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March 16

How can a pedestrian bridge fail so quickly?

Tragic news from Florida. I know it'd be speculation at this point, but surely this is one of the surest things to design for? Imagine Reason (talk) 01:57, 16 March 2018 (UTC)

I've been warning people to avoid speculation at the article, but there are several possibilities. One of the supports could have shifted or been pushed out of alignment (one appears to be tilted, but that could be an effect rather than a cause). There could have been an undetected flaw in the reinforcing connections, or a post-tensioning cable could have failed. Post-tensioned structures are designed to withstand cable failures within reason, but a failure is a spectacular event that could have unforeseen results - they release a lot of energy. Failures like this usually are the result of a bad connection of some kind. The design appears to be a kind of cable-stayed structure, but the component that failed was apparently designed to support itself during construction without the stays. This newspaper article [1] contains a great deal of speculation, but it describes some of the common adjustments that are made to this kind of structure, such as camber adjustment. Construction is often the most hazardous time for a structure, as the redundant systems of a fully-completed design are not in place and the structure may be subject to unusual stresses. Acroterion (talk) 02:10, 16 March 2018 (UTC)

Your link doesn't work, you lost an l at the end [2]. I also came across this from the same paper which IMO is a little better as while it's speculates on possible reasons, it makes it clearer it is just speculation and doesn't really suggest anything is the cause of he collapse [3]. Nil Einne (talk) 05:31, 16 March 2018 (UTC)

Oops, sorry. Here's another article that's fairly measured in its coverage [4]. Our article on L'Ambiance Plaza collapse is worth a read, as it also involved the placement of prestressed concrete structure that was fabricated elsewhere - in that case the movement was vertical, not horizontal. At L'Ambiance it was apparently a problem with evenly distributed support that led to a failure at a connection point. We can expect reports from construction standards organizations in the long run for the FIU accident. Acroterion (talk) 12:22, 16 March 2018 (UTC)

According to this report, the cables were being tightened after a stress test. Akld guy (talk) 21:40, 16 March 2018 (UTC)

Courtesy link to thus unfamiliar: Florida International University pedestrian bridge collapse. Matt Deres (talk) 12:47, 18 March 2018 (UTC)

It sounds like people are surprised it failed so quickly. Bathtub curve suggests an early failure is likely, as is a late failure. RJFJR (talk) 16:26, 20 March 2018 (UTC)

sending probe on Sedna

In 2076, planetoid 90377 Sedna reaches perihelion. Why dont the humans try to land a probe on sedna? I mean, if it lands successfully; it will be a free journey for a very very long distance. All have to be done is to tell the probe to "stay still" and "do nothing" for a few centuries. Once sedna is far long, the probe cab be awaken. Then it can study the outer rim, and edge of solar system. It will take around 12,000 years for sedna to reach the perihelion again. During that time, the probe can go to sleep again. It doesnt need to return by itself. In 12,000 years humans might have gone extinct, or must have built faster space ships. One of such ship can pick-up the probe.

Are there any plans of a mission similar to this? If not, why? —usernamekiran(talk) 12:44, 16 March 2018 (UTC)

It's not that much of a "free journey". It takes considerable energy to match the velocity of such a body. If you can do that, then the probe could have simply put itself into such an orbit anyway. A rendezvous and sampling mission is interesting in itself, see Philae et al, but it doesn't really change the orbital dynamics of a mission. Andy Dingley (talk) 13:02, 16 March 2018 (UTC)

To amplify Andy's point, landing a probe does not provide a "free journey" at all. There is a way to get assistance from an orbiting body such as a planet or asteroid, but it doesn't involve a landing; you have to do a slingshot maneuver. If the probe was able to bounce off a hard object like an idealized rubber ball, then bouncing off the asteroid would give the same benefit that a slingshot maneuver does. (That's because in the asteroid's frame of reference the slingshotting probe is in a hyperbolic orbit.) --69.159.62.113 (talk) 20:15, 16 March 2018 (UTC)

My gut feeling is that for a slow trip to put a permanent memorial on Sedna with minimum fuel expenditure, it would be better for it to be moving away from the Sun. That's because our probe, when it eventually reaches the dwarf planet, will need to slow down and otherwise change velocity to match its orbit, and that is easier if it's moving away than if it is moving sideways or coming toward Earth. The idea is delta v. This would not be the case if you wanted as fast a trip as feasible with a lot of fuel to burn, in which case the distance is what matters most. Wnt (talk) 21:48, 16 March 2018 (UTC)

Voyager 1 crossed the Heliosphere of our sun on August 25, 2012. Voyager 2 is currently still in the Heliosheath, aiming to measure its thickness. So the "study the outer rim, and edge of solar system" is already under way and done.

The United States dont seem as dedicated to the Space Race anymore tho its participation in the joint venture James Webb Space Telescope, in preparation to be launched 2019, will probably deliver more new science than all of the historic "spaceprobes" together. Other states dont seem to have very big plans too. Maybe that changes, maybe someone is secretly planning to become/send the first martian (Mr. Musk?). Btw. planning for the second space telescope (James Webb) started in 1996, more than 20 years(!) ago, so its not like finding a target and starting a probe to it some month later. The process to get politics to decide about the money for it can easily take a decade alone. --Kharon (talk) 23:13, 16 March 2018 (UTC)

Aphelion versus perihelion at those distances actually doesn't have much of an effect on the minimum fuel cost of the mission. The vast bulk of the fuel is simply spent getting off of Earth, and then out of its orbit [5]. It would certainly be faster to go when Sedna is closer to the Sun, but still crazy expensive. Also, from an engineer/scientist/politician's perspective, it would be kind of silly to launch something now, intended to go into hibernation for 60 years (and we can't possibly test in advance if it can survive for that long), when instead would could spend money on something that would yield results now, and not even think about visiting Sedna until its much closer. Someguy1221 (talk) 01:43, 17 March 2018 (UTC)

I'm assuming that the OP is thinking about planning/preparing for this now rather than necessarily launching it now although I agree it's way too far advanced. That said, in addition to all the problems highlighted above there seems to be another big challenge. If putting something into hibernation for 60 years is tricky, and I agree it is, imagine trying to put it into hibernation for a few centuries! I'm not convinced our current tech is able to produce electronics that can survive that long, especially in outer space, even given some protection by the planetoid. Even the power source is likely to be tricky. The Radioisotope thermoelectric generator plans using Americium-241 would probably do although with RTGs being in hibernation is irrelevant to the power source (don't know about the other components); hence most plans don't propose that. And as you've indicated there's both producing things that can last that long, and being confident they will last that long. Nil Einne (talk) 04:03, 17 March 2018 (UTC)

In theory Sedna could contribute useful resources, such as water for propellant and ores for solar panels and mirrors. There is no way to rule out some kind of geothermal energy, even toasty warm spots, without a better look-see. But you'd better bring a very big nuclear plant with you if you need power to mine enough material to make enough mirror to concentrate sunlight at 80 AU out (1/1600 the sunlight!). It might be best to see some bots making fully equipped living space on the Moon and Mercury before we try that one. But yes, as a base, Sedna can be seen as providing free passage to a tremendous amount of raw materials for future space intrigues. Wnt (talk) 21:19, 17 March 2018 (UTC)

1/6400 the sunlight at 80 AU, ~1/875,000 at aphelion... 93.136.39.109 (talk) 02:46, 18 March 2018 (UTC)

To build on the basic point already expressed, but not fully explained: in space, objects do not have the same familiar drags on them that they do on earth. Your scenario suggests a hitch-hiker jumping aboard a passing train to get a free ride out to the Oort cloud. A satellite attempting to land on Sedna would have to match Sedna's speed (i.e. the hitch-hiker would have to run as fast as the train). However, unlike earth-bound hitch-hikers and trains, the satellite and Sedna do not have to worry about friction - they obey Newton's first law of motion in a very different environment than what we experience in everyday life. Once a satellite gets up to the same speed as Sedna, hitch-hiking gives you no benefit as that speed is easily maintained - the satellite will keep going that speed forever (until something else interferes with it). Matt Deres (talk) 13:00, 18 March 2018 (UTC)

Slightly related, if humanity ever want to leave the Milky Way then getting a ride with (probably not on) exiled stars [6] leaving the galaxy may be of help in a distant future – but only because there may be useful resources to stay alive for a really long time. Your spaceship still has to catch up with the speed of the star like with Sedna, so there is no "free ride" advantage. PrimeHunter (talk) 02:57, 20 March 2018 (UTC)

FWIW, there are Jupiter gravity assist windows for 2033 and 2046 launches to go to Sedna (mentioned at 90377 Sedna#Exploration, actually). But no space agency is currently considering launching a probe to go there. Double sharp (talk) 14:36, 20 March 2018 (UTC)

Would babies and toddlers need less nap time in the day if they sleep more hours at night?

If pre-school children all sleep enough hours during the night time, do they really need "nap time" during the day? SSS (talk) 23:48, 16 March 2018 (UTC)

There seems to be a link: " For example, one toddler may sleep 13 hours at night with only some daytime catnapping, while another gets 9 hours at night but takes a solid 2-hour nap each afternoon.". Count Iblis (talk) 02:36, 17 March 2018 (UTC)

Sleep Duration From Infancy to Adolescence: Reference Values and Generational Trends. Alansplodge (talk) 12:33, 17 March 2018 (UTC)

Since retiring, I've drifted back to being polyphasic. --Aspro (talk) 19:38, 17 March 2018 (UTC)

Be aware that the amount and duration of naps and sleep anyone should get is a contentious issue and - at the very least - varies with age. Quick example here. Parents of newborns are inundated with conflicting information, but it's usually formulated the opposite way of the OP. For example, the received wisdom when my child was a newborn (15 years ago) was that having naps improved night-time sleep habits; babies that missed their nap(s) would be too cranky to properly get to sleep at night. That was the theory, anyway. Matt Deres (talk) 16:10, 18 March 2018 (UTC)

March 17

Continuous, emission and absorption spectrums

Does anyone know where I can find find the continuous, emission and absorption images for the elements? I am creating a periodic table of the elements and really want to have these added. I found the visible spectrum images on wikimedia, which is fine, and am content on using them. I have looked and searched and found nothing really, except for hydrogen mostly. The kalzium program on Linux distros has the emission and absorption images, but they are small (and the program hasn't been updated for 12 or more years). 68.68.64.65 (talk)

• You want images? Abductive (reasoning) 06:47, 18 March 2018 (UTC)
• Are you only interested in simple atomic spectra? Even the helium and rubidium spectra have features in them due to diatomic molecules. In the ultraviolet there may also be a continuum due to ionization. In the microwave region there might be lines due to nuclear spin. Even so, don't expect them to be completely characterized for all different conditions, eg temperature, fluorescent, electric discharge. There are publications with big lists of numbers for frequency and line strength around. There are numerous publications on the topic. If you get something good, you can update User:Graeme Bartlett/spectrum of calcium and User:Graeme Bartlett/spectrum of magnesium! Graeme Bartlett (talk) 07:27, 18 March 2018 (UTC)

Yes, Abductive, the images. Here are the ones I have been using so far: https://en.wikipedia.org/wiki/Spectral_line 68.68.64.65 (talk) 03:14, 19 March 2018 (UTC) — Preceding unsigned comment added by 69.63.88.54 (talk) 22:30, 18 March 2018 (UTC)

They seem to be a high quality and complete set of emission spectra in the visible range. Other than that you also want absorption spectra, how do those compare to what you want (spectral range, spectral resolution, graphical size, etc.)? DMacks (talk) 03:29, 19 March 2018 (UTC)

DMacks, I am trying to follow the makers of kalzium (in a way). I just wanted to add the 3 mentioned spectrums (like they had the emission and absorption in kalzium). I am satisfied with what I have now, I just thought that having the 3 would make it look "better". But I never realized looking for visible spectra without lines, just emission lines and just absorption lines spectra would be so difficult. Very grateful for what I found on Wikipedia. I couldn't find them anywhere else. Thanks folks for all your replies and help. 68.68.64.65 (talk) 14:28, 19 March 2018 (UTC)

March 18

Does light knock at least some electrons off from any material?

When light hits a material (any material), can we assume that at least some electrons will be displaced? If we put the material in the dark and illuminate it unevenly, how could we analyze the surface to find spots where light hit? --Doroletho (talk) 14:05, 18 March 2018 (UTC)

First of all, not all light photons will have any effect. They need to be of a high enough energy to do so, which means a high enough frequency - see photoelectric effect and Einstein's first, and Nobel-winning, 1905 paper. (light travels at a constant speed, so its energy depends on its frequency, not like the speed and kinetic energy of a massive particle.)

Secondly, metals and conductors will allow electrons to move freely over their surface. So even if they were displaced, they'd rearrange almost immediately. If the material is a semiconductor though, the effect of the light can be to render that spot conductive. As an insulator, the pattern of electrical charge is fixed in place; first as an even distribution, then as the remainder which wasn't exposed to the light. This is the basis of xerographic photocopying. Andy Dingley (talk) 15:40, 18 March 2018 (UTC)

• And what instrument is used to analyze the surface of an insulator or semiconductor? --Doroletho (talk) 16:15, 18 March 2018 (UTC)

• That would depend a bit on the context, and how fine a spatial resolution you want.

Classically, the instrument was the electroscope, mostly the gold leaf electroscope. It's also possible to make an electroscope with a fine wire probe, which can be scanned across an area to 'read' the charge over it. Like most sensitive charge-reading instruments, the reading process is both destructive (it removes the charge that was there) and also needs resetting after each measurement where charge was found. An electronic electrometer is a more modern version of this, and more convenient.

To see the spatial distribution of the charge, xerography can be used. With a semiconductor-coated metal drum (selenium or a semiconductor doped organic polymer) place a constant charge over the whole surface. Then write on it with light (either laser spot or a reflection of the photocopying target), then dust with a fine pigment powder. Light makes the semiconductor conductive, dissipating the charge through the metal drum. The pigment is attracted to the remaining charge, but only in the unexposed areas. Pressing a sheet of paper over this transfers the pigment, giving an image of the remaining charge, and the pigment can be fixed in place by heat fusing it to the paper. Andy Dingley (talk) 17:26, 18 March 2018 (UTC)

Lots of equipment is used to study a material's surface - there's even a name for the entire field of study: surface physics!

Wikipedia has an article, Surface metrology. I have seen commercial equipment for these purposes, called by any of various generic names: "surface metrology station," "laser metrology machine," "profiling machine," "surface roughness meter," ... and so on. For example, you can purchase a "Panasonic Advanced Metrology System Solution" from your local ... place ... that sells semiconductor fabrication test-equipment. Just don't ask how much it costs. As the promotional literature reminds you, "...what does poor quality really cost?"

Other equipment that can be used to study material surface physics of course includes the conventional optical microscope; the electron microscope in all its forms; the atomic force microscope; the Raman spectrometer; the four point probe; and many other unique and specialized types of equipment. Nimur (talk) 17:59, 18 March 2018 (UTC)

Semi-important nitpick: light only always travels at the same speed in a vacuum. It travels more slowly through a medium, and this is what gives rise to things such as refraction. I'm noting this because it appears to me that "light always travels at the same speed" is a common misconception. More than once I've seen people introduced to things like Cherenkov radiation express confusion because they believe this. --47.146.60.177 (talk) 03:01, 20 March 2018 (UTC)

See Ionizing radiation for the cut-off for appreciable effects of photon energies on electrons, broadly speaking. Acroterion (talk) 18:02, 18 March 2018 (UTC)

Well, if you use that term, proceed with caution: while it is true that "ionizing radiation" knocks electrons out of their atomic orbit, even "non-ionizing" radiation can add energy to an electron: as Andy linked above, that is called the photoelectric effect. In some materials, like metals and semiconductors, an energized electron has greater electron mobility and may migrate, even if its "parent atom" is not "ionized." If we're not extremely careful with terminology, we can lead to great confusion: when mobile electrons flow in a crystal lattice, we do not usually say that the individual atoms are "ionized." In detailed study of solid-state crystal lattices, we often use the term electron gas or "free electrons" to describe sufficiently-mobile electrons that are not specifically associated with individual nuclei. Importantly: these electrons and their weakly-associated nuclei are not ionized: the total amount of energy is too low to separate the electrons completely. In specific: there is not enough energy to move the electrons to infinite distance from the material lattice. Broadly speaking, ionizing radiation provides exactly enough energy to move the electrons to an infinite distance from their atomic nuclei: this exact quantity of energy is called the ionization potential. Nimur (talk) 18:17, 18 March 2018 (UTC)

Agreed, that's why I qualified my answer. Since the question was about displacement, as opposed to raising an electron's energy level, it seems relevant if the OP isn't familiar with the concept. Acroterion (talk) 18:20, 18 March 2018 (UTC)

Yes, thank you for the clarification. The key distinction is "how far" the electron can be displaced. Nimur (talk) 18:24, 18 March 2018 (UTC)

Some substances have a very high ionization energy. For example helium and neon. These need vacuum ultraviolet to shift an electron. So normal light does nothing too them and they are transparent. Many other transparent materials will need electromagnetic radiation in the ultraviolet to knock off electrons. Graeme Bartlett (talk) 22:37, 18 March 2018 (UTC)

• Note that we're operating in the realm of quantum mechanics, where particles have a nonzero probability of escaping from an energy well, even with no boost at all. A boost too small to kick the particle out of the energy well by itself will nevertheless increase this probability. So the basic answer is that when we're dealing with Avogadro's number of particles, pretty much any incident light, regardless of how low its energy is, will increase the number of escaping electrons to some degree. Looie496 (talk) 01:02, 20 March 2018 (UTC)

March 20

Cognitive bias for thinking everything a science field produces is positive?

Is there a term for folks in particular science fields who are so enmeshed in what they're doing that they develop a cognitive bias for being optimistic about everything contributed to the world from their field?

Take me, for example, as someone with a software engineering background, I frequently found myself thinking in terms of software being able to solve any problem. But as I've distanced myself from the field, it becomes clearer how unobjective I was in thinking like that.

Is Pro-innovation bias the best fit? I am asking a user of the Wikipedia rather than as an editor -- I don't intend to use any answer for editing purposes. Stevie is the man! ^{Talk • Work} 15:28, 20 March 2018 (UTC)

That is difficult because a priori its also a central nature of science to test and refute theories when they are wrong. There are early greek idealized philosophical theories about the reign of the wise, today collected in the term Noocracy and there is also Technocracy as a similar concept. Taylorism and Fordism are something similar too. These all imply the believe that pure science is the best of all ways to rule, judge and organize. However again, science includes questioning its ideas, altho it may take some time to conclude it was not good like Albert Einstein about his help to develop the nuclear bomb or Alfred Nobel inventing Dynamite. Science even recently "invented" Technology assessment as a new science! --Kharon (talk) 16:37, 20 March 2018 (UTC)

Thank you. That goes in the direction I intend, but I am trying to focus on a particular cognitive bias of the scientist (as an imperfect human being) in a specific field (which can be any specific field), whereas they become so idealized by their field that they in a sense become PR or evangelists for it. They ignore (or pretend to ignore) the negative ramifications of what their field produces, while seizing opportunities to trample on critics, no matter how well-founded the criticism. They make continual efforts to mold the minds of others toward their "positives only" view of their field. Maybe such scientists are indeed PR folks with a scientific background, but I don't want to assume they are PR. I wonder if there is a true cognitive bias here. Stevie is the man! ^{Talk • Work} 17:08, 20 March 2018 (UTC)

It also depends on what you mean by science. I would consider software engineering to be a branch of engineering. Engineers typically think they can solve the world's problems. And they can do it better than most other people too, but you might end up with something like China with them in charge. Or did you mean computer science? Scientists tend to be a bit more modest. Dmcq (talk) 18:28, 20 March 2018 (UTC)

I'd rather not say what specific field I'm thinking of, as I would like to avoid a debate about that field. But I will say that a field where science is applied fits into this model. So you could say I'm referring to a cognitive bias of a scientist or engineer. Stevie is the man! ^{Talk • Work} 18:50, 20 March 2018 (UTC)

Lots of biases could be used to describe this, but no one single phrase says it all. In-group_favoritism is relevant. As for the science/engineering thing Dmcq mentions, see[7], which specifically calls out Physicists as thinking they can do anything. On the other hand, I know plenty of Mechanical and Electrical engineers that don't think software is engineering. Anyway, I agree that many members of any group can get carried away overvaluing the contributions and abilities of that group. You can see it with computer folks, you see with with some academics. You can find fashion experts who will look you straight in the eye and say that fashion is all that matters. It's a little bit of in-group favoritism with sort of a group-level Dunning-Kruger effect. Which is not to say that everyone in these groups does it. I agree with Dmcq insofar as most of the very accomplished scientists I have met tend to be very cognizant of their field's limitations. As for the value of getting some distance from the field, that is also much discussed, though under many different terms (and in different fields). See e.g. Field_theory_(sociology), Standpoint_theory, Overview_effect, Cognitive_shift, this blog post [8] on the value of outsider's perspectives. SemanticMantis (talk) 18:46, 20 March 2018 (UTC)

There's scientism and technoutopianism. Sagittarian Milky Way (talk) 18:55, 20 March 2018 (UTC)

March 21

Big toe anatomy

I am looking for information of the anatomy of the big toe, but all the pictures and info I can find only show the big toe joint close to the foot, I am looking for info on the first big toe joint that bends close to the nail, what is it called? I am also wondering if this is an actual joint with cartilage or just bones.--User777123 (talk) 04:04, 21 March 2018 (UTC)

That joint has a synovial membrane. The end bone has a double concavity. The bone is called Pedal distal phalanx 1. The big toe has no middle phalanx. There are collateral ligaments on either side. It is connected to plantar ligament connected to articular capsule. Info from Cunningham's Text-Book of Anatomy, 9th ed, by James Couper Brash. A picture of the bone is here: http://www.eskeletons.org/boneviewer/nid/12537/region/feet/bone/pedal_distal_phalanx_1 . It connects to Extensor hallucis longus muscle. Graeme Bartlett (talk) 07:40, 21 March 2018 (UTC)

I'll add that the joint closest to the nail is called the interphalangeal joint of the hallux (aka digit 1 of the foot). -- Flyguy649 ^talk 22:13, 22 March 2018 (UTC)

Weighing scale

Can anyone please point me to a weighing scale which can measure mass up to 1 kg (or, if not, then at least up to 500 g), with the highest possible precision (if possible, ±10 mg or better, but if not, then the best available) and with the greatest possible accuracy? I bought one which meets these specs (or so I thought) on eBay, but it turned out to be a piece of junk -- just today I tried to weigh a 2 L beaker full of water, and it showed the weight as 1410 kg (and that right after calibration!), and as far as smaller weights (like empty beakers, or even a 1 L beaker half full of alumina powder), it remains stubbornly stuck at 0.00 g! 2601:646:8E01:7E0B:0:0:0:9ED6 (talk) 06:36, 21 March 2018 (UTC)

It appears that 2kg broke your 1kg limit scale. Anyway you want accuracy to 1 part in 100,000. You will have to take into account air buoyancy, which will vary at your precision with temperature and pressure. So you will need to weigh in a vacuum or have extra compensation. I can see examples that should be a good enough quality here: https://www.instrumentchoice.com.au/instrument-choice/other-meters/digital-scales-1/precision-and-analytical-balances and another here: http://www.scaleshop.com.au/adam-nimbus-nbl-e/ . Some of the scales here https://www.austscientific.com.au/product/balances/ have compensation for pressure and temperature (but I did not see humidity) and also prevent electric fields that also affect measurements. But they mostly do not meet your requirements for maximum. Graeme Bartlett (talk) 08:10, 21 March 2018 (UTC)

Actually, all 3 of them are good, but far too expensive -- I guess I'll have to settle for using 2 scales, one for weighing small amounts of chemicals (up to, say, 100 g) with high precision, and another one for weighing large amounts (up to 1 kg, preferably higher) with rather lower precision. 2601:646:8E01:7E0B:0:0:0:9ED6 (talk) 08:52, 21 March 2018 (UTC)

I'm slightly confused, did you scale support up to 1kg or did it supposedly support a higher weight limit? If it was only up to 1 kg and you tried to weigh something heavier like a 2L beaker full of water, it's not completely surprising it would break perhaps permanently as Graeme Bartlett has said. With the cheap scales I've bought from AliExpress I've generally gotten away with a fair amount of abuse. But if I was buying a expensive high precision scale from a proper manufacturer, I wouldn't be testing the limits especially to such an extreme. You can only go so far until you actually damage the strain gauge or whatever is used or something else. (In any case, for something from eBay particularly if it's new and from an uncertain manufacturer, you should always consider the possibility it may be DOA. So test it properly when it first arrives and report it to the seller if it is so you can get a replacement.)

Anyway I have seen ultra high precision, supposedly 1 mg scales, supporting up to 1 kg and with an enclosed measuring surface on AliExpress e.g. www.aliexpress .com/item/1kg-0-001g-1mg-electromagnetic-scale-calibration-weights/32810440445.html. And high precision, supposedly 10 mg, scales supporting up to 1kg are very common both the cheap dime a dozen ones which don't even have an enclosed measuring surface (so you know that at a minimum, the accuracy is very questionable) e.g. www.aliexpress .com/item/1PC-Professional-Pocket-1KG-0-01G-Scale-Digital-Jewelry-Precision-Balance-Weighing-Scale-With-Backlight-Keyboard/32819638592.html and those that do e.g. www.aliexpress .com/item/New-APTP457B-Precision-Jewelry-gold-food-weighing-counting-kitchen-scale-1KG-x-0-01g-Laboratory-analytical/32622805989.html.

I'm not suggesting you get these if you actually need these for an important reason (I do actually use a 0.01g scale from AliExpress/eBay but only really for fun, e.g. cooking, in fact 0.1g would do I mostly want something more than 1g and it's a choice between 3kg or so 0.1g and 1kg or so and 0.01g). But these suggest to me you should be able to get one from a decent manufacturer. As GB has said, if you actually need that level of precision you'll need to consider carefully compensation measures.

Nil Einne (talk) 08:36, 21 March 2018 (UTC)

For the record, the scale I have is rated to measure weights up to 3 kg (in fact, the calibration weights are 2 kg in all) -- therefore, my weighing the beaker could not possibly have anything to do with the malfunction (and in any case, it was stuck at 0 for smaller weights even before I weighed the beaker). 2601:646:8E01:7E0B:0:0:0:9ED6 (talk) 08:42, 21 March 2018 (UTC)

Does it have a zero button to re-zero it with no mass on the platform? Graeme Bartlett (talk) 09:58, 21 March 2018 (UTC)

Your description sounds like it was DOA. Even the best brands have this, so I wouldn't conclude from a single instance that the scale model is no good. If it sold as a new & working scale, the seller should set this right especially if they are a commercial seller. Nil Einne (talk) 10:40, 21 March 2018 (UTC)

What is "DOA"? 92.8.216.101 (talk) 13:41, 21 March 2018 (UTC)

The first sense at DOA—that is, dead on arrival. In the context of manufactured products, it means that the item was critically defective as-delivered. TenOfAllTrades(talk) 15:24, 21 March 2018 (UTC)

To get within 10 mg you want a jewelers' scale, which typically goes to 100g at the most. You can buy them in head shops among other places. You might be better off with a lab scale that has a draft shield; i.e., after putting your sample on the scale, you put a transparent box over the whole scale to prevent air currents in the room from disturbing the measurement. Digital scales measure weight using strain gauges which can be damaged if you put too much weight on them, so yes your scale might have gotten messed up if you overloaded it. Also, obviously, they measure force (F=M·g) rather than mass. If you really want to measure mass directly, you want a balance-type scale. I like typing "Ohaus" into craigslist search (sfbay.craigslist.org) just to see pictures of nice old ones, though I don't feel likely to buy one). 173.228.123.121 (talk) 19:56, 21 March 2018 (UTC)

Reflector telescope design

I'm looking for a reflector telescope design where the secondary mirror is offset out of the telescope tube. My motivation is (1) aperture control and (2) better bokeh. thanks! --Masatran — Preceding unsigned comment added by 117.251.229.98 (talk) 07:33, 21 March 2018 (UTC)

I think they're called off-axis reflectors. A curved spider in an on-axis reflector would get rid of the diffraction spikes at least (Maksutovs have a small secondary mirror but have a corrector plate that can't be 100.000% transparent). Sagittarian Milky Way (talk) 08:02, 21 March 2018 (UTC)

How long after last frost before lawns need mowing?

What temperature do lawns consider a frost? 32? 30? 28? 107.77.173.12 (talk) 17:56, 21 March 2018 (UTC)

The growth rate of the grass depends on too many factors to give a simple answer - temperatures, moisture levels, grass species, soil nutrient levels, sunlight levels. If it stays dull, and just above freezing, the grass will grow a lot more slowly than if the temperature shoots up very quickly and there is plenty of sunshine. Wymspen (talk) 18:18, 21 March 2018 (UTC)

Today, many people cut their grass too short. If you keep it no shorter than 2 inches (and I don't mean bowling greens but garden lawns). That too, will help prevent weeds colonizing (so avoid the need for lawn weed treatments). When in the spring, the grass grows to 3 inches, mow it on a day when there is no 'frost'. It is only when the ground is frozen that damage is done because grass grows from the base and any physical trauma will damage the leaf buds. --Aspro (talk) 19:11, 21 March 2018 (UTC)

Another thing to potentially consider 93.136.50.38 (talk) 00:29, 22 March 2018 (UTC)

Interesting question. I went ahead and asked on https://gardening.stackexchange.com/q/37656/13670 and was pointed to a UK news article differentiating between air frost and ground frost. The basic answer I got was that you can mow when the grass needs it (e.g. is >2inch tall) and it and the ground is not frozen. Generally the ground only freezes during extended day and night sub-freezing temperatures, similar to pipes freezing; it isn't immediate but takes extended cold temperatures, with shorter time required the colder it gets. A 32F night can result on frosty surfaces but the ground would not be necessarily be frozen. If the ground is not frozen and the grass itself doesn't have frost on it, the grass can handle a mowing, but if the ground is frozen or the grass itself has frost on it you're better off leaving the grass alone as the disturbance of cutting or even just bending the frosted plant material could injure it into the growing season. Cr0 (talk) 00:33, 22 March 2018 (UTC)

March 22

Did any cultures or parts of history not mentioned in the article have an international date line?

If you asked a say Medieval French or Classical Athenian scholar where the date changes is there any evidence giving a suggestion of what they might've said? Sagittarian Milky Way (talk) 01:14, 22 March 2018 (UTC)

This is a purely intuitive answer, but surely one has to undertake a circumnavigation, or at least cross the Pacific, before a date line becomes necessary? Alansplodge (talk) 09:29, 22 March 2018 (UTC)

Well, a thought experiment leading to the conclusion that there must be a date line is quite conceivable at any time after the Earth was known to be spherical. Double sharp (talk) 10:27, 22 March 2018 (UTC)

Well maybe, but the OP seems to be suggesting a culture which actually had a date line rather than who just thought it might be something that would one day be needed. Nil Einne (talk) 17:19, 22 March 2018 (UTC)

According to one researcher the earliest known texts mentioning gaining or losing a day by circumnavigating the Earth date to the 12th century. Whether someone thought of it earlier is hard to tell if they didn't pen it down. 85.76.72.88 (talk) 14:30, 22 March 2018 (UTC)

Thanks for the link! According to that site the first references to such a thought experiment demonstrating the need for a date line occur in the works of Abu'l-Fida and Nicole Oresme in the 14th century. The thought experiment in question is termed the circumnavigator's paradox, in which two people circumnavigate the globe in opposite directions while a third stays put at their starting point; when they meet each other again at that starting point, a different number of days have passed for each of them. Double sharp (talk) 14:59, 22 March 2018 (UTC)

The Medieval French scholar would probably have said "at the edge of Asia", since they didn't know about the Americas. (No, they didn't think the Earth was flat; this is a still-widespread myth in the English-speaking world.) For the Classical Athenian scholar, the idea was effectively meaningless to them. They had their calendar, and the barbarians had theirs, and the day starts for you when the Sun comes up. This only became something really necessary to consider with the advent of globe-spanning empires in the 1500s, and, the International Date Line article seems to indicate that Europe quickly settled on a line in the Pacific. Even then it only mattered for official record-keeping and similar things. Until the telegraph, you couldn't interact "live" with anyone whose solar time differed noticeably from yours, so why would the average person care what day it was on the other side of the globe? --47.146.60.177 (talk) 06:28, 23 March 2018 (UTC)

Oresme was a medieval French scholar, which would answer the first part of the question, except that the site previously linked does not show that he had a suggestion for where exactly the date line should be, only that he knew there had to be one: "one ought to assign a definite place where a change of the name of the day would be made". Double sharp (talk) 07:31, 23 March 2018 (UTC)

Yes, it is no coincidence that the closely allied concept of time zones is tied directly to the advent of railways long enough and quick enough to make it matter. Matt Deres (talk) 14:12, 23 March 2018 (UTC)

Of course, the exact Greenwich time around the world was known long before then Timeline of time measurement technology. Greenwich mean time has been around since 1675. 92.31.142.218 (talk) 14:52, 23 March 2018 (UTC)

Robert van Gent (who is one of our contributors) doesn't get it quite right [9]. The eleventh century view was that the date line was located six hours east of Jerusalem. Support for this theory comes from the fact of the start of the month being delayed should the conjunction occur at noon or later - at that moment the date will be the same everywhere. Also, with the sun being created at the beginning of the fourth day and the belief that it was on the Jerusalem meridian at that moment there is no other place for the date line to be. 92.31.142.218 (talk) 15:11, 23 March 2018 (UTC)

The question kind of assumes that the entire world uses the same calendar. The question would be, when did that happen? ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:04, 23 March 2018 (UTC)

The Gregorian Calendar was adopted by most Roman Catholic nations in 1582. Britain and its colonies, including what is now the United States, adopted it in 1752. Akld guy (talk) 22:22, 23 March 2018 (UTC)

When did China, et al, adopt the western calendar? ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:31, 23 March 2018 (UTC)

The whole world not using the same calendar didn't stop a date line from existing for Medieval Judaism. Also it doesn't have to be the same day on side X, Russian Alaska used the Julian calendar, and Canada used Gregorian and everyone using either calendar would agree that the date line is on their shared border. Sagittarian Milky Way (talk) 23:41, 23 March 2018 (UTC)

March 23

Gegenschein

The Submillimeter Array atop Mauna Kea in Hawaii

Is the bright spot in this image Gegenschein? I would like to add the image to the Wikipedia article for Gegenschein if so, but I want to be sure. Thanks! Steevven1 ^{(Talk) (Contribs) (Gallery)} 04:22, 23 March 2018 (UTC)

It looks too low to for it to not be twilight and ruin the picture, it's probably the zodiacal light. Sagittarian Milky Way (talk) 04:34, 23 March 2018 (UTC)

If I'm reading zodiacal light correctly, the first question to ask is whether the sun is immediately below the horizon in this image (in which case, it's zodiacal light) or is behind the photographer (in which case, it's gegenschein). Matt Deres (talk) 14:16, 23 March 2018 (UTC)

It can't be too immediately below the horizon, it has to be 40% of the way towards diagonally down or almost to be as dark as that photo. Sagittarian Milky Way (talk) 23:51, 23 March 2018 (UTC)

As an aside it is amusing that astronomers whinge on about light pollution but apparently their radio dishes are afraid of the dark and have to be illuminated at night. Greglocock (talk) 17:08, 23 March 2018 (UTC)

Joking apart, I'd guess that during the long exposure used for the sky, the photographer (or an assistant) walked up to each telescope in turn and illuminated it with a flash. [Which I now see is mostly confirmed by the photo's Summary data – duh!]

I concur that this is the Zodiacal light; one can clearly see its pyramidal shape, whereas the Gegenschein is discernably oval, and also less bright compared to the background sky, although the long exposures necessarily used for both can make this less obvious that to the naked eye. {The poster formerly known as 87.81.230.195} 2.218.14.51 (talk) 21:45, 23 March 2018 (UTC)

Thanks everyone! I'll add this image to the Zodiacal light article in a bit! Steevven1 ^{(Talk) (Contribs) (Gallery)} 00:41, 24 March 2018 (UTC)

Ultrasound repellent devices

Do these devices work? Could they work? Maybe insects are immune due to their lack of ears. But couldn't an ultrasound siren ward off at least cats and dogs? Dogs get normally scared by loud noises (like those of firecrackers), but would a 100-150 dB ultrasonic boom (not perceptible by humans) also scare them? Does ultrasound spread as well as sound in terms of range or intensity/distance? That is, would producing ultrasound through a tweeter be equivalent to producing normal sound through a loudspeaker? — Preceding unsigned comment added by Doroletho (talk • contribs) 23:08, 23 March 2018 (UTC)

Tilting lens extension tube

Is a tilting lens extension tube available for any camera mount? I am interested in this since tilting lens adaptors do not provide autofocus. A tilting lens extension tube can trivially provide autofocus. I'm aware that distant focus would be impossible. --Masatran — Preceding unsigned comment added by 117.246.89.134 (talk) 00:26, 24 March 2018 (UTC)

It depends on your camera. If you have say a Nikon or a middle of the road Canon with standard screw threads on the lens mount then you have choices. Don't worry about focus. That just needs a bit of experimentation to become familiar with titling lenses and manual focus. Start by using a very small aperture (for maximum depth of field), then open up and use your eyes to check for focus on the main subject. --Aspro (talk)