Wikipedia:Reference desk/Archives/Science/2013 May 16

= May 16 =

Use of degree symbol with Fahrenheit
I was taught (told) in college that it was not proper to use the degree symbol with temperatures in Fahrenheit. In other words, you'd say -40°C but -40F. Now, I can't seem to find any credible (or non-credible) resource to tell me that that might be true.

I know Kelvin is abbreviated "K" only, but why would my professor teach that Fahrenheit was, too? I'm not dismissing the possibility that he was an idiot, but I also don't want to switch back without knowing that there's no legitimate source claiming "F" is the right way to go. Please give me your scientific/engineering input. Jared  (t)  &ensp; 18:30, 16 May 2013 (UTC)


 * See Degree symbol.—Wavelength (talk) 18:33, 16 May 2013 (UTC)


 * ACS Style Guide (3rd ed, 2006) editorial-style guideline states "degree (use °B, degrees Baumé; °C, °F, but K)". DMacks (talk) 18:34, 16 May 2013 (UTC)


 * All right, I kind of knew that, I'm more looking for an explanation as to why "F" was taught in lieu of "°F." Jared   (t)  &ensp; 18:36, 16 May 2013 (UTC)


 * Your professor was mistaken. Dauto (talk) 19:11, 16 May 2013 (UTC)


 * Maybe so, but he is not alone. I remember being told the same thing, and it even made it into a | Thermodynamics textbook from Cambridge University Press: "degrees Celsius (°C) ... degrees Fahrenheit (F) ... the degree symbol is used only with the Celsius unit to avoid confusion with the coulomb". - Lindert (talk) 21:14, 16 May 2013 (UTC)
 * Well, people can publish any random words they want into a book of any sort, but the fact that it is published doesn't make it authoritative. Most sources that discuss this issue note that the degree symbol should be used with Celsius/Centigrade and Fahrenheit and no degree symbol for Kelvin.  This style guide from Penn State University and The National Geographic Style Guide and The U.S. Metric Association and The (U.S.) National Institute of Standards and Technology, which quotes the General Conference on Weights and Measures, which in 1967 established the current practice of NOT using the degree symbol with kelvins.  The CGPM (the french acronym for this group) is the international body which maintains the standards for measurement worldwide; it is non-binding of course, but still considered the premier authoritative body on these issues, and many national standards-making bodies conform themselves to their standards.  If the CGPM says its so, it's so.  That's probably as authoritative as we need here.  -- Jayron  32  23:57, 16 May 2013 (UTC)


 * Check out how a major newspaper reports its temperatures. The "F" is understood, of course. ←Baseball Bugs What's up, Doc? carrots→ 23:51, 16 May 2013 (UTC)
 * What American institutions do is not a good guide except in so far as they may reflect Amaerican common practice - the USA is notorious for either ignoring international standards, or claiming that international standards are a wicked European plot to reduce American competitiveness. What newspapers do means nothing - the average journo and average sub-editor is pretty ignorant of science and standards.  (The Sunday Times, a major newspaper, consistently printed silicone every time for years whenever talking about computer chips (silicon).  And every newspaper I've ever seen, when mentioning the poles that carry electricity distribution, calls them "telegraph poles", as in A fatal accident occurred in Black Stump last night when a car collided with a telegraph pole).  What matters is what is standardised by SI and IUPAC.  See Quantities, Units and Symbols in Physical Chemistry (any edition, current edition is 3rd), International Union of Pure and Applied Chemistry (IUPAC). The standard is use the degree symbol with F, R, or C, but never K.  That school teacher had it wrong (not that uncommon with school teachers, and most school texts are written by teachers) - or maybe the OP Jared remembered it wrong.  Wickwack 120.145.81.185 (talk) 00:22, 17 May 2013 (UTC)
 * Fahrenheit is superior to Celsius, as it's a finer gradient. And I don't recall my teachers ever saying that the degree symbol shouldn't be used with Fahrenheit. Maybe the OP had a teacher with his own idea of how things should be. It happens. ←Baseball Bugs What's up, Doc? carrots→ 06:25, 17 May 2013 (UTC)
 * "A finer gradient?" I'm not sure what your teachers meant there, but if they meant what I think they meant (that it provides a finer gradation), then why don't they post highway speeds in inches/year? Also, large parts of the world now make use of the amazing new concept of the decimal mark that allows us to specify quantities with arbitrary precision quite independent of the concrete unit used. --Stephan Schulz (talk) 07:27, 17 May 2013 (UTC)
 * That's what I meant by a finer gradient: In everyday usage, there's no need to specify the temperature in decimal points. If today's high is 71, that's close enough - as opposed to somewhere between 21 and 22. ←Baseball Bugs What's up, Doc? carrots→ 08:07, 17 May 2013 (UTC)
 * That's just nonsense, as it's false precision. There is no need whatsoever in for instance a weather forecast to have decimals, even in Celcius, as nobody can feel the difference between 21, 22 or 23. In field where such accuracy matters (science, medicine) they use..., yep you've guessed it, Celcius or Kelvin. 131.251.133.27 (talk) 10:54, 17 May 2013 (UTC)
 * I have nothing against Celsius (when I lived in Canada I learned to think in Celsius, but never in kilometers). But I don't agree with the second sentence at all.  23 is too warm to sleep comfortably, whereas 21 is fine. --Trovatore (talk) 16:30, 17 May 2013 (UTC)
 * Phileas Fogg would be proud of you, Trovatore. --   Jack of Oz   [Talk]  19:38, 17 May 2013 (UTC)

Just to be picky, the ISO 31-0 specifies that you should leave a space between the number and the unit. Aaadddaaammm (talk) 06:13, 17 May 2013 (UTC)
 * Unsourced attacks on various ethnicities are not helpful. μηδείς (talk) 01:37, 17 May 2013 (UTC)


 * To throw in my two penn'orth, I was taught that temperature is indicated with the ° but temperature difference isn't - that has to specify {em|degree}} or  Unbuttered parsnip  (talk) mytime= Thu 21:11, wikitime=  13:11, 6 November 2014 (UTC)

Dimensional analysis of a punch
I was reading about the awesome Shi_Yan_Ming, but I'm confused by the units given in the article. It says "Shi's punching power has been measured at 772 lbf (3,430 N) of force" -- so far so good, (though I might have expected units of impulse). The article continues "...while his one-inch punch was measured to have 1.78 vC" What is a vC? I can't see how one-inch punch could be measured in Coulombs, and v isn't an SI prefix anyway. The sentence cites a TV show as reference. Does this make sense to anyone, or is it just some weird typo? SemanticMantis (talk) 21:09, 16 May 2013 (UTC)
 * One-inch punch makes a similar claim, but without units, and comparing to car crashes... SemanticMantis (talk) 21:12, 16 May 2013 (UTC)
 * No idea what a vC is, but that portion of the article was added on 31 August 2010 by an anon editor (69.160.245.131) in this edit: http://en.wikipedia.org/w/index.php?title=Shi_Yan_Ming&diff=382143742&oldid=370427550 . They quoted as a reference a History Channel "Stan Lee's Superhumans" show aired on 12 August 2010. I can't find any other reference to a "vC" in this context anywhere, apart from this, which from the date comes from WP. Looks to me like utter nonsense to be honest, but going back far enough that nobody can be asked about it. Treat it like those hair product adverts about pro-trimethoxygoomtribble liposides, ie a complete bunch of old tosh. Tonywalton Talk 23:38, 16 May 2013 (UTC)

A punch delivers kinetic energy, not force - it ought to be measured in Joules (or watt-seconds). Wickwack 120.145.81.185 (talk) 00:28, 17 May 2013 (UTC)
 * Well, not strictly true. It does deliver a force and kinetic energy, insofar as kinetic energy is a force which is applied over a distance.  -- Jayron  32  02:54, 17 May 2013 (UTC)
 * And it also has impulse, as force applied over time. So what it ought to be measured in will depend on what we're trying to assess. SemanticMantis (talk) 03:47, 17 May 2013 (UTC)


 * Alright, that bit is clearly a mess. This is outside ref-desk domain, but since we're here: should I/we just delete all this nonsense? I don't have the time/interest to make it better, but is this a case where no info is better than poor quality/wrong info? SemanticMantis (talk) 01:51, 17 May 2013 (UTC)
 * I went ahead and deleted the vC bit. At least the estimate for the force provided can be interpreted as the average force (even if the reference is a bit dodgy), while I have absolutely no idea as to what a vC could possibly measure (the average speed of a velociraptor, perhaps?) -Anagogist (talk) 14:14, 17 May 2013 (UTC)


 * The 1.78 VC doesn't appear to be total nonsense, it's just obscure jargon that would only be familiar to someone involved with crash testing cars. I found the "Superhumans" clip in question.  I'm hesitant to provide a link to what I presume is a copyright violation, but you can find it by searching for "Shi Yan Ming Superhuman" on youtube.  The impact was measured of a strike to a crash test dummy's chest.  It's clear from the video that the 1.78 VC means 1.78 times the limit allowed for what a crash test dummy will experience under certain conditions as specified in the Federal Motor Vehicle Safety Standards.  I don't know for sure, but I'm guessing the VC stands for "Vehicle Code".  However, it's not clear to me precisely what the limit involved is.  Section 6.4(a) of the Federal Motor Carrier Safety Administration's Standard No. 208; Occupant crash protection allows a maximum compressive deflection of a standard crash test dummy's sternum relative to the spine of 76 mm, but section 6.4(b) allows a maximum of 63 mm, and I don't know which of those two values was being used in the 1.78 VC quote.  Red Act (talk) 03:35, 18 May 2013 (UTC)

Shadows cast by moonlight
Can moonlight shadows be seen in urban areas or can they only be seen in open countryside with no other light source? Also in such countryside areas, how much lighting does a full moon provide in an area with no other light source? Does it become as light as it does in a city at night with the street lamps? Clover345 (talk) 23:27, 16 May 2013 (UTC)


 * They can be seen in cities. The Moon provides for a lot more light than street lights, if you walk outside during a full Moon it feels almost like daytime, you don't get that with street lights. Believe it or not, well away from cities during clear Moonless nights, you can even see the shadow cast by the Milky Way, see here. Count Iblis (talk) 23:41, 16 May 2013 (UTC)


 * Does this depend on where you are on the planet though? Clover345 (talk) 00:15, 17 May 2013 (UTC)


 * (ec) I'm not sure what sources Count Iblis is relying on, but the full Moon is not brighter than typical streetlight illumination. Typical moonlight provides around 0.2-0.3 lux at full Moon; that can get as high as perhaps 1 lux under ideal conditions (perfectly clear sky, tropical latitude with the Moon directly overhead).  If you Google for street lighting standards or similar keywords, you'll find a number of different local and national standards from various jurisdictions.  Typical recommended minimums I've seen are no less than 2 lux for low-traffic residential streets and footpaths, on up to as much as 50 lux for high-traffic, complex intersection (see, for example, the British standards in table 2.1 of this document).
 * In practice, that means you might see a faint shadow cast by moonlight in an urban area if you're in a dark alley, a poorly-lit park, a sheltered backyard, or otherwise protected from normal street and building lighting, but it would take a bit of doing. Under no circumstances should you expect the Moon to offer as much light as even minimal street lighting.  TenOfAllTrades(talk) 00:24, 17 May 2013 (UTC)
 * Don't you guys every go out at night? Air pollution must be realy bad where you live.  Wickwack 120.145.81.185 (talk) 00:26, 17 May 2013 (UTC)
 * Yeah, and a lot of it depends on whether you are a vampire. μηδείς (talk) 01:32, 17 May 2013 (UTC)


 * Where I live we don't have streetlights placed so close to each other that you can't escape being very close to one. In my garden I'm not bothered by streetlights, I would have the right to compain about streetlights if that were the case. Count Iblis (talk) 13:46, 17 May 2013 (UTC)
 * @ Wickwack - it's not air pollution that's the problem it's light pollution. In the UK the only time I see the Milky Way is when we're on holiday on a camp site in a rural location such as in The south west of England or in Scotland. If I remember rightly you live in Australia, which is a massive country with a relatively small population, whereas Britain is a small crowded island where you are rarely far away from a town or city with street lighting. And in the reference given it says of the US "Thirty years ago one could find truly dark skies within an hour's drive of major population centers. Today you often need to travel 150 miles or more." Richerman ''   (talk) 14:17, 17 May 2013 (UTC)
 * You remind me of a cousin that flew out from the UK to visit us. She thought that we lived in a desert, despite all the photos we had sent over the years.  If you divide Australia's population (~24 million) by the total land area (7.7 million sq kilometres), you get 3 persons per sq km - very low.  But very misleading.  Very very misleading.  You see, nearly all of us live in dirty great sprawling coastal cities, and it's all lit up just like any modern city, mate.  Where I live, it is a suburb zoned r4, which means about 4 families per hectare, or about 1600 persons per sq km.  We are not completely stupid, so don't live in the deserts.  So, we have plenty of light polution, and plenty of chemical polution.  But not as bad as Los Angeles or Bejing, and we most definitely can see shadows in moonlight.  Try Google Earth.  Wickwack 60.228.253.98 (talk) 15:11, 17 May 2013 (UTC)
 * Sorry, I misunderstood your point about air pollution. And yes, I do know that most Aussies live in towns and cities around the coast but of course I have no idea if you live in a city or in the middle of the outback. We Brits aren't all completely thick :) Richerman ''   (talk) 16:13, 17 May 2013 (UTC)
 * Also compare that with the population density of London which is 5,206/km2, where you certainly can't see moonlight shadows. I remember spending an evening just outside Bendigo in Victoria admiring an astonishing night sky - you would have to go to the Scottish Highlands to see one as good in the UK. Alansplodge (talk) 19:06, 17 May 2013 (UTC)
 * I get that every night of the year where I live these days. Whenever I spend overnight time in a metropolis, I have to ask "What happened to the stars?".  (Good name for a novel.)  --   Jack of Oz   [Talk]  19:31, 17 May 2013 (UTC)
 * Class 6 skies where I live: " No trace of the zodiacal light can be seen, even on the best nights. Any indications of the Milky Way are apparent only toward the zenith. The sky within 35° of the horizon glows grayish white. Clouds anywhere in the sky appear fairly bright. You have no trouble seeing eyepieces and telescope accessories on an observing table. M33 is impossible to see without binoculars, and M31 is only modestly apparent to the unaided eye. The naked-eye limit is about 5.5, and a 32-cm telescope used at moderate powers will show stars at magnitude 14.0 to 14.5." Count Iblis (talk) 16:55, 17 May 2013 (UTC)

I have taken pictures of the skyline in London, Paris and some other cities with time exposure at night; many of them look as if they were taken during daytime. Count Iblis (talk) 19:51, 17 May 2013 (UTC)


 * That's not an indication of anything. This image was photographed in a class 1 location under a full moon; apart from the stars, it looks like a typical daytime picture. --Carnildo (talk) 00:13, 18 May 2013 (UTC)


 * I live in a major city under class 8 nighttime skies, yet I can see shadows cast by the moon on my apartment balcony - probably because the balcony is not lit by street lights. Of course it is hard to compare that to the illumination from cloudy skies reflecting street lights.  Quite a contrast to the best night sky I've ever seen from near Silverton, NSW.  Astronaut (talk) 18:47, 20 May 2013 (UTC)