Re: North Pole

[Gary Turner <turner@MORNINGSIDE.EDU>]

Maybe clarify the posts b/w John and Michael (then again, I could be
completely wrong!)-

The precession is the rotation of one vector about another.  The precession
of the Earth's spin axis is typically considered to be the rotation of that
angular momentum vector (associated with the Earth's spin) about a vector
perpendicular to the ecliptic (defined by the Earth's orbit) - the Earth's
orbital angular momentum vector

To talk about the precession of the orbital plane, you would have to have a
second reference vector - maybe the orbital angular momentum of the entire
solar system - for the Earth's orbital angular momentm vector to precess
about.

[Sidenote]
The orbital angular momentum vector of the entire solar system could be
fairly accurately determined - Jupiter plays a huge part in that.
Undetermined mass beyond pluto would probably be a minor role because it is
moving so slowly (1/sqrt(r)) and, more importantly, most probably has a
very small mass.

Could this be determined analytically?  Not likely.
Could this be observed?  Even less likely

The observation would have to be that the precession "circle" is not
circular at all, but offset some amount.  Then in 26,000 years, the "North
Pole" would not be pointing to the same place in the heavens as it is
today.  The circle would not overtrace itself, but would be offset some
amount.  However, motions of the reference stars are probably more
significant than that difference, making it virtually impossible to measure.

The answer, to the best of my knowledge, is "we don't know".

The angle between the ecliptic and spin axis does not change
(significantly).  The angle between a plane and a vector always causes
problems IMHO, but the smallest angle will be the same.  The orientation
changes on that plane, but the angle above the plane remains the same.

Of course, eventually a spinning top falls over ...


On Sat, 5 Oct 2002 17:01:42 -0600, Jim Green <JMGreen@SISNA.COM> wrote:

> Has anyone noticed that if the question is about hyperbolic a general
> relativistic space warp, we do just fine.  But if the question is simple,
> the conversation per force gets involved in a hyperbolic space warp all on
> its own.  What am I missing?
>
> Let me try again:
>
> > The Earth's spin axis precesses
> >  such that we get a moving polar star.
>
>  ie the current North Star is Polaris but it was not in 1AD.
>
> Is it also the case that
>
> > ... the orbital axis precess?
>
> {for the senile Yes or NO?}
>
> or is it that
>
> > ... the angle between the orbital plain and the spin axis changes?
>
> {for the senile Yes or No?}
>
> TX
>
> Jim
>
>
> Jim Green
> mailto:JMGreen@sisna.com
> http://users.sisna.com/jmgreen
>
> This posting is the position of the writer, not that of SUNY-BSC, NAU or
the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.