Wikipedia:Reference desk/Archives/Science/2015 December 16

= December 16 =

list of stars about 30000 ly distant please
Hi - it's for a novel - I need some sunlike stars that are about 30,000 lightyears away - ones with curious names/designations especially welcome. Thanks in advance Adambrowne666 (talk) 00:37, 16 December 2015 (UTC)


 * The thing is that modern astronomers don't generally give stars cute names - they're generally just numbered in some manner. Named stars tend to be the ones that have been known for a very long time - typically they are naked-eye objects.  30,000 ly is about a third the way across the galaxy - and stars that are that far away aren't usually visible without a telescope.  There are also an immense number of stars out at 30,000 ly - at least 100 billion of them.  Now, clearly we're not going to name any of them unless they are REALLY special - like maybe super bright.  Why would be bother to name a boringly typical sun-like star out of the billions out there?
 * So the answer is "None" - there are no named stars out at those distances - and certainly no sun-like stars with names at anything like that distance. SteveBaker (talk) 00:56, 16 December 2015 (UTC)


 * Thanks, Steve - names would have been a bonus, but any sort of designation would do. Adambrowne666 (talk) 01:00, 16 December 2015 (UTC)
 * For example, if you take the text dump of the Guide Star catalogs, a set of "all-sky optical catalog(s) of positions and magnitudes of approximately 19 million stars and other objects" that are specifically used by scientists to aim the Hubble Space Telescope, you'll see things like:

N003-AAJE 46.13902  84.36855  0  99.999  99.999  3  16.759  16.685  3  16.082  16.086 N003-AAAF 45.84270  84.48269  0  99.999  99.999  3  14.565  15.112  3  14.730  14.753 N003-AABF 45.75025  84.36574  0  99.999  99.999  1  16.999  17.003  1  16.262  16.315
 * ...and so on, for 19 million entries. The "name" of one of these stars is, for example, "N003-AAJE."
 * Unless that specific entry is scientifically interesting for any other purpose besides its relative location, chances are very high that no scientist has ever bothered to study that star to determine its distance, or its spectral type, or anything else about it. A tiny fraction of the entries in these catalogs have been subjected to intensive scrutiny, but most of them are known only as dots with positions and approximate brightnesses.
 * What's more: if you use a different coordinate system, or even if you just a different sky catalog - that dot might be listed by a totally different designation. It might not even be listed in a different catalog.
 * Nimur (talk) 01:15, 16 December 2015 (UTC)
 * Only the first field seems like a designation. The next two seem like declination and right ascension in degrees in some order and the last four seem like magnitudes in some order (probably B, V, R, and I)? Sagittarian Milky Way (talk) 01:19, 16 December 2015 (UTC)
 * And the Guide Star Catalog II has 1 billion stars instead of 19 million (down to magnitude 21) and per absolute magnitude and interstellar extinction the average sunlike star that distance might be too dim to appear even there. Space is Big! If it did appear it would probably be unusually far from the galactic plane for a Sun-like star as the Sun is fairly metallic by star standards. Sagittarian Milky Way (talk) 01:27, 16 December 2015 (UTC)
 * To be honest, I'm not sure exactly how to interpret every column in the database. The text file I pulled those data from did not include a labeled header; and the database in this particular text-format does not seem to match the documentation in the published literature linked from the main website.  This text-dump is evidently intended for consumption by a very specific computer program - it's really not aimed at human readers.
 * If you're very interested, you can read about this specific catalog at the The Guide Star Photometric Catalog I, from the Space Telescope Science Institute.
 * Anyway, for our OP: if you are writing a hyper-unrealistic Space Trek fictional story, you can just give your star a great name that will enthuse your readers (like Proxima Nimur VII) ; but if you're writing hyper-realistic fiction, you need to re-think the way you portray designated objects in deep space. Space is really big.
 * Nimur (talk) 01:29, 16 December 2015 (UTC)
 * Of course, if this is happening in the future we might be able to tell if it's interesting enough to name (if it has one of the most Earth-like planets known, for instance). That might be done with a telescope in space many miles wide with a spectroscope, or suspended animation and fast automatic spaceships that can function for ages so you could go there where it takes much less equipment to find out how Earth-like it is. Your list might be available to Earthlings before a few decades from now but you could give it an even more robot-sounding catalog number from a bigger alien or future human catalog if it seems likely that the characters would be using it. Sagittarian Milky Way (talk) 01:54, 16 December 2015 (UTC)
 * Or a smaller alien catalog from a planet much closer to the star than we are. Sagittarian Milky Way (talk) 01:58, 16 December 2015 (UTC)
 * Why 30,000 light years by the way? Any particular reason or cause it sounds good? Sagittarian Milky Way (talk) 02:02, 16 December 2015 (UTC)


 * Thanks, everyone. It's 30000 ly so that the protagonists on a world circling one of those billion unnamed stars can imagine looking back at Earth with a putatively perfect telescope and see something (in their imagination) happening 30000 years ago.Adambrowne666 (talk)


 * You can check List of most luminous known stars - some are around 30,000 ly away. One that stands out is the Pistol Star -- if you're writing a culture of gunslingers influenced by American cowboy movies, I think you're in luck! :) Wnt (talk) 14:00, 16 December 2015 (UTC)
 * To be clear, I didn't say that there were no named stars out that far - only that there are no named SUN-LIKE stars that far away. Pistol Star is about as un-sun-like as it gets - it's a  blue hypergiant and is one of the most luminous known stars in the Milky Way.  That means that if your protagonists are anywhere remotely close to it, and your story is remotely believable, they are very, very dead.  SteveBaker (talk) 15:42, 16 December 2015 (UTC)
 * D'oh, I forgot about that part! But as it happens the Pistol Star is "at least double" with much fainter "point sources" near the blue hypergiant  though apparently not confirmed (?).  Of course, we have no idea what aliens can live on really, so I'm prone to stretch the sunlike requirement.  I'll say one thing ... if the emanations of that hypergiant don't kill you, I imagine on the planets of any nearby sunlike companion, they must look pretty awesome. :) Wnt (talk) 20:33, 16 December 2015 (UTC)


 * I tell you, for all that we're stuffing up the world and each other, you people on the Wiki Ref Desks give me heart. Thank you. Adambrowne666 (talk) 02:00, 17 December 2015 (UTC)

What is the precise length of a day?
It is somewhat of a "fiction" that the year is 365 days long. Actually, it is something like 365.25 days long (or so). And that is why we have the need for a leap year every four years. So, what about the length of the day? Is it exactly 24 hours? Or is that also a "fiction"? It seems a little too convenient to be 100% accurate, no? 2602:252:D13:6D70:14DE:69F5:F4C:EAE3 (talk) 02:28, 16 December 2015 (UTC)
 * Depends on what you mean by a "day". There's a Civil day, solar day, a sidereal day, etc.  The civil day is defined as exactly 24 hours, and the sidereal day is defined a 1/365.24 of the year.  The solar day varies slightly depending on the specific day of the year.  All three days are within a few minutes of each other.  -- Jayron 32 02:51, 16 December 2015 (UTC)
 * To be more specific, the Julian calendar set 365 days with a leap day every four years, making 365.25 days. In the year 1582, Pope Gregory made an adjustment to correct an error that had accumulated to about 10 days. In that year, and for that year only, October 4th was followed by October 15th, thus advancing the date by 10 days. The intervening days (5th -14th) simply do not exist. There was also specified that 3 of every 4 centesimal years (ending in 00) would be common years, not leap years. Thus 1600 and 2000 were leap years but 1700, 1800, and 1900 were not. This is where the 365.24 length comes from, and it is this Gregorian calendar that we live under today. Most of Europe adopted the Gregorian calendar immediately in 1582. Great Britain and its colonies (including what is now the US) did not until 1752, by which time another day's error had occurred, so September 2 1752 was followed by September 14, an advancement of 11 days. Akld guy (talk) 03:17, 16 December 2015 (UTC)
 * The sidereal day is 1/366.25 (ish) of a year, rather than 1/365.25. —Tamfang (talk) 03:17, 17 December 2015 (UTC)

Hmmm. OK, now my head is spinning. I thought that a year was how long it takes the Earth to revolve around the sun. So, if the Earth starts at one point (Point A), makes a full revolution around the sun, and ends up at the same starting point (Point A again), that whole trip will take 365.25 days. I thought that a day was somehow similar. Something like how long it takes the Earth to rotate on its axis for one full spin (or something like that). And that was calculated to be 24 hours, which is where we get our "calendar day" from (e.g., the day we call December 15 versus the day we call December 16, etc.). Am I on the right track? In other words, our calendar "day" is derived by some celestial happenings (with the Earth or sun or moon or whatever). So is that celestial event exactly 24 hours? Or is it "off" by a bit, but we just use the "fiction" of 24 hours, for convenience? (Like, for example, in actuality it is really 24 hours and 3 minutes and 18 seconds, or whatever.) Thanks. 2602:252:D13:6D70:14DE:69F5:F4C:EAE3 (talk) 05:58, 16 December 2015 (UTC)
 * In basic terms, yes: a year is how long the earth takes to go round the sun, and a day is how long the earth takes to spin. The problems that make things a bit more complicated are:
 * 1) How do you actually tell when the earth has got back to its starting position? There is more than one way of doing this (depending on what you use as a reference point), which give slightly different answers.  Hence we have concepts like the stellar day and sidereal day
 * 2) The earth/sun system isn't like a machine that does exactly the same thing all the time. Its a slightly wonky lump of rock being pulled round a big ball of plasma by gravity, while other lumps of rock and gas tug at them with their own gravity.  This means things wobble a bit, and don't always move exactly the say way or at the same speed.  Which means the time take to go round the sun or rotate on its axis can change slightly.  This in turn means that - for people who need to define time really precisely, they need to use a consistent definition of year, day, second etc that aren't dependent on what the earth is doing right this moment. Iapetus (talk) 13:14, 16 December 2015 (UTC)
 * The day is currently 24 hours and 1 millisecond. It was 24 hours when the second was first defined to be something other than 1/86400th of a non-fictional day (the late corset era). We are now 1 minute and 8 seconds behind then as a result of the rotation slowing down until a day is 24 old hours and 1 millisecond. We can't change the second, it would screw up precise scientific work so we have to add leap seconds every so often. It takes 365.256 days for the Earth to revolve once but the axis isn't lined up as it wobbles very slowly (26,000 years per wobble) so the seasons (which is what we care about most) take under 365.2422 days to repeat (but well over 365.2421). Those are 365.2422 real days, not the 24 hour ones that're fictional now. Sagittarian Milky Way (talk) 06:15, 16 December 2015 (UTC)
 * The Earth actually takes 23 hours and 56 minutes and 4.091 seconds to rotate but by the time it revolves once it has "canceled" one rotation by the revolving so it needs to make up one rotation every year. This is why it's 24:00:00.00 on average between noons or midnights — it needs to make up 1 day's worth of rotating per year so it takes 24 hours. Sagittarian Milky Way (talk) 06:24, 16 December 2015 (UTC)


 * The "celestial day" you refer to is what is called the sidereal day, this is the length of time it takes for the earth to spin 360 degrees around it's own axis. This is equal to 23h56m. The "solar day", which is how human beings historically defined the day, is the length of time it takes the sun to reach the same position in the sky from one day to the next. Historically, this is how we defined the "day" that we cut up into 24 hours, so the solar day is exactly 24 hours long by definition (some complication: it varies because the earth's rotation and its orbit around the sun are not perfectly even, circular, or fixed). Why is the solar day slightly longer than the sidereal day? Because in the time it took the earth to rotate once around its own axis, it has orbited slightly along its path around the sun. Which means it needs to rotate slightly further for the sun to again be at the same point in the sky since yesterday. The earth's orbit around the sun has essentially caused the sun's position in the sky to "un-orbit" the earth by 4 minutes, so it is 4 minutes behind where it needs to be since yesterday. Thought experiment: if you play this through for an entire year, can you see that the earth's 1 orbit around the sun means that the sun has fully "un-orbited" the earth by 1 whole day over the course of the year? This means that for the 365.24 days we measure by the solar day, the earth has actually rotated exactly 366.24 times on its own axis. If you take 24h*365.24/366.24, you get the 23h56m number. I hope this helped and didn't confuse you further! Zunaid 06:19, 16 December 2015 (UTC)
 * Yes, the "real rotation" and "real revolution" is 23:56:04 and 365.256 days, but the "real day" and "real year" are 24:00:00.001 and 365.2422 day. The fake day and fake year are 24 hours and 365 days respectively. Sagittarian Milky Way (talk) 06:30, 16 December 2015 (UTC)
 * There are other types of days and years but those would just be good for confusing you. They're very specialised and trivia-like, like the human-powered underwater speed record. Sagittarian Milky Way (talk) 06:38, 16 December 2015 (UTC)

Thanks. So, the "real day" is not 24 hours, but is 23:56 (or, in other words, 4 minutes short of a full 24 hours). So don't all of those "four-minute deficiencies" build up and accumulate over time? So, after 2 days, we have an 8-minute deficiency. After 7 days (one week), we have a 28-minute deficiency. After 30 days (one month), we have a 60-minute deficiency. And so forth. And doesn't that accumulated "error" make the days/times get out of sync? Which is why we add that extra leap year, I think? So that the "accumulation" of that "extra" 0.25 of a day beyond the flat 365 days per year does not accumulate too much and throw the "year" off cycle. If we did not have a leap day, then every four years, we have an extra "day" accumulated that never gets accounted for. The way that we "account for" that extra accumulation is by adding a leap day. So, wouldn't it be the same idea for the 4-minute deficiency gap for the "day"? Thanks. 2602:252:D13:6D70:14DE:69F5:F4C:EAE3 (talk) 07:47, 16 December 2015 (UTC)


 * No, you have misunderstood. The 4 minutes per day add up to one full revolution per year, which is accounted for by the revolution of the earth round the sun.  A day is the average time taken from one noon to the next (see Equation of time for why this isn't constant), and the only correction necessary is the occasional leap-second.  The leap year is to keep the seasons from drifting from their usual dates (see Gregorian Calendar).    D b f i r s   08:14, 16 December 2015 (UTC)


 * Some of Dbfirs' disingenuities have been corrected in "Sunrise and sunset" above.  A day is the average time taken from one noon to the next as he says but this average is the "mean solar day" and no further correction is required for "leap seconds".   Joe Public (i.e. the person to whom this resource is directed) does not use leap seconds.   The only people who use leap seconds are scientists who need units of measurement which are not constantly changing in length, and have therefore devised an artificial "SI second" which is the length of the mean solar second as it was back in 1820. 89.240.30.73 (talk) 10:17, 16 December 2015 (UTC)
 * Hold the insults! I was simplifying to help the OP, but here in the UK, we use the SI second.  It is broadcast by the Greenwich Time Signal, and an extra "pip" signifies the leap second for rare minutes that have 61 seconds.  Many other countries use the same system.    D b f i r s   19:30, 16 December 2015 (UTC)
 * The sidereal day is aligned with the stars. So if Taurus rises at a certain time within one sidereal day, it will rise at a certain time on any sidereal day.  Problem is, the Sun might be in Cancer, Libra or Capricorn, so while you might know what time of sidereal day it is, you don't know if it's light or dark out!  So people don't like the 23h56min day very much, even though it's always the same length.
 * The easy thing to do is to make up for how much the sun moves, but here's the trick... it doesn't move at the same rate all the time! Well, technically, it doesn't move much at all, relative to the barycenter of the solar system or whatever, but that's another story - what I mean is, the Earth's orbit is a tad elliptical and so sometimes the planet is whipping around the perihelion a little faster, sometimes further out and a little slower, so the Sun makes a whole analemma thing that can really confuse the hell out of you even if you actually understand everything I've written so far.  But using the same average correction is the sanest way to do things so that your computer video plays back at the same rate regardless of what season it is. Wnt (talk) 14:09, 16 December 2015 (UTC)
 * Let's simplify this for the OP with a few key bullet points.
 * The second is a precisely defined unit of time. Other units you use (including minutes, hours, days, months, years, etc.) on a daily basis are defined precisely as multiples of the second.  So the amount of time you mean when you say "a day" or "a year" in common usage is precisely defined and unchanging and means exactly the same thing.  A day means "86400 seconds" and that's all it means, and it will never change, because the second is precisely defined in a way that it will never change.
 * There are other, highly specialized, meanings of words like "day" and "year" that always come with qualifiers(like "sidereal" or "solar" or "celestial" etc.) that let you know they are defining it in a different way than as multiples of the precisely defined second. Most of these are tied to the physical movements of celestial objects, and since those movements are NOT regular and repeatable (i.e. everything wobbles a bit, as noted above), those definitions are not immutable or precisely defined.  Roughly speaking, the "day" is about equal to the amount of time it takes the Earth to spin once around its axis, and the "year" is roughly equal to the amount of time it takes the Earth to move a full orbit around the sun.  However, because of the myriad of variables involved, defining the physical movement of the earth in terms of "days" and "years" results in numbers that are close to the precisely define times noted above, but which are neither exact nor repeatable forever in the way that the "second" and its derivatives are immutably defined.
 * I hope that helps a bit. -- Jayron 32 15:06, 16 December 2015 (UTC)


 * For the sake of explanation, let's pretend that the earth stays still and the sun goes around us...yeah, I know...but let's pretend because it doesn't change the length of a day/year - and it makes understanding all of this much easier:
 * The time it takes for some distant star to rise above the horizon, go across the sky, sink back under the horizon, and then rise again is the 23 hours and 56 minutes and 4-ish seconds. That's how long it takes the earth to rotate through 360 degrees on it's axis.
 * But that's not "a day". We humans don't give a damn when stars rise and set - we need to know what time to get up and go to work...and that's to do with the sun - not the stars.
 * The time it takes for the sun to rise, go across the sky, under the earth and then rise again is 24 hours, zero minutes and zero-ish seconds...and THAT is what we call "a day".
 * The difference is because (in our imaginary geo-centric solar system) the sun orbits the earth every 365-ish days while the distant stars are "fixed" (ish). The direction that the sun "orbits" the earth is such that while we spend 23 hours and 54 minutes to spin 360 degrees, the sun has moved around it's orbit by 1/365th of 360 degrees...so it's a little bit below the horizon when the earth has spun 360 degrees - and the extra time it takes the earth to rotate the extra 1-ish degrees to get the sun to rise again adds 1/365th-ish to the length of a day.
 * The 23 hours 56 minutes is 1436 minutes - 1436/365 is 4-ish minutes - which *perfectly* explains the difference between 23 hours 56 minutes and the 24 hours we expect.
 * All of those "-ish" numbers conspire to make things a bit messier, leap years and so forth.
 * The fact that the earth doesn't spin precisely evenly (the moon, tides, global warming, volcanoes, weather, relativity (probably!))...explains the odd stray millisecond here and there - which we ignore until we get a whole second off, then we toss in a leap-second and half of the computers on the planet have a hissy-fit about it.
 * Of course the earth rotates around the sun and not the other way around - but that makes no difference whatever to the math. The ancient civilizations that worked all of this out had no idea that the earth orbits the sun and not vice-versa - and it takes a bit of science to figure that out - but it didn't matter because the answer comes out the same either way.
 * SteveBaker (talk) 15:31, 16 December 2015 (UTC)
 * There is no "gold standard" for intervals which track celestial events.  There are various months ranging in length from 28 to 31 days and longer.   Legally a "month" was 28 days.   A synodic month (new moon to new) is longer than a sidereal month (star back to same star) is longer than a tropical month (equinox to equinox) is longer than an anomalistic month (nearest approach to Earth back to same) is longer than a draconic month (intersection with Earth's orbit back to same).   None of these is "correct".   It's the same with the year and the day.   If you're grading the different varieties according to which is most useful then the synodic month comes out top, as does the tropical year (equinox to equinox) and the mean solar day (maximum elevation of sun to same).   Same goes for hours, minutes and seconds.   The day is always divided into 86,400 seconds of whatever timescale you're using.   So the comment that the length of the second is "precisely defined" should be read in that context.   What you must not do is mix units.   To say the day is 24 hours and 1 millisecond is confusing and wrong. 86.151.51.24 (talk) 16:11, 16 December 2015 (UTC)
 * To clear up a possible misconception, the use of leap seconds to convert the grand total of atomic seconds accumulated since the system was devised about 1960 to the familiar hours, minutes and seconds recorded by our clocks is nothing to do with the occasional 29 February which adjusts for the difference between a fixed 365 - day year and the actual time between successive equinoxes.  That four minutes per day you have to allow for if you are using the stars to tell the time when the sun is below the horizon actually amounts to two hours per day (24 hours over a year), which is why if you are using the star charts printed in newspapers to do some stargazing you have to correct to allow for the fact that the stars rise two hours earlier each month. 86.151.51.24 (talk) 16:43, 16 December 2015 (UTC)
 * A day is actually 24 hours and 1 millisecond near this decade and since OP wanted the "non-fictional day" then it has to be said. This is not unit-mixing, noone uses the 1/86400th of a real solar day second anymore. The 24 hour day isn't any more "real" than the 365 day year. If we kept using the fictional units then the seasons would be 1 month wrong in only a century and change and the time would be 8 hours off about 3 or 4 millenia ago or a similar distance in the future. A 24 hour day is clearly close enough most of the time, though. Sagittarian Milky Way (talk) 17:15, 16 December 2015 (UTC)
 * It seems clear 86 is using some weird definition of a second to be 1/86400 of a solar day. (I.E. The length of a second varies.) While 86 is free to use whatever weird definitions they want to, they shouldn't confuse the issue by pretending their definition is normal. It's not. The second which most people i.e. "Joe Public" use, including the UK government and people (and not just the BBC) is the SI one. On other words, this definition is not some weird definition that ony scientists use, it's the definition that nearly everyone uses. It is for example what nearly all modern clock (with the except of sundials) etc aim for. While I believe it's fairly unlikely a clock you're using has the precision that it makes a difference (although I'm not an expert on how long the length of a solar day varies), this doesn't mean such confusion is acceptable. Nil Einne (talk) 19:46, 16 December 2015 (UTC)
 * I don't think Nil Einne lives in Britain.  There was a discussion a few days ago about what all those books are on the table of the House of Commons.   They are law books.   If you look up Halsbury's Laws or Halsbury's Statutes (which is basically what the clerks do) title "Time" you will see that this country has been on Greenwich Mean Time since about 1880.   Now a mean time scale doesn't have a place for leap seconds, so there aren't any (don't need 'em).   Everybody uses Greenwich Mean Time because that's the law of the land.   Saying they use leap seconds implies that  once every eighteen months or thereabouts the whole population rushes about adjusting its timepieces by one second and if you actually live in this country you will see that they don't.   There is a mechanism for disseminating time which does involve leap seconds - the Greenwich Time Signal - but if you asked the BBC if they disseminate the legal time they will say "no, we don't."   If you ask them why they don't disseminate the legal time they will say "just after we insert a leap second the time we disseminate is a fraction of a second behind the legal time, and just before we insert a leap second the time we disseminate is a fraction of a second ahead of the legal time.   Clocks are made to run to mean solar time and the legal time is in line with what is displayed by clocks."
 * Some people are making a big issue of this and saying that there is a huge discrepancy between BBC time and legal time but this is simply not the case.  Every century the number of days in a year reduces by about 1/2 - second.   There are roughly 30,000,000 seconds in a year so after 100 years the length of the second increases by a factor of 1 in sixty million.   After one year the second has grown by just one part in 600,000,000,000.   Why are we getting het up about this? 86.151.51.24 (talk) 21:43, 16 December 2015 (UTC)
 * I agree that leap seconds are not a big issue since the discrepancy between UTC and UT1 is always less than one second. I disagree that clocks are made to run to mean solar time.  They are synchronised to UTC.  Perhaps you are thinking of sun dials?  By the way, UTC is not just BBC time, it is understood world-wide, and often (incorrectly in your view) called GMT. UTC "is the primary time standard by which the world regulates clocks and time".  I agree that GMT is taken to be UT1 for navigational purposes, but GMT is often considered identical to UTC for civil purposes, despite what old laws said. Why are we arguing over a fraction of a second?   D b f i r s   23:01, 16 December 2015 (UTC)
 * I don't get this.  If there is an EC directive relating to "apples" and a shop sells "apples", people coming in wanting to buy will ask for "apples".   Nobody is going to call them pears.   Mean time (without leap seconds) is as different from atomic time (with leap seconds) as apples are from pears.   I note the weasel words creeping in:

"old laws".

The law against killing dates back to Biblical times. It doesn't matter how old it is - if you break it you go to jail.

"Nearly all modern clock".

How is my bedside quartz alarm clock going to assimilate a leap second? Or my wind - up alarm clock, come to that. Or my Apple watch, mobile phone or computer? Computers aren't programmed to accept a leap second. If you want them to do so you have to patch them, but nobody bothers. People adjust their clocks and watches primarily from their mobile phones, or a timecheck from a radio announcer, or a platform clock or (in London) the clock on a bus. If you look at these platform clocks you will see that the seconds displays are not synchronised, so the suggestion that they are influenced by leap seconds is ludicrous.

As for

UTC"is the primary standard by which the world regulates clocks and time"

that unsourced claim was removed from the article with the following result:


 * Another editor edit - warred it back in (with no source)
 * The Arbitration Committee protected the article for a very long time.

So what price "Wikipedia:The free encyclopaedia that anyone can edit" and "all contributions must be sourced"? 81.151.101.74 (talk) 14:09, 17 December 2015 (UTC)


 * ... so you admit to being a sockpuppet of User:Vote (X) for Change? Do you really have only old wind-up clocks and non-radio quartz clocks in your house?  Most modern mobile phones have a GPS receiver and so pick up GPS time.  They then make the correction to UTC by subtracting 17 seconds.  That's a fourth source of UTC to add to my three above. Fifthly, my computer seems to pick up UTC by automatically synchronising with an Internet Time server.  It's only eight years old, so I suppose this is "modern".  It seems to me that UTC is becoming more common on modern equipment.   D b f i r s   16:07, 17 December 2015 (UTC)


 * It doesn't matter what the exposure to UTC is.  Digital radio is new but receivers are incapable of receiving an accurate time signal.   Your argument is based on the sole premise that modern clocks are manufactured to keep atomic time as opposed to mean solar time.   Given that the common quartz unit is accurate to ten seconds a month, what's the basis of that claim?   The typical non - mechanical clock/watch does not contain a radio receiver.   Do you know how much more expensive a radio - controlled clock is compared to one with a standard motor?   The typical non - battery timepiece has an electric motor which drives the hands according to the frequency of the current passing through it.   Such a clock is incapable of adjusting for leap seconds.   That is why the master clocks of the electricity companies show mean time, not atomic time.


 * Adjusting GPS time to UTC is a joke.  The time between leap seconds is so long that the processors overflow and produce spurious readings.   The companies that provide time signals to internet - connected devices do not send leap seconds because the devices cannot handle them.   The reality is that most, if not all the timepieces in homes and offices show GMT/BST and when leap seconds come along no effort is made to adjust them. 81.151.101.74 (talk) 17:01, 17 December 2015 (UTC)


 * On that last point, that's only because your average person doesn't care about a second here or there. But if the leap seconds were to accumulate to 60 and thus into the next minute, rest assured that plenty of us would adjust our clocks so as to not miss the bus or the start of our favourite TV program. Akld guy (talk) 18:32, 17 December 2015 (UTC)


 * You seem to misunderstand how these things are done. Mains synchronous electric clocks are not accurately adjusted to any particular time because the frequency varies according to load. The distribution companies do try to keep the average frequency to 50 Hz (in the UK) so that a synchronous electric clock matches their reference clock as closely as possible (usually within 30 seconds).  The reference clock is most probably a radio clock (UTC) rather than a quartz oscillator clock that is adjusted to UT1, since this is much simpler than trying to adjust the oscillator to an exact fraction of a day.  Do you really have no modern timepieces in your home? The companies that provide time signals to the internet send the time (UTC), with SI seconds, and include leap seconds when necessary.  Radio clocks have come down in price remarkably over the past few years.  Mobile phones with GPS receive GPS time which uses the SI second.  It is possible that there is a delay (seconds, hours, days, or until the next software update?) before a leap-second is implemented since the adjustment is within the phone.  I agree that if shops and offices use older timepieces, then they probably don't adjust them to the second, but if leap seconds become more common (as Akld guy says above), then they will eventually need to be adjusted by hand to take account of these seconds, or the regulator adjusted to the new UT1 if exact time is important.    D b f i r s   20:47, 17 December 2015 (UTC)
 * It's also worth considering how people set their clocks in the first place. while perhaps the OP has clocks which never run out of batteries, most people including in the UK almost definitely aren't like that. If they aren't simply using their computers, mobile phones and tablets; but instead do have wall clocks, desk clocks, watches etc running on batteries or perhaps on mains they will undoubtedly set them on occasion. Not because of leap seconds but because the clock lost accurate time either because of a loss of power or because the clock isn't very accurate long term and so is no longer in sync. When they do that, as their source of time, they may use the radio or TV which due to transmission delays may in some cases be out of sync but which will undoubtedly (whether they are BBC or Sky or ITV or whatever) be set to UTC or UTC+1. Maybe a few people will use some telephone service presuming you still have that in the UK. Again, this will very likely be set to UTC or UTC+1. Or probably most likely, people will simply use their phones or tablets or computers. And these will almost definitely be using UTC or UTC+1 synchronised from the internet or perhaps GPS or the mobile network. (As has been mentioned, these may not add leap seconds immediately but they will at some stage.) The number of people using a UT1 or UT1 +1 time source must surely be fairly small. The difference itself is very small, but that doesn't change the fact everything is computerised and almost definitely using UTC as the ultimate base source. Nil Einne (talk) 16:01, 19 December 2015 (UTC)


 * 81, you seem to be conflating 2 different things; namely the existence of leap seconds and the definition of a second. My post here basically only referred to the definition of the second. (Although I'm pretty sure you're also wrong about the UK not using leap seconds.) The definition of the second used in the UK is in most cases the SI one. And it is indeed what nearly all modern clocks aim for although as I said, pretty much all of them lack the precision that it makes any difference. In other words, whether or not you choose to add leap seconds, your clock is still trying to follow a second that is "the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at rest at a temperature of 0 K." It's not trying to follow a second who's precise length varies in a way that the clock itself can't even know unless it's a very fancy clock (but in which case it's even more likely to be following the definition of a second that aims for consistency). Just to repeat, whether or not you choose to add leap seconds to your clock is somewhat besides the point, albeit as mentioned by others if you don't do so in the long term you would find yourself out of sync with the rest of the world including the rest of UK. Although in most cases it likely lacks the precision or long term accuracy that this really matters and since we're talking about very long term here we don't really know how things will change in the future. BTW, it doesn't necessarily matter when the law itself was written. If the law was written to say the second is "1/86 400 of the mean solar day" or whatever, then yes this would need to be changed or updated or it would still apply. However if the law was written to say "as defined by X government body" or whatever, then the definition would be in government regulations and could change without a law change. Nil Einne (talk) 15:40, 19 December 2015 (UTC)


 * From the original poster - Thanks, all. 2602:252:D13:6D70:258E:2FDC:D3C8:55C9 2602:252:D13:6D70:CD48:DA6A:4989:2B3D (talk) 16:30, 18 December 2015 (UTC)

Will global warming slow down or speed up the earth's rotation speed?
Some news outlets (for example, , ) affirm that global warming will speed up the earth's rotational speed while others (for example , , )affirm that it will slow it down. They can't very well both be right. Contact Basemetal   here  03:51, 16 December 2015 (UTC)


 * First let's kill the echoes. The first two that say faster cite Felix Landerer at Max Planck Institute ( http://onlinelibrary.wiley.com/doi/10.1029/2006GL029106/full ) as of 2007; the latter four that say slower  link to http://advances.sciencemag.org/content/1/11/e1500679 more recently.  My assumption is the argument for slower is that glaciers are near the poles and when they melt the water has to go somewhere further out; the argument for faster is that the ocean overall expands, and water presently in deep basins ends up washing into the shallower Arctic Sea, North Atlantic etc.  Both studies are what they are and we'd be hard pressed to settle the issue amongst ourselves. Wnt (talk) 04:45, 16 December 2015 (UTC)


 * They can't both be right, but they could both be wrong. Reminds me of National Lampoon's newspaper satire, in which articles on consecutive pages had different scientists claiming that we were on the virge of global boiling and global freezing. ←Baseball Bugs What's up, Doc? carrots→ 05:30, 16 December 2015 (UTC)


 * Global boiling: Only 1 billion more years. Global freezing: a few billion after that. Sagittarian Milky Way (talk) 05:35, 16 December 2015 (UTC)


 * Actually they can both be right. When melted, water becomes much more dense, but then it becomes less dense.  Once the Antarctic ice is all melted, bringing rotation to its slowest rate, the oceans will continue to expand and thus redistribute themselves, speeding it up again.  Whether it takes a a billion years or a hundred, I'd guess global boiling should reverse the sign of things a third time by moving the water far up into the atmosphere. Wnt (talk) 09:55, 16 December 2015 (UTC)
 * They might both be right at future points in time, but not at the same point in time. ←Baseball Bugs What's up, Doc? carrots→ 09:57, 16 December 2015 (UTC)
 * At the same time it can be true both that it "will speed up" and "will slow down". Wnt (talk) 13:54, 16 December 2015 (UTC)


 * Expands should correlate with slower. The mass doesn't change. When an ice skater puts her arms out she spins slower, when she brings them in by her side she spins faster. However looking at the arguments I think I agree the redistribution of the water would make a bigger difference than the ice melting so my opinion is on the faster side. I think it should just about be possible to measure the changes so far and figure out who is probably right. Dmcq (talk) 12:49, 16 December 2015 (UTC)


 * Why would redistribution of ice have any noticeable affect? It isn't like the ice on the surface amounts to much. Mt. Everest, the largest feature jutting out of the surface above sea level, sticks out 0.1% of the mean radius of the Earth. That is negligible. I feel that many people (even those who are very educated) picture the surface of the Earth as a very thick layer. It is more like the peel of an apple. If you add a few crystals of ice to the outside of an apple, the spin speed won't change significantly. Similarly, if you redistribute a very very thin layer on the outside of the Earth, it won't mean much as far as spin goes. 209.149.113.52 (talk) 13:42, 16 December 2015 (UTC)
 * The effect is, of course, exceedingly small; however, the length of day is something that can be measured with exceedingly high precision. It is not uncommon for people to write about the effect of an earthquake on the length of a day, and this is more than that.  Because measurements really can be taken, it is also a way to probe or at least verify models of the distribution of the oceans, etc.  It is only important in an academic sense. Wnt (talk) 13:54, 16 December 2015 (UTC)


 * This is the problem with popular reporting of science. It is particularly egregious with health and medical reporting (especially nutrition) but it clearly evident above.  When science reports "cause X produces effect Y", the media reports this as "ONLY cause X produces effect Y, and there are no other causes or effects, and here's everything you need to know, and nothing else matters!!!".  So you get reports like this complete and utter bullshit titled "A Glass Of Red Wine Is The Equivalent To An Hour At The Gym, Says New Study"  No, it fucking doesn't, so stop that.  The study showed that one ingredient in red wine, reservatrol, had certain physiological effects on a few body systems that mimic strenuous exercise.  What the study does NOT SAY is "There is no difference in your body between drinking red wine and physical exercise, so just drink red wine and it's all the same thing".  The science never said that, but the popular press comes out and basically says exactly that.  Because journalists (as a subset of the entire population) have no idea how experimental science works.  A properly constructed experiment " typically include controls, which are designed to minimize the effects of variables other than the single independent variable." to quote the Wikipedia article.  That means, that when scientists publish a study, the study is almost always focusing on a single independent variable and carefully controlled to reduce, minimize, or control all other variables.  So when scientists run an experiment as they did in the above studies regarding climate change and the speed of the earth, what they do is cafefully control for one aspect of climate change; or in these cases one aspect of warming oceans, and then see what the effect of that one single variable is on one single effect.  In this case, the rotation of the earth.  It turns out that depending on how you define your single variable, you can produce speeding and slowing effects.  So which is actually happening?  The answer is: both speeding and slowing, because both of those variables come into play, as well as probably dozens or hundreds of other variables that we didn't test for in these experiments"  So, when you ask science a vague question like "Will the earth speed up or slow down because of climate change" the honest answer is either "It will do X so long as nothing else happens as well" or "We can't predict exactly because there are lots of factors at play, but we do know that this cause would lead to it speeding up because of XYZ, or this other cause could lead to it slowing down because of ABC."  The sort of "tell us the future perfectly so we can stop worrying!" expectation of science is a complete misunderstanding of how knowledge grows... -- Jayron 32 13:43, 16 December 2015 (UTC)


 * Not to lose sight of the elephant in the room, tidal friction dwarfs all other effects, so that in the long term the earth's rate of rotation always slows down.  There are short - term fluctuations - unpredictable events such as earthquakes and tsunamis have an effect as also, I believe, do wind systems.   In the 1920s the rotation rate speeded up quite considerably.   If that were to be replicated, instead of leap seconds delaying our clocks they would speed them up.   Thus headlines like "Wait a second, 1999 is going to be a little late" would change to something like "2019 is going to arrive a little early". 86.151.51.24 (talk) 17:02, 16 December 2015 (UTC)

Journal Articles
How to find journal articles for free about agriculture? Googled, but still hard to find. Any specific sites'd be much helpful... Thanks!

Learnerktm 10:14, 16 December 2015 (UTC)
 * Have you tried https://scholar.google.com/ ? It's not part of the regular google search, and google no longer advertises the service on their main page (they used to), so it is hard to find, but Google Scholar is where I go to search academic journals.  -- Jayron 32 12:52, 16 December 2015 (UTC)
 * Yes, google scholar is great, but it's also important to mention that many articles will be hosted behind the paywalls of academic publishers. So, when I search for e.g. /wisconsin corn/, I see that some of the article listings say "[PDF] from (place)" on the right hand side. Those articles have PDFs that are freely accessible at places like researchgate, but you have to click on the PDF link, not the article title.
 * However, additional articles may also be free! For instance many journals have an "open access" option, wherein someone pays an additional fee to make sure the article is freely accessible. It is hard to tell which articles in a google scholar search have this property. One way is to click through to the journal, and then see if you can download it. Another (often better) way is to look at a specific journal. For example Agroecology and Sustainable Agriculture has an easy way to browse their open access content . PLoS One and PLoS Biology are not focused on agriculture, but they do publish some agricultural research, and all their articles are freely accessible.
 * Taking this one step further, a regular google search for /open access agriculture journal/ gives some good hits, and the open access journals search engine for agriculture  will probably be very useful.
 * Finally, if OP finds an article that is not accessible, they can ask for a copy at WP:REX. Sometimes, once you know the article you want, a full-text search of the title on regular google will find an accessible copy of the article. SemanticMantis (talk) 14:56, 16 December 2015 (UTC)


 * Let's not forget other sorts of interlibrary loan, whether conventional or via Sci-Hub or Libgen... Wnt (talk) 22:18, 16 December 2015 (UTC)

Energy expended per newton per second
Hello. Considering that human muscles are very inefficient, how many joules does an average human being have to actually expend to exert a force of 1 N for a duration of 1 second? Thanks.Leptictidium (mt) 14:01, 16 December 2015 (UTC)
 * Read the Wikipedia article Muscle, which has all the necessary figures to help you answer the question, and probably answers it directly anyways. -- Jayron 32 14:07, 16 December 2015 (UTC)

What sense is my betta fish using to detect the arrival of food?
I put this on the science desk because I guess it's a biological question. My bowl is basically a large sphere about 10" in diameter and when the fish, which is about 2" in length is anywhere, even down at the bottom, when I drop its food, which is 3-4 pellets of betta fish food, each pellet being maybe 1mm in diameter, within about a second, the fish comes up to where the pellets are. Is it likely by smell, i.e., some molecules from the pellets propagating through the water, by sight (there's a fake plastic plant in the middle of the bowl that doesn't seem to slow it down), or by feeling that something has been dropped on the surface? 131.131.64.210 (talk) 14:08, 16 December 2015 (UTC)
 * I don't know directly (and it may vary between different species of fish), but I will direct you to reading the article lateral line. The first sentence of that article "The lateral line is a system of sense organs found in aquatic vertebrates, mainly fish, used to detect movement and vibration in the surrounding water."  So, IF the fish is using the vibrations caused by the food hitting the water to know it is being fed, it would be the lateral line that is doing that.  It could also be a combination of several senses, because you do this too i.e. you know dinner is coming BOTH because you saw your mom go into the kitchen, and you heard her rummaging in the cabinets for pots and pans, and you smelled the dinner cooking.  It wasn't just one, but all of these senses that led to your conclusion.  It could be the same for the fish.  But the lateral line article is an interesting read, not the least of which is because it's a sense system unlike anything humans have.  -- Jayron 32 14:12, 16 December 2015 (UTC)
 * Any angler knows many a fish will bite at a shiny lure just after it strikes the water, so the scent is not, at least, required for all of them. But there are some kinds for which live fish bait seems to work better.  Hearing among fish varies widely - some have inner ear and lateral line integrated in a way that suggests a common octavolateralis organ; others have otolith and lateral line separate.  If a swim bladder is present, it may be used as a sort of tympanic membrane.  I haven't looked into the betta fish at this point... from a first search, it looks like there might be some assocation of the labyrinth organ (a lung) and the lateral line, but I'm not sure yet. Wnt (talk) 14:47, 16 December 2015 (UTC)
 * What about sight? Fish can certainly learn to recognize when food is coming.  My fish come to the top of the tank whenever I pick up the food container because they have learned that this (or some other visual cue) means they are about to get food.  Maybe your fish is just seeing that something was dropped onto the top of the water and it knows this means dinnertime.  Deli nk (talk) 14:55, 16 December 2015 (UTC)


 * I did some searching, and I'm convinced you won't find published results for this scenario with your type of fish, your type of bowl, etc. The above comments are all apt, but they don't tell us which sense is most important, or if all are used, or if some are not important at all. However, you can get empirical and do some experiments! You may not come up with a publishable, fully controlled scenario, but you can manipulate things like what the fish can see outside of the bowl, placing in unscented, inert pellets, blowing air on the surface to cause pressure waves without dropping anything in, etc. This is just a sketch, if you are actually interested in experimental design for this situation, let us know, and I think we could help with that too. My gut feeling (WP:OR) is that sight, sound/pressure, and scent are all used to varying degrees. So if you dropped in some dirt pellets, the fish may well come up to investigate, but no eat. Most animals use several sensory cues to find food, not just one. SemanticMantis (talk) 15:36, 16 December 2015 (UTC)
 * But I'd start by getting something bigger than 10" and replace the plastic plant with a real one, to make it more fun for the fish. Contact Basemetal   here  17:12, 16 December 2015 (UTC)


 * Most likely it's mainly visual. Standard fish behavior is to investigate anything that looks like it might be food.  If it still looks like food close up, the fish takes it into its mouth and tastes it.  If it tastes like food the fish swallows it, otherwise spits it out. Many types of fish also have taste buds on their skin, but my guess is that they don't come into play in this situation. Looie496 (talk) 18:22, 16 December 2015 (UTC)

How did Races/ phenotype features originate?
I was wondering how humans from different parts of the world came about with different features. The answer probably isn't short and sweet and may deal with complex cellular interactions but I am interested in at least learning to gain some insight from someone on here or a at least a secondary source where I could gain more information.

I was wondering if changes in a person's DNA that leads to genetic expression of facial features, melanin regulation, skin folds around eye, etc. was factors of environment, diet, etc.

Say if I were to travel to Africa as a Caucasian male, what traits would I have a proclivity to acquire?

Thank you — Preceding unsigned comment added by 99.229.130.56 (talk) 16:52, 16 December 2015 (UTC)


 * None. Being in a different climate won't change your genetics. ←Baseball Bugs What's up, Doc? carrots→ 16:55, 16 December 2015 (UTC)


 * He meant "his descendants", in the long run. Contact Basemetal   here  17:08, 16 December 2015 (UTC)


 * His descendants will also be the descendants of the person(s) he has sex with. Their genetic make up (and the make up of whoever those descendants have sex with) will also come into play.  Assuming no pedigree collapse, his grandchildren will only have 1/4th of his genes, great-grandchildren will only have 1/8th of his genes and so on.  After several centuries, his unique contribution to the genetic pool of his descendants will be diluted to where his contribution becomes less and less relevant.-- Jayron 32 17:41, 16 December 2015 (UTC)


 * You guys... What the guy wanted to know is what genetic traits would be tend to be acquired in the long run by a Caucasian population migrating to Africa. Is that clearer? Contact Basemetal   here  17:50, 16 December 2015 (UTC)


 * If he meant that, he should have said that. I have no way to read his mind beyond what he typed.  He used the singular pronoun "I" and repeatedly referred himself in the singular moving to Africa, not an entire population moving to Africa.  If he wants to clarify, he can do so, but it is a disservice to him (and rather rude too!) to accuse him of saying something he didn't say, and not taking his question at face value.  It is inappropriate to try to guess at what people mean, instead of what they actually say.  -- Jayron 32 18:00, 16 December 2015 (UTC)


 * You mean the OP is a proponent of Lamarckism and I am being rude? Contact Basemetal   here  18:09, 16 December 2015 (UTC)


 * There you go again, saying words that someone else didn't say, this time me. I didn't say those words either.  I said entirely different words.  Please re-read them again.  -- Jayron <b style="color:#090">32</b> 18:13, 16 December 2015 (UTC)


 * Some relevant articles - the general notion of races splitting apart is similar to that of speciation. So things like sexual selection could be involved in some cases, while things like allopatric speciation could be involved in divergence of other traits. This line of reasoning is in terms of Race_(biology), which is basically equivalent to subspecies.


 * However, keep in mind that Race_(human_categorization) is primarily viewed by modern scientists as a social construct. That is not to say that different lineages don't have different phenotypes, it just means that dividing people by skin color isn't biologically meaningful or useful (much like dividing dogs by coat color is not biologically meaningful. We all readily accept that a black lab is very different from a black poodle, and it would be silly to consider all black dogs as one breed).


 * Moving on, Afro-textured_hair talks a bit about why that might be useful from an evolutionary perspective. Human_skin_color talks a bit about why we think there may have been evolutionary benefits to different skin colors in different places. Our article on white people mentions with references that it is whiteness that is the derived trait - Europeans were darker skinned rather recently.


 * For a more general perspective on variation in human traits, you might be interested in Recent_African_origin_of_modern_humans and Early_human_migrations - these set the stage for different lineages developing different phenotypes, and the resulting Human_genetic_variation. Just for fun, you might also take a look at Archaic_human_admixture_with_modern_humans, and notice that most of us have some Neanderthal or Denisovian alleles, in addition to our "modern human" alleles. SemanticMantis (talk) 17:44, 16 December 2015 (UTC)


 * The eye fold is called the Epicanthic fold and it is not known why some populations have them. Richerman    (talk) 18:57, 16 December 2015 (UTC)


 * Some features that would develop in a population living in Africa, if people still died in large quantities from not having them:


 * 1) Dark skin, as this protects against skin cancer. However, skin cancer can now be prevented with clothing, sunscreen, and staying indoors most of the time, and can be treated as well, so any evolutionary pressure for skin color to change over generations is now minimal.


 * 2) Wide nostrils. This allows for breathing in more air which in turn provides cooling and the oxygen needed to run for extended periods of time, say to bring down game.  There is also little evolutionary pressure to develop this feature now, as herding or getting your food from the store eliminates this necessity.  Of course, being able to run for extended periods of time might save your life during a war, so this feature might still be important in areas with constant war.  As for people living outside tropical climates, breathing in air that fast in cold weather could lead to hypothermia, frostbite, etc., so isn't advantageous there.


 * 3) Curly hair. This transforms sunlight into heat away from the skull, where most of the heat radiates away, thus keeping the brain from overheating.  Less important now that we have hats, air conditioning, etc.  StuRat (talk) 16:23, 17 December 2015 (UTC)


 * Any references for ANY of that? -- Jayron <b style="color:#090">32</b> 16:45, 17 December 2015 (UTC)


 * Maybe he got it from a bit that I think Richard Pryor used to tell. It involved a dark-skinned man asking God about these various features he has, and God explaining them, as StuRat did. The punch line was the man asking God, "So what am I doing in Harlem?" ←Baseball Bugs What's up, Doc? carrots→ 17:31, 17 December 2015 (UTC)


 * I'll believe wide nostrils allow Africans to breathe in more air right around the time when I start seeing white athletes wearing some kind of butt plug like device to try to stretch out what they have. Until then... :) Wnt (talk) 16:18, 18 December 2015 (UTC)


 * Not athletes, but sleepers use strips to open their nostrils more, so they can breathe better: . As for athletes, getting more air past the nose would only help if the rest of their respiratory systems had also evolved to make use of that extra air. StuRat (talk) 07:06, 19 December 2015 (UTC)


 * Here's a picture of retired Belgian striker Émile Mpenza who, not satisfied with his African nostrils, is using a breathing strip on top. Cheater! Contact Basemetal   here  07:34, 19 December 2015 (UTC)

What's the difference between an optician and an optometrist?
The second sentence sounds like a complicated version of the first. What's the real difference? &mdash; Sebastian 20:09, 16 December 2015 (UTC)
 * ❝Optometrists are trained to prescribe and fit lenses to improve vision [...]❞
 * ❝Opticians determine the specifications of various ophthalmic appliances that will give the necessary correction to a person's eyesight.❞


 * As our articles sort of suggest, the precise difference varies from country to country. However optometrist normally have more training and would generally be better at assessing eye health and detecting diseases (often for referral to an opthamologist). In a number of countries, an optician may only be able to dispense an existing prescription from an optometrist. Nil Einne (talk) 21:13, 16 December 2015 (UTC)


 * In my experience in Canada, an optometrist prescribes what glasses (or contact lenses) you need, and an optician sells them to you according to the prescription. It is just like the relationship of doctor and pharmacist as regards prescription drugs.  (You can also go to an ophthalmologist for the prescription; that's an actual doctor specializing in the eyes.) --76.69.45.64 (talk) 01:19, 17 December 2015 (UTC)
 * What you describe is what my ophthalmologist does; and that also fits to what Nil Einne writes. Does ophthalmology equal, include or overlap optometry? If it's true that an optician does not write prescriptions then I'd say we should delete that second sentence I quoted above. &mdash; Sebastian 02:19, 17 December 2015 (UTC)
 * Our article about opticians says:
 * "Recent changes to the British Columbia Opticians regulations allow qualified opticians in that province to test a persons vision and prepare an assessment of the corrective lenses required for a client. Using the results of the assessment an optician is able to prepare and dispense eyeglasses or contact lenses. Opticians in Alberta are also permitted, under certain conditions, to refract and prepare and dispense eyeglasses and contact lenses."


 * Which seems to imply that they can actually determine what sort of correction is needed in certain conditions in certain places in Canada. I hesitate to say they can "write prescriptions". Although they sort of are, it may be that they are only able to test the vision, decide on what sort of correction is required and prepare the suitable appliances and then fit and test these appliances on the wearer. While they would have written a prescription internally, it may be that they aren't actually able to give this as a written prescription.
 * Our article doesn't seem to mention any other areas where opticians can assess and provide corrective appliances without an existing prescription, however it only mentions 3 other countries and in one of them (Ghana) it's not clear to me what opticians can do.
 * But anecdotally, I can say it's fairly common for opticians in Malaysia to carry out the assessement etc themselves. I haven't checked the law, and Malaysia is a place where stuff can happen despite clearly being against the law. But I wouldn't be that surprised if the law does allow this. I would expect some other developing countries have similar systems.
 * I probably should also mention that while in NZ, only optometrists can make prescriptions, there have been suggestions at various times that this should be changed and opticians should be allowed to tests the eyes and effectively make prescrptions albeit perhaps only after the patient has seen an optometrist (or I guess opthamologist) to assess eye health  sometime recently. Nil Einne (talk) 15:23, 17 December 2015 (UTC)
 * Nil Einne (talk) 15:23, 17 December 2015 (UTC)


 * My experience is more or less Nils' and 76's:
 * an optician can make glasses and such, but not prescribe them.
 * an optometrist can prescribe glasses and such
 * an ophthalmologist is a physican qualified to diagnose diseases of the eye and treat them.
 * Anything an optician can do, an optometrist can do; anything an optometrist can do an opthalmologist can do.
 * I suspect the second statement and the first are not meant to be (or sound like) identical; I would guess that the second statement meands that opticians can, for example, determine the distance between pupils in order to fit glasses. - Nunh-huh 02:47, 17 December 2015 (UTC)

Why do progressive lenses have peripheral zones?
Zeiss states that peripheral zones are because of "aberrations" (presumably monochromatic aberrations, but which?) which in turn are caused when "the difference between the radii of curvature in the horizontal and vertical directions grows". Why does it grow to begin with? Can't they just make it so it doesn't grow? &mdash; Sebastian 20:09, 16 December 2015 (UTC)

A progressive lens tries to minimize the visible boundary between different regions, unlike a bifocal lens for example. So everything has to change gradually from one zone to the other ... I think. But optics is an art, and I'm not an artist. Wnt (talk) 22:17, 16 December 2015 (UTC)

Voice pitch of brunette and blonde women
I got an impression that naturally brunette women (roughly beyween ages 18 and 35) have slightly higher voice pitch than natural blondes of comparable age (regardless of language differences). Is it to a certain extant true or bullshit? 93.174.25.12 (talk) 21:10, 16 December 2015 (UTC)
 * I suspect this might be a case of confirmation bias. Not suggesting that it's completely impossible for two relatively disparate traits to be correlated, just that if they were, we'd probably have noticed by now. Googling what does voice pitch correlate with, it looks like it's mostly down to height, taller people have lower voices. Maybe most brunettes you know are shorter than average? Vespine (talk) 21:43, 16 December 2015 (UTC)
 * I'm thinking blondes are Nordic, Nordic is tall, etc., but ... it's not really science. I mean, because it matters what town you live in, where the blondes and brunettes came from, what their population structure is etc.  You could probably find some lists of single nucleotide polymorphisms for these things in 23andme data and try to do a raw correlation coefficient, but all you'd get is the all too common sort of near-irreproducible result that links a genetic trait to something else in one experiment only. Wnt (talk) 22:14, 16 December 2015 (UTC)
 * I've recalled Reference_desk/Archives/Science/2009_October_28. 23:49, 16 December 2015 (UTC)

Perception of own health in an isolated group
Let's say we have a person with some health defect from birth, such as congenital blindness, urinary incontinence, etc. For the sake of experiment, he/she is raised in a group isolated from the rest of society (similar to the Allegory of the Cave) and is being told that blindness or whatever he/she suffers from is normal, that everyone is like him, it's a normal state, etc. Provided the rest in the group suffers from the same defect and there's a continued isolation, would he/she realize by himself/herself at some point that he/she should actually see in case of blindness, contain his urine in case of urinary incontinence, etc? Maybe there were such experiments already? Brandmeistertalk  23:27, 16 December 2015 (UTC)
 * This idea was explored in The Country of the Blind, for example. ←Baseball Bugs What's up, Doc? carrots→ 01:38, 17 December 2015 (UTC)


 * In principle, such a collection of blind people could develop science, perform tests and experiments - and wonder why they have these organs in their heads that appear to have no function whatever - they might note that there are large nerve bundles and huge sections of the brain that connect to them. They might even notice that there is a light-focussing lens at the front of these two structures.
 * I don't think it's unreasonable that they might deduce that these organs were evolved to collect and focus light. If they had access to study animals that can see - then they might eventually come to some conclusions about their own deficiencies.
 * Sure, we can conclude that without vision, it might be much harder for them to develop this science - but it's clearly not impossible and if it's not, then I think they'd eventually realize that they've somehow failed to have all of the connections necessary for these organs to function properly.
 * SteveBaker (talk) 03:17, 17 December 2015 (UTC)
 * Does it mean that we indeed perceive our health through what healthy people around tell us or there's some innate, internal indicator thay may give us clues even in an isolated group? Brandmeistertalk  09:45, 17 December 2015 (UTC)
 * Well, it's a bit tricky to say. We know, for example, that most color-blind people don't know that they're color-blind until some aspect of their behavior is noted by a normally-sighted person - or until they hear descriptions of the world from a normally-sighted person.  So in that specific case, we can be fairly sure that color-blind people don't have some inherent "I know there is something wrong with my vision" sense that tells them that there is a problem "inherently".
 * Another example is people with Asperger's syndrome (I'm one of them) - and most of us don't realize the extent to which we're "emotion-blind" until we're told about it. Until I was diagnosed, I really had no idea that the rest of humanity has this betazoid-like ability to sense emotions in other people from insanely subtle body language and voice inflection.  A population of 'aspies' on a distant island would certainly never know they had something broken in their brains.
 * But then, for conditions that cause pain in some situations - I could certainly imagine people wondering why they feel pain under that situation and not others. There might be other indicators - for example if one foot is painful to step on and the other one isn't - some way that lack of symmetry would provide an indication that something isn't right.
 * But it's hard to tell because there aren't usually large groups of people with identical problems that are so completely cut off from the rest of the world that they never (even in the past) had contact with people who are "normal".
 * One of the great examples I like to consider is Lactose Intolerance. We think of the lactose intolerant as having some kind of a disability - but in truth, all mammals are lactose intolerant after they are weaned from their mothers.  A fairly recent genetic mutation has provided much of humanity with the ability to consume milk after that age - so in a sense, all of us except the lactose intolerant have a broken gene.  Now you have to argue whether the lactose-tolerant mutants have noticed that they have this "problem" or not?  Clearly not.  Of course we now have other societal means to ensure that older siblings stop drinking their mother's milk when they get old enough to find food independently - and lactose intolerance is a pain in a society that farms animals for their milk.  But we certainly don't have an "inherent" indicator that most of us are freakish mutants. SteveBaker (talk) 18:21, 17 December 2015 (UTC)
 * The difficulty might be that health is a 'spectrum', there is no "line" which a healthy person crosses to become unhealthy. For example, someone who weights 80kg is healthy, but someone who is 81kg is not healthy, even though we can quite easily say someone who is 160kg is almost certainly unhealthy. Look at it this way, In the past, if you lived to 55 you might have been considered quite old, but in a lot of places now, if you die at 55 it's quite premature. In the future, it's not at all difficult to imagine everyone living healthy to 100, then we will consider dying at 85 quite premature. Vespine (talk) 00:45, 18 December 2015 (UTC)
 * Yes, but I think our OP's point is that if everyone weighed 160kg and died at 55 - and everyone we'd ever known or heard about had been the same - would we consider ourselves "unhealthy"?  I don't think we would - but it's hard to reason about.  I maintain that in this mysteriously cut-off collection of people, they could develop scientific techniques that would inform them that if they could all lose weight, they'd live a lot longer - and therefore they might start to consider themselves "unhealthy" even though they had no healthier people to compare themselves to.  But is there some "innate" sense of "I am unhealthy" if everyone looks exactly the same to you?  I don't think so. SteveBaker (talk) 15:15, 18 December 2015 (UTC)
 * Agreed, and I would take it a step farther. Humans love to rationalize things - give them explanations, even if those explanations are based on myth or hearsay. My bet is that this hypothetical group, even if some individuals in it figured out something might be "off" in some way, would instead rationalize that they were indeed just fine. Consider the folks even today who consider the human body a temple created by an unerring god rather than a hodge-podge of best-fits and make-do projects left over from the wrath of natural selection. 99.235.223.170 (talk) 18:05, 19 December 2015 (UTC)