Off Topic: Would it be possible for an Earth-Like Planet to have Two Moons that could BOTH eclipse the Sun?

[JoeSchmuckatelli]

I have no idea what the right sub-forum would be for this, but thought I'd ask (in case anyone knows):

 

Would it be possible to have an earth-like planet with two moons, where both moons are approximately the same apparent size when viewed from the surface - i.e. where both moons could eclipse the sun - as our moon does?

 

I've figured out that the farther moon must be substantially larger than the near moon - but have no idea whether a system like this could be stable.

Edited January 22, 2016 by JoeSchmuckatelli

[Snark]

This forum here's probably the right place for a discussion like this:

http://forum.kerbalspaceprogram.com/index.php?/forum/44-science-spaceflight/

Really, there are two questions here:  first, is it possible to have a system like that, which is stable; second, is there some realistic way that such a system could form in the first place.

I see no reason why you couldn't have two moons in a stable system.  For example, give Earth a second moon, which is the exact same size as the the first one, and in the same orbit, but 60 degrees ahead or behind it (i.e. so they're in each other's L4 / L5 points).  That ought to be gravitationally stable.

How likely it is that such a system could form in the first place is a bit more dubious.  To have not one but two large moons, both forming, seems iffy to me-- they condense out of large amounts of material floating around the planet, and I would think that it would be more likely for the bigger one to eat the smaller one's lunch.  But that's a matter for a more astrophysicsy discussion. 

[Plusck]

Unfortunately, I don't think the L4/L5 Lagrange points are stable if they are occupied by something very massive. As far as I am aware, all lagrangian objects are significantly smaller than the secondary body, and I have the feeling that the L4/L5 points are increasingly stable with increasing orders of magnitude of mass for the difference between the three bodies (so Earth's L4/L5 would be stable for very big rocks, but not so much a moon, while the Moon's L4/L5 would be stable for pebbles or dust and not much else - and Hiten didn't find any dust there either).

[Snark]

2 hours ago, Plusck said:

Unfortunately, I don't think the L4/L5 Lagrange points are stable if they are occupied by something very massive. As far as I am aware, all lagrangian objects are significantly smaller than the secondary body, and I have the feeling that the L4/L5 points are increasingly stable with increasing orders of magnitude of mass for the difference between the three bodies (so Earth's L4/L5 would be stable for very big rocks, but not so much a moon, while the Moon's L4/L5 would be stable for pebbles or dust and not much else - and Hiten didn't find any dust there either).

You may be right.  Still, I wouldn't be surprised if there's room for more than one large moon, for example in different orbits with a resonance (like 3:2, or some other ratio) that results in stability.

The how-did-it-form-in-the-first-place questions seems like more of a sticking point to me.

[Zhetaan]

My gut reaction is to say that the volcanoes of the inner moon will be more spectacular than the eclipse.  You're looking at a lot of tidal action here.

My less-gut reaction is to offer you a link to a similar topic on another forum here.  (I'm fairly sure that's not against any rules.)  The discussion is from 2008, but that just goes to show you that this is something that has intrigued people before.  The point made there about retrograde orbits is an interesting one; it complicates the question of formation, but appearances suggest that your system is possible.

Edited January 23, 2016 by Zhetaan

[Snark]

3 hours ago, Zhetaan said:

My gut reaction is to say that the volcanoes of the inner moon will be more spectacular than the eclipse.  You're looking at a lot of tidal action here.

Not necessarily.  Tidal force goes up with the inverse cube of the distance, and with the radius; so a moon half the distance and half the size would experience tidal forces 4 times greater.  Bigger, yes, but not stupefyingly so.  And it would have double the surface-area-to-volume ratio available for dissipating heat over time.

Our own moon is pretty darn dead, tectonically speaking.  I'd guess that you could double or quadruple the tidal load on it without causing volcanoes.

Just to put things in perspective, the one really good example of tidal volcanism we have is Jupiter's moon Io.  Compared with our Moon, it's 1.05 times the diameter, 1.11 times the orbital radius, and a primary that's 317 times as massive.  So the tidal forces it experiences are on the order of 240 times what our Moon does.

So our hypothetical half-the-distance, half-the-size moon wouldn't even vaguely be in Io's ballpark, in terms of tidal heating.