Wikipedia:Reference desk/Archives/Science/2018 May 27

= May 27 =

Sunset
How long does it take the sun to completely disappear below the horizon, since the bottom edge of the Sun's disk disappears below the horizon?

Our article Sunset claims that it takes "about 10 minutes, as seen from temperate latitudes".

How about other parts of the world?

More important: Are there tables presenting the exact duration, as a function of where we are in the world (and of the dates of the year)? HOTmag (talk) 08:38, 27 May 2018 (UTC)


 * Absolute minimum: about 2 minutes. 40°N: about 3, often a bit under, shortest at equinox, slightly longer at winter solstice, longest at summer solstice wait that's only twilight length, the solstices would be very similar if the Sun was the same distance and winter or summer can be longer depending on location. Pole: About a day. Sagittarian Milky Way (talk) 08:50, 27 May 2018 (UTC)


 * 10 minutes seems long for me but then again I'm from 41°N. It'll always be close to 2 minutes if it goes straight down like at the equator on the equinox. 10 minutes would be about a sqrt26:5:1 right triangle hypotenuse path down which would need the help of the horizon slanting away to happen at sane latitudes (picture the declination parallels crossing the horizon, the intersection angles go from 90 minus absolute value of your latitude to 0 at due north and south) Sagittarian Milky Way (talk) 09:10, 27 May 2018 (UTC)
 * How about 48° 25′ N ? HOTmag (talk) 12:10, 27 May 2018 (UTC)


 * 4.1 minutes at winter solstice, 4.0 minutes at summer solstice (cause Sun disc is smaller from distance and noon to noon is less than usual from the orbital speed being slightly closer to 0 which would cause 23 hours 56 minutes=Earth rotation long days (if Earth didn't fall to the Sun which it would)), 3 1/4 minutes at equinox. Sagittarian Milky Way (talk) 15:03, 27 May 2018 (UTC)
 * Have you got a formula for every x (x°N being the latitude)? HOTmag (talk) 18:12, 27 May 2018 (UTC)
 * I would check the Astronomical Almanac. It's the official almanac of astronomy (first published 1766 AD) and has tons of positional astronomical formulae and pre-made tables and the Sun's one of its most covered objects. If it isn't there I'd check the Explanatory Supplement to the Astronomical Almanac which has the things that don't become obsolete every year. I think the table or formula to adjust the standard 0° 34' refraction for temperature was in the yearly almanac though so sunset duration might still be there too. Sagittarian Milky Way (talk) 19:56, 27 May 2018 (UTC)
 * What latitude does it take the sun one hour to disappear since the bottom edge disappears? HOTmag (talk) 07:13, 29 May 2018 (UTC)
 * I will check later. Also I have a correction, I checked 2018 more accurately and it takes a fraction of a second under 4 minutes 3 seconds at 48°25'N so 4.1 minutes is wrong. In 10000 AD the winter solstice sunset will be at least several seconds shorter there even before adding the decrease cause by the lessening axis tilt and eccentricity. Sagittarian Milky Way (talk) 17:42, 29 May 2018 (UTC)
 * Thank you for the correction. I, too, have a correction: Not one hour, but rather 58.5 minutes. HOTmag (talk) 18:11, 29 May 2018 (UTC)
 * Why 58.5 minutes? Sagittarian Milky Way (talk) 22:05, 29 May 2018 (UTC)
 * 58.5 is a result of some calculation I did (which is too complicated to explain here). Anyway, also one hour (60 minutes) can meet my needs (even though it will be less exact). HOTmag (talk) 22:37, 29 May 2018 (UTC)
 * Gotcha. Sagittarian Milky Way (talk) 22:55, 29 May 2018 (UTC)
 * Somewhere around 72 to 70 degrees is about the closest you can get to the Equator and still have a 58.5 minute long sunset without the help of aircraft or moving. The Sun will disappear for an instant then rise for 12 hours (about an hour of which will be just to get the bottom to the horizon). A latitude that starts with a 6 would probably require a poleward horizon kilometers below you and maybe unusual atmospheric conditions like lapse rates, barometric pressure and temperature. Sagittarian Milky Way (talk) 00:12, 30 May 2018 (UTC)
 * Thankxs. HOTmag (talk) 17:46, 30 May 2018 (UTC)
 * Refraction through the atmosphere will have a lengthening effect, and, of course, if you live in a hilly area, this complicates the calculations.  Dbfirs  10:43, 27 May 2018 (UTC)
 * Refraction will raise the bottom of the Sun i.e. 17/30° at the horizon but it'll raise the top of the Sun the same amount when it's at the horizon. This is why the Sun is squished, the top of the disk is refracted less than the bottom so it has to be closer than the width which isn't differentially refracted. It'll make its path vertically displaced by roughly 17/30° (typical) or at least like a degree (the coldest sea level sunset in the world) which might even make it take shorter if it's winter (topography permitting). In actuality the temperature is more likely to drop slightly in those few minutes than rise making it refract marginally more and take longer (3°F/hr is 0.05°/minute and it takes Arctic cold and the help of strong temperature inversions to raise from 17/30° to over 1°) Sagittarian Milky Way (talk) 14:19, 27 May 2018 (UTC)


 * The time is almost entirely determined by the angle at which the Sun is moving when it strikes the horizon. An observer at the equator sees the Sun strike the equator vertically, and gets the minimum time, which as noted above is about 2 min. (The Sun is half a degree across and moves across the sky at about 1/4 degree per minute.) On the other hand, an observer at one of the poles sees the Sun move horizontally, so it never does cross the horizon on a daily basis. For latitudes in between, the time is given approximately by the formula (2 min) × sec (latitude), where "sec" is the secant function. Looie496 (talk) 18:40, 29 May 2018 (UTC)


 * At only 48°25'N it's already 3 1/4 minutes to 4:02.x depending on time of year, then the angle will not be constant and the rate of change of the angle will not be constant. 2*sec(latitude) will break down at latitudes the OP is interested in (hour long sunsets). Sagittarian Milky Way (talk) 22:05, 29 May 2018 (UTC)

worth putting candles in greenhouse?
Hi all, I'm an amateur gardener in the UK. My greenhouse is 12 feet long, 8 feet wide and 8 feet high (3.65x2.44x2.44 metres). Is it worth putting a couple of small candles (tealights) to improve the photosynthesis of my plants (strawberries, tomato and cucumber plants)? Additionally, I should note that my cat lives in my greenhouse (she has a comfy bed, don't worry!), so I have to leave the doors slightly open so that she can get in and out as she wishes, I'm guessing this would result in some/most of the carbon dioxide produced by the candles simply diffusing out of the greenhouse and not getting used by the plants.

I tried googling 'candles in greenhouses photosynthesis' but couldn't really find anything useful.

Many thanks!RichYPE (talk) 09:31, 27 May 2018 (UTC)
 * My understanding is that in typical conditions, the availability of CO2 is not usually the limiting factor on plant growth (see Liebig's law), and due to diffusion and ventilation (necessary to prevent conditions conducive to plant pathogens) I doubt that a couple of candles would actually have a significant effect on the level inside the greenhouse – CO2 enrichment is used commercially, but such greenhouses are large, highly controlled and supplied with CO2 from industrial sources. Plant physiology is complex, and in a given situation some plants may benefit from increased CO2 while others may not. I believe the reason that some domestic gardeners light candles in greenhouses is to provide sufficient extra heat as to guard against frosts at night (a similar technique is sometimes even used outdoors in orchards and vinyards, though direct heat is not the only useful effect in such cases). {The poster formerly known as 87.81.230.195} 2.221.82.140 (talk) 10:56, 27 May 2018 (UTC)


 * I have heard the use of decomposing manure in greenhouses to supply extra carbon dioxide, and also heat. But not that extra carbon dioxide may have undesired effects on the plant, such as extra wood, or more unpleasant tasting toxins. Graeme Bartlett (talk) 11:09, 27 May 2018 (UTC)


 * No. The levels needed to do this are rather more than that. If you look at commercial growers (and these are some of the species, especially tomatoes) where it's done, then it's done with CO2 gas.  The major costs are the rental costs of the gas bottles (it's hard to own outright gas cylinders in the UK), not the gas itself. There are also issues about safety interlocks - CO2 suffocation is a major cause of the deaths in farming accidents (usually animal manure or silos) as it's an insidious and often undetected asphyxiant.
 * You could do this (one of my neighbours is a commercial grower, they have a system) but I can't see it as worthwhile for the amateur greenhouse, especially not if there's a cat. (I've not tested cats for CO2 poisoning, although did once confirm my own carbon monoxide poisoning by dropping a cat. When it landed on its head, I realised we both had a problem.) Much of the use for CO2 enrichment isn't to increase yields, it's to increase yield density - packing more plants in a smaller volume. If you're not at those volumes, you just don't need it. What would be more use in the UK would be to go for some lighting (red/blue LED lamps are now dirt cheap - try eBay) and warmth, by avoiding draughts. Maybe a proper catflap? Andy Dingley (talk) 13:15, 27 May 2018 (UTC)


 * Or let the cat run on a treadmill which is used to power a led lamp. Count Iblis (talk) 13:17, 27 May 2018 (UTC)
 * You don't "let" a cat do anything. You might possibly bargain with it. Andy Dingley (talk) 13:30, 27 May 2018 (UTC)
 * Also, cat + open flame = housefire. - Nunh-huh 13:35, 27 May 2018 (UTC)
 * Old joke: "How do you make a cat go woof?" Alansplodge (talk) 19:13, 27 May 2018 (UTC)
 * I've never heard that before. Guess: pour some really nasty hypergolic rocket fuel/oxidizer on it? Ouch. Sagittarian Milky Way (talk) 20:10, 27 May 2018 (UTC)
 * You're in the right area; I was thinking just apply naked flame to fur. Alansplodge (talk) 15:23, 28 May 2018 (UTC)

A big thanks to everyone who kindly replied. I won't bother lighting candles in my greenhouse for the extra CO2 any more, as it seems it won't have any demonstrable effect. You can therefore rest assure that the cat won't be catching fire any time soon! RichYPE (talk) 20:49, 27 May 2018 (UTC)
 * Okay, now you just need to make sure the cat doesn't set itself on fire, as explained here. --76.69.47.185 (talk) 06:37, 28 May 2018 (UTC)

What is the actual English name for scientific name "Cucumis melo var. chate"?
I am looking for the casual name for this vegetable. The problem is that I don't know it even in my native language so I can translate it. I've tried googling but there are no really results. Maybe there's an article for it on Wikipedia? I didn't find something like that (the Armenian cucumber article refers to it a little bit in its end, but it's not clear what this species is exactly. 15:05, 27 May 2018 (UTC) — Preceding unsigned comment added by 93.126.116.89 (talk)
 * Looks like this: https://en.wikipedia.org/wiki/Armenian_cucumber --TammyMoet (talk) 15:42, 27 May 2018 (UTC)
 * Cucumis melo var. chate is a synonym of Cucumis melo, the muskmelon. See The Plant List DuncanHill (talk) 15:53, 27 May 2018 (UTC)
 * "Var." is the standard Linnaean abbreviation for Variety (botany): "a taxonomic rank below that of species and subspecies but above that of form". So a "var." should help find the exact plant - in theory.  (Sidenote: I wish we C21st encyclopedists could abolish the custom of abbreviations, or certainly those uncommon to the general reader. We do not suffer under the restrictions of paper-writers.) Carbon Caryatid (talk) 16:16, 27 May 2018 (UTC)
 * Here's the Wikicommons link for images of Cucumis melo. There are clearly many varieties - some I recognise and have tasted as quite distinct fruits (NB not vegetables), while others are beyond my experience. Unfortunately, none are labelled as "chate". Carbon Caryatid (talk) 16:20, 27 May 2018 (UTC)
 * An example of the chate variety is Carosello Barese which is a longish green melon. Abductive  (reasoning) 17:09, 27 May 2018 (UTC)
 * I'm pretty sure that it is called adzhur. Abductive  (reasoning) 07:52, 29 May 2018 (UTC)
 * Looks like we're getting closer, according to Wikspecies (but Wikianything is not a reliable source). I love how the next classification along from "adzhur" is "ambiguus". Carbon Caryatid (talk) 09:29, 29 May 2018 (UTC)
 * We're not going to get closer here. The literature, since Lineaus, shows tremendous doubt and confusion. My additions to the article are about as far as one can get without hitting conflicting primary sources. Abductive  (reasoning) 15:58, 29 May 2018 (UTC)

Evolution of brain image processing
I'd like some reading on how our brains evolved the ability to process raw images. According to this, an unprocessed refracted image that hits the retina is inverted, flat, two-dimensional and color-wise unextrapolated. Also does the underdeveloped baby vision imply that our distant ancestors saw the world raw and unprocessed and were fine with it? Are there still such animals? Brandmeistertalk  19:34, 27 May 2018 (UTC)
 * Ontology does not recapitulate phylogeny in most cases. Evolution of the eye does a decent job of explaining what we do know. Abductive  (reasoning) 22:06, 27 May 2018 (UTC)
 * For those who aren't familiar with the phrase, the above post is referring to recapitulation theory. --47.146.63.87 (talk) 07:38, 28 May 2018 (UTC)
 * And ontogeny, not ontology. Andy Dingley (talk) 16:58, 29 May 2018 (UTC)
 * My eyes see one thing, my hands type another. I processed the raw image wrong. Abductive  (reasoning) 18:33, 29 May 2018 (UTC)