Can a moon have its own moon? - now we know!

Woah, thanks Val!

Cool, thanx! Don't know if it'll work it's way into any future stories yet, but any odd info can help!

So I was browsing the internet and came across this article about whether it is possible for a moon to have its own moon: http://curious.astro.cornell.edu/question.php?number=679

In summary, yes! It is very possible! Sci fi writers everywhere may rejoice. Or not. But I've wondered this for several years, and I got a definitive answer from someone who knows, so I'm content.

They do mention that it's likely that the sub-moon's orbit will decay and it will crash into the parent moon. But this can only happen if the parent moon is in synchronous rotation (same side always faces the planet)

A prime example is the Earth-Moon setup. We always see the same side of the moon when we look up at night. The tidal action holds it that way, and anything that orbits our moon would eventually crash into it because that tidal action would decay its orbit. Thankfully, for story purposes, this can take anywhere from thousands to millions of years.

The Sun-Earth-Moon setup is an example of how you can have a stable sub-moon. The Earth orbits the sun, and therefore is its "moon", if you will. But obviously the same side does not always face the sun! Whew. Because Earth spins independently of the sun's forces, the moon's orbit is not decayed by any tidal action, and is stable.

I'm sorry if I bored you guys. It always helps me memorize stuff by writing it out, and I didn't want some of you to TL,DR: this article when you really want to know what it said, so I summarized.

Also, I suppose this would work just as well in a fantasy setting. No one could get into why the science of it worked, though, I guess.

Anyway, there ya go.