Flatness of the setting Sun

摘要

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