Talk:Laplace plane

Added info/corrections/rewrite/citations
Information has been added/corrected as a result of discussion on the talk page of the article Invariable plane, and references discussed there. Terry0051 (talk) 18:46, 28 August 2009 (UTC)
 * Hi Terry. It appears to me that you have added some helpful things, but also removed some helpful things and repeated some things that were already there.  Tonight I can only do first-order corrections because I'm busy with something else.  --BlueMoonlet (t/c) 03:25, 30 August 2009 (UTC)

[From Terry0051]: Hallo BlueMoonlet, thank you for your comments. I note that you accepted in your new edit the previous corrections removing inappropriate references to the ecliptic or invariable solar system plane. But your edit also restores an initial "definition" that is unsourced and is at least ambiguous -- if not positively incorrect and circular in logic. Specifically, what is meant by 'precession cycle' in the context of this 'definition'? Bear in mind (as I'm sure you're aware), that the motion of the pole of the orbit of any real satellite has a number of different periodic components, not just one, and it would give the 'definition' a hopelessly circular logic to explain the ambiguous term by reference to the plane to be defined. I don't mind at all, by the way, that you were savage with my previous edit (seeing that nobody owns anything here on WP), but the latter did at least have the advantage, that it avoided circularity of logic and relied on a description as clear as it gets (so far). I also follow the common and useful practice of placing disambiguations at the top.

Would you like to do the work of mending this opening 'definition' (or supporting it with a suitable citation), or shall I?

With good wishes Terry0051 (talk) 19:37, 30 August 2009 (UTC)


 * Terry, just because I didn't change something doesn't necessarily mean I accept it. As I said, I'll get back to this article in more detail when I have time.
 * Briefly, I see no circularity or other problem in the current definition. In real applications, orbital precession tends to be dominated by one particular frequency.  If there were multiple frequencies of roughly equal strength, then you would be right that defining a Laplace plane might be difficult.  But when one frequency is dominant, the Laplace plane is the mean plane for the dominant precession mode.  The existence of much smaller superposed perturbations does not diminish the usefulness of the concept.
 * On the other hand, it needs to be clear that the Laplace plane does not by its nature have anything to do with the mean angular momentum (though the two correspond closely when perturbers are sufficiently far away). The Laplace plane is more defined by the mean plane of each important perturbation, weighted by the relative importance of that perturbation. --BlueMoonlet (t/c) 00:31, 31 August 2009 (UTC)

I'm afraid I have to call errors here. Ambiguity is not dependent on physical strength of perturbation terms, it arises because the words do not make clear which perturbation terms or other features are indicated. The Laplace plane of a satellite has little to do with dominance or strength of terms, the Moon is a counterexample contradicting that, the term corresponding to its Laplace plane is very tiny, almost everything else is more dominant. Then again the Laplace plane doesn't tend to the invariable plane when the perturbers are sufficiently far away, it tends to the solar orbital plane of the parent planet, see ESAA p.327. Terry0051 (talk) 02:29, 31 August 2009 (UTC)


 * Your second point is correct, though the two are generally very close to each other. I don't understand what you mean by your first point.  The dominant mode of the Moon's precession is about its Laplace plane (basically the ecliptic in this case).  This is clear enough that Giovanni Domenico Cassini was able to figure it out in 1693 (see Cassini's Second Law).  --BlueMoonlet (t/c) 14:06, 31 August 2009 (UTC)