APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

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APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by APOD Robot » Wed Mar 18, 2020 4:05 am

Image Anticrepuscular Rays over Florida

Explanation: What's happening behind those clouds? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a Sun setting on the other side of the sky. Pictured here 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. Featured here is a particularly striking display of anticrepuscular rays photographed in 2016 over Dry Tortugas National Park in Florida, USA.

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don kurtz

Re: APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by don kurtz » Wed Mar 18, 2020 5:32 am

The crepuscular rays and anti-crepuscular rays are sunbeams and shadows, usually cloud shadows. The Sun is so far away that they are effectively parallel in the Euclidian sense, not great circles. Their divergence (crespuscular) and convergence (anti-crepuscular) is a parallax effect. This is the same as standing on railroad tracks and seeing the tracks diverge coming towards you, and then converge when you turn around and look at them going away. There is no need to think of them as projected onto a spherical sky into great circles.

Alex_515

Re: APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by Alex_515 » Wed Mar 18, 2020 8:44 am

Projective geometry at its best.

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orin stepanek
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Re: APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by orin stepanek » Wed Mar 18, 2020 11:09 am

AntiCrepRays_Goff_960.jpg
Very nice: Mother Nature does it again! :D
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Ann
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Re: APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by Ann » Wed Mar 18, 2020 1:20 pm

orin stepanek wrote:
Wed Mar 18, 2020 11:09 am
AntiCrepRays_Goff_960.jpg

Very nice: Mother Nature does it again! :D
She sure does!

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Chris Peterson
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Re: APOD: Anticrepuscular Rays over Florida (2020 Mar 18)

Post by Chris Peterson » Wed Mar 18, 2020 1:34 pm

don kurtz wrote:
Wed Mar 18, 2020 5:32 am
The crepuscular rays and anti-crepuscular rays are sunbeams and shadows, usually cloud shadows. The Sun is so far away that they are effectively parallel in the Euclidian sense, not great circles. Their divergence (crespuscular) and convergence (anti-crepuscular) is a parallax effect. This is the same as standing on railroad tracks and seeing the tracks diverge coming towards you, and then converge when you turn around and look at them going away. There is no need to think of them as projected onto a spherical sky into great circles.
Where I live, it's common for crepuscular rays to be created by mountains rather than clouds. This leads to the interesting phenomenon of repeating structures. That is, patterns of rays that I recognize when they recur, rather than the random patterns that clouds produce.
Chris

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