by Chris Peterson » Fri Jan 24, 2020 5:55 pm
MarkBour wrote: ↑Fri Jan 24, 2020 5:03 pm
This is an amazing combined sequence of images.
I'll confess that (as a person who has not much studied the aspects of a lunar eclipse) I had the following theory in my head:
- When the Earth's umbra first strikes the Moon, because of the stark contrast, the human eye, or human psyche, sees it as black on white (or gray or yellow, depending on the color we're seeing for the full Moon at that point.)
- Then, I had many times seen that as it all gets dark, the Moon takes on a reddish hue (or sometimes orange). I assumed this was always the actual hue of the darkened Moon (and had learned that the reddish light is scattered light from earth's atmosphere, bathing the Moon in a very dim shade based on the light that best passes through our skies to reach the eclipsed Moon.
So, why would the edge of the umbra appear black in this sequence? And I never suspected there was any place in the shadow that would have a blue hue!
So, what is this sequence teaching me in today's APOD ?
- Clearly, as the caption pointed out, the edge of the umbral shadow actually has a faint blue hue, and they have an explanation I'm happy to accept -- that Earth's stratosphere has a realm that actually transmits blue and scatters red, as opposed to the lower, main part of our atmosphere. I wonder if it is quite consistently there, or if it is a variable phenomenon in lunar eclipses, depending on Earth's atmospheric conditions.
- But I'm not at all sure about the seemingly black edge of the umbra in these images. Is the outer edge of the umbra actually blacker than the rest, or is it that those images had to be adjusted to much lower contrast due to the overpowering moonlight from the uneclipsed part of the Moon in those images? In other words, does that edge band appear black in the photos for the same reason I thought it appeared black to the human eye -- because of the need to lower the exposure in those images?
I believe that a photometrically consistent image would have as its lightest pixels newly exposed material on the uneclipsed Moon, and as its darkest pixels the center of maria in the fully eclipsed Moon. So yes, I think the processing of the shadow edge represents a non-linearity. If you follow a single pixel near the limb it goes from bright to dim and then its brightness increases again towards totality. That doesn't seem realistic. That said, while photometrically inaccurate, it perhaps gives a better representation of the visual experience than a more accurate processing flow would achieve.
[quote=MarkBour post_id=299013 time=1579885382 user_id=141361]
This is an amazing combined sequence of images.
I'll confess that (as a person who has not much studied the aspects of a lunar eclipse) I had the following theory in my head:
[list][*]When the Earth's umbra first strikes the Moon, because of the stark contrast, the human eye, or human psyche, [i][b]sees[/b][/i] it as [b]black[/b] on white (or gray or yellow, depending on the color we're seeing for the full Moon at that point.)
[*]Then, I had many times seen that as it all gets dark, the Moon takes on a reddish hue (or sometimes orange). I assumed this was always the [i]actual[/i] hue of the darkened Moon (and had learned that the reddish light is scattered light from earth's atmosphere, bathing the Moon in a very dim shade based on the light that best passes through our skies to reach the eclipsed Moon.[/list]
So, why would the edge of the umbra appear black in this sequence? And I never suspected there was any place in the shadow that would have a blue hue!
So, what is this sequence teaching me in today's APOD ?
[list][*]Clearly, as the caption pointed out, the edge of the umbral shadow actually has a faint blue hue, and they have an explanation I'm happy to accept -- that Earth's stratosphere has a realm that actually transmits blue and scatters red, as opposed to the lower, main part of our atmosphere. I wonder if it is quite consistently there, or if it is a variable phenomenon in lunar eclipses, depending on Earth's atmospheric conditions.
[*]But I'm not at all sure about the seemingly black edge of the umbra in these images. Is the outer edge of the umbra actually blacker than the rest, or is it that those images had to be adjusted to much lower contrast due to the overpowering moonlight from the uneclipsed part of the Moon in those images? In other words, does that edge band appear black in the photos for the same reason I thought it appeared black to the human eye -- because of the need to lower the exposure in those images?[/list]
[/quote]
I believe that a photometrically consistent image would have as its lightest pixels newly exposed material on the uneclipsed Moon, and as its darkest pixels the center of maria in the fully eclipsed Moon. So yes, I think the processing of the shadow edge represents a non-linearity. If you follow a single pixel near the limb it goes from bright to dim and then its brightness increases again towards totality. That doesn't seem realistic. That said, while photometrically inaccurate, it perhaps gives a better representation of the visual experience than a more accurate processing flow would achieve.