THE DANJON LIMIT OF FIRST VISIBILITY OF THE LUNAR CRESCENT

By Louay J. Fatoohi, F. Richard Stephenson & Shetha S. Al-Dargazelli Department of Physics, University of Durham

When the distinguished French astronomer Andre Danjon was the director of Strasbourg Observatory, he became engaged in determining the light curve of the Moon. In 1931 he noticed that the Moon of August 13, which was only 16.2 hr before new, extended only 75-80Β° from cusp to cusp. In other words, Danjon found that the outer terminator of the crescent was considerably less than a complete half-circle, which it should have been theoretically. This was not an isolated observation because other observations, and also examination of previous records, showed that this shortening of the crescent was a general and real phenomenon. Danjon also noticed that the shortening diminishes as the angular distance of the Moon from the Sun increases.