To: WSFAlist at keithlynch.net
Date: Sun, 24 Mar 2002 00:39:09 -0500
Subject: [WSFA] Re: the earth's tilt
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Sat, 23 Mar 2002 11:08:46 -0500 Steve Smith <sgs at aginc.net> writes:

> I seem to remember a report that the north *magnetic* pole was
> moving around more than expected.

The magnetic poles normally wander around within a confined area, in a
quasi-predictable manner, over time spans of a few years.  Predictable to
the extent that maps are published showing the isoclines of magnetic
declination for a given year, accompanied by a table of corrections
running several years out.  Presumably the south magnetic pole wanders as
much as the north magnetic pole, but the wanderings of the north magnetic
pole are of greater interest to people using magnetic compasses in North
America.

The physical pole would be a lot harder
> to move -- the Earth is a muckin' big gyroscope.

Angular momentum should be conserved.  It requires external forces to
change the direction of the earth's axis.  Such forces do act, and over
time the axis does change direction, slowly.  The axis processes due to
the Sun's gravity and the earth's equatorial bulge, completing one cycle
in about 25,000 years.  That is sometimes called 'the precession of the
equinox'.  Also, there is a slow oscillation in the obliquity of the
ecliptic, caused (I think) mainly by Jupiter's gravity.

Theories of how the
> Earth generates its magnetic field are, uhh, incomplete.
>

The field might be caused by convection in the mantle.

> One of the (kook) theories of why the Earth's crust "tips over"
> every now and again depends on the reversals in the Earth's magnetic
> field.  Basically, at the crust-mantle discontinuity, there is a
magnetic
> effect that holds the crust onto the rest of the planet.  When the
> magnetic field goes away during a reversal, the crust isn't held on any
more
> and it slides around.  I believe the "Pole Shift" book that somebody
> mentioned discusses this.
>

Of course the crust does move around slowly in pieces (continental drift,
plate tectonics).  The land that we now call Antarctica once had jungles,
and thus was presumably at much lower latitudes than it is now.  And it
is generally accepted that magnetic pole reversals occur almost
regularly.  But that (kook) theory has the whole crust as a unit swapping
north for south.  As it is, the crust already floats on the mantle.
Perhaps the theory presumes that the crust is magnetized, so when a
magnetic pole reversal occurs, it drags the crust on around with the
field.  Of course the ferromagnetic minerals in the crust are magnetized,
so the crust is lightly magnetized, but the fly in the ointment is that
the feeble magnetic forces would not actually be able to move the crust
much, given the viscosity of the mantle, just as an ant pushing on a
battleship can't move the ship much, given the viscosity of the water in
which the ship floats.

.

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