Wikipedia:Reference desk/Archives/Science/2023 March 13

= March 13 =

hyphen v long dash
The Blood-brain barrier article has a long-dash rather than a simple hyphen between "blood" and "brain". Is this a simple redirect or is it better to revise the title to use just the hyphen? Could an admin make that change, please? Thanks. Zefr (talk) 15:08, 13 March 2023 (UTC)
 * This is better discussed at Talk:Blood–brain_barrier. --OuroborosCobra (talk) 15:25, 13 March 2023 (UTC)
 * That has been done. It seems a simple, obvious solution to correct the title to a hyphen when conventional typing for the common user and the article text use hyphens rather than long dashes. Zefr (talk) 15:33, 13 March 2023 (UTC)
 * The relevant MOS section is MOS:DASH, which says that in cases like this (where the two terms are symmetric and the first is not a modifier of the second), a long dash should be used rather than a hyphen. Directly addressing this question, it says In article titles, do not use a hyphen (-) as a substitute for an en dash, for example in eye–hand span (since eye does not modify hand). Nonetheless, to aid searching and linking, provide a redirect with hyphens replacing the en dash(es), as in eye-hand span. There is indeed already a redirect from Blood-brain barrier to Blood–brain barrier, so I think everything is fine as it is. But if you disagree, go ahead and open a discussion on the article talk page. (Although personally I'd prefer to eliminate en dashes and use hyphens everywhere. Hyphens are easier to type and the barely perceptible visual difference doesn't seem worth it to me to deal with all the rules, redirects and confusion that results from having two types of dashes.) CodeTalker (talk) 16:33, 13 March 2023 (UTC)

Mankind seen from space
Let's say that a random alien is standing on the surface of the Moon. He does not know if there is a civilization of intelligent beings here. Is there something that can be seen with the naked eye (no telescopes allowed) that would reveal our presence? Is any man-made object (including cities) visible from that distance? Cambalachero (talk) 18:58, 13 March 2023 (UTC)
 * It depends what you mean by visible. What sort of visual acuity do these aliens have? What EM frequencies are they sensitive to? With high enough visual acuity they'd be able to read a newspaper from the Moon. And if they're able to 'see' radiowaves, for instance, they'd know something was going on.
 * If you're asking what manmade things a human could see on Earth from the Moon I would guess nothing. Even the glow of our cities on the night side would be indiscernible. nagualdesign 19:18, 13 March 2023 (UTC)
 * Could you provide a reference or some other justification for your last statement? Do you think that is true even during a solar eclipse (lunar eclipse as seen from earth) when there is no glare from the sun or the earth's day side? --Wrongfilter (talk) 19:35, 13 March 2023 (UTC)
 * I can't provide a reference for my guess, but I can give you some justification; the Apollo astronauts couldn't even see the stars. I hadn't considered what it would be like during an eclipse. I suppose you'd have to work out whether the glow of our cities would be bright enough to be seen over the glow of the Milky Way. I suspect that the stars would be very bright when viewed from the Moon during an eclipse. nagualdesign 15:43, 14 March 2023 (UTC)
 * What's your source for the astronauts not being able to see stars?  — Preceding unsigned comment added by Baseball Bugs (talk • contribs) 16:44, 14 March 2023 (UTC)
 * Apologies, I assumed it was common knowledge thanks to conspiracy theorists and didn't think that every statement made here had to conform to the same rigorous standards as articles. How about this? nagualdesign</b></b> 18:08, 14 March 2023 (UTC)
 * I was under the impression that it was the conspiracy theorists who were saying the stars weren't visible. ←Baseball Bugs What's up, Doc? carrots→ 19:42, 14 March 2023 (UTC)
 * Yes, they've been harping on about it for years, claiming it as proof that the Moon landings were fake, but the astronauts genuinely couldn't see the stars during daytime on the Moon. <b style="font:1.3em/1em Trebuchet MS;letter-spacing:-0.07em"><b style="color:#000">nagual</b><b style="color:#BBA">design</b></b> 20:02, 14 March 2023 (UTC)
 * Neil said none of the three saw stars without optics while on the Moon or above the day side, Gene Cernan said he saw some while outside in the shadow of the LEM Sagittarian Milky Way (talk) 20:28, 14 March 2023 (UTC)
 * They had a tight schedule but I think I remember they saw some at least when looking up. Photos back then had much worse brightness ratio than say looking at a sunset and looking down. The dim but not black "pixels" in view fractions of a second after looking at the Sun would've been pitch black if human vision couldn't move its range up to 2 orders of magnitude in a fraction of a second. Sagittarian Milky Way (talk) 17:43, 14 March 2023 (UTC)
 * They can't see the stars because 100,000 full moons of light are in their view, and they had translucent gold plating to make the mid-morning sun less dazzling don't remember if that was a slidable outer visor or not or if they ever slid it out of view outside. The Milky Way is very dim and during the eclipse you can hide the less dim Sagittarian part with the horizon. Or look at the crescent Earth through a V-shaped horizon valley that shows the middle but hides the day and twilight, from a part of the dark edge of a waning waxing gibbous that allows that, when Guangzhou is in the middle (it will be evening there at this time, 85 million people live in the Hong Kong indentation delta, though its light will be spread out among several 20/20 "pixels", I would try during the biggest fireworks night of Chinese New Year during the nightly Hong Kong light show). Sirius does not affect dark adaption from the darkest skies on Earth, though a few planets can, the middle of the Earth is a degree from the edge, that should be enough to be able to avoid the "bright double stars hide dimmer companions effect". I don't know if you can hide the dark-adaption affecting parts of the Earth AND moonscape AND the brightest part of the Milky Way AND look down on Guangdong AND have it be evening and Chinese New Year there at the same time. Maybe. A quick estimate with a globe couldn't tell. If I had a large dark room and a physically small flashlight and someone with a ladder to direct to move and point the light based on my view of the globe and tell me what's the center of the globe from the light's point of view when I ask and a thing or person to hold the globe and tried different angles then I could tell. Sagittarian Milky Way (talk) 17:12, 14 March 2023 (UTC)
 * To be clear, when I mentioned the Milky Way I wasn't just refering to the visible swathe of stars, I was referring to our entire galaxy. Anyone who's seen the night sky from a decent vantage point knows that it's incredibly bright and full of stars to the dark adapted eye. From the Moon during a lunar eclipse the Earth would look similar to how a new moon looks from Earth on a very starry night (only 4 times the size). I doubt you could see any details other than a glowing ring of atmosphere around the horizon and millions of stars surrounding it. <b style="font:1.3em/1em Trebuchet MS;letter-spacing:-0.07em"><b style="color:#000">nagual</b><b style="color:#BBA">design</b></b> 18:08, 14 March 2023 (UTC)
 * John Bortle said that in the darkest Earth skies even Jupiter degraded dark adaption and the Scorpius and Sagittarius regions of the Milky Way cast obvious shadows but didn't mention stars. There are a buttload of stars on the celestial sphere but only about 10^4 <6.50 magnitude, 10^4.5 <7.50 magnitude and 10^5 <8.50 magnitude, not millions naked eye unless you also count the milky band of unresolved stars that is only visible collectively. I suppose the stars would be a little brighter than looking straight up at sea level. The average sea level atmospheric light extinction is about 0.33 magnitudes per airmass (which is not added to star magnitudes maybe cause so many things affect the brightness gap with space (altitude, distance from zenith, humidity, pollen etc)) Sagittarian Milky Way (talk) 19:58, 14 March 2023 (UTC)
 * Also you can avoid seeing the twilight by looking up a specific area to search taking into account libration and driving around till you find a point where the horizon is shaped just right to block the crescent Earth and its twilight but not the center when the city you hope to see is there in its evening. The Moon has no atmospheric twilight so it can get very close to full before it becomes hard to find a place where you can do that while not seeing sun or sunlit moon for over half hour (though it's alleged that many hours of uninterrupted dark is slightly more dark adapted than only a few hours) Sagittarian Milky Way (talk) 20:10, 14 March 2023 (UTC)
 * The city you hope to see? Let's not forget that the random alien described in the OP does not know if there is a civilization of intelligent beings here, let alone where exactly to look and at one particular time. If the alien knows about Chinese New Year and wotnot all bets are off. <b style="font:1.3em/1em Trebuchet MS;letter-spacing:-0.07em"><b style="color:#000">nagual</b><b style="color:#BBA">design</b></b> 20:40, 14 March 2023 (UTC)
 * Right, that would be an idea for the slightly different question of Moon tourists trying to see a city naked eye just to be able to say they did it. I bet anyone who interstellar traveled all the way here would already know about the interesting spectral lines (the canceled Terrestrial Planet Finder could've seen life lines 29 light years away!) Sagittarian Milky Way (talk) 21:21, 14 March 2023 (UTC)
 * However Chinese New Year is near New Moon so that wouldn't add to the light. Maybe one of the Full Moon holidays has fireworks? Sagittarian Milky Way (talk) 21:39, 14 March 2023 (UTC)
 * @Nagualdesign You say the glow of cities on the night side of the Earth would not be visible.  You may be right, but that surprises me.  There are so many pictures of the Earth at night,  which show glowing  cities.   But these were taken from a distance much closer than the moon.  Hmmm.  What is the "angular size" (whatever it's called) of the Earth as seen from the moon? David10244 (talk) 09:18, 20 March 2023 (UTC)
 * The Earth is just about 4 times the diameter of the Moon, so viewed from the Moon, the Earth would be 4 times wider (and thus 16 times the apparent surface area) as the Moon viewed from Earth. The Moon viewed from the Earth is a half a degree wide (about the diameter of a pea held at arm's length), and the Earth viewed from the Moon is two degrees wide (about the size of a US quarter held at arm's length). CodeTalker (talk) 20:34, 21 March 2023 (UTC)
 * The Great Wall of China is supposedly visible "from space". If that's true, how far out would an observer have to be for it to become non-discernible? And where is that point in relation to the Moon? --  Jack of Oz   [pleasantries]  19:40, 13 March 2023 (UTC)
 * I would think that the Great Wall is long enough to be visible, but it's certainly not wide enough. Which leads me to this article: Artificial structures visible from space, which I'm going to peruse now. (Two minutes later...) Okay, not particularly helpful for this question, but the article might profit from the outcome of this discussion. --Wrongfilter (talk) 19:43, 13 March 2023 (UTC)
 * You can see the wall with a 116 millimeter wide telescope out to about 206,265 times the wall's width. But you have to paint it white and 1+ wall-width on each side black or vice versa and know where to look. You need to be about 60 times closer to see the dazzle-painted system with 20/20 vision. Sagittarian Milky Way (talk) 21:10, 13 March 2023 (UTC)
 * I recall that in previous discussions of this question in astronomical circles, it was argued that although the Wall itself is too narrow to be seen from orbit/the Moon, the changes to the surrounding terrain around it (roads, cleared vegetation, etc.) make it effectively much wider, and therefore discernable as a feature from much further away. {The poster formerly known as 87.81.230.195} 90.205.108.171 (talk) 21:26, 13 March 2023 (UTC)
 * I didn't think of that, though technically that's just knowing it's in that line not actually resolving it. Sagittarian Milky Way (talk) 21:43, 13 March 2023 (UTC)
 * You know what's easy to see from space? The Tokyo Dome. It's dozens of times wider than the wall and white while the city around it is gray. It could be seen naked eye in the day up to about low Earth orbit. Lots of artifacts are at least that easy. i.e. Central Park, it's a green rectangle 1 kilometer wide in a gray city. The color darkens when the buildings get higher (probably shadows are at least a major cause). Tokyo itself is an elongated gray blob many kilometers wide in green land next to a 10+ kilometer wide bay. I don't know what's the most distance a current city can be seen from with 20/20 and what's the best combo of viewing target and sun/astronaut positionetc, I wish I knew. Sagittarian Milky Way (talk) 21:41, 13 March 2023 (UTC)
 * With good weather conditions I guess one could see the glasshouses of Almeria as a spot from the moon, but what could one tell from that? NadVolum (talk) 20:41, 13 March 2023 (UTC)
 * I wonder what's the furthest you see the Sun reflecting off something man built that has existed by now? When liberal interpretation of human built maybe the furthest is a dam reservoir. Sagittarian Milky Way (talk) 21:49, 13 March 2023 (UTC)
 * 20/20 vision can see a black square in a white field at 10,800 over pi diameters. The Moon is 384,401 kilometers away on average, 356,000 and change on rare occasions and slightly further or better once a month. Cut those distances by 1 long Moon radius plus a few km for the mountain that gets closest to Earth if at the closest part of the Moon instead of the core or side, another c. 6,384.4 kilometers if looking at the Ecuadorian volcano Chimborazo when it's underneath which is the point on Earth that gets closest to the Moon. The naked human eye visible part of a city will be smaller than the metro area and it won't contrast with the surrounding farms or nature as much as the black squares on white field of the Snellen chart and will probably be rounder than the Snellen squares. The exact right retina pixel would also be hard to find in all that clutter. Clouds and shade variations everywhere, is that pixel it? The other one? I have a strong suspicion sapiens can see cities from the Moon during a deep enough lunar eclipse. Just wait for one that's not terrible like 28° 26' S mid-Pacific (streetlights get brighter the closer it is to looking from below but atmospheric extinction and twilight increases exponentially near the edge of the total Earthal eclipse but the city is squished in one direction making it brighter I guess but there could be more reflected streetlight looking down at the city but cities 29 degrees from the equator or more can't be directly overhead and those tend to be third world and poor (less light pollution per capita)), figure out how to point your helmet straight up and be as dark adapted as you can at mid-eclipse. If you can easily see a city from cruising altitude with probably a shitty window and not max dark adaption then you can probably see it it's till much less than one retina pixel if the ring of twilight isn't too bright. The surface brightness doesn't go away just cause it's further away. Sagittarian Milky Way (talk) 21:04, 13 March 2023 (UTC)
 * I got my figures wrong for that greenhouse area, I don't believe it would be visible from the moon at only 100 square miles. I think you'd need an area more like a hundred times that size or a hundred miles across. It might be possible to see some dim light from some large dense urban regions but it is by no means certain considering how many fields and trees there are around. The light of cities can be seen by astronauts, but the pictures of cities from the ISS use long exposure film. NadVolum (talk) 23:26, 13 March 2023 (UTC)
 * I forgot to divide by 2, one minute of arc is only 10,800 diameters divided by pi. Orders_of_magnitude_(illuminance) provides a way to tell. 252 microcandelas per square meter is 21.6 astronomical magnitudes per square arcsecond. Multiply by 3600 to get magnitudes per square arcminute.(100^0.2)^8.890756252=3600 so subtract 8.890756252 from 21.6 to get 12.7092437 magnitudes per square arcminute. It says typical photographic scene lit by full moon is ~1.4 millicandelas per square meter and other table says that's 2.5 decilux or "less than 1 lux". Google says typical streetlighting levels are about 10 lux. 1.4/0.252*(10/.25)=222.2222222 or 5.866968715 magnitudes. 12.7092437-5.866968715=6.84227499. Oversized from resolution star images are still round so account for only pi/4ths being filled (pi/4)*(100^0.2)^0.2622753=1 so add 0.2622753 magnitudes. 6.84227499+0.2622753=7.10455029. Limiting magnitude is about 6.5 without averted vision but the direct streetlights in sight are much brighter than the ground that lit them but some of what you see is unlit roofs, treetops etc but the eye can see lower surface brightnesses when extended than just one pixel maybe there's a large conglomeration New York latitude or closer somewhere on Earth (you can't see New York at less than 12 degrees tilt from the Moon causing shading of some lights by the mostly unlit roofs). Tokyo is about 35 degrees from the equator, LA is a little closer and the streetlit area is fucking huge. Shanghai is closer still. You can see the Guangdong-Macao-Hong Kong megalopolis from directly overhead from the Moon and it's even more populous than Tokyo and has casinos and the nightly Hong Kong lightshow. East Asians really like making skyscrapers look like Vegas at least part of the night. And probably Dubai too but I don't know if they have enough, it's a small population. Another idea: Find a part of the Moon where the crescent Earth will slowly rise dark first (from the libration) with a horizontal crescent and Guangzhou will rise at the bottom of a V-shaped horizon indentation and blink into view at the center of the Earth without twilight being visible, you'll be in pitch dark and can use averted vision. Pick the city is in evening side of the Moon not the city is in early morning side and wait for their biggest fireworks and light show of the year to coincide with this time. Times Square had a "Space Cannon" searchlight for the millennium, I don't know if that's worth even 10% of the brightness of a rich city of 20-85 million of pointed at you on the Moon or if any other party in a big city does that. Sagittarian Milky Way (talk) 01:41, 14 March 2023 (UTC)
 * For a tongue-in-cheek point of view, see this. --Ouro (blah blah) 05:27, 14 March 2023 (UTC)


 * As noted in Artificial structures visible from space, the notion that the Great Wall can be seen from space is mostly a myth. ←Baseball Bugs What's up, Doc? carrots→ 07:40, 14 March 2023 (UTC)


 * A bit pedantic, but if an alien was standing on the Moon and looking for evidence of human activity, wouldn't something like the reflectors we've left on the moon be the most visible evidence? 12.116.29.106 (talk) 14:55, 14 March 2023 (UTC)
 * Good point. Martin of Sheffield (talk) 15:12, 14 March 2023 (UTC)
 * The beams are miles wide by then, I wonder what they look like and if they're infrared or green or red. They're all in the middle of the Moon, if they never lase a total lunar eclipse there might be no chance of seeing it with human-like eyes. Do they ever lase total lunar eclipses? Sagittarian Milky Way (talk) 15:35, 14 March 2023 (UTC)
 * Thinking of being pedantic...solar eclipse is when the moon blocks the sun (blocking its view from earth: S-M-E) and lunar eclipse is when the earth casts its shadow on the moon (blocking its illumination: S-E-M). When one is on the moon, is S-E-M called a solar eclipse? And what is the term for S-M-E when looking from the earth that is in the moon's shadow? DMacks (talk) 18:20, 14 March 2023 (UTC)
 * It's also a solar eclipse. Browsing the List of missions to the Moon I couldn't find a mission or rover that spent a lunar night on its nearside. However, the Hakuto (“white rabbit” in Japanese) mission launched Dec. 11 and should be landing in April with rovers that can see the nighttime Earth phases with stars, although the rovers are not alien. Modocc (talk) 18:49, 14 March 2023 (UTC)
 * Awesome! Yet another cool never seen before space point of view. Also good for showing flat Earthers (I wonder if any responses will NOT be Photoshop! fake! CGI! I can tell cause X! or similar) With that budget I'm sure they wanted to they could afford a camera with huge electron wells and/or enough megabytes per second for frequent electron well discharge with good counting and/or make a multiple ISO composite (multiple cameras or one at a time) and show the uneclipsed Sun disc with stars. Really you could do that even with film with a coronagraph that's like an eclipse filter except much stronger instead of completely opaque and a polar mount long exposure. Sagittarian Milky Way (talk) 19:27, 14 March 2023 (UTC)
 * Correction: that should be "looking at the earth. The earth-phases article is interesting, thanks! DMacks (talk) 19:08, 14 March 2023 (UTC)
 * SE2024Apr08T.gif obvious term for an S–M–E eclipse, looking from the Moon to the Earth, is "terran eclipse". If the Moon casts a shadow on the surface of the Earth, Earth dwellers in the shaded spot experience, at the same time, a solar eclipse – and if all sunlight is blocked, it is a total solar eclipse. Viewed from the Moon, the terran eclipse is never total. The darkness of a total solar eclipse covers only a minute fraction of Earth's surface as it traces its path across the globe. The penumbra covers a larger part; unlike what is seen in the animation, it does not have a sharp edge. The decrease in the total luminosity of the Earth is not spectacular. --Lambiam 20:38, 15 March 2023 (UTC)
 * Total solar eclipse from Moon naked eye: Meh.
 * Total solar eclipse from Moon enough optical aid: Oh my God, the mountains look 3-D! Where is this? Oh Morocco! Gibraltar looks cool! Lisbon looks cool! Pyrenees look cool! Paris looks cool! Thames looks cool! Umbra landfall site looks cool! Earth's rotation looks cool! Here comes the umbra! It's huge! Sagittarian Milky Way (talk) 01:50, 16 March 2023 (UTC)
 * I think the sensible term for a S-M-E eclipse would be a solar eclipse, since it's an astronomical event describing the alignment of the Sun, Moon and Earth, regardless of where you are in the Universe or whether you witness it personally. And a S-E-M eclipse should always be called a lunar eclipse for similar reasons. <b style="font:1.3em/1em Trebuchet MS;letter-spacing:-0.07em"><b style="color:#000">nagual</b><b style="color:#BBA">design</b></b> 19:38, 16 March 2023 (UTC)
 * Maybe a majority of early humans would call them that but probably those who lived on the Moon their whole lives would call them "terran", "Earth" or similar and solar respectively. If their parents don't they'll especially want to rebel against them and flex how moonar/native they are. Sagittarian Milky Way (talk) 20:37, 16 March 2023 (UTC)


 * Also Venus superimposing the Sun at Earth is called a transit, Venus' view of that is technically a penumbral eclipse of the Earth. Sagittarian Milky Way (talk) 20:48, 16 March 2023 (UTC)