  Atmospheric refraction is responsible for the bending of light-rays in the
atmosphere. It is a result of the continuous decrease in the refractive index
of the air as a function of altitude. A well-known consequence of this
phenomenon is the apparently elliptic shape of the setting or rising Sun (or
full-Moon). In the present paper we systematically investigate this phenomenon
in a standard atmosphere. Theoretical and numerical calculations are compared
with experimental data. The asymmetric rim of the Sun is computed as a function
of its inclination angle, observational height and meteorological conditions
characterized by pressure, temperature and lapse-rate. We reveal and illustrate
some extreme and highly unusual situations.
