A scale measuring the brightness (or rather, the darkness) of lunar eclipses, devised by the French astronomer André Louis Danjon (1890-1967). A table is provided
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}