Wikipedia:Reference desk/Archives/Science/2012 June 7

= June 7 =

== How do these points work on the Douglas Bay Horse Tramway? ==

I guess the question title doesn't really need much expansion; I live on the Isle of Man but oddly haven't actually bothered taking the Horse Tram at any point (I don't live in Douglas and there's no real point in it for normal transport anyway). I have been held up by it when driving along the promenade many times but never seen it traversing points.

So... the question is fairly simple. How on earth does it USE these points? Do they use "ramps" in the tracks? Do they force the horses to swerve violently with the coaches? Something else?

Much appreciate your response  Egg   Centri  c  00:34, 7 June 2012 (UTC)


 * P.S. As a secondary point, the original google maps link affected the spam filter. That's cause I chose short url which shortened it using goo.gl... anyway it was a nuisance wiki complaining about this! Egg   Centri  c  00:34, 7 June 2012 (UTC)


 * It seems most likely that those points are no longer used, and they've simply removed the blades and tarmacked in the gaps. Alternatively, the only option I can see is lifting the tram at each end and moving the wheels across to the point rail, which I can see being a reasonable alternative to mechanical points (which would need to be quite complex if they're set in a road surface safe for pedestrians and other vehicles) if the trams are light and those points aren't used often or by in-service trams.  I had a zoom up to the end of the line on Google Earth and the crossover at the terminus (the roundabout near the ferry terminal) seems to have normal switchblades.FiggyBee (talk) 00:57, 7 June 2012 (UTC)


 * That makes considerable sense, thank you. I also wonder hwo they do change the working points. Tell ya what, I'll try to get on one during the weekend and ask the folk involved what they're doing. And then incorporate it into the wiki article.
 * (Note: I am very busy this coming weekend so if I don't manage this, sorry in advance!) Egg   Centri  c  01:22, 7 June 2012 (UTC)

Sorry
I didn't have time. I hope I will be forgiven and my body parts will remain intact. Egg  Centri  c  23:53, 10 June 2012 (UTC)

chlorhexidine gluconate
will chlorhexidine gluconate react with eugenol, zinc oxide, or clove oil to form anything harmful? --Wrk678 (talk) 07:51, 7 June 2012 (UTC)


 * And what are you planning to do with such a mixture? Someguy1221 (talk) 08:09, 7 June 2012 (UTC)


 * May be the OP is concerned their chlorhexidine containing mouthwash will react with their Zinc oxide eugenol containing dental work? If so I suggest they contact the person who did the dental work or some other suitable professional. 2001:0:5EF5:79FD:20CB:1C04:833A:FA41 (talk) 15:53, 7 June 2012 (UTC)
 * I can't answer that medical question, but chlorhexidine and eugenol have been used in conjunction in research, e.g.  Note that, as explained at, eugenol is not seen as entirely benign in some situations all by itself, and eugenol-free periodontal dressings have been developed.  The real problem with any answer is that "harmful" depends on the context.  A bottle of cyanide is not harmful... provided it stays in the bottle or under a fume hood.  While water toxicity really does occur.  The dose (and circumstances) makes the poison. Wnt (talk) 18:19, 7 June 2012 (UTC)

Drug Delivery
I have had this doubt for a long time. Suppose I have a pain in certain area of my body - say X. I take a pain killer tablet. So the chemicals in the tablet dissolve into my blood stream. Now how does the drug get absorbed exactly in the painful area and relieve the pain ? What is the mechanism that makes the drug to get absorbed at the exact painful spot ? A even more localized example is when you have a sore throat and a tablet works wonders ! — Preceding unsigned comment added by 117.193.139.99 (talk) 08:14, 7 June 2012 (UTC)


 * A systemic painkiller (taken orally and then circulating in your blood) doesn't have to target the physical location or cause/origin of the pain for which you are taking it, it could just dull "your sense of pain". DMacks (talk) 08:17, 7 June 2012 (UTC)

Yes, I expected that reply. But say, there is some problem with the liver/ spleen and tablets need to be taken ( I mean some localized disease), how does it work. It would be a real waste to have the drug in the entire blood stream rather than localize its concentration at the target site. So how does it happen. — Preceding unsigned comment added by 117.193.139.99 (talk) 08:29, 7 June 2012 (UTC)


 * Your typical non-steroidal anti-inflammatory painkiller (tylenol, ibuprofen, aspirin) functions by broadly inhibiting your body's ability to transmit localized pain signals. In this sense, it is acting locally, but it's acting locally everywhere. Opioids, on the other hand (as well as GABA analogues, although less dramatically) block your brain and spinal cord's ability to register pain signals. In that sense, the drugs are acting non-locally from one place, but are still present throughout your entire body assuming you took a pill. As you are probably aware, some painkillers can simply be injected directly to where they need to work, such as for dental procedures. In this case, since the drug stays and acts locally, much higher local concentrations of the drug can be achieved with minimal side effects.


 * As for liver effects, it turns out that most drugs you consume actually wind up in your liver, given its purpose of breaking down most unusual chemicals that you consume. But everything else, unless it is injected locally, pretty much ends up everywhere. It is a goal of medical science to to develop drugs that target to specific tissues, actually. But that's not to make the drugs cheaper. Rather, the hope is that since the drug will only be going where it is needed, there will be fewer side effects from what the drug would do to tissues that don't need it. Someguy1221 (talk) 08:36, 7 June 2012 (UTC)


 * Some drugs are designed to bind selectively to certain types of tissues or cells. For example, they could have a high affinity for chemicals (and hence cells with these chemicals on their surface or vicinity) that are known to be produced in the case of a specific biochemical situation (inflammation, cancer, etc). In this situation, the drug does localize and concentrate to a certain site where it acts, it just disperses and circulates on the way to getting there. Or else they circulate randomly, but only act on cells that are in a certain state and therefore concentrate their effect there. DMacks (talk) 08:48, 7 June 2012 (UTC)


 * In my country, Australia, one of the commercial products based on Ibuprofen runs advertising clearly designed to suggest that the drug somehow knows where the pain is and goes directly there, rather than everywhere in one's body. I suspect it's bullshit. HiLo48 (talk) 16:53, 7 June 2012 (UTC)


 * We get the same adverts in the UK and I've been told by a doctor that it is nonsense. Perhaps it only acts near the site of the pain, but it goes everywhere your blood goes, just like anything else. --Tango (talk) 21:07, 7 June 2012 (UTC)


 * Since most substances taken orally affect your entire body evenly (with some rare exceptions, like radioactive iodine to treat thyroid cancer), this always seemed like a poor way to treat localized problems, to me. One of the worst ideas, IMHO, is the pill you take to help you grow hair, thus risking serious side effects to solve a cosmetic problem.  This obvious alternative is to deliver a hair-growth med with a topical liquid or foam, applied to the areas with hair loss.  StuRat (talk) 06:42, 8 June 2012 (UTC)

Carpal tunnel syndrome
would carpal tunnel prevent a career as a computer programmer? — Preceding unsigned comment added by 59.189.220.235 (talk) 14:08, 7 June 2012 (UTC)


 * I moved your question to its own section.Anonymous.translator (talk) 14:13, 7 June 2012 (UTC)
 * Not if you had voice command software like dragon breath165.212.189.187 (talk) 15:49, 7 June 2012 (UTC)


 * No, especially if it is treated. --TammyMoet (talk) 16:37, 7 June 2012 (UTC)


 * Also depends on whether it affects both hands severely. That's often not the case. HiLo48 (talk) 16:48, 7 June 2012 (UTC)


 * A programmer can reduce the risk of carpal tunnel syndrome by use of ergonomic equipment (wrist rest, mouse pad), taking proper breaks, and using keyboard alternatives such as digital pen and voice recognition. DriveByWire (talk) 20:54, 7 June 2012 (UTC)


 * Severe repetitive stress injury (of which carpal tunnel syndrome is one) can certainly force someone to change careers. If you're experiencing pain on a regular basis, you should definitely do something about it. I've been struggling with a relatively minor case for a while, and I believe that I tend to get better when I'm spending large chunks of time without touching a computer keyboard or pointing device. I sometimes use Dragon NaturallySpeaking to browse the web and read my email, but I don't think there's any way I could stand programming with it. I've seen a doctor and physical therapist through workers' compensation, which I highly recommend. I use Workrave to take breaks from computer usage, and I've started getting exercise to try to improve my overall health. I believe that I'll be completely back to normal eventually, but if I'd not taken action, I'd probably be permanently injured and no longer be a programmer. Paul (Stansifer) 22:04, 7 June 2012 (UTC)


 * Just speaking from experience, I've had CTS in various grades for 15+ years. In the beginning, when it was very painful, I wore wrist splints at the advice of a doctor, and worked to correct many orthopedic problems with my workspace (for a long time I had my mouse on a different level than the keyboard — a real no-no). Anyway, over the years it improved on its own with these adjustments, and today it only rarely manifests as a dull ache. (And the fact that I can't bowl — for whatever reason, bowling triggers every weird CTS symptom in me, and I temporarily lose feeling in multiple fingers. I wasn't any fan of bowling anyway, so no big deal as far as I'm concerned.) I still have a career in which I am constantly using computers and constantly typing. Results will vary given the individual and the severity of the case, but anecdotally, on the face of it I wouldn't conclude that CTS would make computer-based careers inaccessible, but one would really need to be proactive about managing the CTS and correcting the conditions that have created it in the first place. If you have CTS or suspect you do, definitely talk to your doctor about it, there are lots of relatively simple things that can be done to mitigate it, along with non-simple things if those don't work. Separately, I couldn't imagine coding with voice command software, personally, though I wouldn't be surprised to hear that some people can manage it. --Mr.98 (talk) 00:27, 8 June 2012 (UTC)

density
Is density more/less proportional to the distance between nuclei or the number of protons and nutrons in the nuclei or are they equally proportional. What s the relationship? — Preceding unsigned comment added by 165.212.189.187 (talk) 15:18, 7 June 2012 (UTC)


 * Density is mass per volume. The nucleus is essentially all of the mass and that mass is essentially the total of the proton and neutron masses, so the mass (and therefore the density) is proportional to the number of protons and neutrons assuming changing these does not change the distance from one nucleus to the next (i.e., the ionic or covalent radius in the material). Volume is the third power of length, so the density is inversely proportional to the cube of the distance between nuclear centers. DMacks (talk) 15:51, 7 June 2012 (UTC)
 * "inversely proportional to the cube of the distance between nuclear centers." is just the simple-case situation of assuming a cubic lattice, etc. DMacks (talk) 18:59, 7 June 2012 (UTC)

Could adding protons or neutrons ever decrease density of a material?165.212.189.187 (talk) 18:44, 7 June 2012 (UTC)

Density implies a finite amount of free space in each atom. Someone on this ref desk said there is no way to measure the amount of free space in an atom. Could someone clarify?165.212.189.187 (talk) 15:24, 7 June 2012 (UTC)


 * Density does not require finite free space, merely the ability to say "the vast majority of the mass is somewhere within a certain volume". That is, it doesn't matter whether it's a high-mass point particle (nucleus) surrounded by perfect vacuum, or if the mass is evenly distributed for a certain size, or a fuzzy blob that has no distinct boundary as becomes less dense as it extends outward, or even if we cannot actually describe in "real world" macroscopic ideas what is happening a little ways away from the center. Given a large enough space, we can still confidently say "the total mass is X and the total volume is Y within it" and calculate the average density for that object. When we talk about density of a chemical (unless you're doing x-ray crystallography or something), we're talking macroscopic, not just one or two atoms, so the mass distribution at the atomic level is orders of magnitude too small to make a noticeable difference. DMacks (talk) 15:57, 7 June 2012 (UTC)

How does the "certain volume" not imply a "certain volume (of free space)"?165.212.189.187 (talk) —Preceding undated comment added 18:41, 7 June 2012 (UTC)


 * Again, it doesn't matter what is in the space, whether it's extended nucleus or a field generated by nuclear or electronic wavefunctions, or virtual particles or "nothing at all". DMacks (talk) 18:46, 7 June 2012 (UTC)

DMacks, I don't think you understand the question.165.212.189.187 (talk) 18:48, 7 June 2012 (UTC)


 * That's true and I'm not trying to answer the question. My response is to dispute your "Density implies a finite amount of free space in each atom" premise on which the question and confusion appears to rely. DMacks (talk) 18:59, 7 June 2012 (UTC)


 * OK, I'll try. To define density you need only define the mass and the volume of the total "thing" you are trying to get the density of. The distribution of matter within the volume is irrelevant. When people talk about the density of a nucleus, they are defining a somewhat arbitrary volume within the atom to be "the nucleus". Someguy1221 (talk) 18:56, 7 June 2012 (UTC)

I am not concerned with the density of the nucleus, just the atom or any amount of a certain element for that matter. once you define the volume haven't you also determined the (free)space that atom/material occupies?165.212.189.187 (talk) 19:20, 7 June 2012 (UTC)
 * (linking back to the previous discussion for context) True, once you have defined a volume of interest, you have (rather tautologically) determined the space that things within that volume reside in.  Some of it is probably "free space", though it's not clear what you mean by that phrase, and as noted in the prior discussion, "free" will depend not only on the things themselves but also what they interact with -- see the prior discussion's example about neutrinos being able to consider pretty much anything to be "free space".  There's also the problem, as noted before, that things at the quantum scale do not have precisely defined boundaries.  How big is an atom?  We can only speak statistically.
 * Ultimately, though, I think it will be most helpful if you clarify your intent/meaning regarding "free space" and its specific inclusion in your questions, as it appears to me that the rest of the questions have been ably answered. &mdash; Lomn 20:09, 7 June 2012 (UTC)
 * One thing to consider is that allotropes are made of the same "stuff" but can have very different densities, for example Allotrope_of_carbon. In these cases, "number of protons and neutrons" doesn't change but density does. Vespine (talk) 23:12, 7 June 2012 (UTC)

165, it just gets back to the problem that there is no obvious definition of "free space" in modern physics. At some level, you have to arbitrarily define the volume of a piece of matter. You can do this based on the statistical probability of locating a particle in a certain region of space, or on the distance over which a particle can exhibit a certain type of interaction, or some other equally arbitrary boundary. You can stuck at both ends, actually. Even when you're probing an empty vacuum, something you would consider "free space", there is always a probability of finding an electron that shouldn't be there. And if you probe right at the center of what you think is a proton, there is always a probability of finding nothing. Someguy1221 (talk) 00:20, 8 June 2012 (UTC)

Thanks, although you question my definition of free space (point taken), I question your definition of "nothing". Really, the electron that you find "shouldn't" be there?165.212.189.187 (talk) 12:53, 8 June 2012 (UTC)
 * Someguy is simply illustrating the problem with simplistic models when compared to the real world. In the context of a discussion about "empty vacuum", anything found there (such as the electron) "shouldn't" be there in terms of the model (else it's not empty).  This is not a statement imputing motive to the electron, nor is it a statement that electrons shouldn't be found in nature, but rather a recognition that models tend to be imperfect abstractions. &mdash; Lomn 13:21, 8 June 2012 (UTC)

Acupuncture
Does acupuncture really work, or is it just a placebo effect? --108.227.31.161 (talk) 19:54, 7 June 2012 (UTC)


 * Most of the evidence is consistent with a very strong placebo effect. There's a lot of research outlined at Acupuncture. Some of the bigger findings include evidence that the location of needle placement is unimportant (evidence against the importance of meridians or particular points being associated with any particular malady) and that actual needles need not be used at all (poking, but not breaking, the skin with toothpicks performed as well as inserted needles). Add to that the fact that meridians and qi have never been shown to exist and that most of the successful trials involve only subjective outcomes (e.g., pain reports) and/or inadequate control groups, the fingerprint is one of a placebo treatment. &mdash; Scientizzle 20:09, 7 June 2012 (UTC)
 * I'll add that "working" and "a placebo effect" are not necessarily exclusive. If a treatment can reliably and repeatedly achieve the desired effects, then it "works", even if we are pretty sure there is no valid underlying mechanism. SemanticMantis (talk) 20:12, 7 June 2012 (UTC)
 * I know the traditional reason for why acupuncture works is BS, but that doesn't automatically mean that it doesn't work. --108.227.31.161 (talk) 20:29, 7 June 2012 (UTC)
 * I've seen a summary of published research linking acupuncture to the release of endorphin. It was found that when test subjects were given something that blocks the action of opiates, the pain-relieving effect of acupuncture disappeared. I don't have a reference to the research but the endorphin article has a section that seems to say the same thing. So it seems that acupuncture is not (entirely) placebo. IMO, the reasoning of experiments using supposedly-sham acupuncture to show that acupuncture is based on placebo effect is flawed. The observations don't necessarily support the conclusion that the effects of acupuncture has no physiological basis. An alternate conclusion is that the traditional procedures and emphasis on the meridians are unnecessarily specific. It could be that the same physiological mechanism is triggered by the "sham" procedures. --98.114.146.125 (talk) 12:36, 8 June 2012 (UTC)


 * We do have an article on veterinary acupuncture, the subjects of which one might imagine would be less prone (although perhaps not immune) to the placebo effect. The article is dismally referenced, however, so all you can really take from it is that people think it works on animals too. But then people believe all kinds of nonsense. -- Finlay McWalterჷTalk 20:39, 7 June 2012 (UTC)
 * Pets can be prone to a kind of placebo-by-proxy. The owner things the pet should be getting better, so behaves differently, and that difference in behaviour makes the pet better (or seem better). You need a blind study, with the owner not knowing if the treatment has been done or not, to get useful results. I'm not sure if any of those have been done - our article doesn't say. --Tango (talk) 21:12, 7 June 2012 (UTC)
 * People, and animals, will often get better from a malady by themselves. A treatment may often be given the credit for an improvement in condition that would of happened anyway. This is one of the reasons why a double-blind placebo-controlled study is much, much better than anecdotal evidence. LukeSurlt c 22:19, 7 June 2012 (UTC)
 * If you are really interested and want to read more, the science based medicine blog has a number of very good posts evaluating acupuncture studies and their interpretations. Vespine (talk) 23:04, 7 June 2012 (UTC)
 * Strong OR notice here: I can not speak to whether acupuncture is legitimate science or a placebo effect, but having had numerous treatments, it has been effective for me in relieving pain, stress and other conditions.   → Michael J Ⓣ Ⓒ Ⓜ 06:20, 8 June 2012 (UTC)

I have had treatments and I liken it to the feeling you get after working out. They also say that the placement of the needles does matter because it causes a "micro wound" which triggers the white blood cells and other "healing" chemicals in our body to concentrate there to begin to heal that area.165.212.189.187 (talk) 12:58, 8 June 2012 (UTC)
 * I had accupuncture administed by my physiotherapist to treat a tear in my anterior supraspinatus tendon. The rationale was that tendons heal poorly due to limited blood flow, and inserting a foreign object triggers an immunoresponse that leads to an increase in blood moving into the area. 203.27.72.5 (talk) 07:10, 9 June 2012 (UTC)

Is this proof correct for the geometry file?
— Preceding unsigned comment added by Mitch the amateur scientist (talk • contribs) 21:18, 7 June 2012 (UTC)


 * It looks like a description of the square-cube law. But it doesn't make a formal claim, nor follow any kind of formal reasoning, so you can't really call it a proof. -- Finlay McWalterჷTalk 21:34, 7 June 2012 (UTC)
 * It's also abusing standard terminology: increasing a quantity "by addition" or "by factors (multiplication)" is not a clear or common way to phrase a geometric operation. Factorization has a very precise mathematical definition, explained in our article.  Nimur (talk) 23:22, 7 June 2012 (UTC)
 * It looks like it is trying to say the increases are "additive" and "multiplicative", however it's all multiplicative just with different powers. So, it's neither a proof nor correct. I'm not sure what "the geometry file" means, either. --Tango (talk) 23:24, 7 June 2012 (UTC)
 * I think the person writing this appreciates that the area of a sphere is two-dimensional, and thus, when the radius is increased by a factor n ("additive"?) its area increases by a factor of n2. The "proof" is that he has sketched two perpendicular axes to represent this area.  Of course, this falls short of a clear mathematical proof of the proposition, though someone's intuition is on the right track.  I think it would be taken as more of a proof if he considers that the sphere can be divided into many little nearly-square sectors (square in the limit as it is divided up infinitely fine) in which case the proportionality to r and r2 can be proved plainly since they're a defined shape; otherwise the proof needs to incorporate a proof of the area of a sphere of a given radius. Wnt (talk) 11:25, 8 June 2012 (UTC)


 * Pythagoras_algebraic2.svg]] I would compliment the artist on the nice diagram, and point the OP to the notion of "proofs without words", such as this proof of the Pythagorean theorem. For more, see here, and/or google it. Indeed, the originally-posted image could be modified into an essentially rigorous proof, as others have suggested. Basically, it is not necessary to make formal verbal claims to have a "proof", if one is willing to rely on the reader's background a bit (after all, very few proofs are entirely self-contained). SemanticMantis (talk) 15:21, 8 June 2012 (UTC)

Is Skin Cancer More Prevalent Now Than in the Past?
It seems that every summer we are warned of the harmful and dangerous effects of sunlight on unprotected skin. SPF numbers increase every year. Even the slightest exposure to sunlight is discouraged, often in a nearly hysterical tone.

What seems odd about this is that, until very recently, constant exposure to the sun was the norm for mankind. Whether it was building the pyramids, growing crops, sailing, etc., people spent their entire lives working in direct sunlight, without any more protection than clothing and a hat, if that.

I know that scientists, using modern medical technology, are able to determine the ailments of mummified Egyptian pharaohs. Is there any evidence that historic skin cancer rates were comparable to today’s cancer rates?Phidias007 (talk) 22:42, 7 June 2012 (UTC)
 * It might be interesting to look at the average age that skin cancer is likely to occur vs. the average length of life. That is, if the skin cancer rate were lower, it might be simply that something else bit them first. ←Baseball Bugs What's up, Doc? carrots→ 22:54, 7 June 2012 (UTC)
 * My first thought was the same as Bugs', but according to our article on skin cancer people under 19 are the most likely to get skin cancer (at least children from UK). The bigger factor perhaps is ozone depletion. Our ozone depletion article discusses the resultant skin cancer increase at length, but for some reason our skin cancer article only mention ozone depletion once, at literally the very last sentence. Anonymous.translator (talk) 23:26, 7 June 2012 (UTC)
 * Oops, forgot to mention tanning beds as well. Anonymous.translator (talk) 23:28, 7 June 2012 (UTC)
 * You've misunderstood the article. It's comparing children in the UK with children elsewhere in Europe. There is no comparison between different ages (I've checked the source). I'm reworded that bit of the article to make it clearer. My first thought was also the same as Bugs' and I suspect we are all correct. Serious infectious disease is so much rarer now that pretty much every other medical condition is more common than it used to be simply because more people are surviving long enough to get it. --Tango (talk) 23:37, 7 June 2012 (UTC)
 * My apologies. The wording was very confusing.Anonymous.translator (talk) 23:48, 7 June 2012 (UTC)
 * Northern Europeans, with their light skins, are far more vulnerable than any other population. It is only recently that large numbers of light-skinned Caucasians have lived at tropical and subtropical latitudes.  See our melanoma article for more information.  The ancient Egyptians were actually pretty dark-skinned. Looie496 (talk) 23:19, 7 June 2012 (UTC)
 * It wouldn't surprise me if skin cancer was quite prevalent in the past and people just ignored it. People get all sorts of harmless things in their skin, and get left with all sorts of scars after catching various poxes, that they wouldn't have noticed melanomas as anything remarkable. Melanomas usually only become a serious problem when they metastasise, and you're not going to know that the symptoms of the new tumour(s) have anything to do with the skin lesions. --Tango (talk) 23:41, 7 June 2012 (UTC)


 * We are told here in Australia that we have the highest prevalence of skin cancer in the world, largely because we've plonked a whole bunch of people with northern European ancestry into the sunniest continent on Earth. We've also had a sun, beach and outdoor sports loving culture for most of the life of this nation. And yes, we live longer than our ancestors, so the skin cancer has time to appear before we die of something else. HiLo48 (talk) 23:47, 7 June 2012 (UTC)


 * Another issue is that in pre-industrial times most Caucasians had constant exposure to the sun, so they built up tans gradually, and did not burn, which is what causes the most damage. Nowadays people live and work indoors and burn on weekends and holidays. μηδείς (talk) 03:18, 8 June 2012 (UTC)


 * Also, is it possible that skin cancer was confused with other diseases of the skin, in ancient times ? StuRat (talk) 03:56, 8 June 2012 (UTC)


 * Two broad thoughts: 1. Increasing SPFs is a matter of marketing as much as anything else. Beware of confusing advertising with reality. 2. Increased incidence has to be squared away with increased diagnostic ability or changed diagnostic criteria; just because you suddenly measure more or something doesn't mean its base rate has changed. --Mr.98 (talk) 11:47, 8 June 2012 (UTC)


 * Any data on UVA, UVB, and UVC energy [W/m²] over the last 100-years?, that ought to give a serious hint. Electron9 (talk) 17:48, 8 June 2012 (UTC)
 * The amount of UV incident in a given area at the "top of the Earth's atmosphere" is a function of the sun's sunspot activity. This runs in cycles - over the last 100 years, the approx 11-year cycle on top of the gradual rise since the last Maunder Minimum.  The variation due to this in the context of skin cancer is just about negligible.  More important factors are waht affects attenuation of UV in the atmosphere.  This is mainly the rise in particulate polution since WW2, which decreases UV incident at the ground, and currently significant only in certain cities, and the depletion of the ozone layer in the last 20 years of so, which increases UV incident at ground level, more in some locations that at others. See http://en.wikipedia.org/wiki/Ozone_depletion, which does not include the actual UV increase due to ozone depletion, but does give data on the consequent increase in skin cancer.  Incident UV affects the output of photovoltaic electricity generatiion.  In some areas where ozone depletion is significant, output is in recent years of the order of 2 to 5% higher than expected, but in a quick search I could not find a definitive online reference.  Wickwack124.178.139.104 (talk)  —Preceding undated comment added 11:33, 9 June 2012 (UTC)