AstroQ: The Ultimate Cause of Nutation?

[Jinx]

According to the Peterson Field Guide entitled “Stars and Planets” © 1983, changes in the the moon’s orbit cause very minute tugs on the earth which, in turn, cause the earth’s axis to “nod” as it moves through a very small ellipse (with axes of only 18.5" and 13.7") every 19 years.

But, what is the cause of these changes in the moon’s orbit that cycle every 19 years? I WAG the longer story is that tugs on the moon from being in the earth-sun-moon system seem to tug the moon, this way then back that way, appearing to balance out every 19 years…and I’d have to bet that even that’s not a perfect net balance of zero, but such changes must be really, really, REALLY miniscule…taking perhaps eons to add up?

So, ultimately, would it be correct to say that nutation is a measurement accouting for the imperfections of the three bodied problem (ignoring the virtually non-effects from other celestial neighbors)? - Jinx

[moriah]

Nutation of the moon… isn’t that lunacy?

[Chronos]

I don’t recall all of the details, but I think that the rotations of the bodies involved (well, of the Earth and Moon at least) and the mass distributions (neither is perfectly spherically symmetric) are relevant as well. So it’s actually somewhat more complicated than a pure three-body problem, which treats all three bodies as symmetric point masses. I’m not sure if a more detailed answer exists, short of a textbook and extensive calculations.

[David_Simmons]

@Chronos

I don’t recall all of the details, but I think that the rotations of the bodies involved (well, of the Earth and Moon at least) and the mass distributions (neither is perfectly spherically symmetric) are relevant as well. So it’s actually somewhat more complicated than a pure three-body problem, which treats all three bodies as symmetric point masses. I’m not sure if a more detailed answer exists, short of a textbook and extensive calculations.

I’m a rank amateur at this but I don’t see how mass distribution of the earth and the moon could result in an 18.6 year cycle. Surely the motions of some other major, or nearby bodies (like Venus and Jupiter) come into it.

[Bytegeist]

It’s the Sun’s precessional effect on the Moon. Quoting from Fundemental Astronomy, by Kröger, Poutanen, and Donner (one of my old university texts):

Nutation. The Moon’s orbit is inclined with respect to the ecliptic, resulting in precession of its orbital plane. One revolution takes 18.6 years, producing perturbations with the same period in the Earth’s precession. This effect, nutation, changes ecliptic longitudes as well as the obliquity of the ecliptic. …[ Fortunately ] perturbations are relatively small, only fractions of an arc-minute.

[Freddy_the_Pig]

@Jinx

So, ultimately, would it be correct to say that nutation is a measurement accouting for the imperfections of the three bodied problem? - Jinx

No. The precession of the moon’s orbit (also called “regression of the nodes”), and the resulting “nutation” (irregularity of precession) of the Earth’s orbit, do indeed arise from the interaction of three bodies (Earth, sun, and moon). These interactions are well understood–not perfectly understood, as that is the “three body problem”–but more than well enough understood to account for the observed effects.