# Thread: Orbital Dynamis of Stelar Objects

1. ## Orbital Dynamis of Stelar Objects

Hi!

I'm starting to add some details to my world of Ubora. One of the things I'm looking to add is info on the planetary dynamics. Right now I'm trying to reconcile orbits for three moons. One large and two small. I want them to be similar to ours but not too close. Does anyone know of any easy to use (Free) software where I can input and test to see if the orbits are stable and see what the phases of the moons are (if any)? Or should I just say screw it and make it up and physics be damned! I would like to have something I can point to and say "Yes this is correct and see I can even show you calenders with the moon rises and phases."

2. I don't know about easy to use, but Celestia will do that (and let you see what it will look like to have all 3 in the sky at once). Creating a system involves some scripting, but there are several very detailed tutorials out there that walk you through the process.

3. Originally Posted by Gidde
I don't know about easy to use, but Celestia will do that (and let you see what it will look like to have all 3 in the sky at once). Creating a system involves some scripting, but there are several very detailed tutorials out there that walk you through the process.
Ahh Celestia! Of Course!

4. OK never mind! I looked at Celestia and how to make things and when I saw number strings and other programing gobbledy gook my eyes glazed over. I push a mouse around and go click, that's the extent of my programing ability so I'm going with the F it and make it up as I go. If my stuff is ever seen by an astrophysicist and he objects I'll worry about it at that time!

5. Nexis, dunno about software and programming, but there is a high-school maths formula for working out orbits, if you're up to it:

T = 2 pi sqroot(a^3/GM) Where T is the orbital period in seconds, a is the semi-major axis (radius for a circular orbit) in metres, G is the Gravitational Constant (6.7 e-11) and M is the mass of the planet in kg.

It'll keep most astrophysicists happy.

You could use this for both the planet around the star and the moon around the planet and then figure out the phase angles using a spreadsheet, but if you're looking for a 'point and shoot' version, I can't help, sorry.

However, I'm sure such software exists - try googling for 'orbital calculator' or 'orrery calculator' or something like that. There is probably some online thingy that will crunch the numbers for you.

Edit: This looks like a simple one:

http://www.calctool.org/CALC/phys/as...y/planet_orbit

Hope that helps.

6. There is also the Gravity Simulator (located here : http://orbitsimulator.com/gravity/ar.../download.html ) which has several simulations already available. One of them happens to be a simulation of that results in two "large" moons orbiting Earth after several collisions of 100 smaller moonlets already trapped in Earth orbit. (The downloadable simulations are located here : http://orbitsimulator.com/gravity/articles/simu.html "Moonbuilder" located near the bottom of the page) I understand you are looking for three moons, but this might help for better visualization of your goal and it might also (depending on how much work you're willing to put into it) give you three moons with a bit of alteration. The software and simulations are free.

GW

7. Thanks. Interesting but I have no idea how I'm gona be able to use it for what I want.

8. You're making unnecessary work. Basic circular orbits are stable. Look at the Solar System: tons of planets orbiting the sun, tons of moons orbiting the planets.

THREE MOONS is NOT A PROBLEM unless you are going all exotic somehow.

Just spread them out a little, is all, and you'll be fine.

9. Thanks. Interesting but I have no idea how I'm gona be able to use it for what I want.
Nexis, dunno if you were replying to me or GW here, but for the calculator I linked...

1. Decide roughly how quickly you want your moon(s) to orbit the planet (how long is your 'month'?) and how quickly you want your planet to orbit its sun (length of year).

2. Google for typical masses and distances in our solar system to give you some ballpark figures.

3. Trial and error some inputs of mass and distance to get a period that's about right. (distance can be used to tell you how big the moons will appear in the sky and whether eclipses will occur, if that's useful to you).

4. Start at a particular 'time zero' (perhaps a few months before your campaign starts) and place your planet and moons all at 12 o clock to the sun.

5. Use a spreadsheet to figure out how many revolutions and fractions of a revolution each body will have made by 'time X' and sketch the positions out on the back of an envelope.

6. Look at the relative positions of the sun and moons to figure out what phase each moon will be showing at that time.

Or...

Google for some calculator that might do the whole thing in one go.

A lot depends on how important this is to you and how much effort you're prepared to expend to achieve it. No pain, no gain, as they say.

10. Originally Posted by icosahedron
Nexis, dunno if you were replying to me or GW here, but for the calculator I linked...

1. Decide roughly how quickly you want your moon(s) to orbit the planet (how long is your 'month'?) and how quickly you want your planet to orbit its sun (length of year).

2. Google for typical masses and distances in our solar system to give you some ballpark figures.

3. Trial and error some inputs of mass and distance to get a period that's about right. (distance can be used to tell you how big the moons will appear in the sky and whether eclipses will occur, if that's useful to you).

4. Start at a particular 'time zero' (perhaps a few months before your campaign starts) and place your planet and moons all at 12 o clock to the sun.

5. Use a spreadsheet to figure out how many revolutions and fractions of a revolution each body will have made by 'time X' and sketch the positions out on the back of an envelope.

6. Look at the relative positions of the sun and moons to figure out what phase each moon will be showing at that time.

Or...

Google for some calculator that might do the whole thing in one go.

A lot depends on how important this is to you and how much effort you're prepared to expend to achieve it. No pain, no gain, as they say.
Hate to admit it but I have never used a spreadsheet in my life. LOL. OK time to find out.

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