Brightness of Lunar Features

The Moon is a place of strong brightness variations, sometimes seen in one view while looking at a brilliant peak sticking up from a black shadow. But the only way to make consistent and reliable determinations of lunar brightness is to observe at full Moon, when the Sun and the Earth see the Moon from nearly the same angle.

There are two ways to determine lunar brightness - visual estimates and photometric measurements. The great German observer Schroeter developed the visual scale that was popularized by Thomas Elger in the late 1880's. Photoelectric photometry started in the 1920's and is now done not just for a single spot on the lunar surface, but with ccd detectors, for broad areas. The weakness of the visual estimates is their subjectivity. Nonetheless, the old visual estimates and old spot measurements still have value. Here is a comparison of the visual brightness scale, N (0 is the absolute black of shadows and 10 is the brightest spot on the Moon - the central peaks of Aristarchus) and measured albedos - the measured reflectivity of the surface (0 means incident light is totally absorbed - like a black hole; 1.0 means 100% of incident light is reflected). This table comes from the article "Photometry of the Moon" by VG Fessenkov in the 1962 book Physics and Astronomy of the Moon (edited by Z Kopal).

There is a mathematical relation between visual brightness estimate, N, and measured albedo, A:

A = 0.047 - 0.136 x N