Wikipedia:Reference desk/Archives/Science/2013 December 7

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December 7

Recoil force and momentum

When a gun fires, the recoil it produces is equal and opposite to the momentum of the projectile. But since recoil is a force, why isn't recoil calculated using force? ScienceApe (talk) 03:11, 7 December 2013 (UTC)

Recoil, at least as defined by our recoil article, is not a force -- it is the result of a force. Looie496 (talk) 04:12, 7 December 2013 (UTC)

The law of physics that most easily applies here is "Conservation of linear momentum". The gun and bullet are initially stationary and have zero momentum. When you pull the trigger, the bullet goes one way and the the gun the other way. Because the directions of their motions are opposite, the total momentum still adds up to zero and the conservation law applies...and naively, you can say that the mass of the bullet multiplied by the muzzle velocity is equal to the recoil velocity of the gun times the mass of the gun. This isn't quite true - as I'll explain in a moment.

Doing the calculations using Newtons' law (that every action has an equal and opposite reaction_ is also applicable at the instant that the gun fires - but force causes acceleration and it's a complicated calculation because the bullet is exposed to the forces of the expanding gasses over some amount of time and you'd have to integrate that continually varying force curve to calculate the final velocity of bullet and gun. That's a perfectly valid calculation to do - but it's complicated and relies on knowing a heck of a lot about gunpowder, bullets, barrels and so forth. Using conservation of momentum is much simpler - and gets you the answer in one step without having to know whether the gun is powered with gunpowder, a big spring, compressed air or an elastic band! You could use Newton's law to arrive at the same answer - but it would be painful to crunch the numbers.

Of course, in the case of any real gun, simply calculating the momentum of gun and bullet won't get you the correct result because the gasses that expand and leave the barrel also have momentum. So the recoil of the gun will actually be a little more than you'd predict from the mass and velocity of the bullet alone. Note that firing a blank round (where there is no bullet) still produces a recoil due to the expanding gasses.

So the actual FORCE of the recoil will depend on the nature of the gun. The force might start off as a huge force and gradually reduce - or it might be a continuous smaller force depending on ugly details of the propellant's chemical reaction - it's certainly a complicated matter and cannot possibly be stated as a single number "The force of the recoil is X" because you can only say "The force of the recoil Y milliseconds after the trigger is pulled is X".

However, the VELOCITY of the recoil by the time the bullet has left the barrel (and neglecting friction, etc) is simply stated as a single number and easily calculated.

SteveBaker (talk) 04:41, 7 December 2013 (UTC)

To put that more simply, or at least more sillily, suppose you have a witty bitty gun - in fact, it is the same weight as the bullet and has the same cross-section/frictional force in the air. What happens if you fire it? Well, if it is capable of firing the bullet properly, it will go back at the same speed as the bullet, teaching the shooter a valuable but perhaps final lesson in physics. On the other hand, suppose you shoot the bullet out of a monstrous solid gold illuminated blunderbuss that weighs two hundred pounds. Then the force of the recoil will be no more than the force felt by the guy standing behind the person you shoot when he gets hit by the bullet, provided it doesn't go through. Of course the total force felt over time is still F delta t, the change in momentum. You can change how much you get at any given moment, much as you can change the force you feel when moving something by using a lever. Wnt (talk) 06:20, 7 December 2013 (UTC)

Muzzle velocity are usually available quantities. A bolt action rifle or revolver is going to be the most "Classical Newtonian" calculation with the least amount of variables. The force felt by the shooter is very dependent on the mass of the firearm and the ratio to the bullet weight. In semiautomatic weapons, the discharge is countered by a spring or a piston. For a pistol, the spring holds the slide in batter and the discharge is actually moving the slide back against the spring until it hits the block. If the spring is too strong or the shooter doesn't hold it as the spring compresses, the slide won't travel far enough to eject the spent cartridge or load the next and can lead to a feed failure. For a given caliber, the heavier the gun, more comfortable it is to shoot. It's a lot like car crash physics. One things for sure, that hollywood misses, is if the gun didn't knock done the shooter, it's not going to knock the shootee 6 feet backwards. "blown away" is not something that happens. --DHeyward (talk) 05:16, 8 December 2013 (UTC)

NASA's recent picture of Saturn

Commons copy of the OP's picture

A similar photo

NASA recently released an image of Saturn they put together from images recorded by the Cassini probe. You can see the picture here: http://apod.nasa.gov/apod/ap131113.html It's a lovely picture, but there are some features here that I do not understand. Our viewpoint is from the dark side of Saturn looking back towards the sun, which is obscured by the planet.

1. Why is the face of the planet not just completely black? Some people have suggested that it is being illuminated by light being reflected from the rings. I find it hard to believe that there would be enough light reflected from the portions of the rings that are in sunlight to illuminate the entire face of the planet, much less produce an image that could be recorded by our spacecraft.

2. The black band: there is a narrow white band encircling the face of the planet. It is interrupted in some places, but it is basically a circle. Around the upper half of this white ring there is a wider black band of varying width, but it is generally several times wider than the white ring. I presume the white ring is sunlight being refracted through the atmosphere, but what is the black band?

I'm beginning to think this is just a publicity shot put together by the marketing department and has no bearing on reality. 50.43.12.61 (talk) 05:12, 7 December 2013 (UTC)

See ringshine. The Moon illuminates Earth, and while the particles surrounding Saturn are much smaller, the Sun never penetrates the interior of a rock - only their surface area matters.

The black band mystifies me also, now that you point it out - good question! I'm tempted to speculate about refraction, but I'd be blowing smoke. (see below) Wnt (talk) 06:11, 7 December 2013 (UTC)

Everything there seems consistent with NASA's caption.

The rings glow from transmitted and reflected light - except where they are in the shadow of the planet. The planet softly glows in reflected light where it lies close to the glowing sections of the ring (which is exactly what NASA say in their caption, I don't think they'd lie about such a thing!).

The shadowed parts of the ring are occluding that dim light and making black bars across the planet. Bear in mind that the sensors on this spacecraft are quite sensitive - and they may have left the shutter open for a long time in order to capture enough light to form an image. So it really doesn't matter that this twice-reflected sunlight is very dim...the camera could easily capture it if it took the photo over many hours.

Bear in mind that the sun isn't a zero-sized dot...so it actually illuminates slightly more than a hemisphere of the planet...that, along with atmospheric scattering easily explains the bright white line...which is also occluded by the thicker sections of the rings.

The black region is harder to explain. Personally, I suspect that it's an artifact of the motion of the spacecraft while the picture was taken. Cassini is in a polar orbit around Saturn - so it's entirely plausible that it's moving in the vertical direction of the photograph. There is no other explanation that I could imagine for why the black bar would appear at the top of the planet and not the bottom.

The existence of such an imaging artifact strongly suggests that NASA did not in fact heavily doctor the photo because they could easily have removed it had they wanted to.

SteveBaker (talk) 06:16, 7 December 2013 (UTC)

I doubt it's an artifact - think of how high-res Cassini images can be! And it was stitched together from hundreds of individual shots. [1] Nay, we're still stumped! Wnt (talk) 06:24, 7 December 2013 (UTC)

No, that's EXACTLY my point. Because it's stitched together from a lot of images, the spacecraft must have moved between shots. Certainly you can process the image to align the photos together to minimise this effect, but in order to do that, you are (in effect) picking a particular point of view from which you're assembling the composite. But there are guaranteed to be parts of the sky that are visible from that chosen point of view that were never present in any of the images...and because we know that the craft was moving vertically - it makes perfect sense that this produces a section of the image at the top edge of the planet for which no data is available. Rather than falsify this data, NASA leaves it blank. SteveBaker (talk) 06:49, 7 December 2013 (UTC)

Shadow of Saturn on the rings

OK, I got it. I thought about this backwards on my own, but this set me back on the right path. The thing is, the rings that you see blocking the light on the rear side of Saturn are between Cassini and the planet, on the far side from the Sun. Also on that side: Saturn's shadow. Now Saturn's shadow is going to be about the same size as Saturn (a little smaller for totality, a little bigger for penumbra) but Cassini is closer to the farside part of the rings than it is to Saturn. So from its perspective the shadowed part of the rings will look a little bigger than Saturn itself. Wnt (talk) 06:34, 7 December 2013 (UTC)

That makes no sense. SteveBaker (talk) 06:49, 7 December 2013 (UTC)

Well, look at the source I cited, #4 on the picture. That is the black ring you mean, right? (It's substantially larger in that photo because the sun is less perfectly aligned with the planet and probe) Wnt (talk) 12:18, 7 December 2013 (UTC)

The axis of saturn and its rings are not aligned perpenedicular to the with the equatorial plane. Cassini looks like it might be on the ecliptic plane while Saturn tilts away from Cassini towards the sun. We see the shadow as an effect of the tilt. Light that is reflected off the rings on to the backside favor the northern hemisphere. The rings that are in the direct path create a shadow. The rings are banded so transparancy varies. The south is dark due to tilt and the contrast the tilt creates. --DHeyward (talk) 05:37, 8 December 2013 (UTC)

It looks to me like there are dark rings which absorb light from Saturn. And, as far as the light-colored rings reflecting enough light, consider that we can see them from Earth with a minimal telescope, so they must be reflecting lots of light to be visible at that distance. StuRat (talk) 09:32, 7 December 2013 (UTC)

Does the fact that Saturn is a gas giant, i.e. not solid, have a bearing on why we see the black ring? --TammyMoet (talk) 13:32, 7 December 2013 (UTC)

TammyMoet, I think not. The artifact you see as a black ring is more probably a local contrast artifact, because what we are looking at here is a high dynamic range photograph whose parameters would blow the minds of most earthly photographers. The tone mapping algorithm in that area is being asked to combine imagery where adjacent pixels have photopic intensities that correspond to thousands of times - several dozens of bits, or hundreds of decibels - of dynamic range. Even NASA's greatest imaging experts cannot realistically create an algorithm that does so with zero artifacts.

A few months ago, when Cassini turned to photograph us for NASA's "Wave At Saturn" public relations program, I wrote a very lengthy article on Cassini's imaging system that I distributed to my friends and coworkers. I will dig into my archives and extract a few pieces that may be relevant to today's discussion. Nimur (talk) 16:30, 7 December 2013 (UTC)

Excerpted from my summary, and based on these resources:

• http://saturn.jpl.nasa.gov/news/waveatsaturn/
• http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20040618/
• http://photojournal.jpl.nasa.gov/catalog/PIA00452
• http://www.ciclops.org/sci/docs/CassiniImagingScience.pdf

"Cassini's narrow-angle camera is a 1-megapixel 1024x1024) CCD with 12-micron radiation-hardened pixels, manufactured by Loral Corp. It shoots through an f/10.5 2-meter Cassegrain-style Ritchy-Chretian mirror. Each of the 24 individual color filters are captured sequentially with an optical filter wheel. Q.E. ranges from 10% to 40% across visible wavelengths. The pixel well is 120,000 electrons per pixel, with optional 2x2 and 4x4 analog binning. Pixels are sampled at 12-bits. Sensor read-out takes a minimum of 12 seconds. For transmission to Earth, pixels are optionally gamma-adjusted and truncated to 8-bits; and optionally transmitted in JPEG form."

"...Saturn is blocking the sun; and each exposure will probably last several minutes, depending on the AE algorithm, so the total photograph will take several hours."

For a horrifying glimpse at the mathematical constraints, read: A Modern Tour of Image Filtering, which was published in IEEE Signal Processing Magazine a few months ago. The difficulty in astrophotographs is that the contrast is so incredibly high, any stable algorithm must clip or blur. This is state of the art. You probably won't find better HDR algorithms outside of the Jet Propulsion Lab that could produce a nicer image with less artifact. (Though, I encourage any enthusiasts to download Hugin (software) and try!) Alternately, we could have a digital artist go in and "fill" the dark circle - but at that point, our output image is no longer rendered from real data. Nimur (talk) 17:13, 7 December 2013 (UTC)

This is partly true in the sense that the photo is clearly normalized so that the dim farside can be compared directly to the illuminated rings. But the black band itself is not an artifact. It's the shadow of Saturn on the ring behind it. Which looks bigger than Saturn itself because it's closer to the probe. Wnt (talk) 21:46, 7 December 2013 (UTC)

Wnt, the entire dark side of the planet is in "the shadow!" The dark ring is an artifact. It is rendered darker, but that is not an accurate representation of the number of photons from that area. If it were physically darker than the unilluminated side of Saturn, that would mean the penumbra is more shadowed than the umbra, which is not physical. That dark region rendered to a lower pixel value than some of its neighbors, even though it corresponds to a region that had more photons. Sounds like an image artifact to me!

Cassini FAQs from JPL: "The camera measures light from an object at each point in an image and assigns it a number from zero to 4095 depending on its brightness. Sometimes the scientist can't afford to send this amount of data for each pixel because of the amount of storage it takes. The camera has the ability to convert this range of values to those from zero to 255. The camera does this according to a preset table of values designed by the scientists. This table devotes many of the 256 levels for less bright things and less levels for brighter pixels. Part of calibrating an image on the ground is to reverse this table and get back pixels in the range of zero to 4095. Because you're looking at the raw data, images sent back in this mode will have dimmer things look brighter compared to the brighter parts of the image than in images not in this mode...The ideal use of this mode is for image scenes that are dark with almost all of the pixel values less than 255. If the scene is simple with gradual increases in brightness, then even if the original values get over 255 and go dark again, the scientists can figure out what the real value was. If the scene is very complicated or the original values are much brighter than 255, the image can have many bright and dark transitions with strange contours. In this case, the image will look very bizarre but not have much scientific value."

This dark region is an artifact, in the sense that the rendered image not have a monotonic tone map. You can experiment with this yourself: play with some images in GIMP, for example. Grab some photographs with really high contrast. Set up a nonmonotonic luminosity curve. Watch what it does to dark regions, bright regions, and high-contrast edges. Nimur (talk) 23:19, 7 December 2013 (UTC)

This picture explains the black and light bands in the ringshine (see caption)

Did you look at the source I linked above, feature #4? The point is, there's a gap between the nightside of Saturn and the shadow on the rings, because the shadowed part of the rings is closer to Cassini than the backside of Saturn. I agree that the nightside of Saturn would not look as bright as the directly sunlit rings, but the black gap is real. On one side you have the backlighting from the nightside of the planet; on the other you the sunlit rings; in the middle you have only the bright edge of the planet shining. If you look at the brightest ring at the top of the main set of rings, you see it crosses into the black zone, and even trails faintly on the left side into the part backlit by the planet, but it's not blindingly bright there, it's barely visible at all. It's not like they turned up the brightness a thousand fold in the backlit region. I'll admit - the brightness of what looks like the innermost rings relative to the region directly below them, or the black region, confuses me. It's as if there's some purely dark band of dust that blocks the backside right next to the surface of Saturn, but doesn't show up as a ring in the light at all ... that I don't get. But that's a different issue which doesn't actually go away even if you suppose a huge difference in contrast, because there's no lit ring to match either the bright part or the dark part of the backlit rings at the very innermost extreme. (or I can look at the picture above...) Wnt (talk) 02:29, 8 December 2013 (UTC)

"It's not like they turned up the brightness a thousand fold in the backlit region..." In fact that's exactly what they did: high dynamic range images are combined together in exactly that way. The input imagery had massively different exposure parameters - ranging across hundreds of decibels of effective photopic sensitivity. Cassini's camera can expose for hours when it is producing images of dark areas. The transition between these highly-exposed regions and low-exposed regions is as smooth as possible, but as I said before, it must either clip or blur in some areas (because of the Gibbs phenomenon). In this image, the image processing engineer chose to clip to zero, which is why you see dark black (rendered pixel value = 0). Nimur (talk) 03:55, 8 December 2013 (UTC)

Are we talking about the same thing? You do see that there is a part of the rings which is on the back side of Saturn that is in shadow, with nothing behind it but empty space? Is the black region you describe somewhere else? Wnt (talk) 06:00, 8 December 2013 (UTC)

There is another planet edge around the top hemisphere (you have to zoom in). Saturn ends, there is a very bright, white ring. Above that in the northern hemisphere is a black ring. since the layer order planet, white, black, space - the black is not explained. --DHeyward (talk) 10:41, 8 December 2013 (UTC)

When they said a picture is worth a thousand words, I think it was an underestimate. And Commons had all this stuff, so I've added two. Wnt (talk) 18:10, 8 December 2013 (UTC)

Wnt, I understand your statements. The planet shades the rings from the sun. This effect is visible in many photographs and makes physical sense. Nobody is disputing that a shadow exists.

Let's be even more specific: there is an umbra and a penumbra. And there is a planetary limb. For example, look very closely at the limb of the planet. In the variety of images now present in this section, you can see the illuminated rings outside the umbra, partially illuminated inside the penumbra, rings crossing the limb, and rings completely in the umbra. The transition from umbra to penumbra need not be the same as the position of the limb, because of parallax. Photographs from certain angles demonstrate that effect very dramatically. Thus far, Wnt, we are in complete agreement.

Now, if you look very closely at the limb, you will see that it has a dark ring, too - distinct from the other features I just listed. The planet edge clips bright, and then clips to zero, very close to the area where the big change in exposure happens. This is an artifact of combining multiple very different exposures, and is distinct from the real, physical shadow. Nimur (talk) 22:44, 8 December 2013 (UTC)

Seeing the penumbra is a challenge. A solar eclipse can have a penumbra of 6400 km, but the rings of Saturn are less than a fifth of the distance from the planet as the Moon is from Earth. To recognize a 1300 km feature against 121000 km of Saturn - that's 1/100 of the planet's diameter, or about 27 pixels at a maximum. In practice I'm only seeing about 15 pixels near the top and 4 near the bottom, except that one very bright outer ring which continues well beyond where the penumbra must end. The only absolutely black pixels I see, even zooming in in Paint, are a few artifactual lines running up and down, which are obvious jpeg damage. (We've passed around a few versions by now - the one I'm looking at is directly from [2] - the one on Commons I should upgrade because it's much lower resolution and shows more jpeg damage) Wnt (talk) 00:50, 9 December 2013 (UTC)

Just an observation: we see the rings from earth. That's a lot of light reflected. Second, we often see the full outline of the moon when it is a crescent. that light is a reflection of earthshine on the moon being reflected nack and is visible during bright conditions. That's a lot of light too. Ring shine doesn't seem far-fetched. Saturn also has a number of moons including titan. --DHeyward (talk) 05:25, 8 December 2013 (UTC)

Be aware sun-synchronous is not the same as equally-irradiating. The sun insolates the higher latitudes less than the lowers ones on a yearly basis. Some different orbit and/or tilt would be needed. μηδείς (talk) 05:30, 9 December 2013 (UTC)

Hamiltonian

How does one calculate the Hamiltonian of a system? Hamiltonian (quantum mechanics) lists a lot of special cases. How are these derived, in general? 150.203.188.53 (talk) 09:27, 7 December 2013 (UTC)

They are derived by calculating the Hamiltonian in terms of generalized orthogonal coordinates. The machinery of the mathematics is very straightforward, albeit quite lengthy to write: the Hamiltonian is just kinetic plus potential energy; and to solve the dynamics, calculate the differential of the Lagrangian with respect to the differential of the generalized coordinates with respect to time; and subtract that from the same, premultiplied by the generalized velocities. What is this doing ? It's constructing a general representation of action, because according to physicists like LaGrange and Hamilton, minimization of action is a more fundamental statement that encapsulated conservation of energy and conservation of momentum.

The hard part, though, is not the mathematics: it is the transformation of a real physical system into some generalized form that is actually valid. When you look at an automobile, your gut instinct is to desrcribe it in cartesian coordinates. It has an x,y,z position. If you're an advanced physics student, you actually append a general velocity for x,y,z that is represented independently from the position, because you care more about the phase space of the car's momentum than you do about the time derivative.

But if you're a very advanced student of physics and engineering, you recognize that this is insufficient. You need to postfix your coordinate vector with new coordinates describing, say, the angular position of each of the wheels. You need coordinates to represent harmonic modes of vibration of the sheet metal. You need to write coordinates for how much potential energy is in the crumple zone. You need to write everything that can store mechanical potential energy in the car, so that you can express the kinetics of the car in all cases. And, you must make those coordinates orthogonal so that you are can explicitly solve for mechanical coupling. Now you need an applied mathematician to help you write those vectors! You'll probably need a lot of matrix algebra to verify that your representation is valid. Most of all, you have to know which physical action is important enough to write down in your coordinate vector.

In the quantum case, you have to do all of the above, and satisfy additional constraints - some of your vectors do not satisfy regular mathematical relations, because they are physically quantized. You have to replace basic operations like addition and subtraction with physical operators, so that you ensure the system remains mathematically quantized.

Once you have represented the system in those coordinates, your math is trivial: plug and chug, as they say. The formula is splattered throughought every physics textbook and most of our articles on the topic,; and soon it will be burned indelibly into your brain. It's just a shorthand way of writing out some vector math. If you take a simple quantum mechanics class, you'll solve the Hydrogen atom, and somebody will tell you "use these four quantum numbers." If you proceed to the more general case, nobody will tell you which quantum numbers you need - and just like the car example, you have to know. Nimur (talk) 16:10, 7 December 2013 (UTC)

Well, the Hydrogen atom is a very simple case because the potential energy is given by Coulomb's law and the kinetic energy is as usual for a many body system. You don't even need to know the four quantum numbers a priori. 150.203.188.53 (talk) 07:36, 8 December 2013 (UTC)

The Hamiltonian is just the energy of the system. For most cases, it's just the potential energy plus the kinetic energy of every object. --Bowlhover (talk) 07:48, 8 December 2013 (UTC)

Yes, but I wanted to know how you engineer a system to have a particular Hamiltonian. 150.203.188.53 (talk) 08:15, 8 December 2013 (UTC)

Life cycle of frog

Could you please tell me about the life cycle of a frog?Mominjawad13579 (talk) 10:21, 7 December 2013 (UTC)

See Frog#Life_cycle, and come back and ask us if you have any specific questions. Rojomoke (talk) 10:47, 7 December 2013 (UTC)

An orbit that any point on the planet gets the same amount of radiation

Hi,
I would like to ask if there is an orbit, that when you sum up the amount of radiation in each point on the planet you get the same result.Exx8 (talk) 15:51, 7 December 2013 (UTC)

Radiation from the sun around which the planet orbits I assume? if you talk about the amount received during a complete orbit, then you may get close to the same value, I think, when the axis of rotation of the planet lies in the orbital plane and the orbit is circular. Could be I'm forgetting something obvious due to sleep deprivation, but apart from small parallax differences (because the poles stay on the orbital plane, the rest of the planet goes above and under it, so the mean distance from the sun would be somewhat larger plus a small difference in the angle of incoming radiation), the total amount of incoming radiation should be the same everywhere. I think... Ssscienccce (talk) 21:38, 7 December 2013 (UTC)

This is an interesting question, and the Sun most certainly does not insolate the Earth's surface equally at every latitude. I am not sure that there is any orbit that will satisfy this condition. The OP should close this and ask agaian at the Math Desk. They will be much more likely to know or find a solution. μηδείς (talk) 04:02, 8 December 2013 (UTC)

This is a perfectly sound Q for the Science Desk. Math may be used in finding answers, but this is true of most science Qs. StuRat (talk) 04:23, 8 December 2013 (UTC)

Absolutely not! I accept your challenge! I will fight you to the death over my suggestion! μηδείς (talk) 18:47, 8 December 2013 (UTC)

I agree with the comment that the axis of rotation of the planet must be in the ecliptic plane. Also, you would want the planet to have a circular orbit, as an elliptical orbit would mean more light on the side of the planet facing the Sun when it was closest to the Sun. StuRat (talk) 04:23, 8 December 2013 (UTC)

I made a mistake. Represent the position of the planet by an angle from 0° to 360° degrees, with the poles getting maximum radiation at 0° and 180°. The average radiation a pole receives during a full orbit is the same as the average that a place on the equator receives during a full revolution of the planet while its position is 90° or 270°. At all other positions it will be less (at 0° and at 180° the equator receives no radiation at all), so the total average must be smaller than the one for the poles. Ssscienccce (talk) 12:04, 9 December 2013 (UTC)

There is no orbit that will give all latitudes the same radiation. If the tilt of the planet's axis if 0 degrees relative to the ecliptic plane, or the orbit is circular (regardless of axis tilt), then the annual average radiation at a latitude of +X degrees will the same as at a latitude of -X degrees. Dragons flight (talk) 09:16, 8 December 2013 (UTC)

Not with an atmosphere. Even a circular, perpendicular orbit, I think it's always a longer/lossy light path for latitudes above the equator. I think your question is related to spherical aberration from a mathetical standpoint point. Light hitting a real, instead of ideal, spherical surface has aberrations. --DHeyward (talk) 06:22, 8 December 2013 (UTC)

This page has graphs of annual average insolation for axial tilts of 23° (Earth) and 90° (roughly Uranus). At 90° the poles actually get more radiation on average than the equator (see also Uranus#Axial tilt, after the table), which is the reverse of the situation here (0° would be more extreme). There should be a tilt where the differences in insolation are minimized (would make a nice calculation, I'm sure), but it's not clear whether it would be constant as a function of latitude. This assumes that the rotational period of the planet is much shorter than the orbital period, so one does not have to worry about longitudinal effects, as would be the case with bound rotation, for instance. --Wrongfilter (talk) 13:00, 8 December 2013 (UTC)

Do we have a reliable source for the claim that both poles receive more radiation on average than the equator? Seems to me that the eccentricity of the orbit and the orientation of the planets rotational axis relative to the orbits semi-major axis would play a role, with the pole towards the sun in perihelion receiving more radiation than the other one. On the other hand, the planet moves faster in perihelion than in aphelion. Then again, the flattening of the planet may be the most important factor. It's complicated... Ssscienccce (talk) 15:56, 8 December 2013 (UTC)

Come to think of it, the angular velocity in an eliptical orbit is proportional to the inverse of the square of the distance (from Kepler's second law). Given that the energy flux is proportional to the square of the distance, an eliptical orbit may not make any difference when it comes to energy received per angular displacement. At aphelion it will receive less energy, but it will travel slower, making the amount of energy received while traveling 1 degree the same as in perihelion. No?

I misread that completely, thought the x-axis was the position in orbit, I now see it's the latitude. I bow my head in shame... Ssscienccce (talk) 16:40, 8 December 2013 (UTC)

Sure - you can write equations to show whether the effect due to the elliptical orbit cancels out - but that's assuming the planet is a point particle. More time is spent at greater distance; so the radial falloff (r²) in incident radiation is counterbalanced by a longer duration of exposure. Do these exactly counterbalance? Well, write out the partial differential rate of change of intensity with respect to radial distance; and write the rate of change of radial distance with respect to time; and ascertain whether the total derivative of incident radiation with respect to time is exactly zero. Is it? Next, you really need to work out the details of a rotating, spherical planet. I think the cancellation is not so simple as you hope. I would add a simplifying assumption that the rate of rotation is much much faster than the rate of revolution; that is a good approximation for a planet like Jupiter; but this is not a very safe assumption for a planet like Earth, and certainly not for a planet like Mercury.

The last time we spewed voluminous algebra on the reference desk, we all got lost into pedantic details and small disagreements. I do not want to conduct original research, and in my opinion that includes posting an original solution to a special case of standard equations. But my recommendation is that if you enjoy this sort of recreational math, you should be able to solve these equations in ten or fifteen minutes, and satisfy your own curiosity. Plus, you have the added bonus of choosing exactly which simplifying assumptions you make.

It's worth saying that there is a practical implication to working out this math: if you have a satellite in a highly elliptical orbit around Earth, is there a preferred point in its orbit when bidirectional communication is easier? Some years ago when I visited the Geophysical Institute in Fairbanks, I saw a very large satellite dish perform a very very fast angular slew. Those satellite dishes are used to downlink science data from spacecrafts in polar orbit. The explanation I heard was that spacecrafts in polar orbits are visible above the horizon for more hours each day at very northern latitudes. But what implications does that have to the engineering requirements for the ground station's dish size and the dish angular slew rate? Is there any benefit to putting the polar orbit perigee at the north pole? The rocket engineers who launch and control such spacecraft can delay a motor burn until any desired time, so they have the ability to easily control such an orbital parameter... but should they bother? Nimur (talk) 18:13, 8 December 2013 (UTC)

"polar orbit" satellites as I understand it, are not always absolutely polar and can be tilted against the polar axis a bit and still be considered polar. A purely polar orbit would cover every latitude and the altitude would determine how fast it covered it and what longitudes are covered and how fast. (i.e what longitudes are crossed and directly overhead and how long). An angled polar orbit gives the "sine wave" look of coverage bounded by latitudes and I believe the angle also specifies a long/lat point where the satellite is neary visible for a long period.. Spy satellites for example cover a certain latitude range (including polar) and the altitude covers how fast and how far apart each pass is. Depending on which pass, the slew rate would be large for close passes and slow for far away. High gain is necessary for far away, speed is necessary for close. Interesting design tradeoffs. See Sun-synchronous orbits for more refs as that is likely what they were tracking. Every so often, the satellite crosses the station (or very near it). — Preceding unsigned comment added by DHeyward (talk • contribs)

Here is a list of current Alaska Satellite Facility missions. Some spacecraft are in sun--synchronous orbits; others are in low-altitude polar orbits. An alarming typo on that webpage implies that IRIS is flying a bit too low... that ought to say "390 miles," not 39,000 feet. I'll email the site administrator. Nimur (talk) 05:12, 9 December 2013 (UTC)

Fixed! Nimur (talk) 01:07, 10 December 2013 (UTC)

Good catch. The "typo" was because of how it was launched. The rocket was deployed from an aircraft, L-1011, Stargazer at 39,000 feet. --DHeyward (talk) 13:44, 10 December 2013 (UTC)

• I have a feeling (intuition of a non-mathematician) that a wobble in the planet's rotation set up just right to reciprocate to a radiation source in a just the right (not necessarily circular or equatorial) orbit might fit the bill. Of curse that's a math question, so maybe someone at the strange and mysterious Math Desk can answer it. μηδείς (talk) 19:35, 8 December 2013 (UTC)

I will Survive!!!!

• http://survinat.com/2013/03/darpa-is-a-strange-way-to-survival-of-soldiers/
• http://www.youtube.com/watch?v=pT1d6jxKwpk
• http://www.darpa.mil/NewsEvents/Releases/2012/12/10.aspx

This abdominal cavity foam is really a weird way to stabilize a patient with blunt trauma.

I wonder if there are actual stories of persons, whether in a battle field or in the wild, who suffered very serious internal bleeding blunt trauma, managed to survive by crawling or hiking a long way to rescue. -- Toytoy (talk) 16:31, 7 December 2013 (UTC)

ROFL - I thought the stuff in the video looked like polyurethane foam spray insulation that I've seen put around the inside of a garage. And... it is! Folks, don't try this at home... but maybe you could... :) Wnt (talk) 21:43, 7 December 2013 (UTC)

It's basically Gorilla Glue used on human being. However, if you inject Gorilla Glue into your body, it may be poisonous and unremovable. Maybe one day, the maker of Gorilla Glue would invent a Human Glue .... -- Toytoy (talk) 14:50, 8 December 2013 (UTC)

The Gorilla Glue article mentions cyanoacrylate, which is used in surgery, but that doesn't foam. Does Gorilla Glue foam? The link I gave mentioned polyurethane, admittedly though I used that term in my search. Wnt (talk) 17:55, 8 December 2013 (UTC)

Gorilla Glue is basically polyurethane (PU) + some kind of foaming agent. Cyanoacrylate can be used in surgery to adhere two pieces of meat. The PU foam injected into someone's belly is to press the injured area and stop bleeding. It can be removed later by surgery. It cannot be glued to your intestines! PU is used everywhere. Your sports shoes may have PU soles. -- Toytoy (talk) 03:40, 9 December 2013 (UTC)

Let me guess, they didn't (nor will they) bother to test the Ecuadorian fungus Pestalotiopsis, or it's enzymes for the ability to consume the polyurethane in the wound after the injury has stabilized, even in the "animal" trials. Nor did they bother to see how the polyurethane would interact with anything that actually regenerates the damaged tissue. Dirty deeds done cheep. CensoredScribe

List of experiments performed in space

Is their a convenient list you can browse which compiles all the experiments preformed by all the worlds space agencies currently? I had a few I was interested in; like whether the conditions of a burning building have been approximated on a space station (I know that fire+space station=extremely dangerous) I was also wondering whether someone has already investigated the effects of space on Saprophytic fungi and Black carbon, either alone or mixed together. CensoredScribe (talk) 17:30, 7 December 2013 (UTC)

Here's NASA's list anyway. Clarityfiend (talk) 23:52, 7 December 2013 (UTC)

Thank you for that list Clarityfiend; I don't suppose there is a similar one from the Russian space agency Roscosmos, formerly RASA? I'd be interested in if they've taken that radiotrophic chernobyl mold into space yet. CensoredScribe (talk) 00:41, 8 December 2013 (UTC)

Still looking for that one. In the meantime, here's the ESA database search screen; clicking on Space Stations, Space Shuttle, etc. actually shows you lists of missions, and clicking on each mission, its experiments. Clarityfiend (talk) 03:11, 8 December 2013 (UTC)

My assumption is that all experiments performed on the ISS are recorded in the NASA database. While the space agencies do have their own segments, I suspect it's intended that as a shared station, they share their plans for research. Definitely if you look at the NASA list, you can see see experiments performed by their partners. There is even a category option to sort by partner [3] and if you look at an example [4] you can see many of them don't really seem to involve NASA (this particular example was performed I believe at least in part by Sheikh Muszaphar Shukor who was listed as a Spaceflight participant by NASA and Roscosmos, and travelled due to an agreement with Roscosmos).

Anyway if you follow the links in an example you should eventually find [5] which has the list (look at the links on the left) for Roscosmos. Unlike the NASA list, they don't seem to have bothered to keep a list of most of those performed outside the Russia segment, although they do include those performed by spaceflight participants who went up on behalf of Roscosmos even if carried out in other segments e.g. [6]. We also have an article Scientific research on the International Space Station although as the header says, it needs work.

This doesn't help you with experiments performed on Mir or the earlier Russian space stations, perhaps if you poke around you will be able to find something. Although I wonder if all the records were ever properly made available on the web, particularly perhaps those done aboard the military ones which may not have been revealed at the time, and may or may not have been revealed later. Incidentally, it looks to me like the NASA list is only for the ISS so lacks anything aboard Skylab and are you including stuff done aboard Spacelab or otherwise done in what's considered space but not aboard something generally considered a space station? (And either way, none of this helps with stuff performed on Tiangong 1 or commercial stuff like Genesis I or Genesis II, although it doesn't sound like any of that is relevant to your question.)

Nil Einne (talk) 18:51, 8 December 2013 (UTC)

Many experiments are performed on space vessels other than stations, even on unmanned ones. Any full lists would be quite large. Rmhermen (talk) 04:01, 9 December 2013 (UTC)

Tsunamis from non-earthquakes.

How often do tsunamis come from other sources than earthquakes? --78.156.109.166 (talk) 20:07, 7 December 2013 (UTC)

The article Tsunami lists various ways they can be triggered. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:22, 7 December 2013 (UTC)

Mahuika crater is one - possible - example. --Cookatoo.ergo.ZooM (talk) 23:26, 7 December 2013 (UTC)

Presumably the impact causing the Chicxulub crater would have triggered a significant tsunami. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:37, 7 December 2013 (UTC)

Yes, although meteor-tsunamis probably don't happen all that often. Volcanic caldera collapses are probably the most common non-tectonic causes (Krakatoa and Thera are famous examples), and undersea landslides probably next (the Storegga Slides are examples). Looie496 (talk) 04:01, 8 December 2013 (UTC)

Non-earthquake tsunamis happen rarely enough that it won't be possible from the recent historical record to give a good estimate of a comparative rate. These tsunamis are random in relation to one another--a meteor strike has no causal relationship with a volcanic island collapse or a mountain landslide like the 1958 Lituya Bay megatsunami Just finding evidence of prehistoric tsunamis is difficult, as is tracing them to their causes. μηδείς (talk) 18:44, 8 December 2013 (UTC)

The commonest non-earthquake trigger for tsunamis is landslides, indeed many earthquake related tsunamis are actually caused by landslides, like the 1958 Lituya Bay event, the 1998 Papua New Guinea earthquake and the 1929 Grand Banks earthquake, rather than motion of the seafloor during the earthquake itself. To put it another way, many tsunamis are a result of landslides (either submarine of coastal) but most of these landslides are triggered by earthquake shaking. An example of tsunamis unrelated to an earthquake is the 1979 Nice events. Mikenorton (talk) 19:10, 10 December 2013 (UTC)