<p><strong>Partial answer:</strong> the planet will appear roughly three times as bright as the Moon does in our daytime sky! (if the sky is like ours) and the part of the disk not sunlit will look &quot;invisible&quot; or &quot;missing&quot;.</p>
<p>How big it will appear, or the fraction illuminated vs time will depend on details like the ratio of the moon's orbital distance to the planet's diameter. I'll leave that for other answers to address, but it's going to vary <em><strong>a bit like</strong></em> the phases of Earth's moon as seen from Earth.</p>
<p><a href="https://i.sstatic.net/0rBPP.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0rBPP.jpg" alt="A Christmas day morning view of the Moon with a blue sky taken at 8:53am from the village of Trimingham, Norfolk, England. Date Taken on 25 December 2018" /></a></p>
<p><a href="https://commons.wikimedia.org/wiki/File:-2018-12-25_The_moon_in_the_early_morning,_Christmas_Day.JPG" rel="nofollow noreferrer">A Christmas day morning view of the Moon with a blue sky taken at 8:53am from the village of Trimingham, Norfolk, England. Date: Taken on 25 December 2018</a></p>
<hr />
<p>This is an interesting question and it can actually be answered fairly independently of which moon, which orbit or which gas giant!</p>
<p>What we really need to nail down is <em>which atmosphere!?</em></p>
<p>Since both the moon's atmosphere and the planet are illuminated by the same Sun at essentially the same distance, that drops out of the problem. We just need to look at the <em>ratio</em> of the sky brightness to the planet's apparent surface brightness.</p>
<p>Luckily our Moon presents us with an excellent example.</p>
<ul>
<li>The same way that a wall does not get brighter or dimmer as we walk towards or away from it, the average <a href="https://en.wikipedia.org/wiki/Surface_brightness" rel="nofollow noreferrer">surface brightness</a> of a resolved, extended object like the disk of a planet or moon does not really increase or decrease if we are closer/further from it. The total light varies as 1/r<sup>2</sup> because the <em>apparent size</em> varies like that.</li>
<li>The albedo of the Moon is pretty low actually, roughly of order 10%. It's actually slightly orange/brown because it's less reflective in blue and more reflective in red. For more on that see <a href="https://astronomy.stackexchange.com/a/28862/7982">this answer to <em>Why is this moon red?</em></a></li>
<li>The albedo of gas giants in our solar system is 3 to 4 times higher than the Earth's moon!</li>
<li>At the same time, the light in Earth's <a href="https://en.wikipedia.org/wiki/Diffuse_sky_radiation" rel="nofollow noreferrer">Diffuse sky radiation</a> (i.e. blue sky) is quite blue due to <a href="https://en.wikipedia.org/wiki/Rayleigh_scattering" rel="nofollow noreferrer">Rayleigh scattering</a> so while it might interfere with the blue hues of the planet's disk, the rest of the visible spectrum will shine brightly through. If the planet somehow had a &quot;great blue spot&quot; it might be nearly invisible behind a bright blue sky. For more on that see <a href="https://space.stackexchange.com/q/54083/12102">Why does Unity look transparent?</a> and answers there.</li>
</ul>
<p>So as long as the Moon's atmosphere has a blue sky that isn't overwhelmingly brighter than Earth's,</p>
<h3>The gas giant would have a surface brightness ~3-4 times as bright as the Moon in our sky! (if the sky is like ours)</h3>
<p>And there's the rub. A moon will likely have a lower surface gravity <span class="math-container">$g$</span> than Earth, which means that the <a href="https://en.wikipedia.org/wiki/Scale_height" rel="nofollow noreferrer">scale height</a> of the atmosphere will be proportionally larger: <span class="math-container">$$H = \frac{kT}{mg}$$</span> where <span class="math-container">$m$</span> is the average mass of a molecule of atmosphere.</p>
<p>So if the moon has a surface gravity of 0.1 and everything else is the same (it won't be) then the scale height will be more like 80 kilometers compared to Earth's ~8 km. With an atmosphere 10x thicker the sky will be much much brighter.</p>
<p>So let's hope the two effects cancel; albedo 3-4x higher and Rayleigh scattering 3-4x stronger making the planet's surface brightness relative to the &quot;blue sky&quot; in front of it about the same ratio as the Earth's moon behind our daytime sky.</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>Body</th>
<th>Bond Albedo</th>
</tr>
</thead>
<tbody>
<tr>
<td>Moon</td>
<td>0.11</td>
</tr>
<tr>
<td>Jupiter</td>
<td>0.343</td>
</tr>
<tr>
<td>Saturn</td>
<td>0.342</td>
</tr>
<tr>
<td>Uranus</td>
<td>0.300</td>
</tr>
<tr>
<td>Neptune</td>
<td>0.290</td>
</tr>
</tbody>
</table>
</div>
<p><a href="https://i.sstatic.net/Fv5Bc.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Fv5Bcm.jpg" alt="Day moon (16669185823).jpg" /></a> <a href="https://i.sstatic.net/HPRIW.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HPRIWm.jpg" alt="Daylight moon (4087378758) (3).jpg" /></a></p>
<p><a href="https://i.sstatic.net/oyjN2.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oyjN2m.jpg" alt="Луна в дневное время.jpg" /></a> <a href="https://i.sstatic.net/Bj3z2.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Bj3z2m.jpg" alt="A Day View of Moon.jpg" /></a></p>
<p>Sources (Wikimedia):</p>
<ul>
<li>top row: <a href="https://commons.wikimedia.org/wiki/File:Day_moon_%2816669185823%29.jpg" rel="nofollow noreferrer">1</a>, <a href="https://commons.wikimedia.org/wiki/File:Daylight_moon_%284087378758%29_%283%29.jpg" rel="nofollow noreferrer">2</a></li>
<li>bottom row: <a href="https://commons.wikimedia.org/wiki/File:%D0%9B%D1%83%D0%BD%D0%B0_%D0%B2_%D0%B4%D0%BD%D0%B5%D0%B2%D0%BD%D0%BE%D0%B5_%D0%B2%D1%80%D0%B5%D0%BC%D1%8F.jpg" rel="nofollow noreferrer">3</a>, <a href="https://commons.wikimedia.org/wiki/File:A_Day_View_of_Moon.jpg" rel="nofollow noreferrer">4</a>,</li>
</ul>

CC BY-SA 4.0
2022-07-30T22:49:49.890Z
2022-07-30T22:49:49.890Z
uhoh
<p>If you were standing on the proplanetary side of a habitable moon of a gas giant, and the moon had a thick enough atmosphere to make the sky blue, how would the gas giant look during the day? Would it look faint? Would you see the entire planet or just whatever side is more illuminated by the sun (I'm imagining the sun peaking out from behind the planet and illuminating one side, while the other fades to blue)? How would the planet look during the nighttime, when the proplanetary face of the moon is facing away from the sun and the planet is illuminated? Would it be bright and more saturated? And does this all depend on the density/thickness of the moon's atmosphere?</p>

CC BY-SA 4.0
2022-07-29T06:19:53.873Z
I don't think there could be a definite answer to this question. It is too generic. How a gas giant would appear as seen by its satellite depend on the size of the planet, it's albedo, the brightness of the star, the distance of the star, the distance of the satellite, the thickness and composition of the atmosphere...
Prallax
CC BY-SA 4.0
2022-07-29T06:23:31.210Z
Maybe you could help by editing the question and fixing some of these parameters. You could say, for instance, how does Saturn look like when seen from Titan. Or, how would Jupiter look like if seen from a satellite orbiting at X distance and with the same atmosphere as Earth
Prallax
CC BY-SA 4.0
2022-07-30T17:17:20.200Z
@Uhoh the question specifies a habitable moon.  Which habitable moon in our solar system should Elhammo choose as his example as you suggest?
M. A. Golding
CC BY-SA 4.0
2022-07-30T18:41:13.300Z
M. A. Golding - Exactly. I can just look up a photo of Jupiter from Europa if I want, but I'm interested in how a gas giant would appear through an atmosphere (at different times during its orbit).
Elhammo
CC BY-SA 4.0
2022-07-31T20:47:08.557Z
You can actually see Jupiter in the daytime (from Earth), it's just too small to see any detail.  But a good approximation would be what it looks like through a scope in the daytime: https://www.google.com/search?q=jupiter+in+daytime
Greg Miller
<planet><exoplanet><natural-satellites><gas-giants><hypothetical>
If you were standing on a habitable moon of a gas giant, what would the planet look like during the day vs the night?
Elhammo
# If you were standing on a habitable moon of a gas giant, what would the planet look like during the day vs the night?

(Asked by: username_0 on 2022-07-28)

If you were standing on the proplanetary side of a habitable moon of a gas giant, and the moon had a thick enough atmosphere to make the sky blue, how would the gas giant look during the day? Would it look faint? Would you see the entire planet or just whatever side is more illuminated by the sun (I'm imagining the sun peaking out from behind the planet and illuminating one side, while the other fades to blue)? How would the planet look during the nighttime, when the proplanetary face of the moon is facing away from the sun and the planet is illuminated? Would it be bright and more saturated? And does this all depend on the density/thickness of the moon's atmosphere?

**Tags:** planet, exoplanet, natural-satellites, gas-giants, hypothetical
> **Comments:**
> **username_1**: I don't think there could be a definite answer to this question. It is too generic. How a gas giant would appear as seen by its satellite depend on the size of the planet, it's albedo, the brightness of the star, the distance of the star, the distance of the satellite, the thickness and composition of the atmosphere...  
> **username_1**: Maybe you could help by editing the question and fixing some of these parameters. You could say, for instance, how does Saturn look like when seen from Titan. Or, how would Jupiter look like if seen from a satellite orbiting at X distance and with the same atmosphere as Earth  
> **username_2**: @Uhoh the question specifies a habitable moon.  Which habitable moon in our solar system should Elhammo choose as his example as you suggest?  
> **username_0**: M. A. Golding - Exactly. I can just look up a photo of Jupiter from Europa if I want, but I'm interested in how a gas giant would appear through an atmosphere (at different times during its orbit).  
> **username_3**: You can actually see Jupiter in the daytime (from Earth), it's just too small to see any detail.  But a good approximation would be what it looks like through a scope in the daytime: https://www.google.com/search?q=jupiter+in+daytime  

---

## 1 answers

### Answer by username_4 on 2022-07-30. Score: 2

**Partial answer:** the planet will appear roughly three times as bright as the Moon does in our daytime sky! (if the sky is like ours) and the part of the disk not sunlit will look "invisible" or "missing".

How big it will appear, or the fraction illuminated vs time will depend on details like the ratio of the moon's orbital distance to the planet's diameter. I'll leave that for other answers to address, but it's going to vary ***a bit like*** the phases of Earth's moon as seen from Earth.

[![A Christmas day morning view of the Moon with a blue sky taken at 8:53am from the village of Trimingham, Norfolk, England. Date Taken on 25 December 2018](https://i.sstatic.net/0rBPP.jpg)](https://i.sstatic.net/0rBPP.jpg)

[A Christmas day morning view of the Moon with a blue sky taken at 8:53am from the village of Trimingham, Norfolk, England. Date: Taken on 25 December 2018](https://commons.wikimedia.org/wiki/File:-2018-12-25_The_moon_in_the_early_morning,_Christmas_Day.JPG)

---

This is an interesting question and it can actually be answered fairly independently of which moon, which orbit or which gas giant!

What we really need to nail down is *which atmosphere!?*

Since both the moon's atmosphere and the planet are illuminated by the same Sun at essentially the same distance, that drops out of the problem. We just need to look at the *ratio* of the sky brightness to the planet's apparent surface brightness.

Luckily our Moon presents us with an excellent example.

* The same way that a wall does not get brighter or dimmer as we walk towards or away from it, the average [surface brightness](https://en.wikipedia.org/wiki/Surface_brightness) of a resolved, extended object like the disk of a planet or moon does not really increase or decrease if we are closer/further from it. The total light varies as 1/r2 because the *apparent size* varies like that.
* The albedo of the Moon is pretty low actually, roughly of order 10%. It's actually slightly orange/brown because it's less reflective in blue and more reflective in red. For more on that see [this answer to *Why is this moon red?*](https://astronomy.stackexchange.com/a/28862/7982)
* The albedo of gas giants in our solar system is 3 to 4 times higher than the Earth's moon!
* At the same time, the light in Earth's [Diffuse sky radiation](https://en.wikipedia.org/wiki/Diffuse_sky_radiation) (i.e. blue sky) is quite blue due to [Rayleigh scattering](https://en.wikipedia.org/wiki/Rayleigh_scattering) so while it might interfere with the blue hues of the planet's disk, the rest of the visible spectrum will shine brightly through. If the planet somehow had a "great blue spot" it might be nearly invisible behind a bright blue sky. For more on that see [Why does Unity look transparent?](https://space.stackexchange.com/q/54083/12102) and answers there.

So as long as the Moon's atmosphere has a blue sky that isn't overwhelmingly brighter than Earth's,

### The gas giant would have a surface brightness ~3-4 times as bright as the Moon in our sky! (if the sky is like ours)

And there's the rub. A moon will likely have a lower surface gravity $g$ than Earth, which means that the [scale height](https://en.wikipedia.org/wiki/Scale_height) of the atmosphere will be proportionally larger: $$H = \frac{kT}{mg}$$ where $m$ is the average mass of a molecule of atmosphere.

So if the moon has a surface gravity of 0.1 and everything else is the same (it won't be) then the scale height will be more like 80 kilometers compared to Earth's ~8 km. With an atmosphere 10x thicker the sky will be much much brighter.

So let's hope the two effects cancel; albedo 3-4x higher and Rayleigh scattering 3-4x stronger making the planet's surface brightness relative to the "blue sky" in front of it about the same ratio as the Earth's moon behind our daytime sky.

| Body | Bond Albedo |
| --- | --- |
| Moon | 0.11 |
| Jupiter | 0.343 |
| Saturn | 0.342 |
| Uranus | 0.300 |
| Neptune | 0.290 |

[![Day moon (16669185823).jpg](https://i.sstatic.net/Fv5Bcm.jpg)](https://i.sstatic.net/Fv5Bc.jpg) [![Daylight moon (4087378758) (3).jpg](https://i.sstatic.net/HPRIWm.jpg)](https://i.sstatic.net/HPRIW.jpg)

[![Луна в дневное время.jpg](https://i.sstatic.net/oyjN2m.jpg)](https://i.sstatic.net/oyjN2.jpg) [![A Day View of Moon.jpg](https://i.sstatic.net/Bj3z2m.jpg)](https://i.sstatic.net/Bj3z2.jpg)

Sources (Wikimedia):

* top row: [1](https://commons.wikimedia.org/wiki/File:Day_moon_%2816669185823%29.jpg), [2](https://commons.wikimedia.org/wiki/File:Daylight_moon_%284087378758%29_%283%29.jpg)
* bottom row: [3](https://commons.wikimedia.org/wiki/File:%D0%9B%D1%83%D0%BD%D0%B0_%D0%B2_%D0%B4%D0%BD%D0%B5%D0%B2%D0%BD%D0%BE%D0%B5_%D0%B2%D1%80%D0%B5%D0%BC%D1%8F.jpg), [4](https://commons.wikimedia.org/wiki/File:A_Day_View_of_Moon.jpg),


---

