Wikipedia:Reference desk/Archives/Science/2016 September 5

= September 5 =

Earthquake risk and political boundaries
I was looking at maps of earthquake risks and noticed that the risk somehow follow the the US-Canadian border around the Northeastern United States. The state of Maine has a much lower earthquake risk than the surrounding Canadian provinces. Is there a reason for this? AFAIK the political border doesn't correspond to any tectonic plate boundaries. Multiple maps show this behavior so I don't think it's a mapping error. Pizza Margherita (talk) 00:29, 5 September 2016 (UTC)


 * The St. Lawrence Valley is there because of a giant KRAK® in the continent I think. The New Madrid, Missouri quake site too. Sagittarian Milky Way (talk) 01:22, 5 September 2016 (UTC)
 * These kinds of maps are usually built on county-level subdivisions and based on historical earthquake data. That's why it tends to follow political boundaries.-- Jayron 32 02:17, 5 September 2016 (UTC)
 * There's a tendency for political boundaries to follow geographical features -- obviously not an ironclad rule, but fairly common. See Border. Shock Brigade Harvester Boris (talk) 02:34, 5 September 2016 (UTC)
 * Is it the case for the Maine border? I don't see any obvious geographical features that coincide with the Maine border. Pizza Margherita (talk) 02:55, 5 September 2016 (UTC)


 * See Saint Lawrence rift system. Mikenorton (talk) 07:43, 5 September 2016 (UTC)
 * Here's some more detail on the zones. Mikenorton (talk) 08:04, 5 September 2016 (UTC)


 * The Maine border was determined by treaty mostly with the commercial interests of the British as a concern, The US actually had the better claim at arbitration for a more northern border, but really didn't care, while the Brits needed a more sothren border for better access to the St, Lawrence. This should be in the article or at google.  If it isn't, I'll spend a few hours looking it up, if you ping me.  (My working computer was mfctrd in 2004.) μηδείς (talk) 03:19, 9 September 2016 (UTC)


 * Webster-Ashburton treaty map-fr.svg negotiated the shit out of that treaty. No wait, that was the Versailles Treaty.  Sagittarian Milky Way (talk) 04:18, 9 September 2016 (UTC)   ]]


 * Webster-Ashburton Treaty. Sagittarian Milky Way (talk) 04:23, 9 September 2016 (UTC)

Pictures in the night
Why do most advance cameras are unable to take clear pictures in the night?

I can see the moon and stars clearly in the night, but when I try to take their picture, what I get is dark picture. -- Marvellous Spider -Man 14:02, 5 September 2016 (UTC)


 * Most cameras are designed to be used during the day or with the lights on. The amount of light coming from a star is an insignifcant in comparison but the eyes are very good at adapting to extremes. Do a search on the web on something like 'photographing the night sky', there's loads of sites that will help with that sort of problem. You need a tripod and a camera that lets you leave the aperture open for some seconds. Dmcq (talk) 14:45, 5 September 2016 (UTC)


 * Those a very weak light sources - and the human eye is a lot more sensitive than most cameras.. You need a camera which can do a long exposure - and you need to be able to fix the camera to a tripod or other solid object, and use a remote rather than pressing the button, so that there will be absolutely no movement during the exposure. Wymspen (talk) 14:47, 5 September 2016 (UTC)
 * Along these lines: see Dynamic_range and Human_eye. High-dynamic-range_imaging is about how to use image processing to get digital images that look more like what humans perceive. Scotopic_vision is about the specifics of how we perceive low light. Adaptation_(eye) is about how our eyes change in low light conditions. SemanticMantis (talk) 15:21, 5 September 2016 (UTC)

I'd guess you're using a camera that's not up to this sort of task. The image I took here was using automatic exposure, and has come out well. --Phil Holmes (talk) 18:06, 5 September 2016 (UTC)

You'll have to put your camera on a tripod, put the aperture at F/4 or F/2.8 and expose (using the timer or remote control) for tens of seconds at an ISO of 800 or higher. Count Iblis (talk) 21:28, 5 September 2016 (UTC)
 * Companies are starting to delve into millions of ISO sensitivity] making clear night time video possible. Vespine (talk) 22:40, 5 September 2016 (UTC)
 * If you look at the specs you'll see how as well. It uses a 35mm sensor so is able to collect a lot more light than a human eye. That is why it is so expensive as well. Dmcq (talk) 22:38, 6 September 2016 (UTC)


 * Not necessarily. The moon is quite bright, and the photo I posted was taken at 1/200s, F5.6 ISO 400.  IIRC it was hend held.--Phil Holmes (talk) 10:26, 6 September 2016 (UTC)


 * That's quite a narrow field of view with decent magnification - you must have very steady hands, or a shaky recollection. Wnt (talk) 14:37, 6 September 2016 (UTC)


 * It's pretty plausible - I've taken similar pictures handheld with a Canon PowerShot SX410 IS superzoom camera (I don't have any moon pics on Commons, but here's a plane snapped in twilight conditions, with a shutter time of 1/125s - despite being a moving target shot handheld at almost full zoom, it's still pretty clear). The moon is bright enough to snap with a short exposure time, and modern digital cameras also implement Image stabilization to sharpen up the picture. Smurrayinchester 14:51, 6 September 2016 (UTC)


 * It's a 500mm lens with a 1.4x Teleconverter - at 1/200 with image stabilisation you can get good hand held shots (although you can also get some pretty bad ones, too).--Phil Holmes (talk) 08:27, 7 September 2016 (UTC)


 * See our article Astrophotography. One extra note - you can snap the moon and some of the brightest planets and stars with a camera and tripod (or even just a steady hand), but for most faint stars, you need a longer exposure, and even over the course of a couple of minutes, the stars move by a noticeable amount. If you want more detail, you need some way to move the camera so it follows the stars. This normally means an equatorial mount with a motor. Otherwise you get star trails, and the points of light turn into arcs. Smurrayinchester 14:51, 6 September 2016 (UTC)

rust sensor
what is rust sensor — Preceding unsigned comment added by 182.186.88.254 (talk) 17:05, 5 September 2016 (UTC)


 * Some stuff here. Oddly enough, when I started searching I got a lot of G-hits on censorship, despite spelling it sensor. Matt Deres (talk) 18:55, 5 September 2016 (UTC)


 * Without context, it is hard to tell. It could be an auto-body device to check that there are no filled rust holes or other patched damage. A quick Google shows one model called "The Filler Detector" and another called "Crash Check." Personally, I'd just use a magnet to see if it is metal or filler under the paint. 209.149.113.4 (talk) 19:06, 6 September 2016 (UTC)


 * That would take a long time, and the seller might object based on the risk of the magnet scratching the paint. StuRat (talk) 20:38, 6 September 2016 (UTC)

Sonic booms
Was the mechanism behind sonic booms predicted or otherwise known about before supersonic flight? Or was it a surprise when people heard the first one? Now we identify whip cracks and some other things as sonic booms- was that identification made before supersonic flight became possible? Staecker (talk) 17:17, 5 September 2016 (UTC)


 * Photography_of_bow_shock_waves_around_a_brass_bullet,_1888.jpg took a fancy picture of a sonic boom in 1888, predating supersonic flight.]]Thanks for asking, because I don't think I've ever been able to offer such a perfect reference as this: History of Shock Waves, Explosions and Impact A Chronological and Biographical Reference . Another great references is The Curious Events Leading to the Theory of Shock Waves .
 * The main gist is that shock wave is the general phenomenon behind sonic boom, and the math/science of these phenomena far preceded supersonic flight. William_John_Macquorn_Rankine and Pierre_Henri_Hugoniot were key early figures, responsible for the Rankine–Hugoniot_conditions in the nineteenth century. Recall that far prior to manned supersonic flight, projectile regularly broke the sound barrier, and Hugoniot may have had first hand experience with that in his time with Naval Artillery. So no, I don't think anyone sufficiently educated was surprised when they heard the first sonic boom from a manned airplane (This is mostly my understanding from vague memory, but the two sources above will give you as much detail as you need, and correct any of my mistakes). SemanticMantis (talk) 18:17, 5 September 2016 (UTC)
 * I have a few comments. First, on the one hand, the fact that the crack of a bullwhip is a sonic boom doesn't seem to have been known until the late twentieth century (after supersonic flight).  However, second, the distinction between subsonic and supersonic flight was known before supersonic flight was achieved.  In fact, accidentally going sonic was a cause of air crashes.  (I don't have a source for that.)  Third, as noted, projectiles (pistol bullets, rifle bullets, cannonballs) travel at supersonic speed.  That the whine of a shell or bullet is a sonic boom would have been known at least since the work of Mach and others on shock waves.  So I don't think that the sonic boom from an aircraft was ever really a surprise.  The report of a firearm is also a sonic boom.  Robert McClenon (talk) 18:47, 5 September 2016 (UTC)


 * According to this NASA source (which cites other sources) Mach presented the paper that included the shock-wave photo ("shadowgraph") in 1887, not 1888. 107.15.152.93 (talk) 19:47, 5 September 2016 (UTC)
 * Could be a distinction between presenting the paper vs. publishing it. But feel free to correct the caption at Ernst Mach using this reference, as the 1888 date I quoted has no citation at all. SemanticMantis (talk) 19:54, 5 September 2016 (UTC)
 * ✅ 107.15.152.93 (talk) 20:23, 5 September 2016 (UTC)
 * Thanks! SemanticMantis (talk) 20:48, 5 September 2016 (UTC)
 * By the way, sonic booms have been used in warfare on purpose. See https://news.google.com/newspapers?nid=860&dat=19700130&id=zepOAAAAIBAJ&sjid=3ksDAAAAIBAJ&pg=1708,1562265&hl=en.

Robert McClenon (talk) 21:01, 5 September 2016 (UTC)
 * Great info- thanks all. Staecker (talk) 22:20, 5 September 2016 (UTC)

Supersonic propeller tips were a well-known issue long before supersonic aircraft were developed, See [ http://digital.library.unt.edu/ark:/67531/metadc60441/ ], [ https://info.aiaa.org/Regions/MW/SL/Newsletters/AIAA_1996_11.pdf ] and [ http://naca.central.cranfield.ac.uk/reports/1957/naca-tn-4059.pdf ]. Also see Republic XF-84H and Tupolev Tu-95 --Guy Macon (talk) 16:53, 6 September 2016 (UTC)


 * As for whether it was a surprise when people first heard them, it may well have been, to those people, say when produced by a WW2 prop plane in a dive. That is, even if academia knew about them, the average person probably didn't.  There's often a significant delay between when something is discovered and when it becomes common knowledge, especially prior to the internet. StuRat (talk) 20:43, 6 September 2016 (UTC)

Health guidance on websites
Why is it that health advice on websites are generally a lot more excessive than what statistics or healthcare professionals would suggest? They seem to assume worst case scenarios? 2A02:C7D:B903:6C00:3D7C:3554:86A3:BFFA (talk) —Preceding undated comment added 21:46, 5 September 2016 (UTC)


 * Wouldn't it just come down to the "better safe than sorry" principle? But to be honest, I haven't really found that to be the case at all. At lest in what I'd consider remotely reputable websites, maybe you're looking up crackpot alt med sites? I have a young child so I've been on loads of websites looking up symptoms and stuff in the past couple of years, and if anything, the vast majority of times, the websites have been quite reassuring. Quite recently my son had a blood nose during the night, we freaked out a little bit, but most websites I read said it was quite common and perfectly normal and generally nothing to worry about, UNLESS the child has other symptoms, like bleeding from anywhere else (ears or gums etc) or headache, or lack of appetite, etc...Vespine (talk) 22:35, 5 September 2016 (UTC)


 * It really depends on what you are reading. I would think it's just part of the general trend toward sensationalism the media has suffered from for all time. Facts and statistics and honest professional opinions tend to be boring. Someguy1221 (talk) 22:38, 5 September 2016 (UTC)


 * Sites like WebMD match symptoms against a database of possible conditions, and they don't always discriminate between common and rare diseases properly. If I tell WebMD I have a cough, it puts "common cold" top of the list of possible causes, but it puts Chronic obstructive pulmonary disease above hayfever, and it considers ricin poisoning and irritation caused by a swallowed object equally likely! Smurrayinchester 15:00, 6 September 2016 (UTC)
 * There's some more serious discussion on symptom checkers here . Speaking of WebMD, a common meme or joke is that WebMD will diagnose everything as cancer (potentially copyvio but probably short enough for a resonable fair use claim so I'm still linking to it),, , , , , . Nil Einne (talk) 13:32, 7 September 2016 (UTC)


 * One thing to beware of is if the web site is trying to sell something. If so, then they will make whatever minor complaint you have into a major "syndrome".  Pharmaceutical companies do this as much as "snake oil salesmen". StuRat (talk) 20:46, 6 September 2016 (UTC)


 * Indeed. Count Iblis (talk) 20:52, 6 September 2016 (UTC)
 * We actually have a nice article on snake oil. SemanticMantis (talk) 22:25, 6 September 2016 (UTC)