship between arcsec, au, and pc (and the conversion factor of 206,265) allows really easy conversion between measured angular sizes, proper motions, distances, etc. in one's head. But that's not something that most people will ever have to care about, so there's not a strong reason to use pc on Wikipedia, except that that is the value that most primary sources report. I like the suggestion above to use `order=flip` in the converter, since that helps make it clear where the value came from: we have a terrible problem of un- and poorly-sourced numbers in astronomy articles.
  Metric prefixes for meter are right out: most astronomical scales are well beyond what any typical reader would be familiar with; I work computing, and still have to remind myself what the exponent for peta is. Prefixes above giga would be completely unfamiliar to most readers.
  To the question of whether astronomers are, or should be, using Wikipedia as a data source: I mostly hope that they do not. We have our own curated sources for data values (e.g. NED, SIMBAD, or the SDSS value added catalogs), and it's almost always not as simple as just grabbing the "top" value from a catalog. This often happens on Wikipedia, and results in long arguments by non-experts about e.g. which star is biggest. I've tried at various points to get colleagues to edit wiki pages when they notice something incorrect without much success; you don't get tenure or grant funding editing wiki! We're not going to turn Wikipedia into a preferred source of numerical values without essentially re-creating the work that went into something like SIMBAD, and that took *significant* funding and buy-in from institutions. Trying to re-create that work without experts onboard is not worth our time. - Parejkoj (talk) 03:58, 31 July 2024 (UTC)
• Light years as primary. I would include a conversion to parsecs (using order=flip if it helps). Light-years are widely recognised by the lay reader, which will be most of our audiences. Parsecs are more likely to be found in sources (so including them helps with verification) and will be needed if we aspire to have an audience of professionals. I would not strongly object to a third conversion to kilometres where space allows, but I would strongly object to the use of obscure metric prefixes that would not be readily understood even by professional astronomers or other professional scientists. Shoot, even names of large numbers are more understandable to more people (even if we restrict ourselves to professionals) than most of the larger metric prefixes. At least you can work out how big a quintillion is from its name. You can't do that with exa-, zetta- or yotta-. Kahastok talk 21:40, 2 August 2024 (UTC)

  I feel it's worth adding something on the kilometres point. My view is that conversions from light years to kilometres are unnecessary in general. Such conversions are almost never found in other sources on the topic. And that's because systematically converting interstellar distance to kilometres is as useful as systematically converting the lengths of running races into light-years. In general, I believe that light-years are sufficiently well-known that, per my point below, most readers will understand them. However, I do not strongly object. I do see some small amount of value in helping readers quantify some of these very large distances in niche circumstances, and I don't see a huge harm in using them occasionally - in infoboxes but not in prose, for example. Of course, no such value arises from units using obscure SI prefixes to create things like exametres and zettametres, as these are less likely to be understood than the units they're supposed to be trying to explain. Kahastok talk 17:14, 3 August 2024 (UTC)

• Lightyears first, with parsecs (second choice: parsecs + lightyears). Lightyears first, for our general readers. Parsecs second, for those more familar with astronomy at that scale, readers and editors both, and for the sake of our reputation. Not kilometres, for several reasons; my big two are that few have the skill of comparing values in exponent notation at a glance, without having to separately scale left and right portions, and that any comparison to terrestrial dimensions or speeds tells us only that each sidereal distance is vastly greater and would take inordinate time to cross at any ordinary travel speed, but nothing about this or that particular sidereal distance – or to put it another way, kilometre sidereal distances are good for the sort of fantasy math that we might try once or twice in a lifetime but not for regular use across the 'pedia. NebY (talk) 17:01, 21 August 2024 (UTC)

Summing up

This RFC has had its 30-day run, so it seems time to wrap it up. Trying to boil the comments above down to editor preferences, my ordered tally is:

• Ly, none or km
• Pc or ly, km
• Ly, pc
• Ly, none
• Ly, source
• Ly, source
• Pc, none or ly
• Ly, pc
• Ly, none or km
• Pc, Pm or km
• Ly, pc
• Ly, pc in prose
• Ly, pc
• Ly, pc

If I'm counting correctly (feel free to double-check me), out of 14 participants, that's:

• Primary: 11 ly, 1 pc or ly, 2 pc
• Secondary: 5 pc, 2 none or km, 1 km, 1 none (but pc in prose), 2 source, 1 none or ly, 1 Pm or km
• Tertiary: one non-objection to km, some objections in previous discussion

It seems there's a strong preference (79%) for light-year as primary unit, 86% if you count the vote for parsec or light-year.

For the secondary unit, taking the greatest number of supporters for each option (given the "or" votes), there are at most: 5 pc, 4 km, 4 none, 2 source, and 1 Pm. If we add in 3 votes from people who wanted parsec to be the primary unit, that's a solid 8 supporters for parsec, which is 57%, plus the preference for parsec in prose.

So, any objection to closing this RFC with light-year winning for primary unit and parsec for secondary unit? My proposed sub-bullet-point to add to the "Generally, conversions...except:" bullet point in MOS:CONVERSIONS would be the following. -- Beland (talk) 05:23, 6 September 2024 (UTC)

---

• For distances between star systems and galaxies, use "light-years" or "ly" as primary units, with a conversion to "parsecs" or "pc". Link the units on first mention and do not convert to SI units. For larger quantities, use large number words in prose (million or billion but not thousand) and metric prefixes (kly, Mly, Gly, kpc, Mpc, Gpc) in compact contexts. Examples:
  • First mention in prose: 34.6 ± 2.3 million light-years (10.6 ± 0.705 Mpc)
    • Wikitext: {{convert|34.6|±|2.3|e6ly|Mpc|sigfig=3|lk=on}}
  • Infobox or table: 2.50 Mly (765 kpc)
    • Wikitext: {{cvt|765|kpc|Mly|order=flip|sigfig=3|lk=on}} (use "order=flip" when source uses parsecs)

---

I think it's a fair summary of the outcome. But since we generally don't give commands, I'd suggest rewording the first two sentences to something like: "For distances between star systems and galaxies, "light-years" or "ly" should be used as primary units, with a conversion to "parsecs" or "pc", but no conversion to SI units. The units should be linked on first mention." Gawaon (talk) 08:24, 6 September 2024 (UTC)

Added to the MOS with your tweaks. -- Beland (talk) 17:26, 6 September 2024 (UTC)

Conversion of non-SI metric units

One part of MOS:CONVERSIONS says "conversions to and from metric units and US or imperial units should be provided". Another part says for units "not part of the SI or US customary systems (e.g. zolotnik), supply a parenthetical conversion into at least SI units". Should the latter part say "metric" or "modern metric" instead of "SI"? I may have been the one to put that in and merely chose the wrong words. I ask now because I came across jansky, which is a non-SI metric unit. It seems a bit silly to convert janskys to 10⁻²⁶ W⋅m⁻²⋅Hz⁻¹, which is apparently more strictly SI.

By "metric units" I assume it's clear we mean anything that's a named combination of SI units or can be related to them with factors of 10, which also includes the liter, hectare, and metric ton. Those three I assume we clearly don't want to convert, but to the degree that they are no longer used, it does seem like we'd want to avoid CGS and MTS units where there are drop-in SI equivalents, like erg or dyne or sthène? -- Beland (talk) 01:18, 30 July 2024 (UTC)

It's a bit tricky since "metric" would be very broad, while "modern metric" and SI seem to be largely treated as synonymous. (Metric system say "The International System of Units is the modern metric system"; similarly, International System of Units calls it "the modern form of the metric system".) Units officially accepted for use with the SI should be fine in general (though I'd still like to see the astronomical unit converted to km, but maybe others will disagree). For others, such as the jansky, it could be decided on a case-by-case basis – that one seems reasonable enough. I think rather than changing the text of that section, adding a note on other acceptable units might be a better solution – to be extended, if the need arises, after a short discussion on this page, or through WP:EDITCON. Gawaon (talk) 06:37, 30 July 2024 (UTC)

I don't think that a "metric unit" is necessarily related to an SI unit by an integer power of 10. The article List of metric units includes multiple examples that do not satisfy this criterion. Examples include the CGS-ESU electromagnetic units statcoulomb and statmho. Are these not metric units? Dondervogel 2 (talk) 07:36, 30 July 2024 (UTC)

CGS is obsolete. I assume it is the reason Beland mentioned "modern metric". Tercer (talk) 08:00, 30 July 2024 (UTC)

Yes that's why I was leaning toward "modern metric", though it's interesting to find out about non-base-10 metric units, so, well spotted!

It does seem like a specific list of exceptions to "convert to SI" is needed for clarity, especially since upon further research there are some units that meet my description that are so obscure I'd probably want to convert anyway. Maybe "metric" in the first sentence should actually be replaced by "SI"?

AU and eV have come up at Wikipedia:WikiProject Astronomy/Manual of Style. I was expecting to have a separate discussion about AU after we finalize a decision for light-years. For electronvolt and dalton, it seems like they are actually too small and fiddly, and are actually well-suited to the domains in which they are used, without conversions that would require scientific notation. If no one objects, I think it would make sense to say units officially accepted for use with the SI (except possibly for AU) don't need to be converted.

List of metric units was very helpful to look through for possible exceptions to the "otherwise convert to SI" rule. I tried to find the ones that would be awkward to convert into SI, either because the combo unit is very complicated, or the non-SI name is very common.

• Solar flux unit is complicated but doesn't look like it's used outside of articles about units.
• Rad (radiation unit) looks like it could be replaced in our articles with Gray (unit)
• Rutherford (unit) could perhaps be replaced with MBq
• M for molar concentration seems like a good candidate for leaving unconverted, due to common use.
• Rayleigh (unit) is complicated, but only used in a handful of articles; should probably be discussed.
• Currently, English Wikipedia articles on some planets have atmospheric pressure given in bar, atm, and Pa. Since 1 bar is exactly 100 kPa, that seems like overkill, and a good topic for the astronomy MOS. Measuring in standard atmospheres is nice because it's an intuitive comparison to Earth, though I guess given that the other scales are calibrated with 1 or 100 at Earth standard, maybe only one of these scales is actually necessary? I don't have a good sense of how well people in metric-using countries know bar and Pa, and if one would be better to use over the other or if they are both fine and could be used based on the field or article history or whatnot. Here in the US, the weather is in inHg, my bike tires are in psi, and I have no intuitive sense of how they relate to each other and vague memories of bar and torr from high school chemistry class. (Hlep!) -- Beland (talk) 08:42, 30 July 2024 (UTC)

The electronvolt and dalton are among the units officially accepted for use with the SI, so I think they should be fine to use without conversion. Molar as mol/L seems fine to me, since the mol is an official and the litre an accepted unit. As for rayleigh, I don't know – there doesn't seem to be any other common unit of photon flux, so it probably needs to be kept? Pressure should preferably be given in or converted to Pa, since that's the standard. Gawaon (talk) 09:18, 30 July 2024 (UTC)

Having lived in several countries that use SI, the kilopascal is the common and only unit for pressure in tires in Southern Africa, Australia and New Zealand. The hectopascal is used worldwide for atmospheric pressure (Canada uses kPa), the USA uses inches of mercury for surface pressure and millibars for upper air pressure. The bar and millibar are deprecated and not SI. What they are trying to do in the MOS is have SI primary for every country except the USA, Britain is a mix. Avi8tor (talk) 10:48, 30 July 2024 (UTC)

Here in Spain bar (and milibar) is used for pressure. In informal speech it is used interchangeably with atm. Pascal only exists in textbooks. Tercer (talk) 13:13, 30 July 2024 (UTC)

I was surprised to see erg (as sole unit) and erg/s (with a conversion to W) in Proxima Centauri, one of the examples in the discussion above. Is that common among astronomers?

Some use hectopascals (hPa) rather than mbar for terrestrial weather, as the values are identical, but in science ant technology generally it's Pa, kPa and Mpa with no glimpse of hPa. For inHg/psi, you probably see barometers go up to about 30 inHg and know standard atmospheric pressure's about 14.7 psi, so 2:1 is good enough for an intuitive relationship? NebY (talk) 17:54, 30 July 2024 (UTC)

Weather for pilots is in hPa worldwide because older altimeters are calibrated to millibars and nothing needs changing. Most airplanes now have a switch to change the display from inHg to hPa depending on where you are on the planet. Avi8tor (talk) 08:59, 31 July 2024 (UTC)

My point is not to develop an intuition about US pressure units, but to point out that for the vast majority of Americans that doesn't exist. -- Beland (talk) 19:20, 31 July 2024 (UTC)

Part of the problem here is that previous discussions have broadly used "SI" as a shorthand for the conventional systems of units used in most of the world outside the US and UK. It was never intended that we strictly use the SI interpretation of the metric system purely for its own sake, regardless of what the rest of the world uses. The people writing this probably weren't thinking of units used by scientists at all. They were thinking of things like Scandinavian miles, pennyweights and chains.

My rule of thumb for scientific articles would be to ask whether the reader (judged e.g. according to WP:ONEDOWN) is likely to be familiar with the unit. If so, no conversion is needed, just link it. If not, then you should convert to a unit that they will be familiar with. Kahastok talk 15:14, 3 August 2024 (UTC)

Sure, which is why a conversion into kilometres (with scientific notation) seems like a good idea. Gawaon (talk) 15:40, 3 August 2024 (UTC)

Depends what you're converting. If you're starting from something in Scandinavian miles, sure. If the starting value is in angstroms, I'd suggest metres or nanometres instead. Kahastok talk 16:56, 3 August 2024 (UTC)

Hehe, sure! Gawaon (talk) 17:37, 3 August 2024 (UTC)

Not all non-SI metric units can easily be converted to SI. For example, in the CGS system, the units of time and of temperature are the second and kelvin respectively, and no conversion is necessary. For some other units, there is a simple conversion by multiplication: the CGS units of mass and of length are the gram and centimetre respectively, and conversion of these to kilograms and metres involves multiplication factors of 0.001 or 0.01 respectively. But anything involving electricity or magnetism is fraught with danger: there are at least two CGS units of electric current, the abampere and statampere, one of which has the speed of light as a term; and one abampere (CGS) corresponds to, but is not equal to, 10 amperes (SI). --Redrose64 🌹 (talk) 15:50, 3 August 2024 (UTC)

All the more reason to always convert them to SI, lest the reader not know what we are talking about. This is a rather abstract concern, though, I have never met anyone that actually wanted to use the electrostatic/electromagnetic CGS units. Tercer (talk) 16:03, 3 August 2024 (UTC)

Draft treatment

Thanks, everyone, for the help in collecting observations about which units are in use around the world. The situation is complicated and I'm not sure what the best solution is for readers that would also satisfy the most editors here. To keep making progress, I've drafted some language, taking Gawaon's suggestion of explicitly listing the units we're talking about. In the process of doing this, I started to realize the great degree to which "core" metric units the general public uses are different than the "core" metric units defined by SI and whatnot, and that affects what level of explanation or linking is needed. That results in somewhat longer lists. Please take my draft only as a starting point for discussion; I don't actually have strong feelings about how these units should be treated and for some of them I just guessed or made an arbitrary choice.

The current text says:

• For units of measure that are obsolete, obscure outside of a particular specialty or geography (e.g. furlong), or not part of the SI or US customary systems... supply a parenthetical conversion into at least SI units

We could change "SI" to "metric" in both places and add list sub-items to clarify:

---

(version 1, edited based on below comments)

• Metric units should be commonly known, and commonly-known units approved for use with SI can be used without conversion in science-related articles or as a conversion target in other articles: meter (m), gram (g), second (s), ampere (A), liter (L), Pascal (Pa), hertz (Hz), degrees Celsius (°C), minute (min), hour (h), day (d), degree (of angle, °), volt (V), watt (W)
• Metric prefixes used should be commonly known, namely "pico" thru "tera", preferably in commonly-used combinations (e.g. 2,000 kilometers not 2 megameters).
• Metric units commonly used in certain fields but less familiar to the general public can be used without conversion in science-related articles or as a conversion target in other articles, as long as the unit name, abbreviation, or symbol is linked on the first instance and the property being measured is clear from context (e.g. that K is for a temperature): kelvin (K), mole (mol), candela (cd), hectare (ha), metric ton (t), joule (J), coulomb (C), radian (rad), steradian (sr), decibel (dB), arcminute (′), arcsecond (″), molar concentration (M), electronvolt (eV), dalton (Da), neper (Np), jansky (jy), gray (Gy), becquerel (Bq), sievert (Sv) tesla (T), farad (F), ohm (Ω), lumen (lm), lux (lx), siemens (unit) (S), henry (unit) (H)
• Obscure metric units should be linked and defined in basic SI units on first use: rayleigh (R), katal (kat), weber (Wb)
• Metric units other than those listed (e.g. in the CGS or MTS systems) should either be replaced with listed units (e.g. joules instead of ergs), or they should be treated as obscure units in the same way as furlongs.
• Some quantities expressed in listed metric units should also be converted into more intuitive field-specific units; see Wikipedia:WikiProject Astronomy/Manual of Style#Units
• (Light-year and parsec and astronomical unit will need to be added depending on the outcome of the RFC.)

---

Later we would either add light-year and astronomical unit or link to Wikipedia:WikiProject Astronomy/Manual of Style, with the specific content depending on the outcome of the RFC. "Pico" thru "tera" are in everyday use according to Metric system, though that may be pushing the boundary of what Americans can cope with in STEM articles where there are no conversions to US units.

-- Beland (talk) 09:10, 14 August 2024 (UTC)

I would recommend using the pipe trick to remove (unit), e.g., [[mole (unit)|]] rendering as mole rather than [[mole (unit)]] rendering as mole (unit). -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 09:56, 14 August 2024 (UTC)

Sure, I wasn't trying to be tidy on the first pass. I just tweaked the draft to pipe as appropriate, not link where linking is not recommended, and show the symbol for each. I kind of grouped the units by topic and frequency and kind of just dropped them in as I came across them. Maybe alphabetical would work better instead? -- Beland (talk) 16:43, 14 August 2024 (UTC)

I needed some extra coffee, so can I check I understand the principles?

• Some common/familiar SI units and units approved for use with SI can be used without being linked.
• Less familar metric units should be linked on first use.
• Obscure metric units should be defined in base SI units on first use.
• Only familiar metric prefixes should be used, preferably in commonly-used combinations.
• Re conversion, effectively insert thus into the current text or rephrase to this effect: For units of measure that are obsolete, obscure outside of a particular specialty or geography (e.g. furlong), or not part of the SI (or units approved for use with SI) or US customary systems... supply a parenthetical conversion into at least SI units.

Are those the key points, in outline, or have I missed some? — Preceding unsigned comment added by NebY (talk • contribs) 7:13, 14 August 2024 (UTC)

Yes, except on the last item, change "SI" to "metric". -- Beland (talk) 16:20, 14 August 2024 (UTC)

Ah, that would be rather different. Do you mean this? For units of measure that are obsolete, obscure outside of a particular specialty or geography (e.g. furlong), or not metric or US customary units ... supply a parenthetical conversion into at least metric units. NebY (talk) 18:26, 14 August 2024 (UTC)

I should add now, because I might seem to be trying to entrap you, that this would not be wise.

Some units in three variants of the metric system

Quantity	SITooltip International System of Units/MKSTooltip Metre–kilogram–second system of units	CGSTooltip Centimetre–gram–second system of units	MTSTooltip Metre–tonne–second system of units
acceleration (a)	m/s2	gal (Gal)	m/s2
force (F)	newton (N)	dyne (dyn)	sthene (sn)
pressure (P or p)	pascal (Pa)	barye (Ba)	pièze (pz)
energy (E, Q, W)	joule (J)	erg (erg)	kilojoule (kJ)
power (P)	watt (W)	erg/s (erg/s)	kilowatt (kW)
viscosity (μ)	Pa⋅s	poise (P)	pz⋅s

The "metric system", unless specified as SI plus units approved for use with SI, comprises several different coherent variants and a number of, ahem, incoherent units too. It would be unacceptable to provide conversions into most of them. This table, copied from Metric system#Development of various metric systems, shows some. There are also calories, statcoulombs, abamperes (biots), the gauss, maxwells, apostilbs (blondels), skots, brils, stères, and variants on the variants – ampere-turn, international volt, millimetre of mercury (mmHg), metre head (mH₂O), metric horsepower, daraf, debye, demal (a measure of conductivity - our redirect is unhelpful), Einstein (unit) and on, and on. We mustn't go there. The SI units and the units approved for use with SI are the only metric targets we need or want, and have the virtue of being comprehensively documented in reliable sources, fully supported by {{Convert}}, and adopted by many countries and many standards organisations even in places that haven't fully metricated or metrified – NIST, ASME, ANSI and more.

If I've misunderstood you and this is all a straw man - phew! But let's be careful with this. NebY (talk) 20:10, 14 August 2024 (UTC)

You are exactly right; that was what I intended to convey, and I think I failed to do that clearly. Presumably for any units not explicitly listed, we want articles to either not use them or convert them into one of the units listed. I will add another bullet point to make that explicit. Tweaks and further discussion welcome. -- Beland (talk) 22:16, 14 August 2024 (UTC)

Oh, and yes, that was the new wording I intended. -- Beland (talk) 22:18, 14 August 2024 (UTC)

That sentence isn't right yet, or your first bullet isn't. The sentence would require units accepted for use with SI to be converted to SI – litres to cubic metres, degrees Celsius to kelvin, minutes, hours and days to seconds, and so on. It would also stop units accepted for use for SI being sufficient as conversion targets - no converting degrees F to degrees C unless also converted to K. I don't think that's your intention and it's not what your first bulletpoint says. NebY (talk) 13:22, 15 August 2024 (UTC)

• I can't tell whether this is meant to apply to scientific articles, or general articles. If the latter, the idea that meters, kilos, and degrees C will be given without conversion is a nonstarter. EEng 17:09, 14 August 2024 (UTC)

  Ah, yes, my wording is definitely muddled. I did not intend to change the circumstances under which units need to be converted, only to say which units we mean by "metric" when we say they can be used without conversion on scientific articles and as conversion targets for articles that use US and imperial units. I will change the phrasing to clarify; further tweaks welcome. -- Beland (talk) 21:54, 14 August 2024 (UTC)

• Generally this reads good to me, impressive work! Though I don't quite understand why it says "metric" – wouldn't "SI" work too? Gawaon (talk) 06:28, 15 August 2024 (UTC)

  @Gawaon: Whilst all SI units are metric, not all metric units are SI. This has been extensively discussed already. --Redrose64 🌹 (talk) 07:30, 15 August 2024 (UTC)

  While that's trivially true, it misses the point. Where in Beland's listing are the non-SI metric units? SI units should usually be preferred, so I just wonder if these non-SI units deserve the treatment suggested for them. Gawaon (talk) 12:57, 15 August 2024 (UTC)

  Liters, hours, Daltons, and decibels, for example are non-SI units accepted for use with the SI, though not all of them are based on units of 10 from the base units. Light years, parsecs, or AU may get added to that. Not all of those are what I think of when I think of the metric system; dalton (unit) doesn't even say that unit is part of the metric system, so maybe "metric" isn't the best terminology, either. -- Beland (talk) 16:29, 15 August 2024 (UTC)

  Dalton (unit)#Adoption by BIPM already correctly describes it as a non-SI unit accepted for use with the SI and the following sections go into more detail. The lead is unhelpful; we don't need another demonstration that Wikipedia is not an RS so I'll fix that. SI's coherence is one of its great virtues compared to earlier metric systems, but metric units have had many relationships with each other. NebY (talk) 16:50, 15 August 2024 (UTC)

  I think it should be fine to talk about "SI units and units approved for use with the SI", especially since we then go on and list all relevant units anyway. So there is no need to mention "metric units". Gawaon (talk) 17:14, 15 August 2024 (UTC)

  If I'm understanding correctly, I find myself agreeing with Gawaon (talk · contribs). All non-SI units should be converted to SI, except those units accepted by BIPM for use with SI. Dondervogel 2 (talk) 13:10, 15 August 2024 (UTC)

  Just so. We shouldn't require degrees C to be converted to K, or insist that mL be converted to m³. We should normally convert °F to °C not K, and fl.oz. to mL is usually enough,but it would be excessive to start specifying pairs. NebY (talk) 13:33, 15 August 2024 (UTC)

  That makes sense. There should also be no requirement to convert logarithmic units like the byte, shannon or decibel to SI. Dondervogel 2 (talk) 18:01, 15 August 2024 (UTC)

  Bytes and shannons are units of information and information entropy. As far as I can tell, they cannot be converted to SI units because they are nonphysical, but are also not accepted for use with the SI. Presumably all English- speaking countries use the same units, so I think we don't need to say anything about those. -- Beland (talk) 20:40, 15 August 2024 (UTC)

  On second thoughts, some variants of the decibel, like the dBm or dBV, really do need conversion, in these two cases into watts and volts, respectively. Dondervogel 2 (talk) 21:46, 15 August 2024 (UTC)

• Beland's fourth bullet Obscure metric units should be linked and defined in basic SI units on first use: rayleigh (R), katal (kat), weber (Wb) is unnecessary and not useful to our readers. Very few who don't understand weber (Wb) will find (kg⋅m²⋅s⁻²⋅A⁻¹) useful; most will find the eruption of gobbledegook into a sentence disruptive. We do want to make the point that Less familar units should be linked on first use, which does sit in the middle of Beland's third bullet about conversion but I think would stand better alone.

That third bullet ends with a long list of units. Do we want such itemisation? It could be summarised instead as "other SI units and units approved for use with SI", "units in SI or approved for use with it", or similar. NebY (talk) 13:56, 15 August 2024 (UTC)

I like that idea. The three units in the last list are used by hardly any articles, so presumably they will explain some amount of more helpful context. If we just say those should be linked like the second list, then the only list we'd need would be the ones that don't need to be linked. -- Beland (talk) 17:29, 15 August 2024 (UTC)

---

(version 2, edited based on below comments)

Change hatnote to:

See also: § Currency conversions, and Wikipedia:WikiProject Astronomy/Manual of Style § Units

Change bullet point and add sub-points:

• For units of measure that are obsolete, obscure outside of a particular specialty or geography (e.g. furlong), or not part of the SI/SI-accepted or US customary systems (e.g. zolotnik), supply a parenthetical conversion into at least SI or SI-accepted units. [...remainder of line unchanged...]
  • Metric units not part of SI or accepted for use with SI should either be replaced with SI or SI-accepted units (e.g. joules instead of ergs), or they should be treated as obscure units in the same way as furlongs.
• Metric prefixes used should be commonly known, namely "pico" thru "tera", preferably in commonly-used combinations (e.g. 2,000 kilometers not 2 megameters).
• (Light-year and parsec and will need to be added depending on the outcome of the RFC.)

Under the bullet point "Units unfamiliar to general readers", add:

• Commonly-known SI and SI-accepted units that can be used without linking are: meter (m), gram (g), second (s), ampere (A), liter (L), Pascal (Pa), hertz (Hz), degrees Celsius (°C), minute (min), hour (h), day (d), degree (of angle, °), volt (V), watt (W)

---

Version 2 above is an attempt to incorporate the above comments. If we accept that as a general rule, it implies there is no need to convert AU to kilometers, though we could add an exception for that later. -- Beland (talk) 18:23, 15 August 2024 (UTC)

I'm not sure I like the part about the AU not being converted all that much, but in general, that draft makes a lot of sense! Gawaon (talk) 18:37, 15 August 2024 (UTC)

This is much tighter, easier to read and easier to use, and sound. I like it. I'm not sure we need "(e.g. in the CGS or MTS systems)" - it could be seen as contradictory, because there is a lot of overlap between SI and those two (metres, (kilo)grams, seconds), but that's about the biggest nit I can pick. AU->km might be an exception, but metrology always has edge cases; better to have a clear core. NebY (talk) 11:54, 16 August 2024 (UTC)

Okey, dropped that parenthetical. -- Beland (talk) 04:55, 17 August 2024 (UTC)

What specifically is meant by the "SI/SI-accepted unit and an intuitive field-specific units" clause? I don't think the link to WikiProject Astronomy is all that helpful here. Maybe it would be possible to list the relevant units right here instead? Gawaon (talk) 06:34, 17 August 2024 (UTC)

Wikipedia:WikiProject Astronomy/Manual of Style#Units has like ten additional points, so I thought it would get less out of sync if it only exists in one place? The relevant units would be radiuses and masses of the Sun, Jupiter, Earth, and Moon; luminosity of the Sun; and standard gravity. Maybe better phrasing would be "Some properties of planets and stars need additional conversions; see Wikipedia:WikiProject Astronomy/Manual of Style#Units"? -- Beland (talk) 20:05, 17 August 2024 (UTC)

Frankly I'm not convinced they need to be mentioned here at all. The important point is that at least a conversion into SI units is present; that they are additionally also expressed in alternative units is not forbidden by our rules. So if that's not mentioned here, but only on the Project Astronomy page, there's no conflict. Gawaon (talk) 20:21, 17 August 2024 (UTC)

By "they" do you mean the specific units to be converted to like "mass of the Sun"? I was assuming this MOS page should at least link to any topic-specific advice related to units, for completeness and ease of navigation. -- Beland (talk) 23:14, 17 August 2024 (UTC)

I'm rather sceptical of linking from this page to the WikiProject since it would seem to make the WikiProject's rules an "official" extension of this guideline, which actually they are not. A "See also" hatnote might be more appropriate and would reduce the risk of giving that impression. Gawaon (talk) 06:36, 18 August 2024 (UTC)

Well, if you ask me, they are just as important to follow, because the consequences will be the same - the project will be inconsistent, and people will potentially be upset and potentially revert your edits. But a "see also" link is fine. I'll edit the draft. -- Beland (talk) 15:17, 18 August 2024 (UTC)

Discussion seems to have died down, so I put the revised-as-above version 2 on the live page, except for the light-years part which is still closing out in the above thread. -- Beland (talk) 05:37, 6 September 2024 (UTC)

Jansky and Rayleigh and friends

The above changes have taken care of a lot of cases (hurray!) but as I was tidying up, I realized some units seem to have fallen through the cracks, namely:

• jansky (jy), rayleigh (R), katal (kat), weber (Wb)

These seem to be the only metric units not approved for use with SI which are nevertheless still in modern use, and awkward to convert into SI units in a way that is easy to understand when presented briefly. I think the new wording of MOS:CONVERSIONS tells us to treat these units "as obscure units in the same way as furlongs" which would require a conversion to SI units. But given the discussion above, it sounds like we don't actually want a conversion, but would be happy with just linking to the defining articles? If so, I assume we can simply add these as exceptions under the light-year/parsec exception? -- Beland (talk) 18:14, 6 September 2024 (UTC)

I'm not sure about katal and weber (their respective articles claim they are both SI units), but jansky and rayleigh are unfamiliar and should be converted to an equivalent SI unit. Dondervogel 2 (talk) 18:41, 6 September 2024 (UTC)

Ah, you're right; katal and weber are both listed on International System of Units as derived units. I totally missed those on the list. The conversions for the others would be:

• 1 jansky (10⁻²⁶ W⋅m⁻²⋅Hz⁻¹)
• 1 rayleigh (1/4π 10¹⁰ photons s^-1 m^-2 sr^-1)

This looks like the "gobbledygook" NebY was objecting to for weber; does that objection also hold for jansky and rayleigh given that unlike weber they are not official SI units? Beland (talk) 19:39, 6 September 2024 (UTC)

The jansky's expression in SI isn't bad, as these things go; it's brief, the units give a fair indication of what sort of quantity's being measured, and the converted values might well be meaningful to those interested. We don't seem to use it often, or if we do we aren't linking it - most of the instances at Special:Whatlinkshere/Jansky seem to be uses of Template:Radio astronomy, and one of the few applications is in the opposite direction a peak X-ray flux density of 2.3×10⁻²³ W/(m²⋅Hz) (2.3×10³ jansky). If someone went wild and inserted SI conversions throughout, at worst it'd cause little offence to editors and little or no difficulty to readers; and it might even be helpful. Let's treat them like furlongs.

The rayleigh's expression looks much more intimidating in either version, using 1/4π sr-1 as above or using extended columns of a centimetre cross-section. We link to it in 9 articles, with actual values in only two. IMAGE (spacecraft) has The sensitivity is 1.9 count/second-Rayleigh. Student Nitric Oxide Explorer has The sensitivity of channel A at 130.4 nm is 23 counts/second/Rayleigh and the sensitivity of channel B at 135.6 nm is 26 counts/second/Rayleigh. Is counts/second/rayleigh equal to 4π^-1 10^-10 m² · steradian counts per photon?? Is it too late to pretend we never heard about it? NebY (talk) 21:30, 6 September 2024 (UTC)

OK, I've added jansky and rayleigh as affirmative examples of obscure metric units that should be converted to SI. The rayleigh equations didn't make much sense to me either in terms of dimensional analysis or visually, so I'll tag those two articles and rayleigh (unit) itself and ask for some help from experts. -- Beland (talk) 00:25, 7 September 2024 (UTC)

The weber (Wb) is one that I learnt at school (but have had little use of it since). It is the SI unit of magnetic flux, and 1.0 Wb = 1.0 V s, or 1.0 J s C^-1, or 1 × 10⁸ maxwell. --Redrose64 🌹 (talk) 20:56, 6 September 2024 (UTC)