'Best time' for the visibility of the lunar crescent

The concept of "best time" for the first visibility of the thin crescent moon developed by Bruin, Schaefer and Yallop did not consider the elevation of the site of observation. Our first estimation -- after analyzing some documented observations -- is that "best time" is directly proportional to site elevation and inversely proportional to moon altitude. For moderate elevation sites (less than 1000m) the crescent could be first seen shortly after sunset. However, for higher elevations (around 2000m) the crescent could be first seen shortly before moonset.

By using our first visibility photometric model, the extensive data of Blackwell 1946 experiment and the measured twilight sky brightness of our site (1990m), we find that the optimum lunar altitude for first visibility is about 2 degrees, no matter what the lunar elongation is.