Planetary Physics: Two orbitally locked moons

[SidusLupus]

The Sun also has a major impact on the tides, and it doesnt seem to be mentioned in this discussion, and i thought it would be prudent to point it out.

I'm not an astronomer or a physics major but it seems to me you would still have tides but they might be more pronounced than on earth depending on how big you make the moons in relationship to the planet.

On a side note, have you considered making it a flat world where having 2 moons orbiting equidistant from one another wouldn't really matter?

 

[LightPhoenix]

Of course, when you talk about instability - how long would it take a two-moon set-up to degrade to the point where it would self-destruct? If the system would last for millions of years, slowly decaying, then over a couple of hundred years there's not going to be much of a detectable change, and then the world is sound. It will eventually end in catastrophe, if I'm correct in assuming the moons would hit either each other or the planet (but I'm not an physicist by any means). The thing is that catastrophe is not now, but thousands or tens of thousands of years in the future, and not entirely applicable. Even if the system only lasts for tens of thousands of years, that's more than enough time for a campaign to take place without worrying too much about the ultimate fate of the system.

It could even be a cool epic quest, to put back the moons' orbits so that the planet and moons will exist for another cycle.

 

[Mishihari Lord]

I think it could work in a stable fashion, especially if as you said the moons are locked either side of the planet. The only way I can think of that a moon would get locked like that is if the mass distribution of the planet were uneven. If there were high-density locations, one each on the day and night side, the moons could get dragged along by the gravity differential, eventually syncing with the planet's rotation. Add a tiny bit of damping from the hydrogen present in space and you've got a fairly stable system.

Under this scenario there would be no noticable tides on the planet. While the tidal bulges would be twice that produced by a single moon, the bulge would be locked to the planet's surface an hence unnoticable.

 

[Mustrum_Ridcully]

I think it could work in a stable fashion, especially if as you said the moons are locked either side of the planet. The only way I can think of that a moon would get locked like that is if the mass distribution of the planet were uneven. If there were high-density locations, one each on the day and night side, the moons could get dragged along by the gravity differential, eventually syncing with the planet's rotation. Add a tiny bit of damping from the hydrogen present in space and you've got a fairly stable system.

Under this scenario there would be no noticable tides on the planet. While the tidal bulges would be twice that produced by a single moon, the bulge would be locked to the planet's surface an hence unnoticable.

Though this would not allow for a Day and a a Night moon, unless the planet itself doesn´t have a day/night cycle. (as when completing the axis-rotation in the same time as the rotation around the sun - and in the same direction)

 

[Bran Blackbyrd]

I would enjoy things less if the physics didn't work and the GM said "It's magic, suck it."

If I had a nickel for every time I said that...
And $150 for each time I needed bail because I said it.

Incidentally, my campaign world has two moons, one visible and one that is hidden from view except for a day or two every so many years.

 

[Nyeshet]

Incidentally, my campaign world has two moons, one visible and one that is hidden from view except for a day or two every so many years.

I assume your second moon has a comet-like orbit (highly eliptical, small size, only near enough to be easily seen from the surface of the planet a few days each year)?

 

[Bran Blackbyrd]

I assume your second moon has a comet-like orbit (highly eliptical, small size, only near enough to be easily seen from the surface of the planet a few days each year)?

Not a lot of real-world logic went into my second moon. It's actually obscured from view by the visible first moon (and possibly something a bit more... magical). I'm open to suggestions though, since not much hangs in the balance, story-wise, if I were to change why it's hidden.

 

[SpiralBound]

Here's an odd thought... What if one of the two moons was a "ghost" moon, from a moon that was destroyed(?) aeons ago by some terrible sorcery gone awry... It could even be that the ghost moon is actually partially in the ethereal/shadow/"whatever equivalent you use" plane and thus technically exists, but can only be seen as a ghostly image of its former self.

With such a scenario, all the varying proposed planetary gymnastics above won't be needed. You just use the same setup as Earth currently has. The ghost moon isn't physically there anymore and thus doesn't have any standard gravitational effects. What "ethereal gravational" effects it may or may not have are likely right up there with how many angels can dance on the head of a pin.

This would allow the "night moon" to be REALLY spooky. It's a ghost moon!!

You could even have an epic campaign event where the ghost moon is threatening to "drift" out of its ghostly state and revert to its previous "unghostly" condition. This of course would now cause catastrophic destruction as a second moon "popped" into existance in orbit around the planet. (one can only imagine how bad it was when the moon first left! Perhaps that's why that event marks the beginning of a dark age, with little or no knowledge surviving of what was before then...) Of course, it would be the PC's mission to prevent this! Now if only anyone were around who had any clue what magical accident it was that made the moon disappear in the first place...

 

[Lonely Tylenol]

Oh, yeah. Earth's moon is frickin huge for the size of the planet!

This was an issue in an old Terry Pratchett (pre-discworld) sci-fi novel. The moon, being so ludicrously huge--improbably so--turns out to be a clue that the world was created by an earlier civilization, who left the moon there for just that purpose. Humanity figures out the ruse, and in doing so become the next world-building race, learning to build new planets complete with buried dinosaurs, rock strata, etc.

 

[Vocenoctum]

Here's an odd thought... What if one of the two moons was a "ghost" moon, from a moon that was destroyed(?) aeons ago by some terrible sorcery gone awry... It could even be that the ghost moon is actually partially in the ethereal/shadow/"whatever equivalent you use" plane and thus technically exists, but can only be seen as a ghostly image of its former self.

Sounds like a good way to make the "daymoon" a big black sphere, instead of a normal moon.