Wikipedia:Reference desk/Archives/Science/2009 June 18

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June 18

Question on the Scientific Method

What's the difference between the Control and the Constant? 71.196.63.33 (talk) 05:29, 18 June 2009 (UTC)

It's unclear without context, but i would guess you mean control as being the comparison experiment in which the indepentant variable in not varied or at a standard level (eg. If measuring the effect of various additives on bacterial growth rates, the control should be a growth experiment with no additive, but the rest of the conditions kept the same.) The constant would then mean the parts of the experiment that always stay the same (eg, no matter what additive is used, the agar growth medium/temparature/pH/time should always be the same).09:16, 18 June 2009 (UTC)

surface tension

what is surface tention? —Preceding unsigned comment added by 218.248.80.112 (talk) 05:48, 18 June 2009 (UTC)

Hi, take a look at surface tension and for some interesting reading, contact angle. ZabMilenko^{How am I driving?} 05:55, 18 June 2009 (UTC)

What causes surface tension, essentially, is molecular attraction among the molecules of a liquid (it could be Van der Waals forces, or hydrogen bonding like in water or in concentrated HF, or ionic interactions, etc.) When a liquid molecule is surrounded on all four sided by other molecules, these forces cancel out; but when the molecule is on the surface of the liquid, it's only surrounded by other liquid molecules on the other three sides, so their molecular attraction results in a net force toward the interior of the liquid phase, which is what causes surface tension. Quite simple, really. :-) 76.21.37.87 (talk) 06:26, 18 June 2009 (UTC)

Battery. 2.3Ah, 3.3V. How many Watt-hours it could hold?

Was never understanding that ampere thing(why it even mentioned? rhetorical question). From search it looks like I need to multiply mAh and Voltage. But, no way, it could not hold 2300mAh*3.3V == 7.59kW·h!!! Sorry. DeadlyPenguin (talk) 08:42, 18 June 2009 (UTC)

No, 2.3 Ah x 3.3 V = 7.59 W·h, not kW·h. --Heron (talk) 09:27, 18 June 2009 (UTC)

... and, in real life, the capacity depends on the rate of discharge (and never seems to achieve the claimed figure). Dbfirs 12:50, 18 June 2009 (UTC)

Points on a plate ...no idea????

hi friends ....

can any one help me how should i model a mathematical problem for the following problem:

a thin metal sheet is supplied with heat from external source (such as a welding machine etc.) and i want the heat at various points in the plate considering one dimensional heat flow only. —Preceding unsigned comment added by Sameerdubey.sbp (talk • contribs) 09:29, 18 June 2009 (UTC)

This can be modelled using the one-dimensional heat equation. Gandalf61 (talk) 10:47, 18 June 2009 (UTC)

hey thanks a lot .. for quick reply ... but i want to consider heating of previous point and next points . —Preceding unsigned comment added by Sameerdubey.sbp (talk • contribs) 11:07, 18 June 2009 (UTC)

I think the heat equation allows you to do that, if you want it done as a series of steps you could use an iterative formula. Elocute (talk) 15:08, 18 June 2009 (UTC)

Hepatic filtering

In case of cyanide poisoning or something similar, is it a good idea to start exerting physically as much as possible before the poison starts acting? I think it could be idea, since it would increase the speed of your blood flow, and therefore the rate at which your liver detoxifies the toxin. Have I overlooked possible negative consequences of exercising while poisoned? --83.56.176.138 (talk) 10:21, 18 June 2009 (UTC)

It would really be a nice possibility to detoxify in this way. But in reality, for most toxins this won't work. By exercising, you are not only speeding up your blood flow, you are raising your overall rate of metabolism. So your liver would probably detoxify a bit faster, but the toxins would also poison you faster, because of quicker distribution and uptake. So your method could only work for toxins whose speed is not limited by distribution in blood and uptake by the cells. These would need another distribution method in the body, which would render the liver rather useless (because the liver detoxifies blood). --TheMaster17 (talk) 11:18, 18 June 2009 (UTC)

Also, exercise will probably cause vasodilation in the muscles, leading to increased absorption there (although I'm guessing vasodilation would also occur in the liver for glycogenolysis). --Mark PEA (talk) 15:31, 18 June 2009 (UTC)

In any case, if you get poisoned with cyanide, the poison will act too quickly for you to do much of anything about it. 76.21.37.87 (talk) 05:02, 21 June 2009 (UTC)

Lightning & thunder

We all know that getting hit by lightning probably is not very good for you, but what about a near miss? Thunder is sound energy cause by the shockwave of the rapid heating of the air the lightning passes through, so if you are close to lightning (very close), could you be hurt by the thunder, even if the lightning itself missed? Ruptured eardrums, etc? 65.121.141.34 (talk) 13:17, 18 June 2009 (UTC)

At a guess, probably not (save perhaps the hearing damage). Mythbusters recently did a bit on breaking windows with sonic booms, which I would expect are comparable to thunder. They had no problem with the hosts standing 200 feet from a supersonic fighter jet (with good ear protection). I would expect similar effects from nearby thunder. — Lomn 13:26, 18 June 2009 (UTC)

Our articles lightning strike and Roy Sullivan don't mention the possibility of ruptured eardrums even from a direct lightning strike, so I doubt there's much risk. Algebraist 13:29, 18 June 2009 (UTC)

While we're usually good, we're not that good. A Google search for ruptured eardrum or ruptured tympanic membrane and thunder will pull in a substantial number of reputable reports and case studies describing ruptured eardrums following a direct lightning strike; a bit of sieving will identify cases caused solely by thunder. TenOfAllTrades(talk) 13:38, 18 June 2009 (UTC)

Absolutely. Any sufficiently loud noise can rupture eardrums, including a clap of thunder. The British Medical Journal reported such cases all the way back in 1864, also noting that it may be accompanied by otorrhagia (bleeding from the ears). TenOfAllTrades(talk) 13:32, 18 June 2009 (UTC)

I can attest to that. I once had lighting strike nearby me (30 yards or so away). I thought I had permanent hearing damage after that, but thankfully it was temporary. ~Amatulić (talk) 17:10, 18 June 2009 (UTC)

Thanks all. Good to know. 65.121.141.34 (talk) 18:08, 18 June 2009 (UTC)

There was an incident when players were playing football on slightly wet grass when the ground was hit by lighting, due to the conductivness of the water on the grass 4-7 players were electrocuted i suppose this could countChromagnum (talk) 13:51, 20 June 2009 (UTC)

Earth sized cyclotron/rail-gun for human relativistic stasis?

Supposing a cyclotron rail-gun with a capsule large enough for a terminally ill human was created that circled the Earth. Would it be theoretically possible to accelerate the person to relativistic speeds with present day technology (assuming no funding limits) to slow time down for them until the cure for their disease became available, bearing in mind that surviving cyronic suspension is a highly speculative proposition to say the least? (Trevor Loughlin) —Preceding unsigned comment added by 80.2.205.61 (talk) 13:46, 18 June 2009 (UTC)

In short, no. By my calculations, at light speed, you will circle the earth at a rate of 7,500 times each second. Using the equation for centripetal force, you are talking acceleration on the human in question of 1.44 billion g's of acceleration. Considering that amount of acceleration, I can confidently say that he will be unrecognizable as a living thing long before he gets to relativistic speeds. 65.121.141.34 (talk) 14:05, 18 June 2009 (UTC)

My slide rule, confirmed by a back of the envelope calc, says that if the circumference of the Earth is about 24000 miles, and the speed of light would carry you around it 7500 times per second, then the speed of light would be 180 million miles per second. When did it increase in your universe? Edison (talk) 18:57, 18 June 2009 (UTC)

Did you not get the memo? --Tango (talk) 14:20, 20 June 2009 (UTC)

I agree, the g-forces for that kind of circular path would be prohibitive. If you had unlimited funding, you could probably put them on a space ship and send them at relativistic speeds in a straight line. In order to keep the g-forces reasonable you would need to accelerate fairly slowly (by relativistic standards), so I doubt you could get the proper time elapsed for the patient below a year or two (including accelerating, decelerating, accelerating in the other direction and then decelerating again). The elapsed time for the Earth could be arbitrarily high, though, you just keep going for longer before you turn around. --Tango (talk) 14:13, 18 June 2009 (UTC)

Unfortunately for the straight line relativistic rocket, we find that at high speeds collisions with dust become extremely energetic and resultingly very destructive. Making the practicalities design of such a vehicle out of the scope of modern technology. Elocute (talk) 15:04, 18 June 2009 (UTC)

If money is no object, you can just put lots of ablative armour on the front (give me a minute and I'll try about work out how much). That would dramatically increase the energy requirements, but that's just more money (prohibitively more money in real life terms, but the OP isn't talking about real life). --Tango (talk) 15:55, 18 June 2009 (UTC)

Ok, apparently we're talking about something on the order of 1 atom per cubic centimetre. I'll assume that is actually one proton per cubic centimetre, because that's close enough (it's going to be mostly hydrogen). Let's say we want a time dilation factor of 100, that gives the energy per atom as about 15nJ. That time dilation factor requires a speed of 0.99995c. One square centimetre of the front of the ship would sweep out a volume of 3*10¹⁰ cubic centimetres every second, so would have 3*10¹⁰ collisions per second. That's an energy of 450J per second. So you would need to dissipate 450W per square centimetre of cross section all the time you are at top speed. That is a lot, but not astronomical amounts. Impossible to do with existing technology at any remotely reasonable cost, but I don't think it would be impossible with unlimited funds. (Ablative armour probably isn't the best idea, but it was the first I thought of. Something that can just turn the energy into heat combined with a refrigeration system would probably be better.) --Tango (talk) 16:08, 18 June 2009 (UTC)

Because the guy can't orbit the earth (see above argument about centrifugal forces) - he'd be rocketting away from us at some large fraction of the speed of light in more or less a straight line. When we find the cure and send the message to tell him to come home and get fixed up - the message is going to take an alarming amount of time to get there. At a 100x time dilation factor, (as Tango points out), he has to travel at 99.995% of c - he covers 99.995% of a light-year every year. From our perspective our message (at the speed of light) is only catching him up at a relative speed of 0.005% of c. So it might take a few thousand years for our message to reach him! Then he has to slow down, stop, and fly back to us for another thousand years. Even with the 100x time-dilation factor - he's going to be dead before he can get home. I think it's possible to show that there is no way for him to cheat death no matter how clever the technology is. SteveBaker (talk) 18:16, 18 June 2009 (UTC)

Well, I suppose you don't need to fly the ship in a perfectly straight line. If you flew in a circle with a 1 ly radius, you are only pulling 1.5g, and could get the message in only 1 year, but even 1.5g long term might be really bad for you. (Maybe you would look like the incredible hulk when you were done). 65.121.141.34 (talk) 18:37, 18 June 2009 (UTC)

Make the circle a little bigger and you only need 1g, which would be perfect. No worries about high g-forces and no worries about weightlessness. That requires constant thrust, though, which requires even more fuel. In this money-no-object scenario, though, that's a good option. --Tango (talk) 18:46, 18 June 2009 (UTC)

Hang on... I get the radius required for 1.5g proper centripetal acceleration as a little over 6000ly, not 1ly. Either I can't Google relativistic formulae correctly, or you made a mistake. --Tango (talk) 19:03, 18 June 2009 (UTC)

I was thinking you would just guess how long it would take to figure out a cure, add a little on for a margin of error, and come back at that time. If you come back later than necessary, no great problem (it wouldn't be much later from your perspective), if you are too soon then you could always do it again, although you would need to spend more time accelerating back up to speed. --Tango (talk) 18:46, 18 June 2009 (UTC)

mechanical device

Is there any mechanical device like a gear box of a car,in which the output shaft rotates in the same direction independent of the direction of rotation of the input shaft i.e. the output shaft always rotates in clockwise direction when the input shaft is rotated either clockwise or anticlockwise? —Preceding unsigned comment added by 113.199.184.1 (talk) 15:26, 18 June 2009 (UTC)

how about the gear box of a car? —Preceding unsigned comment added by Alaphent (talk • contribs) 16:02, 18 June 2009 (UTC)

I'm not sure a car's gear box behaves the way you describe (else how else does a car reverse?), but a winch on a boat (like this one) commonly has a ratchet system that behaves this way. — Lomn 16:08, 18 June 2009 (UTC)

Ratchets usually just stop the output shaft turning at all if you turn the input shaft in the wrong direction. You could build a system with two sets of ratchets, one in each direction, with one direction being reversed (that just requires one extra gear). I can't see anything difficult about such a set up. (This may be what some boat winches do, I'm not much of a sailor.) --Tango (talk) 16:13, 18 June 2009 (UTC)

All sailboat winches I've worked with advance the line in one direction regardless of which way the handle is turned. Changing which direction you turn the handle affects the mechanical advantage provided, much like gears on a bicycle. I suppose this means that there are a pair of ratchet setups, but not being a ratchet expert, I opted for the generic "ratchet system" phrase. — Lomn 17:34, 18 June 2009 (UTC)

Ok. "Ratchet system" would be an accurate, if imprecise, description. Having different gearing for each direction would be trivial to achieve in the kind of setup I described. --Tango (talk) 17:56, 18 June 2009 (UTC)

Let the input shaft "jiggle" a Self-winding watch. This is probably what goes on in this watch winder. The output shaft is what turns any of the hands of the watch. Cuddlyable3 (talk) 00:32, 20 June 2009 (UTC)

Need additional scientific quotes

I asked my friend what he was doing and he said he was "just sitting around converting oxygen to carbon monoxide". I thought that was funny and I wanted to think of other quotes to answer him back but I can't think of any. --Reticuli88 (talk) 15:56, 18 June 2009 (UTC)

You shouldn't talk to your furnace. People will look at you weird. 65.121.141.34 (talk) 16:12, 18 June 2009 (UTC)

For your information humans don't breathe out Carbon Monoxide, which is in fact poisonous. We breathe out Carbon Dioxide. See Breathing. Rkr1991 (talk) 16:22, 18 June 2009 (UTC)

Tell him that you're doing your part to increase entropy. Tempshill (talk) 19:42, 18 June 2009 (UTC)

How about these: "I'm just sitting here converting glucose to glycogen" "I'm just sitting around converting glucose to ATP" and if you are going to be drinking "I'm going to put my hardest working enzymes to good use" (alcohol dehydrogenase and aldehyde dehydrogenase "Running around converting glucose to carbon dioxide" --Reticuli88 (talk) 19:46, 18 June 2009 (UTC)

"I'm just sitting here keeping my synapses from weakeining."68.208.122.33 (talk) 21:28, 18 June 2009 (UTC)

There are thousands of things you could say, but one will sum them all up: "Maintaining homeostasis". --Mark PEA (talk) 21:58, 18 June 2009 (UTC)

I am currently resting between hydrous oxide downloads. Cuddlyable3 (talk) 00:19, 20 June 2009 (UTC)

Tell him not to be such an oxygen thief and do something usefullChromagnum (talk) 13:45, 20 June 2009 (UTC)

Fan Displacement

Does anyone have any figures for the average displacement of a desk fan (preferably around 40 cm in diameter), or alternatively, any figures for the revolutions per minute and depth of such a fan, so I can work it out myself? Thanks. --80.229.152.246 (talk) 16:07, 18 June 2009 (UTC)

Desk fans at 40 cm (15-16 inches) are rather rare. 12-13 inches are more common. Size is not necessarily in direct relation to efficiency. The math is rather complex. 68.208.122.33 (talk) 19:07, 18 June 2009 (UTC)

Examples of Thermodynamic Laws

Please give me some practical examples of each of the Thermodynamic Laws: Zeroth, First, Second and Third. I read the Wiki article but having problems on how I would apply it. Thanks! --Reticuli88 (talk) 18:45, 18 June 2009 (UTC)

• The zeroth can be roughly summarized as "if a=b and b=c, then a=c". As a result, we can have meaningful conversations about temperature and energy, since we can discuss common points of reference. Alternately, as Max Planck said, we can construct a thermometer.
• The first law notes that you cannot create or destroy energy, ruling out perpetual motion generators. Rather, energy is transformed.
• The second law notes that energy transformations are inefficient. Over time, the usable energy of a system is lost as waste heat. This rules out remaining perpetual motion machines.
• The third law defines absolute zero as a reference point, but notes that you never actually reach it.

Depending on how much you've read, I'll note that we've got an overview article of the laws of thermodynamics as well as individual articles on each law. However, you may also need to clarify what you don't understand and what sort of applications you're looking for. — Lomn 19:01, 18 June 2009 (UTC)

Let's have a shot at some examples:

• Zeroth law: Take three bits of metal called A,B and C that are at different temperatures. Put A,B and C together until the temperature of A is the same as B - and B is the same temperature as C. I don't think anyone would be surprised to find that A and C are at the same temperature as each other. The Zeroth' law is really just elementary math...it almost doesn't need to be stated. It's so "obvious" that it wasn't really named/stated until about 100 years after the first/second/third laws.
• First law: A refrigerator makes your food colder - it's taking energy out of the food. But the first law says that we can't destroy that energy - it's got to go someplace - so at the back of your fridge - you'll see some exposed metal pipes. These are HOT! The reason being that they are where the energy from your warm food went to as it was cooled down.
• Second law: There is no such thing as a 100% efficient machine - there's always some kind of lossage when you convert energy from one place to another. That's why you have to put electrical energy INTO your refrigerator in order for it to move energy OUT from the food. Some people wonder why you can't use the energy that you take out of the food to run the refrigerator...well, the second law is the problem. The result is even more waste heat coming out of those pipes at the back.
• Third law: Heat energy moves from hot things into cold things...not the other way around. The only way for your refrigerator to make your food colder is to use a coolant that's even colder than the food. However, to get things to absolute zero (where there is ZERO heat energy left) you'd have to get the very last bit of energy out by putting something even colder than absolute zero next to it. But you can't have less than zero heat energy - so there is no way to get to zero.

I've heard it said that the four laws can be summarized as the following depressing look at life:

• First law: You can't win.
• Second law: You can't even break even - except when you have nothing left to lose.
• Third law: There's always something else to lose.
• Zeroth law: You can't even leave the game.

That's a bit of a stretch - but it's memorable!

SteveBaker (talk) 00:55, 19 June 2009 (UTC)

Another version of this (though lacking the zeroth law) is:

1. You can't win, you can only break even.

2. You can only break even at absolute zero.

3. You can't reach absolute zero.

AndrewWTaylor (talk) 17:23, 19 June 2009 (UTC)

why is carbamic acid unstable?

Is it because the OH protonates the NH2 group, making it NH3+COO-, that decomposes into ammonia and carbon dioxide? What is the exact mechanism of decomposition, electron transfer, etc. and how fast and exergonic is the reaction? If there was an alkyl group on the nitrogen (making it a secondary or tertiary amide), would decomposition still occur? John Riemann Soong (talk) 19:53, 18 June 2009 (UTC)

As we discussed in the quesiton about carbamic acid a few months ago, decarboxylation of N-alkyl carbamate is fairly rapid., and is part of the removal of amine protecting groups like t-BOC, Cbz, and Fmoc. I don't remember at what point/rate in actually performing the various methods the CO2 is released, but note that they cover the range of pH, being cleaved under strongly acidic, neutral, and basic conditions respectively. An alternate mechanism could be first protonating the N and then carboxyl collapse/fragmentation to give O=C=OH⁺ (BOC cleavage with neat TFA is often complete within a few minutes). The carbamate page mentions some equilibrium-constant information about carboxylation of amine (i.e., the reverse of the mechanism you propose), but doesn't mention pH effect. Interestingly, solid phase peptide synthesis notes "Fmoc deprotection is usually slow because the anionic nitrogen produced at the end is not a particularly favorable product, although the whole process is thermodynamically driven by the evolution of carbon dioxide." The carboxylate form does exist at least long enough to trap it if you're careful:

RNH-C(=O)-OTBS + TBAF + BnBr → RNH-C(=O)-OBn

and TBAF is usually considered strongly basic, so the fragmentation might be most efficient not under such conditions. DMacks (talk) 21:39, 18 June 2009 (UTC)

Solar radiation

At what distance is the radiance of the Sun still blinding enough to obscure other stars or planets? Is there anything to relate the brilliance of a star to the vastness of the space it is in? (please, if there is no answer, that is good an answer as any) ~ R.T.G 23:57, 18 June 2009 (UTC)

On Earth we can't see stars during the day because the Sun's light gets scattered by the atmosphere. Without an atmosphere, even if you were really close to the Sun, say on Mercury, you would be able to see stars during the day as long as the Sun (or anything lit by the Sun) wasn't in your field of view. So, if you look straight up so you can't see the ground and shield the sun with something then, if you give your eyes time to adjust, you'll be able to see stars (admittedly, you'll be burnt to a crisp, but let's not worry about that!). If you want to be able to see stars while the Sun is in your field of view you have to be quite far away. I'm not sure how far... About 20 times further away from the Sun than Pluto would definitely be far enough, since the Sun there would be about as bright as a full moon here, and you can see stars during a full moon. So it is something less than that, but I'm not sure how much less (it could be a lot less, you may be able to see stars during the day from some planets in the solar system, I'm not sure). Is any of that helpful? I'm not sure I understood the question, so that's my best attempt at answering what I think you mean. If I've misunderstood you, please clarify! --Tango (talk) 00:18, 19 June 2009 (UTC)

Did Armstrong or the others try this on the moon? 65.121.141.34 (talk) 13:10, 19 June 2009 (UTC)

The lunar surface itself is brightly enough lit by sunlight that it is very hard to see stars from the surface of the Moon. There are people who think the lunar landings were faked and one of the bits of "evidence" they cite is that photos taken from the Moon don't show stars. But see item 4 in this section of the article. --Anonymous, 18:45 UTC, June 19, 2009.

Indeed. As I said, you have to make sure there is nothing lit by the sun in your field of view. All the photos taken include large amounts of lunar surface, so it isn't at all surprising there are no stars. Even if they pointed the camera straight up they would need a long exposure to pick up stars, just as you do at night on Earth. --Tango (talk) 21:10, 19 June 2009 (UTC)

20 times farther than Pluto? NASA says that from Pluto, the Sun just looks like a bright star. (It's a kids' page, but, still, a NASA page which I assume is accurate.) Tempshill (talk) 03:44, 19 June 2009 (UTC)

It's angular diameter is very small (so it would look like a point of light), but it is still brighter than a full moon. Apparent magnitude says the Sun is 449,000 times brighter than a full moon. That means for the Sun to appear the same brightness as a full moon you need to be ${\sqrt {449000}}$ times further away. That's 670AU, or just under 20 times more than Pluto's semi-major axis. --Tango (talk) 16:05, 19 June 2009 (UTC)

It's interesting to think about: at Pluto's distance, the Sun appears no bigger than Venus does to us, yet it's still 100 times brighter than the full moon. --Bowlhover (talk) 00:51, 20 June 2009 (UTC)

Actually, it's nearly 400 times brighter. I'd never really thought about it before doing that calculation, it really is an interesting fact. --Tango (talk) 01:17, 20 June 2009 (UTC)

In a total eclipse, you can see stars pretty close to the edge of the sun. That's how Sir Arthur Eddington measured how much the light from a star is bent by the sun's gravity - and thereby was the first to verify Einstein's theory of relativity. In normal daylight conditions, it's impossible to see any stars with the naked eye.SteveBaker (talk) 00:31, 19 June 2009 (UTC)

Impossible, you say? As for the OP's second question, there's the inverse square law: brightness drops off as the square of the distance. A twofold increase in distance corresponds to a fourfold decrease in brightness, for example. --Bowlhover (talk) 03:50, 19 June 2009 (UTC)

Well, technically - you can see Sirius from the top of a tall mountain at just the right time of day and year IF the sky is perfectly clear...but that's an awful stretch - and Sirius is the only star that's anywhere close to being that bright. However, re-reading RTG's question, we're also being asked about planets - and there is no doubt you can see Venus in daylight around dawn or dusk...even when it's pretty close to the sun. SteveBaker (talk) 04:22, 19 June 2009 (UTC)

You can see Venus in full daylight without needing to be at a high elevation, when it's at an angle far enough from the Sun, i.e. near elongation. I've done it. Of course, when it's in such a position, it's not the Sun that's "competing" for your eyes' attention, but only the sky. --Anonymous, 18:51 UTC, June 19, 2009.

I am supposing, so, that even as far as Mars or Jupiter our eyes would be much more able for looking directly at the Sun (correct me if that is wrong!) Thanks. 11:22, 19 June 2009 (UTC)

More able, certainly, but I expect it would still blind you if you did it for too long. Jupiter is only 5 times further away from the Sun than Earth, so the Sun would be 25 times dimmer. That sounds like a lot, but the eye actually works on a logarithmic scale, so really it's just a difference in magnitude of 3.5, which isn't much when you consider than the magnitude of the Sun from Earth is nearly -27. You can look directly at the Sun from Earth, as long as you don't do it for long (I don't recommend you try it, though, because I don't know how long "too long" is - a second would probably be enough to cause at least temporary damage). --Tango (talk) 16:13, 19 June 2009 (UTC)

The statement is actually wrong: it's a fact that the brightness per unit area of a light source remains constant as you move farther away from it -- only its area changes. That means that you wouldn't be able to look at the Sun safely until you are so far away that it appears as a point which the eye can't fully resolve. And that's far beyond the orbit of Pluto. From Pluto, the Sun would be a very small circle, but each point on its surface would look just as bright as it does from Earth. Looie496 (talk) 23:55, 19 June 2009 (UTC)

The angular size of the sun seen from Earth is about 32'. Block that disk and you are left with the corona. In visible light the corona seems insignificant outside twice the sun's radius. That implies that if your view of the sun is blocked by a disk of twice its angular size as seen from wherever you are, and there is no intervening atmosphere, then the sun does not affect star viewing at all. Cuddlyable3 (talk) 00:08, 20 June 2009 (UTC)

Is brightness per unit area the sole consideration? It seems that it should be a consideration, since it determines the amount of light hitting a given part of the retina, but is there no harm done by having a large part of the retina exposed to a level of light that isn't quite enough to harm any part of it individually? (If that made any sense.) --Tango (talk) 00:12, 20 June 2009 (UTC)

Well, if you move your eyes around, the larger the bright area, the more time a given part of your retina is likely to be exposed to it. I didn't mean to imply that size doesn't matter at all. As far as I know there isn't a harmful "total exposed area" effect though, unless you are so close that the sun heats the interior of your eye appreciably. Looie496 (talk) 02:59, 20 June 2009 (UTC)

Okay, now I am thinking that at 100 times the brightness of the moon in the Pluto region, the Sun would still be quite blinding (as regards trying to pick out stars etc.) and also that it is something that is not often calculated. ~ R.T.G 15:46, 21 June 2009 (UTC)

Yes, although it would be so small that it would be very easy to block it out so you could see stars very close to it if you wanted to (as long as there wasn't any illuminated ground in you field of view, as mentioned above). --Tango (talk) 16:28, 21 June 2009 (UTC)

As long as the disc of the Sun can be resolved by the human eye (possible down to about 1 arc minute, corresponding to a distance of about 30 AU), the brightness per unit area stays the same. If you travel farther away, the spot on the retina doesn't become smaller (see Airy disc), and it is roughly the size of 1 cell of the retina. If you multiply the distance at which you can no longer resolve the Sun's disc by the square root of the ratio of the Sun's and the full Moon's apparent brightness (about 384*10³), you get the distance at which the Sun looks as bright as one "pixel" of the full Moon (about 19*10³ AU). Icek (talk) 23:26, 22 June 2009 (UTC)

I can almost smell the cookies now but for me that says 19 multiplied by 1000 (* = computer multiply sign and 10³ = 1000, but I suspect you are saying "Somewhere in the Delta Quadrant"). ~ R.T.G 00:09, 23 June 2009 (UTC)

Writing "19,000" would imply more precision than I want to imply ;-) -Icek (talk) 11:12, 23 June 2009 (UTC)

The square root of 384e3 is 62e1, not 19e3. —Tamfang (talk) 17:48, 26 June 2009 (UTC)

The distance at which the sun cannot be resolved by the eye is 30 AU, not 1 AU ;-) Icek (talk) 07:24, 29 June 2009 (UTC)

Now I see how I didn't read carefully enough. —Tamfang (talk) 15:19, 1 July 2009 (UTC)