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The precession of the equinoxes (AKA general precession) is composed from two components:

  • A dominate component which is caused by the the tilt of the Earth's axis - which is called "precession of the equator", or "Lunisolar precession".
  • a minor component which is caused by the very movement of the ecliptic plane itself. this component is called "precession of the ecliptic" or "planetary precession".

I'm not sure I understand what is the nature of the minor component, but the impression I get is that it is equivalent observation-wise to the major component (in very much the same manner that static Sun and a moving Earth is equivalent to moving Sun and static Earth). Is this is the case?

If indeed this is so, can we somehow distinguish between the components physically by actual observation, or it is only by theoretical gravity calculation that we know the correct share of each component in the precession of the equinoxes?

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  • $\begingroup$ "Precession of the equator" is mostly the Earth wobbling like a spinning top. "Precession of the Ecliptic" is the Earth's orbit changing. $\endgroup$
    – Greg Miller
    Mar 24, 2022 at 1:03
  • $\begingroup$ @GregMiller, what is "Earth's orbit changing"?, and how it results in different observable universe with respect to Earth axis wobbling $\endgroup$
    – d_e
    Mar 24, 2022 at 8:11

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"Precession of the equator" refers to long-term changes in the Earth's rotation axis relative to the mean (average) orbital plane of the Earth-Moon barycenter about the Sun, aka the ecliptic. "Precession of the ecliptic" refers to long-term changes in the mean orbital plane itself.

The Earth's orientation with respect to the ecliptic changes primarily because of torques exerted on the Earth by the Moon and the Sun, but also because of torques exerted by other planets. Because of those other torques, it is not quite correct to call this "lunisolar precession".

The mean orbital plane of the Earth-Moon barycenter about the Sun also changes slowly due to the influences of other planets (and also the Sun's not-quite spherical shape, and also relativity). The mean orbital plane of the Earth-Moon barycenter about the Sun is tilted a bit (about 1.57°) from the solar system invariable plane.

There are also short term effects, with frequencies less than 20 years. These short term effects are lumped into two groups. Nutation models the short term effects that can be predicted for hundreds of years. Like general precession, nutation also results from torques on the Earth's rotation and its orbit. Polar motion describes short term effects that cannot be modeled accurately beyond a year or so. Polar motion includes what physicists call torque-free precession. Polar motion also includes effects due to transfer of angular momentum between the Earth's core, mantle, oceans, and atmosphere. These small amplitude motions are observed. Predictive models don't cut it after a year.

From the perspective of physics, precession and nutation are aspects the same phenomenon. The frequency gap between the Earth's precession and nutation is so vast that it makes sense to model them separately.

In theory, other stars also exert an influence on both the Earth's orientation and its orbital plane, but these effects are so small that they are not modeled.

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  • $\begingroup$ Thanks. much clearer now I think. Just little one more question: I can see how/in what manner Earth's axis rotates in 26000 years; but I still don't see how the The mean orbital plane of the Earth-Moon barycenter "rotates". that's basically the main thing I am asking. does it keep the 1.57deg tilt? (pretty much like the obliquity is kept at 23.5deg [yes, I know it also not accurate and it also moves between 22.5deg to 24deg - but for the sake of the question]). $\endgroup$
    – d_e
    Mar 24, 2022 at 19:39
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    $\begingroup$ @d_e Just as the Moon and Sun (and the planets, to a much lesser extent) cause the Earth's rotational axis to rotate about the Earth's orbital angular momentum vector, the planets cause the Earth's orbital angular velocity vector to rotate about the solar system's angular momentum vector. In particular, the planets can cause the Earth-Moon barycenter's right ascension of ascending node with respect to the "celestial intermediate pole" to precess. $\endgroup$
    – David Hammen
    Mar 24, 2022 at 21:09

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