# What is the lower boundary of the "sky"? **URL:** \ **Category:** Factual Questions\ **Created:** [July 6, 2002, 11:47pm UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700 "2002-07-06T23:47:11Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Cubsfan](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@Cubsfan](https://boards.straightdope.com/u/Cubsfan)\ **Post date:** [July 6, 2002, 11:47pm UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/1 "2002-07-06T23:47:11Z")
My little girl and I were walking to check the mail tonight and she mentioned something about trying to touch the sky and it got me thinking. Does the sky end where the earths crust begins? Does this mean that the air in my house is “sky”? Or is there a lower boundary to the “sky”? Is all air part of the “sky” even if it is seperated from the rest of it by the walls and ceiling of my house? ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [July 7, 2002, 12:25am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/2 "2002-07-07T00:25:46Z")
Yes, all _outside_ air is sky. Not in buildings, or caves, or such, but in the open. Don’t remember where I read that, but it make’s sense to me. In a poetic sense anyway. 🙂 Tell your daughter that the sky touches her all over when she’s outside. Peace, mangeorge ---
**Author:** ![JRootabega](https://avatars.discourse-cdn.com/v4/letter/j/8e7dd6/32.png) [@JRootabega](https://boards.straightdope.com/u/JRootabega)\ **Post date:** [July 7, 2002, 12:55am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/3 "2002-07-07T00:55:18Z")
Y’know, on second though, you probably _shouldn’t_ tell your daugher that… 😉 ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [July 7, 2002, 1:14am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/4 "2002-07-07T01:14:37Z")
> [@](#): > > \*Originally posted by mangeorge \* > \*\*Yes, all _outside_ air is sky. Not in buildings, or caves, or such, but in the open. Don’t remember where I read that, but it make’s sense to me. In a poetic sense anyway. 🙂 > Tell your daughter that the sky touches her all over when she’s outside. > Peace, > mangeorge \*\* True. The sky, at least on a clear day, is just all the scattered, blue light that comes to us from all directions. I suppose at night the sky is the starry matrix. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [July 7, 2002, 1:16am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/5 "2002-07-07T01:16:19Z")
OK, if all outside air is ‘sky’, what is the absolute limit of vision on a perfectly clear day (close to zero percent humidity, as on a winter day)? At what point does the blue coloration from diffracted sunlight block continued sight? ---
**Author:** ![Manduck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manduck/32/256_2.png) [@Manduck](https://boards.straightdope.com/u/Manduck)\ **Post date:** [July 7, 2002, 1:30am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/6 "2002-07-07T01:30:24Z")
You can see the sun. So you can see at least that far. As for the sky, I don’t think there is an “official” boundary. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [July 7, 2002, 3:24am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/7 "2002-07-07T03:24:05Z")
> [@](#): > > \*Originally posted by Derleth \* > \*\*OK, if all outside air is ‘sky’, what is the absolute limit of vision on a perfectly clear day (close to zero percent humidity, as on a winter day)? At what point does the blue coloration from diffracted sunlight block continued sight? \*\* Well, in the absence of smoke from forest fires, when flying at about 10000 ft. you can see both Charleston Peak, just west of Las Vegas, NV and Humphries Peak just north of Flagstaff, AZ from halfway in between them. I can nearly always see past the peak of Mt. Whitney from here and it is about 80 mi. away. Flight Service Stations often report visibilities over 100 mi. in the southwest on a clear day. ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [July 7, 2002, 3:34am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/8 "2002-07-07T03:34:41Z")
> [@](#): > > \*Originally posted by Stinkpalm \* > \*\*My little girl and I were walking to check the mail tonight and she mentioned something about trying to touch the sky and it got me thinking. \*\* “Why is my little girl listening to Jimi Hendrix music?” ---
**Author:** ![snermy](https://avatars.discourse-cdn.com/v4/letter/s/d6d6ee/32.png) [@snermy](https://boards.straightdope.com/u/snermy)\ **Post date:** [July 7, 2002, 3:37am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/9 "2002-07-07T03:37:24Z")
I’m going to go again prevaling theory and say it’s all sky until it hits the ground. Why is why we say “[_up_ in the sky” to indicate things like planes, birds, and supermen… ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [July 7, 2002, 4:14am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/10 "2002-07-07T04:14:52Z")
I think the sky’s limits are poetic, rather than physical. I think it’s only sky when you can’t touch it, otherwise it’s air. So you have to touch the sky with your mind, or your heart, or you imagination. And it doesn’t stop being sky when it stops being air, either, since the stars are definitely in the sky, and so are the sun, and the moon. That means the space is part of the sky. The sky is what is over you, if you are not inside, or underground. But, if you fly, then you are in the sky, until you fall out of it. ## Tris “It should be possible to explain the laws of physics to a barmaid.” ~ Albert Einstein ~ “You should see the place where Einstein used to drink!” ~ Triskadecamus ~ ---
**Author:** ![JohnClay](https://avatars.discourse-cdn.com/v4/letter/j/c37758/32.png) [@JohnClay](https://boards.straightdope.com/u/JohnClay)\ **Post date:** [July 7, 2002, 4:20am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/11 "2002-07-07T04:20:05Z")
> [@](#): > > \*Originally posted by Derleth \* > \*\*OK, if all outside air is ‘sky’, what is the absolute limit of vision on a perfectly clear day (close to zero percent humidity, as on a winter day)? At what point does the blue coloration from diffracted sunlight block continued sight? \*\* I think it just depends on how bright the object is (as seen from the earth). We would be able to see stars in a blue sky if they are bright enough (due to their distance, they’d have to be really, really, really bright). Maybe if there was a big supernova or something then we’d be able to see it in the day-time. ---
**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\ **Post date:** [July 7, 2002, 4:49am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/12 "2002-07-07T04:49:24Z")
[The sky is a revisionist myth](http://www.adequacy.org/?op=displaystory;sid=2001/11/25/101153/25) ---
**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\ **Post date:** [July 7, 2002, 4:55am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/13 "2002-07-07T04:55:34Z")
[The sky is a revisionist myth](http://www.adequacy.org/?op=displaystory;sid=2001/11/25/101153/25) ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [July 7, 2002, 5:09am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/14 "2002-07-07T05:09:09Z")
_The sky is what is over you, if you are not inside, or underground. But, if you fly, then you are in the sky, until you fall out of it._ I’d say that nails it. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [July 7, 2002, 5:29am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/15 "2002-07-07T05:29:57Z")
> [@](#): > > \*Originally posted by Shalmanese \* > \*\*[The sky is a revisionist myth](http://www.adequacy.org/?op=displaystory;sid=2001/11/25/101153/25) \*\* :smack: Bullshit. Not you, **Shalmanese** , but **em**. At the link you posted. The sky is all that enrobes the earth, just as **Triskadecamus** so eloquently stated. Try telling **my little girl** there is no sky. Peace, mangeorge ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [July 7, 2002, 5:41am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/16 "2002-07-07T05:41:21Z")
[ul]\*\*[sup]the[/sup]\*\*\*\* HORIZON \*\*[/ul] ---
**Author:** ![Cubsfan](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@Cubsfan](https://boards.straightdope.com/u/Cubsfan)\ **Post date:** [July 7, 2002, 6:39am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/17 "2002-07-07T06:39:34Z")
So basically the sky is not measurable. It reaches from the top of the toppest structure to infinity? I made up the word “toppest” just for this post. Pretty sweet eh? ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [July 7, 2002, 6:54am UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/18 "2002-07-07T06:54:16Z")
As long as we’re being sweet, Infinity = “forever and forever”. 🙂 “Structure”, anyone? ---
**Author:** ![ianzin](https://avatars.discourse-cdn.com/v4/letter/i/ed655f/32.png) [@ianzin](https://boards.straightdope.com/u/ianzin)\ **Post date:** [July 7, 2002, 3:14pm UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/19 "2002-07-07T15:14:53Z")
I just wanted to tell **Stinkpalm** I think ‘toppest’ is a fine word, and it’s remarkable that it hasn’t been coined before. Isn’t there another ongoing thread about words you’ve invented? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 7, 2002, 7:19pm UTC](https://boards.straightdope.com/t/what-is-the-lower-boundary-of-the-sky/117700/20 "2002-07-07T19:19:55Z")
The problem here is that folks in this thread seem to think that the sky is a three-dimensional object, when it’s only two dimensional. Further, its two dimensions are both angular dimensions, not length-type directions. If you ask “how big is the sky”, for instance, the answer would not be in kilometers, but something like “180 degrees across, and 360 degrees around”. The lower boundary of the sky is the horizon, a one-dimensional object which is likewise measured in angles. The distance to the sky is the same in all directions, but is otherwise not well-defined. Alternatively, one might call the sky a sphere of infinite radius, and the horizon a circle of the same radius, but this doesn’t make it any easier to touch.