APOD: The Last Full Moon (2023 Dec 30)

[APOD Robot]

The Last Full Moon

Explanation: Known to some in the northern hemisphere as December's Cold Moon or the Long Night Moon, the last full moon of 2023 is rising in this surreal mountain and skyscape. The Daliesque scene was captured in a single exposure with a camera and long telephoto lens near Monte Grappa, Italy. The full moon is not melting, though. Its stretched and distorted appearance near the horizon is caused as refraction along the line of sight changes and creates shifting images or mirages of the bright lunar disk. The changes in atmospheric refraction correspond to atmospheric layers with sharply different temperatures and densities. Other effects of atmospheric refraction produced by the long sight-line to this full moon rising include the thin red rim seen faintly on the distorted lower edge of the Moon and a thin green rim along the top.

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[MelvzLuster]

A good last full moon is a nice transition from the year 2023 to 2024. It signifies good luck because it's the year of the Dragons in the Chinese calendar. I am on the side of the Golden Dragons.

[johnnydeep]

So why is there a green rim on the top and a red rim on the bottom? I couldn't find anything explaining that in the links. I would have thought that the mere 0.5° width of the moon could not be enough to cause the difference due to the differing lengths of atmosphere that light from the top and bottom would experience.

[Chris Peterson]

So why is there a green rim on the top and a red rim on the bottom? I couldn't find anything explaining that in the links. I would have thought that the mere 0.5° width of the moon could not be enough to cause the difference due to the differing lengths of atmosphere that light from the top and bottom would experience.

It's not about different path lengths, but about dispersion, the variation of refractive index across different wavelengths. It's what makes a prism separate different wavelengths, and the atmosphere has non-zero dispersion, like any medium light travels through. Light refracts as it passes through the atmosphere, changing the path slightly. And the path is different for short wavelengths and long wavelengths. The longer the path, the further the paths separate and the more chromatic fringing we see. So the closer to the horizon, the worse it is. This effect is why we see the Green Flash at sunset.

It can be corrected to a degree in some images, by slightly shifting the red channel up and the blue channel down. There are also prism-like adapters for telescopes that can compensate partly for atmospheric dispersion before the light hits the eyepiece or camera.

[johnnydeep]

So why is there a green rim on the top and a red rim on the bottom? I couldn't find anything explaining that in the links. I would have thought that the mere 0.5° width of the moon could not be enough to cause the difference due to the differing lengths of atmosphere that light from the top and bottom would experience.

It's not about different path lengths, but about dispersion, the variation of refractive index across different wavelengths. It's what makes a prism separate different wavelengths, and the atmosphere has non-zero dispersion, like any medium light travels through. Light refracts as it passes through the atmosphere, changing the path slightly. And the path is different for short wavelengths and long wavelengths. The longer the path, the further the paths separate and the more chromatic fringing we see. So the closer to the horizon, the worse it is. This effect is why we see the Green Flash at sunset.

It can be corrected to a degree in some images, by slightly shifting the red channel up and the blue channel down. There are also prism-like adapters for telescopes that can compensate partly for atmospheric dispersion before the light hits the eyepiece or camera.

Well, a prism causes diffraction for different wavelengths of light because of the length of glass that the light travels through. It seems like you're saying that the different lengths of atmosphere that the light travels through at the top and bottom of the moon does the same thing through dispersion rather than diffraction, right? Although I'm unclear about the difference between diffraction and dispersion.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).

---

[That proves it]

Proof at last that the "Moon" is really a giant balloon launched every night from a secret mountain base.

[Chris Peterson]

So why is there a green rim on the top and a red rim on the bottom? I couldn't find anything explaining that in the links. I would have thought that the mere 0.5° width of the moon could not be enough to cause the difference due to the differing lengths of atmosphere that light from the top and bottom would experience.

It's not about different path lengths, but about dispersion, the variation of refractive index across different wavelengths. It's what makes a prism separate different wavelengths, and the atmosphere has non-zero dispersion, like any medium light travels through. Light refracts as it passes through the atmosphere, changing the path slightly. And the path is different for short wavelengths and long wavelengths. The longer the path, the further the paths separate and the more chromatic fringing we see. So the closer to the horizon, the worse it is. This effect is why we see the Green Flash at sunset.

It can be corrected to a degree in some images, by slightly shifting the red channel up and the blue channel down. There are also prism-like adapters for telescopes that can compensate partly for atmospheric dispersion before the light hits the eyepiece or camera.

Well, a prism causes diffraction for different wavelengths of light because of the length of glass that the light travels through. It seems like you're saying that the different lengths of atmosphere that the light travels through at the top and bottom of the moon does the same thing through dispersion rather than diffraction, right? Although I'm unclear about the difference between diffraction and dispersion.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).

---

Refraction is the bending of a light ray when it passes from a medium with one index of refraction (speed of light in that medium) to one with another. Light rays coming to us from outside the atmosphere always bend except when they come directly from the zenith. The closer the source is to the horizon, the greater the bend angle. (It's actually more of an arc, because the index of refraction of air changes as you get closer to the ground). When we see the Sun set, the actual Sun is already well below the horizon. We would have seen it set a few minutes earlier if we had no atmosphere to bend its rays.

Dispersion (which is quantified with a value called the Abbe number) measures the variation of the index of refraction as a function of wavelength. A prism does not work because different wavelengths pass through different amounts of glass. It works because red light bends (refracts) less when it crosses the air/glass boundary than violet light does. Dispersion is what causes rainbows, because different wavelengths are bent through different angles when they pass into and out of water droplets.

Diffraction is an interference phenomenon typically associated with how light interacts with apertures and edges, and doesn't have anything to do with the current discussion.

[johnnydeep]

It's not about different path lengths, but about dispersion, the variation of refractive index across different wavelengths. It's what makes a prism separate different wavelengths, and the atmosphere has non-zero dispersion, like any medium light travels through. Light refracts as it passes through the atmosphere, changing the path slightly. And the path is different for short wavelengths and long wavelengths. The longer the path, the further the paths separate and the more chromatic fringing we see. So the closer to the horizon, the worse it is. This effect is why we see the Green Flash at sunset.

It can be corrected to a degree in some images, by slightly shifting the red channel up and the blue channel down. There are also prism-like adapters for telescopes that can compensate partly for atmospheric dispersion before the light hits the eyepiece or camera.

Well, a prism causes diffraction for different wavelengths of light because of the length of glass that the light travels through. It seems like you're saying that the different lengths of atmosphere that the light travels through at the top and bottom of the moon does the same thing through dispersion rather than diffraction, right? Although I'm unclear about the difference between diffraction and dispersion.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).

---

Refraction is the bending of a light ray when it passes from a medium with one index of refraction (speed of light in that medium) to one with another. Light rays coming to us from outside the atmosphere always bend except when they come directly from the zenith. The closer the source is to the horizon, the greater the bend angle. (It's actually more of an arc, because the index of refraction of air changes as you get closer to the ground). When we see the Sun set, the actual Sun is already well below the horizon. We would have seen it set a few minutes earlier if we had no atmosphere to bend its rays.

Dispersion (which is quantified with a value called the Abbe number) measures the variation of the index of refraction as a function of wavelength. A prism does not work because different wavelengths pass through different amounts of glass. It works because red light bends (refracts) less when it crosses the air/glass boundary than violet light does. Dispersion is what causes rainbows, because different wavelengths are bent through different angles when they pass into and out of water droplets.

Diffraction is an interference phenomenon typically associated with how light interacts with apertures and edges, and doesn't have anything to do with the current discussion.

Thanks. But darn, so many new questions...which I won't bother you with...at least not for a while. But here's a start: so then why is a prism triangular? Could it instead be just a tilted flat piece of glass? Does it even have to be tilted?

[Chris Peterson]

Well, a prism causes diffraction for different wavelengths of light because of the length of glass that the light travels through. It seems like you're saying that the different lengths of atmosphere that the light travels through at the top and bottom of the moon does the same thing through dispersion rather than diffraction, right? Although I'm unclear about the difference between diffraction and dispersion.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).

---

Refraction is the bending of a light ray when it passes from a medium with one index of refraction (speed of light in that medium) to one with another. Light rays coming to us from outside the atmosphere always bend except when they come directly from the zenith. The closer the source is to the horizon, the greater the bend angle. (It's actually more of an arc, because the index of refraction of air changes as you get closer to the ground). When we see the Sun set, the actual Sun is already well below the horizon. We would have seen it set a few minutes earlier if we had no atmosphere to bend its rays.

Dispersion (which is quantified with a value called the Abbe number) measures the variation of the index of refraction as a function of wavelength. A prism does not work because different wavelengths pass through different amounts of glass. It works because red light bends (refracts) less when it crosses the air/glass boundary than violet light does. Dispersion is what causes rainbows, because different wavelengths are bent through different angles when they pass into and out of water droplets.

Diffraction is an interference phenomenon typically associated with how light interacts with apertures and edges, and doesn't have anything to do with the current discussion.

Thanks. But darn, so many new questions...which I won't bother you with...at least not for a while. But here's a start: so then why is a prism triangular? Could it instead be just a tilted flat piece of glass? Does it even have to be tilted?

When light passes from air to glass an angle, it bends towards the normal; when it passes from glass to air, it bends away from the normal (refraction). Shorter wavelengths bend more (dispersion). If you shine light at an angle into a flat piece of glass, the same thing happens, but when the light exits, it all bends back to be parallel (but slightly offset) to the original rays. So the spreading stops, and the resulting spectrum is probably not separated enough to be visible (but you may see fringing around the exit beam similar to that around the Moon in this APOD). You don't need a triangle as such, but you want your two faces to not be parallel. That way the different wavelengths exit at different angles, and the farther away you get, the more separated they are.

Light that enters parallel to the normal doesn't refract, so there is no bending and no dispersion. (Well, technically there is dispersion, because the speed of light is different for different wavelengths... so there is a change in the phase of different wavelength components passing through a planar optic. That property is used in a lot of optical configurations.)

[johnnydeep]

Refraction is the bending of a light ray when it passes from a medium with one index of refraction (speed of light in that medium) to one with another. Light rays coming to us from outside the atmosphere always bend except when they come directly from the zenith. The closer the source is to the horizon, the greater the bend angle. (It's actually more of an arc, because the index of refraction of air changes as you get closer to the ground). When we see the Sun set, the actual Sun is already well below the horizon. We would have seen it set a few minutes earlier if we had no atmosphere to bend its rays.

Dispersion (which is quantified with a value called the Abbe number) measures the variation of the index of refraction as a function of wavelength. A prism does not work because different wavelengths pass through different amounts of glass. It works because red light bends (refracts) less when it crosses the air/glass boundary than violet light does. Dispersion is what causes rainbows, because different wavelengths are bent through different angles when they pass into and out of water droplets.

Diffraction is an interference phenomenon typically associated with how light interacts with apertures and edges, and doesn't have anything to do with the current discussion.

Thanks. But darn, so many new questions...which I won't bother you with...at least not for a while. But here's a start: so then why is a prism triangular? Could it instead be just a tilted flat piece of glass? Does it even have to be tilted?

When light passes from air to glass an angle, it bends towards the normal; when it passes from glass to air, it bends away from the normal (refraction). Shorter wavelengths bend more (dispersion). If you shine light at an angle into a flat piece of glass, the same thing happens, but when the light exits, it all bends back to be parallel (but slightly offset) to the original rays. So the spreading stops, and the resulting spectrum is probably not separated enough to be visible (but you may see fringing around the exit beam similar to that around the Moon in this APOD). You don't need a triangle as such, but you want your two faces to not be parallel. That way the different wavelengths exit at different angles, and the farther away you get, the more separated they are.

Light that enters parallel to the normal doesn't refract, so there is no bending and no dispersion. (Well, technically there is dispersion, because the speed of light is different for different wavelengths... so there is a change in the phase of different wavelength components passing through a planar optic. That property is used in a lot of optical configurations.)

Thanks again! In summary: light traversing a triangular prism is subjected to refraction (bending) and when it exits, the different wavelengths are dispersed (widened and increasingly so).

Is there a use for a thick cube of glass that causes dispersion at the first surface, but stops it at the final surface (as you've described), but nevertheless resulting in a spreading of all the frequencies? Perhaps not...