Designing detailed Astronomy?

[Talroth]

I often use maps as the basis of story writing, and this is one of the biggest reasons I do anything related to them. By starting with a map I can flesh out geographic settings, that leads to geo-political, and societies in general. Personally I spend far more time setting up a foundation to build a story on, than I spend on the story itself. (Grew up helping build houses, has odd effects on people I guess.)


I've been trying to think of better ways to model and track details such as motions of moons and planets that could be seen, as well as key stars in the night sky. As a computer science student in university, I enjoy making digital models, and having the physics of my world grounded in real world math and physics so that I can actually use real world math and physics for the characters. That is to say, I don't simply want to throw out four or five moons, and say they rise and set at these times, with these motions.

I've taken a few classes in astrophysics, but not enough to generate a stable model that lasts over a long period of time. (A few centuries always sees something like an inner planet flung out of the system. often the one I want my novel set in)

Has anyone else tried designing a system that has an earth sized planet, but multiple smaller moons? Any suggestions on how best to do it, and ways to compute current state of everything if given time as an input?

[HFP]

I did it, but completely by hand. It was a nightmare.

The world I did it for has two moons. The both have different orbits. The problem was that only once a year can they both be full or new moons at the same time. That is, only once a year are both moons full moons, and only once a year are both moons full moons. This is important for the calendar and religious festivals.

It took me a week to figure it all out on paper, and then I lost it. The basement got flooded and all my notes had to be thrown out. I would love for someone to come up with a program that stuff out for me - let me input 2 moons, the period of their orbits, and to specify custom events.


Lisa

[Ascension]

My main quibble with multiple moons is how they would affect the tides. Writing about sea voyages would be hard to figure out if there never was a low tide or neap tide, etc. Also, would multiple moons throw the 24-hour cycle out of whack by the gravitational pull affecting the rotation? Maybe if they were small moons there would be no problem...I dunno. Then there is the orbit path...wouldn't want the moons bumping into each other and gravity would pull them closer together over time...especially with different size moons. So much to think about and I'm nowhere near smart enough to do the math...I failed college physics 101. A model would most certainly be useful and a neighbor kid I used to babysit works for NASA so I'll ask him if he comes up for the holidays.

[Talroth]

Those are very good points Ascension, and some of the key reasons why I want better modeling of the subject Multiple moons would change tides a lot, but might have some interesting properties, such as very minor tides in general, and then massive tide changes on proper alignments.

[Redrobes]

I was interested in this question so did a bit of searching...

There was this paper

EVOLUTION OF A TERRESTRIAL MULTIPLE-MOON SYSTEM

which seemed like just the sort of thing we need...

http://www.iop.org/EJ/article/1538-3...0139.text.html

and lets skip all the work and go straight to the conclusions

The purpose of this work has been to conduct a preliminary study of the stability of a terrestrial multiple-moon system, using both analytical techniques and numerical integrations. In particular, we are interested in the stability of multiple-moonlet configurations predicted by modeling of lunar accretion from an impact-generated disk. Our results indicate that all of the systems produced in ICS97 will likely yield a single moon for reasonable values of tidal parameterizations.

So it seems 'no' you cant have a stable multi moon scenario - at least not by what they reckon.

Just to be clear they reckon that its not possible for Earth type planets but not necessarily a bad idea for others especially gas giants.

Wouldn't have wanted to do all that work to figure that out myself tho

[Notsonoble]

That particular article talks about the idea of moons formed from impacts (which is the current theory of our moon)... since then a theory of two moons existing prior to our current one provides a stable bi-moon model. The first was in '99, the new one (about the trojans) is from '08.

[Redrobes]

Ohhh good - I always like the thought that you could. Do you have any links to those papers / sources etc ?

[Notsonoble]

http://www.thaindian.com/newsportal/..._10045903.html

That's one of several articles that say the same thing, I'm having a really hard time finding any paperwork on it though.

The neat thing about the Trojans is their fixed location and non-catastrophic formation. While the Trojans were small, it's quite easy to see that larger ones could form in other solar system formations.

It looks like that's the downside though. They kinda have to be there to start with, and still be far away or small enough not to have large effects on a tidal system, because then the torque has to go somewhere. This would probably limit multi-moon systems to big hollow or low density things.

[Talroth]

Another option that I just remembered. If you have a large primary moon, say something the size of our moon, then it should be mathematically possible to place something like mars's two moons at the Lagrangian Points, 3, 4, and 5. If rather improbable to have this happen, at least it should remain mostly stable for a good amount of time. Now, mars has two moons and are in slightly unstable orbits, but that is mostly due to how low they are. The hardest part is working out just how, and where to form moons. Perfect orbital captures of larger objects are possible, just highly unlikely.

[Greason Wolfe]

I haven't experimented with this yet, so I'm not sure if it will be of any specific help, but maybe you'll find it useful.

http://www.orbitsimulator.com/gravit...cles/what.html

If you have AstroSynthesis, you might be able to find results similar to what you are looking for. I have the older version and got the following results for earth by regenerating system contents until I got two moons.

Moon One
Orbital Distance - 34952 km
Radius - 128 km
Gravity - 0.02G
Orbital Period - 0.75 days
Rotation - Tide Locked

Moon Two
Orbital Distance - 518415 km
Radius - 1644 km
Gravity - 0.1G
Orbital Period - 42.72 days
Rotation - 884 hours

Such results would suggest that it is possible for two moons to exist around an earth-like body, but working out tidal effects as well as geothermal effects is well beyond the scope of my abilities.