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# Planar Inclination confusion

## Question

Hi

I have managed to get by with my understanding of inclination for a long time, but more and more it's starting to bother me that I don't thoroughly grasp it.

I would really like to know:

I assume that the orbital plane of a planet is solely related to the star, so...

How is the 0.0 plane derived/defined in a solar system?

Are satellites around planets also calculated from the Stars defined plane? With highly eccentric planets moving up and down across the plane, am I right to think that the satellite's plane would actually always be the same when read as a parallel to the star's plane?

Minmus has a 6deg inclination. If you were preparing a launch to a coplanar orbit with it from Kerbin, setting off on a 6deg inclination on the ascending or descending side has a seemingly very different appearance. If you were to use MEchJeb and ask it to send you off on a 6deg inclination on the ascending side it looks fine, but why isn't it a -6 inclination or 354deg if taking off on the descending side of the planets rotation?!

Thanks o/

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In KSP, there is an absolute 0 degree plane. It is the same plane that Kerbin orbits the Sun, and the same plane that Mun orbits Kerbin. Also, it is parelell to the planes that ALL planets and moons rotate (their equators are parallel to this plane). So, "North" in the KSP universe is ... well ... universal. Every planet's North Pole points exactly north on the Navball, and every planet's South Pole points south.

So, when we say Minmus is tilted 6 degrees, it's both 6 degrees from Kerbin's equator, Sun's equator, Kerbin's orbit around Sun, Mun's orbit around Kerbin, and even Minmus' equator. Becuase ALL of those things are coplanar.

I can't answer the Mechjeb question as I don't use it, but if you can correctly launch 6 degrees northward from KSC at the right time to reach Minmus' exact orbital plane, the exact same launch to but 6 degrees southward 3 hours later will also get into that plane.

Note this has almost nothing in common with real life. Planets in reality can be tilted any which way and orbit with any inclinations. "0 degrees" is always relative to something, even if the person doesn't specifically say it it's implied somwhere.

Edited by 5thHorseman

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How is the 0.0 plane derived/defined in a solar system?

A reference plane is chosen. For our solar system they chose the plane of Earth's orbit.

For moons and satellites you usually (but not always!) chose the equator plane of the planet.

With highly eccentric planets moving up and down across the plane, am I right to think that the satellite's plane would actually always be the same when read as a parallel to the star's plane?

The inclination of a satellite is measured from the planets equator reference plane which itself can be quite different to the solar system reference plane. Think about Uranus. Its rotation axis is tilted 97.7Ã‚Â° compared to his orbital plane (which itself has an inclination of 0.77Ã‚Â° compared to the solar system reference plane). That means the equatorial plane of the planet is tilted 97.7Ã‚Â°, too!

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Thanks 5th Horseman and Aqua

I now see more clearly!

So what happens when defining a satellite of Earth, as Earth has a "wobbling" axis I suppose to make coplanar launches to the space station etc they define the ISS inclination at the time of needing it?

Maybe they have a more advanced Mechjeb at SpaceX and NASA : )

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For real-life astrophysical applications, using Earth as the zero point for all inclination references isn't a bad idea. Like many bodies, Earth's orbit is very close to the standard along the plane of the ecliptic. This is the plane along which most of the junk in the solar system orbits, and most of it in the same direction around the sun. This plane of our solar system also is very close to the plane of the Milky Way galaxy. Many solar systems are likewise not very inclined, while some are more inclined than others. You can see this effect on a smaller scale with the rings of Saturn, pretty strongly inclined with respect to the rest of the solar system and the Milky Way, but all those ice bits orbit along one flat plane, right along Saturn's rotational equator.

And that's what causes everything to line up so nicely: rotation.

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And that's what causes everything to line up so nicely: rotation.

Well, more precisely it's that if you have a lot of dust particles in "random" orbits, then over time they'll bump into each other and gradually average out into a nice, clean plane orbiting in the same direction. Accretion takes slightly longer than that, which is why planets end up in roughly the same plane when they form. Also, planet formation is awesome (and complicated when you get to the details) .

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In KSP, Kerbin's plane is set to 0 inclination.

IRL, I think they usually define the 0 inclination plane in a couple different ways. The sun rotates and has poles, so the 0 inclination plane can be set at the sun's equator (where the solar surface travels fastest) or perpendicular to the poles. Another way is LaPlace's "invariable" plane, which is a weighted average of the planets' rotation and orbits. Most planets are within 2 degrees of invariable plane, except Venus (just barely) and Mercury (way off). Because it's weighted by planet mass, the gas giants are hugely influential in determining this plane, but those planets (especially Jupiter) are hugely influential in how the solar system works anyway.

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Another point that deserves some explanations is the mechjeb question : actually, mechjeb will put you at the desired inclination from the one you are at the moment you launch your vessel. Problem is "relative inclination" isn't always the same from anywhere on earth (I use RSS so let's call it "earth"). I you want to use mechjeb, you'll have to set the target and then use the auto ascend option to match planes with your target and wait for mechjeb to launch you at the right date.

You can't simply use your target inclination and send your vessel to that inclination anytime, it have to be at its AN or DN for this inclination to have a sense when you want to match planes, or you have to calculate a corrective inclination AND course, just because your target's inclination (let's say moon for example) is calculated from a reference plane that is only "matched" by both your target and you at its AN or DN. Pure inclination definition is the angle of course when compared to 0 plane when it reaches AN. More details here

So, if you want to use your target's inclination and match planes, you have to launch when your target reaches its AN or counter inclination when it reaches its DN. Then, your inclination will match your target one AND your AN and DN will match as well, so you'll match planes.

EDIT : sorry, english isn't my mother tongue so explanations aren't that great...

Edited by Yzangard