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 75-80° 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.
By Bradley E.Schaefer NASA/Goddard Space Flight Center, Code 661 Greenbelt, MD 20771, USA
Prediction of the first visibility of the lunar crescent is a difficult problem involving astronomy, meteorology, and physiology. Historically, this problem has been attacked by an empirical approach where some set of observations is used to deduce a criterion for visibility. In this paper, I present a list of 201 observations and their observing circumstances for use in deriving and testing prediction algorithms. I find that criteria involving the moonset lagtime and the Moon’s age are quite bad in their predictive ability. Criteria involving the relative altitude and azimuth of the Moon at sunset are better, yet still can yield incorrect predictions within a zone of uncertainty with a width of over 105 degrees in longitude. The new theoretical model of Schaefer ( 1 988) is found to have a zone of uncertainty with an average total width of 47 degrees in longitude.