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I want to have a fantasy setting with two suns and two moons. Ideally, the stars would be 5 million km apart, orbit each other every 10 days, and pass in front of each other every 5 days, creating an eclipse of one sun over the other. I’d like the planet to have two moons that appear in the sky at night, how could they have a stable orbit?A diagram of a circumbinary system, with two suns orbiting each other and a planet on an orbit path around those suns. The planet has two moons orbiting it.

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2 Answers 2

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Yes-ish

The moons could not be stable in the same orbit around the planet or in close orbits around the planet. They could be stable if they are in 3:2 orbital resonance with each other or if the 2nd moon has a much larger orbit than the first moon.

If you want both moons to be in the sky at the same time, it's best if you have one moon close to the planet and the other much further out.

The orbit of the planet around the stars is an S-type orbit. These can be stable, but the orbit of the planet has to be several times the diameter of the largest distance between the two stars. Your stars are plenty close enough to have stable orbits in the zone of habitability.

Note, though, that you should not think of the stars as orbiting on the same circle. Even if their orbits are near-circular, the more massive star is going to have a smaller orbit around their barycenter.
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Assume stability

Whether this is viable in real life does not matter. If this would not happen in real life it is still a possibility for you to include. There is no impact on the setting whether these orbits are realistic, as long as they seem realistic. Fortunately, this is also a fantasy world which eliminates some additional constraints.

In short, unless you care particularly about whether these orbits would be stable or not, it is easiest to assume stability.

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