PDA

View Full Version : Designing detailed Astronomy?



Talroth
12-09-2008, 11:44 AM
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
12-09-2008, 01:23 PM
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
12-09-2008, 04:08 PM
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
12-09-2008, 04:33 PM
Those are very good points Ascension, and some of the key reasons why I want better modeling of the subject :P 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
12-09-2008, 05:50 PM
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/1538-3881/117/1/603/980139.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 (http://www.xtec.net/recursos/astronom/moon/canupe.htm) 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
12-09-2008, 06:07 PM
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
12-09-2008, 06:10 PM
Ohhh good - I always like the thought that you could. Do you have any links to those papers / sources etc ?

Notsonoble
12-09-2008, 06:17 PM
http://www.thaindian.com/newsportal/entertainment/earth-might-have-had-multiple-moons-in-the-past_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
12-09-2008, 06:57 PM
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
12-09-2008, 08:23 PM
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/gravity/articles/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.

Steel General
12-10-2008, 06:13 AM
This is a interesting discussion but way over my head (no pun intended).

jezelf
12-10-2008, 08:34 AM
it is for me too - in both respects . Nevertheless I was thinking of having a dual moon system in a story I'm writing and I did find this (http://curious.astro.cornell.edu/question.php?number=247) from a quick Google.

Part of my world building is that the planet my story will be on is in our universe and in the here and now - that they see the same stars we see.

Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

Talroth
12-10-2008, 09:03 AM
Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

For the most part, unless the characters were on a planet orbiting a star very near earth, picking out the stars that belong to given constellations would be rather useless. They are based on what appears to be some of the brightest stars To Us, but not what are actually the brightest stars. Things like the north star would be useless, becoming just another random star out there. Bright stars would dim, dim stars could brighten. If you really wanted to and you had the data, then it wouldn't be all that hard to calculate the primary visible stars from the new location, and then high light the primary stars as seen from earth.

Now, that is actually a far easier task than orbits, if you didn't want things overly fancy or precise. Did you have a project that you wanted such details for? If I can find a set of galaxy data that is compact enough to work with, but nicely detailed, then I might be able to do a program to render your new night sky view from any given star within the data set.

torstan
12-10-2008, 10:02 AM
Now I haven't done this stuff for a while so this is only loosely remembered. As far as I can remember if you have two large moons then you have to fully calculate the effects of a three-body gravitational system - which is a serious headache. It is also pretty chaotic as the gravitational potential felt by either of the moons will change as their relative distance changes.

There are clear exceptions:
1. The moons are precisely opposite each other on either side of the planet.

Therefore the gravitational pull of the other moon is directly through the centre of the planet and the orbit acts as if the planet is just a little heavier than it is.

This case is really unstable, because if the moon is perturbed from that perfect alignment then the system once again becomes chaotic and degenerates (something crashes into something else). Another issue with this instability is that the likelihood of it being created in the first place is really small.

2. The moons are very small compared to the planet.

This means that the gravitational force is dominated by the planet and the moons have a negligible effect on the force felt by the other - hence teh gas giant moons.

3. One moon is close, the other is far away, and if one moon is really light, that helps too.

This means that the moon close to the planet feels the gravitational potential of the planet with only a small effect from the other moon (better if the far away moon is small as well). Then you can approximate the situation as single body rotation. The far away one feels the planet and moon gravitational forces combined, however it is so far away, the variation of the potential due to the orbit of the closer moon is very small.

4. There is some deeply complex chaotic orbit that is stable over long periods of time and never allows one body to hit another.

I'm sure that these sort of orbits will exist as solutions. However the chances of being left in such a configuration from random creation of moons from impacts is pretty tiny. If you want to go down this route I would suggest justifying it by divine intervention :) This would make for a cool planet though as the tides would be random and unpredictable. You'd need some fairly serious mathematical understanding to evolve before you could predict the future behaviour of the moons. Until then your tides and weather are going to seem like cruel choices of fickle gods and the moons can be seen as heralds of that fate (the moon grows large in the sky and the tides rise to swallow the village - you'd think the moon was an evil god too).

Greason Wolfe
12-10-2008, 10:09 AM
Is there a program that can extrapolate constellations we see and look at them from another location? I would expect the alignment would be very different from another point of view, but my characters would will be looking at the same stars.

cheers

There is a program called Celestia (a free download, or at least it was when I got it and it had several add-ons) that might help you with this. I had it for a while, but haven't downloaded it again since the great crash of October. It is basically an interactive planetarium that allows you to "travel" to other stars and take a look at things from their point of view. It wouldn't be entirely accurate from a planetary perspective, but you could "turn on" the constellations and see what they might look like from another location in the neighborhood. And, if I remember correctly, it defined and rendered the stars based on luminosity and tried to represent their real visibility from every point of view and distance.

http://www.shatters.net/celestia/

Greason Wolfe
12-10-2008, 10:34 AM
Now, getting back to the original question . . .

I suppose that one approach you could take would be to think of it as a binary star system with a planet orbiting one of the stars (there are a few "rules" governing this possibility). Alternatively, you could approach it from the view point of that same binary system with a planet orbiting the central mass of both stars. Again, there are some rules about the mechanics of this and, at one point, I had a link to some information about this sort of thing, but alas, the great October crash did much more damage that I thought.

In any event, you might look into things from that angle and see if you can find a suitable solution. There is still a lot of mathematics involved, but following one of these two solutions might do the trick for you.

Redrobes
12-10-2008, 02:17 PM
A lot of these theories that the moon orbits degenerate because of collision and stuff is based on a small but significant probability that there is a collision so running for a long time makes the orbit not possible. But if your in a fantasy setting then I would say that if the orbit is not probably going to cause that collision then just fudge it and say they don't collide even tho there is a small chance they might. It would be like saying that in my fantasy setting with earth and moon setup there are never any solar eclipses. It wouldn't drastically affect the game to make that assertion.

Talroth
12-10-2008, 02:51 PM
A lot of these theories that the moon orbits degenerate because of collision and stuff is based on a small but significant probability that there is a collision so running for a long time makes the orbit not possible. But if your in a fantasy setting then I would say that if the orbit is not probably going to cause that collision then just fudge it and say they don't collide even tho there is a small chance they might. It would be like saying that in my fantasy setting with earth and moon setup there are never any solar eclipses. It wouldn't drastically affect the game to make that assertion.

Well, this is kind of the interesting thing about planets and their moons. Even Earth's moon is degrading and will eventually leave orbit (I forget if it is coming toward us, or going to move away and out of orbit) and ones that orbit around Mars are even more unstable.

For a large moon like Earth's, that most likely starts as a major impact, then you ideally have it stable for several million years. But for small ones, they can be 'stable' for far shorter periods of time, even if they're only stable for hundreds of years before going back out of orbit they can still have an impact on society and life.

Ascension
12-10-2008, 05:37 PM
The Science Channel used to show Carl Sagan's "Cosmos" quite frequently until being retired this summer. One of his shows was about constellations and how they look totally different from different perspectives...not only in space but also in time and culture. Run the program backwards and Ursa Major doesn't look like Ursa Major at all. Viewed by the ancient Chinese, it was actually three figures (a dragon, a cart, and a tax collector) instead of a bear. Since everything is moving, over time the shape will change and for us the bear will become something else (although that time is thousands of years off).

As to the moon retreating away from us, there was a History Channel show called "If We Had No Moon" (narrated by Patrick Stewart) that says the moon is moving away from earth at about 1 mile per year.

jezelf
12-11-2008, 08:14 AM
Talroth: yeah - the stars would be brighter and dimmer, thanks for pointing that out. Thanks for offereing to help out. At the moment, it's nothing more than ideas and note making, structuring creation etc. I might do some illustrations in time with a night sky, but I will also be tempted to work out the night sky per season.

Greason Wolfe: Cool program- Thanks! I had a play about with it. Need to explore more, but that could be just what Im looking for now.

Ascension: That sounds good. I'll have to look into that. The stars representing something else is something I did think off too. I did have a Night Sky program, you could look back and forward in time and see the stars in a similar way

My story is a fantasy epic. Just liked the idea on something going on in our unvierse on another planet in our galaxy, right now. Its more for a world building note to self than appearing in the story. The stars might crop up in the story as wizards or characters naviagte and I'll give them mythical names etc.

I thought it might be cool to work out what those constellations would look like and see if I can create new ones for my characters based on their persective. Might also inspire mythical creatures for my story's history.

I guess as Im creating a fictional planet, putting it in a known part of space might conflict with any suspension of reality...so it might be a flawed idea. - and probably too much detail, but I'm a sucker for it, even if it's all just for my own reference.

thanks for the help!

Greason Wolfe
12-11-2008, 03:51 PM
Greason Wolfe: Cool program- Thanks! I had a play about with it. Need to explore more, but that could be just what Im looking for now.

No problem, glad to hear that it might be useful to you. I found it by accident one day when I was researching some stuff for a javascript application I was writing for some personal astrophysics stuff. I thought it was a pretty sharp program and I'm going to have to go get it again one of these days.

Talroth
12-29-2008, 09:58 PM
This would be so much easier if my last physics class that covered the math I actually need for designing the simulation wasn't over 4 years ago,...


But by playing with fairly low density moons (2/3 that of our moon) and the orbitsimulator program Greason Wolfe posted, I've managed to get 7 moons ranging in visible size from 50% to 150% of our current moon. Sadly that simulation has several issues in step size, as patterns highly depend on speed the simulation is run at. That is, running ultra high speeds produces very stable systems, where as lower speeds have far more wobble to them.

Mind you, I didn't actually try it with standard density moons, and it would have produced several small moons that would travel across the sky at fairly high speeds. I'll try to find the time to play with these problems over the next week or so and see if I can't come up with some better modeling tools.


*Edits*
Very odd, systems can die very violently after displaying very steady patterns for several thousand years. I'm still thinking it might be an issue with their stepping algorithm, as running a burst of high speed, then restarting and waiting for it to run at a slow speed produces very different positions. Anyone else that uses the program noticing this?

EDG
12-30-2008, 03:24 AM
Looks like I joined just in time :).

Orbit Simulator (aka Gravity Simulator, or GravSim) is a godsend for this sort of thing, I've used it a lot (I post on their forums as Mal).

Celestia is also a good visualisation tool.

Gravity Simulator is a tricky one to use properly, but is pretty much unparalleled and incredibly useful. The timestep size has to be much smaller than the smallest orbital period involved in the simulation to get accurate results - I think I usually go for 5 to 10% of the shortest orbital period. So if you have a moon with a 10 day orbital period, you'll need to have your step size at a maximum of 32k-64k seconds. The step size does affect things a lot since for larger step sizes two bodies can influence eachother for only one or two steps, whereas for smaller step sizes they'd influence eachother for many steps - and that changes everything that comes afterwards.

So you need to run systems slowly in GravSim - and if that means running it for a few hours (or overnight) then that's what you'll have to do. Fortunately you can save a data file, and there's a really good visualisation tool for the results available that one of the users wrote and has posted on the gravsim discussion boards too.