APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

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APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by APOD Robot » Tue Feb 21, 2012 5:07 am

Image Anticrepuscular Rays Over Wyoming

Explanation: What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Pictured above is a particularly striking set of anticrepuscular rays photographed last month near Cheyenne, Wyoming, USA.

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Beyond » Tue Feb 21, 2012 5:26 am

WHOA! APOD should stay away from the CIA! :ninja: They may be the cause of why there is no video in the last Anticrepuscular link. The account for it has been terminated.
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Dan » Tue Feb 21, 2012 12:27 pm

Just to clarify, this photo was taken at sunset facing east?

physicsflyer

Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by physicsflyer » Tue Feb 21, 2012 2:47 pm

Please note that the photographer stopped driving and pulled to the side of the road to take this shot!

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by alphachapmtl » Tue Feb 21, 2012 3:24 pm

physicsflyer wrote:Please note that the photographer stopped driving and pulled to the side of the road to take this shot!
Maybe he was driving off the road while being busy taking the picture!

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Chris Peterson » Tue Feb 21, 2012 3:42 pm

Dan wrote:Just to clarify, this photo was taken at sunset facing east?
Taken at sunset, facing ENE. The antisolar point at that location and time was azimuth 61°.
Chris

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by RADDAD » Tue Feb 21, 2012 4:00 pm

I am sorry, but the APOD explanation is erroneous. It has nothing to do with Great Circles. Great Circles are curves around a sphere - these rays are parallel straight lines. I found a better explanation a few years ago: Parallel straight lines appear to converge looking in either direction. If you are standing on a long straight section of railroad tracks, the tracks will appear to converge when you look in either direction. The thing that throws us off about rays from the Sun is that we always associate them with the Sun and thus we see them converging toward the Sun. Because of the Sun's great distance, it appears as a point of light rather than the huge disk that it really is, hence the straight parallel rays seem to be converging at the Sun. We forget that if we could see them going away from the Sun, they would naturally appear to converge in that direction also.

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Chris Peterson » Tue Feb 21, 2012 4:42 pm

RADDAD wrote:I am sorry, but the APOD explanation is erroneous. It has nothing to do with Great Circles. Great Circles are curves around a sphere - these rays are parallel straight lines.
By constructing a plane that intersects the ray and the center of the Earth, a great circle is projected on the Earth. But I agree with you that the concept of a great circle is really of no use in understanding crepuscular rays.
I found a better explanation a few years ago: Parallel straight lines appear to converge looking in either direction. If you are standing on a long straight section of railroad tracks, the tracks will appear to converge when you look in either direction.
I think today's caption basically says that... but by conflating it with great circles, it makes the explanation rather confusing.
We forget that if we could see them going away from the Sun, they would naturally appear to converge in that direction also.
I don't think we forget that at all! That's exactly what today's image is showing. We can see the rays going away from the Sun, and we do see them converging.
Chris

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Coyne

Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Coyne » Tue Feb 21, 2012 6:38 pm

Are those *really* anticrepuscular rays? They look a lot like chem trails to me. :wink:

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by drollere » Tue Feb 21, 2012 6:47 pm

i agree the explanation is correct but confusing ... and the link from "straight lines" to "light cones" even moreso.

the illusion of converging rays applies specifically to the perspective projection of parallel lines onto a plane perpendicular to the line of sight. the spherical projection explains why the plane projection works no matter which direction the line of sight is directed.

http://www.handprint.com/HP/WCL/perspect1.html#rule7

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by gdreiber » Tue Feb 21, 2012 7:25 pm

Doesn't light travel on the great curve of gravity in fabric of space/time?
Is light not bend by the "lens" of the earth atmosphere?
Why nit pick by half? :roll:
Do we have a convergence of opinion?
I enjoy the picture, the descriptions and the discussions.

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by orin stepanek » Tue Feb 21, 2012 7:49 pm

I like the photo with the light rays the windmills and the highway! pretty much the way I remember Wyoming! Nice picture Nate! 8-)
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by neufer » Tue Feb 21, 2012 8:08 pm

drollere wrote:
Chris Peterson wrote:
RADDAD wrote:
I am sorry, but the APOD explanation is erroneous. It has nothing to do with Great Circles. Great Circles are curves around a sphere - these rays are parallel straight lines.
By constructing a plane that intersects the ray and the center of the Earth, a great circle is projected on the Earth. But I agree with you that the concept of a great circle is really of no use in understanding crepuscular rays.
i agree the explanation is correct but confusing ... and the link from "straight lines" to "light cones" even moreso.

the illusion of converging rays applies specifically to the perspective projection of parallel lines onto a plane perpendicular to the line of sight. the spherical projection explains why the plane projection works no matter which direction the line of sight is directed.

http://www.handprint.com/HP/WCL/perspect1.html#rule7
The Great Circles referenced are projections of straight rays onto the celestial sphere but the links do confuse the issue.
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The "Bamboo Forest" Model

Post by BobStein-VisiBone » Wed Feb 22, 2012 1:22 am

I like to explain crepuscular (and anticrepuscular) rays by imagining a bamboo forest.

Imagine you have climbed halfway up a bamboo stalk in a tall tall bamboo forest. Stalks above you, stalks below you. You would see a vanishing point above you, where all the stalks seem to point to your zenith. And you'd see a vanishing point below, where all the stalks seem to point to your nadir.

The sun corresponds to the zenith, the anti-solar point to the nadir. Sunbeams are like bamboo poles, very nearly parallel to an Earthling, as RADDAD very properly explains.

viking

Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by viking » Wed Feb 22, 2012 1:53 am

An easier way of explaining "anticrepuscular" rays is to simply state that they emanate upward from where the sun is, and tha crepuscular rays (often called "the Sun drawing water") seem to rise up towards the sun. Bowditch is a good reference.

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by TNT » Wed Feb 22, 2012 2:41 am

I've never heard of that. "The Sun drawing water"-makes no sense to me at all.
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by alter-ego » Wed Feb 22, 2012 4:54 am

TNT wrote:I've never heard of that. "The Sun drawing water"-makes no sense to me at all.
It is an old phrase. The last time I heard was probably my mother.
Wiki wrote: Sun drawing water - from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)
(same link as above)
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by Chris Peterson » Wed Feb 22, 2012 5:28 am

viking wrote:An easier way of explaining "anticrepuscular" rays is to simply state that they emanate upward from where the sun is, and tha crepuscular rays (often called "the Sun drawing water") seem to rise up towards the sun. Bowditch is a good reference.
Crepuscular rays normally rise upwards and away from the Sun. Anticrepuscular rays normally converge downwards on the horizon opposite the Sun. They are the same thing... sometimes you can see them all the way from the Sun to the opposite horizon. There name just changes when they cross the meridian.
Chris

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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by neufer » Wed Feb 22, 2012 6:04 pm

Chris Peterson wrote:
Crepuscular rays normally rise upwards and away from the Sun. Anticrepuscular rays normally converge downwards on the horizon opposite the Sun. They are the same thing... sometimes you can see them all the way from the Sun to the opposite horizon. There name just changes when they cross the meridian.
Interaction with the meridian causes the photons to transform into antiphotons.
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by bystander » Wed Feb 22, 2012 6:43 pm

neufer wrote:Interaction with the meridian causes the photons to transform into antiphotons.
Wouldn't that cause annihilation along the meridian?
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Re: APOD: Anticrepuscular Rays Over Wyoming (2012 Feb 21)

Post by BobStein-VisiBone » Wed Feb 22, 2012 8:05 pm

bystander wrote:
neufer wrote:Interaction with the meridian causes the photons to transform into antiphotons.
Wouldn't that cause annihilation along the meridian?
In fact, yes, causing energy to erupt in the form of crepustular rays.