Planetary Physics: Two orbitally locked moons

[RandomUsernamehmimo71]

This kind of orbit you are describing is unstable over the long run. Sorry.

I guessed that it might be, but I'm curious how long I could keep it going

You can get close by placing a moon in one of the gas giants lagrange points if you like, but they cannot orbit the giant. L1 would give you a light moon and L2 would give you a dark moon for your purposes.

I guess I was was misunderstanding things badly.. Then again, I did lead with the fact that I'm no good at this. It seemed like L3 would be a stable point for another moon, but maybe that's only if it's stationary at that point, and not relative to the first moon?

Wikipedia says

A third Lagrangian point, L3, exists on the opposite side of the Sun, a little further away from the Sun than the Earth is, where the combined pull of the Earth and Sun again causes the object to orbit with the same period as the Earth. The Sun-Earth L3 point was a popular place to put a "Counter-Earth" in pulp science fiction and comic books.

Wouldn't that work for the moon problem, just on a smaller scale?


-E "But if not, I'll accept it"

 

[Len]

If you have two moons on opposite sides of the planet, you get extra big tides. It's like having a permanent "spring tide" except more so.

If the two moons are synced with the planet's rotation ("daymoon" and "nightmoon") then yes the planet will bulge slightly. In fact it's because of the drag on this bulge that our moon always points the same side at us.

Even if it was said that "system X would fall apart without the influence of Y doing Z" I'd be happy as long as Y doing Z would reasonably fix X.

How about (close your eyes, Kemrain):
[sblock]X = the moons remaining balanced on opposite sides of the planet,
Y = a bunch of powerful wizards or psions, and
Z = regular telekinetic nudges to keep the moons from wandering out of orbit.
I'm pretty sure it's easy to keep the moons in place, but if this isn't done for some reason then the moons will start attracting each other and it's hard to get them back in place. Maybe they'll even collide, causing a worldwide cataclysm![/sblock]

 

[Hawkshadow]

Read the section in the Wikipedia article about stability. L1 - L3 are only marginally stable, while L4 and L5 are much more stable.

You can keep an object at L1, L2, or L3 by continually adjusting the object to keep it "ballanced" at or around the point.

Also note, the two objects which create the L points are assumed to be substantially bigger than the object at the L point.

 

[TheAuldGrump]

Also, how much mass is present in these moons?

Strange as it sounds, Earth's moon is not a moon, we are part of a dual planet pair. (Personally I think tht the moon should have been chosen as the measure of 'moondom', with lesser bodies being called something like 'sublunar objects'.)

The Auld Grump

 

[Abstraction]

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

 

[the Jester]

Even if it was said that "system X would fall apart without the influence of Y doing Z" I'd be happy as long as Y doing Z would reasonably fix X.

Y and Z being an epic spell and its effects, for instance.

 

[Umbran]

Wouldn't that work for the moon problem, just on a smaller scale?

Yes, but only if the moons are rather small compared ot the planet, and there's nothing else perturbing the system - like, say, a nearby sun. Unfortunately, the star the planet is working with probably isn't negligible...

I will also give backup to others - having two moons on opposite sides of the planet would not cancel tides. Their tidal effects would add, rather than subtract form each other.

Mind you, what's the point of a "day moon"? Ever look up during the new moon? Not much to see there. You could just get rid of the sun, and have that day moon be the planet's light source...

 

[ZuulMoG]

X = the moons remaining balanced on opposite sides of the planet,
Y = a bunch of powerful wizards or psions, and
Z = regular telekinetic nudges to keep the moons from wandering out of orbit.
I'm pretty sure it's easy to keep the moons in place, but if this isn't done for some reason then the moons will start attracting each other and it's hard to get them back in place. Maybe they'll even collide, causing a worldwide cataclysm!

This strongly reminds me of Larry Nivens depiction of the Atlantis situation, with wizards getting together reguarly to cast an epic spell to protect Atlantis from geological instability. A venal ruler prevented the wizards from performing this crucial task, and Atlantis sank beneath the waves.

This is an ideal setting for a fantasy world. The ritual that keeps the moons at bay (and they would be seen as inimical by their world, responsible for all sorts of catastrophes, like Venus was back in Babylonian times) needs to be reenacted periodocally, so a cabal of mages, priests, or whatever exists to perform this ritual at the appointed times. If the period is infrequent enough (say 500 years or so between rituals), the possibility of common folk forgetting about it and seeking to 'overthrow the useless parasites' rises.

This baby has adventure hook written all over it for epic characters. Some artifact required for the ritual goes missing, the high priest of the ritual has his soul stolen by an archdevil, etc...

 

[Nyeshet]

You want the moons to cancel out the tides? There is a leading (and trailing also, for that matter) Lagrange point 60 degrees fore and aft of the planet. Give it a large normal moon and a couple notably smaller moons at the lagrange points mentioned. If their combined mass is enough, I think it would be able to cancel out most of the tidal effects on the planet. Of course, if they are that massive they may not be stable in those particular lagrange points.

This would also result in three moons. No matter the time of day or night, there would always be either one or two visible (although the smaller would be less bright). Perhaps the two smaller moons could be considered younger sisters - or children - of the larger moon in popular mythology on your world?

 

[painandgreed]

Actually, if the moons were EXACTLY equal mass and EXACTLY on the opposite side of the planet AND they moved in a perfect sphere... Then their center of gravity would be the center of the planet. Tides would be cancelled out.

Doesn't work like that as the tides would be within the radius of the moons and thus you can't reduce it to a single center of gravity problem. Only when you're outside the radius of the mass can you equate the problem to a single center of gravity. No, with moons on either side of the planet, you're going to get very large tides as the two faces closest to the moons are going to be gravity wells that the water from the sides facing away from the moons will flow into. Because gravity is a r squared force even though the moons are of equal mass, each side of the planet will be affected the most by the moon closest to it. The only place that the gravity of the two moons will cancel each other out is in the plane that is equidistant from each of them.