<blockquote>
  <p>1) From the moon, how will the sky look like?</p>
</blockquote>

<p>If the moon is tidally locked in a circular orbit then the giant gas planet will always be in the same spot in the sky. It wouldn't move. It would always be there. </p>

<p>But if the orbit is eccentric, as is that of Mercury, then it would turn and expose all its sides to its mother planet, even though it is tidally locked.</p>

<p>The mother gas giant planet would change its phases and its atmospheric storms. Its hugeness and nearness would take up a really large fraction of the sky for anyone on the side of that moon which permanently faces the planet. You couldn't have a better GPS...</p>

<blockquote>
  <p>2) What will the equivalent of a "day and night cycle" (sun-up to
  sun-up) look like?</p>
</blockquote>

<p>Day, as in dawn to dusk, for a moon would be its orbital period. For most large moons in the Solar system, their orbital periods are a few days to a week or so. Not extremely different from our Earth day rhythm, but likely a few times longer.</p>

<blockquote>
  <p>3) Will the winds on the moon have any "non-earthlike" patterns?</p>
</blockquote>

<p>Titan of Saturn is the only moon which has atmosphere, and thus winds. And its winds seem to be very calm. And the winds on Mars too move grains of sand quite slowly most of the time. It's more of a geological phenomenon than a weather phenomenon, if I've understood it correctly. But weather patterns in gas giant planets can be enormous. As in the atmospheres of stars. And then there's the Venusian atmosphere. There are four atmospheres of rocky/icy worlds in the Solar system, and they are all very different. Very different. So your fiction may be believable almost anyway you make it in that respect.</p>

<p>Here's a SETI talk about tidally locked exoplanetary insolation which I think might interest you:
<a href="http://www.seti.org/weeky-lecture/patterns-sunlight-extra-solar-planets" rel="nofollow">http://www.seti.org/weeky-lecture/patterns-sunlight-extra-solar-planets</a>
13 minutes into it maybe it gets more interesting.</p>

<p>And I think that the solar wind from the star would keep all the interstellar medium anti-anti-matter from interacting with its planets. ISM is about one atom per cubic centimeter. If its matter or antimatter I think it wouldn't change much as our Sun plows through it.</p>

CC BY-SA 3.0
2014-07-01T19:11:12.307Z
2014-07-01T19:54:17.703Z
2014-07-01T19:54:17.703Z
LocalFluff
<p>First it will be important to consider the term 'relativistic speed'. If by that you mean something like 0.1c, it will only change the colour of the stars as you mentioned in the bounty description. However, if it means something with higher Lorentz Gammas (like 0.9c or 0.99c), all other relativistic effects come into play. There's relativistic beaming and aberration combined, which means that all the light that you receive from systems with respect to which you're moving at relativistic speeds will only be received in small beams towards the direction you're moving in and those will be strongly blueshifted and intensified. Watch this animated video for an illustrated view of what I mean - <a href="https://www.youtube.com/watch?v=JQnHTKZBTI4" rel="noreferrer">https://www.youtube.com/watch?v=JQnHTKZBTI4</a>. (It's not a high quality video, but it is very effective in demonstrating what I just wrote.)</p>

<p>So, in effect, you will possibly see only three major sources of light in the sky:</p>

<ol>
<li>The parent planet</li>
<li>The parent star</li>
<li>The bright patch of the sky in the direction of motion, highly blueshifted, with brightness dependent on your speed</li>
</ol>

<p>Their relative brightnesses will depend on:</p>

<ol>
<li>distance to parent star</li>
<li>distance to parent planet</li>
<li>albedo of the parent planet</li>
<li>speed of motion through the ism</li>
</ol>

<p>For a day and night cycle:</p>

<ul>
<li>The parent planet will obviously be at the zenith the whole time (or fixed location in the sky if you're at a different location).</li>
<li>The parent star will appear to move around the sky with the same period as the revolution period of the moon - say a few days at the most (will be fairly small as compared to the 12 year revolution period of the planet, so you can ignore 'synodic' effects to the first order for a single day/night cycle).</li>
<li>The sky will also move with roughly the same period as the sun, ignoring synodic effects.</li>
</ul>

<p>Winds will be very complexly affected by the three sources (since heating due to radiation affects winds). There will also be tidal effects due to the parent planet which also play a role in the winds (similar to how tides work on the earth). So, you will have to take into account heating from the star, heating from the planet (which is constant due to tidal locking), tidal effects of the planet (again constant) and heating from the sky. Now the radiation from the sky can also act in a weird way, since blueshifting can give rise to strong emissions in UV, X-ray and Gamma ray photons, plus relativistic cosmic rays (everything you encounter will be in the form of cosmic rays due to the relativistic speed of your motion). These can ionize the atmosphere, heat it, cause there to be particle showers and aurorae, which can all affect wind patterns in complex ways, which I cannot exactly say much about, given my limited knowledge in atmospheric sciences.</p>

<p>As I mentioned above, yes, all interactions will be relativistic and will be similar to cosmic ray bombardment onto the moon you live on. Things will get nasty and you'll be having geomagnetic storms all the time. Satellite communications will probably never work. Expect these particles to arrive in the previously mentioned bright region of the sky due to aberration still applying to this case. You will see similar aurora like regions on the parent planet (<a href="http://www.spacetelescope.org/static/archives/postcards/screen/hst_postcard_0008.jpg" rel="noreferrer">http://www.spacetelescope.org/static/archives/postcards/screen/hst_postcard_0008.jpg</a>), and the star will probably not be affected that much.</p>

<p>Oh and by the way, due to the antimatter nature of your system and the matter nature of the ism, the atmosphere would thin over time, and later these antimatter cosmic rays will start bombarding the moon. So if you stay in the ism for long enough, living on the moon will be difficult. Also, I am assuming that life evolved on your moon in an antimatter ISM and you have entered the matter ism only recently, as otherwise, the moon won't be stable for long enough for life (intelligent life) to evolve.</p>

<p>I will try to see if I can think of some other effects, but for now, I guess this will do?!</p>

CC BY-SA 3.0
2014-07-02T19:36:05.800Z
Wow @Takku - great answer, I'm giving it a couple more days to give a chance others suggest improvements (and a fair shot at the bounty if they can give a better answer...), otherwise, you take the cake :). Thanks for your effort!
G0BLiN
CC BY-SA 3.0
2014-07-02T19:48:59.323Z
Regarding the day/night cycle. Is the following description correct (disregarding the "bright patch")? At "sunrise" the sun appears at the "east" and starts moving up the sky. After a while, it'll be eclipsed by the planet, and be obscured for quite a while (so it's a dark, "psuedo-night"), after this eclipse, the sun'll gradually appear on the other side of the planet, proceeding "west" until "sunset". at this point It'll become darker, but since now the visible planet side is becoming more illuminated by the sun, this "true-night" is actually brighter since there's "planet-light".
G0BLiN
CC BY-SA 3.0
2014-07-02T19:56:08.343Z
Regarding life on this moon - the campaign assumes this wired system is accessed and settled recently by humans using a completely-non-physically-sound "star-gate" technology, living only several generations before the story proper begins - the radiation is planned to be part of an approaching dooms-day scenario, so I'm glad this is something a smart player can deduce when they realize where they are. I didn't mention this to avoid over-complicating this question, which isn't very simple or straight-forward as it is...
G0BLiN
CC BY-SA 3.0
2014-07-02T22:19:17.820Z
Yes, the day/night cycle will indeed be as you mentioned, if the orbits of the planet and the moon are aligned (otherwise it will be similar to  how the earth-moon system and lunar eclipses work - there will be eclipses when the moon is close to the nodes). Also, as I mentioned, the relative brightnesses of the three sources (including albedos etc. and other complications) will decide how dark the night is and how bright the day is. In any case the pseudo-night/eclipse will be darkest, since there will be no light received from the planet or the sun.
Takku
CC BY-SA 3.0
2014-07-04T11:55:46.690Z
Also worth noting is the libration (https://en.wikipedia.org/wiki/Libration) of the moon as seen from the earth. If such a thing happens for the moon in question, the position of the parent planet in the sky as seen from the moon may vary over time (most likely oscillating or elliptical motion around the zenith).
Takku
CC BY-SA 3.0
2014-07-04T12:16:11.713Z
Regarding the particle bombardment - will the star and planet shield the moon from these at certain points? Put differently - is there a region in the planet's orbit where most of the particles will be either diverted or absorbed by the sun, so the planet and moon are relatively "safe"?
G0BLiN
CC BY-SA 3.0
2014-07-04T18:56:18.500Z
Sure. This would happen when the sun or the planet occult the 'bright spot' in the sky. However, this only happens if the spot (or direction of motion) is in the plane of the orbit of the planet (for sun occulting the spot) or the plane of the orbit of the moon (for planet occulting the spot).
Takku
CC BY-SA 3.0
2014-07-01T13:53:16.113Z
2014-07-01T14:05:33.373Z
2014-07-01T14:05:33.373Z
Takku
<p>(Hope this is the right Stack-exchange site for this question)<br/>
I'm working on a sci-fi RPG campaign, set on a very atypical location. Since this is a work of fiction, there's enough room for speculation and artistic license. But I'd still like to start from a more astronomically sound perspective.<br/>
<br/></p>
<h3>Background</h3>
<p>Consider the following system:</p>
<ol>
<li><p>A single <strong>moon</strong>, with properties roughly comparable to Earth's so that it can support human life (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water etc.). This moon is tidally locked with it's planet.</p>
</li>
<li><p>A single gas-giant <strong>planet</strong>, with a size roughly comparable to Saturn. Orbiting it's star so that it completes a cycle about every 12 earth years.</p>
</li>
<li><p>A <strong>Star</strong>, larger than Sol and emitting enough energy to keep the moon in a comfortably warm temperature.</p>
</li>
</ol>
<p>The features above may be altered to ensure possibility of human survival on the moon, and if possible, to keep the moon tidally locked with a large planet.</p>
<p>Now, the complications (inspired by a short story by Larry Niven):</p>
<ol>
<li>The whole system (star, planet and moon) are <strong>traveling at a relativistic speed</strong> across the galaxy.</li>
<li>(Based on the theory that some parts of the universe might be made from anti-matter) The whole system is <strong>made from anti-matter</strong>, but is currently traveling through a normal-matter area of space.
Note: the &quot;human life&quot; on the moon are also based on anti-matter...</li>
</ol>
<p>Finally, the story will begin with a small community of primitive &quot;anti-matter humans&quot;, living on the moon in a place from which the planet fully visible, and at the zenith.</p>
<h3>The Questions</h3>
<p>I'd appreciate ideas, insights and comments on the following:</p>
<ol>
<li>From the moon, how will the sky look like?</li>
<li>What will the equivalent of a &quot;day and night cycle&quot; (sun-up to sun-up) look like?</li>
<li>Will the winds on the moon have any &quot;non-earthlike&quot; patterns?</li>
<li>Considering that at relativistic speeds, the odds of the bodies in the system hitting hydrogen particles in the vacuum of interstellar space are significant (is that even correct?), will that create any special aura or energy streams from the sun or planet?</li>
</ol>
<p>Thank you for your help.</p>

CC BY-SA 3.0
2014-06-28T22:24:43.057Z
p.s. - I'm not sure I tagged this question right - by all means fell free to add and remove as necessary.
G0BLiN
CC BY-SA 3.0
2014-06-28T22:48:14.780Z
What is the orbital period of the moon around the gas planet? How long is the moon in the shadow of the gas planet (without the energy of the sun to warm it)? What is the albedo of the moon (high albedo means slow heating and fast cooling).
LDC3
CC BY-SA 3.0
2014-06-28T23:00:38.487Z
The life sustainability of the moon is the crucial factor here - it has to be warm enough while in the shadow of the planet and not overheat while in direct sunlight. Fulfilling that, I'd rather have a light/darkness cycle of no longer then several days (If I'm right there should be two periods of darkness every rotation cycle - one "night" and one "eclipse", but I'm waiting for actual answers for things like that). So that will mean an orbital period of between 4 to 8 Earth days, right?
G0BLiN
CC BY-SA 3.0
2014-06-28T23:04:24.103Z
Wikipedia tells me that Earth has an average albedo of about 0.3 - so unless it impacts survivability, I'd go with a similar value for this moon.
G0BLiN
CC BY-SA 3.0
2014-07-03T16:26:01.950Z
If my theory is correct that antimatter has negative gravity, the star would explode because gravity holds stars together.  The gas on the gas giant would "fall up" into space, and it would become a gigantic diffuse cloud.  The planet and moon would break apart (not sure how big the chunks would be).  However, if my theory is wrong and antimatter has normal gravity, the "real" answer below is probably pretty well on track :-)  Sounds like this could shape up to be a really interesting game!
Jonathan
CC BY-SA 3.0
2014-07-03T21:19:07.423Z
@Jonathan - I never heard about the idea of anti-matter having reversed gravitational effect, this sounds like a good basis for a different question either here or on physics exchange :) (if you end up asking that, you can leave a link in a comment - I'd like to know if this has any basis) - at any rate, I think that if antimatter masses reject each other, there wouldn't be an anti-matter star (nor planet nor moon) in the first place, just diffuse "gas" - so this isn't applicable to my campaign. Meaning I'll use "artistic  license" to ignore it even if it is a valid theory...
G0BLiN
CC BY-SA 3.0
2014-07-04T11:30:07.963Z
Agreed, and as far as I know, the general view is that antimatter gravitates normally.  Some, including me, think that it does something else.
Jonathan
<tidal-forces><earth-like-planet><habitable-zone><antimatter>
Help in determining the features of an unusual, fictional star system
G0BLiN
# Help in determining the features of an unusual, fictional star system

(Asked by: username_0 on 2014-06-28)

(Hope this is the right Stack-exchange site for this question)  
I'm working on a sci-fi RPG campaign, set on a very atypical location. Since this is a work of fiction, there's enough room for speculation and artistic license. But I'd still like to start from a more astronomically sound perspective.

### Background

Consider the following system:

1. A single **moon**, with properties roughly comparable to Earth's so that it can support human life (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water etc.). This moon is tidally locked with it's planet.
2. A single gas-giant **planet**, with a size roughly comparable to Saturn. Orbiting it's star so that it completes a cycle about every 12 earth years.
3. A **Star**, larger than Sol and emitting enough energy to keep the moon in a comfortably warm temperature.

The features above may be altered to ensure possibility of human survival on the moon, and if possible, to keep the moon tidally locked with a large planet.

Now, the complications (inspired by a short story by Larry Niven):

1. The whole system (star, planet and moon) are **traveling at a relativistic speed** across the galaxy.
2. (Based on the theory that some parts of the universe might be made from anti-matter) The whole system is **made from anti-matter**, but is currently traveling through a normal-matter area of space.
   Note: the "human life" on the moon are also based on anti-matter...

Finally, the story will begin with a small community of primitive "anti-matter humans", living on the moon in a place from which the planet fully visible, and at the zenith.

### The Questions

I'd appreciate ideas, insights and comments on the following:

1. From the moon, how will the sky look like?
2. What will the equivalent of a "day and night cycle" (sun-up to sun-up) look like?
3. Will the winds on the moon have any "non-earthlike" patterns?
4. Considering that at relativistic speeds, the odds of the bodies in the system hitting hydrogen particles in the vacuum of interstellar space are significant (is that even correct?), will that create any special aura or energy streams from the sun or planet?

Thank you for your help.

**Tags:** tidal-forces, earth-like-planet, habitable-zone, antimatter

> **Comments:**
> **username_0**: p.s. - I'm not sure I tagged this question right - by all means fell free to add and remove as necessary.  
> **username_1**: What is the orbital period of the moon around the gas planet? How long is the moon in the shadow of the gas planet (without the energy of the sun to warm it)? What is the albedo of the moon (high albedo means slow heating and fast cooling).  
> **username_0**: The life sustainability of the moon is the crucial factor here - it has to be warm enough while in the shadow of the planet and not overheat while in direct sunlight. Fulfilling that, I'd rather have a light/darkness cycle of no longer then several days (If I'm right there should be two periods of darkness every rotation cycle - one "night" and one "eclipse", but I'm waiting for actual answers for things like that). So that will mean an orbital period of between 4 to 8 Earth days, right?  
> **username_0**: Wikipedia tells me that Earth has an average albedo of about 0.3 - so unless it impacts survivability, I'd go with a similar value for this moon.  
> **username_2**: If my theory is correct that antimatter has negative gravity, the star would explode because gravity holds stars together.  The gas on the gas giant would "fall up" into space, and it would become a gigantic diffuse cloud.  The planet and moon would break apart (not sure how big the chunks would be).  However, if my theory is wrong and antimatter has normal gravity, the "real" answer below is probably pretty well on track :-)  Sounds like this could shape up to be a really interesting game!  
> **username_0**: @username_2 - I never heard about the idea of anti-matter having reversed gravitational effect, this sounds like a good basis for a different question either here or on physics exchange :) (if you end up asking that, you can leave a link in a comment - I'd like to know if this has any basis) - at any rate, I think that if antimatter masses reject each other, there wouldn't be an anti-matter star (nor planet nor moon) in the first place, just diffuse "gas" - so this isn't applicable to my campaign. Meaning I'll use "artistic  license" to ignore it even if it is a valid theory...  
> **username_2**: Agreed, and as far as I know, the general view is that antimatter gravitates normally.  Some, including me, think that it does something else.  

---

## 2 answers

### Answer by username_4 on 2014-07-01. Score: 5 | Accepted answer

First it will be important to consider the term 'relativistic speed'. If by that you mean something like 0.1c, it will only change the colour of the stars as you mentioned in the bounty description. However, if it means something with higher Lorentz Gammas (like 0.9c or 0.99c), all other relativistic effects come into play. There's relativistic beaming and aberration combined, which means that all the light that you receive from systems with respect to which you're moving at relativistic speeds will only be received in small beams towards the direction you're moving in and those will be strongly blueshifted and intensified. Watch this animated video for an illustrated view of what I mean - <https://www.youtube.com/watch?v=JQnHTKZBTI4>. (It's not a high quality video, but it is very effective in demonstrating what I just wrote.)

So, in effect, you will possibly see only three major sources of light in the sky:

1. The parent planet
2. The parent star
3. The bright patch of the sky in the direction of motion, highly blueshifted, with brightness dependent on your speed

Their relative brightnesses will depend on:

1. distance to parent star
2. distance to parent planet
3. albedo of the parent planet
4. speed of motion through the ism

For a day and night cycle:

* The parent planet will obviously be at the zenith the whole time (or fixed location in the sky if you're at a different location).
* The parent star will appear to move around the sky with the same period as the revolution period of the moon - say a few days at the most (will be fairly small as compared to the 12 year revolution period of the planet, so you can ignore 'synodic' effects to the first order for a single day/night cycle).
* The sky will also move with roughly the same period as the sun, ignoring synodic effects.

Winds will be very complexly affected by the three sources (since heating due to radiation affects winds). There will also be tidal effects due to the parent planet which also play a role in the winds (similar to how tides work on the earth). So, you will have to take into account heating from the star, heating from the planet (which is constant due to tidal locking), tidal effects of the planet (again constant) and heating from the sky. Now the radiation from the sky can also act in a weird way, since blueshifting can give rise to strong emissions in UV, X-ray and Gamma ray photons, plus relativistic cosmic rays (everything you encounter will be in the form of cosmic rays due to the relativistic speed of your motion). These can ionize the atmosphere, heat it, cause there to be particle showers and aurorae, which can all affect wind patterns in complex ways, which I cannot exactly say much about, given my limited knowledge in atmospheric sciences.

As I mentioned above, yes, all interactions will be relativistic and will be similar to cosmic ray bombardment onto the moon you live on. Things will get nasty and you'll be having geomagnetic storms all the time. Satellite communications will probably never work. Expect these particles to arrive in the previously mentioned bright region of the sky due to aberration still applying to this case. You will see similar aurora like regions on the parent planet (<http://www.spacetelescope.org/static/archives/postcards/screen/hst_postcard_0008.jpg>), and the star will probably not be affected that much.

Oh and by the way, due to the antimatter nature of your system and the matter nature of the ism, the atmosphere would thin over time, and later these antimatter cosmic rays will start bombarding the moon. So if you stay in the ism for long enough, living on the moon will be difficult. Also, I am assuming that life evolved on your moon in an antimatter ISM and you have entered the matter ism only recently, as otherwise, the moon won't be stable for long enough for life (intelligent life) to evolve.

I will try to see if I can think of some other effects, but for now, I guess this will do?!

> **Comments:**
> **username_0**: Wow @username_4 - great answer, I'm giving it a couple more days to give a chance others suggest improvements (and a fair shot at the bounty if they can give a better answer...), otherwise, you take the cake :). Thanks for your effort!  
> **username_0**: Regarding the day/night cycle. Is the following description correct (disregarding the "bright patch")? At "sunrise" the sun appears at the "east" and starts moving up the sky. After a while, it'll be eclipsed by the planet, and be obscured for quite a while (so it's a dark, "psuedo-night"), after this eclipse, the sun'll gradually appear on the other side of the planet, proceeding "west" until "sunset". at this point It'll become darker, but since now the visible planet side is becoming more illuminated by the sun, this "true-night" is actually brighter since there's "planet-light".  
> **username_0**: Regarding life on this moon - the campaign assumes this wired system is accessed and settled recently by humans using a completely-non-physically-sound "star-gate" technology, living only several generations before the story proper begins - the radiation is planned to be part of an approaching dooms-day scenario, so I'm glad this is something a smart player can deduce when they realize where they are. I didn't mention this to avoid over-complicating this question, which isn't very simple or straight-forward as it is...  
> **username_4**: Yes, the day/night cycle will indeed be as you mentioned, if the orbits of the planet and the moon are aligned (otherwise it will be similar to  how the earth-moon system and lunar eclipses work - there will be eclipses when the moon is close to the nodes). Also, as I mentioned, the relative brightnesses of the three sources (including albedos etc. and other complications) will decide how dark the night is and how bright the day is. In any case the pseudo-night/eclipse will be darkest, since there will be no light received from the planet or the sun.  
> **username_4**: Also worth noting is the libration (https://en.wikipedia.org/wiki/Libration) of the moon as seen from the earth. If such a thing happens for the moon in question, the position of the parent planet in the sky as seen from the moon may vary over time (most likely oscillating or elliptical motion around the zenith).  
> **username_0**: Regarding the particle bombardment - will the star and planet shield the moon from these at certain points? Put differently - is there a region in the planet's orbit where most of the particles will be either diverted or absorbed by the sun, so the planet and moon are relatively "safe"?  
> **username_4**: Sure. This would happen when the sun or the planet occult the 'bright spot' in the sky. However, this only happens if the spot (or direction of motion) is in the plane of the orbit of the planet (for sun occulting the spot) or the plane of the orbit of the moon (for planet occulting the spot).  

---

### Answer by username_3 on 2014-07-01. Score: 1

> 1) From the moon, how will the sky look like?

If the moon is tidally locked in a circular orbit then the giant gas planet will always be in the same spot in the sky. It wouldn't move. It would always be there.

But if the orbit is eccentric, as is that of Mercury, then it would turn and expose all its sides to its mother planet, even though it is tidally locked.

The mother gas giant planet would change its phases and its atmospheric storms. Its hugeness and nearness would take up a really large fraction of the sky for anyone on the side of that moon which permanently faces the planet. You couldn't have a better GPS...

> 2) What will the equivalent of a "day and night cycle" (sun-up to
> sun-up) look like?

Day, as in dawn to dusk, for a moon would be its orbital period. For most large moons in the Solar system, their orbital periods are a few days to a week or so. Not extremely different from our Earth day rhythm, but likely a few times longer.

> 3) Will the winds on the moon have any "non-earthlike" patterns?

Titan of Saturn is the only moon which has atmosphere, and thus winds. And its winds seem to be very calm. And the winds on Mars too move grains of sand quite slowly most of the time. It's more of a geological phenomenon than a weather phenomenon, if I've understood it correctly. But weather patterns in gas giant planets can be enormous. As in the atmospheres of stars. And then there's the Venusian atmosphere. There are four atmospheres of rocky/icy worlds in the Solar system, and they are all very different. Very different. So your fiction may be believable almost anyway you make it in that respect.

Here's a SETI talk about tidally locked exoplanetary insolation which I think might interest you:
<http://www.seti.org/weeky-lecture/patterns-sunlight-extra-solar-planets>
13 minutes into it maybe it gets more interesting.

And I think that the solar wind from the star would keep all the interstellar medium anti-anti-matter from interacting with its planets. ISM is about one atom per cubic centimeter. If its matter or antimatter I think it wouldn't change much as our Sun plows through it.


---

