Planetary Physics: Two orbitally locked moons

[RandomUsernamehmimo71]

I'd appreciate the assistance of Enworlders smarter than I but particularly those better in Planatary physics..

One of the ideas that I'm playing with in my game design is that of a planet with two moons- One on either side of the planet, revolving in a locked orbit. Essentially, a daymoon, and a nightmoon.

I'm running into some interesting (To myself and my cohorts) questions in understanding the implications, and I could use some insights.

One obvious thought is tides.. If we assume the moons have similiar mass, tidal effects on the planet should be minimized, at least in to my understanding.. This would seem to have implications as if there we no moon in this regard- No tidal pools, Very different erosion, and the like.

Another question is as to the stability of such an orbit.It would seem to me that it would be possible for such an orbit to exist over an extended time period, but it would be extraordinarly difficult for it to form.

What other implications should I be thinking of with a design such as this? Might the gravitational pull on the planet cause the planet to be slightly less spherical?

Any thoughts would be highly appreciated- While I have an interest in planatary physics, I am not as knowledgable as I would like, or I feel I should be.

 

[painandgreed]

Having two moons won't cancel out the gravitational effects of the moons. That would require a ring arond the entire planet. Also, I suspect that having two moons such as this would actually result in even greater tides. High tide takes place twice a day. It's always on the spot facign the oon and the spot facing away from the moon. Placing a moon on both sides is just going to create even larger gravity effects under both of them that will draw the water away from the faces perpendicular to the moons where the low tides are even more. Even without any moons, you'll still have soem tides as wind and air pressure can raise and lower they ocean's water level although such effects will be pretty much random but still vary the sea level by a few feet.

 

[Psion]

If you are worried about whether it would be stable, I think you need a Klemperer rosette, which requires 3 or more bodies. And even then, tidal forces might screw it up.

Google Kemplerer rosettes for more info. Try this page for visuals:
http://burtleburtle.net/bob/physics/kempler.html

 

[Simplicity]

Having two moons won't cancel out the gravitational effects of the moons. That would require a ring arond the entire planet. Also, I suspect that having two moons such as this would actually result in even greater tides. High tide takes place twice a day. It's always on the spot facign the oon and the spot facing away from the moon. Placing a moon on both sides is just going to create even larger gravity effects under both of them that will draw the water away from the faces perpendicular to the moons where the low tides are even more. Even without any moons, you'll still have soem tides as wind and air pressure can raise and lower they ocean's water level although such effects will be pretty much random but still vary the sea level by a few feet.

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.

Now, whether that's the actual scenario... probably not in reality. But it may approximate that.

 

[Abstraction]

Uh, is this a world with magic? Are any of your players physicists? Then it works, causing very high and low tides. Or whatever.

 

[Abstraction]

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.

Now, whether that's the actual scenario... probably not in reality. But it may approximate that.

Actually, no. Don't forget that the force of gravity drops off. So, the side of the planet facing Moon A feels the gravity of Moon A far more than it feels the gravity of Moon B. The opposite is also true.

 

[jeff37923]

Another question is as to the stability of such an orbit.It would seem to me that it would be possible for such an orbit to exist over an extended time period, but it would be extraordinarly difficult for it to form.

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

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.

Hope that helps.

Also, your questions are not stupid in the least. Anyone who thinks that they have the right to flame you for asking is just covering up their own inadequicies.

 

[jeff37923]

Uh, is this a world with magic? Are any of your players physicists? Then it works, causing very high and low tides. Or whatever.

I second this statement. I watched a guy on RPGnet with a brilliant idea get it utterly destroyed by a planetology grad student when he asked about some scientific accuracy for a fantasy setting. These things we create are pieces of interactive fiction and if the story must be sacrificed for the science to work then damn the science. As long as you and your players are having a good time, then that is all that matters.

 

[Psion]

I second this statement. I watched a guy on RPGnet with a brilliant idea get it utterly destroyed by a planetology grad student when he asked about some scientific accuracy for a fantasy setting. These things we create are pieces of interactive fiction and if the story must be sacrificed for the science to work then damn the science. As long as you and your players are having a good time, then that is all that matters.

If you are referring to Evil Dr. Ganymede, he's actually mellowed out over the past few years.

But for some people, the realism is what they ground their story in. For some of us, the game world is the real world + magic.

 

[Kemrain]

But for some people, the realism is what they ground their story in. For some of us, the game world is the real world + magic.

As a player in e1ven's game, I must chime in in agreement with Psion's statement. I would enjoy things less if the physics didn't work and the GM said "It's magic, suck it." I want to be able to understand the planet with physical models, and having some of this explained by the folks of EN World will go a long way towards making me like it.

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.

- Kemrain the Physics Monkey.