I can find axial tilt of planets easily, but that doesn't specify the direction of that tilt, i.e. planet's rotation axis may be anywhere in circle defined on a sphere by axial tilt value. And I can't google for obliquity direction as it only gives me obliquity value, not it's direction. Even NASA's HORIZONS only gives obliquity value. I expect there should be another angle, either from main body equinox, Earth's equinox, ascending node or maybe periapsis, that with axial tilt and orbital elements would define direction of planet's rotation axis. I know that rotation axis is often if not usually precesses, but that takes thousands of years, and compared to time from J2000 it's practically neglectable (or at least it's easier to find precession rate and make a correction for it, knowing some starting value).

  • $\begingroup$ I'm no physicist, but wouldn't the axis of rotation for the planet vary relative to the star the planet is orbiting over the coarse of the planet's orbit? The only alternative meaning I think you might mean is the axis of rotation relative to the rest of space outside the solar system in question, is this the case? $\endgroup$ Aug 24, 2021 at 13:02
  • $\begingroup$ Obliquity (axial tilt) is set as angle from perpendicular of orbital plane and axis of rotation. It does depends on main body (star, in case of planets) only as it's in one of orbit's foci, but not defined by it in any way. That's why I've said that there's whole circle of possible values for any given obliquity - imagine plane (which is orbital plane), perpendicular vector from it and any unit vector tilted from that perpendicular at some specific angle, that gives a whole range of vectors, all of which would be correct, but only one would represent an actual axial tilt of a planet IRL. $\endgroup$
    – Aberro
    Aug 24, 2021 at 13:15
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    $\begingroup$ The orientation of a planet's rotation axis (its polar axis) changes very slowly. Earth's axis is tilted at an angle of ~23.5°, and its orientation takes ~26,000 years to go through a cycle. Currently, it points near Polaris (the pole star). Please see en.wikipedia.org/wiki/Axial_tilt $\endgroup$
    – PM 2Ring
    Aug 24, 2021 at 13:29
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    $\begingroup$ I just discovered a table of north poles of the Sun & all the planets: en.wikipedia.org/wiki/Axial_tilt#Solar_System_bodies $\endgroup$
    – PM 2Ring
    Aug 24, 2021 at 13:32


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