<p>No.</p>

<p>Polar orbits aren't what you think they are. A polar orbit is a (more or less) circular orbit that travels over <em>both</em> poles. A satellite in a polar orbit doesn't hover over one pole.</p>

<p>An orbit with greater "range" (ie further from the planet) will always be slower. This is Kepler's third law.</p>

<p>You can't get exactly what you want in real physics.  Can I suggest a simpler alternative: A moon that is (for various unlikely reasons) in a highly elliptical orbit and tilted orbit. It would appear to grow, and move dramatically (for a moon) across the sky during perigee. Then shrink and fade and move much more slowly for most of the time of the orbit. It can't cross the orbit of the other moon (or they will eventually collide). The regular approach followed by becoming smaller justifies gives it the appearance of "fleeing".</p>

CC BY-SA 4.0
2019-03-14T00:29:20.423Z
Hmm, I like this.  I think I am going to adjust my vision in this direction.  It fits my intentions for the moon in question yet is more doable from an orbital dynamics standpoint.  Thank you.
Aren Yashar
CC BY-SA 4.0
2019-03-14T02:07:26.893Z
@Aran Bear in mind that, in general, highly eccentric orbits tend to become less eccentric (i.e., more circular) over time. Unless there's something interacting with them to maintain the eccentricity, like a larger moon. ;) The tricky thing is to organize that setup so it's stable and doesn't fling the eccentric moon out of the system, or crashing into the planet.
PM 2Ring
CC BY-SA 4.0
2019-03-14T06:54:07.333Z
If the eccentric moon is as small as you suggest then it might be a relatively recently  (few million years)  captured asteroid or comet.  If it's orbit takes it too far away, though the gravity of the Sun becomes an issue and it is unlikely to stay in orbit long at all.
Steve Linton
CC BY-SA 4.0
2019-03-14T07:23:01.993Z
@PM2Ring and Steve Linton - I had the same concerns about stability. But that orbit doesn't have to be stable at astronomic time scales for OP's purposes. It only needs to be stable at historic time scales. Thousands of years instead of billions. I'm sure it could be stable even longer than thousands of years - probably millions of years would be doable, as Steve said. But this is just intuition. So a body that was captured, say, 1 million years before might work.
Florin Andrei
CC BY-SA 4.0
2019-03-14T07:28:56.897Z
@JamesK I think it's better to just say it would grow at perigee and shrink at apogee. "Brighten" and "fade" may suggest all kinds of wrong things - although I believe I know what you're thinking. But surface brightness would remain the same, only the object would grow as it gets closer, and shrink as it flees the planet's vicinity. If the apogee is far enough, it may even become a point-like object like a star. I've no idea if that orbit would be stable. Something to do with the Hill sphere I guess.
Florin Andrei
CC BY-SA 4.0
2019-03-14T08:32:00.477Z
@Florin Agreed. I wasn't suggesting it's impossible, just tricky, and hard to stabilize long term. Thousands of years, no problem. Millions might be stretching it though. I guess someone should do a simulation... ;)
PM 2Ring
CC BY-SA 4.0
2019-03-14T15:10:15.657Z
This is a variation of the 3-body dynamics problem. If you start with 3 non-coplanar bodies, the orbits will go "all over the place" as the system progresses.
Carl Witthoft
CC BY-SA 4.0
2019-03-13T23:07:56.090Z
2019-03-14T20:58:42.583Z
2019-03-14T20:58:42.583Z
James K
<p>Orbits can follow certain patterns, as I am aware.  Some include a circular orbit around the bulk of the planet, a polar orbit spinning about the pole...</p>

<p>What I'd like to know, for a planet with two moons (one large one in a circular orbit at the equator, more or less), could the smaller moon, at a larger range but higher velocity, be pulled into an orbit that looks like a polar orbit for part of it's cycle, then transition towards a circular one before "flipping" back towards a polar orbit on the opposing pole, dancing between the three states either in sequence or seemingly at random as the smaller moon approaches certain criteria, being tugged on by the planet, the larger moon, the sun, maybe other planets in the system.</p>

<p>Is this feasible, or does it break the laws of orbital dynamics to have such a situation occur?  This is for a story I am writing centered on a planet other than Earth, but is relatively Earth-like in it's properties.</p>

<p>Size and density the same as Earth.  Orbit a bit tighter and faster than Earth's in relation to it's sun.  The smaller moon is perhaps 200 square miles in surface area, tidally locked stone and ice.  The larger is perhaps 42 million square miles in surface area.</p>

<p>Any thoughts on this?  The basic idea is that this smaller moon, Eamor, is going to be seen in the night's sky almost like a star to the naked eye, racing across the skies as if scared of it's own shadow, sometimes disappearing under the horizon only to dart back up again.  Hence the name, which means, in the language of one culture on the planet, "one who is terrified".</p>

CC BY-SA 4.0
2019-03-13T22:42:23.003Z
Polar orbits aren't what you think they are. A polar orbit is just a circular orbit where the circle is "tipped" so it passes over (or at least close to) both poles in alternation.
Steve Linton
CC BY-SA 4.0
2019-03-13T23:10:01.667Z
This is quite "worldbuilding".  I note that a planet-wide monoculture is almost as unlikely as a moon darting anywhere.
James K
CC BY-SA 4.0
2019-03-14T00:38:34.833Z
@Steve Linton, you are right.  If a polar orbit is not defined as a tight circular orbit around a line stretching from the center of the planet through the pole, then you are quite correct.  Which is perhaps a good reason for me to have come here to ask more knowledgeable people.  Thank you for the input.
Aren Yashar
CC BY-SA 4.0
2019-03-14T00:39:54.673Z
Thank you for the compliment @James K.  Though I would hardly call Iolara a monoculture.  Does it have a dominant culture that inflicted some of its names of things on the world as a whole?  Yes.  One that is still dominant?  That's arguable...heh.
Aren Yashar
CC BY-SA 4.0
2019-03-14T07:37:37.960Z
This is quite clearly a question for https://worldbuilding.stackexchange.com/ but hopefully you'll get some good ideas here as well, since the issues you're raising are almost entirely within the realm of orbital mechanics, and there are folks here who know this stuff.
Florin Andrei
CC BY-SA 4.0
2019-03-14T07:50:04.547Z
The big moon is a little bigger in diameter than our Moon. The little moon is quite tiny, about 13 km diameter. For the little moon to appear in the sky as big as we see our Moon, it would have to get as close as 1500 km to the planet. I suspect that's high enough for the orbit to be stable long enough for your purposes. Make it get much closer and then stability issues would occur over a long time. It may even make tides on the planet if it gets close enough (but it's late for me and I can't do the math now).
Florin Andrei
CC BY-SA 4.0
2019-03-14T07:51:22.147Z
Objects as small as 13 km diameter are not necessarily spherical. They could be, but that would be pure chance. It could have any shape - like a potato for example.
Florin Andrei
CC BY-SA 4.0
2019-03-14T07:54:37.930Z
There's a problem. The Roche limit for the Earth is at 9500 km for something like our Moon, and more like 18000 km for a comet. Any moon descending closer than that would break up. You have to keep your little moon above 20000 km. Make it bigger, so it would look bigger to the people on your planet. Say, make it 300 km in diameter. Even this big it may not necessarily be spherical - it might be, but there's no guarantee.
Florin Andrei
CC BY-SA 4.0
2019-03-14T07:59:10.323Z
With a perigee at 20000 km, it would not move super-fast. It would be faster than our Moon, maybe 100x faster at perigee (estimate - can't do math now), but it would not zip across the sky like a satellite.
Florin Andrei
<orbit><natural-satellites>
Various moon orbits
Aren Yashar
# Various moon orbits

(Asked by: username_0 on 2019-03-13)

Orbits can follow certain patterns, as I am aware. Some include a circular orbit around the bulk of the planet, a polar orbit spinning about the pole...

What I'd like to know, for a planet with two moons (one large one in a circular orbit at the equator, more or less), could the smaller moon, at a larger range but higher velocity, be pulled into an orbit that looks like a polar orbit for part of it's cycle, then transition towards a circular one before "flipping" back towards a polar orbit on the opposing pole, dancing between the three states either in sequence or seemingly at random as the smaller moon approaches certain criteria, being tugged on by the planet, the larger moon, the sun, maybe other planets in the system.

Is this feasible, or does it break the laws of orbital dynamics to have such a situation occur? This is for a story I am writing centered on a planet other than Earth, but is relatively Earth-like in it's properties.

Size and density the same as Earth. Orbit a bit tighter and faster than Earth's in relation to it's sun. The smaller moon is perhaps 200 square miles in surface area, tidally locked stone and ice. The larger is perhaps 42 million square miles in surface area.

Any thoughts on this? The basic idea is that this smaller moon, Eamor, is going to be seen in the night's sky almost like a star to the naked eye, racing across the skies as if scared of it's own shadow, sometimes disappearing under the horizon only to dart back up again. Hence the name, which means, in the language of one culture on the planet, "one who is terrified".

**Tags:** orbit, natural-satellites

> **Comments:**
> **username_1**: Polar orbits username_0't what you think they are. A polar orbit is just a circular orbit where the circle is "tipped" so it passes over (or at least close to) both poles in alternation.  
> **username_2**: This is quite "worldbuilding".  I note that a planet-wide monoculture is almost as unlikely as a moon darting anywhere.  
> **username_0**: @username_1, you are right.  If a polar orbit is not defined as a tight circular orbit around a line stretching from the center of the planet through the pole, then you are quite correct.  Which is perhaps a good reason for me to have come here to ask more knowledgeable people.  Thank you for the input.  
> **username_0**: Thank you for the compliment @username_2.  Though I would hardly call Iolara a monoculture.  Does it have a dominant culture that inflicted some of its names of things on the world as a whole?  Yes.  One that is still dominant?  That's arguable...heh.  
> **username_3**: This is quite clearly a question for https://worldbuilding.stackexchange.com/ but hopefully you'll get some good ideas here as well, since the issues you're raising are almost entirely within the realm of orbital mechanics, and there are folks here who know this stuff.  
> **username_3**: The big moon is a little bigger in diameter than our Moon. The little moon is quite tiny, about 13 km diameter. For the little moon to appear in the sky as big as we see our Moon, it would have to get as close as 1500 km to the planet. I suspect that's high enough for the orbit to be stable long enough for your purposes. Make it get much closer and then stability issues would occur over a long time. It may even make tides on the planet if it gets close enough (but it's late for me and I can't do the math now).  
> **username_3**: Objects as small as 13 km diameter are not necessarily spherical. They could be, but that would be pure chance. It could have any shape - like a potato for example.  
> **username_3**: There's a problem. The Roche limit for the Earth is at 9500 km for something like our Moon, and more like 18000 km for a comet. Any moon descending closer than that would break up. You have to keep your little moon above 20000 km. Make it bigger, so it would look bigger to the people on your planet. Say, make it 300 km in diameter. Even this big it may not necessarily be spherical - it might be, but there's no guarantee.  
> **username_3**: With a perigee at 20000 km, it would not move super-fast. It would be faster than our Moon, maybe 100x faster at perigee (estimate - can't do math now), but it would not zip across the sky like a satellite.  

---

## 1 answers

### Answer by username_2 on 2019-03-13. Score: 1 | Accepted answer

No.

Polar orbits username_0't what you think they are. A polar orbit is a (more or less) circular orbit that travels over *both* poles. A satellite in a polar orbit doesn't hover over one pole.

An orbit with greater "range" (ie further from the planet) will always be slower. This is Kepler's third law.

You can't get exactly what you want in real physics. Can I suggest a simpler alternative: A moon that is (for various unlikely reasons) in a highly elliptical orbit and tilted orbit. It would appear to grow, and move dramatically (for a moon) across the sky during perigee. Then shrink and fade and move much more slowly for most of the time of the orbit. It can't cross the orbit of the other moon (or they will eventually collide). The regular approach followed by becoming smaller justifies gives it the appearance of "fleeing".

> **Comments:**
> **username_0**: Hmm, I like this.  I think I am going to adjust my vision in this direction.  It fits my intentions for the moon in question yet is more doable from an orbital dynamics standpoint.  Thank you.  
> **username_4**: @Aran Bear in mind that, in general, highly eccentric orbits tend to become less eccentric (i.e., more circular) over time. Unless there's something interacting with them to maintain the eccentricity, like a larger moon. ;) The tricky thing is to organize that setup so it's stable and doesn't fling the eccentric moon out of the system, or crashing into the planet.  
> **username_1**: If the eccentric moon is as small as you suggest then it might be a relatively recently  (few million years)  captured asteroid or comet.  If it's orbit takes it too far away, though the gravity of the Sun becomes an issue and it is unlikely to stay in orbit long at all.  
> **username_3**: @username_4 and username_1 username_1 - I had the same concerns about stability. But that orbit doesn't have to be stable at astronomic time scales for OP's purposes. It only needs to be stable at historic time scales. Thousands of years instead of billions. I'm sure it could be stable even longer than thousands of years - probably millions of years would be doable, as username_1 said. But this is just intuition. So a body that was captured, say, 1 million years before might work.  
> **username_3**: @username_2 I think it's better to just say it would grow at perigee and shrink at apogee. "Brighten" and "fade" may suggest all kinds of wrong things - although I believe I know what you're thinking. But surface brightness would remain the same, only the object would grow as it gets closer, and shrink as it flees the planet's vicinity. If the apogee is far enough, it may even become a point-like object like a star. I've no idea if that orbit would be stable. Something to do with the Hill sphere I guess.  
> **username_4**: @username_3 Agreed. I wasn't suggesting it's impossible, just tricky, and hard to stabilize long term. Thousands of years, no problem. Millions might be stretching it though. I guess someone should do a simulation... ;)  
> **username_5**: This is a variation of the 3-body dynamics problem. If you start with 3 non-coplanar bodies, the orbits will go "all over the place" as the system progresses.  

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