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".

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    $\begingroup$ 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. $\endgroup$ Mar 13, 2019 at 22:42
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    $\begingroup$ This is quite "worldbuilding". I note that a planet-wide monoculture is almost as unlikely as a moon darting anywhere. $\endgroup$
    – James K
    Mar 13, 2019 at 23:10
  • $\begingroup$ @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. $\endgroup$ Mar 14, 2019 at 0:38
  • $\begingroup$ 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. $\endgroup$ Mar 14, 2019 at 0:39
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    $\begingroup$ This is quite clearly a question for 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. $\endgroup$ Mar 14, 2019 at 7:37

1 Answer 1



Polar orbits aren'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".

  • $\begingroup$ 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. $\endgroup$ Mar 14, 2019 at 0:29
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    $\begingroup$ @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. $\endgroup$
    – PM 2Ring
    Mar 14, 2019 at 2:07
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    $\begingroup$ 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. $\endgroup$ Mar 14, 2019 at 6:54
  • $\begingroup$ @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. $\endgroup$ Mar 14, 2019 at 7:23
  • $\begingroup$ @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. $\endgroup$ Mar 14, 2019 at 7:28

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