> Assuming an astronomical object is a member of a two-body system and
> has a spin axis and an orbital plane, can the object's spin axis
> precess around an axis /other/ than the normal to its orbital plane?

> Doesn't seem likely, not with only two bodies.

> Okay, add a few more bodies.

> Take the Moon, for example.  Its spin axis precesses about the normal
> to its orbital plane.  

> Why couldn't its spin axis also precess around the normal to the
> ecliptic?

> Or does it, but to a very much smaller degree?

> Is that what nutation is?  A smaller and faster precession around some
> other axis superposed on a somewhat larger and slower "main"
> precession?

Quadibloc <jsav...@ecn.ab.ca> wrote:

>On Jul 1, 10:31 pm, Dave Typinski <möb...@trapezium.net> wrote:
>> Assuming an astronomical object is a member of a two-body system and
>> has a spin axis and an orbital plane, can the object's spin axis
>> precess around an axis /other/ than the normal to its orbital plane?

>> Doesn't seem likely, not with only two bodies.

>> Okay, add a few more bodies.

>> Take the Moon, for example.  Its spin axis precesses about the normal
>> to its orbital plane.  

>> Why couldn't its spin axis also precess around the normal to the
>> ecliptic?

>> Or does it, but to a very much smaller degree?

>> Is that what nutation is?  A smaller and faster precession around some
>> other axis superposed on a somewhat larger and slower "main"
>> precession?

>To find out what precession can do, this Wikipedia article seems like
>a good one...

>http://en.wikipedia.org/wiki/Precession

>Since nutation, described on another Wikipedia article, is largely
>tied to the advance of the Moon's nodes, no doubt it is a response to
>the Moon's gravity even as ordinary precession is a response to the
>Sun's.  It's probably simpler to combine the torques and get the
>resultant precession than to add precessions, though.