The south pole is the point on a sphere with minimum 
-coordinate for a given coordinate system.
For a rotating sphere like the Earth, the natural coordinate system is defined by
the rotation axis, with the south pole given by the point in the southern hemisphere
that is farthest from the equator (i.e., midplane of the sphere).
The 
coordinate in spherical
coordinates is measured from the north pole and
takes on the value 
at the south pole.
