Re: Nutation

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Jim G's original questions on this thread:

> Do I understand the following :
>
> Currently the spin axis of the Earth is ~23o off perpendicular to
> the orbital plane.

It's about 23.439 deg and currently is decreasing.

> The spin axis precesses with a period of ~23kyr

Give or take a couple of millennia.

> So I ask the following:
>
> How far off the perpendicular does this axis get?

There is a long term nutation in the obliquity of the spin axis
whose amplitude is about 2 deg and whose period is about 41 kyr.
The current value of 23.4 is about in the middle of this range
which goes from around 22 deg to about 25.5 deg.

> Why does the axis precess?  ie What is the external force?

We already discussed this in other posts.

> Does this axis also nutate?

Yes.  See above.  Also besides this 41 kyr  +/-2 deg nutation there
is supposedly a much shorter term nutation of about 9.18 arcsec in
amplitude and has an 18.6 yr period.  I don't know anything about
what causes it, though.

Besides these effects there are smaller tiny effects resulting in
the wandering in the locations of the North & South poles on the
order of a few 10s of meters over the course of a few years.
These wanderings have both a seasonal component and a longer term
component and are presumably related to changes in the Earth's
inertia tensor due to such things as seasonal changes in the
Earth's mass distribution and slow climatic changes that
redistribute mass over the earth over somewhat longer time scales
(melting pack ice, changes in ocean currents), and other things
like maybe tectonic motions & such.

A significant contribution to the shift in the location of the
South pole relative to the marker indicating its position is that
the glacial ice at the south pole is moving over the subtrate rock
fast enough to make the marker's location obsolete in relatively
short order.

> Jim Green

David Bowman