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In apsidal lunar precession, the moon's elliptic major axis precesses eastward and completes one revolution eastward in 8.85 years. Because the orbit is inclined relative to the ecliptic, basic geometry dictates that the ascending and descending nodes must also rotate about the ecliptic at the same rate (all else constant.) But then there's nodal precession, rotating the nodes in the opposite (west) direction, one revolution every 18.6 years. So what is the net effect of these two precessions on the positions of the nodes? Subtract the two?

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(Hope I’m not too late, two and a half years later… Just saw this post now—and I wasn’t a member of Astronomy SE back then…)

The short answer is no. These two effects don’t occur in the same plane. Imagine the lunar orbit as an elongated ellipse—much more elongated than it is in real life. Imagine the Earth as one of the foci, closer to one tip than to the other. Imagine the plane of the ecliptic as, if you wish, parallel to your floor or your desk. Now, the Moon’s orbit is tilted—by a small amount, about 5.14°—to the ecliptic (NOT to the Earth’s equatorial plane, like other planetary satellites, but that’s a different story). So imagine your first ellipse tilted with respect to your floor or desk.

The first movement, the precession of the perigee, makes it so the ellipse pivots in is own plane around the Earth. So its plane doesn’t change at that point.

The nodal precession, though, changes the orientation of the plane. The movement is still around the Earth, but parallel to the plane of the floor or the desk.

The closest everyday parallel I can find is if you own a ventilator (fan) that pivots on itself to aerate in different directions. The movement of the fan blades would represent the apsidal precession (the first you mention), while the swiveling of the fan head left and right would represent the nodal precession.

Hope this helps!

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  • $\begingroup$ Thanks for clarifying...I think I'm following you...nodal precession is like a coin spinning on a table (but staying at the same angle off the table and not flattening down.). but if the two motions don't occur on the same plane, on what plane does the apsidal precession occur? Is it projected onto the ecliptic? $\endgroup$ Commented May 27, 2021 at 20:19
  • $\begingroup$ Actually, in nodal precession, the coin would be rolling along its edge, but staying in the same place—a little like hula hoop—and the writing would always be pointing in the same direction. Apsidal precession is in the plane of the coin, to keep the same coin analogy, but it would be like the coin turning on itself, with the writing eventually becoming upside-down, then continuing to turn on itself in the same direction, eventually with writing rightside-up, then continue… $\endgroup$ Commented May 27, 2021 at 21:10
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If there was only nodal precession the Moon would the apogee of the Moon's orbit would appear to be moving to the West and if there was only apsidal precession the apogee would appear to be moving to the East. While these precessions are not in the same plane, the Moons orbit plane is fairly shallow (varying from around to 18.5 degrees to 28.5 degrees) relative to the Earth's equatorial plane. This means to a very rough approximation, they can be simply subtracted with the apsidal precession dominating the nodal precession, because it is very roughly twice as fast, so it would be reasonable to estimate the net effect is that the apogee is slowly moving Eastward. If the Moon's orbit was orthogonal to the equatorial plane, it would not make sense to use this approximation. If a more exact result is required, then this can found using a bit of geometry and trigonometry and the calculation of the monthly precession of the apogee position around the Earth's rotation axis would require both the apsidal and nodal precession to be taken into account.

Edit: I slightly misread your question. You said:

So what is the net effect of these two precessions on the positions of the nodes? Subtract the two?

Taking this statement literally, the nodal precession is the motion of the nodes and the apsidal precession has nothing to do with it. What I said before about the precession of the Moon's apogee around the axis of the Earth, being dependent on both the nodal and apsidal precession still stands and I assume that is what you meant.

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