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wd40
2011-Apr-24, 10:17 PM
How much brighter and clearer would the heavens appear at night to an unaided eye (from inside an optically perfect space helmet) standing on the Earth if there was no atmosphere at all?

Would the unaided human eye be able to see many more stars, or just a few more? Would it be able to resolve better the Andromeda galaxy?

Or is the 80% Nitrogen of the existing air already so optically clear, that apart from some twinkling, the night sky viewed in a vacuum wouldn't seem much different?

Grashtel
2011-Apr-24, 10:56 PM
How much brighter and clearer would the heavens appear at night to an unaided eye (from inside an optically perfect space helmet) standing on the Earth if there was no atmosphere at all?

Would the unaided human eye be able to see many more stars, or just a few more? Would it be able to resolve better the Andromeda galaxy?

Or is the 80% Nitrogen of the existing air already so optically clear, that apart from some twinkling, the night sky viewed in a vacuum wouldn't seem much different?
Next time you have a clear night see for yourself, look at how much brighter the stars straight up are than the ones about thirty degrees from the horizon the light from which has gone through twice as much air. Based on the time I have done it the answer is not enough to be noticeable.

slang
2011-Apr-24, 11:23 PM
How much brighter and clearer would the heavens appear at night to an unaided eye (from inside an optically perfect space helmet) standing on the Earth if there was no atmosphere at all?

IIRC that question was answered in your thread about how location would influence astronomy in societies over the centuries. I don't remember its title, maybe you do?

astromark
2011-Apr-25, 09:20 AM
A dark sky sight... can confuse the city dweller. From personal experience tis' true.

But to remove the atmosphere would improve the light image...

This is why the 'Hubble' space scope has been so successful.

At only 2.4 metres it has given us images unavailable from the surface from much larger scopes.

However doing this would anger a awful lot of your friends....:razz:

ngc3314
2011-Apr-25, 11:55 AM
The stars would be brighter, certainly. Across the visible band, atmospheric absorption even vertically ranges from about 10-25%, increasing toward the horizon. In addition, getting rid of airglow darkens the "background" sky, although that's not as big an effect as one would think at first (in the visible - this gain becomes enormous in the near-IR). While we tend to think of the sky from a clear mountain location as inky black, in fact almost half of that sky brightness is sunlight and starlight (yeah, redundant) scattered from interplanetary and interstellar dust which sets a floor level even above the atmosphere. The number of stars to be seen does rise quickly as one can go fainter. The Tycho catalog from HIPPARCOS lists (http://www.stargazing.net/david/constel/howmanystars.html) 8800 stars brighter than V=6.5, and 18,000 from 6.5 to 7.5. Following the Euclidean ~3x increase in number of stars per one-magnitude bin, a typical 0.5-magnitude improvement in limiting magnitude would mean one could see 23,000 stars around the sky; since there would be no elevation effect, at one time one could see half of those compared to the often-quoted (and individually variable) 2000 stars at once from "good" locations (for a ~6x gain).

Hornblower
2011-Apr-25, 12:10 PM
As has been pointed out already, the bright stars would be only slightly brighter. The big difference would be a significantly darker background, because the upper-atmosphere airglow would be eliminated. The Milky Way would show higher contrast between its bright and dark structures, and its faint extremities would be seen over a wider area. The zodiacal light and gegenschein would be more vivid. Faint stars at the threshold of visibility would be more numerous. All of these faint features would be unchanged right down to the horizon, instead of fading out the way they do as seen from the ground even in the best locations.

A few years ago I made some dark sky simulations with my computer artwork and a good CRT monitor, which could go almost totally black. I made progressively darker swatches and measured the light level with an exposure meter as far as it would go. For darker patches I used a neutral density filter over a previously calibrated swatch to evaluate the darker patch. I was able to set a reference swatch to the published skyglow levels quoted in Sky and Telescope. In a blacked out room with my eyes fully dark adapted, the skyglow stood out boldly from the black background. It gave me a good idea of the potentially increased Milky Way contrast I could expect out in space.

Unfortunately my present flat screen monitor cannot do this demonstration. For all of its virtues it is incapable of true black. In a dark room it looks more like the light pollution over greater Washington. I am glad I still have the old CRT for old times sake.

I remember a childhood view of the summer Milky Way in rural Texas nearly 50 years ago. The Milky Way and the profusion of faint stars actually made it harder to see the familiar pattern of Cygnus. Deneb did not look any brighter, and it did not stand out the way it does from a light-polluted sky. I would predict that this effect would be even more pronounced out in space.

chornedsnorkack
2011-Apr-25, 12:27 PM
How does plasma screen black compare against LCD and CRT blacks?

Also, how much skyglow would come from an atmosphere being 100 mbar oxygen and little or no nitrogen?