# How far away is the sky "blue"? **URL:** \ **Category:** Factual Questions\ **Created:** [September 13, 2000, 6:23pm UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054 "2000-09-13T18:23:33Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![SuaSponte](https://avatars.discourse-cdn.com/v4/letter/s/67e7ee/32.png) [@SuaSponte](https://boards.straightdope.com/u/SuaSponte)\ **Post date:** [September 13, 2000, 6:23pm UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/1 "2000-09-13T18:23:33Z")
I know why the sky is blue (scattering of light, etc.) My question is - how far away does the visible horizon have to be before we humans “see” the blue? For example, I can look all the way down Broadway from the street outside my office, and the view doesn’t get bluer in the distance (and please, no smog jokes ;)) Final thought: Does the background make a difference? From inside my office, I can see distant hills in NJ. They look bluish and indistinct. If I were on the plains at the base of the Rockies, would I have to go farther away to “see the blue” than I would at the beach? Sua ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [September 14, 2000, 12:38am UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/2 "2000-09-14T00:38:32Z")
Atmospheric haze can make a big difference in seeing distant objects as blue. Under some conditions distant mountains will be blue but quit distinct. At sea it’s often very hard to see distant objects clearly but not always. Hard to quantify though because normal photo film is more sensitive to UV light than your eyes are. Often what looks clear in the viewfinder is hazy in the photo. A stright UV haze filter helpos but sometimes a faintly reddish “skylight” filter is needed. When it gets extremely bad it’s time to switch to B&W film with a deep red filter. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [September 14, 2000, 12:43am UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/3 "2000-09-14T00:43:38Z")
I’ve seen pictures taken from SR-71s (they fly so high the pilots wear astronaut’s wings) at the edge of space, and it’s blue _below the plane_. All you can see is the black emptyness of space and a few stars, then the camera looks _down_ at blue sky. Very surreal. I’ll see if I can dig one of those pics up on the web. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [September 14, 2000, 12:52am UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/4 "2000-09-14T00:52:46Z")
I think you’re mistaken on the astronaut wings for SR-71 pilots. During the X-15 program the AF set the threshold at 50 miles (statute I think) to earn those wings. The SR-71 flies high but not half high enough for that. SR-71 do wear pressurized gear similar to space _suits_ though. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [September 14, 2000, 1:28am UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/5 "2000-09-14T01:28:49Z")
Padeye: I seem to remember hearing that on the Discovery Channel, but I can’t find a site with a cite. I’ll keep looking. I did find [this](http://www.sr-71.org/blackbird/miscinfo.htm), which says the suit is the same one the space shuttle crews wear. Getting back to the original subject, if you wanna try to find one of the edge-of-space pics I mentioned, use NASA’s picture search engine thingy [here](http://nix.nasa.gov/nix.cgi?s=a). I looked, but haven’t sifted thru all the results yet. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [September 14, 2000, 1:37am UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/6 "2000-09-14T01:37:20Z")
Previous post kinda didn’t work. I meant to say this: (insert the following at “I did find” in previous post) I did find [http://www.sr-71.org/blackbird/miscinfo.htm](http://www.sr-71.org/blackbird/miscinfo.htm), which says the suit is the same one the space shuttle crews wear. Getting back to the original subject, if you wanna try to find one of the edge-of-space pics I mentioned, use NASA’s picture search engine thingy [here](http://nix.nasa.gov/nix.cgi?s=a). I looked, but haven’t sifted thru all the results yet. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [September 14, 2000, 1:40pm UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/7 "2000-09-14T13:40:21Z")
> [@](#): > > \*Originally posted by SuaSponte \* > \*\* > From inside my office, I can see distant hills in NJ. They look bluish and indistinct. > \*\* IMHO… I suspect that this is part of your answer. The distance to those mountains is where the atmospheric scattering of blue light begins to become significant. True-blue would be farther than that (probably farther than the horizon?) ---
**Author:** ![SuaSponte](https://avatars.discourse-cdn.com/v4/letter/s/67e7ee/32.png) [@SuaSponte](https://boards.straightdope.com/u/SuaSponte)\ **Post date:** [September 14, 2000, 2:16pm UTC](https://boards.straightdope.com/t/how-far-away-is-the-sky-blue/33054/8 "2000-09-14T14:16:50Z")
> [@](#): > > \*Originally posted by Padeye \* > \*\*Atmospheric haze can make a big difference in seeing distant objects as blue. \*\* Am I correct in interpreting this to mean that pollution affects the “blueness”, and/or how far away objects are before they get a blue tint? Sua