What are orbits relative to?

[tetryds]

I was thinking to myself, the planet earth spins on its axis at a certain speed, so you use that as a push to get to orbit.

If you have a very eccentric orbit, it will always point toward the same direction, so at a point your apoapsis will be closer to the Sun, half a year later it will be further from the Sun (I guess the problem is here).

Now, the sun is moving around the galaxy at ~60 degree angle, so an orbit on the exact plane of the Earth/Sun plane orbit will remain on that plane... or will it?

Ok so if i was orbiting the Moon, the same effect of periapsis going near Earth at a point, and far from it at another will happen to the Sun too?

What am I missing here?

Relativistic physics say that the motions happen from the observer POV, but thinking this way it seems that the orbit might be relative to something...

Is the orbital speed relative to the center of mass or is it relative to something else?

Because then the apoapsis wouldnt stay pointed towards some direction, it would be spinning.

But if its spinning, there must be a force causing it to change it's course, i mean, the velocity at AP and PE would be different from what is expected from a stable fixed position orbit.

Or that effect belongs to every orbit and is just not described for sanity sake?

If yes, then the angle relative to the Earth/Sun plane would change because of different orbit speeds or something?

What am I missing here?

Edit: to simplify:

What are the spinning axis of an orbit relative to?

How are those axis given?

And everything that comes with it.

Edited January 28, 2014 by tetryds

[K^2]

Motion is relative. Rotation is absolute. So is acceleration. Which is why an orbit which has a particular orientation of its major axis will maintain that orientation regardless of your choice of observer.

P.S. I'm neglecting various sources of precession for simplicity.

[goldenpeach]

Acceleration is not absolute nor rotation.

Nothing is absolute, everything is relative(including colour, time, mass and dimension).

[tetryds]

Acceleration is not absolute nor rotation.

Nothing is absolute, everything is relative(including colour, time, mass and dimension).

I know all that stuff, but still, something absolute must be relative to something.

If there are 3 cars, one accelerating at 1m/s², other at 2m/s², and another at 3m/s² they will notice different accelerations relative to each other.

A car at 1m/s² feels that because the acceleration is relative to the speed of itself, then orbits are relative to themselves?

So I could have an orbit that spins around its AP/PE axis just like that?

It would change its orbit speed on that rotation axis, which will change everything else.

Then, that might be relative to something, because that is speed, not acceleration...

Edited January 28, 2014 by tetryds

[sjwt]

Edit: to simplify:

What are the spinning axis of an orbit relative to?

How are those axis given?

And everything that comes with it.

About the only thing spin etc could be relative to is clockwise/counter clockwise, that then can be used to give you NSEW etc.. but those in them selves are only relative to that one plain.

[K^2]

Acceleration is not absolute nor rotation.

Nothing is absolute, everything is relative(including colour, time, mass and dimension).

Fantasy is fine, but General Relativity yields absolute acceleration and rotation. And seeing how GR itself is a consequence of local symmetries, I don't see how you plan of getting out of that.

One can always define an inertial frame, making acceleration and rotation absolute quantities.

[tetryds]

About the only thing spin etc could be relative to is clockwise/counter clockwise, that then can be used to give you NSEW etc.. but those in them selves are only relative to that one plain.

Ok, so, if i accelerate upwards on the midway between AP and PE of a eccentric orbit, i will start to spin on that axis?

That would cause the AP and PE to change vaules then, since part of the gravity force will be "mantaining" that rotation.

Or does it?

Edit: A planet spins on it's own axis when there is a smaller force on the surface then what would be expected from it's mass.

And that orbiting is "spinning on a timespace distortion funnel grid".

So what if the grid was spinning relative to the reference of another planet that is also spinning on it's axis?

The relative speed of the surface between them would differ from the relative speed of themselves on their rotation.

If that is not possible, you can then only say that the grid is fixed for all bodies, thus creating the proven-wrong universal relative point (or universal relative timespace grid).

Edited January 28, 2014 by tetryds

[Z-Man]

If you want to get an answer that is actually useful, drop the Relativity stuff, at least General Relativity. For full GR treatment, the answer has to be "None of the things you want information on exists." (or at least the bits I think you are talking about). From the viewpoint of the orbiting body, it just moves on a spacetime geodesic. For the kind of situations you talk about (multiple relevant bodies), there are no orbits to talk about. There is no globally well defined way to talk about directions, either.

So you have to simplify the situation: The only scenario we can talk about is one heavy central body that your small craft is moving around. The next thing we need to do is pick a suitable coordinate system that both works with GR and is intuitive; spherical coordinates with the heavy body at rest at the center and the distant galaxies at time fixed angular coordinates are the best choice. That coordinate system is also a reasonably good choice if your central body is orbiting an even heavier body, by the way, as long as your keep r small. Assume for now the central body does not rotate. And in that coordinate system, all your intuition and what you learned from KSP essentially applies. The plane of an orbit is fixed. Apoapsis and periapsis distance do not change over time. What does happen is that over time, they slowly rotate in the orbital plane.

There are two reasons any "But RELATIVITY! There is no absolute!" do not apply. The first, as K^2 already stated, relates to rotation. Rotation speed is something you can measure, it is absolute *. You know you are rotating when you feel centrifugal forces. The second relates to motion. You have a very heavy central object that you can measure your speed relative to. And as the main problem you are facing is the gravity of said object, the frame in which the object is at rest is special in the sense that it makes calculations easier.

So, to answer the thread title question: Orbits are relative to the orbited body. Because if the orbited body's rest frame is not a sensible frame to work in, you are not in orbit around it.

* locally absolute. There is frame dragging near rotating bodies and of course general geometric distortions. What does not exist is an absolute reference frame for directions. If you take a small, nonrotating object and move it around without applying torque, it will always stay nonrotating**. However, if you bring it back to its starting position, it may have changed its orientation.

** Ok, pulling that out of my rear right now. But a rotation vector (or rather tensor) should be parallel transported as you move it around, and parallel transport of zero always stays zero.

[tetryds]

Thanks, Z-Man, that made a lot of sense to me.

So basically the rotation of the orbit depends on the orbit itself and geometric distortions, thus it can be calculated geometrically and always happens.

Still, the "gyroscope" effect persists.

If you apply relativistic physics it remains as is since you are not orbiting around the wireframe, it's not like you orbiting around a wireframe while the body rotates on its own axis on the wireframe.

Just consider the frame fixed relative to you and the distortion is what moves around you, and the planet spins on it's COM.

So you cannot consider two different wireframes, you consider the one relative to you, and the rotation/translation/whatever of the other bodies relative to their COM will behave as expected.

[K^2]

It's relative to you, so long as you experience no acceleration. The key here is inertial frame of reference.