APOD: Video of a Green Flash (2021 Nov 10)

[APOD Robot]

Video of a Green Flash

Explanation: Many think it is just a myth. Others think it is true but its cause isn't known. Adventurers pride themselves on having seen it. It's a green flash from the Sun. The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green. The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A green flash is also visible for a rising Sun, but takes better timing to spot. A dramatic green flash was caught on video last month as the Sun set beyond the Ligurian Sea from Tuscany, Italy. The second sequence in the featured video shows the green flash in real time, while the first is sped up and the last is in slow motion. The Sun itself does not turn partly green -- the effect is caused by layers of the Earth's atmosphere acting like a prism.

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[alter-ego]

I don't know if post-processing was done and if it affected color over a wide spectrum, but sure appears like there's some violet tints visible, and increasing, around the perimeter as the end of the green flash approaches. It's weak enough that maybe there is actually a subtle violet flash occurring?

[migueloz]

In Thule Greenland at the end of the 24 hour sunlight time of summer the sun grazes the northern horizon horizontally at midnight and rises. Then you can change the visual position and therefore the colour of the sun by walking up and down a north facing hillside. Descending, the last bit of sun changes from white to green and blue before disappearing. Walking up the hillside reverses the sequence.

[Ann]

I don't know if post-processing was done and if it affected color over a wide spectrum, but sure appears like there's some violet tints visible, and increasing, around the perimeter as the end of the green flash approaches. It's weak enough that maybe there is actually a subtle violet flash occurring?

There was definitely some blue at the end of that green flash, and some violet, too.

Ann

[rj rl]

In Thule Greenland at the end of the 24 hour sunlight time of summer the sun grazes the northern horizon horizontally at midnight and rises. Then you can change the visual position and therefore the colour of the sun by walking up and down a north facing hillside. Descending, the last bit of sun changes from white to green and blue before disappearing. Walking up the hillside reverses the sequence.

now that's something I'd love to experience

[madtom1999]

I wouldnt call that a "Green Flash" OK there was a green 'prominence' above the sun but a green flash lights up the sky (or somehow floods your eyeballs). When you see one you will be rocked by it. The one I saw was associated with a green 'prominence' detaching from the main solar image but the light intensity shoots up but seems to come from the sky, not the actual solar/prominence area and last milliseconds at most.

[orin stepanek]

Neat; I never seen one except in photos!

Kinda looks like a green cloud!

Pretty kitty; never saw a cat hold it's ears back like that

[Chris Peterson]

I wouldnt call that a "Green Flash" OK there was a green 'prominence' above the sun but a green flash lights up the sky (or somehow floods your eyeballs). When you see one you will be rocked by it. The one I saw was associated with a green 'prominence' detaching from the main solar image but the light intensity shoots up but seems to come from the sky, not the actual solar/prominence area and last milliseconds at most.

Whatever you're describing is not what we call a "green flash". Or at least, is not typical of an ordinary green flash, which is precisely what this video shows.

[Sa Ji Tario]

I don't know if post-processing was done and if it affected color over a wide spectrum, but sure appears like there's some violet tints visible, and increasing, around the perimeter as the end of the green flash approaches. It's weak enough that maybe there is actually a subtle violet flash occurring?

The atmosphere is acting as a prism but it is not still, so other colors appear here and there.
In addition, the climate must be in accordance with the angle of the Sun so that only the green color is filtered

[Chris Peterson]

I don't know if post-processing was done and if it affected color over a wide spectrum, but sure appears like there's some violet tints visible, and increasing, around the perimeter as the end of the green flash approaches. It's weak enough that maybe there is actually a subtle violet flash occurring?

The atmosphere is acting as a prism but it is not still, so other colors appear here and there.
In addition, the climate must be in accordance with the angle of the Sun so that only the green color is filtered

If the green is dispersed, so are the shorter wavelengths. They usually just aren't visible because the sky background is too bright and our eyes have low sensitivity to them.

[neufer]

I don't know if post-processing was done and if it affected color over a wide spectrum, but sure appears like there's some violet tints visible, and increasing, around the perimeter as the end of the green flash approaches. It's weak enough that maybe there is actually a subtle violet flash occurring?

The atmosphere is acting as a prism but it is not still, so other colors appear here and there.
In addition, the climate must be in accordance with the angle of the Sun so that only the green color is filtered

If the green is dispersed, so are the shorter wavelengths. They usually just aren't visible because the sky background is too bright and our eyes have low sensitivity to them.

• Blue, Indigo & Violet visible at the very end of the video are not clearly separated from Green.
  
  However, Yellow & Orange are blocked by stratospheric ozone absorption:

Chappuis ozone band absorption responsible for yellow/orange gap below Green Flash

---

Click to play embedded YouTube video.

How did Ozzie Measure Ozone in the Air Outside His Office?
By Forrest M. Mims III

<<As Ozzie watched the traffic light cycle though its colors of green, yellow and red, his boss's [challenge to measure ozone in the air outside his office window] kept cycling through his mind. Suddenly he shouted, "Yes!" and immediately searched online for more information about the absorption of visible light by ozone.

Ozzie soon learned that James Chappuis had long ago discovered the ozone absorption feature named for him. The Chappuis ozone absorption band extends from the blue to the red region of the visual spectrum with twin peaks in the yellow at around 575 nm and the orange at around 603 nm, as shown in this chart:

This chart is a simplified version of measurements of the Chappuis band by many separate teams. References to all of them listed at here

Ozzie learned that the absorption by ozone in the visible spectrum is much less than in the UV, but with care, it is definitely measurable. He compared this new information with the colors emitted by the traffic light and found that the yellow light was closest to the peak absorption of the Chappuis band. Ozzie then designed a simple photometer that used a silicon solar cell as a photodiode...>>

[alter-ego]

The atmosphere is acting as a prism but it is not still, so other colors appear here and there.
In addition, the climate must be in accordance with the angle of the Sun so that only the green color is filtered

If the green is dispersed, so are the shorter wavelengths. They usually just aren't visible because the sky background is too bright and our eyes have low sensitivity to them.

• Blue, Indigo & Violet visible at the very end of the video are not clearly separated from Green.
  ...

I disagree with you on that. I agree with Chris that the violet/indigo colors are dispersed, and although weak and don't constitute a true flash visible to the eye, the shorter wavelengths separate enough and in the right direction (above the sun) twice during the video (0:17 and 0:29). I expect the clarity and degree of color separation would vary depending on the details atmosphere layering, and that sharply defined colors are less common. I think the violet seen could be primarily dispersed, but the "flash" fizzled because the conditions weren't good enough. This video suggests to me that violet flashes a indeed rare.

[neufer]

If the green is dispersed, so are the shorter wavelengths. They usually just aren't visible because the sky background is too bright and our eyes have low sensitivity to them.

• Blue, Indigo & Violet visible at the very end of the video are not clearly separated from Green.
  ...

I disagree with you on that. I agree with Chris that the violet/indigo colors are dispersed, and although weak and don't constitute a true flash visible to the eye, the shorter wavelengths separate enough and in the right direction (above the sun) twice during the video (0:17 and 0:29). I expect the clarity and degree of color separation would vary depending on the details atmosphere layering, and that sharply defined colors are less common. I think the violet seen could be primarily dispersed, but the "flash" fizzled because the conditions weren't good enough. This video suggests to me that violet flashes a indeed rare.

The green "flash" stands out primarily due to:

• 1) our eyes' sensitivity to green light and
  2) stratospheric yellow/orange absorption of the Chappuis ozone absorption band.