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November 29

Sky of Venus

Lets say you are standing on Venus on a "clear" day (if that's possible on Venus). What color would the sky be? ScienceApe (talk) 03:33, 29 November 2011 (UTC)[reply]

WHAAOE: Extraterrestrial skies#Venus. Dragons flight (talk) 03:56, 29 November 2011 (UTC)[reply]

The high pressure and temperature of the venusian atmosphere would cause the horizon to have negative curvature due to extreme optical refraction. When the sun is located low over the horizon as at sunrise and sun set, it should have a bean shape due to that same effect. This is just a supposition, as the sun cannot actually be seen in the visibile range of the EM spectrum. Plasmic Physics (talk) 09:39, 29 November 2011 (UTC)[reply]

More spectral sky info from the Russian probes here.[1]--Aspro (talk) 12:09, 29 November 2011 (UTC)[reply]

Note that we also have an article Atmosphere of Venus.

As this page points out, the question is non-physical, since due to the extreme pressures the clouds are unavoidable. But if somehow you had a "clear" column, I would expect that Rayleigh scattering would cause the extinction of blue light (clear skies at Earth pressure are blue regardless of their composition due to this phenomenon!) to the point where the higher-frequency light would be completely absent from the surface, and you would be left with a very dim, yellow-to-red illumination.-RunningOnBrains^(talk) 17:28, 29 November 2011 (UTC)[reply]

One hypothesis is that the extra colour absorption is due to trisulfur gas. Graeme Bartlett (talk) 09:58, 30 November 2011 (UTC)[reply]

I remember several times reading the claim that if its atmosphere were perfectly clear and you had a perfect telescope, you could see the back of your head in it from all the way around the planet. But I never really believed it... probably a moot point to argue though, like how much the Pope would weigh if he were a cherry tree. Wnt (talk) 16:04, 30 November 2011 (UTC)[reply]

Nanotechnology to improve the efficiency of heat engines

Are there any ideas on using nanotechnology to improve the efficiency of heat engines? ScienceApe (talk) 20:08, 29 November 2011 (UTC)[reply]

Maybe with better lubrication. ApNano apparently makes a line of "nanotech" lubricants. --Jayron32 20:51, 29 November 2011 (UTC)[reply]

The second law of thermodynamics makes it very hard to improve heat engines to levels beyond what's already currently achieved (with or without nanotech). Dauto (talk) 02:05, 30 November 2011 (UTC)[reply]

At least until we can build these guys. :) Franamax (talk) 05:58, 30 November 2011 (UTC)[reply]

One way to improve is to make a better spark. If the whole vapour air mixture could be ignited simultaneously in an ICE it would be more efficient. Perhaps lasers could make 1000 bright spots, or even one ignition point at the optimal position, that could start burning in the cylinder at the same time. Or perhaps your nanobots could move in and start ignition at the right places and times. Graeme Bartlett (talk) 10:02, 30 November 2011 (UTC)[reply]

The Moon

Look at this picture, for example: http://commons.wikimedia.org/wiki/File:Apollo17.jpg How come you can actually see the curvature of the Moon, i.e. you can basically tell that it's round? You can't really see it here on Earth. Is it because the Moon is smaller? 80.122.178.68 (talk) 21:17, 29 November 2011 (UTC)[reply]

Quite a few of the photos that I've seen that were taken on the Moon used a wide angle lens, especially those that were taken from the lander/rover, which causes a "fish eye" distortion. I'd chalk it up to that. Dismas|^(talk) 21:19, 29 November 2011 (UTC)[reply]

Yes, it's definitely not due to the actual curvature of the Moon. The Moon isn't that small! For comparison, here's a photo taken on Earth that definitely doesn't show Earth's curvature: http://www.photoanswers.co.uk/Advice/Search-Results/Photopedia/Fisheye-lens-/ --140.180.15.97 (talk) 21:25, 29 November 2011 (UTC)[reply]

This is image AS17-134-20387. It is a very famous photograph. NASA's Apollo 17 Lunar Surface Journal leaves little to the imagination: the photograph was taken in 1972 (118:26:38 into the mission, during EVA-1, at the LM); you can read a transcript during which Gene Cernan and Harrison Schmitt discuss the actual conditions of taking that photograph; the original image was captured on Magazine 134, color film, in the Hasselblad camera with a 60mm f/5.6 lens. I would not call this a "fish-eye lens" or even "wide angle" - even on medium format. Of course, some visible curvature is attributable to the geometric distortion of the image; but let's make clear: this is a telephoto lens - and it's widely considered the finest of its type ever made. For comparison, have a look at these terrestrial photos using a similar (not identical) camera and lens: Hasselblad 60mm f/3.5 CF T photo gallery. It's probably more correct to say that the apparent curvature of the horizon in AS17-134-20387 is due to the unusual camera angle, not due to lens imperfection.

Transcript during the setup for this photograph

18:25:37 Cernan: Well, I want to get something here.

118:25:46 Schmitt: What's that?

118:25:47 Cernan: I want to get the Earth.

118:25:49 Schmitt: Okay. Let me get over here.

118:25:51 Cernan: Get around on that side.

[Jack had moved west of the pole, and now moves back. He overshoots his mark and sprays dust as he stops.]

118:25:54 Schmitt: I don't think it's going...You're a little close, maybe, to have them both in focus. That might do it.

[Gene starts to bend his knees and, in an effort to get Earth in the picture along with Jack and the flag, almost gets down on his knees. His first effort, AS17-134- 20383 gets the flag but very little of Jack and the Earth, his second photo, AS17-134-20384, is much more successful. After he gets up, Gene gives Jack the camera and they trade places.]

[Using planetarium software, we see that, had cloud cover over the southwestern Pacific been lighter, the Antarctica would have been visible at the left and Australia would have been coming into view over the top.]

118:26:08 Cernan: Try that one time, then we'll give up and get to work. (Long Pause)

[Jack holds the camera in his hand and gets it as low to the ground as he can without kneeling.]

118:26:26 Cernan: Point it up a little...Yeah. (Pause)

[This is AS17-134- 20385.]

118:26:32 Schmitt: Let me try it again.

118:26:33 Cernan: Okay.

[This is AS17-134-20386. This photo shows the "red apple" actuator for the purge valve reasonably well.]

118:26:35 Schmitt: I don't know, Geno.

118:26:36 Cernan: Okay.

118:26:38 Schmitt: Let me get over here closer to you. (Pause)

[Once again, Jack almost goes to his knees.]

118:26:43 Schmitt: Okay. That might have got it.

[This photo is AS17-134-20387 and, in addition to having Earth in the picture, it shows Gene's checklist and watch/mirror band on his left arm, the OPS actuator on the right side of his RCU, and the OPS antenna on the top of his PLSS. The red bands on the suit and helmet show that this is Gene and not Jack.]

118:26:45 Cernan: Okay, very good.

118:26:47 Schmitt: Okay. All right, let's do it. (Pause)

Nimur (talk) 22:23, 29 November 2011 (UTC)[reply]

As can readily be seen from several photographs in the Apollo 17 article, the mission landed in a hilly area. The apparent "curved moonscape" behind Gene Cernan is, I'm pretty sure, just a small hill. {The poster formerly known as 87.81.230.195} 90.197.66.159 (talk) 22:31, 29 November 2011 (UTC)[reply]

In fact, you can see the ridge of a closer hill, sloping across the diagonal at the bottom right of the photo. One might even find AS17-134-20386 instructive. Nimur (talk) 22:36, 29 November 2011 (UTC)[reply]

And a wider view of the landing site, too. Nimur (talk) 22:52, 29 November 2011 (UTC)[reply]

Oh snap, it's just a hill. Thanks guys! 80.122.178.68 (talk) 23:38, 30 November 2011 (UTC)[reply]

Colour shifts in field of vision

I have noticed, in the context of driving at night and thus moving quickly relative to passing car headlights, static motorway street and tunnel lights, etc, that as lights move backwards out of the areas of foveal vision and into the areas of peripheral vision they become noticably redder. This is not apparent when moving one's head while stationary and looking at stationary lights, only when the relative movement is very swift (and the direction of gaze is kept fixed) as in the night-driving situation.

Is there a known name and/or explanation for this phenomenon? I have considered the possibilities of differential distributions of "red" v "blue" cone cells on the retina (with proportionally more red-sensitive cones further from the fovea, though references suggest that if anything the opposite is true, with the fovea being blue-cone poor), of chromatic aberration in the eye varying with distance from the axis of vision (which I believe exists, but is slight), and of it being an obscure consequence of the Purkinje effect (which as an erstwhile astronomer I'm familiar with, but would think should result in the opposite), but I haven't been able to find any definitive information.

(And no, I'm not driving fast enough for it to be Doppler effect :-) .) {The poster formerly known as 87.81.230.195} 90.197.66.159 (talk) 23:01, 29 November 2011 (UTC)[reply]

I've got a stupid question : Do you wear glasses? I have particularly strong lenses and I've noticed a few unusual color effects near the edges where the lenses are thickest. APL (talk) 23:53, 29 November 2011 (UTC)[reply]

Not at all a stupid question, but although I have indeed worn glasses for the last 48 years (for modest myopia and astigmatism), I'm quite sure the effect is not due to my (not very thick) lenses, as it would be quite apparent when looking at lights or extended light objects (such as a computer screen!) to the side of the visual field when in a static situation. I do experience very slight 'barrel' distortion at the periphery of vision through my glasses - that is, vertical lines | | look very slightly bowed out ( ) - but there is no perceptible colour effect, and certainly no marked reddening such as in the lights zipping by as described above. At the next opportunity I will double check by removing my glasses (momentarily!) while night driving, but I'm almost certain that the effect is innate to the eye or perception rather than an artefact of spectacle lenses. {The poster formerly known as 87.81.230.195} 90.197.66.159 (talk) 01:15, 30 November 2011 (UTC)[reply]

[Addendum: It occurs to me that this apparent effect could have influenced some scientific observations, namely of the colours of meteors moving from nearer the centre of the visual field to the edge - they would appear to progressively redden. However, although I have myself made visual observations and counts of meteor showers and have naturally also read a good deal about doing so, I don't recall any mention of it. {The poster formerly known as 87.81.230.195} 90.197.66.159 (talk) 01:25, 30 November 2011 (UTC)][reply]

Assuming you are looking through the front windshield/windscreen, it will be made of safety glass. Perhaps you are seeing a chromatic aberration due to the plastic layer sandwiched between the glass? The same effect would be visible if you were sitting in the car at the side of the road, but would disappear if you stepped out of the car. Or you could stick your head out the sunroof, but that would be less advisable. Franamax (talk) 05:52, 30 November 2011 (UTC)[reply]

Particularly as I don't have a sunroof - Ouch! I had considered this possibility briefly, but if it were the case I think I would long ago have noticed the effect when parked - I've had this particular car for over 4 years. Nevertheless, I'll make further observations with this in mind tomorrow (Thursday) night, which is the next time I expect to be driving after dark. {The poster formerly known as 87.81.230.195} 90.193.78.16 (talk) 10:21, 30 November 2011 (UTC)[reply]