<p>The only potentially valid way to cause a long-lasting eclipse that I can think of is to have your moon orbit at the planet's L1 <a href="https://en.wikipedia.org/wiki/Lagrangian_point" rel="nofollow noreferrer">Lagrange point</a> between the planet and the star (and the moon has to be the right size). If it had just the right orbit and was the right size, it could then cause a long-lasting eclipse. Note I'm not saying permanent because such a system wouldn't be <em>permanent</em>. I'm not going to run through the math$^1$ to determine if such a system is stable or for how long. I'd guess it may be stable for a few orbits at least.</p>

<p>The main problem is getting your moon there in the first place. One could conceivable say that by some freak chance a third fairly massive body came close to your planet/moon (either from outside the solar system or near the edge) and gravitationally perturbed your Moon's orbit such that it ended up in the L1 point. But the chances of such an event are so slim that it would be incredulous if it actually happened.</p>

<p>And just to add a technicality, once your moon moved to the L1 point, it would now be orbiting the Sun and wouldn't be a moon anymore according to the definition of a moon.</p>

<hr>

<p><sub>$^1$ Out of curiosity, I ran through some of the math very loosely and found that if you put the Moon in Earth's L1 position, the Moon survives for about 3-5 months before perturbations knock it out of the L1 position. Of course our Moon isn't the right size to cause an eclipse at that distance, but this gives a <em>very</em> rough estimate for how "permanent" the scenario I described above really is.</sub></p>

CC BY-SA 3.0
2017-11-03T12:59:41.740Z
Naaaaahhh,, it just has to be a really really big satellite :-)
Carl Witthoft
CC BY-SA 3.0
2017-11-02T16:53:59.743Z
2017-11-03T13:26:15.700Z
2017-11-03T13:26:15.700Z
zephyr
<p>I'm afraid this hypothetical object blocking the sun would have to be artificial. If the object is in synchronous orbit, and if it is to perpetually eclipse the sun, then the planet would have to be tidally locked to the sun. If that's the case, then the object blocking the sun would have to be at the sun itself.<br />
If you want to put an object in orbit around the sun that's large enough to block the sun's view from the planet, the object would have to have a delta-v lower than the amount required to sustain that orbit. Because of this, the object blocking the sun would require extra energy to sustain it's orbit in between the planet and the sun. </p>

<p>To answer the three questions,</p>

<ol>
<li><p>I guess it depends on the civilizations technological ability to detect it. Theoretically, nothing can truly be hidden in space. There should always be some method to detect anything, up until the inverse square law diminishes the information to nothing. </p></li>
<li><p>Yes. Mercury is smaller than Callisto, but has more mass than two Callisto's combined. </p></li>
<li><p>Depends on the kind of planet. If it is very volcanic, maybe? Anyhow, I wouldn't think it would last long. There's a lot of surface area on a planet to radiate heat out into space. It would get really cold, really fast. </p></li>
</ol>

<p>Edit: I forgot about the Langrange points. The object could orbit on the point between the star and the planet, but I'd suspect the orbit would not be very stable for such a large object. At least, on a cosmic timescale. </p>

CC BY-SA 3.0
2017-11-03T13:00:07.933Z
Not true, at least in SciFi - unlimited technology terms.
Carl Witthoft
CC BY-SA 3.0
2017-11-04T00:39:50.270Z
I'm not sure I understand why the object blocking the sun should be at the sun in order to eclipse it. As big as it might be, should it not be closer to the planet to eclipse the sun ?
Lnou
CC BY-SA 3.0
2017-11-04T00:50:42.000Z
The object cannot orbit the planet and perpetually eclipse the sun-facing side of it at the same time. That orbit only exists either at the sun, or at the L1 Langrange point between the planet and the sun, presumably millions of miles away from the planet. If that's the case, then the object would have to be larger than most gas giants can feasibly be.
BenjaminF
CC BY-SA 3.0
2017-11-04T00:53:57.430Z
It may be better to just put an artificial object at the Langrange point that acts like a sun shade. I don't have figures for the world you're building, so I can't do the math, but here on Earth, an object that large must be artificial. https://en.m.wikipedia.org/wiki/Space_sunshade
BenjaminF
CC BY-SA 3.0
2017-11-02T16:58:34.860Z
2017-11-03T13:20:05.717Z
2017-11-03T13:20:05.717Z
BenjaminF
<p>So, to add a bit of context, I’m doing the NaNoWriMo challenge, which consists of writing a novel of 50k words in one month. My whole idea is based around the following question: What would happen if the sun were to suddenly disappear?</p>

<p>To make it short, the story is about a colony on a faraway planet, which is settled for few generations already. And one day or let’s say, in less than a month, the sun disappear. So far, so good. Except I need a valid reason as to why the sun disappear… I have found some ideas (dust from volcanic eruption, change of orbit, dying sun, etc…) but none of them looked good enough. (I’m open for suggestion if any.)</p>

<p>I’m now investigating the possibility that the planet would gain a satellite big enough to hide the sun on the whole planet (like an eclipse but lasting forever). This <a href="https://astronomy.stackexchange.com/questions/6188/can-a-natural-satellite-exist-in-a-geostationary-orbit">topic</a> lead me to think that if the satellite is in geostationary orbit, it is possible. <strike>But there is still some questions I can’t get my head around.</p>

<ol>
<li>How fast a new satellite can “appear”?</li>
<li>Is it possible that a bigger but “lighter” (with less gravitational force) satellite can orbit around a smaller but “heavier” planet?</li>
<li>This <a href="https://astronomy.stackexchange.com/questions/22265/long-term-temperature-effects-of-an-eclipse">topic</a> reminded me that no sun means no heat. Can the geothermic activity of a planet be enough to prevent the surface from being impossible to live on?</strike> (Off topic.)</li>
</ol>

<p>So, basically, the question is, if it's possible to have a forever eclipse, how ?</p>

<p>I’m sorry in advance if the questions are not clear enough, even though I understand the basics, I don’t necessarily have all the technical terms.</p>

<p>Thank you !!</p>

CC BY-SA 3.0
2017-11-02T18:07:40.887Z
Maybe a large, forming ring system would come close to a forever eclipse,  Once fully formed it would flatten out but forming it could shade quite a bit of the planet.  The planet would need a very small axial tilt, but . . . just maybe you could get a ring system to permanently block the sun.    You'd still get some reflection.   There's also the problem of ring debris raining down onto the planet.   Not ideal, but just maybe.
userLTK
CC BY-SA 3.0
2017-11-02T19:29:17.143Z
I think that if a star has the right size it could in principle collapse directly into a black hole. Nothing will change to the orbiting planet except for no daylight anymore. For something fictional it can help. Just investigate here or in physics about the possibility .
Alchimista
CC BY-SA 3.0
2017-11-02T20:00:09.303Z
I'm voting to close this question as off-topic because this would be much better on the worldbuilding platform. You might also want to trim it down to one question or it may be closed as too broad.
James K
CC BY-SA 3.0
2017-11-03T12:59:02.377Z
Have you read Ringworld?
Carl Witthoft
CC BY-SA 3.0
2017-11-04T00:15:39.807Z
@Kozaky I didn't even know it existed. But since I'm not really creating a new world, but mostly using "rules" of our own, I'm not sure it fits either...
Lnou
CC BY-SA 3.0
2017-11-04T00:19:55.137Z
@Alchimista Won't the black hole destroy (or anything else bad) the planet ? But thanks too !
Lnou
CC BY-SA 3.0
2017-11-04T08:54:26.567Z
Lnou. No.the black hole will have the gravitational effect of the very big star from it formed. Really explore this possibility . Perhaps ask "can a star with just the right mass collapse into a black hole without passing supernova or any explosive stage?".I am pretty sure that in principle is possible but I have no data nor refes. Try to Google too. It could be the right scenario for your novel
Alchimista
CC BY-SA 3.0
2017-11-04T08:58:43.397Z
https://arstechnica.com/science/2017/05/vanishing-star-hints-at-direct-collapse-to-black-hole/
Alchimista
<the-sun><planet><solar-eclipse><natural-satellites>
Is it possible that a satellite creates a forever eclipse?
Lnou
# Is it possible that a satellite creates a forever eclipse?

(Asked by: username_0 on 2017-11-02)

So, to add a bit of context, I’m doing the NaNoWriMo challenge, which consists of writing a novel of 50k words in one month. My whole idea is based around the following question: What would happen if the sun were to suddenly disappear?

To make it short, the story is about a colony on a faraway planet, which is settled for few generations already. And one day or let’s say, in less than a month, the sun disappear. So far, so good. Except I need a valid reason as to why the sun disappear… I have found some ideas (dust from volcanic eruption, change of orbit, dying sun, etc…) but none of them looked good enough. (I’m open for suggestion if any.)

I’m now investigating the possibility that the planet would gain a satellite big enough to hide the sun on the whole planet (like an eclipse but lasting forever). This [topic](https://astronomy.stackexchange.com/questions/6188/can-a-natural-satellite-exist-in-a-geostationary-orbit) lead me to think that if the satellite is in geostationary orbit, it is possible. But there is still some questions I can’t get my head around.

1. How fast a new satellite can “appear”?
2. Is it possible that a bigger but “lighter” (with less gravitational force) satellite can orbit around a smaller but “heavier” planet?
3. This [topic](https://astronomy.stackexchange.com/questions/22265/long-term-temperature-effects-of-an-eclipse) reminded me that no sun means no heat. Can the geothermic activity of a planet be enough to prevent the surface from being impossible to live on? (Off topic.)

So, basically, the question is, if it's possible to have a forever eclipse, how ?

I’m sorry in advance if the questions are not clear enough, even though I understand the basics, I don’t necessarily have all the technical terms.

Thank you !!

**Tags:** the-sun, planet, solar-eclipse, natural-satellites

> **Comments:**
> **username_1**: Maybe a large, forming ring system would come close to a forever eclipse,  Once fully formed it would flatten out but forming it could shade quite a bit of the planet.  The planet would need a very small axial tilt, but . . . just maybe you could get a ring system to permanently block the sun.    You'd still get some reflection.   There's also the problem of ring debris raining down onto the planet.   Not ideal, but just maybe.  
> **username_2**: I think that if a star has the right size it could in principle collapse directly into a black hole. Nothing will change to the orbiting planet except for no daylight anymore. For something fictional it can help. Just investigate here or in physics about the possibility .  
> **username_3**: I'm voting to close this question as off-topic because this would be much better on the worldbuilding platform. You might also want to trim it down to one question or it may be closed as too broad.  
> **username_4**: Have you read Ringworld?  
> **username_0**: @Kozaky I didn't even know it existed. But since I'm not really creating a new world, but mostly using "rules" of our own, I'm not sure it fits either...  
> **username_0**: @username_2 Won't the black hole destroy (or anything else bad) the planet ? But thanks too !  
> **username_2**: username_0. No.the black hole will have the gravitational effect of the very big star from it formed. Really explore this possibility . Perhaps ask "can a star with just the right mass collapse into a black hole without passing supernova or any explosive stage?".I am pretty sure that in principle is possible but I have no data nor refes. Try to Google too. It could be the right scenario for your novel  
> **username_2**: https://arstechnica.com/science/2017/05/vanishing-star-hints-at-direct-collapse-to-black-hole/  

---

## 2 answers

### Answer by username_5 on 2017-11-02. Score: 6

The only potentially valid way to cause a long-lasting eclipse that I can think of is to have your moon orbit at the planet's L1 [Lagrange point](https://en.wikipedia.org/wiki/Lagrangian_point) between the planet and the star (and the moon has to be the right size). If it had just the right orbit and was the right size, it could then cause a long-lasting eclipse. Note I'm not saying permanent because such a system wouldn't be *permanent*. I'm not going to run through the math$^1$ to determine if such a system is stable or for how long. I'd guess it may be stable for a few orbits at least.

The main problem is getting your moon there in the first place. One could conceivable say that by some freak chance a third fairly massive body came close to your planet/moon (either from outside the solar system or near the edge) and gravitationally perturbed your Moon's orbit such that it ended up in the L1 point. But the chances of such an event are so slim that it would be incredulous if it actually happened.

And just to add a technicality, once your moon moved to the L1 point, it would now be orbiting the Sun and wouldn't be a moon anymore according to the definition of a moon.

---

$^1$ Out of curiosity, I ran through some of the math very loosely and found that if you put the Moon in Earth's L1 position, the Moon survives for about 3-5 months before perturbations knock it out of the L1 position. Of course our Moon isn't the right size to cause an eclipse at that distance, but this gives a *very* rough estimate for how "permanent" the scenario I described above really is.

> **Comments:**
> **username_4**: Naaaaahhh,, it just has to be a really really big satellite :-)  

---

### Answer by username_6 on 2017-11-02. Score: -1

I'm afraid this hypothetical object blocking the sun would have to be artificial. If the object is in synchronous orbit, and if it is to perpetually eclipse the sun, then the planet would have to be tidally locked to the sun. If that's the case, then the object blocking the sun would have to be at the sun itself.  
If you want to put an object in orbit around the sun that's large enough to block the sun's view from the planet, the object would have to have a delta-v lower than the amount required to sustain that orbit. Because of this, the object blocking the sun would require extra energy to sustain it's orbit in between the planet and the sun.

To answer the three questions,

1. I guess it depends on the civilizations technological ability to detect it. Theoretically, nothing can truly be hidden in space. There should always be some method to detect anything, up until the inverse square law diminishes the information to nothing.
2. Yes. Mercury is smaller than Callisto, but has more mass than two Callisto's combined.
3. Depends on the kind of planet. If it is very volcanic, maybe? Anyhow, I wouldn't think it would last long. There's a lot of surface area on a planet to radiate heat out into space. It would get really cold, really fast.

Edit: I forgot about the Langrange points. The object could orbit on the point between the star and the planet, but I'd suspect the orbit would not be very stable for such a large object. At least, on a cosmic timescale.

> **Comments:**
> **username_4**: Not true, at least in SciFi - unlimited technology terms.  
> **username_0**: I'm not sure I understand why the object blocking the sun should be at the sun in order to eclipse it. As big as it might be, should it not be closer to the planet to eclipse the sun ?  
> **username_6**: The object cannot orbit the planet and perpetually eclipse the sun-facing side of it at the same time. That orbit only exists either at the sun, or at the L1 Langrange point between the planet and the sun, presumably millions of miles away from the planet. If that's the case, then the object would have to be larger than most gas giants can feasibly be.  
> **username_6**: It may be better to just put an artificial object at the Langrange point that acts like a sun shade. I don't have figures for the world you're building, so I can't do the math, but here on Earth, an object that large must be artificial. https://en.m.wikipedia.org/wiki/Space_sunshade  

---

