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# June 21

## Lines in front of Moon

I took these photos of the Moon a few days ago. What are the thin lines in front of the Moon - contrails? clouds? (Ignore the UFO in the lower left.) Bubba73 You talkin' to me? 01:30, 21 June 2022 (UTC)

Obviously, the "UFO in the lower left" is a hologram projector. 2603:6081:1C00:1187:1807:C437:3907:422A (talk) 07:25, 21 June 2022 (UTC)
It's either a Jupiterean lens flare or he's a time traveler. Slight time travel. Sagittarian Milky Way (talk) 07:39, 21 June 2022 (UTC)
Or it's a lightbulb. Sagittarian Milky Way (talk) 07:42, 21 June 2022 (UTC)
Oh it's not today. The time on the camera is probably wrong. Or it's probably south of USA for that moon to be rising so early. Sagittarian Milky Way (talk) 08:01, 21 June 2022 (UTC)
Mountain Time Zone and near southeastermost Arizona? Hawaii? Sagittarian Milky Way (talk) 08:09, 21 June 2022 (UTC)
Judging from the position of the terminator, the picture was taken on 16 June around 4 UTC, give or take a few hours. 21:30 local time on the 15th somewhere at the longitude of the USA is plausible. If the moon was rising that early, the picture must have been taken at low latitude. I just compared the pictures to the simulation in Stellarium. Judging from the orientation of the moon and the time it rose, Jacksonville FL is plausible, or somewhere near Austin TX or in the southwest of New Mexico. For each time zone, there's a line running southwest-northeast where the moon rose at that time and the orientation of the moon tells us it must have been around 30° North latitude. Unfortunately, the pictures aren't very sharp. For Pacific time zone, it appears that this line reaches the coast too far north, all assuming the picture was taken this month. A nice puzzle, but OP can tell us exactly where the picture was taken.
As for those clouds, the thin lines look like a far-away contrail. It gets wider from the first image to the last. The cloud appears pretty much stationary with the moon rising behind it, an effect of the large distance of around 500km to the cloud. It's only about a degree above the horizon. There is some forward-scattering of light in some cirrostratus clouds visible around the moon, possibly older contrails. The other blob looks like a remnant of a cumulus cloud only a few tens of kilometres away or a man-made cloud from some chimney. It's clearly moving, without any significant forward-scattering of light. PiusImpavidus (talk) 10:48, 21 June 2022 (UTC)
Good analysis of the time and position! The photos were taken from St. Simons Island, on the east coast of Georgia (US state), latitude about 31 degrees. Those times came from my camera - I need to check to see if they are accurate. The Moon had just risen, so it is low in the horizon, with a lot of atmospheric distortion. Bubba73 You talkin' to me? 15:39, 21 June 2022 (UTC)
The times were about 1 hour off (camera still had EST) - corrected now (EDT on the evening of June 15). Bubba73 You talkin' to me? 15:46, 21 June 2022 (UTC)
Right, I found that Stellarium didn't properly apply DST either, so the position still fits. PiusImpavidus (talk) 16:17, 21 June 2022 (UTC)
Maybe there are two thin lines because this was a twinjet. Vertical separation between the lines appears to be around 1/40 of the altitude or about 250m. That's a bit much, but the wingtip vortices can move the trails apart and if the aircraft was banking, one trail would move up and the other down. My guess is that it was Air Europa flight 98 from Miami to Madrid, a Boeing 787-9 Dreamliner at 37000 feet, which made a right turn at about the right time and place. All those flights to the Norteastern US only fly about 300km away from your observing location and would appear a bit higher in the sky, at about 2 degrees. After a while, the trails from both engines merge. PiusImpavidus (talk) 17:01, 21 June 2022 (UTC)
• You did some really good work on this! BTW, about the UFO - I couldn't tell what it was with the naked eye. With my strong telephoto lens on my camera, I could tell that it was a shrimp boat, fishing at night with lights on. Bubba73 You talkin' to me? 00:56, 23 June 2022 (UTC)
Identified non-flying object. Sagittarian Milky Way (talk) 02:24, 23 June 2022 (UTC)

## How can rectum be hotter than core?

[1] Sagittarian Milky Way (talk) 07:45, 21 June 2022 (UTC)

It is not as if there is a heat-producing core reactor, with the heat being spread to the rest of the body by thermal conduction. All living tissue produces heat, mainly through metabolic activity. I don’t know how the "core" temperature measurements presented in the Welch Allyn chart were obtained; perhaps from Houdas, Y., Ring, E. F. J; Human Body Temperature, Its Measurement and Regulation; 1982; pp. 81–87, which they cite as a source.[2] I imagine that the temperatures in the liver and the pancreas are not necessarily the same. In any case, I cannot think of an argument why the rectal temperature should not exceed core temperature.  --Lambiam 08:41, 21 June 2022 (UTC)
The difference is consistently 0.4 degrees F? That's quite small. Could it simply be explained by sampling error? Martinevans123 (talk) 08:46, 21 June 2022 (UTC)
That page nowhere defines 'core' temperature. Where and how is it measured? 08:50, 21 June 2022 (UTC)
Houdas and Ring give this definition, from the standard established by the International Union of Physiological Sciences and published as a "Glossary of Terms for Thermal Physiology"[3]: Core temperature or deep body temperature    The mean temperature of the tissues at a depth below that which is affected directly by a change in the temperature gradient through peripheral tissues.[4] The "Glossary" adds this: Mean core temperature cannot be measured accurately, and is generally represented by a specified core temperature, e.g., that of the rectum. [°C] That really raises the question where Welch Allyn got their numbers from.  --Lambiam 14:00, 21 June 2022 (UTC)
A rare case in which the numbers were literally pulled out of their arses?
(I'll get me coat) {The poster formerly known as 87.81.230.195} 90.201.73.76 (talk) 22:58, 22 June 2022 (UTC)
I incorrectly assumed core temperature was the point of highest temperature in the human. Where would that be? Liver? Furthest point from the skin? Where's that? Sagittarian Milky Way (talk) 20:07, 21 June 2022 (UTC)
If the environment is very hot it will be the skin? Presumably all these standard temperatures are measured in a defined benign environment? Martinevans123 (talk) 20:11, 21 June 2022 (UTC)
If the skin is the hottest, the body temperature must be rising (unless there's some internal cooling, like breathing really cold air or pumping cold water through the intestinal tract). There would be nowhere for the waste heat to go. Not something you can survive for long. PiusImpavidus (talk) 18:42, 22 June 2022 (UTC)

I just saw this today, see what you think! https://www.sciencealert.com/new-research-shows-your-brain-can-run-a-lot-hotter-than-we-ever-realized 211.30.161.13 (talk) 07:08, 22 June 2022 (UTC)

It gives a new sense to the term hothead.  --Lambiam 07:59, 22 June 2022 (UTC)
Goodness me... if that means getting a flattened Rubik's cube stuck in me swede, I think I'd rather put up with a hot rectum, thanks. Martinevans123 (talk) 18:47, 22 June 2022 (UTC)

## Precordial catch syndrome

While the pain of costochondritis can be severe, it does not appear suddenly like the sharp, stabbing pain of precordial catches. Bouts of the latter also disappear quickly, in a matter of minutes, whereas the pain of costochondritis tends to linger for extended periods, possibly for several months. Since the same symptoms (for either of these conditions) may be caused by conditions that are a medical emergency, it is cautious, if experienced, to see a medical professional, who can also give advice on pain management.  --Lambiam 20:02, 21 June 2022 (UTC)

## JP-7

Did JP-7 contain any natural or synthetic diamantane homologues? 2601:646:8A81:6070:F5AD:972D:6C45:ADB3 (talk) 22:49, 21 June 2022 (UTC)

The original JP-7 recipe dates to 1955; the first laboratory synthesis of diamantane was in 1965—so I'm going to go out on a limb and speculate that the JP-7 spec didn't call for synthetic diamantane derivatives.
I can't find a copy of the original 1955 specifications, but the relatively recent MIL-DTL-38219D, DETAIL SPECIFICATION: TURBINE FUEL, LOW VOLATILITY, JP-7 doesn't mention diamantane or other diamondoids and their derivatives specifically.
The JP-7 spec does call for low volatility and high thermal stability (which is unsurprising, given its working environment). Naturally-occurring diamondoid compounds certainly wouldn't hurt in achieving those necessary stability and volatility thresholds, so I wouldn't be surprised if there were trace amounts of natural diamandoids (including diamantane species) in JP-7. TenOfAllTrades(talk) 12:00, 23 June 2022 (UTC)
Thanks! So, probably not in the original recipe, is that correct? (But if we need to make a new fuel with similar specs to JP-7, then synthetic diamantanes would very likely be one of the ingredients!) 2601:646:8A81:6070:640D:F1C9:3B1D:1044 (talk) 02:25, 24 June 2022 (UTC)

# June 22

## When is a group too large?

Is there a scientific study on the maximum size a group of people can be in order to organize themselves?--2A02:908:422:9760:3487:FE15:1D22:12D6 (talk) 20:10, 22 June 2022 (UTC)

There's Dunbar's number (about 150), and here's some academic paper which is trying to back that number up, by looking at the size of people's Christmas card mailing lists.  Card Zero  (talk) 21:17, 22 June 2022 (UTC)
While I'm not aware of specific scientific studies, an immediate issue is to give an operational definition of self-organization in a sociological sense. It is not at all evident that self-organization requires that every individual member of the group knows who each person is and how each person relates to every other person in the group. Viewing the group abstractly as a dynamic system, the evolution of internal relations may bring the system to a relatively stable equilibrium, or it may remain chaotic. I suppose – without specific theoretical backing – that much depends on whether the group already shares a sense of a common destiny, a common purpose or even a vision grounded in equality, or whether it is "each to their own" from the beginning. I further suppose that one can train people in efficient methods of self-organization, so that a much larger group of people, if many have received such training, will be able to readily organize themselves than without such training.  --Lambiam 11:39, 23 June 2022 (UTC)
There's about 40,000 active editors on English Wikipedia, but we can't organize ourselves, so I guess that's too many.  Card Zero  (talk) 12:33, 23 June 2022 (UTC)
• For a first approach of the subject, I recommend checking out the work of fr:Mehdi Moussaïd if you can understand French (there’s a few articles in English on his website but much is untranslated). He is best-known for the Youtube vulgarization channel "Fouloscopie" (foule = crowd, so a rough translation of the channel name is "Crowdscience"), but he is a serious researcher at the Max Planck Institute, so he does give bibligraphies and such.Now that he’s Youtube-famous, he can leverage his community to run experiments with lots of people without paying them - quite a smart move if you ask me. TigraanClick here for my talk page ("private" contact) 13:44, 23 June 2022 (UTC)
And speaking of Dunbar's number, it is probably not a coincidence that a company is usually not more than 100-150 men! 2601:646:8A81:6070:640D:F1C9:3B1D:1044 (talk) 02:28, 24 June 2022 (UTC)

## Star color

What causes the change in color as stars get hotter from red to yellow to white to blue? Despite many online sites stating the fact that as temperature increases the color changes I cannot find what makes temperature change the color of the star. 172.112.210.32 (talk) 20:56, 22 June 2022 (UTC)

It's pretty much straight blackbody radiation. It's not just stars, but any matter that tends to follow the same changes in color with temperature. TenOfAllTrades(talk) 21:39, 22 June 2022 (UTC)
And just to make the point explicit, the temperature that matters is the temperature of the star's visible surface, not the layers hidden below it, which generally are much hotter. --174.95.83.56 (talk) 04:02, 23 June 2022 (UTC)
Apparent temperature may be affected by relativistic Doppler effect; (i.e.: redshift / blueshift)*  --2603:6081:1C00:1187:5D0E:1FD1:561A:AB20 (talk) 07:36, 23 June 2022 (UTC)
*not to be confused with redfish / bluefish
See also Planckian locus (not to be confused with the Black locust).  --Lambiam 11:19, 23 June 2022 (UTC)
Also see Wien's displacement law. It's just blackbody radiation. As the temperature of the gas gets higher, so does the temperature of the radiation it emits. The relevant temperature is the temperature of the photosphere, which is the layer from where radiation can escape to space as the mean free pathlength of the light gets longer than the scaleheight of the atmosphere. The apparent temperature can be affected by Doppler shift, but this effect is so small that under normal circumstances you cannot see the effect on the broad-band colour. You can see the effect of movement on the spectral lines and in case of white dwarfs the gravitational redshift may be visible in spectral lines too. PiusImpavidus (talk) 11:41, 23 June 2022 (UTC)

# June 23

## Redox Double Displacement reaction

A double displacement reaction can never be a redox reaction. Is this true only for the Modern Concept (electronic concept) or also for the old concept(addition and reduction of Oxygen and hydrogen)?--ExclusiveEditor Notify Me! 13:13, 23 June 2022 (UTC)

As the article salt metathesis reaction makes clear, this sort of reaction is usually ${\displaystyle {\ce {AB + CD -> AD + CB}}}$, driven by the insolubility of one of the products. These are not redox reactions as typically no change in valency of the individual components occurs. I would hesitate to say they could "never" involve a redox change, for example if one of the products AD and CB were very susceptible to, say, oxidation by air in an experiment where air had not been excluded. Mike Turnbull (talk) 16:33, 24 June 2022 (UTC)
If the bonds are all ionic, then there's clearly no redox because the four entities (A, B, C, D) each are unchanged. If we get into covalents though (which, oddly, salt metathesis reaction includes in-scope), then redox is certainly possible (simple example is Halogenation#Free radical halogenation). User talk:ExclusiveEditor, could you give us the source of this "can never" claim, so we can see its context/scope and academic level? DMacks (talk) 17:23, 24 June 2022 (UTC)

@DMacks: Actually, I heard that somewhere, still a quick search on the internet provides this source- Resource1 and many answers to this Quora query say no - Qoura1. --ExclusiveEditor Notify Me! 13:34, 25 June 2022 (UTC)

Those seem to fit well with the "ionic"(-like) concept. DMacks (talk) 14:01, 26 June 2022 (UTC)

## Testicular motion

I got home from work about an hour ago. It's been a really hot day so to cool down I took my clothes off and reclined on my bed. Although I was motionless apart from my breathing, I noticed that my testicles were constantly moving back and forth around every 20~30 seconds. It's not something I've ever noticed before. I'm aware that testicles obviously move, and I've just been reading our article about the cremasteric reflex, but I didn't realize just how much they move. Or rather, I wasn't aware that they moved continuously. What's going on here? Is it just because I'm a bit too hot? nagualdesign 15:05, 23 June 2022 (UTC)

Yes. Your genetic survival depends on the temperature of your testicles. 21:30, 23 June 2022 (UTC)
I'm aware of that, but why the continuous motion? I would have thought that they'd reach some sort of equilibrium. Is it easier to reach equilibrium at a lower temperature, or do they always move? I'm in my 40s and have never witnessed it before. nagualdesign 22:16, 23 June 2022 (UTC)
They are drawn in towards the body to get warmer, released outwards to cool. Presumably they are also in motion to take advantage of cooler areas of the scrotum where air circulation (and sweating, in animals that sweat) has allowed more cooling. Since the body does not know exactly where these cool areas are, moving them around decreases the odds of one testicle or one area of a testicle will overheat and kill cells. I recall hearing that once the scrotum has relaxed entirely in the heat, the movement mechanism (which has a name, I think) is engaged. 23:32, 23 June 2022 (UTC)
See Cremaster muscle (which is the "movement mechanism") and Testicle#Temperature regulation and responses. Martin of Sheffield (talk) 06:13, 24 June 2022 (UTC)
Thank you for the responses but none of them tell me anything I didn't already know. Everyone knows that they move up when it's cold and down when it's warm. What I'm asking is do they always move up and down continuously? Abductive gave the most interesting answer but it seems like mostly supposition. If anyone can provide links to more concrete information that would be great. It might even be worth adding some information to Cremaster muscle and Cremasteric reflex if any is forthcoming.Pun intended nagualdesign 21:58, 24 June 2022 (UTC)
...Perhaps I should add that about 20 years ago I found a lump on my left testicle. I went to see a doctor and she squeezed it, which hurt like a motherfucker, and dismissed it as a cyst. I then suffered from severe inguinal pain for almost a year and struggled to walk (tip: do NOT see a female doctor for a men's problem!) until I had an ultrasound scan, which revealed that it was actually a varicocele, and was told how to flush out any "melancholic blood" to stop the pain. A few years earlier my brother had also suffered from epididymal orchitis (extremely painful!), so he and I are keenly aware of testicular physiology and have even seen the inside of our respective testes. All this is to say that I'm looking for information that's perhaps less commonly known. I've tried googling several things but all you get is the same information that should be very obvious to any adult male. nagualdesign 22:17, 24 June 2022 (UTC)

## Asteroid speed variation in it's hyperbolic trajectory from infinity and passing near the sun

I find a lot of article describing asteroid hyperbolic trajectory, comming from infinity and passing near the sun, in a two body problem. But nothing describing it's speed variation from speed Vi (at infinity and not relativistic) to it's maximum speed Vr when it is closest to the sun, and again to it's original speed at infinity. Malypaet (talk) 21:31, 23 June 2022 (UTC)

I think you'll find Mr Kepler figured that out for you. Greglocock (talk) 23:23, 23 June 2022 (UTC)
Its speed and trajectory depend on its initial speed and location. Bubba73 You talkin' to me? 02:42, 24 June 2022 (UTC)
Let ${\displaystyle E_{\text{k}}}$ and ${\displaystyle U}$ denote, respectively, the kinetic energy and the gravitational energy of the body. Their sum, ${\displaystyle E_{\text{k}}+U,}$ is constant. Let ${\displaystyle G}$ denote the gravitational constant and ${\displaystyle M_{\odot }}$ the solar mass. The variable ${\displaystyle R}$ stands for distance of the body from the sun, and ${\displaystyle v_{R}}$ for the magnitude of its velocity at that distance. Dividing ${\displaystyle E_{\text{k}}+U}$ by the mass of the body, we have:
${\displaystyle {\frac {1}{2}}v_{R}^{2}-{\frac {GM_{\odot }}{R}}=C,}$
in which ${\displaystyle C}$ is a constant, not varying with ${\displaystyle R.}$ Plugging in ${\displaystyle R=\infty ,}$ we find ${\displaystyle C={\frac {1}{2}}v_{\infty }^{2},}$ and so:
${\displaystyle v_{R}=\left(v_{\infty }^{2}+2{\frac {GM_{\odot }}{R}}\right)^{\frac {1}{2}}.}$
--Lambiam 11:10, 24 June 2022 (UTC)
Or you can write
${\displaystyle v_{R}=\left(v_{\infty }^{2}+v_{esc}^{2}\right)^{\frac {1}{2}}.}$
where ${\displaystyle v_{esc}}$ is the escape velocity at the distance ${\displaystyle R}$. Ruslik_Zero 20:18, 24 June 2022 (UTC)

# June 24

## Is it at least theoretically possible to turn CO2 into fuel?

Could we, at least in theory, capture the CO2 of a running internal combustion engine and turn it back into some sort of fuel? Obviously, I see that you'll have to add energy to process CO2 back to fuel, but is the idea as crazy as time travel or simply in the ballpark difficulty of fusion energy or Mars travels? --Bumptump (talk) 12:04, 24 June 2022 (UTC)

It's similar to why you cannot have perpetual motion energy generators: C + O2 -> CO2 + energy; in order to reverse the reaction CO2 + energy -> C + O2, you would have to give it back the energy you got from it in the first place. Hence, no energy is left over, and because nothing is 100% efficient, you would lose energy overall. 12:12, 24 June 2022 (UTC)
• That's why I said: " Obviously, I see that you'll have to add energy to process" The idea is to use it as a sort of battery. --Bumptump (talk) 17:07, 24 June 2022 (UTC)
See Carbon capture and utilization for how it is done. Graeme Bartlett (talk) 12:26, 24 June 2022 (UTC)
Some researchers are hyping this up as a serious prospect, see this website. To be practical, you would need some very inexpensive energy source to drive the process. Mike Turnbull (talk) 17:22, 24 June 2022 (UTC)
It can be done. Capturing the CO2 from a running internal combustion engine is a bit hard (you use an ICE because you have some mobile application; CO2 is a gas and hard to store on such an application), but you can blow the CO2 into the air and capture it later. That's even less efficient, but if this captured CO2 is the only viable or legal source of hydrocarbon fuel and hydrocarbon fuel is still the only technically viable aviation fuel, it will happen. Renewable electricity is very cheap at peak production times. The price often goes negative these days. You can build an electrolyser next to your wind/solar farm to avoid having to sell the electricity at a negative price; you only switch it on at peak production times. PiusImpavidus (talk) 18:23, 24 June 2022 (UTC)
You can do this by replacing the combustion engine with a horse that eats grass and pulls your wagon. But you have to accept a big reduction in your energy usage. If you want to use a combustion engine and an artificial process to turn the CO2 back into fuel, that is also possible, but you'll still have to accept a much lower energy usage than what we're accustomed to. To go with the battery analogy, all the petroleum in the ground is an enormous battery that has been slowly charged up over a very long time, and we are currently discharging it very, very quickly. We can recharge it, but we don't have a way to recharge it as quickly as we're draining it now. --Amble (talk) 19:03, 24 June 2022 (UTC)

Well, carbon fusion and oxygen fusion both exist, in the cores of massive stars late in life, so yes, "at least in theory" you can use CO2 as fuel. --Trovatore (talk) 19:46, 24 June 2022 (UTC)

## Request for the research and thesis suggestions

Hello, I hope this is good place to ask this question as well. As a student doing masters in geoinformatics I will try to do thesis in crop remote sensing. I am doing another masters in agronomy (other being online) , what thesis area in agronomy will complement me later? Will it be appropriate if I choose precision agriculture in the agronomy thesis. I have general ideas what to be done. I was requesting how both thesis later in overall suits each other developing me as a more fluent in two domains.

I am not from other developed nations, there would probably have been graduate and career advisors. In my case, there are none, so I genuinely wanted some suggestions. If anyone wishes to suggest more, personal advice, there is email associated with the account. Any kind comments are welcomed. Even if none, thankyou. - Learnerktm 12:31, 24 June 2022 (UTC)

The central issues in the areas of remote sensing in geoinformatics are quite distinct from those of precision agriculture, so it seems to me that the domains are largely complementary. Since remote crop sensing can certainly find use in precision agriculture (check this out), it seems to me that the combined mastery of both can prove a valuable asset.  --Lambiam 22:49, 24 June 2022 (UTC)

not just in precision agriculture, do you have other kind suggestions that would suit each other? Learnerktm 11:03, 25 June 2022 (UTC)

Not exactly career advice, but I feel that while writing a Master's Thesis requires getting deep into a subject, it pays off in the end to keep a broad interest while pursuing the things you believe to be of interest, not what others tell you to be interested in. Some people can develop theory, pushing its boundaries, which is very valuable. Other people can use theory to create methods that are applied in practice, and that is equally valuable. What should you aim at? Find out where your passion is, and follow your passion – that is the best way to make a difference.  --Lambiam 20:56, 25 June 2022 (UTC)
Really thankyou for the nice advice. I will keep this in mind. :) Learnerktm 21:41, 25 June 2022 (UTC)

## Does E=MC^2 violate conservation of energy?

This has always confused me. The first law of thermodynamics says that energy cannot be created or destroyed. But E=MC^2 says that energy can be created from mass. Assuming my understanding is correct, I've never heard anyone explicitly state that energy cannot be created or destroyed except for general relativity. A Quest For Knowledge (talk) 20:01, 24 June 2022 (UTC)

It is not created from mass. It just always exists. Ruslik_Zero 20:09, 24 June 2022 (UTC)
Conservation of Energy explains this, albeit with a lack of decent citations. Classical physics says energy is conserved and mass is conserved. Relativistic physics (probably there's a more accurate term) says mass-energy is conserved. --Floquenbeam (talk) 20:11, 24 June 2022 (UTC)
Indeed, in classical (Newtonian) physics we say mass is conserved and energy in conserved. In special relativity, we say that mass-energy is conserved, i.e., mass–energy conversion can occur just as one form of energy can be converted into another (e.g., gravitational potential energy into kinetic energy), but the total mass–energy of a closed system must be conserved and thus ${\displaystyle E=mc^{2}}$ does not violate the conservation law. ComplexRational (talk) 20:21, 24 June 2022 (UTC)
The obvious way of solving the apparent contradiction is to consider mass as a form of energy.[5]  --Lambiam 22:57, 24 June 2022 (UTC)
Individual photons do not have an invariant mass and are therefore massless and only carry an energy in accord with the relativistic Doppler effect. That said, in general, mass and energy are equivalent per mass-energy equivalence and neither are created or destroyed. See center-of-momentum frame. If the COM frame exists for a system or body of particles, the system has a total energy and an equivalent invariant rest mass. Moreover, the equivalency holds whether or not particles are stable or undergo fission, fusion, or creation/annihilation. Even a system of photons, such as the gamma rays emitted after an antimatter-matter annihilation event may have a COM frame thus invariant mass. Modocc (talk) 01:40, 25 June 2022 (UTC)
COM frame always exists. Ruslik_Zero 20:05, 25 June 2022 (UTC)
From our article on center-of-momentum frame: "Systems that have nonzero energy but zero rest mass (such as photons moving in a single direction, or equivalently, plane electromagnetic waves) do not have COM frames, because there is no frame in which they have zero net momentum." Modocc (talk) 21:54, 25 June 2022 (UTC)
Ok, for a system with a non-zero invariant mass it always exists. Ruslik_Zero 12:17, 26 June 2022 (UTC)

# June 26

## Water lilies

Are water lilies edible? If so, have there ever been attempts to cultivate them for food, or is this only relevant in the context of wilderness survival? 2601:646:8A81:6070:D151:4B2:4CF:6657 (talk) 01:37, 26 June 2022 (UTC)

I found that the seeds and flowers and rhizomes of the Fragrant water-lily Nymphaea odorata are eatable (traditionally by N American tribes) and the roots are used medicinally as a tea to treat coughs and diarrhea, but my Peterson Field Guide warns that large doses can be toxic. Modocc (talk) 02:20, 26 June 2022 (UTC)
I've eaten Lotus Root Chips (Americans would probably call them fries) "served with lemon & herb seasoning and a side of Wasabi mayo" at the Blue Lotus Water Garden, a tourist attraction not far from where I live in Australia. But I have now discovered that lotuses are no longer considered to be water lilies. So maybe not so helpful. HiLo48 (talk) 02:34, 26 June 2022 (UTC)
Our Lotus-eaters article has water lilies as a possible candidate for the eponymous lotus plant that they are supposed to have lived on. Greek mythology is not a good source for survival skills however. Alansplodge (talk) 07:31, 26 June 2022 (UTC)
So, some are edible and some are not -- got it! So, one of 2 questions answered -- the other one being, whether water lilies have been cultivated for food? (And yes, for the purpose of this question, all 3 plant families on the dab page are considered water lilies, so lotuses do count!) 2601:646:8A81:6070:4C2C:9973:74AD:CB94 (talk) 07:15, 27 June 2022 (UTC)
The N American tribes knew how to cultivate plants but had their lands taken from them. According to Peterson, the Ojibwa ate the fragrant water-lily's flower bud which has been attributed to preventing scurvy among them. Modocc (talk) 11:04, 27 June 2022 (UTC)
Nymphaea nouchali is cultivated for food in Sri Lanka. Modocc (talk) 14:42, 27 June 2022 (UTC)
Thanks! So the answer to the second question is also yes -- they have been occasionally cultivated for food (which makes sense, in swampy areas they could be grown instead of potatoes)! 2601:646:8A81:6070:6DDC:7C7B:BDD3:2E97 (talk) 01:19, 28 June 2022 (UTC)

I wonder did you see this video? Regards. Martinevans123 (talk) 14:47, 27 June 2022 (UTC)

It's my understanding that unchecked and abandoned cultivars (if such water lilies exist/ed) can revert back to their wild forms so as to become inedible, which might explain any differences between the historical and contemporary accounts assuming they are accurate. Modocc (talk) 17:05, 27 June 2022 (UTC)
From what has been said above, some wild species of water lily are indeed edible, and there has been cultivation but not selective breeding of these, so the question of cultivars vs. wild forms is not applicable here. 2601:646:8A81:6070:6DDC:7C7B:BDD3:2E97 (talk) 01:19, 28 June 2022 (UTC)
Selective breeding is ancient and the fragrant water lily has not been completely free of toxin. Moreover cultivars can typically cross-pollinate with wild varieties so the cultivated plants are lost due to natural selection. In others words, little Fido is unlikely to survive in the wild. Like I said, the natives lost their traditional productive lands. Perhaps genetic studies will reveal lost subspecies/cultivars. My field guide is both new and a recent edition but it focuses on the plants' medicinal qualities, so I cannot confirm the sources that our articles are relying on. Modocc (talk) 01:49, 28 June 2022 (UTC)

## What's the lowest frequency broadcast by a skyscraper antenna?

The 129 meter tall World Trade Center antenna has VHF and UHF but not AM MF for some reason, maybe it'd be too impractical. Sagittarian Milky Way (talk) 08:13, 26 June 2022 (UTC)

There's an anticorrelation between AM and VHF-UHF, but it's not a hard rule: aviation comms are AM in the VHF band. But the LF-MF-HF broadcasters are going out. Lower frequencies have longer range, which is nice if your target audience is spread over a large area, but it also means there's only room for a limited number of transmitters in the world, or they would interfere. What's more, the available bandwidth is limited. Nowadays, if a radio station wants to reach people far away, they offer a live stream on the web. Ships and aircraft use satellite. The only real use left for lower frequency AM broadcasting is to spread independent news or propaganda to unfriendly countries.
There's only a small number of lower frequency transmitters as their range is very large. They need tall antennas and cause significant interference with nearby electrical equipment. Putting them on a skyscraper may not be so practical. PiusImpavidus (talk) 09:31, 26 June 2022 (UTC)
Maybe in some countries but in the United States there's so many stations between 1710 AM and 540 AM it's near capacity and every American radio can receive. A big metro area has dozens of them. It's used by people who want to watch sports games or talk shows but can't, old people, and a fairly popular far right (ish) radio network with sometimes offensive content like playing Barack the Magic Negro all the time (there's also National Public Radio for the left to center (public as in non-commercial donation-funded, not public like the BBC, though that's retransmitted in U.S. MF too I think)). Due to greatly increased ionospheric range at night only a few stations per clear channel per continent can use more than a few watts at night and those must use 10-50kW 24/7 so they can be heard far enough to get the special license. But if you want to follow a major sports team in a mundane game within a thousand kilometers of them while driving at night then you probably can. Sagittarian Milky Way (talk) 12:10, 26 June 2022 (UTC)
The aerials for lower frequencies are proportional to the wavelength. Droitwitch (which transmits at 198 kHz in the long wave band) has an aerial which is 590 foot (180 m) long for example. You'd need a pair of skyscrapers with the aerial stretched between them for LW! Martin of Sheffield (talk) 10:00, 26 June 2022 (UTC)
Do you know why parts of the world still have LW AM but not Oceania or the Americas? Those frequencies are used for extra non-directional beacon channels here. We do have a LW station or two in the States for atomic clocks but you probably can't hear LW on most of them so most US radios only go down to about 525 kHz. Sagittarian Milky Way (talk) 12:10, 26 June 2022 (UTC)
Well one of the reasons is to carry the shipping forecast. LW reaches far out to sea and is invaluable as a adjunct to NAVTEX, or for small boats as the only source. 198 kHz (used to be 200kHz) has carried shipping forecasts since 1911. Martin of Sheffield (talk) 13:12, 26 June 2022 (UTC)
I've heard about it. Never having been lucky enough to sail I don't know what's the American equivalent, I know there's frequencies for maritime use only. Our popular 60 kHz clock station is apparently not intended for humans, so the clocks don't let you hear the station. Sagittarian Milky Way (talk) 14:16, 26 June 2022 (UTC)
Because Great Britain and Ireland are small-ish islands, the BBC broadcasts of the Met Office's Shipping Forecast for the sea areas adjacent to land, and the "inshore waters" addendum that circles the British coastline in segments, are of interest to a great many landlubbers in the "NW Europe archipelago". Some of the main forecast's areas extend to the Atlantic, North Sea, Channel and Biscay coastlines of (South East) Iceland, Norway, Denmark, Germany, The Netherlands, Belgium, France and Spain, so there may be some interested non-mariner listeners there, too. {The poster formerly known as 87.81.230.195} 90.201.73.76 (talk) 19:18, 26 June 2022 (UTC)
(edit conflict) However, BBC Radio 4's long wave goodbye says that the world's supply of giant glass valves has been bought up to keep the Droitwich transmitter operating, and when the last of those breaks, BBC Radio 4's long wave transmissions in England and Wales will cease (estimated at 10 years in the article published 11 years ago). Scotland and Northern Ireland have smaller LW transmitters that are easier to maintain. Alansplodge (talk) 14:18, 26 June 2022 (UTC)
Hmm. Fleming's diode valves, 1904 look familiar ... compare martian from War of the Worlds, 1906. /OR  Card Zero  (talk) 15:41, 26 June 2022 (UTC)
Have you noticed that the cathode is outside (the loop) and the anode at the centre (the plate or squiggly bit). All diodes I've ever seen have the cathode in the centre (where it can be indirectly heated if required) and the anode surrounding it. Triodes, tetrodes and pentodes follow the same inside to outside construction (though obviously a little more complex when you get double-diode-triodes or similar). Martin of Sheffield (talk) 17:02, 26 June 2022 (UTC)

## Anatomical feature

What is this protrusion on the side of upper leg below the knee (marked with arrow)? From what I've searched, it could be either tendon of biceps femoris or iliotibial tract, but not sure. Thanks in advance. 212.180.235.46 (talk) 17:36, 26 June 2022 (UTC)

That's the biceps femoris tendon, partly shaded by the tassel of her handbag, making it look a bit like it protrudes more. nagualdesign 20:02, 26 June 2022 (UTC)

## Oh, the humidity!*

I have a building where the humidity needs to be fairly constant; ideally between 45~55% (realistically, 40~60%) Since the humidity is typically 60%+ year round, and 80%+ is not uncommon in summer, I had a dehumidifier installed with the HVAC. In the winter, however, when the (gas) heater is operating, the inside humidity can drop well below 40% despite it being around 65% outside. Why is that? Common sense might suggest that the heater somehow "dries the air", but since humidity is a measure of water vapor (H2O in gas phase), I don't see how heating the air somehow converts water vapor to something else. Surely it doesn't consume or "burn" the hydrogen and oxygen, right? If that were so, then we'd have a nearly inexhaustible supply of cheap energy. --2603:6081:1C00:1187:813A:ACFE:62DF:42DE (talk) 22:51, 26 June 2022 (UTC)

The quantity of water vapor air can contain is lower at lower temperatures. We can use a NOAA moisture calculator to find that a cubic meter of air at 5°C has a relative humidity of 50% then it contains about 3 g of water. If that air moves into a home, it will expand (I haven't figured out how much). If the air warms to 20° the amount of water in the air will still be 3 g, but the relative humidity will be about 20%. Jc3s5h (talk) 23:06, 26 June 2022 (UTC)
Hmmm... I'll have to think about that a bit. 2603:6081:1C00:1187:813A:ACFE:62DF:42DE (talk) 23:41, 26 June 2022 (UTC)
We need to clarify a couple of terms here. Absolute humidity is the amount of water vapour in a given volume of air. Typically measures in gm-3. Relative humidity takes saturated air as 100% and relates the current value to it. For example, air at 30 °C (86 °F) can hold 28 gm-3. A sample of air with 14 gm-3 at 30 °C therefore has a relative humidity of 50%. Now let's consider what happens in winter. Outside air might be at 8 °C (46 °F) and lets assume 100% RH. The absolute humidity is 8 gm-3, a cold dank day. The air is brought in and warmed up to 30 °C (a bit hot, but I have the figures to hand). It still contains 8 gm-3, but this is now 8÷28 of the maximum or 29% RH. The reverse effect is common experience in freezers. Room temperature air enters when you open the door. As the air cools it passes its dew point (100 %RH) and water vapour comes out of the air and builds up as ice on the inside. The article humidity explains this in far more detail. Martin of Sheffield (talk) 07:27, 27 June 2022 (UTC)
Since the inside and outside pressure are the same, you can use Charles's law to calculate the expansion from 5°C (278.15 K) to 20°C (293.15 K); it is by a factor of 293.15/278.15 ≈ 1.054, an increase in volume by 5.4%.  --Lambiam 08:52, 27 June 2022 (UTC)
Thanks, y'all. So, when air is heated, the proportion of H2O molecules remains the same; and, since the air expands, the number per unit of volume decreases (right?). 2603:6081:1C00:1187:9D25:B1B4:4C95:1B58 (talk) 07:04, 28 June 2022 (UTC)
No. Despite expansion being mentioned here, it's close to irrelevant to the change in relative humidity when air is heated up. As explained earlier, warm air can carry more water vapour than cold air. Thus, when cold air at 100% RH is warmed up, it is no longer carrying the maximum water vapour that it could, so its RH is below 100%. Remeber, RH is the actual amount of water vapour in the air divided by the maximum it could carry at that temperature.--Phil Holmes (talk) 07:40, 28 June 2022 (UTC)
The actual amount of water vapor doesn't change? 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 14:59, 28 June 2022 (UTC)
Only if it condenses out as its cooled. That's how air is dried in an air conditioning unit. Martin of Sheffield (talk) 15:18, 28 June 2022 (UTC)
I grasp the gist of "what", but the "how" and "why" still eludes me. Empirically, when outside cold and damp air is heated indoors, not only does the metric for humidity drop, the air feels much drier (and stuff that should not be exposed to too-dry air for long becomes vulnerable). 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 15:35, 28 June 2022 (UTC)

Remember this is the relative humidity that drops, the absolute humidity remains the same, there is the same amount of water in the air. As the air warms the relative humidity drops, and the air is able to take up more water, this is what evaporating is. For instance the moisture in your skin evaporates and the skin feels dry. When breathing very dry air, then the moisture in your lungs and passages evaporates and you experience a dry mouth or nose. Moisture on surfaces is also drawn into the air making them better insulators, with increased ability to store static electricity and deliver shocks. Martin of Sheffield (talk) 16:08, 28 June 2022 (UTC)

So, because warmed air has an increased capacity for water vapor (for some mysterious reason), water in liquid phase has a tendency to fill the gap (so to speak)? 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 19:15, 28 June 2022 (UTC)
Approximately. The real answer is tied up with vapour pressure and partial pressure, and that's a whole different ball game. Martin of Sheffield (talk) 20:24, 28 June 2022 (UTC)
Thank you for helping to fill the gap between knowing something and (mostly) understanding it.
Resolved
– 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 21:13, 28 June 2022 (UTC)

# June 27

## Why doesn't everyone go into eachother's Roche limit when their in crowds?

I mean, they are really close together, maybe in eachother's Roche limit! AtomicSphere 19:59, 27 June 2022 (UTC)

The simple computation of the Roche limit at Roche limit#Rigid-satellite calculation assumes that the satellite is in hydrostatic equilibrium, meaning in particular that it is held together only by its own self-gravity. That assumption is radically wrong for humans. Their self-gravity is completely negligible; they are held together by their tensile strength. --Trovatore (talk) 20:27, 27 June 2022 (UTC)
By extensional metaphor, we could model complicated behaviors using some kind of effective potential, describing the crowd's behavior in aggregate "as if" it were controlled by attractive or repulsive forces between individuals...
Such modeling methods are used in both practical- and artistic- applications. For example, big-budget motion pictures might use crowd simulation to animate large groups of people for visual effects in filmmaking. Some of these methods are specifically particle system models. If one were inclined, one could tease apart the equations and work the similarities- and differences- from an inverse-square-law model. Similar methods could be used for non-filmmaking purposes to model things like foot-traffic for architectural design, fire-safety, and so on.
But the big caveat is that the dynamics of a crowd of people are more complicated - a simple inverse square law would make for a poor and unrealistic model of the crowd. Therefore, a Roche limit-like horizon - at least, one that is a consequence of an inverse square law - would probably not emerge from a well-designed model.
Applied mathematics is full of techniques for studying complex systems - and equally so, it is full of attempts to extract simplified group-dynamics from diverse governing equations. A recurring theme is explored in chaos theory: even very simple governing equations lead to unpredictability. These emergent properties would make it hard, in any non-trivial system, to rigorously define any 'limit' analogous to the Roche limit we compute for an inverse-square-law system.
Nimur (talk) 21:01, 27 June 2022 (UTC)
Nimur, I'm not sure I've understood what you're getting at, but I don't think it's what the OP meant. As I understood it, the OP was asking why you don't get torn apart by tidal forces when you walk too close to another person. --Trovatore (talk) 21:49, 27 June 2022 (UTC)
Obviously we're in agreement that the actual force of gravity is irrelevant in these scenarios ... gravity is very weak.
But the point I was making is that we can make mathematical models for emergent behaviors - we can write relations to express whether characters walk toward- or away from each other. That is to say - we can model groups of people using methods that are normally used for particle-dynamics. Then, we can use these models to study how crowds form and dissipate. These kinds of approaches aren't (necessarily) tracking any "physical law" - the governing equations are purely synthetic, and usually selected because they make cool results. Anything we conclude from the results is only as good as our computer-model. Real human individuals don't obey simple mathematical laws governing their trajectories - but animated characters in a video-game or film might do!
Here's an example - this is a tutorial for animating crowds using Unreal Engine. Around the 1 minute mark, the animator shows "traditional" crowd simulations using targets and collision dynamics; ... around the 19 minute mark, you can see the animator/programmer playing with a particle simulator by controlling parameters like effective collision radii, which is much more performant...
These kinds of approaches can be used to model or simulate how a crowd forms (or disperses). If the collision radii is too small, the characters collide in an unrealistic way; but if that same parameter is too large, the crowd disperses!
I sort of imagine you could describe a "Roche limit" metaphor to describe the circumstances that cause a group to "disperse"...
In case it's not clear, I'll reiterate: these are not physically valid models of crowds - they're dramatic simplifications that follow simple programmed rules or equations. They're basically just simple equations that are useful for creating cool animations. If you were mathematically inclined, you could manipulate those equations ad infinitum to see how the emergent behaviors compare with other particle-dynamics models, like an n-body gravity simulation; and you could try to find analogous modalities.
Nimur (talk) 00:39, 28 June 2022 (UTC)