Wikipedia:Reference desk/Archives/Science/2009 May 24

Science desk
< May 23	<< Apr | May | Jun >>	May 25 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

May 24

Cold air

If I was to put a bottle of ice in front of a fan, would the fan blow air over the bottle which would cool, and would then cool down the surroundings? Would the air about a foot in front of the fan be colder with the bottle there than without the bottle? Would it make much difference to the temperature of the air around the fan, or would it be negligible? Thanks.—Preceding unsigned comment added by 86.177.122.34 (talk) 02:47, 24 May 2009 (UTC)

If the ambient air temperature is above the temperature of the ice (I presume it is) - then the fan will gradually warm up the bottle - eventually melting the ice - and the air blown over the bottle will be cooler than ambient. Heat is moved from the air into the bottle - so the bottle warms up and the air cools down.

Important Note: Fans don't make things colder.

They stir up the air and they make lightly clothed humans feel cooler - but they don't reduce the air temperature at all. The reason that standing in front of a fan makes you feel cooler is because your body produces heat which warms up the air next to your skin. That layer of warm air insulates you somewhat from feeling the ambient air temperature directly. The fan moves that layer of warm air away so that you can feel the cooler air that's all around you - which (because the ambient air temperature is lower than body heat) makes you feel cooler. This is what meteorologists mean when they talk about "wind chill factor".

But when the ambient air temperature is above body temperature - fans don't make you feel cooler - they make you feel hotter! It's like opening an oven door! (I speak from experience - it gets HOT in Texas!) SteveBaker (talk) 03:18, 24 May 2009 (UTC)

This is all true, except that Steve forgets that air circulation also cools human skin by helping sweat evaporates faster. However, most of it has nothing to do with the question, which is about cooling of the air due to ice as affected by the fan.

The bottle of ice will absorb heat and thus cool the air around it, at a rate which depends on the difference in temperature between the bottle's surface and the air nearby. The surroundings will be cooled as the cooled air in turn cools the air around it. If a fan is mixing the air, it will bring warmer air into contact with the bottle. This increases the difference in temperature and therefore the bottle will absorb heat, and cool the air, faster. So, yes, the fan will promote cooling of the surroundings (until all the ice is gone, which will also happen faster). However, I think the difference would be negligible, perhaps not even enough to make up for the heat added by the fan motor. --Anonymous, edited 04:31 UTC and again 07:27 UTC, May 24, 2009.

"But when the ambient air temperature is above body temperature - fans don't make you feel cooler - they make you feel hotter! It's like opening an oven door! (I speak from experience - it gets HOT in Texas!) "

Are you sure that this is true as a general statement? I would only expect it to be true at near 100% humidity. APL (talk) 07:43, 24 May 2009 (UTC)

Well, I see where you're coming from - removing the humid air from around the body theoretically gives sweat a better chance to evaporate into drier ambient air...but in the height of summer, it's rarely more than 30% humidity where I live - and fans definitely seem to make matters worse when the temperature hits 100F. Fortunately, we have air conditioned houses, cars, offices and shopping malls...so it's rarely a practical problem! SteveBaker (talk) 14:18, 24 May 2009 (UTC)

Thanks for the answer. The air conditioning point raises another question. Are most houses in the USA built with air conditioning pre-installed, or do most people have to have it installed afterwards? Are there many places in America where it isnt hot enough to warrant air conditioning in the summer months. Here in the UK we can have summers that have high enough temperatures, but those summers are an exceptional rather than the rule. As it is im sure the vast majority of houses are built without air conditioning because it usually isnt hot enough. My question is how much warmer would the average summer have to be in order for most houses to have air conditioning built-in,as a general rule?

In my personal experience, the existence of central air conditioning depends on several factors:

1. The average temperature. When visiting Florida, even the smallest homes had central air conditioning, but around my home in New England, a lot of people don't.
2. The age of the house. Back in the day, only large buildings had central air, with most having window-mounted air conditioners. Nowadays the technology's gotten a bit cheaper.
3. The price of the house. Let's face it: if you're rich, and it gets hot sometimes, you're going to buy a house with central air. If you're not rich, installing the window-mounted units, though a hassle, is worth the cut in cost. From my personal experience, the richer my friend's parents, the more likely they were to have central air.
4. The size of the house. This is slightly related to factor #3, but important: the bigger the house/building, the harder it would be to cool it with window/wall-mounted air conditioners.

Hope I've answered at least part of your question. -RunningOnBrains^{(talk page)} 15:46, 24 May 2009 (UTC)

In Texas, all houses less than maybe 20 years old will have central air conditioning. Older - low-budget houses might only have one or two window-mounted units - but very, very few people would have none. We get temperatures over 100F every year - 110F is not all that unusual. I doubt you could sell a car here that didn't have A/C. However, it wasn't always this way. Quite a few people who were born here recall living in houses with "swamp coolers" which is basically a fan blowing over a wet surface - the evaporation of the water cools the air - however it also increases the humidity and promotes unhealthy mold growth in the house - so people wouldn't turn them on unless they absolutely had to. Most of the deaths from high temperatures are amongst the poor and elderly who either don't turn on their A/C because of the cost - or can't afford to get their A/C repaired when it breaks. SteveBaker (talk) 16:13, 24 May 2009 (UTC)

Cartophilia

Is there an actual cartoneurosis called "cartophilia", or is it some sort of injoke? Ottre 03:52, 24 May 2009 (UTC)

Cooler to wear long sleeves in the sun?

Is it cooler to wear a long sleeved cotton shirt or a short sleeved cotton shirt when it's say 80 or 90 deg F outside in the sun? —Preceding unsigned comment added by 76.25.202.33 (talk) 06:02, 24 May 2009 (UTC)

Arabs traditionally wear long flowing robes in the desert. Loose long baggy sleeves may keep the heat off the skin, as an umbrella would, and circulating air would remove the heat from the underside of the fabric. 89.243.84.208 (talk) 10:05, 24 May 2009 (UTC)

I believe that the 'long flowing robes' thing only really works when the air temperature exceeds body temperature - which is not the case at 80 to 90F - so I'd go with short sleeves. However, if you have light colored skin - it may be wise to cover up in strong sunlight order to avoid the risks of skin cancer. SteveBaker (talk) 14:13, 24 May 2009 (UTC)

I think you are correct, but have your numbers wrong. You don't want core body temperature, you want skin temperature. I think that's about 33C (91F), so at the upper end of the OP's range it might be helpful. Also, if you are in direct sunlight you are being heated by radiation as well as conduction from the air, so being covered from the sun helps you keep cool. Wearing baggy clothes in the shade when the ambient temperature is below skin temperature is probably not a good plan, but either sunlight or high ambient temperature can change that. --Tango (talk) 14:45, 24 May 2009 (UTC)

AIR, skin temperature depends on the distance from the heart/body core, and at room temperature the extremities are around 25 °C (77 °F). Then again talking about the temperature of a non-equilibrium system is always dicey, with molecular sublayers and the such. I suppose the skin temperature would be highly dependent on the external temperature, which then leads us back to the original question (I swear I intended this as a helpful answer initially :-D).-RunningOnBrains^{(talk page)} 15:32, 24 May 2009 (UTC)

Indeed. I think the 33 degrees value I have stored away in my head from somewhere (possibly that question not long ago about radiating heat in space) was an average. For this question, we actually want the temperature of the arms, which I don't know (does anyone have one of those forehead strip thermometers? Put one of them on your arm and let us know what is says (and give us a rough idea of the ambient temperature where you are, and what you are wearing on your arms)!). --Tango (talk) 16:14, 24 May 2009 (UTC)

Thirty three degrees appears to be in the right range. According to one study, "the epidermis layer is uniformly effected by the environmental conditions irrespective of its thickness while the subcutaneous part is more influenced by the body core temperature. Also, the skin and subcutaneous tissues thickness play an important role in the temperature regulation under different environmental conditions." 152.16.16.75 (talk) 00:16, 25 May 2009 (UTC)

It seems to me SB is close to being right and Tango is mostly wrong. 33 and 25 degrees C are irrelevant. Your skin temperature is probably NOT going to be that temparature when the air temperature is 36 degrees C for example and you've been out in the open for the past 5 hours. Sweating does help cool you down, but it's not magic. If your skin is exposed to 36 degrees C air it's not likely to end up much lower then that. In other words, while perhaps a bit lower then 37 degrees C, it seems unlikely it'll be that much lower whereby you'll skin temperature will be the same as or hotter then the surrounding air. Radiation obviously has to be considered which I haven't but the temperature of your skin in a cold environment is irrelevant. Obviously in the short term, if you go from an airconditioned room or car then you should consider it but that's only short term. Going from a non airconditioned but sheltered location will add some complexity but I expect it will have to be at the fairly high end, probably past 32-33 degrees C and again, that's only in the short term. Nil Einne (talk) 22:25, 27 May 2009 (UTC)

Hyperthermia#Prevention may also be of interest but note that Wikipedia does not give medical advice. PrimeHunter (talk) 15:23, 24 May 2009 (UTC)

Remember that sunlight heats up the surface of the skin, possibly causing it to exceed 90F if exposed, and in addition it heats up the air, usually to several degrees warmer than in the shade, which is where temperature readings taken for weather condition reports are usually located. ~AH1^(TCU) 20:31, 24 May 2009 (UTC)

I would say from personal experience the short sleeve shirt. Actually no shirt is probably better. Particularly if there's wind. (And if your under shelter.) As SB says, this may not be a good idea if you have light skin and aren't wearing sun screen however. Note I'm primarily referring to the temperature range of the OP (and high humidity at that). I don't have much experience wiht temperatures much higher thankfully. Nil Einne (talk) 22:11, 27 May 2009 (UTC)

Strange blue sea creature on NSW coastline

Today I found many strange, shiny blue, frond like sea creatures at the beach. I live on the Central Coast, NSW, Australia. I've attached a picture I took of the creature (it was taken by me, on my iPhone, so sorry if it isn't the best quality). Here is a link to the image: http://img.skitch.com/20090524-874rj2bfh41pb9fn4tq3mhcdph.png I was wondering if anyone could identify it for me, Thanks, Sam. —Preceding unsigned comment added by 58.107.134.227 (talk) 06:39, 24 May 2009 (UTC)

I'm thinking Glaucus atlanticus[1]. In which case do not touch; it eats venomous jellyfish, collects and stores concentrated venom, and injects it into enemies. 62.78.198.48 (talk) 07:02, 24 May 2009 (UTC)

Alternative is the closely related and, AFAIK equally dangerous, Glaucilla marginata, how big were they? Hard to tell the size from the photo. Mikenorton (talk) 08:34, 24 May 2009 (UTC)

Quite small really, maybe a centimetre or two. —Preceding unsigned comment added by 58.107.134.227 (talk) 10:35, 25 May 2009 (UTC)

A link in the article on the G. atlanticus says that they can be "up to 6 cm", whereas the G.marginata is shown as being barely a quarter of that size. It's not clear, but I think that the IP above was probably right, some of the atlanticus specimens in google images are clearly of the size that you mention. Mikenorton (talk) 12:11, 25 May 2009 (UTC)

Theoretical lower bound of maximum member load in a truss?

We're building a bridge out of popsicle sticks as part of the Undergrad programme and in the design of the trusses I've found that different designs lead to quite different results. In some cases for a 100N load the maximum member load goes up to 200N while others only at 70N. Which led me asking, what is the theoretical lower bound of maximum member load for a given criteria? --antilived^{T | C | G} 09:22, 24 May 2009 (UTC)

Swimming pigs

A friend of mine was relating the following factoid: pigs can't swim because their front trotters slash across their throats, opening the jugular and killing them. Is there even the slightest bit of truth to this?

Thanks Adambrowne666 (talk) 09:34, 24 May 2009 (UTC)

I doubt it. Vimescarrot (talk) 09:42, 24 May 2009 (UTC)

even better. Vimescarrot (talk) 09:43, 24 May 2009 (UTC)

That's hilarious, thanks Vimescarrot - I'm sitting here with him now, and just remembered that I myself have seen pigs swimming at the Royal Melbourne Show with little or no evidence of throat-self-slashing. Adambrowne666 (talk) 09:50, 24 May 2009 (UTC)

I imagine your friend read Daggie Dogfoot by Dick King Smith as a child :) 80.41.42.73 (talk) 22:59, 24 May 2009 (UTC)

How "lagged" is my television or radio?

When watching a normal "live" broadcast, how long after an event happens before it shows up on my tv screen"?

Since most radio is local, I assume that the delay is very minor. But for a US Cable tv customer watching a "live broadcast from Rome", it might be quite a sizable delay.

I suppose I should clarify-- I'm looking for the actual answer, based on knowing about the electronics involved; I know the time it would take a truly instant broadcast at the speed of light.

The actual answer involves how long does it takes light to enter a camera, be digitized, be broadcast via satellite to New York, be received and altered, and then rebroadcast via satellite to the cable company which travels on wires to my digital cable box to my television which then re-emits the light.

Many bets ride on your answer. --Alecmconroy (talk) 10:03, 24 May 2009 (UTC)

I can only say that it varies by several seconds. My personal experience from switching between two television channels showing the same live events is a difference of up to around 4 seconds between the two channels. This is without a deliberate broadcast delay as far as I know (on European channels with less sensitivity than you Americans). I don't know how big the actual delay on the fastest of the two channels is. PrimeHunter (talk) 11:27, 24 May 2009 (UTC)

In some situations they actually insert several seconds of deliberate delay in live events in order to give themselves a chance to spot something going horribly wrong in the live feed and be able to cut it before it reaches viewer screens. Radio shows with live phone-ins are particularly careful about this - using up to 10 seconds of delay. SteveBaker (talk) 14:08, 24 May 2009 (UTC)

Communications satellites are typically 1.334 light-milliseconds (low earth orbit) to 0.1194 light-seconds (geostationary orbit) from the surface of the Earth. Hence there will always be a delay of at least 0.24 s in a communication via geostationary satellite: this delay is just perceptible in a transoceanic telephone conversation routed by satellite.

One satellite broadcasting system sends two carrier signals with 4-second delay between them. This enables the receiver to maintain a large buffer of the audio stream, which, along with forward error correction, helps keep the audio playing in the event that the signal is temporarily lost, such as when driving under an overpass or otherwise losing line-of-sight of any of the satellites or ground repeater stations. Cuddlyable3 (talk) 14:27, 24 May 2009 (UTC)

In the UK, digital terrestrial channels have about a 2 second delay compared to their analogue equivalents - but I don't know how much of this delay is at the transmission end and how much is due to buffering in the receiver. Gandalf61 (talk) 09:15, 25 May 2009 (UTC)

From the point of view of the speed of the recording and displaying electronics, you wouldn't notice the delay. If you go to a shop with a video camera display and you step into the view of a camera displayed on a TV ahead of you you'll see a very, very, very short between you moving and the image of you doing the same. Of course cable TV may well be encrypted and be decrypted by cheap hardware in your set top box which can add seconds to the delay. --203.202.43.53 (talk) 06:45, 26 May 2009 (UTC)

I have to wonder, how long of a delay could they insert before the show is no longer "live" and their claims to the contrary become legally-prosecutable false advertising? I suppose that's very much a subjective judgement call. Dcoetzee 06:59, 26 May 2009 (UTC)

I'm sure the FCC (or whatever governmental broadcasting body you wish) has some sort of regulation about what can and cannot be considered "live." Livewireo (talk) 15:46, 26 May 2009 (UTC)

My car radio has buttons for KFOG's two transmitters. One time I noticed a couple of seconds' difference between them. —Tamfang (talk) 16:54, 4 June 2009 (UTC)

Why do humans need fat in their diet?

It is commonplace that humans get fat when they overeat. In other words, they can synthesise fat. Why then do humans need fat in their diet? 89.243.84.208 (talk) 10:28, 24 May 2009 (UTC)

Because lipogenesis costs more energy than in taking lipid from the diet? We can also synthesize sugars. --Mark PEA (talk) 10:33, 24 May 2009 (UTC)

Also, quite a few of the vitamins we need are 'fat soluable' - they are dissolved into fats that we eat - so if you have a totally fat-free diet, you can suffer rabbit starvation. 14:04, 24 May 2009 (UTC)

Also and, we can't synthesise all the fatty acids we need - check out Essential fatty acid. Aaadddaaammm (talk) 16:33, 24 May 2009 (UTC)

Fat is an important energy source. It has nine calories per gram, 2.25 times more than carbohydrates and proteins. Fat is necessary to displace carbohydrates as an energy source, and consuming less than twenty percent of calories from fat will almost invariably cause the carbohydrate intake to be too high. Very high carbohydrate diets (such as those obtaining more than two-thirds of calories from carbohydrates) can adversely impact lipid profiles, increasing triglycerides and suppresses HDL cholesterol.

The Institutes of Medicine said this in 2005:

The AMDR for fat and carbohydrate is estimated to be 20 to 35 and 45 to 65 percent of energy for adults, respectively. These AMDRs are estimated based on evidence indicating a risk for coronary heart disease (CHD) at low intakes of fat and high intakes of carbohydrate and on evidence for increased risk for obesity and its complications (including CHD) at high intakes of fat. Because the evidence is less clear on whether low or high fat intakes during childhood can lead to increased risk of chronic diseases later in life, the estimated AMDRs for fat for children are primarily based on a transition from the high fat intakes that occur during infancy to the lower adult AMDR. The AMDR for fat is 30 to 40 percent of energy for children 1 to 3 years of age and 25 to 35 percent of energy for children 4 to 18 years of age. The AMDR for carbohydrate for children is the same as that for adults—45 to 65 percent of energy. The AMDR for protein is 10 to 35 percent of energy for adults and 5 to 20 percent and 10 to 30 percent for children 1 to 3 years of age and 4 to 18 years of age, respectively.(http://books.nap.edu/openbook.php?record_id=10490&page=769) The Institutes of Medicine talk about the harmful effects of both fats and carbohydrates (see the lengthy chapters beginning here and here).

The USDA says:

A low intake of fats and oils(less than 20 percent of calories) increases the risk of inadequate intakes of vitamin E and of essential fatty acids and may contribute to unfavorable changes in high-density lipoprotein (HDL) blood cholesterol and triglycerides.

(http://www.health.gov/dietaryguidelines/dga2005/document/pdf/DGA2005.pdf, see Chapter Six)

"The AHA notes that in the absence of weight loss, diets high in total carbohydrate (e.g., >60% of energy) can lead to elevated triglycerides and reduced HDL cholesterol. These effects do not occur with substitution of monounsaturated or polyunsaturated fats for saturated fat. NCEP suggests that monounsaturated fat can be up to 20% of total energy and polyunsaturated fat up to 10% of total energy."(http://www.diabetes.org/uedocuments/ADACardioReview4.pdf)

Foods that are high in fat tend to be palatable (they taste good) and high in calories per ounce. There are very few foods (with the exception of whole milk) that obtain a huge percentage of their calories from fat and are also high in water. This is why high intakes of fat may be associated with obesity, but studies comparing high-carbohydrate diets have produced conflicting results. Some studies, of course, show that carbohydrates are more likely to cause obesity than fats.75.89.27.94 (talk) 23:13, 24 May 2009 (UTC) —Preceding unsigned comment added by 71.31.105.41 (talk)

Thank you very much for all that, I shall try to study and comprehend that very carefully. I'm still surprised that its beneficial to eat what seems to me quite a lot of fat - I thought it was sinful in dietry terms. 89.241.155.179 (talk) 18:16, 25 May 2009 (UTC)

Why is the Dust bowl no longer a problem?

What has changed about agricultural practices to make the dust bowl of the 1930s something that does not seem to happen since then? Years ago the UK fens had the same problem with soil erosion, but that is never reported in the news or media now either. The fens farmers went through a phase of using very deep plowing with powerful double-engined articulated tractors in an unsuccesful attempt to stop the wind erosion, but I do not think such plows are used anymore. 89.243.84.208 (talk) 10:49, 24 May 2009 (UTC)

If I recall one of the main ecological reasons behind the dust bowl was poor agricultural practices like a lack of crop rotation. I suspect that changing back to more traditional practices will have gone a long way towards that end. I suspect the dust bowl phenomena is one of these exponential tipping-point type things, where once it gets bad it gets worse and worse and worse, but if you keep it from getting bad in the first place then it's not a problem. --98.217.14.211 (talk) 18:31, 24 May 2009 (UTC)

I'm thinking that the dust bowl occured in the first place due to overfarming; natural vegetation was removed (which holds the soil together) and new crops planted during a period of intense drought. The lore back then was that the "rain follows the plow", but that obviously did not occur, leading to the loose, dry soil blowing away. Today, there are a number of reforestation programs, which have helped to hold the soil together, so that a repeat of the dust bowl does not occur. There is one threat, however, that could cause prolonged dust bowl-like conditions in the near future. One degree Celcius (roughly 2F) of further global warming could turn much of the US Midwest into a semi-desert, meaning that the soil may blow away more easily once again, unless massive irrigation and plantation projects are farther exploited. ~AH1^(TCU) 20:28, 24 May 2009 (UTC)

While not quite as bad as the dust bowl, I do recall dust storms in South Dakota about 10 years ago that filled in the roadside ditches with dirt and reduced visibility to about 1/4 mile for 2-3 days. So these storms can still happen on smaller scales. 65.121.141.34 (talk) 13:53, 26 May 2009 (UTC)

Building muscle

Does eating raw eggs have any advantage over eating cooked eggs when it comes to building muscles? —Preceding unsigned comment added by 116.71.34.218 (talk) 11:34, 24 May 2009 (UTC)

No. The cooked egg is better: raw egg white has Ovomucin and Avidin --Digrpat (talk) 16:12, 24 May 2009 (UTC)

So why do bodybulders eat raw eggs? Vimescarrot (talk) 16:22, 24 May 2009 (UTC)

Laziness? So they don't have to chew? Salmonella poisoning would also not be very good for muscle building, I'm sure. Aaadddaaammm (talk) 16:36, 24 May 2009 (UTC)

No, they eat them because they believe it helps in their bodybuilding. Why do they think this? Vimescarrot (talk) 18:33, 24 May 2009 (UTC)

Research indicates that the Raw Egg Eating Meme (REEM) originated circa 1890 with Bernarr Macfadden, the index case. The highly influential Charles Atlas became infected by 1920 and is reguarded by many historians as patient 0. In 1976 REEM went pandemic with the release of Rocky. It is now endemic within bodybuilding community. --Digrpat (talk) 20:33, 24 May 2009 (UTC)

No. Axl ¤ [Talk] 16:40, 24 May 2009 (UTC)

Instrumentation

what are the various aspects of this field instrumentation ? what are the research prospects ? Is instrumentation related to robotics ? if so how? —Preceding unsigned comment added by Srini279 (talk • contribs) 13:00, 24 May 2009 (UTC)

See Measurement instruments unless you mean Musical instruments. Cuddlyable3 (talk) 14:11, 24 May 2009 (UTC)

Instrumentation is a broad term - it can mean anything from TV cameras to little switches that turn on when a robot bumps into something...robotics are heavily dependent on good instrumentation (they'd be more likely to call them "sensors") because the computer inside is blind and deaf without them. With the rise in the applicability of robots, there is considerable research into the sensors they use. Finding novel sensors, making cheaper/lighter/lower-power-consumption/more-robust sensors and finding better ways to interface the sensors to the computer software - these are all hot research topics. SteveBaker (talk) 14:30, 24 May 2009 (UTC)

thanks a lot for the information you provided i now have gained some considerable insight into instrumentation Srini279 (talk) 12:35, 27 May 2009 (UTC)

Powering a Phobos Colony

I read in this article that Phobos, with its orbit only 3000 miles from Mars and synchronized to always face the planet, is a "natural staging area for manned excursions to Mars." My question is: wouldn't it be difficult for a Phobos colony to be powered by solar energy if Mars completely encompasses the sky? Sappysap (talk) 15:02, 24 May 2009 (UTC)

Put the solar panels on the side facing away from the planet then? Or am I oversimplifying? --Kurt Shaped Box (talk) 15:06, 24 May 2009 (UTC)

One side of the moon always faces Mars - so there are several possibilities:

• If you set up camp on the side of Phobos that's nearest Mars - then during your "mid-day", Mars is between you and the Sun and appears as a black disk that's blocking out the sun. During the night - the Sun isn't in your sky. However, there will be times around dawn and dusk when the Sun is low in your sky between the horizon and Mars. However, even during the day - Mars only takes up a quarter of the sky - so you'd get a few hours of sunlight in each 7.5 hour "day". However, even at midnight - Mars would be 2500 times brighter than our moon and 6400 times larger...that's quite a lot of reflected sunlight - so you'd probably get reasonable amounts of power from "marslight"...especially if you made solar panels that were most sensitive to red light.
• If you set up camp on the side of Phobos that's furthest from Mars - then Mars is permenantly below the horizon and you get plenty of sunlight during the day (but no marslight at night).
• You could also build tall, vertical, rotating solar panels on the North or South poles of Phobos then they would benefit from sunlight for all but maybe an hour a day when Mars gets in the way. Phobos has zero degrees of axial tilt - so unlike here on earth, the sun would never set below the horizon at the poles - the only darkness is when Mars gets in the way.

So - either put fixed solar panels on the equator as far as possible from Mars. You'll get full sunlight for almost 4 hours in every 7.5 hour "day"...or...put them on the poles and make them rotate one revolution every 7.5 hours. That's attractive because you only need to have enough battery storage to cover that one hour of darkness - and keeping the panels at 90 degrees to the incoming sunlight maximises their power production. SteveBaker (talk) 15:59, 24 May 2009 (UTC)

I believe Phobos' axial tilt it zero with respect to its orbit around Mars, not the Sun. Phobos' orbit is inclined 1 degree to Mars' equator which has an axial tilt of 25 degrees. So, if I'm visualising this correctly, that's equivalent to an axial tilt that varies between 24 and 26 degrees. I don't see how Mars would ever get in the way of the poles, though. Yes, I do! --Tango (talk) 16:09, 24 May 2009 (UTC)

Being so close to Mars, presumably there is a lot of tidal strain. Could that be used for power? I'm not entirely sure how - there is no ocean or atmosphere, so you would have to use tidal movement in the rock itself, which isn't particularly convenient. --Tango (talk) 16:03, 24 May 2009 (UTC)

That tidal strain will stress and release the rock, causing it to warm (the way a paperclip warms when you bend it back and forward). The insulated centre of the moon should be warmer than the outside, so that sets up a thermal gradient. You could, in theory, use that to run a heat engine (like an OTEC) but surely the thermal gradient will be so shallow that you wouldn't, in practice, break even. 87.114.167.162 (talk) 19:00, 24 May 2009 (UTC)

Incidentally, if my rusty trig is correct, doesn't Mars only occupy about 21 degrees of the sky, hardly "completely encompasses"? 87.114.167.162 (talk) 19:08, 24 May 2009 (UTC)

I doubt there is enough tidal strain in Phobos to be of any use given that

• a) Mars gravity isn't very strong,
• b) Phobos always shows the same face to Mars,
• c) Phobos' orbit isn't very eccentric,
• d) There isn't any large moon around Mars that could periodically tug on Phobos and steadily transfer energy to Phobos libration which would than be dissipated through tidal friction.

Dauto (talk) 02:46, 25 May 2009 (UTC)

flourescent lights inop.

Can anyone explain to me, why, whenever it rains, half of my shop lights work and the others don't? [ 4ft flourescent] Thanks, Bill —Preceding unsigned comment added by 208.100.237.166 (talk) 17:56, 24 May 2009 (UTC)

You probably have two lighting loops. It may be that one of those loops is being exposed to rainwater, causing a short circuit. That's a dangerous condition, and you should have an electrician look at it. 87.114.167.162 (talk) 18:46, 24 May 2009 (UTC)

I'm surprised it hasn't blown a fuze or tripped a circuit breaker - but yeah - this is very dangerous. Somewhere there could be a puddle that'll electrocute you when you step in it...also, as it dries out, there is a good chance of arcing and all sorts of other things that could cause an electrical fire. Definitely get it fixed ASAP. SteveBaker (talk) 21:55, 24 May 2009 (UTC)

Whoa, don't go overboard here! I mean, yeah, if there's an actual rainwater leak, it's dangerous and needs fixing. But it's common for fluorescent lights to malfunction in rainy weather if they're starting to fail -- my house is full of fluorescents and I see this all the time. Presumably the explanation is increased current leakage through the air when it's humid. Bill probably just needs to do the usual things for non-working fluorescents: check if they're properly in the sockets, change the tubes, check the wiring connections, replace the ballast. The latter two things are the sort of work where some people would say an electrician is necessary and others would say "learn how, then do it yourself"; I'll leave it at that. --Anonymous, 21:07 UT, May 25, 2009.

Try moving the ;lamps and starters between good positions and flashing positions. GlowWorm. —Preceding unsigned comment added by 174.130.252.83 (talk) 02:08, 25 May 2009 (UTC)

The intermittent problem could be a leak causing a short in the mains supply, but the lamps would robably keep going until the fuse or breaker interrupted the current. Do you find the fuse/breaker open? They do not, in general, reclose on their own. Absent that, we are left with the ballast, the starter, the socket, or a temporary open in the switching circuit. I doubt it is the lamp itself which is somehow sensitive to humidity or precipitation. If water is leaking into a connection such as a wire nut, insulating crud could build up. A screw terminal or push-in terminal could be loose producing intermittent operation only coincidentally related to rain. There could be an intermittent open in an underground cable, if the feed to the shop is underground, but that would likely affect all the lights. A qualified electrician could check for loose or corroded connections. The circuit should be turned off at the breaker while inspections or repairs are attempted. Edison (talk) 00:53, 26 May 2009 (UTC) Edison (talk) 00:53, 26 May 2009 (UTC)

prognostic markers

Hi I have been given the task of researching the 'prognostic markers' used in the management of breast cancer. I have no idea what the term 'prognostic markers' even means. Can anyone give me a definition please? Theres no article on them and Wiktionary doesn't seem to have heard of them either. Also if you know of any websites which may be of use for my research I would be grateful to hear about them! Cheers RichYPE (talk) 18:50, 24 May 2009 (UTC)

"Prognostic" just means "predictive". "Marker", in this context, may mean genetic marker. If that's so, it means "what genetic sequences exist which can be used to help predict whether a given patient will or will not develop breast cancer". The epidemiology and etiology of breast cancer#Heredity article might be helpful for you. 87.114.167.162 (talk) 19:04, 24 May 2009 (UTC)

A marker in this context is any measurement or indicator that can be used to detect/identify a disease , stage it, predicts its course, or decide on suitable treatment. A genetic marker is just one class of markers, and there are many other clinical markers for breast cancer and other diseases. Here are some (random) links to get you started: [2], [3]. Search for "prognosis marker breast cancer" on pubmed and you'll find 100s of other references. Abecedare (talk) 19:20, 24 May 2009 (UTC)

The Prognosis section of our article on breast cancer provides useful context, definitions, and links. Oddly, the section of that article on Staging includes some of the most relevant information on tumor markers (hormone receptors and HER2 expression) even though they are not used for staging per se, and (appropriately) these markers are not discussed in the Staging section of the main article cited at the beginning of that section. Time to do a bit of editing... --Scray (talk) 21:25, 24 May 2009 (UTC)

I think it's a little better-organized now - relevant section is now Breast_cancer#Classification, third bullet point is relevant to the OP about tumor markers: 'Protein & gene expression status'. --Scray (talk) 21:40, 24 May 2009 (UTC)

We should maintain a distinction between "prognosis" and "prediction" here... Prognosis assumes that a person has a disease, and that we are trying to use "prognostic markers" to tell how well or poorly they are likely to do, or whether they are likely to respond to a particular treatment. Prediction assumes that a person does NOT have a disease and we are trying to use "predictive markers" to tell whether or not they are likely to have the disease at some time in the future. While it is true that prognostication and prediction sometimes use similar assays, these are two VERY different things. --- Medical geneticist (talk) 23:36, 24 May 2009 (UTC)

Excellent point! I missed that in the prior answers, was focused on the OP. Thanks for pointing that out. --Scray (talk) 02:13, 25 May 2009 (UTC)

Ack, the "Breast cancer" article needs a lot of work. I'll have a go. Axl ¤ [Talk] 08:00, 25 May 2009 (UTC)

Saturn floating on water

When I was younger, I recall reading a children's book that stated that the planet Saturn was composed of very light elements, and therefore could actually float on a hypothetical ocean that was large enough. I just did a google search, and I actually turned up this http://coolcosmos.ipac.caltech.edu/cosmic_kids/AskKids/saturnfloat.shtml

I guess I'm just looking for confirmation to this claim, because I figure that the dense core might create problems for this hypothetical scenario. ScienceApe (talk) 20:00, 24 May 2009 (UTC)

Yes, here are a few sources I found: BBC News, Encyclopedia Britannica and NASA. A Quest For Knowledge (talk) 20:31, 24 May 2009 (UTC)

It's true. The average density of Saturn, according to Saturn, is 0.687 g/cm³, compared to 1 g/cm³ for water. There is a slight question regarding how you define the edge of a gas giant, but there is enough margin of error there that any reasonable definition will still give it a density less than that of water. --Tango (talk) 20:38, 24 May 2009 (UTC)

Saturn is mostly hydrogen so it is not surprising that its density is less than water. Jupiter is also mostly hydrogen but has a density greater than 1, but this is because of the great pressure in Jupiter. I expect that water at the same pressure would still have a greater density. As for floating, well no it would not. It is light enough to float, but any body of water large enough to float Saturn in would be larger than the planet. Saturn would lose its separate identity and be gravitationally melded into a new water+hydrogen planet, the combined body possibly being now large enough to ignite nuclear fusion and create a second sun in the solar system. In any case it would be spectacular, not at all like rubber ducks in the bath. SpinningSpark 21:23, 24 May 2009 (UTC)

These kinds of comparisons are always very tortured, approximate and flawed. The problem with a gas giants is that they are kinda soft fuzzy things - and saying how big they are is a bit like asking how big a ball of cotton wool is. So you can come up with all sorts of density numbers depending on where you draw that line. Whether it would actually float is a really silly question - when you consider the gravitational pull of all of that water and of the planet itself - plus the fact that while the outer layers are really light - the core is pretty heavy and would obviously sink...but please - let's not go there! Suffice to say that most authorities say that it's mean density is less than that of water and quietly forget about floating. SteveBaker (talk) 21:52, 24 May 2009 (UTC)

Well, yes, obviously you can't actually float it in water. That would require the water and Saturn to be inside a gravitational field, which, in order for the whole thing to make sense, would have to be much greater than that generated by Saturn itself (just as when you float at apple at Halloween the apple is far smaller than the Earth). If you put Saturn in that gravitational field it would fall apart since it is only held together by its own gravity. "Would float" in this context just means "has a density less than that of water", it shouldn't be interpreted literally. --Tango (talk) 22:22, 24 May 2009 (UTC)

Yeah the question wasn't intended to really mean Saturn. The question was just about density, and buoyancy. Obviously, such a scenario can never exist with the real saturn, but that wasn't really the point of the question. ScienceApe (talk) 18:15, 27 May 2009 (UTC)

Obviously, Saturn would need to be in a giant plastic bag. APL (talk) 02:28, 25 May 2009 (UTC)

The plastic bag would get stuck when the water freezes. Then how would you get Saturn out? Tempshill (talk) 06:33, 25 May 2009 (UTC)

As an aside, I always wondered if that figure for density factored in the rings-- which I assume would be included in that bag with the rest of Saturn, even if the planet was in a rather misshapen form as the result of being in the massive gravitational field necessary for this thought experiment. 69.224.113.202 (talk) 04:24, 26 May 2009 (UTC)

The mass of the rings as a proportion of Saturn is quite insignificant (about 50ppb) SpinningSpark 23:16, 26 May 2009 (UTC)

True, but their volume is quite big, especially if you define Saturn as the ball containing the planet with rings. I don't think the rings are included and including them, however you did it, would reduce the density, so you still have it less than that of water for any reasonable definition. --Tango (talk) 11:50, 27 May 2009 (UTC)

Weird Bug

For the past couple of years I've seen this insect right around springtime, It's fuzzy, fairly large, has a black head and black legs, and the fuzz is mostly a dull yellow with a black stripe running width-wise through the center of his back.

Americanfreedom (talk) 21:47, 24 May 2009 (UTC)

Bee? 78.146.52.86 (talk) 22:25, 24 May 2009 (UTC)

I think so, is there a species of bumblebeethat has a statge in it's life before it grows wings where it can walk around?

Americanfreedom (talk) 00:02, 25 May 2009 (UTC)

No, a bumblebee does not "walk around" before it grows wings :) . Bumblebees have a complete metamorphosis, so their larvae look nothing like the familiar adult (winged) stage. Now, there are distant relatives of bumblebees called "velvet ants" (Mutillidae). Mutillidae females really do resemble wingless bees, or big fuzzy ants, in their adult stage; males, OTOH, have wings. Is that what you've seen? --Dr Dima (talk) 00:44, 25 May 2009 (UTC)

[[4]] The person who took the photo didn't know what it was either. They said it was a caterpillar of some kind. 71.236.24.129 (talk) 10:25, 26 May 2009 (UTC)

Yup, it's a caterpillar; from the image on this website [5], it's probably a spotted tussock moth (Lophocampa maculata). Mikenorton (talk) 12:30, 26 May 2009 (UTC)

Jet fighter sledging along on wet grass

Jet fighters sometimes practice following the terrain at an altitude of just a few feet, called 'hedge hopping'. I'm wondering what would happen if such a jet misjudged things and found itself sliding along on its belly on a flat area of wet grass, like a sledge. If it had no significant damage, would it be able to get into the air again?. That suggests another question - why do aircraft have their wheels on poles? 78.146.52.86 (talk) 23:19, 24 May 2009 (UTC)

To answer your second question first, airplane wheels are setup to hold the airplane some distance (at least a couple feet) from the ground to keep the engines as far as possible from the ground. Propeller planes obviously need this distance to keep from smashing up their propellers. Jets also need this distance to avoid sucking in debris from the ground.

For your first question, you might check out Belly landing. Most planes would not be able to recover from even the softest belly landings under their own power. I'm not sure if a fighter plane could do it, they're supposed to be more rugged and definitely have very powerful engines. I doubt the pilot would attempt it, he would have no way of knowing how bad the damage was. APL (talk) 01:39, 25 May 2009 (UTC)

Oh wow, Check out Malév Flight 262. APL (talk) 01:42, 25 May 2009 (UTC)

Awesome! Tempshill (talk) 06:31, 25 May 2009 (UTC)

You've got to read it carefully - the wings didn't scrape along the ground as the OP is asking - the undercarriage support apparatus evidently worked like skids - protecting the underside of the wings from damage. Still - a pretty amazing happening! SteveBaker (talk) 01:43, 26 May 2009 (UTC)

The OP said "Belly" in his original post. APL (talk) 02:25, 26 May 2009 (UTC)

What about an aircraft where the wings and underneath of the body formed a flat surface - can a wing lift if it has no air underneath it, but is just sliding along a flat surface? 78.146.108.137 (talk) 13:35, 25 May 2009 (UTC)

Well, when the wings get within about half their length of the ground, you get a build-up of pressure under the wing (it's called "ground-effect") that actually increases the amount of lift - but if the wings actually hit the ground, there is so much friction that the plane's speed would rapidly drop to the point where flight is impossible...and the damage that's inevitable means it's going nowhere afterwards. SteveBaker (talk) 13:46, 25 May 2009 (UTC)

(EC) It's called 'nap-of-the-earth' (NOE) flight. In peacetime, they are limited to something like 500 feet above the terrain - but in wartime, they can go much lower - 50 to 200 feet depending on the nature of the terrain and the aircraft type. Most fighters that do this have terrain-following radar - the pilot hands control of the height of the plane over to the computer because human reactions simply aren't good enough to do this at hundreds of miles per hour (and because you often want to do it at night!). Some aircraft avoid using active radar by having accurate digital maps of the terrain instead (I know the Royal Dutch Airforce F16's do this because I worked on that subsystem in a flight simulator we built for them). As for 'belly landings' - if the ground is very smooth - with no large rocks - trees, etc - then it's possible to do a controlled 'wheels-up' landing and not die in the process - but the plane is going to be a write-off. Most fighter pilots would rather eject. SteveBaker (talk) 13:44, 25 May 2009 (UTC)

Belly landing is a bad idea. Very good chance that you will clip some sort of obstruction or bump in the ground. Even if you don't, the airframe is not designed for that kind of stress. You will weaken your aircraft, and the FAA wouldn't want to re-certify the aircraft and risk it falling apart at 10,000 ft. 65.121.141.34 (talk) 16:07, 27 May 2009 (UTC)