  The basic effect of the earth's atmospheric refraction on telescope operation
is the reduction of the true zenith angle to the apparent zenith angle,
associated with prismatic aberrations due to the dispersion in air. If one
attempts coherent superposition of star images in ground-based interferometry,
one is in addition interested in the optical path length associated with the
refracted rays. In a model of a flat earth, the optical path difference between
these is not concerned as the translational symmetry of the setup means no net
effect remains.
  Here, I evaluate these interferometric integrals in the more realistic
arrangement of two telescopes located on the surface of a common earth sphere
and point to a star through an atmosphere which also possesses spherical
symmetry. Some focus is put on working out series expansions in terms of the
small ratio of the baseline over the earth radius, which allows to bypass some
numerics which otherwise is challenged by strong cancellation effects in
building the optical path difference.
