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Is there any way a second moon would appear to hang out near the closer one in the sky? For example always appearing either ahead of or behind throughout the year, or even alternating between those two. I don't think trojan moons would work for this, but I obviously have no idea.

It's still early days so it's not the end of the world (literally) if there isn't a second moon, this is more of a curiosity than anything. Thanks.

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  • $\begingroup$ If by moon you mean our own Moon, the problem is that our own Moon is very large, so large that the Earth-Moon system is almost a double planet. Our Moon is too large to allow for a similar-sized body to share its orbit. If by moon you mean a satellite in general, then in our solar system we have the example of the two co-orbital satellites of Saturn, Jason and Epimetheus. $\endgroup$
    – AlexP
    Commented Jan 14 at 19:44
  • $\begingroup$ I don't see why a trojan would not work. It would have to be tiny, though. $\endgroup$
    – Anton Sherwood
    Commented Jan 16 at 20:47

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The most obvious answer is yes, if the moon had itself a satellite, it would appear either ahead or behind it while moving in the sky.

We observe it rather easily with, for example, Jupiter, Mars, Saturn and their moons.

And if they were larger, we could see it also with our moon and the artificial probes orbiting it.

A rather spectacular case would be if the moons had similar masses, so an observer on the planet surface would see them "dancing" around their center of mass while they move across the sky.

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  • $\begingroup$ How big can the sub-moon be? I guess that depends on density and distance and whatnot so I can probably fudge a good chunk of that. This is helpful, thank you! $\endgroup$
    – Syntal
    Commented Jan 14 at 20:29

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