Is the Sun yellow?

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Ann
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Re: Is the Sun yellow?

Post by Ann » Fri Jul 02, 2010 6:34 pm

Helio George, I love your post! Thank you so very much for it! And I love the links you provided, particularly the last one. Here George Cooper explains so well why the Sun is white, not yellow. He says all the things I have tried to say here and some that I didn't know of, and he says them much better than I do. He included a picture of the Sun and planets in natural color, and I found that picture on the net, so I'm posting it here:

Image

Look at that snow-white Sun! Fantastic, isn't it? I have to wonder how many people have ever seen the Sun portrayed like that.

As for the wavelength where the solar emission peaks as seen from below the atmosphere, Chris said it was at 555nm. I asked for a color sample showing what that color looks like. Well, if you want to know, take a look at yesterday's APOD (July 1, 2010), displaying aurora australis. Almost everybody would call the light from a 557.7nm aurora green, not yellow. I admit it is on the yellow side of green, but most people would surely say that it looks far more green than yellow.

Image

A green aurora, showing off a color that almost exactly matches the peak intensity of the solar spectrum as seen from the ground.

Ann
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Re: Is the Sun yellow?

Post by Chris Peterson » Fri Jul 02, 2010 6:51 pm

Ann wrote:As for the wavelength where the solar emission peaks as seen from below the atmosphere, Chris said it was at 555nm. I asked for a color sample showing what that color looks like. Well, if you want to know, take a look at yesterday's APOD (July 1, 2010), displaying aurora australis. Almost everybody would call the light from a 557.7nm aurora green, not yellow. I admit it is on the yellow side of green, but most people would surely say that it looks far more green than yellow.
This also demonstrates the inadequacy of color names in actually describing colors. Our eyes easily perceive a significant difference in color between 555nm and 558nm. Further complicating things is that neither of these actually defines a color. If you have a monochromator that outputs 555nm, the human eye will perceive thousands of colors as you change the output intensity from sub-threshold to glare-inducing. Not only are these truly different colors, but they will also show a change in the perceived hue. Many people would not recognize a faint green aurora and a bright one as the same color or the same hue.
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Re: Is the Sun yellow?

Post by Ann » Fri Jul 02, 2010 7:41 pm

Chris said:
Our eyes easily perceive a significant difference in color between 555nm and 558nm.
I'm sure you are right about that. But that just means that a green aurora is yellower than the Sun, and the peak intensity of the spectrum of the Sun is greener than a green aurora!
If you have a monochromator that outputs 555nm, the human eye will perceive thousands of colors as you change the output intensity from sub-threshold to glare-inducing. Not only are these truly different colors, but they will also show a change in the perceived hue. Many people would not recognize a faint green aurora and a bright one as the same color or the same hue.
I'm sure you are right about this too, but what relevance does it have for the discussion of the color of the Sun? Surely you aren't trying to say that 555nm will look yellower if it is bright than if it is faint, and therefore the "true color" of the Sun might be described as yellow since sunlight is always so searingly bright when seen from the Earth?

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Re: Is the Sun yellow?

Post by Chris Peterson » Fri Jul 02, 2010 8:04 pm

Ann wrote:I'm sure you are right about that. But that just means that a green aurora is yellower than the Sun, and the peak intensity of the spectrum of the Sun is greener than a green aurora!
No disagreement. Although our eyes are extremely poor at seeing or describing color in absolute terms, we are very good with relative hues. Anybody with normal vision will readily understand (and see) that 558nm is more yellow than 555nm, even though by themselves most people would simply call either "green".
If you have a monochromator that outputs 555nm, the human eye will perceive thousands of colors as you change the output intensity from sub-threshold to glare-inducing. Not only are these truly different colors, but they will also show a change in the perceived hue. Many people would not recognize a faint green aurora and a bright one as the same color or the same hue.
I'm sure you are right about this too, but what relevance does it have for the discussion of the color of the Sun? Surely you aren't trying to say that 555nm will look yellower if it is bright than if it is faint, and therefore the "true color" of the Sun might be described as yellow since sunlight is always so searingly bright when seen from the Earth?
When you change the intensity of a spectrally pure light source, the color changes by definition. But the hue also shifts. So indeed, as the intensity of the Sun changes, so does its apparent hue. This is readily obvious if you simply view the Sun through different neutral density filters. Most anybody who views the Sun directly (briefly!) will critically report that it appears white. Most anybody who views the Sun through an absolutely neutral density filter (which does not change the spectral characteristics at all) will critically report the Sun to be yellow (I'm talking here about viewing it through the atmosphere). As the filter gets denser, an observer will report an apparent shift of the observed yellow, either towards red or towards green- depending both on the observer and on the actual intensity after attenuation.

I'm not arguing with you here about the color of the Sun, only pointing out some of the interesting aspects of human color vision that color the entire matter of describing the appearance of astronomical (and non-astronomical) objects.
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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 2:55 am

Thanks much, Ann! You're very kind considering all the "corn" I dished out in the blog. :)
As for the wavelength where the solar emission peaks as seen from below the atmosphere, Chris said it was at 555nm.
This may be a peak wavelength for a Sun when the sunlight is passing through our atmosphere with an air mass of about 2 (AM2). [AM1, one air mass, is the amount of atmosphere from sea level to the vertical top or zenith]

The peak wavelength of the sun as seen in space is in the blue portion of the spectrum, abour 450nm to 480nm. [Color gets a little too subjective to nail-down each color for each wavelength, but the general range of each color band is far less subjective.]

For instance, using the Wiehrli '85 data set, the peak wavelength is found at 450.5nm with a value of 2146 w/m^2 - nm, but other "peaks" can be found within a couple percent of this value around the 480nm range. At 555nm it is 1900 w/m^2 - nm.

Using the Thuilier '02 data set, which is obviously taken on a different day, the peak is at 456.63 nm with a value of 2130 w/m^2 - nm.

Using the data set from Dec. 10, 2008 from SORCE, the peak is 479.4 nm with a value of 2099 w/m^2 - nm.

Your earlier link to a spectral irradiance graph is helpful since it shows the sp. irr. of the sun as seen from space and as seen terrestrially (probably AM1.5, which is common). Notice that the peaks vary and that there are many similar peaks.
I asked for a color sample showing what that color looks like. Well, if you want to know, take a look at yesterday's APOD (July 1, 2010), displaying aurora australis. Almost everybody would call the light from a 557.7nm aurora green, not yellow. I admit it is on the yellow side of green, but most people would surely say that it looks far more green than yellow.
A rich or saturated yellow takes place aroun 580nm. At 555 or 558nm, you might see a slightly yellowish tint to a rich green, but the spectral sensitivy of our eyes vary from one person to another. This is true of one another's visible range of colors too.

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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 3:03 am

Chris Peterson wrote:This also demonstrates the inadequacy of color names in actually describing colors. Our eyes easily perceive a significant difference in color between 555nm and 558nm.
Some may, but 3 nm will likely make little to no difference. Are there some experiments that demonstrate your view?
If you have a monochromator that outputs 555nm, the human eye will perceive thousands of colors as you change the output intensity from sub-threshold to glare-inducing.
This is highly unlikely. It is known that intensity changes will alter the color perception but not in seeing many other colors, much less thousands.

Green is particullary interesting since at very low intensities it is the last color the eye can distinguish. Green happens to be the most sensitive color for our eyes. At low light levels, both the color cones and rods contribute to the final color determination. Green has become more popular for emergency vehicles here in the U.S. for this very reason.

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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 3:16 am

Chris Peterson wrote:So indeed, as the intensity of the Sun changes, so does its apparent hue. This is readily obvious if you simply view the Sun through different neutral density filters. Most anybody who views the Sun directly (briefly!) will critically report that it appears white.
Surprisingly, many see yellow or yellowish white more than white.

In the past, I did a quick survey of the Sun's color by having each person take a quick glance of a mid-day Sun. [Ths relative size represents the results.]
Image

I see a white sun, though I do see a yellow corona around it.
Most anybody who views the Sun through an absolutely neutral density filter (which does not change the spectral characteristics at all) will critically report the Sun to be yellow (I'm talking here about viewing it through the atmosphere).
That is certainly true whenever the Sun is near the horizon. When it is significantly above the horizon the Sun will appear white when seen at a reasonable intensity. This is best demonstrated by simply projecting the solar disk onto a white sheet of paper by using another sheet with a pin hole in it. These unfiltered projections look white, unless you have serious smog issues.

As for neutral filters, I have yet to find a completely neutral filter. I use a Baader filter and it produces a yellow sun, but it isn't a true neutral filter.
As the filter gets denser, an observer will report an apparent shift of the observed yellow, either towards red or towards green- depending both on the observer and on the actual intensity after attenuation.
This sounds much more like atmospheric scattering, which explains the Sun's color metamorphisis as it drifts over the late afternoon horizon.

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Re: Is the Sun yellow?

Post by Chris Peterson » Sat Jul 03, 2010 4:40 am

Helio George wrote:Surprisingly, many see yellow or yellowish white more than white.
I see white as well. I think the fact that people see yellow has more to do with their observational skills than anything else (they tend to allow the surrounding halo to dominate, or they simply perceive white as yellow given the blue sky, which is a natural illusion). This kind of critical evaluation is something that needs practice.
Most anybody who views the Sun through an absolutely neutral density filter (which does not change the spectral characteristics at all) will critically report the Sun to be yellow (I'm talking here about viewing it through the atmosphere).
That is certainly true whenever the Sun is near the horizon. When it is significantly above the horizon the Sun will appear white when seen at a reasonable intensity. This is best demonstrated by simply projecting the solar disk onto a white sheet of paper by using another sheet with a pin hole in it. These unfiltered projections look white, unless you have serious smog issues.
I disagree. The Sun definitely has a warm cast when viewed directly through a neutral density filter, and it definitely has a yellowish cast when projected, as long as the projection is viewed in an area shielded from skylight. I see this from my observatory at 3000m elevation with very clean, dry air. I also remember it from back when I worked at Big Bear Solar Observatory, again under good atmospheric conditions.
As for neutral filters, I have yet to find a completely neutral filter. I use a Baader filter and it produces a yellow sun, but it isn't a true neutral filter.
There are pretty good neutral density filters over the visible range. In fact, Baader material (and aluminized Mylar in general) is actually quite flat.
As the filter gets denser, an observer will report an apparent shift of the observed yellow, either towards red or towards green- depending both on the observer and on the actual intensity after attenuation.
This sounds much more like atmospheric scattering, which explains the Sun's color metamorphisis as it drifts over the late afternoon horizon.
Not related. You can duplicate the effect easily in a lab with a monochromator. A change in intensity of a single spectral band, or a mix of bands, will produce an apparent shift in hue, separate from the obvious change in color.
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Re: Is the Sun yellow?

Post by Ann » Sat Jul 03, 2010 6:25 am

Helio George wrote:
Thanks much, Ann! You're very kind considering all the "corn" I dished out in the blog. :)
Hah!!! I wondered if George Cooper was you! Helio George (Sun George?) linking to a blog by George Cooper, who takes a great interest in the Sun - that was a bit of a coincidence!

Thanks for the blog, I really appreciated it! :mrgreen:

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Re: Is the Sun yellow?

Post by makc » Sat Jul 03, 2010 8:31 am

on subject of this thread: isn't it kinda obvious that, if our sky scatters away blue light, the color of the sun = yellow we see + blue = emmm, white?

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Re: Is the Sun yellow?

Post by Chris Peterson » Sat Jul 03, 2010 2:55 pm

makc wrote:on subject of this thread: isn't it kinda obvious that, if our sky scatters away blue light, the color of the sun = yellow we see + blue = emmm, white?
Of course... I think that has been brought up a few times. The easiest way to see this is when you have snow on the ground. Open areas exposed to both the sky and direct sunlight are white (sun + sky), areas of light shade are blue (sky), and sunbeams extending into areas of deep shade are pale yellow (sun). If your color discrimination isn't good enough to make this obvious, a camera certainly will, with the white balance set to sunlight for all three shots.
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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 6:22 pm

Chris Peterson wrote: I disagree. The Sun definitely has a warm cast when viewed directly through a neutral density filter, and it definitely has a yellowish cast when projected, as long as the projection is viewed in an area shielded from skylight. I see this from my observatory at 3000m elevation with very clean, dry air. I also remember it from back when I worked at Big Bear Solar Observatory, again under good atmospheric conditions.
This is interesting since it is contrary to all that I’ve seen so far. I would enjoy learning what you would observe if you did a simple pinhole projection onto a white sheet of paper, especially from 3000m.

There is a slight chance you are experiencing yellow induction due to the blue sky. [In the 1700s, Nocolas de Beguelin claimed the yellow light from a candle’s flame would produce a bluish color of any shadow cast upon a white paper. He claimed the blue sky was the reason. Goethe, however, took it further by doing the demonstration in a non-blue room on an overcast day. He showed that blue and yellow are complementary so one color can induce another.]

I am curious if Big Bear uses filters for their Solar projections, or could there be some lens coating issue. I wouldn’t think the extra humidity would be an issue, or is this a possibility?
There are pretty good neutral density filters over the visible range. In fact, Baader material (and aluminized Mylar in general) is actually quite flat.
Transmittance info would be real handy about now. I have looked in the past for this info but gave up for the reasons below. I can say my Baader filter does produce a yellow Sun….

Here is one:
Image

Solar projections at solar observatories – though not always true at Big Bear and perhaps not at others, but at least at Kitt Peak - appear as solid white and without any hint of yellow. Here is an image taken with the help of Dr. Drew Potter and Roy Lorenz at Kitt-Peak’s McMath-Pierce telescope. [The color pieces, of course, were added for color calibration assistance.]

Image

This is also how it appears in person.

There are two important points worth discussing: filtration and CLV.

I verified that they were not using any filters for this projected image, so WYSIWYG, and what we get is all white.

The center to limb variation (CLV) also did not alter the image’s color. This is significant because the CLV is almost 1400K (6390K at the center, 5000K at the limb). If an unfiltered image of the cooler limb region is white then it seems highly unlikely that any yellow sun argument can hold much ground.

However, color is subjective, so you and others may simply see yellow where others see white. If so, it would be interesting to find out what variables contribute to this variability.
Not related. You can duplicate the effect easily in a lab with a monochromator. A change in intensity of a single spectral band, or a mix of bands, will produce an apparent shift in hue, separate from the obvious change in color.
I have never done this, but I must assume that the hue itself does not change but our perception of the colour does based on changes in brightness and saturation. I also assume any variation of a monochrome light, say from a green laser, will never produce a red, orange, yellow, or blue projection without some other major factor like induction, though I doubt induction or anything else is powerful enough to make a monochromatic green beam appear red. A green laser will always look green though a yellowish-tint might be observed as the intensity changes. Is this what you are saying?
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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 6:35 pm

Ann wrote:Helio George wrote:
Thanks much, Ann! You're very kind considering all the "corn" I dished out in the blog. :)
Hah!!! I wondered if George Cooper was you! Helio George (Sun George?) linking to a blog by George Cooper, who takes a great interest in the Sun - that was a bit of a coincidence!

Thanks for the blog, I really appreciated it! :mrgreen:

Ann
I see the cat is out of the bag, though I tend to forget I even have a cat and bag. :)

I have had lots of fun with this topic, enough almost enough for a short book.

An even more serious case of "corn" can be found at Standford's Solar Center's website where they have, surprisingly, installed a zanny multiple choice quiz on the quest for the Sun's color, better known as Heliochromology. [There is a reward at the end. I rarely recommend this link since I can only stand being this much a shill. :)]

You too seem smitten with this colorful topic. Whether you know it or not, I have no doubt that you are an amateur heliochromologist like me! *wink*
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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 7:06 pm

makc wrote:on subject of this thread: isn't it kinda obvious that, if our sky scatters away blue light, the color of the sun = yellow we see + blue = emmm, white?
That is an important viewpoint. The white solar image I gave above comes from sunlight that has been somewhat scattered by our atmospere. A corrected color image to convert this image into a color depiction of the Sun as it would be seen in space would require us to add the scattered light back into this white image.

As you have stated, it is primarily blue light that gets scattered (a 4th power scattering law of Lord Rayleigh, based upon Tyndall's work), so what happens when we add blue to white? We won't get yellow.

If some do see the Sun's projection as yellow, which seems to be the case for Chris, then would adding blue to the yellow disk produce white? Maybe, but it depends upon the amount of blue light that gets added to the yellow.

Fortunately, there is a considerable amount of data on this subject. The more important information relating to the Sun's color may be in the converting of the spectral irradiance data taken from space to a photon flux distribution since our eyes respond to photons.

The following shows the spectral irradiance distribution from th eThuilier '02 data set and it shows the converted distribution for photon flux. Blue photons require more energy because we know that E = hf.
Image

What we see is that the photon flux from the Sun is very flat. Such a distribution is unlikely to allow any one color of the spectrum to stand out enough for us to give the Sun any color assignment, thus by this alone, we can argue that the Sun is white, though for some reason, some may see some color from this, but I can't imagine why.

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Re: Is the Sun yellow?

Post by makc » Sat Jul 03, 2010 7:31 pm

take a sunset for example, where sun light is scattered the most - the sun is getting not only yellow but even red. maybe when a sun is directly above us, it is white, then adding some blue back to it would indeed make it blue in outer space? btw, I think I have seen some webpage that claimed to comute sun color in rgb, let me search for it... oh yeah, here it is (says sun is pink (but agrees with the above conclusion that sun is white + blue if you select D50 as white)).

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Re: Is the Sun yellow?

Post by Helio George » Sat Jul 03, 2010 11:23 pm

makc wrote:take a sunset for example, where sun light is scattered the most - the sun is getting not only yellow but even red. maybe when a sun is directly above us, it is white, then adding some blue back to it would indeed make it blue in outer space? btw, I think I have seen some webpage that claimed to comute sun color in rgb, let me search for it... oh yeah, here it is (says sun is pink (but agrees with the above conclusion that sun is white + blue if you select D50 as white)).


Here is another link we can use. They claim the the Sun is "peachy pink" relative to a more white standard, D65. Is the Sun a girl star? :wink: :mrgreen:

I am very suspicious of taking spectral data and being confident in the color computation that is found. Part of the problem is that the Sun is not next to any other white source, though if a D65 colored star were next to it, I doubt we'd see a pink result.

Another big issue that supports a white sun is that of color constancy. Using an incandescent light bulb or even a candle as your only light source will produce a much more yellow light than the much hotter Sun. But inside a room with just candlelight, what color would you describe white paper, assuming it is white under sulight?

Our brains and digital camera processors know how to take yellow lighting and adjust all the colors so that they are consistent with what we see in daylight. It is one of those remarkable things that we take for granted. [I recall one high quality camera manual encouraging the shooter to always have a white object in the image to serve as a reference for color processing. Perhaps this is well known.]


It would be my guess that the white light of the Sun could be the ultimate white.... uber white. :) But maybe not; our eyes are very complex.

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Re: Is the Sun yellow?

Post by Ann » Fri Jul 09, 2010 8:49 pm

Helio George wrote:
Is the Sun a girl star? :wink: :mrgreen:
Would you believe that the Sun is a feminine entity in Swedish? Nowadays almost everybody in Sweden calls the Sun an "it", but traditionally Swedes have called it a "she". (Where is the Sun? She is high in the sky.")

We have traditionally called the Sun "Mother Sun", just like we have called the Earth "Mother Earth". Apparently we Swedes think that Nature is extremely maternal! And Mother Sun and Mother Earth have cooperated and brought forth... man. Except that "man" is "människa" in Swedish, and would you believe that "människa" is grammatically a feminine entity? "Människa" is a "she"! Mother Sun and Mother Earth cooperated and got a daughter, "människa"!

Don't we Swedes believe in any fathers in nature at all? Well, not fathers exactly, but at least the Moon is (or used to be) a male entity to us Swedes! Well, there is a man in the Moon, so then I guess we just assumed that the Moon itself was male, too!

So...

Image +Image = Image

Mother Sun + Mother Earth = Daughter "Människa"

Image+ Image = Image

Man in the Moon + Male Moon = True Love

I guess we Swedes have long believed in untraditional partnerships and families, no? :mrgreen:

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Re: Is the Sun yellow?

Post by Chris Peterson » Fri Jul 09, 2010 9:02 pm

Ann wrote:Would you believe that the Sun is a feminine entity in Swedish? Nowadays almost everybody in Sweden calls the Sun an "it", but traditionally Swedes have called it a "she". (Where is the Sun? She is high in the sky.")
Hi Ann- not to teach you Swedish <g>, but as a speaker of Danish and a reader of Old Norse, let me comment on an interesting linguistic detail. In old Norse, there were three genders, male, female, and neuter (as in modern German). But in modern Danish, Swedish, and Norwegian, that has been reduced to just two, common and neuter. Male and female have been combined! But with very old words, or old concepts, change is so slow that old forms tend to remain long after the language as a whole has moved on. You can conduct a sort of archaeology on languages by studying these kinds of details.
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Re: Is the Sun yellow?

Post by Beyond » Sat Jul 10, 2010 6:11 pm

Ann wrote:Helio George, I love your post! Thank you so very much for it! And I love the links you provided, particularly the last one. Here George Cooper explains so well why the Sun is white, not yellow. He says all the things I have tried to say here and some that I didn't know of, and he says them much better than I do. He included a picture of the Sun and planets in natural color, and I found that picture on the net, so I'm posting it here:

Image

Look at that snow-white Sun! Fantastic, isn't it? I have to wonder how many people have ever seen the Sun portrayed like that.

As for the wavelength where the solar emission peaks as seen from below the atmosphere, Chris said it was at 555nm. I asked for a color sample showing what that color looks like. Well, if you want to know, take a look at yesterday's APOD (July 1, 2010), displaying aurora australis. Almost everybody would call the light from a 557.7nm aurora green, not yellow. I admit it is on the yellow side of green, but most people would surely say that it looks far more green than yellow.

Image

A green aurora, showing off a color that almost exactly matches the peak intensity of the solar spectrum as seen from the ground.

Ann
Ann, do you have any idea of what the white circle is about bottom center of the Aurora picture?
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Re: Is the Sun yellow?

Post by Ann » Sat Jul 10, 2010 8:43 pm

Unfortunately I have no idea what the white circle might be. Has anyone else got any ideas?

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Re: Is the Sun yellow?

Post by Chris Peterson » Sat Jul 10, 2010 9:26 pm

Ann wrote:Unfortunately I have no idea what the white circle might be. Has anyone else got any ideas?
It's almost certainly Manicouagan crater in Canada.
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Re: Is the Sun yellow?

Post by Beyond » Sat Jul 10, 2010 9:54 pm

Chris Peterson wrote:
Ann wrote:Unfortunately I have no idea what the white circle might be. Has anyone else got any ideas?
It's almost certainly Manicouagan crater in Canada.
I don't think so. There seems to be a lot of vertical rise to the white ring and from what i have been able to find, the ring around the outside of the impact area has worn down to the point where it is not much more than a "bump" in the land scape. Is there any way to find out where the shuttle was when it took the picture of the southern lights??

And what are those 4 Blobs of color in/or under the clouds about 3 circle diameters to the right? And there is also one color Blob to the left at the botton of the picture.
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Re: Is the Sun yellow?

Post by Chris Peterson » Sun Jul 11, 2010 12:53 am

beyond wrote:
Chris Peterson wrote:It's almost certainly Manicouagan crater in Canada.
I don't think so.
Turns out this was an APOD. According to the caption, the ring is, in fact, Manicouagan crater.
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Re: Is the Sun yellow?

Post by Beyond » Sun Jul 11, 2010 2:02 am

Well....It looks like the only two pictures that show the outer crater are Apod pictures about 5 years apart. All i could find on the web was the inner crater circle of water. So it seems to be what it is even though to me it seems to be too high walled. But then I've never flown over Manicouagan crater.
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Re: Is the Sun yellow?

Post by Chris Peterson » Sun Jul 11, 2010 2:25 am

beyond wrote:Well....It looks like the only two pictures that show the outer crater are Apod pictures about 5 years apart. All i could find on the web was the inner crater circle of water. So it seems to be what it is even though to me it seems to be too high walled. But then I've never flown over Manicouagan crater.
I don't see any vertical relief at all in the aurora image. But the Manicouagan crater does have pretty steep walls, as well as a steep interior. And it freezes over completely in the winter. The original ISS image is here, and if you view the high-res version, you can easily match the structure of the crater and its surrounding rivers to some of these images. A couple show it in the winter, and there's a nice 3D image as well.
Chris

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