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November 1

Midnight sun in Norway

Directives for military officers and military commanders in the event of an armed attack on Norway has a completely uncited section discussing an alleged event from 1968 on the Russo-Norwegian border:

On the evening of 7 June, the garrison heard the noise of powerful engines coming from the manoeuvres...Actual observations were not possible over the border in the dark...At daybreak the impressive numbers of the Soviet forces staged along the entire border became visible.

Google Maps says that the southernmost point of the border is about 69°N, and based on Midnight sun, it looks like anywhere north of 67°13'N experiences midnight sun by the end of May. Consequently, this means that all points on the Russo-Norwegian border experience midnight sun on 7-8 June, so the whole scenario is impossible. Am I understanding rightly, or have I missed something? This isn't some recent vandalism; it's present in the first version of the page history, apparently translated from the corresponding article in the Norwegian Bokmal Wikipedia. Nyttend (talk) 18:59, 1 November 2024 (UTC)

The article on the Norwegian bokmål Wikipedia ascribes the difficulty in observing the cause of the hubbub to dårlig vær, bad weather. In the original version on the Norwegian bokmål Wikipedia, the difficulty is said to have been, specifically fog. BTW, in this original bokmål version the alleged incident took place on 7 June 1967. Half a year later, "1967" was changed to "1968" by a user whose only contribution was this change.  --Lambiam 20:33, 1 November 2024 (UTC)

User:Lambiam, there's also a no:Sovjets demonstrasjon av militær styrke ved den norsk-russiske grensen i 1968, with several sources. Do the sources confirm the year, or is 1968 an error? Maybe Theohein was just fixing a typo. Nyttend (talk) 21:25, 1 November 2024 (UTC)

The sources confirm June 1968 but appear to name 6 June 1968 as the date when Norwegian soldiers stationed along the border with the Soviet Union became alarmed by a sudden advance of Soviet tanks and heavily armed soldiers, stopping only within metres of the border. There is no mention of any difficulties in observing this. Apparently, the information has been kept classified for 40 years.  --Lambiam 07:00, 2 November 2024 (UTC)

November 2

Is there a name for 0.001 miles?

I need to work on a software that works in units of one one-thousandth of a mile internally. Is there a name for such a unit? --193.83.24.42 (talk) 00:35, 2 November 2024 (UTC)

American silliness? HiLo48 (talk) 00:45, 2 November 2024 (UTC)

As opposed to French silliness, such as the met-ray. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:09, 2 November 2024 (UTC)

The Roman mile was by definition a thousand paces (milia passuum). catslash (talk) 01:24, 2 November 2024 (UTC)

Actually, that's 1000 double steps, a bit under 1500m. --Stephan Schulz (talk) 22:40, 7 November 2024 (UTC)

The obvious millimile gets a little use. (The author there can't redefine span (unit) so easily though.) I also found it in a more modern book about chemistry, where it seems to be part of a quiz designed to test the reader's understanding of units: Which length is longer, a millimile or a decameter? but archive.org has stopped showing snippet views of in-copyright books. Millimole tends to pollute search results, which is perhaps why a chemist would be inspired to invoke millimiles.  Card Zero  (talk) 01:46, 2 November 2024 (UTC)

See pace (unit). When I worked as a surveyor, we often used this informal unit and with practice it became 99% accurate, good enough for most purposes. Shantavira|^{feed me} 09:21, 2 November 2024 (UTC)

There is an existing more accurate unit, used by the railways, known as the "link". Your unit is 8 links. The link is divided decimally and contains 7.92 inches. Your unit is therefore (7.92 x 8) = 63.36 inches. Before metrication, Ordnance Survey maps were scaled at 1 inch to a mile. The scale was therefore 1/63 360. 2A00:23D0:FFC:3901:90DF:CC65:72B0:11FF (talk) 14:51, 2 November 2024 (UTC)

Can I determine distance from size of a photographed object

If a photograph contains an object of known real size, can I use that to determine how far it was from the camera or other device that took the picture? Also, would this question fit better here or on the math reference desk? Primal Groudon (talk) 16:18, 2 November 2024 (UTC)

For one thing, you would have to know the type of lens. A wide-angle or fisheye lens makes things look farther away than they actually are. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:55, 2 November 2024 (UTC)

Even with a pinhole camera you couldn't, unless you know the distance inside the camera between the pinhole and the photographic plate or film. Define four variables:

• h_ext is the real size of the object;
• d_ext is the distance between the pinhole and the object;
• h_int is the size of the object's image;
• d_int is the distance between the pinhole and the photographic plate or film.

Then h_ext : d_ext = h_int : d_int.

If you have the values of three of these variables, you can determine that of the fourth. With simple fixed lenses, this also gives a reasonable estimate.

To determine the value of d_int, you don't have to look inside the camera. Just take a picture of an object of known size at a known distance and measure h_int. You can now calculate d_int.  --Lambiam 20:16, 2 November 2024 (UTC)

Empirical calibration is surely the best approach, but if your camera has a zoom, then you will need to ensure the same level of zoom is used for the measurement and the calibration. You can do this by checking the 35 mm-equivalent focal length in the exif meta-data in the jpeg files. catslash (talk) 23:49, 2 November 2024 (UTC)

Lambiam, what if the object is a building of known dimensions? If you can see 3 corners couldn't you solve for the relative position of the camera? Or do you need 4 points? fiveby(zero) 04:32, 9 November 2024 (UTC)

In general, we have a projective transformation that projects a 3D point $(x,y,z)$ in real space to a 2D point $(\xi ,\eta )$ on the film by a transformation of the form

$\xi ={\frac {d+ex+fy+gz}{1+ax+by+cz}},~~\eta ={\frac {h+ix+jy+kz}{1+ax+by+cz}}.$

Let $y$ denote the vertical coordinate in the 3D space and $\xi$ the horizontal coordinate on the film. Then, assuming the camera is not tilted, the value of $\xi$ is invariant under changes of $y$ while $x$ and $z$ remain constant, which implies that $f=b=0.$ Let furthermore $x$ denote the horizontal coordinate parallel to the film in the camera. Then the value of $\eta$ is invariant under changes of $x$ while $y$ and $z$ remain constant, which implies that $i=a=0.$ Every point in the straight line of sight from the camera, as well as in the vertical plane through that line, has the same value for $x$, and its image on the film is on a vertical line with constant $\xi .$ We can set both equal to $0,$ which implies that $d=g=0.$ The third 3D coordinate $z$ denotes the horizontal coordinate in the direction of sight of the camera. We can set $\eta =0$ for the height of the horizon on the film. This means that $\eta \to 0$ as $z\to \infty ,$ which implies $k=0.$ Finally, at a constant distance $z,$ real-world squares remain squares on the image, so we know that $e=j.$ A judicious choice of coordinates has led to the simpler projective transformation

$\xi ={\frac {ex}{1+cz}},~~\eta ={\frac {h+ey}{1+cz}}.$

Three unknown coefficients still remain: $c,e,h.$ Relating a real-world point with known coordinates $(x,y,z)$ to a point on the film with known coordinates $(\xi ,\eta )$ gives you two equations, so if my reasoning is correct, in general doing this for two points should suffice. It becomes more complicated if you don't know the coordinates of real-world points but only the distances between them.  --Lambiam 07:34, 9 November 2024 (UTC)

Furthermore, putting

$\lambda ={\frac {e}{c}},~~z_{0}=-{\frac {1}{c}},y_{0}=-{\frac {h}{e}},$

we can rewrite the transformation as

$\xi =\lambda {\frac {x}{z-z_{0}}},~~\eta =\lambda {\frac {y-y_{0}}{z-z_{0}}}.$

Fixing the value of $y$ for the ground level as being zero, $y_{0}$ should be the eye height of the camera, which is presumably easily measured, eliminating one more unknown. This might suggest just one real-world point $(x,y,z)$ needs to be related to an image point $(\xi ,\eta ),$ but the equations you get are not independent, since, if the values of $\xi ,\eta$ and $y$ are known, the value of $x$ can be computed without measuring it, using $x={\frac {\xi (y-y_{0})}{\eta }}.$.  --Lambiam 12:18, 9 November 2024 (UTC)

Here is a scenario, if you have an uncropped picture of the White House and you know the Camera Model and the Fixed Len used. You can buy the Camera and Len and take the same Picture at vairous distance of the White House until the your picture matches up to the target Picture. Then you can estimate the distance. 2001:8003:429D:4100:5CCA:727C:C447:3877 (talk) 22:07, 3 November 2024 (UTC)

There are two factors that change the size of an object in a photo when you fix the lens and zoom. One is the distance to the object. The other is the scaling of the photo itself. If I print a photo as 4x6 it will be much smaller than the same photo printed on 8x10. To measure distance, you really need another photo with known distance from the same camera with same lens and same zoom so you can measure the width of an object of known size on the size of the photo you printed. Then, you know the scaling factor to work out the distance to an object in another photo. Another option is to have two (or more) objects of known size at different distances where you know the distances between them. When in the same photo, you can use the sizes of the objects and the known distance between them to estimate the distance to the camera. 12.116.29.106 (talk) 18:26, 5 November 2024 (UTC)

Projective Geometry can help if you know the length of something in a picture and the shape of something, e.g. a square on the ground. NadVolum (talk) 18:47, 5 November 2024 (UTC)

Magenta vs. purple

How come mixing blue and red with equal gets magenta in modern color theory but a bluer purple (more violet-like) in RYB color theory?? Georgia guy (talk) 16:36, 2 November 2024 (UTC)

No colour theory is perfect, but, more importantly, it is ill-defined what it means to "mix" colours. Different physical procedures will also give different outcomes.  --Lambiam 20:20, 2 November 2024 (UTC)

it depends! Red in RYB is something "not (subtracting) blue" and blue is somewhat "not (subtracting) red". So the red takes the role of the blue in the other system and vice versa. If you constructed a special "blue pigment" with some green in it and a special red pigment with some orange in it, you would mix either the pigments and have a tone between magenta and purple or you would mix the reflected light from both pigments, and have a more bright version of exactly the same tone. I don't know if you can make the brightness the same too. So it doesn't have to be a different colour, but that only works with purple because of the complementarity. Generally the colours in both systems are very differently mixed. 176.2.70.177 (talk) 20:48, 2 November 2024 (UTC)

Reading Subtractive color and Additive color might help. Klbrain (talk) 01:08, 11 November 2024 (UTC)

November 6

Pornography addiction in animals

Has any scientific research ever been conducted showing that non-human animals are capable of addiction to pornographic imagery, in the same or similar manner as mice have been shown to be capable of addiction to cocaine, in that they will prefer cocaine over food and refuse food? 104.171.53.110 (talk) 23:44, 6 November 2024 (UTC)

I'm not sure there is unambiguous evidence that mere imagery can elicit sexual arousal in any non-human animal species.  --Lambiam 08:01, 7 November 2024 (UTC)

"Unambiguous evidence" sounds like the challenge. We do have an article about panda pornography if we're willing to weaken the standard. And rhesus like to look at photos of certain body-parts of the opposite sex of their species (see Animal sexual behaviour#Others for ref). I assume the male researchers involved in that study made plenty of "look at this picture of macaque!" jokes. DMacks (talk) 08:44, 7 November 2024 (UTC)

Somewhere in one of our journals is a study on turkeys. Male turkeys get excited and attempt to mate with anything that makes them think it is female. The researchers began with a wooden female turkey and eventually ended up with a wooden female turkey head on a stick and the males still attempted to mate with it. If it is of interest, I can see if it is still in our collection and get a better reference. It isn't a photo of a turkey, but it is still an artificial substitute for a live turkey. 68.187.174.155 (talk) 18:26, 7 November 2024 (UTC)

They just gobble, gobble, gobble it up. Clarityfiend (talk) 01:06, 8 November 2024 (UTC)

Anyone wanna help me develop a RealHen? DMacks (talk) 04:46, 8 November 2024 (UTC)

November 8

Street lights, rain drops and windows

A few nights ago it was pouring down with rain. I looked out a window to take a look, and I noticed that really beautiful patterns of light appeared as I put my eyes right in front of rain drops that were in front of a street light. The rain drops had interesting 'arms' surrounding them, but the most important part I noticed was that there were so many black lines covering the entirety of the drops.

I was able to take a picture of them with my phone, but unfortunately most of the the lines do not appear in the photos. You can see some on the sides but most of them are missing.

What caused these lines to appear?

―Panamitsu (talk) 05:08, 8 November 2024 (UTC)

See Caustic (optics).  --Lambiam 09:37, 8 November 2024 (UTC)

The repeating black lines are an example of Newton's rings. They occur due to internal reflections in a thin wedge of fluid and are most apparent when the source light is monochromatic e.g. yellow sodium light. The article shows a more reliable way to view the rings using a thin convex lens than relying on chance raindrop spreading. Philvoids (talk) 10:49, 8 November 2024 (UTC)

Yes, can confirm that this is what the lines looked like, although they were not as round as in the article's images. Thanks. ―Panamitsu (talk) 22:11, 8 November 2024 (UTC)

November 9

Black Body emissive power in medium

The black body emissive power in a medium $E_{m}$ is equal to the product of the square of its refractive index $n$ and the emissive power in vacuum $E_{v}$ with the formula:
$E_{m}=n^{2}E_{v}$.
What does this mean in terms of the energy emitted, respecting the principle of conservation of energy and in the case where the energy is emitted in a vacuum, then enters a medium with refractive index $n$? Malypaet (talk) 23:16, 9 November 2024 (UTC)

Power is energy emitted over time. So energy is conserved as it is emitted more slowly. Heat energy turns into electromagnetic energy. Graeme Bartlett (talk) 10:15, 11 November 2024 (UTC)

Yes, but here, if you use the SI units in $Joules\cdot s^{-1}\cdot sr^{-1}\cdot m^{-2}$ for emissive power as radiance, you have $E_{m}>E_{v}$. So, to conserve energy, you cannot use only the velocity for power, as you suggested. Malypaet (talk) 23:31, 12 November 2024 (UTC)

November 10

What is a pipehead dam?

I've seen various things described as a "pipehead dam" (common noun), as well as some specific instances of dams named "... Pipehead Dam", eg Serpentine Pipehead Dam, which is separate to Serpentine Dam. I gather from the text of Serpentine Pipehead Dam that a pipehead dam is a smaller dam fed from a larger dam, with the smaller (pipehead) dam then feeding water into the pipe into the water supply system - but I cannot find anything (including with a Google search) that specifically says that. Mitch Ames (talk) 01:03, 10 November 2024 (UTC)

Wow! This was hard to hunt down. Deep in the results for probably the same set of searches you did, I finally found on page 77 of [https://sitecore9-cm-prod.watercorporation.com.au/-/media/WaterCorp/Documents/Our-Water/Regional-Water-Supplies/water-forever-south-west-final-report.pdf]: "Pipe-head dam — a diversion dam that takes streamflow

from the catchment to another dam for storage." --jpgordon^{𝄢𝄆𝄐𝄇} 01:22, 10 November 2024 (UTC)

That definition appears to be the reverse of what the Serpentine articles say. The articles say water goes from main dam to pipehead dam, but the Water Corp definition suggest the water goes from pipehead to another (main?) dam. Mitch Ames (talk) 10:01, 10 November 2024 (UTC)

The point that might not necessarily come from the easy picking of the water authority or google online materials, is that in the history of the dams, the water can be moved either from the main dam to the pipehead, or vice versa - and in turn can also be distributed to other parts of the system, there is no one way only part of the system, maybe not easily found online but nevertheless the current water corp web space is very poor on the intracies of the dynamics of the water supply system. There could well be a range of security issues attached to the lack of information . JarrahTree 10:55, 10 November 2024 (UTC)

...for the over two centuries that pipe head dams have existed? --jpgordon^{𝄢𝄆𝄐𝄇} 16:37, 10 November 2024 (UTC)

The higher the pitch of the instrument the longer the bow: why?

As everyone has probably noticed, the violin has a longer bow than the viola, which has a longer bow than the cello, which has a longer bow than the double-bass. Why? I'm guessing a given length of bow (irrespective of the instrument) takes the string through a given number of vibrations. Therefore to make the string vibrate for a given amount of time at a higher frequency requires more bow length. But is this correct? Another consequence would be that no matter what the instrument the bows make the string vibrate for roughly the same amount of time and that the violin requires a higher bow speed than the viola which requires a higher bow speed than the cello which requires a higher bow speed than the double bass. Again, is this correct? 178.51.16.158 (talk) 08:15, 10 November 2024 (UTC)

To make the string vibrate with a nice sound, there has to be sufficient (but not too much) friction between the bow and the string, which requires the bow to move at the same speed or just slightly faster than the top speed of the vibrating string, 2π times the product of amplitude and frequency. So higher frequencies at a given level of dynamics require a higher bow speed.  --Lambiam 09:17, 10 November 2024 (UTC)

Ok. But leaving aside variations of the amplitude of the vibration, of the tension of the string, of the tightness or looseness of the bow (which the player can adjust), of the mass of the string and of the bow, of the thickness of the string and of the material it is made of, of the thickness of the bow, of the length of the string, of the force exercised by the hand, of how carefully the player has rubbed his bow with rosin, of the quality of the rosin, etc. etc. is it nevertheless the case that (things being roughly equal) to sustain a string's vibration at a higher frequency for a given unit of time requires more bow length? Clearly in practice there wouldn't be a linear relation between increase in frequency and increase in length. 178.51.16.158 (talk) 17:12, 11 November 2024 (UTC)

(Only) slightly pertinent to this query, you might be amused by Kingsley Amis's 1971 novel Girl, 20, in which a would-be avant-garde classical composer and violinist performs a controversial concert with rock musicians (an actual thing at the time, see for example Concerto for Group and Orchestra). Someone has secretly greased both his violin bows, but he impresses with his technical skills (though not with his actual music) by borrowing and using a double-bass bow. {The poster formerly known as 87.81.230.195} 94.7.95.48 (talk) 17:54, 10 November 2024 (UTC)

The articles about Bow (music) and the archetien who makes them say little about bow length. My survey below does not support the OP's observation. Lengthwise the bows for viola, violin and cello seem nearly interchangeable. The wide variation in longer bows for the double bass is due to the sitting players' preferences and arm lengths.

                |  Viola |   Violin |  Cello  |  Double bass
                |        |          |         |
bow    strings  |        |          |         |
cm   LOW    TOP |        |          |         |
----------------+--------+----------+---------+-------------
80   196    659 |   GE   |          |         |   x
79     .      . |        |          |         |   x
78     .      . |        |          |         |   x
77     .      . |        |    CA    |         |   x
76     .      . |        |   x      |         |   x
75     .      . |  x     |   x      |         |   x
74     .      . |  x     |          |   CA    |
73     .      . |        |          |  x      |
72    41     98 |        |          |  x      |    EG
 cm    Hz     Hz

Philvoids (talk) 12:15, 11 November 2024 (UTC)

Your chart would make the relationships clearer if Violin were in the first column, reflecting the order of relative sizes (hence string lengths and usual ranges) of the instruments. I can see a clear correlation between increasing size and decreasing bow length for the first three instruments. The double-bass may be anomalous because, unlike the other three, it is usually played standing.

I am also puzzled by your quoted figures, as my full-sized violin bow is only 65cm (ribbon length), and I am sure I have seen double-basses played with bows less than 50cm. {The poster formerly kown as 87.81.230.195} 94.7.95.48 (talk) 17:53, 11 November 2024 (UTC)

November 13

Light patterns

When I stare at a ceiling light and use my fingers to very slowly close my eye lids I see a weird pattern emerge. It looks like floaters that are covering my entire vision. I must also add that you can also see it (but with lesser detail) if you position your phone so that you can see the sun's reflection in the camera, and then you bring the reflection right in front of an eye.

What am I seeing? I'm guessing it is something inside my eyes because it looks so much like floaters. ―Panamitsu (talk) 10:38, 13 November 2024 (UTC)

It sounds like you should ask your eye doctor. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:33, 13 November 2024 (UTC)

November 14

Tau propagation

Create redirect Tau propagation to Tau_protein#Tau hypothesis of Alzheimer's disease which section of Tau protein? ExclusiveEditor _{Notify Me!} 20:12, 14 November 2024 (UTC)

It is a controversial hypothesis that cannot be dealt with with a simple redirect.  --Lambiam 05:38, 15 November 2024 (UTC)

November 15

Why are Koalas vulnerable to extinction

Hi. I was wondering why Koalas are vulnerable to extinction unlike Kangaroos, which are way more common, and both animals are found in Australia. Please let me know. Thanks. 2605:B100:142:A3B7:1D63:4EBE:694C:7BCA (talk) 04:22, 15 November 2024 (UTC)

The article has some information on it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:54, 15 November 2024 (UTC)

Habitat loss, especially lack of connected habitats, chlamidia, overcrowding, dogs. I doubt they are anywhere near extinct. Greglocock (talk) 05:17, 15 November 2024 (UTC)

It might have been quicker to Google your question - this was one of the first results; Threats To The Koala. Alansplodge (talk) 11:44, 15 November 2024 (UTC)