Welcome to the science section
of the Wikipedia reference desk.

Select a section:

WP:RD/S

Want a faster answer?

Main page: Help searching Wikipedia

How can I get my question answered?

Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
Post your question to only one section, providing a short header that gives the topic of your question.
Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
Don't post personal contact information – it will be removed. Any answers will be provided here.
Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
Note:
- We don't answer (and may remove) questions that require medical diagnosis or legal advice.
- We don't answer requests for opinions, predictions or debate.
- We don't do your homework for you, though we'll help you past the stuck point.
- We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

June 11

Butterfly ID

In the Valley Water ad which exhorts you to "Bring your yard to life!", did I correctly identify the butterfly at the top right as a Papilio machaon? (Which, BTW, would be a pretty weird choice of species for them, given that it's not native to the area!) Pictures are OK if needed to answer the question, but keep them small! 2601:646:8082:BA0:15AB:5FDE:33D5:2ED4 (talk) 11:37, 11 June 2025 (UTC)[reply]

What ad are you referring to? Matt Deres (talk) 13:41, 11 June 2025 (UTC)[reply]

Presumably this one. Mikenorton (talk) 13:54, 11 June 2025 (UTC)[reply]

I think that image of the ad is too small for a positive ID, but maybe Papilio zelicaon, which is "native to the area"? Deor (talk) 14:04, 11 June 2025 (UTC)[reply]

A better version of the image here - looks like P.machaon. Mikenorton (talk) 14:07, 11 June 2025 (UTC)[reply]

A real head-scratcher

Why do people scratch their heads when they are trying to solve a puzzle or difficult question? 2A00:23C7:533:3C01:C50D:1C:C26D:B845 (talk) 18:36, 11 June 2025 (UTC)[reply]

Do you? ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:46, 11 June 2025 (UTC)[reply]

Not since I was in infants school, I think. But my nephews do. 2A00:23C7:533:3C01:C50D:1C:C26D:B845 (talk) 20:04, 11 June 2025 (UTC)[reply]

A response to some level of anxiety, maybe? ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:30, 11 June 2025 (UTC)[reply]

See Displacement activity. {The poster formerly known as 87.81.230.195} 94.6.40.15 (talk) 22:51, 11 June 2025 (UTC)[reply]

The article describes the phenomenon ("what?") and the conditions triggering it ("when?"), but does hardly explain it ("why?").

Additionally, I think that head-scratching while considering difficult problems is not constrained to having to make a difficult choice. In fact, it is IMO more likely when the thinker does not even know where to start and needs to organize their thoughts to get a grip on the problem. ‑‑Lambiam 00:42, 12 June 2025 (UTC)[reply]

I see this in the form of slow tail wagging in cats during decision making when they are faced with the always-on-the-wrong-side-of-a-door conundrum. I've wondered whether it is something like a way to inject a noise signal into competing neural patterns to do something or other, like enhance differences, speed up resolution. I guess this is testable. Does decision making time or reliability change for tricky questions/choices when people are exposed to a stimulus (like white noise, automated head scratcher or whatever). But since I've only had about 2 original ideas that panned out in my life, I assume someone has already thought of this and tested it. Sean.hoyland (talk) 05:03, 12 June 2025 (UTC)[reply]

I'm very sorry to admit that I've just found this at the Daily Mail: "Why we scratch our heads when we're confused". It was a 2017 study by a Jamie Whitehouse from the University of Portsmouth who looked at why macaque monkeys scratch their heads in certain situation. It was published in Scientific Reports. Martinevans123 (talk) 13:29, 12 June 2025 (UTC)[reply]

Is the Daily Mail claiming that we evolved from ape ancestors? What happened to them? Anyway, here's a more recent interesting paper from Whitehouse et al. Sean.hoyland (talk) 15:15, 12 June 2025 (UTC)[reply]

Yes, seems Whitehouse is now at Nottingham Trent University. Martinevans123 (talk) 15:46, 12 June 2025 (UTC)[reply]

The Trier social stress test sounds very effective, although very arbitrary. Participants must prepare for a clinical psychologist job interview, including a 3-minute speech about why they are an ideal candidate. Our article says "The participant is allowed to use paper and pen to organize their presentation, but this paper is then unexpectedly taken away from them when it is time to begin the presentation." ... participants are given a series of purposefully difficult job interview questions (e.g. ‘What do you consider to be the main advances in the area of Clinical Psychology in the last 20 years?’) and then they have to count backwards in 13s from 1022. Card Zero (talk) 20:39, 12 June 2025 (UTC)[reply]

I only scratch my head when it's counting backwards in 14s. Martinevans123 (talk) 08:14, 13 June 2025 (UTC) p.s. I didn't even get the job.[reply]

I think this is the result of gesture learning. When we see adults doing this as children, we imitate this behavior, and after generations of imitation, this habit is reinforced. Stanleykswong (talk) 17:22, 12 June 2025 (UTC)[reply]

I think many adults often do it deliberately, as a kind of self-parody of confusion. Martinevans123 (talk) 17:37, 12 June 2025 (UTC)[reply]

It might be interesting to see the geographical distribution of head-scratching while decision making to see whether it is actually universal. Gaze direction also changes during decision making and the directionality seems to vary across cultures. Sean.hoyland (talk) 11:11, 14 June 2025 (UTC)[reply]

Is the Barbate lighthouse part of an analemmatic sundial?

The article analemmatic sundial claims that the Barbate lighthouse is the gnomon of one. It also claims, "The gnomon is not fixed and must change position daily to accurately indicate time of day."

As the lighthouse looks stationary, is one of the claims untrue?

Thanks, cmɢʟee⎆τaʟκ 23:50, 11 June 2025 (UTC)[reply]

It isn't, by definition, an analemmatic sundial. The tower does not move. Instead, there are multiple circles. You pick the circle that matches the current date and use that to tell time. So, instead of moving the tower, they are essentially moving the arc around the tower. 68.187.174.155 (talk) 00:15, 12 June 2025 (UTC)[reply]

It's not the prototypical analemmatic sundial (which has nothing to do with the analemma; it's a misnomer), but it is based on the same principle, namely the intersection of the shadow of a vertical rod with an ellipse on the ground. The difference is that the rod doesn't move, but the ellipse moves (and changes size). Or rather, multiple ellipses are shown on the ground and the user picks the right one for the date. How much one can deviate from the prototypical analemmatic sundial and still call it one is a question that has no scientific answer. PiusImpavidus (talk) 11:03, 12 June 2025 (UTC)[reply]

Thanks for explaining. I'll clarify it in the caption on the article. cmɢʟee⎆τaʟκ 20:40, 12 June 2025 (UTC)[reply]

June 12

Butterfly watching question

In New Hampshire and Maine, what are some good places to see Papilio canadensis, and until how late in the summer are they typically on the wing there? (Yes, I'm making plans to travel there for the express purpose of watching these critters -- I've made pretty good progress in my (self-administered) exposure therapy, in fact just today I briefly watched a live 3-inch P. rutulus without any phobic response, and I'm sure I'm ready to go from pictures and pinned butterflies to live small ones (no bigger than 3 inches or so), and that's why I have to travel to New England for it -- that's the only place in the country where I can see P. canadensis and be sure of not running into any bigger species!) 2601:646:8082:BA0:1881:7267:DD7D:636D (talk) 22:07, 12 June 2025 (UTC)[reply]

Butterflies of North America: Butterflies of Maine (small photo) says Flight: One flight from May to mid-July. Habitat: Northern deciduous and evergreen-deciduous woods and forest edges. There is a map. The map includes Greenwood, Maine. Commons has a photo of 20 of them mud puddling in that location taken on 28 May 2015. A June 22, 2020 blog post from Maine Department of Inland Fisheries and Wildlife (picture not loading?) says ... often seen coursing through backyard gardens and along woodland edges at a rapid pace. Named for their bright yellow color and black stripes, tiger swallowtails appear to be especially abundant this year, with dozens of recent social media posts applauding their presence statewide. In summary, go now (in summery?) and find the edge of a woodland, any woodland. Card Zero (talk) 11:32, 13 June 2025 (UTC)[reply]

Thanks! Unfortunately, this will have to wait until a later year -- my calendar for this year is already full until mid-July, so I'll be too late to see them :-/ 2601:646:8082:BA0:49CD:9100:D068:6208 (talk) 22:02, 13 June 2025 (UTC)[reply]

What is the range you are prepared to travel within, if someone could find another good location where you would find only diminutive lepidopterans, OP? SnowRise ^{let's rap} 02:22, 14 June 2025 (UTC)[reply]

Not just any butterflies; only those with a distinctive pattern such as seen on many swallowtails will help in desensitization. ‑‑Lambiam 07:09, 14 June 2025 (UTC)[reply]

Perhaps I am missing something about your reasoning, but I don't see why that would necesarily be the case, as aversions and phobias, whether idiopathic or trauma related, are highly idiosyncratic in presentation. While I don't doubt that the pattern would be a part of the trigger for some persons, I don't think we can know for sure that this is the case with the OP. Although I will grant you, all of the species they mention have somewhat convergent phenotypes/morphologies in this respect. SnowRise ^{let's rap} 00:22, 15 June 2025 (UTC)[reply]

For the record, I will have you know that the tiger-striped pattern similar to that of P. glaucus (i.e. a pattern of contrasting vertical stripes, whether dark stripes on a bright background as on P. glaucus or bright stripes on a dark background as on P. garamas) is indeed a key part of the trigger to my phobia (whereas a presence or absence of tails on the hind wings is not) -- solid black swallowtails like Papilio polyxena (where did the article go -- did I spell it wrong???) or Papilio troilus do not bother me at all, at least as long as they are no bigger than 4 inches (beyond that size, I just don't know -- the only butterflies I've ever seen which were much bigger than 4 inches also had the tiger-striped coloration). And never mind about what I said earlier -- I think I can actually carve out some time in early July to travel to New England! 2601:646:8082:BA0:D5E3:9787:7C90:66A0 (talk) 02:16, 15 June 2025 (UTC)[reply]

P. polyxenes. ‑‑Lambiam 09:13, 15 June 2025 (UTC)[reply]

Very interesting. If I'm not prying, may I ask whether there was a specific formative event or events for your phobia, that you can recall? In any event, enjoy your trip and best of luck! SnowRise ^{let's rap} 12:10, 15 June 2025 (UTC)[reply]

Snow Rise, this OP has been consulting the Ref desk regarding these matters for some years, and has previously described the origin of their trauma – perhaps you could search for that description in the archives? {The poster formerly known as 87.81.230.195} 94.1.137.14 (talk) 13:27, 15 June 2025 (UTC)[reply]

Found here. Wikipedia:Reference_desk/Archives/Science/2021_February_9#M's_butterfly_collection AlmostReadytoFly (talk) 13:28, 16 June 2025 (UTC)[reply]

I see! Thank you both for the context: that explains why Lambiam was so confidant about what species were required for the OP's purposes. I hope the trip goes well. SnowRise ^{let's rap} 06:10, 17 June 2025 (UTC)[reply]

I wonder if someone would like to put information about butterflies into Wikivoyage:, in case anyone else wants to travel to see them. No refs are necessary (at most, paste a URL or description in the edit summary if you think it's important). See voy:Wikivoyage:Welcome, Wikipedians for the usual advice. WhatamIdoing (talk) 18:25, 16 June 2025 (UTC)[reply]

June 16

Medical guidelines

I'm looking for a high-quality reliable source to use in the article Medical guideline (note the many synonyms). The question to be answered is: Do these guidelines, which are frequently but not always published in medical journals, usually/always/ever undergo peer review? (Specifically, I'm interested in pre-publication external peer review, i.e., the normal kind of peer review for a medical journal.)

@CFCF, I think you might be interested in this question as well. WhatamIdoing (talk) 18:29, 16 June 2025 (UTC)[reply]

This depends on what you mean, a consolidated statement by say 100 organizations can not presumably be peer reviewed, because who would be the appropriate peer? So, the short answer is no, the longer answer is: sometimes, it's complicated. For instance when agencies put out guidelines, they often go through the process of stakeholder mapping followed by consultation or review and referral rounds which include other agencies, implementing bodies (i.e. clinicians), expert panels, etc. This can often be far more rigorous than academic peer review.

Guidelines are also not per se evidence, even if a trend since the late 1990s has been to incorporate evidence synthesis such as through systematic reviews in the process - and most high quality guidelines do this today.

There is quite some a bit of literature on the process, and one could start with the following:

A lot of these processes tack on to the HTA-framework, which is often one of the steps in guideline development. CFCF (talk) 21:19, 16 June 2025 (UTC)[reply]

How nuclear energy from the Sun's core emitted as gamma rays is transformed into blackbody radiation

In the Sun's core, a nuclear reaction transforms hydrogen into helium while emitting gamma photons. How can these gamma photons transform into blackbody radiation photons as they travel outward from the Sun? Malypaet (talk) 22:20, 16 June 2025 (UTC)[reply]

The photons get reabsorbed by other particles, (electrons and protons) and then re-emitted many, many times over. As it escapes over the centuries from the core of the Sun it will meet cooler particles and then be reemitted as lower energy photons. There may also be convection of plasma or gas in parts of the Sun, which will transfer heat towards the surface, the but the pressure will lower and temperature decline. What we see is the photons coming out from near the surface, which will reflect the temperature of the gas there. Graeme Bartlett (talk) 23:53, 16 June 2025 (UTC)[reply]

There is convection, in the convection zone. Sagittarian Milky Way (talk) 14:29, 20 June 2025 (UTC)[reply]

June 18

Health lore

Where does the advice to "keep your feet warm and your head cold" (widely believed in Eastern Europe) come from? Is there actual evidence of better health outcomes when following this advice? 2601:646:8082:BA0:196D:8E1:5181:D48F (talk) 02:08, 18 June 2025 (UTC)[reply]

Severe problems due to frozen toes (see Frostbite) or heat stroke are all too common. Of course, one's nose or ears can also be frostbitten, but this is far less common. ‑‑Lambiam 06:29, 18 June 2025 (UTC)[reply]

Might they be speaking metaphorically about the head, as in "keeping a cool head" when under stress? ←Baseball Bugs ^{What's up, Doc?} carrots→ 09:06, 18 June 2025 (UTC)[reply]

Unlikely, given the grammatical construction of this sentence in languages other than English (in particular, in Russian it reads "keep your head in the cold" -- definitely referring to literal thermodynamics, not psychology). 2601:646:8082:BA0:D199:AB8A:25C3:B09B (talk) 23:54, 19 June 2025 (UTC)[reply]

Several sources attribute this advice to Herman Boerhaave (1668–1738): "Houd uw hoofd koel, uw voeten warm en prop niet te vol uw darm." ("Keep your head cool, your feet warm, and don't overstuff your bowels.") I have my doubts this is real; it sounds like a made-up attribution. ‑‑Lambiam 18:45, 18 June 2025 (UTC)[reply]

Thanks! And yes, it does actually sound completely plausible that he was the one who first published this advice -- given that the model of human anatomy he had come up with was essentially hydraulic in nature, it makes perfect sense that he would have believed that maintaining a temperature gradient between the head and the feet would improve circulation, whereas keeping the head excessively warm would hinder it! Now, for the second part of the question: does this actually result in better health outcomes, compared with keeping the whole body uniformly warm in cold temperatures or uniformly cool in hot temperatures? 2601:646:8082:BA0:D199:AB8A:25C3:B09B (talk) 23:54, 19 June 2025 (UTC)[reply]

Treatment

what is the function of lotion _Derma Dew? 223.185.220.224 (talk) 10:08, 18 June 2025 (UTC)[reply]

It is the brand name for a skin cream containing Aloe vera. That article describes its use. You'll also find it if you use a search engine for the brand. Mike Turnbull (talk) 11:02, 18 June 2025 (UTC)[reply]

June 19

Boiling point of nitrogen

Do we have a table for the boiling point of diatomic nitrogen as a function of pressure, especially between atmospheric pressure and its critical point? The infobox at the start of our Nitrogen article says that the critical point is near 3400 kPa and 126 K, but there's no reference given for that. I don't see data for pressures lower than that, other than a boiling point of 77 K at some unspecified pressure presumably near atmospheric. Ideally I'd also like a table of the liquid densities as a function of temperature in this range. I tried to look for other articles on en.Wikipedia, or phase diagrams on Commons, but all I found are two phase diagrams for much higher pressures. I don't need high precision data. Nitrogen is sold in pressurized cylinders for cheap so I expected this data to be widely available. – b_jonas 11:30, 19 June 2025 (UTC)[reply]

doi:10.5075/epfl-thesis-4721 has a nice phase diagram on page 61. It's cited to doi:10.1088/0031-9120/7/4/010, whose full-text I cannot access at the moment, so I don't know if it simply copied the diagram, used experimental or theoretical values from it, or has some other cited underlying source of information. DMacks (talk) 17:24, 19 June 2025 (UTC)[reply]

The labels have been reset in a serif typeface, but the grid and curves, although redrawn, are virtually indistinguishable. ‑‑Lambiam 20:36, 19 June 2025 (UTC)[reply]

Oceani (sic) Foam? 1960s Soviet breakthrough.

(Science seems to be the best here but this is more materials science or consumer science.)

On an episode of Gaslit about Nixon and Martha Mitchell, John Dean's character drops about how the Soviets invented a cushion foam that "feels better than you imagine". [1] (youtube link ap isn't working)

Is this real? How do you spell it? Any other info on this, like why after the cold war no one makes it?

Thanks in advance! 2600:1700:6742:3C00:C8E3:FC65:9FD0:D2C3 (talk) 18:07, 19 June 2025 (UTC)[reply]

Just tried looking it up under several different spellings, and the only refs I found led right back to the show -- so I can safely conclude that this material is purely fictional. BTW, the entire story is highly implausible on several counts: (1) having personally lived in the Soviet block during the last few years of the cold war, I can confirm that their foam cushions were not actually all that good -- they were very saggy and tended to support you in exactly the wrong places; (2) Kennedy would never have risked using Soviet-made materials in Air Force One (of all places) regardless of their supposed quality, for risk of public scandal; (3) I have found confirmation that the cushions on Air Force One were in fact manufactured in the USA (oh, the good old days!); and (4) the properties attributed to this "Poshinaya foam" appear to be similar to those of memory foam, which was invented during that time period (just about the only historically accurate part of this whole thing), but by Nasa, not the Soviets! 2601:646:8082:BA0:D199:AB8A:25C3:B09B (talk) 00:23, 20 June 2025 (UTC)[reply]

Really appreciate your deep dive on this. Hollywood always makes it look real smh. Any other thoughts or comments are welcome but what you are stating makes sense in some ways. 2600:1700:6742:3C00:C8E3:FC65:9FD0:D2C3 (talk) 03:11, 20 June 2025 (UTC)[reply]

Now that I remember this, there were even calls by local health experts to avoid using foam cushions in home furniture (and especially in children's furniture) because they believed them (and probably with very good reason) to contribute to poor posture due to their sagginess -- which shows just how bad Soviet-block foam cushions really were! 2601:646:8082:BA0:D83C:8BF:CC86:2F61 (talk) 12:05, 20 June 2025 (UTC)[reply]

Can you tell me something about two headed snakes?

As far as I know, two headed snakes are relatively common compared to other Siamese twins. Is it true that the two heads often get angry with each other and disagree about where their body is going and start fighting each other? 146.200.107.90 (talk) 22:47, 19 June 2025 (UTC)[reply]

The following article from a reputable source might be of interest; however, I didn't read it because I was required to "Enter your email to read this article", which I'm unwilling to do:

"Life Is Confusing For Two-Headed Snakes". Animals. National Geographic. 20 June 2025. --136.56.165.118 (talk) 00:42, 20 June 2025 (UTC)[reply]

Two quotes from that article:

"Just watching them feed, often fighting over which head will swallow the prey, shows that feeding takes a good deal of time, during which they would be highly vulnerable to predators," said Burghardt. "They also have a great deal of difficulty deciding which direction to go, and if they had to respond to an attack quickly they would just not be capable of it."

...

Snakes operate a good deal by smell, and if one head catches the scent of prey on the other's head, it will attack and try to swallow the second head.

There is no suggestion of anger being involved. ‑‑Lambiam 03:58, 20 June 2025 (UTC)[reply]

Unlike mammals, reptiles lay eggs, and the mother's body cannot detect and terminate severely deformed embryos early in development. Stanleykswong (talk) 20:18, 21 June 2025 (UTC)[reply]

Also, in placental mammals and many other viviparous animals, Siamese twins make delivery harder, endangering the mother. In humans it's almost certain to kill the mother without modern medical care. That increases the selection against genetic defects that cause Siamese twins. Some snakes employ some form of ovoviviparity. PiusImpavidus (talk) 09:03, 22 June 2025 (UTC)[reply]

Minor quibble: some species reptiles are viviparous (even some snakes). Two-headed snakes are probably rare enough that it would hard to look for a correlation with species. DMacks (talk) 12:01, 22 June 2025 (UTC)[reply]

June 21

Very small black holes

Black holes are right now thought to be characterized only by mass, angular momentum and charge. Could weak isospin also be a theoretical characterization? Aso I've heard that black holes could pass through us without our noticing. if they had angular momentum,charge, or hefty weak isospin, would they be more noticeable?Rich (talk) 20:55, 21 June 2025 (UTC)[reply]

Black holes vary in size, from tiny micro black holes to supermassive black holes that reside at the centers of galaxies. In theory, if the black hole is small enough, it is possible for a small black hole to pass through the Earth without us noticing. Although black holes have a high density, if the black hole is small enough, the effect of gravity on the tiny black hole will be negligible and we cannot detect it. Stanleykswong (talk) 11:33, 22 June 2025 (UTC)[reply]

It would be funny if it turned out that neutrinos are actually tiny black holes. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:03, 22 June 2025 (UTC)[reply]

It's not impossible. Neutrinos are extremely tiny particles, with a mass far lower than other elementary particles such as electrons. They have almost no size, so the density can be very high. In addition, they have no charge, so a large number of neutrinos can be concentrated in a very small space. Stanleykswong (talk) 16:13, 22 June 2025 (UTC)[reply]

Who told you that? --Wrongfilter (talk) 17:10, 22 June 2025 (UTC)[reply]

Neutrinos have a lepton number (unlike black holes, unless I'm mistaken) and obey the Pauli exclusion principle (so they can't be arbitrarily concentrated). AlmostReadytoFly (talk) 08:26, 23 June 2025 (UTC)[reply]

Note that micro black holes evaporate within a tiny fraction of a second -- and also note that, even for a Planck-mass black hole (10 micrograms), such near-instantaneous evaporation would release an energy of about 900 gigajoules (which, for comparison, is about the explosive power of a 500-lb high-explosive bomb like the ones we drop on Isis and other terrorist groups). So no, such a black hole still can't pass right through you for 2 different reasons -- (1) it will evaporate first, and (2) in doing so it would blow you to bits if it happened to be close enough! 2601:646:8082:BA0:BDCC:8484:2734:F138 (talk) 13:21, 22 June 2025 (UTC)[reply]

However, if such a hole were travelling close to the speed of light, relativistic time dilation would prolong its lifetime. See also Micro black hole#Expected observable effects. {The poster formerly known as 87.81.230.195} 94.6.41.216 (talk) 15:27, 22 June 2025 (UTC)[reply]

Still, assuming a casualty radius of 11 meters for the explosion caused by the evaporation of the black hole (implying blast effects only, no flying debris), the time dilation would have to be at least by a factor of thirty-six orders of magnitude to allow said black hole to pass through you and explode a safe distance away! (And if the explosion would create a significant amount of flying shrapnel, e.g. if it demolishes a parked car nearby, then that would add an extra order of magnitude for the time dilation required!) 2601:646:8082:BA0:F577:E853:35B:8DD9 (talk) 11:52, 23 June 2025 (UTC)[reply]

June 23

One minute earlier in 151 years

Accounting for DST today, the sun set on Sunday, October 4, 1874, in Oakland, California, at 6:47 PM. This year, in 2025, the sun will set at the same month, day, and location at 6:46 PM, one minute earlier in just 151 years. What factors account for this small difference? Viriditas (talk) 09:51, 23 June 2025 (UTC)[reply]

Milankovitch cycles? Although I suppose there might also be some rounding errors between whole minutes there? I guess we can rule out the unreliability of chronometers in Oakland in 1874? Martinevans123 (talk) 09:59, 23 June 2025 (UTC)[reply]

We have a good calendar that keeps calendar date in sync with the apparent motions of the sun. It ensures that on the same date the sun sets at the same time (with small variations). Or what did you actually expect? --Wrongfilter (talk) 10:08, 23 June 2025 (UTC)[reply]

The solstice - the exact moment that the Earth's axis lines up with the sun - drifts around a little bit due to the fact that the year isn't a perfect 365 days. According to timeanddate.com, summer solstice 1874 was 21 June 15:07, and summer solstice 2025 was 21 June 02:42 UTC. That means that 4 October this year starts - in an astronomical sense, compared to the Earth's rotation around the sun - 12 hours earlier this year compared to 1874. Sunset times changes quite quickly in late September/early October (it looks like in Oakland, it's a minute or two per day), because it's near the equinox, and it's quite possible that that 12 hour difference is enough to push the sunset from one minute to another. In another year, when we have an unusually late solstice, October 4 will have a slightly later sunset, but most of us are unlikely to live to see it. The solstice date naturally gets slightly earlier every year by about 6 hours and 13 minutes, and that's why every 4 years we have a leap year - but the leap year correction still leaves about 13 minutes of drift per year. We try to cancel that out by skipping a leap year once a century (not counting years divisible by 400), so normally you get late solstices following a round century year (1800, 1900 etc) because the leap years are skipped in these years and that resets the calendar, but 2000 was a leap year so the calendar didn't get the usual reset. There won't be another late solstice until 2103, when it will occur at 22:47 UTC on 21 June - and that year Oakland will gain a minute and the sun will set at 6:48 PM (according to Wolfram Alpha). Smurrayinchester 10:35, 23 June 2025 (UTC)[reply]
Thank you! Viriditas (talk) 10:59, 23 June 2025 (UTC)[reply]

Many thanks, Smurrayinchester. Just for my info, are sunrise and sunset times always given in whole minutes? Martinevans123 (talk) 11:02, 23 June 2025 (UTC)[reply]

I think so. You can work it out more precisely, but the terminator (line of sunrise/sunset) moves at 463 metres per second at the equator, and slower the further towards the poles you go - you only need to walk a short distance to change the sunrise by a second. The sun rises and sets approximately 10 seconds earlier at the Tower of London compared to at Big Ben, for example, not taking into account all the other factors like the precise shape of the Earth and atmospheric effects that refract the sun's position. It's just not worth publishing sunrise times in seconds - it would be an example of false precision. Smurrayinchester 12:35, 23 June 2025 (UTC)[reply]

Many thanks for that very clear detail. Is that walking North or South, or East or West? Perhaps both. Obviously, in many places on land, the horizon will be obscured by terrain and/or structures. Is there an agreed official location for the sunset and sunrise in London? I would have guessed Royal Observatory, Greenwich. Sorry to keep asking! Martinevans123 (talk) 12:48, 23 June 2025 (UTC)[reply]

East-west. And I don't know if there's an official single point for astronomical calculations in London, but Greenwich certainly produce their own sunrise and sunset tables for astronomy. Smurrayinchester 15:01, 23 June 2025 (UTC)[reply]

The terminator moves across the Earth's surface with a speed of 360° of longitude in one day of 24 × 3600 s – not uniformly in October's Oakland, but the average is good enough here. The longitudinal width of Oakland is about 0.2°, so for the terminator to glide across Oakland takes about 48 seconds. Can we be certain that the reference point for 1874 is at the same longitude as that for 2025? ‑‑Lambiam 18:22, 23 June 2025 (UTC)[reply]

Yes, it's quite long, isn't it. Martinevans123 (talk) 18:30, 23 June 2025 (UTC)[reply]

There was no standard time in the United States in 1874 (History_of_time_in_the_United_States#Railway_time). So was 6:47 pm recorded in municipal time? Local solar time? Or something else? --Amble (talk) 22:05, 23 June 2025 (UTC)[reply]

(Just to be clear, I think the OP is talking about calculations of the kind you get from modern astronomical software, not measured local clock time. Even for precisely the same coordinates, the calculated sunset times vary for the reasons I listed above). Smurrayinchester 08:48, 24 June 2025 (UTC)[reply]

Your reply was fantastic, those effects can easily explain the variation. You are probably correct about the OP's data source as well. I initially thought it might relate to the long record of timekeeping data from Chabot Space and Science Center, which "served as the official timekeeping station for the entire Bay Area, measuring time with its transit telescope", but Chabot was only founded in 1883. --Amble (talk) 14:54, 24 June 2025 (UTC)[reply]

June 25