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Procyan
2007-Sep-28, 04:03 AM
From the moon looking earthwards, what would you see? A red ring or red image of the sun? I wonder does the atmosphere actually bend the light like a lens?

Earlier in the year there was a thread called RED MOON and Maksutov said "The atmosphere tends to absorb light frequencies from violet to almost yellow when an object is low in the sky. What remains is yellow, orange, and red."

That makes sense, but wouldn't there be at least some shorter wavelengths following the curve like a waveguide?

Also, have any space probes been in a position to get that image? Of Earth in front of the sun at ~lunar distance? Must be a very bright feature considering how brilliant a RED MOON looks from here.

01101001
2007-Sep-28, 04:13 AM
From the moon looking earthwards, what would you see? A red ring or red image of the sun?
[...]
Also, have any space probes been in a position to get that image? Of Earth in front of the sun at ~lunar distance? Must be a very bright feature considering how brilliant a RED MOON looks from here.

Red ring. This has been discussed before. See topic How will look a total solareclipse on the Moon? (http://www.bautforum.com/questions-answers/39924-how-will-look-total-solareclipse-moon.html).

There are some images cited there.

Apollo 12 came close:

Earth - Apollo 12 (http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/a12_h_53_7917.html)

http://nssdc.gsfc.nasa.gov/imgcat/thumbnail/a12_h_53_7917.gif (http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/a12_h_53_7917.html)

tony873004
2007-Sep-28, 04:49 AM
Here's my photoshopped guess:

http://orbitsimulator.com/BA/lunarEclipse.JPG

Procyan
2007-Sep-28, 05:19 AM
Thanks, I missed that thread completely. That photoshop image, thats what gets me, so much light must shoot though such a thin ring. Those surveyor pictures really give a good indication. But to see it that for real...someone will pretty soonish.

01101001
2007-Sep-28, 05:43 AM
[...] so much light must shoot though such a thin ring.

You give the impression that you might think the moon is bright and well-lit during a lunar eclipse. What do you consider "so much" to be?

Even ordinarily, the Moon is not all that bright, being not very reflective. But, it is brighter than anything around, hanging in blackness like it does.

During a deep lunar eclipse, it is lit poorly. It is in mostly shadow, receiving just a little refracted light through the narrow band of Earth's atmosphere. It might still look relatively bright compared to the darkness around it, but it is dim compared to its normal uneclipsed illumination.

Anyone have the numbers? The Danjon scale (http://en.wikipedia.org/wiki/Danjon_scale) seems pretty subjective. Are records kept of illumination -- which must certainly vary by the alignment as well as the condition of Earth's atmosphere.

Procyan
2007-Sep-28, 07:46 AM
I was amazed at the 28 August lunar eclipse. I was told to look for a coppery hue. I'm 53 and it was my first full lunar eclipse!

The actual event, we had partial clouds moving quickly so it was a sort of peep show. But I was amazed at how bright or luminescent that orange/red glow really was. Maybe the contrast against the dark clouds enhanced the effect.

Anyway I got to wondering about the view from the moon and then I thought perhaps the earth actually was a lens. I meant to ask, if the moon were a bit further out, is there a possibility that the hue would change towards shorter wavelengths? Do all planets with atmospheres project a kind of rainbow shadow opposite the sunny side. I'm probably way off but thought ya'll would know for sure.

Tog
2007-Sep-28, 07:50 AM
The closest I can get to actual numbers that might mean something are the exposure settings of the pictures I took during the last eclipse in August.

These were taken at f/6.3 with 400 speed film. The full, uneclipsed moon was shot at 1/1000 second.
The moon at totality was usually 30 to 40 seconds.

That's a difference of 15 "stops" which someone may be able to convert to real numbers.

Tog
2007-Sep-28, 08:27 AM
Anyway I got to wondering about the view from the moon and then I thought perhaps the earth actually was a lens. I meant to ask, if the moon were a bit further out, is there a possibility that the hue would change towards shorter wavelengths? Do all planets with atmospheres project a kind of rainbow shadow opposite the sunny side. I'm probably way off but thought ya'll would know for sure.

Here is a very rough, and nowhere near to scale graphic of the shadow. The lightest yellow is the unobstructed sunlight. The deep red is the area that gets only diffused light from the Earth's atmosphere. The darker yellow is that area that has some sunlight blocked by the Earth, but not really enough to matter. In official terms, the red would be the Umbra of the shadow, while the darker yellow is the Penumbra.

The moon's position (due to poor planning on my part) is well beyond it's real location on the graphic, but is in the right spot to get an annular eclipse. This would be a ring of bright sunlight around the edges, with a diffused center spot. A penumbral eclipse is when none of the moon passes through the umbra, and the change is very minimal in most cases.

I think the answer to your question about the color if it were further out, we need to look only at a penumbral eclipse. I don't think you would ever get a blue ring around the earth, just a variation from deep red to yellow. This might be wrong though, so see below for corrections. :)

grant hutchison
2007-Sep-28, 12:21 PM
If you think about the geometry, what you're seeing reflected from the moon is the light of every sunset and sunrise currently happening on the Earth.
People standing within the "red ring" of the Earth's atmosphere that is visible from the eclipsed moon will have the sun at the horizon in one direction and the moon at the horizon in the opposite direction. Reddened sunlight is streaming past them, being further reddened by another chunk of atmosphere, and then going on to illuminate the moon.
So the only way to change the colour of the light would be to change the amount of scattering in Earth's atmosphere: with a less dense atmosphere you could have a lighter orange or a pale yellow.

Grant Hutchison

01101001
2007-Sep-28, 10:53 PM
That's a difference of 15 "stops" which someone may be able to convert to real numbers.

That seems to be in the ballpark for this, about apparent magnitude (http://en.wikipedia.org/wiki/apparent_magnitude):

Excerpts from Observe Eclipses book (http://www.lpl.arizona.edu/~rhill/alpo/eclstuff/observeeclipses/chapter15.htm):


During totality, the full moon’s usual magnitude of –12.7 may drop by a factor from 10,000 to 1,000,000 times; to anywhere from magnitude –2.7 to +3.3 or fainter.

KaiYeves
2007-Sep-29, 12:46 AM
Yup, my Earth Science teacher always said it would look just like a solar eclipse.
Are Mars' two moons too tiny to create a really stunning solar eclipse or not?

01101001
2007-Sep-29, 01:21 AM
Are Mars' two moons too tiny to create a really stunning solar eclipse or not?

NASA Rovers Watching Solar Eclipses by Mars Moons (http://www.jpl.nasa.gov/releases/2004/81.cfm)


The current rovers began their eclipse-watching campaign this month. Opportunity's panoramic camera caught Mars' smaller moon, Deimos, as a speck crossing the disc of the Sun on March 4. The same camera then captured an image of the larger moon, Phobos, grazing the edge of the Sun's disc on March 7.
[...]
From Earth, our Moon and the Sun have the appearance of almost identically sized discs in the sky, so the Moon almost exactly covers the Sun during a total solar eclipse. Because Mars is farther from the Sun than Earth is, the Sun looks only about two-thirds as wide from Mars as it does from Earth. However, Mars' moons are so small that even Phobos covers only about half of the Sun's disc during an eclipse seen from Mars.

More than you probably care to know at:
Transit of Phobos from Mars (http://en.wikipedia.org/wiki/Transit_of_Phobos_from_Mars)
Transit of Deimos from Mars (http://en.wikipedia.org/wiki/Transit_of_Deimos_from_Mars)

Kaptain K
2007-Sep-29, 07:45 AM
FWIW - A passage of a moon of Mars in front of the Sun would be more accurately termed a transit than an eclipse.

astromark
2007-Sep-29, 09:09 AM
You seem to have missed the point that from the lunar surface the Earth is much larger than is the moon from Earth. So from the moon the sun would be completely obscured. The atmosphere may glow with refracted light and give a ring of red light. I do not know that to be true. Its an image we will see one day soon. All you need is a camera on the moon looking at Earth

01101001
2007-Sep-29, 02:20 PM
The atmosphere may glow with refracted light and give a ring of red light. I do not know that to be true. Its an image we will see one day soon. All you need is a camera on the moon looking at Earth

Did you miss the link to the other thread? More direct:

Excerpt from Observe Eclipses (http://www.lpl.arizona.edu/~rhill/alpo/eclstuff/observeeclipses/chapter15.htm)


[Caption] Photograph 15-3a, b Unusual views of a total lunar eclipse: a total solar eclipse as seen from the moon! Surveyor 3 images of the 24 April 1967 eclipse. The left image was taken at 11:24 UT. a total lunar eclipse was visilbe across East Asia, Austrialia, and the Pacific.

Quality wasn't so hot. And they're grayscale. But, we've had cameras on the moon looking at Earth.

KaiYeves
2007-Sep-29, 06:15 PM
I thought so. Thanks!

Procyan
2007-Sep-29, 08:37 PM
The atmosphere may glow with refracted light and give a ring of red light.

Yes, thats the question, is the sunlight that travels through earths atmosphere refracted or simply scattered? If we know the refractive index of vacuum/atmosphere could we (royal :)) not calculate the ...focal length? or am I completely in the ozone here?

Those surveyor pictures are great. Almost looks like Baileys beads! Of course that can't be, so perhaps it shows the difference between clear light paths and those obscured by clouds? It just seems too bright to be scattered light. Perhaps the moon orbits a bit inside the focal point. Then you would see a the bright red ring, but what would the putative focal point image look like?

grant hutchison
2007-Sep-29, 10:50 PM
Yes, thats the question, is the sunlight that travels through earths atmosphere refracted or simply scattered? If we know the refractive index of vacuum/atmosphere could we (royal :)) not calculate the ...focal length? or am I completely in the ozone here?Bit of both.
At sunset or sunrise (moonset or moonrise) we see the sun or moon lifted by refraction by a bit more than half a degree. So direct sunlight grazing through the atmospheric "red ring" will be deflected by a little more than a degree (half going in, half coming out). That'll delay the disappearance of the sun's disc behind the Earth, as seen from the moon: it'll squash into a very bright sliver of light before disappearing entirely. So refraction will make the Earth's shadow cone a little narrower than it would be based on simple geometry; but the shadow is still several times the moon's diameter at the distance of the moon's orbit. So when the moon is inside the shadow cone, it's necessarily being illuminated by red light that has been scattered into the shadow zone: the equivalent of red sky before sunrise or after sunset here on Earth.

Grant Hutchison