View Full Version : Orbital Dynamis of Stelar Objects

Nexis

11-01-2009, 04:58 PM

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! :lol: 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."

Gidde

11-01-2009, 05:04 PM

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.

Nexis

11-01-2009, 05:18 PM

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!

Nexis

11-01-2009, 06:31 PM

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! :D

icosahedron

11-02-2009, 07:34 AM

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/astronomy/planet_orbit

Hope that helps. :)

Greason Wolfe

11-02-2009, 10:51 AM

There is also the Gravity Simulator (located here : http://orbitsimulator.com/gravity/articles/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

Nexis

11-03-2009, 01:27 PM

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

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.

icosahedron

11-04-2009, 06:56 AM

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.

Nexis

11-04-2009, 07:08 PM

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.

icosahedron

11-05-2009, 09:13 AM

A spreadsheet noob doing orbital dynamics? <sharp intake of breath> ;)

Ok, here's some pointers.

When you enter a formula into a cell, precede it with the Equals sign.

eg if you put =A1*B1 into the cell C1 it will take the value in A1 and multiply it by the value in B1. You'll need to practice a bit first - I'm sure there are some online tutorials.

For this project, and going from memory...

Once you have your orbital period, you need to set up your spreadsheet to do the following:

1. For any time interval (Tx - T0) you need to figure out how many complete and partial orbits your body will have made. Just divide the time interval by the orbital period.

We'll call this the Yearfrac.

2. Use the Modulus function (MOD), entering the Yearfrac and the Period, and this will drop the whole orbits and leave you with the remaining fraction (which is the only bit that relates to the body's position). We'll call this the Remainder.

3. Divide the Remainder by the Period and multiply by 360 to get the angle of the body relative to the initial position.

4. If I've cocked anything up, or you get stuck, just ask. :)

Example:

Cell A1 is Time Zero

Cell A2 is Time Now

Cell A3 is =A2-A1 This is the time interval

Cell A5 is Orbital Period

Cell A6 is =A3/A5 This is the Yearfrac

Cell A8 is =MOD(A6,A5) This is the Remainder

Cell A9 is =A8/A5*360 This is the angle in degrees from the initial (time zero) position in the direction of orbital rotation.

Repeat for all bodies and draw out the results. Figure out the phase by inspection.

NB. Use something more handleable than seconds for your period and time intervals, otherwise you'll get lost in huge exponential numbers. Days might be ok (you might need to decide the length of your day, too).

Let me know if that works. :)

Nexis

11-05-2009, 09:34 PM

Hmmmm...Verrry Interesting.

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