  The displacement of star images by atmospheric refraction observed by an
Earth-bound telescope is dominated by a familiar term proportional to the
product of the tangent of the zenith angle by the refractivity at the ground.
  The manuscript focuses on the torsion of the ray path through the atmosphere
in a model of atmospheric layers above the ellipsoidal Earth surface, induced
by the two slightly different principal curvatures along N--S and E--W pointing
directions. This breaking of the azimuthal symmetry effects apparent places at
the sub-milliarcsecond scale at optical and infrared wavelengths.
