Starship Asterisk*

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.

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

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.

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.

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.

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,

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.