APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05)

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APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05)

Post by APOD Robot » Sun Oct 05, 2014 4:06 am

Image A Total Lunar Eclipse Over Tajikistan

Explanation: If the full Moon suddenly faded, what would you see? The answer during the total lunar eclipse of 2011 June was recorded in a dramatic time lapse video from Tajikistan. During a total lunar eclipse, the Earth moves between the Moon and the Sun, causing the moon to fade dramatically. The Moon never gets completely dark, though, since the Earth's atmosphere refracts some light. As the above video begins, the scene may appear to be daytime and sunlit, but actually it is a nighttime and lit by the glow of the full Moon. As the moon becomes eclipsed and fades, the wind dies down and background stars can be seen reflected in foreground lake. Most spectacularly, the sky surrounding the eclipsed moon suddenly appears to be full of stars and highlighted by the busy plane of our Milky Way Galaxy. The sequence repeats with a closer view, and the final image shows the placement of the eclipsed Moon near the Eagle, Swan, Trifid, and Lagoon nebulas. Nearly two hours after the eclipse started, the moon emerged from the Earth's shadow and its bright full glare again dominated the sky. The next total lunar eclipse will occur this Wednesday.

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by bystander » Sun Oct 05, 2014 3:12 pm

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by Jimless Ear » Sun Oct 05, 2014 6:35 pm

Just a thought: Some of the light that passes through Earth's atmosphere gets refracted toward the moon, but a lot of what lights the moon is simply the glowing circle of atmosphere lit by the sun. This is not refracted light, it is light that has been intercepted by particles in the atmosphere and re-emitted or reflected into the shadow of the Earth.

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by gvann » Sun Oct 05, 2014 6:46 pm

I wonder if it's true that refraction is the cause of the light that makes the Moon glow red during an eclipse. Just by watching sunsets, and noting the shape of the disk of the Sun during a sunset, it is clear that the deflection angle caused by refraction in the atmosphere is a fraction of the Sun's diameter (which is about half a degree). The Earth, as seen from the Moon, has an angular diameter that is four times that of the Sun, such that the umbra has an angular diameter of about 1.5 degrees. Therefore, the deflection angle needed for refracted light to strike the Moon during a total eclipse is considerably larger than the angular diameter of the Sun. If refraction were the cause of the light that strikes the Moon, I would expect the disc of the setting Sun, as seen from the surface of the Earth, to be considerably flattened. It's true that the setting Sun is clearly not a circle, but the distortion is not dramatic, leading me to conclude that refraction by Earth's atmosphere can deflect sunlight by, at most, a fraction of a degree. Yet, the glow of a fully eclipsed Moon is very noticeable even in areas of the Moon that are quite far (more than half a degree) from the edge of the umbra.

I had always assumed that the red light is due to Rayleigh scattering in the Earth's atmosphere. Rayleigh scattering, of course, is the phenomenon that causes the sky to glow after sunset, when the disc of the Sun is fully below the horizon.

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by alter-ego » Mon Oct 06, 2014 2:15 am

gvann wrote:I wonder if it's true that refraction is the cause of the light that makes the Moon glow red during an eclipse. Just by watching sunsets, and noting the shape of the disk of the Sun during a sunset, it is clear that the deflection angle caused by refraction in the atmosphere is a fraction of the Sun's diameter (which is about half a degree). The Earth, as seen from the Moon, has an angular diameter that is four times that of the Sun, such that the umbra has an angular diameter of about 1.5 degrees. Therefore, the deflection angle needed for refracted light to strike the Moon during a total eclipse is considerably larger than the angular diameter of the Sun. If refraction were the cause of the light that strikes the Moon, I would expect the disc of the setting Sun, as seen from the surface of the Earth, to be considerably flattened. It's true that the setting Sun is clearly not a circle, but the distortion is not dramatic, leading me to conclude that refraction by Earth's atmosphere can deflect sunlight by, at most, a fraction of a degree. Yet, the glow of a fully eclipsed Moon is very noticeable even in areas of the Moon that are quite far (more than half a degree) from the edge of the umbra.

I had always assumed that the red light is due to Rayleigh scattering in the Earth's atmosphere. Rayleigh scattering, of course, is the phenomenon that causes the sky to glow after sunset, when the disc of the Sun is fully below the horizon.
As convincingly discussed in this NOAA article, The Colors of Sunset and Twilight, the vivid orange and red sunsets are due to Rayleigh scattering taking out the blues. Even If you ignore any wavelength dependent absorption (attenuating blues more than reds), it seems clear that whatever light that makes to the moon during eclipse totality must be weighted heavily towards red. The key is the light that makes it to the moon during totality isn't explained by refraction alone. Take your example, how long after sunset (or before sunrise) do you see orange or reddish skies? The sun is degrees below the horizon and the sky still shows color. This is the case on the moon too. There's enough color from the thin atmosphere layer to be visible. The most beautiful sunrises/sunsets are those where clouds reflect/scatter the sun's light to flood the land. The collective contribution from high altitude clouds may be present too. I also believe that the variations in color seen on the moon during totality have been influenced by large particle emission events like volcanic eruptions.

Not a stretch at all, really, to get a blood red moon during a deep eclipse,
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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by KiethHoyt » Mon Oct 06, 2014 8:17 pm

alter-ego wrote:
gvann wrote:I wonder if it's true that refraction is the cause of the light that makes the Moon glow red during an eclipse. Just by watching sunsets, and noting the shape of the disk of the Sun during a sunset, it is clear that the deflection angle caused by refraction in the atmosphere is a fraction of the Sun's diameter (which is about half a degree). The Earth, as seen from the Moon, has an angular diameter that is four times that of the Sun, such that the umbra has an angular diameter of about 1.5 degrees. Therefore, the deflection angle needed for refracted light to strike the Moon during a total eclipse is considerably larger than the angular diameter of the Sun. If refraction were the cause of the light that strikes the Moon, I would expect the disc of the setting Sun, as seen from the surface of the Earth, to be considerably flattened. It's true that the setting Sun is clearly not a circle, but the distortion is not dramatic, leading me to conclude that refraction by Earth's atmosphere can deflect sunlight by, at most, a fraction of a degree. Yet, the glow of a fully eclipsed Moon is very noticeable even in areas of the Moon that are quite far (more than half a degree) from the edge of the umbra.

I had always assumed that the red light is due to Rayleigh scattering in the Earth's atmosphere. Rayleigh scattering, of course, is the phenomenon that causes the sky to glow after sunset, when the disc of the Sun is fully below the horizon.
As convincingly discussed in this NOAA article, The Colors of Sunset and Twilight, the vivid orange and red sunsets are due to Rayleigh scattering taking out the blues. Even If you ignore any wavelength dependent absorption (attenuating blues more than reds), it seems clear that whatever light that makes to the moon during eclipse totality must be weighted heavily towards red. The key is the light that makes it to the moon during totality isn't explained by refraction alone. Take your example, how long after sunset (or before sunrise) do you see orange or reddish skies? The sun is degrees below the horizon and the sky still shows color. This is the case on the moon too. There's enough color from the thin atmosphere layer to be visible. The most beautiful sunrises/sunsets are those where clouds reflect/scatter the sun's light to flood the land. The collective contribution from high altitude clouds may be present too. I also believe that the variations in color seen on the moon during totality have been influenced by large particle emission events like volcanic eruptions.

Not a stretch at all, really, to get a blood red moon during a deep eclipse,
That definitely explains it as well as possible, but I want to know why when the sun is setting there is a rainbow-like effect on the opposite horizon. I just boil it down to refraction of light wavelengths. Take a look at this picture and this is what I'm talking about.

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by geckzilla » Mon Oct 06, 2014 8:29 pm

KiethHoyt wrote:That definitely explains it as well as possible, but I want to know why when the sun is setting there is a rainbow-like effect on the opposite horizon. I just boil it down to refraction of light wavelengths. Take a look at this picture and this is what I'm talking about.
As you can see from the image filename, that phenomenon is commonly known as the Belt of Venus. It is not a rainbow effect at all. The blue part is simply Earth's shadow. Above that, you see the pink band caused by the aforementioned Rayleigh scattering.
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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by Chris Peterson » Mon Oct 06, 2014 8:31 pm

KiethHoyt wrote:That definitely explains it as well as possible, but I want to know why when the sun is setting there is a rainbow-like effect on the opposite horizon. I just boil it down to refraction of light wavelengths. Take a look at this picture and this is what I'm talking about.
Nope, that's nothing to do with refraction. It's a scattering effect. The upper part of the atmosphere is still in sunlight, and you're seeing backscattered red from the sunset. The lower is in the Earth's shadow, and all you see is the scattered blue that the sky normally shows.
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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by Chris Peterson » Mon Oct 06, 2014 8:33 pm

gvann wrote:I wonder if it's true that refraction is the cause of the light that makes the Moon glow red during an eclipse. Just by watching sunsets, and noting the shape of the disk of the Sun during a sunset, it is clear that the deflection angle caused by refraction in the atmosphere is a fraction of the Sun's diameter (which is about half a degree). The Earth, as seen from the Moon, has an angular diameter that is four times that of the Sun, such that the umbra has an angular diameter of about 1.5 degrees. Therefore, the deflection angle needed for refracted light to strike the Moon during a total eclipse is considerably larger than the angular diameter of the Sun. If refraction were the cause of the light that strikes the Moon, I would expect the disc of the setting Sun, as seen from the surface of the Earth, to be considerably flattened. It's true that the setting Sun is clearly not a circle, but the distortion is not dramatic, leading me to conclude that refraction by Earth's atmosphere can deflect sunlight by, at most, a fraction of a degree. Yet, the glow of a fully eclipsed Moon is very noticeable even in areas of the Moon that are quite far (more than half a degree) from the edge of the umbra.

I had always assumed that the red light is due to Rayleigh scattering in the Earth's atmosphere. Rayleigh scattering, of course, is the phenomenon that causes the sky to glow after sunset, when the disc of the Sun is fully below the horizon.
Correct. The red of a lunar eclipse is the result of scattering. Refraction isn't a factor.

The deflection of the Sun due to refraction is about 0.5° when it's at the horizon. That is, when you're seeing the bottom of the Sun just kiss the horizon, it has actually just set completely.
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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by gvann » Fri Oct 10, 2014 3:56 pm

The deflection of the Sun due to refraction is about 0.5° when it's at the horizon. That is, when you're seeing the bottom of the Sun just kiss the horizon, it has actually just set completely
I did not know that 0.5 degree was the deflection of the Sun's image at sunset. In that case, refraction is, indeed, the main cause of the light reaching the Moon during an eclipse. If the Sun's image at sunset (as seen from the surface of the Earth) is deflected by 0.5 degrees, it means that the overall deflection observable from space (e.g., from the ISS or from the Moon) is twice that, or 1 degree. Since the size of the umbra is only 1.5 degrees, even if you are in the center of the umbra, a deflection of 0.75 degrees is sufficient.

If you are on the Moon during an eclipse and you look at the disc of the Earth, all around the edge of the disc you are seeing sunsets (or sunrises). If you are only slightly inside the umbra, the light goes through high elevations in the Earth's atmosphere (kind-of like watching a sunset on the Tibetan plateau) and is brighter and less red. If you are in the center of the umbra, the light needs to pass near the surface of the Earth to achieve the 0.75-degree deflection, and it will be fainter and redder, just like sunsets seen from the seashore. So, the redness of the light of the Moon is not due to forward Rayleigh scattering, rather, it's due to the fact that large-angle Rayleigh scattering favors shorter wavelengths (it's the reason the sky is blue) such that the light of the Sun that is not scattered and is refracted by the atmosphere is redder (like the disc of the Sun at sunset).

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Re: APOD: A Total Lunar Eclipse Over Tajikistan (2014 Oct 05

Post by Chris Peterson » Fri Oct 10, 2014 4:10 pm

gvann wrote:
The deflection of the Sun due to refraction is about 0.5° when it's at the horizon. That is, when you're seeing the bottom of the Sun just kiss the horizon, it has actually just set completely
I did not know that 0.5 degree was the deflection of the Sun's image at sunset. In that case, refraction is, indeed, the main cause of the light reaching the Moon during an eclipse.
No, it isn't. It's just light that is being scattered. The refraction angle doesn't really matter.
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