Brightness of the lunar crescent

Martin Elsässer 2018

relative brightness vs. elongation
The brightness of the lunar crescent itself was calculated with a simple spherical model for various elongation values. The diagramm shows the values for the brightest part and for the overall brightness, compared to the situation at 20° elongation.

This is only a RELATIVE evaluation. For comparison with the brightness of other things such as the surrounding sky, some absolute measurement would be required.

This data/model together with the simulated Sky brightness at the position of the crescent explains why thin crescents are so much more difficult to see than wider crescents.