WILL THE REAL SOUTH POLE PLEASE STAND UP!
J. S. Sweitzer
There are actually three "South Poles." Two are near the Amundsen-Scott
South Pole Station and one is not even on the Antarctic continent! Which
one is the "real" South Pole? The one that is out in the Antarctic Ocean
is the south magnetic pole of the Earth. It was last located precisely
at a position of 65.3 degrees south latitude and 140 degrees east longitude
in 1986. Technically, it is called the south magnetic dip pole, and is
at the point on the Earth where a compass needle, which is able to move
vertically as well as horizontally, points straight up. The magnetic pole
is due to magnetic fields that are generated deep in the Earth's core.
These fields change slowly and flip from south to north on a very long
time scale. For that reason, the south magnetic pole is rarely found precisely
at the real South Pole of the Earth.
If you were to visit the Amundsen-Scott South Pole Station one of the
first things you might do is to have your picture taken by the ceremonial
South Pole. It is the one with the barber pole stripe and reflecting globe
on the top. Surrounding it is a circle of flags of the nations that have
signed the Antarctic treaty. This pole is where it is, because it is in
a convenient location -- close to the station and within the aircraft
skiway turn-around circle. But it is also not the true pole of the Earth.
The real South Pole is a couple of hundred meters beyond the ceremonial
pole in the direction opposite of the station dome. It is a stake with
a small brass plaque on top. Next to it is a sign that labels it as the
Geographic South Pole. This one is the real thing. But if you look beyond
it you see an entire string of old pole stakes. Each of them used to be
the Geographic South Pole in years past. What is going on?
The reason the Geographic South Pole needs to be re-staked each year
is not because the Earth's axis itself is moving*, but rather because
the ice beneath the station is sliding 9.9 meters every year. When you
are at the South Pole you are standing atop a nearly 3 km thick ice cap.
Over long time scales, ice slowly moves down hill. In this case downhill
is in the direction of the Weddell Sea, 1,400 km away. At the rate quoted
above, that would mean the current South Pole marker will drop into the
sea in about 140,000 years.
- Technically, I might get caught on this, so let me take a few lines
to describe a fine point in physics for advanced readers. The axis of
symmetry of the Earth stays fixed on the surface of the Earth. The axis
of rotation, however, does not. The rotational axis actually wobbles
in a roughly 12 meter diameter circle about the South Pole over a period
of 430 days. This is called the Chandler wobble and is an example of
what physicists call free-space precession. This precession is a result
of the fact that the Earth is not a perfect sphere.
Don't confuse this precession with an entirely different type of wobbling
-- the precession of the equinoxes, which occurs over a period of 25,600