Wikipedia:Reference desk/Archives/Science/2013 May 12

= May 12 =

Why are there no terrestrial creatures that can emit electricity as a weapon?
Question as topic. I'm aware that this could be answered with 'because they haven't evolved yet', but I'm wondering if there's a reason why the only animals that can emit electricity from their bodies for attack/defence are all aquatic. Is there any reason that the same thing can't work in air? --Kurt Shaped Box (talk) 00:10, 12 May 2013 (UTC)


 * Mainly because when an electric eel tries to shock you the electricity has nice conductive water to flow through, but in air it has billions of ohms of air to get through. Potential divider has a nice diagram, if you imagine almost all of the volts being used up on the air, and an undetectable fraction being used across the much more conductive organism's body.-- Gilderien Chat&#124;List of good deeds 00:15, 12 May 2013 (UTC)


 * Perhaps the OP is refering to contact electrocution? Plasmic Physics (talk) 00:25, 12 May 2013 (UTC)


 * I was referring to either, really. --Kurt Shaped Box (talk) 01:10, 12 May 2013 (UTC)


 * Because water's a bloody good conductor and air isn't. Tonywalton Talk 01:16, 12 May 2013 (UTC)


 * I can imagine such a creature, say one that injects electrical contacts into the target. The question, then, is whether there is an evolutionary path that leads in that direction.  There are plenty of land animals that already pierce the skin of their targets, such as snakes, so that part is already done.  If there was some accidental electrical discharge, say as the result of mixing two components of a toxin which was injected, and that turned out to be useful in stunning the target, then evolution might tend to favor those with this feature.  StuRat (talk) 02:29, 12 May 2013 (UTC)


 * Because there is more than 10 orders of magnitude difference between the conductivity of air and fresh water, more than the difference of magnitude between 1/100th of a millimeter, and 100 kilometers. μηδείς (talk) 02:42, 12 May 2013 (UTC)


 * Could the pressure of water favor the production and use of electricity by creatures in the oceanic environment? It seems the problem is the production and deployment of electricity. Does the aquatic environment in some way favor this over the air/land environment? Bus stop (talk) 02:55, 12 May 2013 (UTC)
 * It's not pressure. It's dissolved electrolytes in the water that accounts for the difference.  -- Jayron  32  03:29, 12 May 2013 (UTC)

Electroreception discusses active electrolocation and electrocommunication, both of which involve and animal emitting electrical signals to interact with the environment. These would not be very useful in a non-conductive medium (air), so they would not have evolved in terrestrial animals. I'm thinking that these abilities would be intermediate steps on the evolutionary path to using electricity as a weapon, and since they did not evolve in terrestrial animals, weapons-grade electricity producing organs didn't either. This would not prevent aquatic animals with electrical shock capabilities from evolving into terrestrial creatures. The shocks might prove useful on land if physical contact could be made, but might cause burns to the shocker as well as the shockee.--Wikimedes (talk) 07:48, 12 May 2013 (UTC)


 * I can imagine an electric toad. Plasmic Physics (talk) 08:03, 12 May 2013 (UTC)

The star-nosed mole detects prey using electroreception. --Fama Clamosa (talk) 10:56, 12 May 2013 (UTC)

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 * Apart from not having the good conductivity of water, there are other reasons why it's not going to work as a weapon for land animals. The most important is that the marine creatures that can issue electric shocks are basically fish (rays, eels) with no limbs.  They have a simple muscular structure.  The electric organs are comprised of modified muscle cells. When they shock prey or other creatures, they shock themselves.  However, their simple limbless structure means nothing inconvenient to them happens physically, and, no doubt, their brains have adapted so they block out their own shocks and so "consciously" don't feel them.  If a land animal, with its complex limbed anatomy with a vast array of muscles was to develop electric organs, the current though its own body would cause all sorts of inconvenient muscle contractions with legs jerking about.  Note that an electric eel or ray is surrounded by a large amount of water stretching out in all directions.  So whenever it makes a shock, the current in its own body is pretty consistant in where in its own body it flows.  A land animal would have to cope with delivering shocks from various points on its own body into prey of all sorts of sizes and orientations.  So the current in its own body would never flow twice in the same way, making it difficult for the nervous system to block out the pain.
 * Another reason is that the sea provides a large area of contact to prey - this means a large current can flow without needing an excessively high voltage. On land, only small areas of direct contact can occur, making it hard to deliver enough current.
 * Most land animals have fur or at least a reasonably thick skin. Dry skin, and especially fur, is a good electrical insulator.  The only way to electrocute them with a reasonable voltage is through their feet or their mouth.
 * Wickwack 143.238.218.186 (talk) 11:27, 12 May 2013 (UTC)


 * Concerning your last statement: that is why I thought toad. Plasmic Physics (talk) 11:53, 12 May 2013 (UTC)


 * They're moist, and things tend to bite or grab them. Plasmic Physics (talk) 11:56, 12 May 2013 (UTC)


 * Probably not wise to electrocute something that's biting you. The shock will force their jaws to close.  And if you kill them with their jaws locked closed, its definitely curtains for both.  Wickwack 60.230.192.64 (talk) 13:22, 12 May 2013 (UTC)


 * Oh yea... Plasmic Physics (talk) 22:24, 12 May 2013 (UTC)


 * There's the possible example of the Mongolian Death Worm. Further research, however, is needed (and I have been indirectly sponsoring some, insamuch as I subscribe to an organisation which has mounted expeditions to search for specimens.) The poster formerly known as 87.81.230.195 212.95.237.92 (talk) 13:16, 13 May 2013 (UTC)


 * According to the Wiki article nobody has ever seen it. Further, it kills with electric discharge at a distance, which would defy the laws of physics, unless it carries a sizable Crocroft-Walton generator around with it.  Sounds like you have been sinking money into finding the Mongolian equivalent of the abominable snowman, or the Australian Cougar, or perhaps the local bogeyman who sneaks in at night and steals naughty children.  Wickwack 124.182.32.113 (talk) 14:13, 13 May 2013 (UTC)

Harbor seal rookeries in Maine & northeast
My spouse likes watching harbor seals. We are thinking of going to the northeast for our vacation in early June, and are wondering where are good places to go where this is a good chance of seeing harbor seals without having to go on some sort of a boat ride or tour. How far north do we need to go? Crypticfirefly (talk) 00:21, 12 May 2013 (UTC)


 * According to this PDF from NOAA, harbor seals are present seasonally as far south as Florida. So if your are in Key West you might have to travel north. :) --Fama Clamosa (talk) 12:09, 12 May 2013 (UTC)


 * My parents travelled to Maine a few summers back, which was the first time my mother saw seals in the US N.E. I'll see if I can find out where. μηδείς (talk) 18:38, 12 May 2013 (UTC)

Homo superior and human brain evolution
Most probably the 1911 novel The Hampdenshire Wonder first mentioned it — superior next generation of Homo sapiens — Homo superior. Three questions— Tito Dutta (contact) 00:54, 12 May 2013 (UTC)
 * 1) How are human species (mainly human brain) going to evolve in future?
 * 2) What are the extra-ordinary/advanced/remarkable features these?
 * 3) Will Homo sapiens evolve to Homo superior at all or it will be Homo inferior? (ref Human Brain size is shrinking?


 * "We don't answer requests for opinions, predictions or debate". Looks like a request for predictions to me... AndyTheGrump (talk) 00:57, 12 May 2013 (UTC)


 * Yes we shouldn't speculate but see Man After Man: An Anthropology of the Future for a book about it and the see also at the end of that article for various other more reasonable articles. Dmcq (talk) 01:04, 12 May 2013 (UTC)
 * Man After Man is a lot of fun, and so is Darwin's Radio, but if the OP expects an answer he should have asked this under the Entertainment desk. μηδείς (talk) 02:44, 12 May 2013 (UTC)

Astrophysics & Geography
1. Earth has an elliptical or exactly circular orbit?

2. If it is elliptical, then there must be apogee and perigee, and at the same time, we know that earth like planets can only exist in a narrow orbit space where the temperatures and energy received from the star are ideal for life (neither too hot, nor too cold) then why doesn't this shift in positions of earth pose a danger to global temperature and existence of life on earth?

3. 1 AU (Astronomical Unit)= Distance at which the Earth revolves around the Sun, has a constant value, how is this derived if the orbit is elliptical? An average is taken?

4. What is the reason for the rotation of earth? What force makes it revolve? — Preceding unsigned comment added by 117.200.253.88 (talk) 10:05, 12 May 2013 (UTC)


 * Your question sounds like homework to me. We are volunteers and it is not our policy to do your homework for you - but if you show evidence that you've tried but got stuck, we'll help you get unstuck.  Have you thought what might be likely answers?  What would be sensible? Having thought aboit it a bit, have you tried googling some likely phrases?  How about looking up "Earth" on Wikipedia?  Hint: if you put a kettle full of water on a stove top, does it boil instantly, or does it take some time while slowly getting hotter?  The Earth's atmosphere and heated surface is a heck of lot bigger than a kettle of water.  Wickwack 143.238.218.186 (talk) 11:01, 12 May 2013 (UTC)


 * 1/2 Our page Earth's_orbit states that the eccentricity of Earth's orbit is 0.0167, the perigee is 147 million km, and the apogee is 152 million km. Perigee occurs on 3rd July. The eccentricity is too little to make much difference to the climate; the differences in light/heat received due to the axis tilt is much more significant in causing Winter and Summer.
 * 3 1 AU is the length the semi-major axis. See Ellipse to see what this means; the Sun is at one of the focuses. (Strictly speaking the focus is the centre of mass of the Earth and Sun; this is inside the Sun).
 * 4 The same as the Earth orbiting the Sun; the cloud of gas that the Solar System condensed from was rotating. See Formation_and_evolution_of_the_Solar_System
 * CS Miller (talk) 11:07, 12 May 2013 (UTC)
 * CS Miller (talk) 11:07, 12 May 2013 (UTC)


 * Note that the apparent rocking of the Earth relative to orbital axis is about + and - 23 degrees. The insolation (the energy per unit area) is proportional to the cosine of the incident angle to the surface, and this means the summer/winter energy variation from this cause, averaged over the whole [surface]{correction:} hemisphere, is about 11% {added to clarify} peak to peak.  Incident energy is inversely proportional to the square of distance - this means the variation of insolation due to the Earth's orbital parameters is about (152/147)2 i.e., about 6%.  This is lower than the mean summer/winter variation, but it is NOT insignificant.  What is important is that atmospheric and ocean currents spread the heat about, so that the total energy recieved over the whole Earth is important, as well as local insolation.  Wickwack 143.238.218.186 (talk) 11:55, 12 May 2013 (UTC)


 * I haven't done the calculation, but the 11% figure seems too low to me. I guesstimate it to be at least 50%. Dauto (talk) 16:39, 12 May 2013 (UTC)


 * It is somewhat non-obvious what the 11% is actually comparing, and it might be helpful for Wickwack to specify where he got that figure from or how it was derived.  (For the purposes of this post, I'm going to use 'summer' and 'winter' to refer to the times of year around the June and December solstices; 'spring' and 'autumn' will refer to the times of year around the March and September equinoxes.)
 * Consider a point on the Arctic circle (or anywhere north of it). On at least one day of the year (at the solstices), the location will receive a full day of sunlight or a full day of darkness.  How should we score that?  Is it a variation of 100%?  At noon on the summer solstice, the sun still gets only 47 degrees above the horizon on the Arctic circle, giving only about 60% of the insolation we would see if it were overhead.  Does that instead score as a variation of 60%?
 * You can also get some counterintuitive effects depending on your location. Consider, for instance, a point on the Earth's equator.  At midsummer or midwinter, the Sun will make the same peak angle with the horizon, about 23 degrees from directly overhead&mdash;so the summer/winter variation in insolation due to axial inclination is actually zero.  On the other hand, the Sun will be directly over head at the equinoxes, resulting in about 8% more insolation.  So when considering variations in insolation due to axial tilt, is it more meaningful to report summer versus winter (0% difference), or annual minimum versus maximum (8% difference)?
 * Or should we integrate over the course of a full twenty-four hour period, to compare the min/max energy received per day, instead of just at noon? (Which then becomes more math than I feel like doing right now.) TenOfAllTrades(talk) 17:55, 12 May 2013 (UTC)


 * TenOfAllTrades has glipsed that an accurate calculation is actually quite complicated - that's one reason why I did not set out all all the math. Note only have I not mastered typing complicated equations in Wikipedia, I'm too lazy to spend the time doing it, and the length of it would turn most Ref Desk readers off, and most likely would not help the OP.  However, some simple concepts are useful:-
 * Clearly, as the heated upper surface of the Earth, sea, and the surrounding atmostphere supplies considerable thermal inertia, for the purposes of the OP's question, we should integrate the recieved energy over 24 hours. The recieved energy has 100% diurnal variation, but it is a sort of half-sine pulse with a period of zero ranging from zero to 24 hours depending on latitude and season.  This makes it more complicated and introduces more error if you just take the noon peak.
 * The received solar energy at the poles is indeed zero throughout winter. The variation in received solar energy summer/winter is obviously 100%.  However, when quoting a mean variation over the whole hemisphere, the poles have little influence due to the small area involved.  In fact, to be accurate, we need to progressivly discount by sine-weighting the variation at any latitude as we move further from the equator.
 * When considering the equator, there is a maximum and a minimum twice a year. So, obviously there is a seasonal variation in received solar energy acording to cos(0) - cos(23o), ie about 8% as TenOfAllTrades said. At the tropics the annual variation is similarly calculated as ~30%, which will sound familiar to those in the photovoltaic power generation industry.
 * When calculating a hemisphere average, while the annual variation for the equator is obviously 8%, we need to take into account equatorial areas peak twice as often and also the peaks occur out of phase with with the peaks elsewhere. Due to atmospheric averaging, non-coincident peaks reduce the hemisphere variation.
 * Don't forget that atmospheric and ocean currents spread the heat around considerably smoothing out temperatures, but have no direct effect on solar power generation.
 * On reviewing what I posted before, I see that my wording was a bit sloppy. I've made some corrections that should make the meaning of the 11% figure clear.


 * There is another interesting complication: The Earth, due to orbital parameters, is closer to the sun on [July 3 as CsMiller said - error; correction follows] January 3.  This corresponds to Northern Hemisphere winter.  If the Earth was a perfectly even geology, you would then expect Northern Hemisphere winters to be milder than southern hemisphere winters, and southern hemisphere summers to be [milder - eror] hotter.  However, this is nicely balanced out by the trade winds and the Northern Hemisphere having a greater land area and less sea surface, affecting re-radiation back out to space.  Every 23,000 years as the Earth's axis precesses, it actually reinforces variation instead of balancing it out - but then winds and perhaps ocean currents will change.  Another reason why I don't have much faith in climate change modelling as it has been done to date - its another complexity that some ignore.
 * Wickwack 124.182.180.184 (talk) 01:30, 13 May 2013 (UTC)


 * July 3rd is the northern hemisphere winter? I could understand it being just a mistake if the rest of the explanation didn't perpetuate the notion, but since it does: am I missing something? I live in North America, which I assume is in the northern hemisphere, and I'm pretty sure that July 3rd is summer here. So I would expect that southern hemisphere winters would be milder, and... as far as I know, they are? If you compare, say, Rio Gallegos to Sept-Iles, the average mean daily temperature from June to September in Rio is 2.73 Celsius, while from December to March in Sept-Iles the same is -11.78 Celsius - clearly the winter is milder in Rio, even though it is actually further south (51°38′S) than Sept-Iles is north (50°13′N). (Those are just two points I picked randomly by looking at google maps trying to find cities approximately equidistant from the equator with WP pages with climate information). Interestingly both cities have very very close average daily means for their respective summer temperatures. So, uh, I guess my point is that I know nothing about that stuff but I think there's an error in your explanation and also I'm not sure about the point it makes even if the error was corrected. 64.201.173.145 (talk) 16:00, 13 May 2013 (UTC)
 * I should have written "The Earth ... is closer to the Sun on January 3 (Perigee; more correctly perihelion - note CsMiller quoted the date for aphelion by mistake). This corresponds to Northern Hemisphere winter." The rest of it then makes sense, or should do.  Good cop, 64.201.173.135!  Having implied two other people recently did not check their facts, I went ahead with combining repeating CsMiller's error, which I didn't spot, with going on memory of this aspect of climate, and not checking my self.  One should always be carefull when comparing the local climate of locations of similar latitude.  Rio Gallego virtualy surrounded by sea on all sides - this generally makes for a milder climate when ocean currents are not an influence.  Sept-Illes in Canada is comparitively land-locked rendered cooler by other factors.  One can easily find coastal cities at the same latitude in the same hemisphere that have different climate. Wickwack 121.221.30.176 (talk) 01:37, 14 May 2013 (UTC)


 * I'm still very suspicious of the 11% figure. Dauto (talk) 18:15, 13 May 2013 (UTC)
 * Perhaps you could explain the basis of your suspicion, Dauto. The figure clearly can't be anywhere near the 50% figure you estimated.  Do you have a new estimate after reading my post?  Wickwack 121.221.30.176 (talk) 01:44, 14 May 2013 (UTC)


 * July 3rd is actually the Earth's aphelium, when the sun is in the farthest point from the Earth, making northern hemisphere's summer milder. The Earth's  perihelium happens in January making northern hemisphere's winter milder. Dauto (talk) 18:15, 13 May 2013 (UTC)