Full Moon next to Venus???? (APOD 23 Apr 2008)

[jshirey]

Looks like a full moon next to Venus... which can never happen. So is this an altered picture without credit to that effect?
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[orin stepanek]

http://apod.nasa.gov/apod/ap080423.html
you make a good point! Would the moon look fuller if it is a time photo?
Orin

[henk21cm]

Looks like a full moon next to Venus... which can never happen. So is this an altered picture without credit to that effect?

I do not think so. When you want to catch the milky way on your image, you will have to expose your image for quite some time. The picture must have been made on December 4th, 2005, around 20 h local time. That was 4 days beyond new moon. The ash gray light must still have been noticeble, (nearly full earth) so when exposing an image for some minutes, it looks like full moon. Note thhat the stars are not points, but small lines, due to a long exposure time. What is open to debate is whether the stray light of the moon in the earths atmosphere, during a long exposure, is so strong that it exceeds the dim light of the milky way.

When i read the comments at Serge Bruniers website and the intentions of the TWAN project, i get the idea that todays images is not a faked. Especially the 12the image, taken in the Atacame dessert, is stunning, notacibly when the conditions are taken into account. 90 minutes exposure time, at 5900 m altitude, harsh conditions, sub zero temperatures, lack of oxygen. The subscript mentions that every movement was difficult, the fatiguing lack of oxygen destroyed his will to continue.

Regards,
Henk

[Axel]

Is that a helicoid cloud in the right half of the photo?

[watch24]

Caption says "your view of the world at night could look like this." Is that true?

I was told by someone who viisted Mauna Kea that at 13,000 feet, you don't see many stars with the naked eye because there's not enough atmosphere to disperse the light. His tour group had to go to a lower elevation to actually see the stars clearly. Can anyone confirm this?

If so, then perhaps one couldn't see any stars from space or from the dark side of the moon--which certainly spoils a favorite fantasy of mine.

[bystander]

Caption says "your view of the world at night could look like this." Is that true?

I was told by someone who viisted Mauna Kea that at 13,000 feet, you don't see many stars with the naked eye because there's not enough atmosphere to disperse the light. His tour group had to go to a lower elevation to actually see the stars clearly. Can anyone confirm this?

If so, then perhaps one couldn't see any stars from space or from the dark side of the moon--which certainly spoils a favorite fantasy of mine.

If atmosphere was necessary for star gazing, the hubble telescope would be useless. (my opinion)

Please don't create multiple threads for the same APOD.

[watch24]

Of course the human eye and brain gather and interpret light in real time and in a much different manor than a telescope, so what I was told is quite possible. I was unable to see stars from a 12,000-ft peak once, but the reason could've been ambient light.

Forgive me for cluttering the forum with a duplicate posting. I just joined an hour ago, so wasn't clear on how all this works.

[bystander]

Of course the human eye and brain gather and interpret light in real time and in a much different manor than a telescope, so what I was told is quite possible. I was unable to see stars from a 12,000-ft peak once, but the reason could've been ambient light.

I don't see how less atmosphere would affect the light gathering power of your eyes. If anything, the stars should be more visible, because of the lack of scattering. (again, my opinion). The night skies while camping in Colorado were amazing.

Art? Chris? Are you watching? What do the physicists say?

Forgive me for cluttering the forum with a duplicate posting. I just joined an hour ago, so wasn't clear on how all this works.

It's ok. Just kind of a pet peeve of mine. Welcome aboard.

[watch24]

Of course the human eye and brain gather and interpret light in real time and in a much different manor than a telescope, so what I was told is quite possible. I was unable to see stars from a 12,000-ft peak once, but the reason could've been ambient light.

I don't see how less atmosphere would affect the light gathering power of your eyes. If anything, the stars should be more visible, because of the lack of scattering. (again, my opinion). The night skies while camping in Colorado were amazing.

How high were you?--I doubt 13,000'. I'm an avid backpacker, but I've always retreated to the valleys to camp below 11,000 feet where the stars are bright. The problem with the eye is that it can only resolve a minimum sized image, and a star in real time may not have enough light to for the retina to resolve. A CCD or photo plate, however, is limited only by the technology.

[watch24]

The minimum-sized image the naked eye, if I recall correctly, is 2 arc seconds (maybe 0.2??), which is much bigger than a star's dia. The atmosphere spreads starlight to a large enough size for the eye to see. My question is, if the point of light is smaller than the eye can resolve, can it resolve it by brightness alone without the aid of atmosphere?

--BTW, I'm obviously still trying to figure out how to respond with quotes properly.

[bystander]

How high were you?--I doubt 13,000'. I'm an avid backpacker, but I've always retreated to the valleys to camp below 11,000 feet where the stars are bright. The problem with the eye is that it can only resolve a minimum sized image, and a star in real time may not have enough light to for the retina to resolve. A CCD or photo plate, however, is limited only by the technology.

Not sure, probably not, certainly above 8,000' (Rocky Mtn Natl Park). But the scattering of light by the atmosphere should reduce the amount of light available, not increase it.

Used to be an avid camper. Camped all over Colorado back when you didn't have to stay in registered campgrounds (70's). Only time the stars weren't brilliant was because of weather. Light pollution wasn't much of a problem in the back woods.

[bystander]

--BTW, I'm obviously still trying to figure out how to respond with quotes properly.

Easy way is just click the quote button on the box you're answering, then edit the text.

The minimum-sized image, if I recall correctly, is 2 arc seconds (maybe 0.2??), which isn't very big. Stars are much smaller than the eye's ability to see, but the atmosphere spreads the light to a large enough size for the eye to see. My question is, if the point of light is smaller than the eye can resolve, can it resolve it by brightness alone without the aid of atmosphere?

Don't know about this. That's why I appealed to our resident physicists, Art Neuendorffer and Chris Peterson.

[watch24]

The quote button I figured out, now I just need to figure out the 'edit' part--when I'm not on company time.

I've never treked the CO rockies, but have been all over northern Idaho, North Cascades, and the Beartooths in WY and MT. Love the night sky there--abosolutely nothing around. Packing my 10" would be rough though.

Meanwhile, there's gotta be a physicist, mountaintop astronomer, pilot, or someone who can answer my dumb question--they just better make the font bigger than 2 arc-seconds or I won't be able to read it!

[henk21cm]

The minimum-sized image, if I recall correctly, is 2 arc seconds (maybe 0.2??), which isn't very big.

The resolution of the eye is about 4 arc minutes. Mizar/Alcor is a double star, in Ursa Major, just visible by the naked eye. It is told (urban legend?) that the native Americans used Mizar/Alcor as a test for the eye.

Stars are much smaller than the eye's ability to see, but the atmosphere spreads the light to a large enough size for the eye to see.

The amount of air at 5 km high is approximatedly half the amount of air at sea level. That is an awful lot of air molecules to spread the light. In meteorology the 500 hPa 'level' is in use. That is about 5 km high. Light is scattered by air molecules. The amount of scattering depends on the wave length and air pressure. The pressure where pressure becomes important is at a level what we call vacuum. Blue light is scattered 15 times more than red light. Thats why the sky is blue. Above the main part of the atmosphere the sky is black, see e.g. yuri planet, April 12th. That is a good approximation fpr vacuum.

Next to scattering of light, which makes a star 'twinkle', any optical lens produces an additional scattering of light, called Fraunhofer diffraction. The smaller the diameter of the lens, the wider the diffraction pattern. This diffraction pattern hampers the separation of two adjacent stars. The separation is roughly equal to the wave length of the light divided by the diameter of the lens, expressed in radians.

With a 0.06 m lens at 600 nm wavelength the separation is roughly 1E-5 radians. 1 radian is 3600 x 180 / pi = 206264 arc seconds. The separation is therefore 2 arc seconds. A good test for this separation are epsilon1 and epsilon 2 Lyrae, which are both dounle stars separated by 2.2 arc seconds. A star is far less than 2 arc seconds in size. Our sun at 10 lightyears distance would have a size of 5 milli arc seconds. That is far less than the smearing of light due to Fraunhofer diffraction. Therefore there is no reason why one should not see stars at high altitudes, e.g. Mauna Kea level.

A good test is picture #12 of Serge Brunier, just above the 500 hPa level:

http://www.sergebrunier.com/gallerie/pa ... ce/12.html

The Nikon camera he used is comparable -regarding to optics- to the human eye.

Regards,
Henk

[watch24]

Thanks for the detailed explanation. I got the gist of what you were saying, but I had trouble keeping up with the optics. I'll have to dig out a textbook tonight. Obviously, my recollections of human eye limitations are equally rusty.

Therefore there is no reason why one should not see stars at high altitudes, e.g. Mauna Kea level.

I'd assumed some stars could be seen, but would one expect to have their vision noticably degraded by the thin atmosphere at 4 or 5km? Also would stars be visible out a spaceship window or from an astronauts visor?

Foolish questions I suppose, but questions I've never had answered. I've never seen the stars as clearly out a jetliner window as I do from my car, even when they darken the cabin for long international flights and I sealed myself against the window with a thick dark blanket. And the some of the best naked eye observation has been near sea level on the back side of Maui and the Osa Peninsula in Costa Rica--better even than wilderness mountaintops in remote Montana. (maybe they just seemed better because I wasn't shivering) Anyway...burning questions of a child coming from an old man.

Thanks again.

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[watch24]

A good test is picture #12 of Serge Brunier, just above the 500 hPa level:

http://www.sergebrunier.com/gallerie/pa ... ce/12.html

The Nikon camera he used is comparable -regarding to optics- to the human eye.

I looked at the image. Spectacular, and I have to admire the fortitude to even take such a shot (almost 20,000'!). Unfortunately, being a time exposure, it doesn't answer my question about naked-eye observation at those altitudes.

Marty

[Axel]

But but guys, that helicoid-looking cloud is really interesting! Is this a well-known phenomenon?

[Mac Coak]

Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

[Mac Coak]

SO - by this discussion one is led to believe that if there is life in the universe on some other planet that has a very thin atmosphere then that life would not "see" any stars or other planets other than their own star. APOD 20 November 2007 (picture taken with a camera but not a time exposure) shows a dark background "sky" behind Earth, the sun is behind the camera obviously.

[watch24]

Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

A little disturbing isn't it? "All" the short-exposure shots I've ever seen from space (and the moon) are also black-skied, but I've never seen an explanation. It could be due to the bright foreground subjects (planets or orbital craft), but I suspect it's due to the lack of atmospheric dispersion of light. It would be nice to get a confirmation from someone who knows first-hand.

[bystander]

Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

A little disturbing isn't it? "All" the short-exposure shots I've ever seen from space (and the moon) are also black-skied, but I've never seen an explanation. It could be due to the bright foreground subjects (planets or orbital craft), but I suspect it's due to the lack of atmospheric dispersion of light. It would be nice to get a confirmation from someone who knows first-hand.

This discussion was had sometime ago, haven't found the link. I think the explanation was oversaturation from the bright foreground. Same with pictures taken on the moon. But, from what henk21cm says, there should be no problem with ISS astronauts seeing stars.

[henk21cm]

Unfortunately, being a time exposure, it doesn't answer my question about naked-eye observation at those altitudes.

G'day Marty,

I cannot answer your question from first hand experience. I live 5 m below see level, and the highest point i ever saw a star was when i was just a few hundreds of meters high.

There are reasons which tell me that the height and thus air pressure is of no importance. We could ask Serge Brunier, he has been there.

Each optical element (mirror or lens) has to endure Airy diffraction. The separation angle i told you yesterday is roughly the angle caused by this type of diffraction. See e.g. http://en.wikipedia.org/wiki/Airy_disc. Even the eye has Airy diffraction. The Airy angle in our eye will be roughly 600 nm / 4 mm = 150 micro radians. The radius of our eye is 2.5 cm. Such an angle will lead to a distance on the retina of 1.5E-4 * 2.5E-2 = 4 E-6 m.

Our eye has approximatedly 100 millions light sensitive cells. The surface of the retina is approximately 5 square centimeters. Each cell will, on average, occupy roughly 5E-8 square centimeters, that is 5E-12 square meters. The size (length, width, radius) of such a cell is therefore about 2E-6 m, that is the same order of magnitude as the center part of the Airy disk. Thus the limitations of optics (i.e. ideal optics) will cause diffraction of a point source, and spread the light out over at least one cell.

So the projection of a point source on the retina 'always hits a light sensitive cell', it never falls in between of the cells. That would be the obvious reason why you would miss a star. You can compare it with a bed of nails, as used by some artists/performers. When you drop from a few meters high a marble on the bed of nails, the marble has a chance to fall between the nails. Now we add optics to the equation. The limitation to optics (Airy diffraction) makes the marble larger: they become melons. A melon -when dropped on a bed of nails- will always hit a nail.

Whether the light is diffracted by the atmosphere, is of no importance, since not the atmosphere is the dominant player, the optical limitations in the human eye are.

I read in a publication of the JPL about the Spitzer telescope (Bachus, Velusamy, Thompson and Arballo, 2005). The Airy diffraction, as projected on the CCD, has a size which is comparable with the size of one element of the CCD. This situation is the same as in the human eye. Much smaller CCD elements are useless, since the Airy pattern would spread out the light over many cells, not adding information.


Regards,
Henk

[watch24]

The size (length, width, radius) of such a cell is therefore about 2E-6 m, that is the same order of magnitude as the center part of the Airy disk. Thus the limitations of optics (i.e. ideal optics) will cause diffraction of a point source, and spread the light out over at least one cell.

So the projection of a point source on the retina 'always hits a light sensitive cell', it never falls in between of the cells. That would be the obvious reason why you would miss a star.

So assuming you are correct in all your calculations (and given my expertise and time resources, I must do so), then the physics indicates we should be able to see stars even in the vacuum of space--assuming our eyes didn't pop out of their sockets first.

Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

My hats of to your expertise in physics, but now maybe we need a neurologist -

Marty

[henk21cm]

Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

G'day Marty,

If light is spread out over more cells, the intensity per cell will decrease. The same amount of energy is spread out over a larger surface, so the energy flux decreases. When an object is too dim, when it emits too little light, the eye can not trigger on the low amount of photons collected. As a result we do not see any stars less luminous than magnitude 5.5 to 6 with the naked eye. When light is spread out over many cells in our eyes, more than under normal circumstances, the energy flux decreases beyond a point that it is too low to trigger our cells.

A simple experiment you can perform yourself at any altitude: when you wear glasses, or contact lenses, look up the sky and count stars in a particular region of the sky, with these glasses or contact lenses. This we call normal situation. Then remove your glasses or contact lenses and count again. We call that blurred. If you don't wear glasses, look up the sky without glasses. We call that normal. Then borrow those of your parents, sisters, brothers or wive, (non plural!) and repeat the count with those borrowed ones, which we call blurred.

My prediction is that you will count more stars under normal conditions than when under blurred conditions: removing your normal glasses or adding glasses that are not yours.

but now maybe we need a neurologist -

I am not such a person. The only one i know and which comes close to a neurologist is a professor in Eindhoven, "Dept. of Biomedical Imaging". I could e-mail him and hope he has an answer and is willing to give the answer. It may take some time, specially since next week is spring holiday.

[henk21cm]

Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

G'day Marty,

Eating is proof of the pudding, Serge Brunier, creator of the APOD of 2008-04-23, has been at high altitude and knows from first hand whether stars are visible at high altitude. I wrote him an e-mail and he has an answer. His reply:

Bien sur, les etoiles sont visibles !!!!!!!!!!!!!!!!!!!!!!!!!!!

A 3000 metres environ, le ciel est fantastique.
A 4000 metre, 5000 metre, c est plus difficile, du fait du manque d oxygene
dans le sang.

Mais, dans tous les cas, les etoiles sont visibles en tres haute altitude...

La scintillation n a rien a voir la dedans : c est absolument ridicule...

Translated: Indeed, the stars are visible!!!!!!!!!!!! The sky at about 3000m is magnificent. At 4000 to 5000 m it more difficult, due to the lack of oxygen in your blood. But in all these cases the stars are visible at these high altitudes. Atmospheric scintillation /dispersion has nothing to do with vision in that matter, that is absolutely ridiculous.