<p>There is no clear-cut boundary and no scientific definition of &quot;sky&quot;. There is no unique and undisputed upper boundary of Earth's atmosphere as it gradually gets thinner with hight, transitioning into the inteplanetary space. Yet arguably, birds are in the sky, and they fly MUCH lower. Thus what exactly is meant with the word 'sky' depends on the speaker and the context.</p>
<p>There are a few vague definitons for 'edge of space', like the <a href="https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line" rel="nofollow noreferrer">Karman line</a>. That defines space to start from 100km upwards. If you want to refer to the atmosphere: usually all of the atmosphere above you is 'the sky'. Yet if you want to refer to something there in particular, you use the layers of the atmosphere which you already mentioned with 'troposphere'.</p>

CC BY-SA 4.0
2021-12-02T17:11:38.093Z
Thanks, that's kind of what I thought but I wasn't sure if there was an official definition. Perhaps this would be a good philosophy question, too!
ehelmick93
CC BY-SA 4.0
2021-12-02T16:36:36.053Z
2021-12-02T16:36:36.053Z
planetmaker
<h3>Start from the basics, the sky we see:</h3>
<blockquote>
<p>If the sky is the limit...where does the sky start?</p>
</blockquote>
<p>I know you don't mean where does the &quot;blue part&quot; begin, but for completeness I'll direct readers to:</p>
<ul>
<li><a href="https://earthscience.stackexchange.com/q/16652/6031">Where does the sky's blueness come from; at what altitudes is it being produced?</a></li>
<li><a href="https://space.stackexchange.com/q/54083/12102">Why does Unity look transparent?</a></li>
<li><a href="https://physics.stackexchange.com/q/650898/83380">How much blue sky comes from each altitude; dependence of atmosphere's dielectric constant and isothermal compressibility with altitude?</a></li>
</ul>
<h3>The Karman line (not):</h3>
<p>Yes there is no easily defined end to the atmosphere, but there are some limits and lines that are handy to talk about.</p>
<p>There's the <a href="https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line" rel="nofollow noreferrer">Karman line</a> at about 100 km. There's certainly plenty of atmosphere above it, but it does roughly delineate where we can think of things as actually pretty much still orbiting versus pretty much entering/reentering the atmosphere (i.e. doomed to burn up very soon)</p>
<ul>
<li><a href="https://space.stackexchange.com/q/29721/12102">What would a &quot;Kármán plane&quot; look like, a bird, or a plane?</a></li>
<li><a href="https://space.stackexchange.com/q/38036/12102">When is/will be the symposium to revisit the Karman line and consider the &quot;McDowell line&quot;?</a></li>
<li><a href="https://space.stackexchange.com/q/29704/12102">Have spacecraft ever dipped below the Karman line and then safely continued spaceflight?</a></li>
</ul>
<p>There's also <a href="https://space.stackexchange.com/q/21604/12102">a knee there</a> above which the density falls off much more slowly. There's a transition between diatomic and monatomic oxygen there and also it's where the <a href="https://en.wikipedia.org/wiki/Turbopause" rel="nofollow noreferrer">turbopause begins</a> as pointed out by <a href="https://space.stackexchange.com/a/49678/12102">@DavidHammen's answer</a> where the motion of molecules becomes more ballistic and gases no longer easily mix by collisions.</p>
<h3>The absolute highest limit is the Hill sphere:</h3>
<p>But that's not the end of the atmosphere. There are gas atoms all the way out to the Moon, but they are very rare. At some point you have to stop calling them &quot;Earth's atmosphere&quot; because their motion is no longer bound to Earth's gravity; the gravity of the Moon and the Sun take over.</p>
<p>That limit can be considered the Earth's <a href="https://en.wikipedia.org/wiki/Hill_sphere" rel="nofollow noreferrer">Hill sphere</a> but you have to calculate it for the Earth-Moon system rather than the Sun-Earth system. Its distance is very roughly where the Lagrange L1 point is for a given system. <a href="https://en.wikipedia.org/wiki/Lagrange_point#Solar_System_values" rel="nofollow noreferrer">For Earth-Moon that's about 300,000 km</a> from the Earth.</p>

CC BY-SA 4.0
2021-12-03T03:39:08.147Z
2021-12-03T03:39:08.147Z
uhoh
<p>Does the &quot;sky&quot; start anywhere that the land is not touching? Does it start after the troposphere? After the atmosphere?</p>

CC BY-SA 4.0
2021-12-03T03:48:33.687Z
The whole point of the expression is that the sky is always above you.  You never reach the sky no matter how high you go.  The answer is "**never**" - but this is not about astronomy, but about English.
James K
CC BY-SA 4.0
2021-12-04T02:23:28.453Z
I’m voting to close this question because this is not an astronomy question but is a about an English phrase.
Nilay Ghosh
CC BY-SA 4.0
2021-12-07T06:00:09.750Z
Where does the sky end and space begins seems like it's arguably on-topic here in Astronomy (adding the amateur observing tag), though Earth Science might be as good a place as any to migrate if it is closed as off-topic. Certainly it will be answerable with facts and citations to the vote to close because it "s likely to be answered with opinions rather than facts and citations." makes no sense.
uhoh
CC BY-SA 4.0
2021-12-07T06:24:46.893Z
@uhoh,  why do you think the sky stops when space starts?  I disagree with that opinion. I think the sky includes space. But perhaps that is just my opinion.
James K
CC BY-SA 4.0
2021-12-07T07:13:59.533Z
@JamesK ya perhaps "sky" has several or more meanings and even more nuances. During the day I think that the sky is the blue stuff, seen looking up from the surface, sideways from high altitude flights and looking down from orbit. But the night sky extends *quite a ways further!*
uhoh
<amateur-observing><planetary-atmosphere>
If the sky is the limit...where does the sky start?
ehelmick93
# If the sky is the limit...where does the sky start?

(Asked by: username_0 on 2021-12-02)

Does the "sky" start anywhere that the land is not touching? Does it start after the troposphere? After the atmosphere?

**Tags:** amateur-observing, planetary-atmosphere
> **Comments:**
> **username_1**: The whole point of the expression is that the sky is always above you.  You never reach the sky no matter how high you go.  The answer is "**never**" - but this is not about astronomy, but about English.  
> **username_2**: I’m voting to close this question because this is not an astronomy question but is a about an English phrase.  
> **username_3**: Where does the sky end and space begins seems like it's arguably on-topic here in Astronomy (adding the amateur observing tag), though Earth Science might be as good a place as any to migrate if it is closed as off-topic. Certainly it will be answerable with facts and citations to the vote to close because it "s likely to be answered with opinions rather than facts and citations." makes no sense.  
> **username_1**: @uhoh,  why do you think the sky stops when space starts?  I disagree with that opinion. I think the sky includes space. But perhaps that is just my opinion.  
> **username_3**: @JamesK ya perhaps "sky" has several or more meanings and even more nuances. During the day I think that the sky is the blue stuff, seen looking up from the surface, sideways from high altitude flights and looking down from orbit. But the night sky extends *quite a ways further!*  

---

## 2 answers

### Answer by username_4 on 2021-12-02. Score: 3 | Accepted answer

There is no clear-cut boundary and no scientific definition of "sky". There is no unique and undisputed upper boundary of Earth's atmosphere as it gradually gets thinner with hight, transitioning into the inteplanetary space. Yet arguably, birds are in the sky, and they fly MUCH lower. Thus what exactly is meant with the word 'sky' depends on the speaker and the context.

There are a few vague definitons for 'edge of space', like the [Karman line](https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line). That defines space to start from 100km upwards. If you want to refer to the atmosphere: usually all of the atmosphere above you is 'the sky'. Yet if you want to refer to something there in particular, you use the layers of the atmosphere which you already mentioned with 'troposphere'.

> **Comments:**
> **username_0**: Thanks, that's kind of what I thought but I wasn't sure if there was an official definition. Perhaps this would be a good philosophy question, too!  

---

### Answer by username_3 on 2021-12-03. Score: 0

### Start from the basics, the sky we see:

> If the sky is the limit...where does the sky start?

I know you don't mean where does the "blue part" begin, but for completeness I'll direct readers to:

* [Where does the sky's blueness come from; at what altitudes is it being produced?](https://earthscience.stackexchange.com/q/16652/6031)
* [Why does Unity look transparent?](https://space.stackexchange.com/q/54083/12102)
* [How much blue sky comes from each altitude; dependence of atmosphere's dielectric constant and isothermal compressibility with altitude?](https://physics.stackexchange.com/q/650898/83380)

### The Karman line (not):

Yes there is no easily defined end to the atmosphere, but there are some limits and lines that are handy to talk about.

There's the [Karman line](https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line) at about 100 km. There's certainly plenty of atmosphere above it, but it does roughly delineate where we can think of things as actually pretty much still orbiting versus pretty much entering/reentering the atmosphere (i.e. doomed to burn up very soon)

* [What would a "Kármán plane" look like, a bird, or a plane?](https://space.stackexchange.com/q/29721/12102)
* [When is/will be the symposium to revisit the Karman line and consider the "McDowell line"?](https://space.stackexchange.com/q/38036/12102)
* [Have spacecraft ever dipped below the Karman line and then safely continued spaceflight?](https://space.stackexchange.com/q/29704/12102)

There's also [a knee there](https://space.stackexchange.com/q/21604/12102) above which the density falls off much more slowly. There's a transition between diatomic and monatomic oxygen there and also it's where the [turbopause begins](https://en.wikipedia.org/wiki/Turbopause) as pointed out by [@DavidHammen's answer](https://space.stackexchange.com/a/49678/12102) where the motion of molecules becomes more ballistic and gases no longer easily mix by collisions.

### The absolute highest limit is the Hill sphere:

But that's not the end of the atmosphere. There are gas atoms all the way out to the Moon, but they are very rare. At some point you have to stop calling them "Earth's atmosphere" because their motion is no longer bound to Earth's gravity; the gravity of the Moon and the Sun take over.

That limit can be considered the Earth's [Hill sphere](https://en.wikipedia.org/wiki/Hill_sphere) but you have to calculate it for the Earth-Moon system rather than the Sun-Earth system. Its distance is very roughly where the Lagrange L1 point is for a given system. [For Earth-Moon that's about 300,000 km](https://en.wikipedia.org/wiki/Lagrange_point#Solar_System_values) from the Earth.


---

