APOD: Planet Earth at Twilight (2022 Apr 22)

[APOD Robot]

Planet Earth at Twilight

Explanation: No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture was taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles. Of course from home, you can check out the Earth Now.

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[orin stepanek]

https://www.youtube.com/watch?v=M26zWFUPGpE
Lovely view of Earth!

[vdix]

You are using nautical miles to measure altitude?... Anyway, cool image and thanks for the 'Earth Now' link.

[De58te]

You are using nautical miles to measure altitude?... Anyway, cool image and thanks for the 'Earth Now' link.

NASA is using nautical miles not necessarily for altitude but they use it for charting longitude and latitude on a globe. It just so happens that a nautical mile is 1/60th of a minute of arc or degree. And there happens to be 360 degrees around a globe. (Don't quote me on this, I am speaking from memory.) Anyways it is really convenient with measurements on the globe. So it would avoid confusion to use nautical miles for everything rather than switching to statute miles for altitude.

[johnnydeep]

Appropriate for Earth Day. But to me, it almost looks like a close-up of the atmosphere of Jupiter (except for the blue limb)! Also, I've "known" that "the sky is blue" because blue wavelengths are scattered more than others. But thinking more about it now, and reading the scatters blue link I'm confused about what's really happening. From the link:

We see the sky as colored because our atmosphere interacts with the sunlight passing through it. This phenomenon is called "scattering." The type of scattering responsible for blue sky is known as Rayleigh scattering. Because this effect becomes sharply more pronounced as the energy of light increases, wavelengths at the blue end of the spectrum, where energy is the highest, are scattered preferentially. The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue.

I would think that more scattering would cause more of the blue light to not make it all the way through the atmosphere to our eyes! And if, say, red light is scattered less, wouldn't we see more of it?

[Chris Peterson]

Appropriate for Earth Day. But to me, it almost looks like a close-up of the atmosphere of Jupiter (except for the blue limb)! Also, I've "known" that "the sky is blue" because blue wavelengths are scattered more than others. But thinking more about it now, and reading the scatters blue link I'm confused about what's really happening. From the link:

We see the sky as colored because our atmosphere interacts with the sunlight passing through it. This phenomenon is called "scattering." The type of scattering responsible for blue sky is known as Rayleigh scattering. Because this effect becomes sharply more pronounced as the energy of light increases, wavelengths at the blue end of the spectrum, where energy is the highest, are scattered preferentially. The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue.

I would think that more scattering would cause more of the blue light to not make it all the way through the atmosphere to our eyes! And if, say, red light is scattered less, wouldn't we see more of it?

If there were no atmosphere we'd see a white Sun and a black sky. Because of Rayleigh scattering we see a blue sky (because the short wavelengths are scattered out of the direct path of the Sun's light, and we see a yellow Sun, because some of the blue has been removed. If the light is traveling through a lot of atmosphere, as near dawn and dusk, even more of the shorter wavelengths are scattered out and we see an orange or red Sun.

[johnnydeep]

Appropriate for Earth Day. But to me, it almost looks like a close-up of the atmosphere of Jupiter (except for the blue limb)! Also, I've "known" that "the sky is blue" because blue wavelengths are scattered more than others. But thinking more about it now, and reading the scatters blue link I'm confused about what's really happening. From the link:

We see the sky as colored because our atmosphere interacts with the sunlight passing through it. This phenomenon is called "scattering." The type of scattering responsible for blue sky is known as Rayleigh scattering. Because this effect becomes sharply more pronounced as the energy of light increases, wavelengths at the blue end of the spectrum, where energy is the highest, are scattered preferentially. The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue.

I would think that more scattering would cause more of the blue light to not make it all the way through the atmosphere to our eyes! And if, say, red light is scattered less, wouldn't we see more of it?

If there were no atmosphere we'd see a white Sun and a black sky. Because of Rayleigh scattering we see a blue sky (because the short wavelengths are scattered out of the direct path of the Sun's light, and we see a yellow Sun, because some of the blue has been removed. If the light is traveling through a lot of atmosphere, as near dawn and dusk, even more of the shorter wavelengths are scattered out and we see an orange or red Sun.

Again, though, why are we able to see the scattered light at all? Why doesn't "scattering" imply it's being bounced around more and so we should see less of it?

[Chris Peterson]

Appropriate for Earth Day. But to me, it almost looks like a close-up of the atmosphere of Jupiter (except for the blue limb)! Also, I've "known" that "the sky is blue" because blue wavelengths are scattered more than others. But thinking more about it now, and reading the scatters blue link I'm confused about what's really happening. From the link:



I would think that more scattering would cause more of the blue light to not make it all the way through the atmosphere to our eyes! And if, say, red light is scattered less, wouldn't we see more of it?

If there were no atmosphere we'd see a white Sun and a black sky. Because of Rayleigh scattering we see a blue sky (because the short wavelengths are scattered out of the direct path of the Sun's light, and we see a yellow Sun, because some of the blue has been removed. If the light is traveling through a lot of atmosphere, as near dawn and dusk, even more of the shorter wavelengths are scattered out and we see an orange or red Sun.

Again, though, why are we able to see the scattered light at all? Why doesn't "scattering" imply it's being bounced around more and so we should see less of it?

What would you expect the sky to look like if no light were scattered? Scattered light is ALL we can see! The only direct light is looking straight at the Sun. We see scattered light because as the shorter wavelengths bounce around off of air molecules, some of those photons get diverted in our direction and make it to our eyes.

[johnnydeep]

If there were no atmosphere we'd see a white Sun and a black sky. Because of Rayleigh scattering we see a blue sky (because the short wavelengths are scattered out of the direct path of the Sun's light, and we see a yellow Sun, because some of the blue has been removed. If the light is traveling through a lot of atmosphere, as near dawn and dusk, even more of the shorter wavelengths are scattered out and we see an orange or red Sun.

Again, though, why are we able to see the scattered light at all? Why doesn't "scattering" imply it's being bounced around more and so we should see less of it?

What would you expect the sky to look like if no light were scattered? Scattered light is ALL we can see! The only direct light is looking straight at the Sun. We see scattered light because as the shorter wavelengths bounce around off of air molecules, some of those photons get diverted in our direction and make it to our eyes.

Still having trouble with this, but...

Hmm, so the light from the Sun that makes it to us "more directly"(*) is responsible for the color of the Sun we perceive, but the scattered light is responsible for the color of stuff (atmosphere) that's causing the scattering?

(*) - I say "more directly" because I would think that most visible photons from the Sun must get scattered somewhat on their way through the atmosphere simply from probabilistic standpoint.

[Chris Peterson]

Again, though, why are we able to see the scattered light at all? Why doesn't "scattering" imply it's being bounced around more and so we should see less of it?

What would you expect the sky to look like if no light were scattered? Scattered light is ALL we can see! The only direct light is looking straight at the Sun. We see scattered light because as the shorter wavelengths bounce around off of air molecules, some of those photons get diverted in our direction and make it to our eyes.

Still having trouble with this, but...

Hmm, so the light from the Sun that makes it to us "more directly"(*) is responsible for the color of the Sun we perceive, but the scattered light is responsible for the color of stuff (atmosphere) that's causing the scattering?

(*) - I say "more directly" because I would think that most visible photons from the Sun must get scattered somewhat on their way through the atmosphere simply from probabilistic standpoint.

Yes, the scattering affects all wavelengths, it's just that the shorter visible wavelengths are more likely to scatter. If you look at the sky spectroscopically, it's a continuum, just weighted towards the shorter wavelengths. Which we see as blue. (Larger particles scatter by a different mechanism, Mie scattering, which isn't very wavelength dependent, which is why we see clouds and fog and overcast as white.)

If you consider the path of a photon, those that come on a direct path from the Sun will have more short wavelength ones knocked out of the path, so we see the Sun appear a bit yellowish. Those that come by any other path will have been scattered one or more times, with each scattering event selectively favoring shorter wavelengths. So we'll see more short wavelength photons than long.

[johnnydeep]

What would you expect the sky to look like if no light were scattered? Scattered light is ALL we can see! The only direct light is looking straight at the Sun. We see scattered light because as the shorter wavelengths bounce around off of air molecules, some of those photons get diverted in our direction and make it to our eyes.

Still having trouble with this, but...

Hmm, so the light from the Sun that makes it to us "more directly"(*) is responsible for the color of the Sun we perceive, but the scattered light is responsible for the color of stuff (atmosphere) that's causing the scattering?

(*) - I say "more directly" because I would think that most visible photons from the Sun must get scattered somewhat on their way through the atmosphere simply from probabilistic standpoint.

Yes, the scattering affects all wavelengths, it's just that the shorter visible wavelengths are more likely to scatter. If you look at the sky spectroscopically, it's a continuum, just weighted towards the shorter wavelengths. Which we see as blue. (Larger particles scatter by a different mechanism, Mie scattering, which isn't very wavelength dependent, which is why we see clouds and fog and overcast as white.)

If you consider the path of a photon, those that come on a direct path from the Sun will have more short wavelength ones knocked out of the path, so we see the Sun appear a bit yellowish. Those that come by any other path will have been scattered one or more times, with each scattering event selectively favoring shorter wavelengths. So we'll see more short wavelength photons than long.

Thanks! I think I finally got it.