Specifics of planetary orbits.

[Apollo7]

Ok so I've been doing these solar systems in Celestia, and for the most part I use 4 main characteristics to plot planetary orbits, they are: Period, SemiMajorAxis, Eccentricity and Inclination. For the basics these four items work reasonably well. However what do I need to do if I want to create something more complex, like the 3:2 ratio of revolution that pluto and neptune share. It is well known that Pluto and Neptune rarely come closer than about 15 AU from each other, but that Pluto, when it swings above the plane of the ecliptic passes within Neptune's orbit and for a time is the eigth planet from the sun.

I know that Celestia uses several orbital arguments with which I am not all that familiar, like the Argument of Pericenter, Ascending Node, Mean Longitude or Rotational Offest. Can anyone explain these things to me? I'd like to create a system where I can have a 3:2 resonance and actually make it work, so that the planets don't cross near each other, but instead avoid near-encounters. Any ideas?

[selden]

I created a crude diagram that shows the orbital parameters at http://www.lns.cornell.edu/~seb/celestia/orbital-parameters.html It doesn't go into a lot of detail in its textual descriptions. Please let me know where it can be improved.

[Apollo7]

Thank you the diagram helps quite a bit however I think I, by chance, solved my problem, let me tell you what I was trying to do.

I have two planets very near a F9 V class star. "A" is an earth-sized world with a radius of 8041Km, a Mass of 1.9Me and a period of 14.086 days. The planet has an eccentricity of .547 and a SemiMajorAxis of .115253. Planet "B" is a gas-giant weighing in at 89.44 Earth Masses, a radius of 45738km and a revolves in a nearly circular orbit with a semimajor axis of 0.05547 and an eccentricity of 0.625.

Now the problem was planet "A" has a period of nearly exactly 3-times that of planet "B" and their perihelia (sp?) are nearly identical (.052167 for A as opposed to 0.051942 for B). The issue was that the planets would collide, literally, if I left the orbits without any modifications. However I found that if I set the AscendingNode for A to nearly 180 degrees. Then, the planets avoid each other at all times. The closest they approach is between .110 and .105 AU. Though these distances are very small the closer approach at .105 occurs while both planets are on opposite sides of the star. The second comes at a time while A is approaching perihelion. Granted this system may not be accurate or plausable, but since both planets come nowhere near each others Hill-radii, for sci-fi purposes I think it works. Now if I could just find out WHY changing the ascending node to 180 fixed things.

Cheers.

[selden]

Apollo7,

An eccentricity of 0.6 is not very circular, so I'm guessing you meant 0.06.

Changing the Ascending Node by 180 rotates the orbit's orientation by 180, putting the apohelion, perihelion and the planet on the opposite side of the star from where they were. With a 1:3 resonance, if the planets were colliding before, they can't possibly collide now: they'll be on opposite sides of the star when at periheiia.

Does this help clarify things?

[Apollo7]

Yes indeed I meant .06, heh sorry for the typo, but thats ok. I think I get it now, by rotating the ascending node, the planets can be kept apart as their perihelia are now on oposing sides of the star. To clarify the ascending node is the point at which the orbit travels above the reference plane, yes? (i.e. the ecliptic in the Solar System).

Thanks for the info again. Cheers.

[selden]

To clarify the ascending node is the point at which the orbit travels above the reference plane, yes? (i.e. the ecliptic in the Solar System).

Well, I'd use the phrase "up through" instead of "above", but I think you've got it.