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As the earth revolves around the sun, does the ground track of a polar orbit change? I realize the oblateness of the earth causes precession of the nodes, I want to know if there's another effect.

As an example, suppose the polar orbit ground track on an equinox follows the 0 deg/180 deg longitude. The earth's axis would be in the plane of the polar orbit. What would the ground track be like on solstice? Would the earth's axis be tilted 23.5 degrees out of the plane of the polar orbit?

Maybe what I want to know is better phrased as: does the plane of a high polar orbit, high enough to not experience much precession of the nodes, change relative to the sun as the earth and satellite rotate around the sun? If the plane of the orbit is facing the sun at an equinox, is it edge on at a solstice? If the plane of the orbit changes, where is the torque coming from that changes it?

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  • $\begingroup$ I've deleted my first answer. This question is a lot more interesting than I realized! $\endgroup$
    – uhoh
    Commented May 17 at 3:32
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    $\begingroup$ Interesting question. Compare & contrast the (major) moons of Uranus. They're in equatorial orbits, but the axis of Uranus is inclined by 97.77° to its orbital plane. $\endgroup$
    – PM 2Ring
    Commented May 17 at 3:35
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    $\begingroup$ Assuming we're ignoring effects due to the Earth's gravitational field not being perfectly spherically symmetrical, and perturbations due to the Moon, I expect that the angular momentum vector of the satellite is invariant. But I'd like to see what happens in a simulation... $\endgroup$
    – PM 2Ring
    Commented May 17 at 3:49
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    $\begingroup$ @PM2Ring Even though it's so far away, there's a small component of the Sun's gravitational force out-of-plane, always pulling the satellite towards the ecliptic plane. I think that will induce a tiny precession, but maybe there's some cancellation. $\endgroup$
    – uhoh
    Commented May 17 at 3:53
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    $\begingroup$ @PM2Ring the component perpendicular to the ecliptic is a lot smaller, but I think that (like Earth's equatorial oblateness) is what induces precession, not the radial component. $\endgroup$
    – uhoh
    Commented May 17 at 4:12

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Ignoring precession and perturbation of the satellite's orbit due to the influence of other masses in the solar system, the satellite's orbital axis and the Earth's rotational axis will maintain their orientation, relative to the rest of the universe, for a considerable time.

Over very long time frames, the orientation of the Earth's axis will change due to Milankovitch cycles. Axial tilt will vary by between 22.1° and 24.5°, over a cycle of about 41,000 years. Axial precession changes the direction of the Earth's axis of rotation over a period of about 25,700 years. Of course, over these time frames the satellite's orbit will be significantly perturbed by the gravitational influence of the Moon, Sun, and other planets.

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  • $\begingroup$ If I understand you, and that's only a maybe, what is applying the torque to the satellite's axis to keep it aligned? My own thought is that the orbit stays in a plane and the earth's spin keeps it gyroscopically stable, so that over a quarter year, the earth's axis gets misaligned with the plane of the axis. $\endgroup$
    – Bruce Ediger
    Commented May 17 at 16:38
  • $\begingroup$ Yes the orbit stays in a plane and the normal to that plane always points in the same direction. Ignoring Milankovitch cycles, the Earth's axis of rotation also always points in the same direction (it may be a different direction). So after a quarter of a year, no change in misalignment will occur. $\endgroup$
    – phil1008
    Commented May 17 at 16:52

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