Wikipedia:Reference desk/Archives/Science/2013 May 9

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May 9

When was the cactus introduced to China?

This recent question informed me that cacti are native to the Americas. They are very popular in China, and the Chinese have bestowed an awesome name upon them: 仙人掌 (literally "immortal's palm"). I'm now very curious to learn whenabouts the first cacti made it to China. Given their prodigious trading history, I assume it was rather early... but even then, it could not have predated Columbus, could it? The Masked Booby (talk) 02:05, 9 May 2013 (UTC)

They could predate Columbus.Try google-ing on "Storm-driven maritime dispersal" --Digrpat (talk) 09:36, 9 May 2013 (UTC)

Most likely it would refer to a cactus-like plant in the Euphorbiaceae - see [1] for example. See [2]. Wnt (talk) 19:24, 9 May 2013 (UTC)

The only reliably sure pre-Columbian contacts between Eurasia and the Americas broadly are Vinland and whatever early Polynesian seafarers brought the sweet potato to the Pacific Islands and New Guinea. The Vikings of Vinland didn't land anywhere that cacti grew, but as there are cacti ranging down the Pacific coast from Canada to Patagonia, it is technically possible that those same Polynesians may have taken Cacti to China. Though unlikely, because I don't know that they dropped off the cacti in the same places they dropped off the sweet potato. It's much more likely than any true cactus in China got there as a result of the post-Columbian trans-Pacific trade of the 16th-17th centuries. --Jayron32 19:32, 9 May 2013 (UTC)

Is there actually a trend in the size of fragments of a broken object or is it a hoax?

Just in some Chinese internet articles I saw something like a "雅各布·博尔碎片规律" （Jacob Bohr?'(I cannot find his exact english name)s trend of fragments）, it roughly says:

In 1942, a Danish college student Jacob Bohr accidentally broke a glass bottle. He studied the sizes of galss fragments, and find the sizes of fragments roughly falls in 4 groups, with each group's average size 16 times that of the next group. This trend applies to other objects of different material and size too, though there are some difference in the proportion between sizes of each group,(16 for cups and vases, 11 for sticks, and 40 for balls). This trend is now applied in archaeology and meteor studies as to estimate positions of the fragments in the original piece.

Another version says that Jacob Bohr categorized the pieces by sizes(largest, medium, smaller, smallest), and calculated the average mass of each group, but it seems too rough and arbitrary. Still another unsourced version gives that he categorized the fragments by mass groups of 10g-100g, 1g to 10 g, 0.1g to 1g and less than 0.1g.--朝鲜的轮子 (talk) 02:12, 9 May 2013 (UTC)

Seems genuine. Or at least, it appears to have been published in a scientific journal: [3] AndyTheGrump (talk) 04:10, 9 May 2013 (UTC)

Actually, that seems to refer to something else - but by the same author, I'd imagine. AndyTheGrump (talk) 04:13, 9 May 2013 (UTC)

Are there any other studies in this area?--朝鲜的轮子 (talk) 06:07, 9 May 2013 (UTC)

And are there more details to this study and its conclusion? I have seen text whichi I have no access to full text[4] such as "pieces between one-tenth of a gram and a gram will be 16 times greater still. The number 16 is the "scaling factor". It looks like what I have in derived articles in Chinese. I am not sure whst it means: greater in number, or greater in mass or volume?--朝鲜的轮子 (talk) 06:33, 9 May 2013 (UTC)

There has been a lot of research into fragmentation because it's important in many fields (and of interest to the military). If you google size distribution of fragments you'll find many interesting papers. Sean.hoyland - talk 06:34, 9 May 2013 (UTC)

Still I have struggled on understanding the "pieces between one-tenth of a gram and a gram will be 16 times greater still". According to the idea in the abstract given above, the Probability density function of a fragment of mass m being found should be like f(x)=A*m^-C, where C is scaling exponent,an integration will show if the mass range is reduced for 10 times, the probability of finding fragments of this size will be 10¹⁵times greater?--朝鲜的轮子 (talk) 07:06, 9 May 2013 (UTC)

No, not 10¹⁵times greater. Just 16 times greater. C is NOT equal to 16. Dauto (talk) 17:45, 9 May 2013 (UTC)

"The probability of finding a fragment scales inversely to a power of the mass; the power, or scaling exponent, was found to depend on the shape of the object rather than on the material." Do I get it right?--朝鲜的轮子 (talk) 00:09, 10 May 2013 (UTC)

I see. Scaling factor and scaling exponent are just two different terms. It is rather confusing withour reading the whole article.--朝鲜的轮子 (talk) 01:41, 10 May 2013 (UTC)

The fragmentation of material has also been described in terms of fractals. This ref gives examples from both naturally and experimentally fragmented materials, many of which have a fractal size distribution with a fractal dimension of about 2.5. I'm not clear whether this is describing a similar distribution to the OP's example. Mikenorton (talk) 19:34, 9 May 2013 (UTC)

The size of sun seen from planets look bigger during sunset?

At 6;30 PM at Pacific Time Zone, I can actually look at the sun setting the sky, the sun looks big going down at the horizon. But at afternoon, I am too afraid to look at the sun, I can estimate the sun at noon is much smaller than it is during sunset. I don't know why is that? Does other planets follow the same way? If we look at the sun at Mercury during sunset will the sun view in horizon look bigger than the sun view in horizon during the noon hours? What about seen from Jupiter's four major moons?--69.233.254.115 (talk) 04:24, 9 May 2013 (UTC)

Only if they have refracting atmospheres. μηδείς (talk) 04:32, 9 May 2013 (UTC)

(edit conflict) The concept is covered at the article Moon illusion which discusses why both the sun and moon appear to be larger when close to the horizon. The sun and moon are not larger; if you measure the size of the sun on the horizon and compare it to the size of the sun at noon, they will be the same. You can do this with the full moon easier since a) it isn't blindingly bright and b) it is subject to the same illusion as the sun. Just hold out a ruler at arms length and measure how big the moon is when it is on the horizon and looks abnormally large (say, near 6-7 PM on the night of a full moon). Then do the exact same thing at around midnight. You'll find them exactly the same size. There is some debate as to what the source of the optical illusion is (and it is just that: an optical illusion), though I suspect the illusion is caused primarily by the proximity to the horizon as a reference. When the sun or moon is higher in the sky, you don't have the horizon nearby, so your brain perceives it to be smaller at the zenith and larger on the horizon. (post EC response to Medeis) The "refraction" hypothesis has been around since at least Aristotle and Ptolemy, and has been summarily discredited since the real size can be very accurately measured, and does not vary. Actually, both the sun and moon are very slightly smaller on the horizon because they are literally farther from the viewer; the difference for the sun is impossibly small to measure, while the difference for the moon can only be detected on very accurate instruments. In reality, to any reasonable measure, the sun is the same size at all points in the sky. It's just an optical illusion. --Jayron32 04:36, 9 May 2013 (UTC)

So is just the human eye is a crappy way to measure size? Is that angle of perception? I looked up refraction article.--69.233.254.115 (talk) 04:54, 9 May 2013 (UTC)

It's not refraction, it's an optical illusion. The illusion is in your brain, not your eyes. Take the classic crazy tables illusion, your eyes aren't diffracting anything or seeing anything "incorrectly", it's your brain's interpretation of the input from your eyes that's causing the illusion. Vespine (talk) 05:41, 9 May 2013 (UTC) (edit, sorry I don't know how I got above Jayron32's reply even though my edit time is 3 minutes later, It did take me more then 3 minutes to write the above)

It's not your eye. The lenses in your eyes aren't to blame. It's your brain. In order to make sense of the world, your brain has to take the input from your eyes and give it meaning; in doing so it takes contextual clues to determine things like relative size and distance. That is, your brain judges the size of an object by the environment that it is in. This is because your brain has to decide between "big and far" and "small and close" and there are ways to construct "optical illusions" which confuse your visual processing center in your brain into messing up the sizes of objects. This is a necessary side effect of being able to (in most cases) be able to accurately judge distance and size; that is the same processes that cause the illusion come "part-and-parcel" with the processes that allow you to see in the first place. Some common illusions that play with your visual perception of size and distance are the Ebbinghaus illusion, the Delboeuf illusion, the Ames room illusion, the Jastrow illusion, the Müller-Lyer illusion, the Ponzo illusion. Whatever processing glitch is responsible for these common illusions, the same thing is likely happening with the size of the sun on the horizon. They're all caused by our brain misinterpreting size due to the contextual clues around the object of study; the brain perceives the size of the object from these cues, and sometimes it gets it wrong. What is striking is how consistent it gets it wrong: nearly all people experience these illusions in the same way, which makes it clear that this is something universal to how the human brain constructs a visual image of the world using the raw data from our eyes. --Jayron32 05:38, 9 May 2013 (UTC)

I (more or less) agree with (nearly) everyone else here - this is just like the moon illusion, the sun looks larger near to the horizon because of a simple optical illusion. However, as the image at right shows, atmospheric diffraction does plays a very, very small part in distorting the shapes of the sun and moon - but not by enough to account for the size changes we perceive. In fact, you can only see the distortion effect at all when the sun or moon are very close to the horizon over the ocean or dead-flat ground. Under those circumstances, you can sometimes see that the orb of the sun is squashed at the bottom just as it touches the horizon due to atmospheric diffraction. Worse still for the atmospheric distortion theory - and as you can see in that picture even if atmospheric distortion were a factor, it would make the sun and moon look smaller at the horizon...not bigger!

Incidentally - I do 3D computer graphics for a living, and am occasionally asked to include the sun and/or moon in my pictures. Its interesting to note that the moon illusion shows up in computer graphics sun and moon rise and set too. Even though we have not simulated any atmospheric distortion in the graphics system - and we know for 100% sure that the diameter of the circle we're drawing for those bodies is precisely the same size no matter what the time of day is - you still get the powerful impression that the sun and moon are twice the size at the horizon than at zenith.

SteveBaker (talk) 13:07, 9 May 2013 (UTC)

The OP seemed to be asking about distortion at sunset, not relative appearance high and low in the sky. I am curious why you say the sun would appear smaller at sunset? μηδείς (talk) 19:26, 9 May 2013 (UTC)

I think you misunderstand.

What we're being asked is: "I can estimate the sun at noon is much smaller than it is during sunset. I don't know why is that?" - so our OP believes (as many people do) that the sun (and moon) are smaller at noon and larger at sunset. We know that this is because of the "moon illusion" effect. The argument that you (wildly incorrectly!) made that this is somehow due to refraction is untrue for two very good reasons:

1. Because the moon illusion is the cause - proven by the fact that the illusion persists even in computer graphics where there is no refraction.
2. Because if it were due to refraction, it would only happen within about a third of a sun/moon diameter of the horizon - *and* the effect of refraction is to decrease the apparent size, not to increase it.

So for both of those reasons, the your explanation and the OP's evident suspicion (that this is related to atmospheric composition) are entirely false - and therefore the followup about other planets is probably not relevant.

If you look at the pair of photos I posted (above), you can see that the top half of the sun is more or less a semi-circle (following the blue line) - and the bottom half is drastically squished upwards (following the green line). You can clearly see from that photograph that the effects of atmospheric distortion only happen VERY close to the horizon (too close to encompass the entire sun or moon) - and the effect is to squash the object, not to stretch it...so even if refraction were somehow the cause - the sun/moon would look SMALLER at the horizon, not bigger.

Despite this explanation, many people refuse to believe that this effect is just an illusion (because it's such a powerful one). To those people, I suggest this practical experiment: A US one-cent coin (or a UK penny), held at arm's length more or less covers the sun (or moon) exactly (depending on how long your arms are!). Knowing this, hold the penny up right next to the sun at noon - then again at sunset - and it'll be obvious that the true size of the sun is indeed exactly the same on both occasions.

QED.

SteveBaker (talk) 20:29, 9 May 2013 (UTC)

I took the OP's question to be, why does the sun look wider at sunset. Your answer sees to be it is not larger, but it is squished top-to bottom. Unless that distortion in proportions is caused by something other than the refractive power of an atmosphere I am satisfied. μηδείς (talk) 21:47, 9 May 2013 (UTC)

The answer, as many other editors have said, is that it's an optical illusion. The Sun is not actually wider at sunset; it just looks that way because the brain is misinterpreting what the eyes are telling it. --Bowlhover (talk) 23:22, 9 May 2013 (UTC)

Not larger over all, squatter. Wider, but not taller to the same extent. μηδείς (talk) 01:02, 10 May 2013 (UTC)

As Jayron32 alluded to in the first post on this question, the brain percieves objects high the sky as smaller than they are. The brain uses objects on the horizon (trees, buildings, hills) to calibrate size perception. When nearby objects of roughly known size are nearby, the brain can accurately judge the size of the sun or moon. When no such objects are nearby, the brain defaults to perceiving the sun or moon's disk, or anything else in the sky, as much smaller than reality. The phenomena is well known to movie directors, and was well illustrated in the Howard Hughes movie The Aviator, 2004 starring Leonardo DeCaprio. Hughes was depicted making a movie featuring a biplane dogfight, and wanting audiences to feel immersed in the action. It didn't work, because the planes in the sky look tiny, and not of the size expected from the lens focal length and shooting distance. Hughes realised that to provide the eye with scale, he had to include ground objects in each scene, and have the planes visually near the objects. Wickwack 124.178.140.2 (talk) 01:13, 10 May 2013 (UTC)

The Moon Illusion changes the apparent size of the moon, not its relative width. The sun gets squat, squatter, and green flash as it transits the horizon. μηδείς (talk) 02:25, 10 May 2013 (UTC)

None of that has to do with the illusion we're talking about here. We keep saying "apples, apples, apples" and you keep saying "But oranges!!!". Try to keep up. The moon illusion doesn't have anything to do with the odd effects happening at sunset/sunrise or moonset/moonrise. It has to do with the full circle of the moon/sun appearing bigger when close to the horizon, and smaller at the zenith. This is a real, documented effect and optical illusion (that is, caused purely in the visual processing of the human brain, not with the actual image itself) which is quite independent and unrelated to all the effects you are talking about. The ones you are talking about is what happens to the disc of the sun as it moves past the horizon itself. Different. --Jayron32 02:54, 10 May 2013 (UTC)

Green flashes?? Medeis needs to get outside more. Wickwack 124.178.140.2 (talk) 02:57, 10 May 2013 (UTC)

The green flash is actually a documented phenomenon, so Medeis is (probably) not pulling this out of his ass. (Or his ass's ass - :) ) Whoop whoop pull up ^{Bitching Betty | Averted crashes} 03:33, 10 May 2013 (UTC)

I'm pretty sure it's her ass, but regardless, I've conceded that the green flash is a real phenomenon, as are the other actual optical distortions noted; just that they aren't relevant to the discussion at hand. --Jayron32 05:02, 10 May 2013 (UTC)

I was referring to 128.178.140.2's comment. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 19:28, 11 May 2013 (UTC)

One more observation, I live on the western edge of a mountain range. So when the sun or moon rises, it has to come up quite high to rise over the mountains. I've driven home in the city when the moon is 20-30 degrees up in the sky and looks quite normal, but when I get close to home and the mountains dominate the horizon, the moon will look huge even though it's higher in the sky then when I started the trip. Vespine (talk) 05:54, 10 May 2013 (UTC)

Excellent observation, but don't forget that it isn't that the moon looks bigger than reality when against the horizon (or montain range); it's that it looks smaller than reality when high in the sky. Wickwack 121.215.25.35 (talk) 08:01, 10 May 2013 (UTC)

It's actually pointless to decide which perspective on the moon is "reality". The entire issue is that the moon looks larger on the horizon and smaller at the zenith, not that either position represents "reality". Both are merely collections of neurons firing in your visual cortex giving you a particular perception of the moon, the illusion is the mistaken belief that the two don't match each other, when in fact, they do. It makes no sense in this context to speak of one collection of firing neurons as "more real" than the other. --Jayron32 15:42, 10 May 2013 (UTC)

I disagree - it may be "pointless" - but it's certainly possible to find out which one is "correct". I've actually tried this with many people (although not a statistically valid sample). If you talk to people about holding various objects at arm's length and using them to cover the sun or moon - they will generally concede that a penny - or at most a golf-ball held at that distance would cover the disk of the sun or moon at zenith - and that a shirt-button would definitely not be big enough and a tennis ball would be much too large (in fact, a penny is big enough). However if you ask them how big the sun/moon looks at the horizon, they tell you things like that your whole hand would only just cover it, or a tennis ball held at arm's length is the right size - only people who know about the illusion, or who have actually tried it will easily believe that even a golf ball is sufficient to cover a full moon at the horizon. So the illusion is definitely inflating the "true" size of the object when it's close to the horizon rather than shrinking it at zenith. That's a testable fact...and "too big" at the horizon and "about right" at zenith is the clear result from my informal testing...although I have to say that people do also assume the sun and moon are larger than they really are at zenith too...just not by such a crazily huge amount!

The fact that they over-estimate the size of the sun and moon everywhere is another interesting thing...coming back to my computer graphics experience, when I draw the sun and moon at the correct size - most people say that they look ridiculously too small - so I tried giving them a slider to adjust that size - and at zenith, down to maybe 30 degrees above the horizon, they generally wanted the sun or moon to be about twice as large as it should be...which is about the size of a golfball held at arm's length...which nicely ties in with what many people tell me when I ask them to estimate the size that way. That suggests that some part of the over-estimate is not so much an optical illusion (which ought to be the same in the computer display as in reality) - but more a matter of memory. SteveBaker (talk) 16:58, 10 May 2013 (UTC)

How Green Was My Valley was a best-selling novel that later became an Oscar-winning movie. The author, Richard Llewellyn, wrote a lesser known sequel called Down Where the Moon is Small, which surely draws on the optical illusion of the moon apparently changing its size. -- Jack of Oz ^[Talk] 20:39, 10 May 2013 (UTC)

I'll say this ONE MORE TIME. When you see at the moon down near the horizon, you see it at its' true size. When you see it up in the sky, you see it appearing much smaller that it actually is. Jyron32 is wrong when he asserts its only relative. SteveBaker is correct when he says we can determine the true size, but then goes on to repeat a common old wives tale about pennies (he obviously hasn't tried it himself) and seems to think it's the sky view that is correct. It is NOT that it looks too big near the horizon, its that it looks too small when up in the sky. And the reason is because up in the sky the brain has no familair objects to calibrate scale perception, and when that is the case, it defaults to perceiving objects smaller than reality.

We known exactly how big the moon should look (or the sun, forgetting for moment that it is too bright to look at up in the sky). It's simple proportion. Ignore for the moment atmospheric distortion (pretty much negligible) and the fact the the moon's orbit is elliptical. The moon's diameter is 3464 km. It's mean distance from Earth is ~380,000 km (neglecting radius of Earth, 12,700 km). Holding your fingers as far away from your face as you can, the distance from your eye is ~900 mm (average male). Therefore, by simple proportion, the moon will just cover a disk held in your fingers of:-

(3464/380000) x 900 = [9.1] sorry 8.2 mm.

So, the moon is equivalent to a disk 8.2 mm diameter held at arms' length. A US penny (one cent coin) is 19.1 mm diameter, and a golf ball 43 mm, which is why I think Steve didn't check before writing. And why all that stuff about computer graphics is more than a bit sus.

If you are surrounded by flat ground, the horizon is about 5 km away. But most people are not - they are surrounded by city building, hills, trees, or whatever. In my case, I can see buildings on top of hills about 2 km away east of me. I've looked when the moon is real low - again by simple proportion the moon is equivalent to a disk 18.2 m diameter at that distance. Guess what - that's just about how big it looks! And regardless of whether it is down on the horizon, or straight up in the sky, it is just covered by the nail on my third finger when held at arm's length. That is, about 9 mm. Go try this yourself, before posting on Ref Desk. Note that, due to orbital parameters and the Earth's radius, the angle subtended by the moon varies about 12%, reinforcing the illusion that it is big when down low - people do notice that it looks bigger sometimes than it does at other times. There is also an optical illusion that makes objects look larger if near the vanishing point if parallel lines are viewed in perspective - as for example roads seen narrowing into the distance, or city blocks stretching out below your vantage point. However this illusion usually is not effective with natural vistas. Perhaps it had something to do with Steve's computer graphics size perception.

Wickwack 121.215.63.160 (talk) 04:31, 11 May 2013 (UTC)

Er: (3464/380000)*900 = 9.1? On what planet is this true? Anyway - it doesn't matter. A penny (as you've kinda shown does indeed cover the sun/moon at arm's length...more easily than perhaps I suggested. But the fact remains that when you ask people whether a penny can cover it, they say "NO!"...and that's true at zenith and at the horizon. You can't prove that with math - you have to go out and ask some people who don't already know the answer. Their estimation of the answer is MUCH closer to being correct at zenith than at the horizon. This means that your speculation as to the underlying reason for the illusion is flawed. SteveBaker (talk) 06:15, 11 May 2013 (UTC)

Erk! It actually = 8.2 mm. I first worked it out for 1 m distance, then thought I should give the average arm's length distance, which when I checked was actaully closer to 900 mm, and forgot to change the result accordingly. However, I still got the rough size right. It's certainly not the size of a US 1 cent coin (penny). Maybe some other country's penny? By no means is my "speculation" as you put it flawed. The phenomena is well known and understood by movie directors as I said before - unless you provide nearby objects of known size, the brain gets it wrong. You just asked the wrong question of your friends. Or maybe you jiggled the answers to fit your pre-determined theory. Next time, ask them to recall the last time they saw the moon down real low, and ask them estimate the size it appeared to be as a disk, in metres or yards. Ask them to think about buildings or trees that were near the moon. I bet they get the size roughly right. Better still, go out and check it yourself. Wickwack 124.182.21.1 (talk) 07:14, 11 May 2013 (UTC)

So you want to ask people an impossible question? "How big does the moon appear in meters?" - I'd have no idea how to answer that. But ask if they believe that they can cover it with a golfball held at arms' length and you've asked a meaningful question with a perfectly valid answer...and when you do that, they always greatly over-estimate the size of an object required to cover it at both zenith and at the horizon - but to a much greater degree at the horizon. (And please, you are required to WP:AGF in your conversations here...your continual accusations that I'm deliberately lying or fabricating some story or other is not acceptable here - and I won't put up with it. You've done it several times now - and if you do so again, I will call in the admins and demand some kind of disciplinary action against you.) SteveBaker (talk) 14:40, 11 May 2013 (UTC)

It's not an impossible question - asking people to compare the moon's disk size against trees or buildings a kilometre or two away is not the slightest bit conceptually different to asking them to compare it to a coin or golfball about a metre away - only the scale changes. People know how big a golf ball is, and they know how big buildings are in their area.

Now, go call the admins. You have revealed yourself as a petulant schoolboy who cannot stand being caught out bullshitting. A more mature person, and a person more certain of his facts, would come back with a response that stands on its own merits. I could easily pick holes in some of your posts on other questions with technical answers but chose not to.

Wickwack 60.228.250.110 (talk) 15:32, 11 May 2013 (UTC)

Does someone need a hug? --Jayron32 18:05, 11 May 2013 (UTC)

I go with Wickwack. Estimating the size of anything by comparing it with buildings is trivial. You'd need to prompt most people to visualise when they last noticed a "huge" moon with buildings in the scene, but Wickwack covered that. What supports Wickwack's explanation that the brain needs nearby familiar objects to subconsciously judge scale is the following: If you watch the moon going down or rising while viewing it out at sea, with no ships visible, the illusion that it is small remains very strong - not necessarily as strong as at zenith, but very strong. The presence of a ship or yacht near the horizon kills the illusion. I've watched the moon rise or fall in flat desert country, and the illusion that it is small is quite stong then as well. I guess it's Steve that needs the hug.

Electromagnetic radiation effect

in memory chip, we can save several information/file. like credit card, pass card for employee, etc. My question is does it right that electromagnetic radiation can affect dammaged on memory chip? — Preceding unsigned comment added by 202.152.199.34 (talk) 04:56, 9 May 2013 (UTC)

The short answer is yes, and it's not just memory chips that can be affected. An Electromagnetic pulse can induce currents in delicate electronics far greater then the device's operating parameters. What kind or how strong a pulse it would take to damage any particular piece of electronics would depend on a very large number of factors. Typically, sensitive electronics are shielded at least to some degree to prevent damage from EMF that might be encountered during normal use, like audio speakers, generators, mobile phones, wifi transmitters, etc... Vespine (talk) 05:23, 9 May 2013 (UTC)

Er. If the answer is "yes," what was the question? Evanh2008 ^{(talk|contribs)} 05:27, 9 May 2013 (UTC)

Paraphrased from non-native English, the question appears to be "Is it correct that EM radiation can damage the information on a memory chip?" SemanticMantis (talk) 14:55, 9 May 2013 (UTC)

Earth's orbital plane and its revolution

The moon revolves around the earth, the earth revolves around the sun and the sun revolves around the center of milky way galaxy. Do these three celestial bodies have the same orbital plane or different orbital plane? The moon always revolves around the earth and everyday it should come in the path of sun and earth. I think the moon comes (once in a day) between the straight line joining the center of the earth and sun. If it comes, then, why don't we see solar eclipse everyday? Concepts of Physics (talk) 08:40, 9 May 2013 (UTC)

The rotation of the Sun around its axis, of the planets around the sun, and of the Moon around the Earth all lie in roughly the plane of the ecliptic (well, the orbit of the Earth lies exactly there, by definition). The key to your question is in the roughly. The moons orbital plane is inclined a bit more than 5% against the ecliptic. So we only get an eclipse when the Moon happens to be near one of the points where its orbital plane and the ecliptic intersect, and when it is also on a line with the Earth and the Sun. The orbit of the Sun in the galaxy is unrelated to the ecliptic. --Stephan Schulz (talk) 08:54, 9 May 2013 (UTC)

(edit conflict)

No, all three bodies have different orbital planes, all three have different orbital periods. Therefore, they do not allign everyday. Plasmic Physics (talk) 08:57, 9 May 2013 (UTC)

(edit conflict) There are three different orbital planes here. The plane of the Earth's orbit around the Sun is called the ecliptic plane. The Moon's orbital plane is at an angle of about 5 degrees to the ecliptic plane, which explains why we don't see a lunar and a solar eclipse every month. The orbital plane of the Sun around the centre of the galaxy is at an angle of about 60 degrees to the ecliptic plane. Gandalf61 (talk) 09:01, 9 May 2013 (UTC)

And to correct one thing, it takes a month for the Moon to orbit the Earth. Even if the orbital planes were exactly aligned, we'd see a solar eclipse once a month (at New Moon), not every day. Rojomoke (talk) 10:23, 9 May 2013 (UTC)

Speaking of which, an annular eclipse is starting in about half an hour's time in northern Australia and parts of the Pacific. C'mon down. -- Jack of Oz ^[Talk] 20:24, 9 May 2013 (UTC)

Wait a minute there! A lunar month is not the time it takes the Moon to orbit the Earth - it's the (average? variable?) time between two equivalent phases of the Moon. Because the Earth is revolving around the Sun, the point in the orbit where the Moon is full changes from month to month. See sidereal month. Wnt (talk) 23:24, 9 May 2013 (UTC)

Question about Science career choice in India

Hello,

I am an Indian Student currently looking at going for a 4 year Under-Graduate course in B.Science at the Delhi University. I've heard from some that doing the UG course is equivalent to doing a Masters course, as you can get Masters jobs and PhD straight after doing the UG. But at the same time, the course has just been introduced in IISc and will be introduced in DU this year onwards.

So could you please tell me whether I should be looking for doing a Masters after doing UG or not. If you can tell me which are the advantages in doing that?

Thanks! 117.194.88.176 (talk) 14:33, 9 May 2013 (UTC)

I don't know much about the system in India, but my general advice for students is to not worry too much about masters/Ph.D before you've even started your undergraduate education! Many things can change during your degree program. Your interests, your goals, your finances, your relationships, etc. Waiting all the way until the end of your second year UG to think about MS/Ph.D is certainly not too late. Just focus on your studies, and discovering what your true interests and motivations are. Good luck! SemanticMantis (talk) 14:48, 9 May 2013 (UTC)

You can ask the following resources.

• (The Times of India) Ask Questions, Get answers to Questions - Question Answers on IndiaTimes QnA :: Ask, Share, Learn
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—Wavelength (talk) 15:18, 9 May 2013 (UTC)

I have no idea how it works in India, but in other countries it depends on what sort of career you want. For example, in Australia (the USA is much the same) if you want to be an Engineer, you first do a Bachelor degree - that is the minimum requirement to work as a professional Engineer. With a Bachelor degree, you can assimilate just about anything in peer reviewed journals (with a bit of effort) and that's really all the academic ability you need. Most Engineers have only a Bachelor degree (4 years), but a few go on and get a Masters or even a Ph.D. It's more useful though to get a Masters in Business Management, not in Engineering, and do it after some years of professional work experience - that is what will help get an Engineer promoted. However, if you want to be a psychiatrist, the minimum requirement to be allowed to see patients is a Ph.D - so students stay at university for the required 7 or 8 years in one long stretch until they either drop out and do something else, or they get their Ph.D.

In any case, the best thing is to make contact with practitioners in fields that you might like, talk to them and find out what they really do, and how they got there. Consult staff at universities and review their syllabus and student handbooks. My advice is pick the course that you think, after diligently investigating (not just do, say, town planning because some school teacher who thinks it is about drawing plans has noticed you are good at art) that will be the most fun, at the best university you can get into. Relying on a few posts on Reference Desk and/or perusing a few websites will not give you enough to make a good decision. Wickwack 124.178.140.2 (talk) 00:47, 10 May 2013 (UTC)

Genetic diversity of dogs vs. other mammals

This blog post makes what I think is an absurd claim about the relative genetic diversity of dogs compared to other mammals. It says "The differences between a Great Dane and a Pug are greater than the differences between a weasel and a walrus." Now, they don't quite specify genetic diversity, but on this point, the claim can't be true. Unless I'm much mistaken, a Great Dane and a Pug are the same species, and thus could reproduce (better make it a male Pug, though). And of course, a weasel can't do this with a walrus. If the claim is more about phenotypes, the dog breeds would still seem to have more in common than the weasel and walrus. Is this claim true in any meaningful way? --BDD (talk) 18:22, 9 May 2013 (UTC)

Some of the genetic changes favored by artificial selection are alterations in really fundamental proteins that evolution normally doesn't touch (that is, doesn't touch and live for long). You could find individual nonconserved positions on specific proteins that would be more similar in weasel and walrus than between dog breeds, but only on the minutest fraction, a few individual basepairs, out of the whole genome. I ought to go hunting for examples in these dog breeds (Manx cat is one I actually remember offhand) but somehow I'm just not up to chasing the squirrel right now. Wnt (talk) 19:10, 9 May 2013 (UTC)

• The claim is utter nonsense, the differences between dog breeds are caused by a few sets of genes that regulate overall and relative size of skelatal organs, hair and skin color and pattern, and a few things which are rather superficial from a developmental point of view. There is no difference at the cellular, metabolic, or early developmental level; they still eat and digest their food in the same way, can interbreed, react the same to the same medicines, etc. On the other hand weasels and walruses are not close enough to produce ofspring. I highly doubt a walrus would do well on a weasel's diet. It's utter nonsense. μηδείς (talk) 19:20, 9 May 2013 (UTC)

• Another vote for total nonsense. I've been trying to think of some odd sense in which it might be true, but you'd have bend over backward like Wnt has above :) I suppose you could hand-type in every url at the bottom of the graphic, and read everything you find in an attempt to discover how they went so wrong, but I'm not that charitable toward lazy "info" graphic makers. SemanticMantis (talk) 20:52, 9 May 2013 (UTC)

Good catch! I didn't even notice the references at the end. I checked three of them and found this Scientific American blog post, which says the skulls of the Pug and Great Dane have more differences than the walrus and weasel. I suppose it's not exactly surprising that a blog took a fun fact and just ran way too far with it. --BDD (talk) 21:56, 9 May 2013 (UTC)

Heh. By that logic, you and I are more different than me and my dog are; you haven't slept as many days in the same house as we have ;) SemanticMantis (talk) 23:04, 9 May 2013 (UTC)

How did our Stone Age ancestors get enough vitamin A?

Some of our foods are fortified with vitamin A and some foods we eat that contain a lot of beta carotene like carrots are not really natural foods, they are the result of centuries long selective breeding. I have checked that I get enough vitamin A, but only about 1.8 times the RDA, despite eating about twice the average amounts of vegetables as recommended. If I subtract the extra vitamin A I get from carrots and from fortified foods, I end up below the RDA for vitamin A. So, I don't see how I could get enough vitamin A if I lived in the Stone Age. Count Iblis (talk) 19:00, 9 May 2013 (UTC)

Liver (food). Internal organs were held in quite high regard as foods in ancient times, and the use of liver for night blindness was known to Dioscorides (see [5]). Wnt (talk) 19:05, 9 May 2013 (UTC)

Indeed, carnivore livers can cause Vitamin A overdose (it's possible to overdose on Vitamin A itself, beta-carotene is excreted without being converted if taken in excess, and isn't poisonous) CS Miller (talk) 19:08, 9 May 2013 (UTC)

(edit conflict) Liver. Some animal livers, like Polar Bear, IIRC, have so much vitamin A that they are toxic even in small amounts. But the liver of any animal contains enough vitamin A to keep you going for some time. Vitamin A is a fat soluble vitamin so you can store it in the long term, to handle those weeks when you don't take down any buffalo for a while. --Jayron32 19:09, 9 May 2013 (UTC)

• Liver. μηδείς (talk) 19:23, 9 May 2013 (UTC)

Apparently Western culture is a bit of an outlier when it comes to diet, meat-wise in particular. We don't eat the good(-for-you) bits. Organ meat is where are the nutrients are. The Inuit diet is a decent parallel to how they got nutrients back then. Mingmingla (talk) 22:16, 9 May 2013 (UTC)

I see, so we are closer to being carnivores than our present typical diets would suggest. Count Iblis (talk) 22:38, 9 May 2013 (UTC)

No, that is not a valid conclusion. Ancient people ate lots of non-meat. What is true is that modern people (and modern USA in particular) eat far less organ meat than our ancestors did. SemanticMantis (talk) 22:59, 9 May 2013 (UTC)

That's what I was going for, yes. Mingmingla (talk) 01:22, 10 May 2013 (UTC)

Yes, I guess I was focussing too much on what I usually eat :) Count Iblis (talk) 12:40, 10 May 2013 (UTC)

It wasn't just liver. The Paleolithic diet included substantial amounts of leafy vegetables (modern day examples of which are spinach, kale and collard greens), a couple hundred grams of which is enough to supply all the vitamin A you need in a day. See Vitamin A#Sources. Red Act (talk) 23:57, 9 May 2013 (UTC)

One can also ask, did they get enough vitamin A? I don't see any reason to assume that they did. They got enough to survive as a species, or we wouldn't be here, but how we know they got enough for the best health outcomes is obscure to me. (It's not impossible that someone here does know that, in which case it would be interesting to hear.) --Trovatore (talk) 00:06, 10 May 2013 (UTC)

Our Life expectancy article gives aUpper Paleolithic lifespan of 33 years. So perhaps they really didn't live long enough to worry about all our modern middle-age health issues. But agree with the points above, that every edible scrap of the animal would have been eaten. Alansplodge (talk) 07:27, 10 May 2013 (UTC)

Life expectancy at birth is almost useless for that sort of consideration. The table says that a Paleolithic person who lived to 15 would on the average die at 54, which is more relevant to your point. But in any case it doesn't address my point particularly. There's no obvious reason to exclude the possibility that it would have been better for them to get more vitamin A, even in their prime, but it just wasn't available. --Trovatore (talk) 08:52, 10 May 2013 (UTC)

This "early people barely lived to 40, therefore they were unhealthy" trope is really common, and equally untrue... there's ample evidence as Travatore points out, that pre-agricultural people had catastrophically high rates of childhood death (even as recently as 100 years ago western cultures had horrific death rates among infants/mothers in childbirth). There's some other suggestive evidence that a lot of that early death was at the hands of other humans. I think most would concede though that some old-age diseases like dementia, certain cardiovascular diseases and cancers (prostate being the most obvious), etc. are old-age diseases. But of all the diseases to associate with age... the youth suffer from malnutrition diseases more than any other population. Alansplodge is approaching this like a video game.... start life at 100% vitamin A and work your way down. That's obviously a colorful exaggeration, but life expectancy has about 0 to do with this. Shadowjams (talk) 03:18, 11 May 2013 (UTC)

The following text comes from Douglas Mawson#Australasian Antarctic Expedition:

It was unknown at the time that Husky liver contains extremely high levels of vitamin A. It was also not known that such levels of vitamin A is poisonous to humans. With six dogs between them (with a liver on average weighing 1 kg), it is thought that the pair ingested enough liver to bring on a condition known as Hypervitaminosis A. However, Mertz may have suffered more because he found the tough muscle tissue difficult to eat and therefore ate more of the liver than Mawson. It is of interest to note that in Eskimo tradition the dog's liver is never eaten.

Dolphin (t) 08:20, 10 May 2013 (UTC)

As a fat-soluable vitamin, you don't need to consume vitamin A on a regular basis -- a short period of massive intake every few years will do. During World War II, as part of the research for their rationing program, the British put a group of people on a vitamin A-free diet to see what level of vitamin A the rationing program needed to provide. The war ended without any of them developing deficiency symptoms. --Carnildo (talk) 01:12, 11 May 2013 (UTC)