The Length of the Lunar Crescent

Observation of the young Moon has from time to time suggested that the outer terminator of the illuminated crescent is less than the theoretical value of 180° often by a substantial margin. This phenomena is not observed at every new Moon. It is shown that deformations of the Moon's figure are insufficiently large to account for the observed effect but it is shown that the effects of seeing in the Earth's atmosphere account for the observations both qualitatively and quantitatively.