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I would like for my world to have two moons. I think it would look simply stunning at night. Also, I came up with a pretty cool idea for a "God Eye" cult. When the smaller moon would have crossed the orbit of the larger one, a lunar eclipse would occur, which would look like an eye. (A smaller black dot against a white moon. )

According to one theory, our planet once had 2 moons. One was around 1/4 of the size of our current moon. They existed together for a few million years before colliding, and our moon was created from the debris.

My planet is supposed to be pretty Earthlike, with different landmasses but similar size and axial tilt.

Could I pull this off?

I'm not really a physics guy and English is not my first language, so I will really appreciate, in-depth answers.

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  • $\begingroup$ The giant impact theory I know states that a planet about the size of Mars impacted the proto Earth and resulted in the formation of the Moon... $\endgroup$
    – L.Dutch
    Aug 9, 2023 at 12:43
  • $\begingroup$ That apart, 1 question per post is our standard. Asking for all ramification makes this overly broad. $\endgroup$
    – L.Dutch
    Aug 9, 2023 at 12:44
  • $\begingroup$ Sorry I edited it a little bit. $\endgroup$ Aug 9, 2023 at 13:01
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    $\begingroup$ There are some theories that say that the Earth actually once had two moons and the second one crashed into the far side of our current one, explaining why the far side is so different than the near side. Also, the fact that the Moon formed much closer than its current position and gets farther 1 cm per year, means that it is plausible that an unstable but longed lived two-moons scenario really could happen. $\endgroup$ Aug 9, 2023 at 14:05
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    $\begingroup$ The L4 and L5 Lagrange points are places you can safely park a smaller moon and expect it to stay there. Unfortunately, that means they'll stay a fixed distance ahead or behind the moon's orbit around the Earth, it will never move directly between Earth and the big moon. I think you can get eclipses of the small moon on the big one, but the shadow will be on the side of the big moon, when seen from Earth. This probably doesn't make it look like a proper eye, like you want for your story. $\endgroup$ Aug 10, 2023 at 7:48

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If the orbits are well separated and/or resonant, there's no reason a second, smaller moon couldn't orbit Earth stably. This is aided by the fact that our Moon is much further from Earth than is the case with other "real" moons in the Solar System -- like those of Jupiter and Saturn and the ice giants, the ones that are big enough not to be captured asteroids or comets are in much closer orbits, despite their primaries being much larger and more massive than Earth.

The complicating factor here might be Luna's chaotic orbit, which is due in part to the fact it's almost as much a co-orbital planet as it is a moon, along with the Earth's axial tilt and the Sun's gravity effects fighting over whether the Moon should orbit over the equator or in the ecliptic plane (with the result that it's neither). If your world has low/no axial tilt, then the orbits of the moons would tend to stay equatorial and a 4:1 resonance (for instance) would tend to keep them in place, as it does for Jupiter's Galilean moons. Your small moon would then eclipse the larger, more distant one (assuming it's in the same orbit as Luna) roughly every 8 days -- and if it's visually smaller, you'd see one moon surrounded by the other.

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  • $\begingroup$ Thank you for your time and your answer! What about axial tilt similar to earth, for example, 22* ? $\endgroup$ Aug 9, 2023 at 13:36
  • $\begingroup$ Guessing, but with orbital height around 95,000 km, you'd get an orbit that wanders, but wanders a good bit less than Luna's and stays closer to equatorial. Eclipses would be rare, probably much rarer than Solar eclipses by the larger/further moon. $\endgroup$
    – Zeiss Ikon
    Aug 9, 2023 at 16:10
  • $\begingroup$ The smaller moon can also be behind the larger moon with the same ease, resulting it its total eclipse should their orbits be in a single plane. $\endgroup$
    – Vesper
    Aug 10, 2023 at 3:48
  • $\begingroup$ @Vesper If they're far enough out to have a week period, never mind a month, the planet's equatorial tilt will ensure they aren't (or don't stay) in the same orbital plane, though. This is why our eclipse cycle is so complex -- our Moon's orbit is chaotic due to the interaction of the Earth's tilted spin and the Sun's gravity. $\endgroup$
    – Zeiss Ikon
    Aug 10, 2023 at 11:07

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